978142001516selüloz

5 Cellulose and CelluloseDerivatives

Donald G. Coffey, David A. Bell, and Alan Henderson

CONTENTS

5.1 Introduction ...............................................................................................................148

5.2 Cellulose..................................................................................................................... 148

5.2.1 Occurrence and Isolation ................................................................................ 148

5.2.2 Structure..........................................................................................................149

5.2.3 Biosynthesis.....................................................................................................151

5.2.4 Chemical and Physical Properties ................................................................... 151

5.2.5 Physically Modified Celluloses ........................................................................ 152

5.2.5.1 Microfibrillated Cellulose ..................................................................152

5.2.5.2 Microcrystalline Cellulose..................................................................152

5.3 Chemically Modified Cellulose Derivatives ...............................................................152

5.3.1 The Manufacture of Cellulose Ethers .............................................................153

5.3.1.1 Preparation of Alkali Cellulose ......................................................... 153

5.3.1.2 Alkylation and Hydroxyalkylation of Alkali Cellulose .....................153

5.3.1.3 Product Purification .......................................................................... 154

5.3.2 Characterization of Cellulose Ethers...............................................................154

5.3.2.1 Degree of Substitution.......................................................................154

5.3.2.2 Molar Substitution ............................................................................154

5.3.3 Sodium Carboxymethylcellulose ..................................................................... 155

5.3.3.1 Effect of DS on Solubility ................................................................. 156

5.3.3.2 Solution Rheology ............................................................................. 156

5.3.4 Methylcelluloses ..............................................................................................159

5.3.4.1 Solution Rheology ............................................................................. 159

5.3.4.2 Thermal Gelation............................................................................... 159

5.3.4.3 Solute-Induced Gelation ....................................................................162

5.3.5 Ethylmethylcellulose........................................................................................163

5.3.6 Hydroxypropylcellulose...................................................................................163

5.4 Applications in Foods................................................................................................163

5.4.1 Fish/Meat ........................................................................................................164

5.4.2 Sauces, Gravies, Soups, and Syrups................................................................ 165

5.4.3 Emulsions........................................................................................................166

5.4.4 Baked Goods...................................................................................................167

5.4.5 Frozen Desserts ............................................................................................... 168

5.4.6 Emerging Technologies: Barrier Films............................................................ 168

5.5 Nutritional Effects of Cellulose Derivatives .............................................................. 169

5.5.1 Dietary Fiber...................................................................................................169

Stephen / Food Polysaccharides and Their Applications, Second Edition DK3085_C005 Final Proof page 147 20.4.2006 4:52pm

147

© 2006 by Taylor & Francis Group, LLC

5.5.2 Water-Holding Capacity ................................................................................. 170

5.5.3 Metabolism ..................................................................................................... 170

5.6 Conclusion .................................................................................................................171

Appendix 1 .........................................................................................................................171

Appendix 2 .........................................................................................................................172

Acknowledgment................................................................................................................ 174

References .......................................................................................................................... 174

5.1 INTRODUCTION

Cellulose is the world’s most abundant naturally occurring organic substance, rivaled only by

chitin. It has been estimated that nature synthesizes from 100 to 1000 billion (1011 to 1012)

metric tons of cellulose every year [1–3]. It is therefore not surprising that humans have made

use of cellulose on a vast scale in the paper, mining, building and allied industries, and as a

source of bioenergy. This applies to cellulose in its natural state, isolated, or as a source of raw

material for modification into products having different properties from those of pure

cellulose. Wood pulp is the main source of processed cellulose, the bulk of which is converted

to paper and cardboard, and about 2%, amounting to just over 3 million tons, into regener-

ated fiber and films or chemical derivatives [3].

(probably a small fraction of 1%) in comparison with that of the animal world, it is symptom-

atic of the importance attached to human existence and activity that the emphasis in this

chapter and book is upon human beings’ adaptation of natural food resources for their own

use and enjoyment. Regarding cellulose, the major carbohydrate nutrient for herbivores,

including insects, enzymatic approaches to modification could change radically the value and

applicability of this abundant polysaccharide to human nutrition [4]. As it is, cellulose performs

production of processed animal feed based on the normally inaccessible lignified cellulose

sources, whereby degrading enzymes are used to enrich the cellulose and related carbohydrate

polymers and render them acceptable to both ruminants and nonruminants [5–8]. Such pro-

cesses may continue to the sugar level, and it is of interest that sugarcane bagasse may serve not

only as a source of paper and industrial ethanol, but also as animal fodder [9,10].

Given that cellulose has been a natural part of the world’s diet since time immemorial, and the

excellent toxicological profile enjoyed by it and its ether derivatives (which constitute the only

food-allowed group of modified celluloses), it is not surprising that these materials have found

wide acceptance within the food industry. Most of this chapter is concerned with their applica-

tions. A brief account is given (considering the vast literature on the subject) about the occur-

rence, biosynthesis, analysis, and properties of cellulose. This leads to a description of the salient

to bring them into a state of solution or dispersion for practical use. An overview of regulatory

the basis of Section 5.4. The reader is referred to the first edition of this book for a list of key

manufacturers of cellulosics, and to additional references describing earlier work [11].

5.2 CELLULOSE

5.2.1 OCCURRENCE AND ISOLATION

Cellulose is the major building block of the cell wall structure of higher plants [12]. Cellulose

constitutes 40–50% of wood, 80% of flax, and 90% of cotton fiber. Green algae also have


148 Food Polysaccharides and Their Applications, Second Edition


Although (as stated in Chapter 1) the total food consumption of humans is minuscule

a critical function as dietary fiber (see Chapter 18). Nevertheless, there has been a surge in the

aspects is given inAppendix 2,whereas details concerning the applications of these products form

features of physically and chemically modified celluloses, and in Appendix 1 to the methods used

cellulose in their cell walls, as do the membranes of fungi. Cyanobacteria [13] represent a

primitive source, and bacterial cellulose is well described [14]. Acetobacter xylinum can

synthesize extracellular pellicles of cellulose from glucose. Simple marine animals such as

tunicates deposit cellulose in their cell walls [15].

Commercial purification of cellulose is centered on cotton linters and wood pulp, in the

first case because of the high cellulose content and, in the second, the relative abundance and

ease of harvesting of wood and straw. Cellulose in the natural state is difficult to purify due to

its insolubility in commercial solvents. Because of their high polarity, amine oxides are used

successfully in a variety of applications, e.g., N-methylmorpholine-N-oxide hydrate as solvent

in the Lyocell regeneration process [3]; 1,3-dimethyl-2-imidazolidinone/LiCl as a system that

is used as solvent in a food product application [17]. Isolation of cellulose in a pure form

usually involves alkaline pulping to remove waxes, proteins, and — in particular from wood —

lignins. Pulps for cellulose ether production often undergo extra alkali extraction steps; these

are carried out to remove low-molecular-weight polysaccharides and so-called hemicelluloses,

as well as to raise the pure (alpha) cellulose content [2,18].

5.2.2 STRUCTURE

One of the certainties in cellulose science is that cellulose is an aggregate of linear polymers of

D-glucopyranosyl residues in the chain form, which are linked together entirely in the b-1,4

configuration [3,19] (Figure 5.1), chemical and enzymatic hydrolyses, acetolysis, methylation

studies, NMR and x-ray analysis affording ample proof of this. Cellulose is an isotactic b-1,4-

polyacetal of 4-O-b-D-glucopyranosyl-D-glucose (cellobiose), as the basic unit consists of two

units of glucose b-1,4 linked [2]. The b-1,4 diequatorial configuration results in a rigid and

linear structure for cellulose. The abundance of hydroxyl groups and concomitant tendency

to form intra- and intermolecular hydrogen bonds results in the formation of linear aggre-

gates. This contributes to the strength shown by cellulose-containing structures in plants and

also to the virtual insolubility of cellulose in common solvents, particularly water.

As a glucan, cellulose may be analyzed, after enzymatic removal of starch, by hydrolysis in

a concentrated aqueous solution of H2SO4, dilution with water, and anthrone assay [20].

Cellulose has a characteristic CP/MAS 13C NMR spectrum [21], and functional groups in

derivatives are readily identified by 1H NMR [22]. The last reference describes the first

application of NMR to the end-group determination of molecular weight of a cellulose

derivative. Other standard methods of spectroscopic analysis are readily applied [23–25].

Molecular size of polymer molecules can be conveniently described in terms of degree of

polymerization (DP), which is an average value of the number of monomer units. By various

physical techniques (intrinsic viscosity measurement [26,27], light scattering, etc.) the DP of

O

O

CH2

CH2

OH

OH

OH

OH

HO

HOHO

HH

H

HH

O

O

CH2

OH

OHHO

HO

H

HH

O

n−2

HH

H HHH

FIGURE 5.1 Cellulose.


Cellulose and Cellulose Derivatives 149


can be used in the analytical steric (size) exclusion chromatography (SEC) mode (see Section

5.2.2) instead of the more common dimethylacetamide (DMA)/LiCl [16]; and an amine oxide

cellulose from various sources can be estimated. A number of examples are given in Table 5.1.

Much attention has been given to molecular weight distribution measurements of cellulose and

derivatives using various adaptations of SEC [16,28–30], andby these techniques, informationwas

obtained about the semiflexible chain structure of cellulose in alkaline urea solution [31], and the

effects of oxidation [32] and heat [33]. Carboxymethylcellulose (CMC), hydroxymethylcellulose,

and more complex derivatives have been analyzed similarly, employing various stationary phases,

elution systems, and detectors devised to improve the reliability of the method [34–38].

In the solid state, highly ordered crystalline areas are interspersed between less-ordered

amorphous zones. These amorphous zones are regions in which the hydroxyl groups are more

readily available for reaction than in the more highly ordered crystalline areas, which are less

reactive [12,39–41]. Cellulose reactivity is thus dependent on the source of cellulose and the

conditions of isolation and purification.

In the native state cellulose is in the form known as Cellulose I, which has a unit cell

containing two cellulose molecules in line with the b-axis, but arranged countercurrently to

each other [42]. A lattice structure known as Cellulose II is obtained after mercerization [43],

i.e., treatment with sodium hydroxide, in which the c-axis is lengthened and the a-axis

shortened. Other forms of cellulose have been described, but a description of these is outside

the scope of this chapter. More specialized discussions are readily available [2,3,44].

Early work employing electron microscopy showed that the linear cellulose molecules are

bound together (through hydrogen bonding and other electronic forces) into long threadlike

bundles called microfibrils. In certain areas, these microfibrils have the chains arranged in

stacked layers. These areas are then sufficiently organized in regular fashion to form discrete

crystalline regions known as crystallites [45] and at a higher level of organization, micro-

tubules [46]. Microfibrils are not necessarily the only mode of organization of cellulose chains,

as in wheat straw the highly oriented crystalline lamellae are arranged perpendicularly to the

tangential direction with respect to the annual rings [47]. A great deal of work has neverthe-

less been done on the detailed structure of microfibrils, with early reviews by Delmer [48] and

of Marchessault and Sundararajan [12]. The article by Klemm et al. [3] admirably summarizes

the various structural levels of cellulose from data derived by crystal analysis using x-rays to

fiber morphology (microfibrils and microfibrillar bands), which includes the crystallites and

amorphous regions, and to the structural design of plant cell walls in which the cellulose is

accompanied by various lignin, hemicellulosic, and pectic components.

Different techniques have been used to establish many facets of cellulose formation

and structure apart from the biosynthetic routes discussed in Section 5.2.3. The effects of

glucomannan and xylan on the cellulosic structure of A. xylinum were defined using NMR

and electron diffraction [49], and the behavior of microfibrils when attacked by cellulases, by

x-ray [50]. Periodic disorder along ramie microfibrils was demonstrated by small-angle

neutron scattering [51], and the ultrastructure of iodine-treated wood (fluorescent markers

TABLE 5.1Degree of Polymerization of Cellulose

from Various Sources

Source DP

Purified cotton 1500–300

Cotton linter 6500

Spruce pulp 3300

Aspen pulp 2500




are also used) by light microscopy [52] ). Improved activated carbon results from retaining the

microfibrillar structure of the pyrolyzed cellulose [53].

5.2.3 BIOSYNTHESIS

For a compound as abundant and important in nature as cellulose, the uncovering of the fine

details of its biosynthesis has proved remarkably difficult. A number of general articles

introduce the observations that follow [14,42,54–60].

The substrate uridine diphosphate glucose (UDP-glucose) splits off glucose to form a lipid–

pyrophosphate–glucose, and from this a lipid–pyrophosphate–cellobiose derivative. The cello-

biose unit is then detached and forms the end part of a water-soluble, short-lived polymer based

on glucose. The enzyme complex responsible for these transformations functions on plasma

membrane surfaces. As they are formed, the chains undergo hydrogen bonding to form the

partially crystalline microfibrils. The absorbing question as to how cellulose has become by far

the main source of organic terrestrial matter has been approached in the basic reviews by Delmer

and coworkers [61,62] on the roles of genes and catalytic proteins in the biosynthetic pathways

leading to cellulose. One aim of such fundamental studies has been to improve cellulose quality

and its suitability for applied purposes, and parallels work on starch in some respects (see

has led to successful cultivation of improved cotton [56,65], better pulp quality in Eucalyptus [66]

and poplar [67], and the modulation of cellulose content of tuber cell walls in transgenic potatoes

[68]. Numerous biochemical aspects have been pursued, such as the requirement of the N-glycan

processing enzyme a-glucosidase [69] for cellulose biosynthesis, and factors in the alignment

of cellulose in microfibrils, the patterns of which govern the direction of cell wall expansion

[70,71].

5.2.4 CHEMICAL AND PHYSICAL PROPERTIES

Cellulose is a hygroscopic material, insoluble but able to swell in water, dilute acid, and most

solvents. Solubility can be achieved in concentrated acids [72] but at the expense of consid-

erable degradation through acetal (glycosidic) hydrolysis. Alkali solutions lead to consider-

able swelling and dissolution of hemicelluloses present.

The chemical reactions of cellulose are dictated by its polymorphic nature. The less-

ordered amorphous regions are more reactive than the ordered crystallite regions, initial

chemical reaction takes place on the less-ordered surfaces of the fibrils. Little or no effect is

observed on the impenetrable crystalline structure.

Caustic alkaline solutions penetrate cellulose by swelling and by subsequent capillary

attraction, which allows entry into the regions between the crystallite zones. Consequently,

the crystallite zones are disrupted. This process, often termed mercerization [43], is used to

(>1508C) cellulose undergoes hydrolysis in basic media, and oxidation may also occur.

Microbiological degradation occurs via enzymatic hydrolytic cleavage of the b-1,4-glucosidic

link [72a]. Substituted cellulose ethers are for steric reasons less exposed to this process, and their

relative stabilities may be considerably greater. It is also known that, as expected, amorphous

cellulose is more susceptible to enzymatic hydrolysis than more highly crystalline cellulose [3,73].

Partial degradation of natural cellulose using multicomponent, degrading enzymes leads to

products having certain practical advantages, including enhanced solubility [73a].




Chapter 1 and Chapter 2). The critical discovery of genes from Arabidopsis encoding for cellulose

activate cellulose for cellulose ether production (see Section 5.3.1.1). At higher temperatures

synthase in 1996, of which ten true and eight weaker subdivisions have been reported [63, cf. 64],

5.2.5 PHYSICALLY MODIFIED CELLULOSES

5.2.5.1 Microfibrillated Cellulose

The preparation of this material has been disclosed in a patent from the ITT Corporation [74].

Essentially, a slurry of cellulose is passed through a small orifice under conditions of

high shear and at a great pressure differential. This treatment disrupts the cellulose into

microfibrillar fragments [45]. The assay for microfibrillated cellulose (MFC) is described in

the Food Chemicals Codex (FCC) [75] and involves titration by ferrous ammonium sulfate of

a potassium dichromate and sulfuric acid-treated solution of the cellulose. The assay specifies

at least 97% cellulose.

MFC has considerably more water-retention capability than normal-grade materials and

is considerably less prone to precipitation in the cupriethylenediamine residue test (cuene

test). The change in DP with respect to the pulp feed stock is kept at a minimum during

microfibrillation. Suspensions of MFC are shear thinning (pseudoplastic), exhibit slight

thixotropic behavior, and apparently do not suffer a viscosity drop on heating. Electrolyte

tolerance is also on a par with that of other commonly used cellulosics.

5.2.5.2 Microcrystalline Cellulose

Microcrystalline cellulose (MCC) is produced by treating natural cellulose with hydrochloric

acid to dissolve the amorphous regions of the polysaccharide, leaving behind the less reactive

crystalline regions as fine crystals. As with MFC, the viscosity of dispersions of this product is

both pH and heat invariant. A number of varieties are available — powdered, bulk-dried

colloidal, spray-dried colloidal (with CMC), and spray-dried with sweet whey [76]. MCC

dispersions have been shown to exhibit both thixotropic and pseudoplastic behavior [77].

MCC can be assayed in a manner similar to MFC by employing the ferrous ammonium sulfate

titration technique; other determinations, including loss on drying and residue on ignition, can

be accomplished as described in the FCC [75]. The assay specifies not less than 97% cellulose.

Particles may be approximately 1 mm in length, and the technique used for dispersion is

in the commercial product. Purified cellulose powders for use as dietary fiber (Section 5.5.2), of

average fiber length varying from 25 to 120 mm, are essentially unmodified [78]. Many import-

ant industrial food applications have been listed [45,79] and patents registered [80–82].

5.3 CHEMICALLY MODIFIED CELLULOSE DERIVATIVES

Despite the wide variety of cellulose derivatives that have been made, notably acetate and

nitrate esters [3,22,83–86], only a few of the cellulose ethers find application (and are

approved for use) in foodstuffs [87,88]. The most widely used cellulose derivative is sodium

CMC [89,90]; other ethers have unique and interesting properties, however, which ensure

their inclusion in a widening array of products. Thus, for example, methylcellulose (MC) and

hydroxypropylmethylcellulose (HPMC) find uses as a result of interfacial activity and their

ability to form gels on heating.

Although a number of cellulose ethers are available, they are all made in essentially the

same manner. Naturally, individual suppliers have their own technologies, but the production

process can be broken down into the generation of alkali cellulose, alkylation or hydroxy-

alkylation, and finally product purification. Alkylation specifically at primary alcohol sites

requires a more elaborate synthetic procedure [91].




important [77]; in the reference cited, some 10% of CMC (see Section 5.3.3) is incorporated

5.3.1 THE MANUFACTURE OF CELLULOSE ETHERS

5.3.1.1 Preparation of Alkali Cellulose

Alkali cellulose is most commonly produced by slurrying, spraying, or otherwise mixing

chemical cellulose chip, pulp, or sheet with aqueous sodium hydroxide solution (35–60%

w/v). Inert organic solvents may be used as slurry media. This mixture is then held for a

predetermined time at a controlled temperature and pressure to ensure complete reaction and

to control the viscosity of the final product through an ageing process. Alkali cellulose is quite

readily degraded by air oxidation; thus the quantity of oxygen present in the alkalization

reaction and the type of cellulose used have a crucial influence on the DP of the final product

and hence its viscosity in solution [2].

Although in theory it would be possible to produce alkylcelluloses by the use of potent

alkylating agents such as diazomethane, the heterogeneous nature of the cellulose leads to

variations in the availability and reactivity of the hydroxyl groups. Treating the cellulose with

caustic alkaline solution disrupts hydrogen bonding between and within the polymeric

strands, making the majority of the hydroxyl groups available for modification, with the

C2 and C6 hydroxyl sites typically more reactive than the C3 site [83]. Additionally, sodium

hydroxide acts as a catalyst in the Williamson etherification reaction.

For high-viscosity products, cotton linters are utilized under strictly controlled conditions

in order to minimize oxidation. These linters have an extremely high DP and a high

a-cellulose content (>99%). Other products with lower viscosity requirements are made

from a variety of wood pulps.

The preparation of alkali cellulose takes place as follows:

RcellOH��!NaOH

RcellOH �NaOH

RcellOH �NaOH ��!RcellONaþH2O

5.3.1.2 Alkylation and Hydroxyalkylation of Alkali Cellulose

As industrial production processes make use of alkyl chlorides (Williamson ether synthesis)

for the alkylation step and epoxides (oxiranes) for hydroxyalkylation, this chapter will deal

only with these routes.

5.3.1.2.1 AlkylationIn the Williamson ether synthesis, a nucleophilic alkoxide ion reacts with an alkyl halide to

give an ether and a salt. Thus, the alkali cellulose slurry reacts with methyl chloride to give

MC and sodium chloride. In a similar manner sodium monochloroacetate is used to produce

sodium CMC.

The preparation of MC and CMC takes place as follows:

RcellOHþ CH3Cl��!NaOH

RcellOCH3 þNaClþH2O

RcellOHþ ClCH2CO2Na��!NaOH

RcellOCH2CO2NaþNaClþH2O

5.3.1.2.2 HydroxyalkylationHydroxyalkylation of cellulose is carried out by treating alkali cellulose with an epoxide.

Propylene oxide is used to prepare hydroxypropylcelluloses (HPCs), ring opening at the




primary carbon leading to a product containing a secondary alcohol function within the ether

moiety introduced.

The preparation of HPC proceeds as follows:

O OHNaOH

RcellOH + CH3CH CH2 RcellOCH2CHCH3 + NaOH

For the production of mixed alkylhydroxyalkylcelluloses, alkylation and hydroxyalkyl-

ation can be carried out either sequentially or concurrently. An important factor in the

hydroxylation reaction is that a new hydroxyl group is generated and is available for further

reaction. The effect of this will be considered later in the discussion.

The properties of the resulting cellulose ethers are a complex function of the molecular

weight average of the polymer and the types and level of substitution. It is here that the skill of

the manufacturer comes into play to produce useful products consistently.

5.3.1.3 Product Purification

Because byproducts are generated in these reactions, purification is required to meet the food

and other premium application standards. Byproducts include alcohols, alkoxides, ethers, and,

in the case of CMC production, glycolic acid and its salts. Generally, the thermal gelling

and hot water-insoluble products such as MC and HPC, respectively, are purified by hot

water washing and filtration procedures. Cold and hot water-soluble products such as CMC

are purified by washing with solvent systems such as aqueous ethanol or acetone. The purified

products are then dried, and their particle sizes are modified by suitable means. Finally, they

are analyzed for premium application compliance and are packaged.

5.3.2 CHARACTERIZATION OF CELLULOSE ETHERS

Besides the type of substituents carried by the cellulose backbone and the viscosity of the

cellulose ether in aqueous solution (normally quoted for a 1 or 2% solution w/v), these

products are described by the degree of substitution (DS) and the molar substitution (MS)

level. Molecular weights and distributions are now readily accessible [28,34–36,38].

5.3.2.1 Degree of Substitution

Each anhydroglucose unit in the cellulose molecule has three hydroxyl groups available for

derivatization. Thus, if all of these hydroxyl groups were substituted, the product would be

said to have a DS of 3. If an average two out of three of these groups were reacted, then the

block reactive hydroxyl groups; reagents that allow further chain growth are characterized

by MS.

5.3.2.2 Molar Substitution

Derivatization of a reactive hydroxyl group with propylene oxide generates on a one-for-one

basis a replacement hydroxyl site for further reaction. Thus, as the reaction continues, chain

extension occurs. Oxyalkyl substitution is thus described by the MS level, i.e., the number of




DS would be 2, and so on (Figure 5.2). The term DS is reserved for those substituents that

moles of alkylating agent per mole of anhydroglucose in the chain (Figure 5.3). The ratio of

MS to DS gives the average chain length (DP) of these side-chain substituents.

5.3.3 SODIUM CARBOXYMETHYLCELLULOSE

acid or its sodium salt or mixtures thereof. The sodium salt is the most common for food use,

as the free acid form is insoluble in water. In this discussion, for convenience, the term CMC

will be used to refer sodium CMC. In the United States the term ‘‘cellulose gum’’ is often used

for food-grade CMC [45]. CMC is assayed by determining the sodium chloride and sodium

glycolate percentages [75] and subtracting these from 100%. The FCC specifies that the assay

be at least 99.5% CMC. The principle of SEC has been applied to the characterization of

CMCs [34,92–94].

CMC was initially developed in Germany as a gelatin substitute. The real drive for its

commercial usage was the discovery in 1935 that CMC could improve the efficacy of laundry

detergents. Food-grade CMC was introduced by the Hercules Company in 1946 [95] and since

then it has become the dominant cellulose ether in terms of its total usage. It was estimated

that in 1983 the consumption rates in the United States, Western Europe, and Japan were

22,200, 93,000, and 17,200 metric tons, respectively, with about 5% of CMC production used

by the food industry, or about 6,600 metric tons per annum globally [96].

O

O

O

OO

O

OH

HH

HHH

H HH H H H

H HHH

HO

HO

HO

HO CH2

CH2

CH3

CH3

CH3

H3C

O

O

CH2

CH3

O

O

CH3

O

n–2

FIGURE 5.2 Methylcellulose with a DS of 2.0.

O

O

O

OO

O

OH

HH

HHH

H

OH

HH H H H

H HHH

HO

HO

HO

HO CH2

CH2

CH2CHCH3

CH2CHCH3

CH3CHCH2

CH3CHCH2

O

O

CH2

CH2CHCH3

O

CH2CHCH3

OH

OHOH

OH

O

OOH

n−2

FIGURE 5.3 Hydroxypropylcellulose with a MS of 2.0.




CMC (Figure 5.4) is an anionic, linear, water-soluble polymer that can exist either as the free

5.3.3.1 Effect of DS on Solubility

Commercially, CMC is available in the DS range 0.4 to 1.5. As the interchain association of

adjacent hydroxyl groups is the most important determinant of solubility, the DS of CMC

(and other cellulose ethers) has a profound effect on the physical properties of solutions. At

low DS the bulk of the etherification has occurred in the amorphous and the crystalline

surface regions of the cellulose. This gives a product with a high residual degree of chain:chain

association and resultant poor water solubility. Thus, CMC with a DS < 0.3 is only soluble in

alkali. At higher DS, the interchain associations are more disrupted, yielding partially soluble

material. As the DS approaches 0.7, the crystalline regions have been sufficiently disrupted to

yield highly water-soluble material. Above a DS of 1.0 the relative concentration of unreacted

hydroxyl sites is so low that little or no interchain association occurs. In the United States,

CMC for food use is limited to DS � 0.95 [97].

5.3.3.2 Solution Rheology

Because CMC and other cellulose ethers are produced by controllable reactions, it is possible

to tailor desirable property features into the polymer. Characteristics such as solution

viscosity, thixotropy, and pseudoplasticity can be controlled by varying the amount of

chain oxidation (molecular weight control), DS, and uniformity of chemical substitution.

As discussed earlier, the viscosity of the final product is a function of the molecular weight

of the polymer. Since the viscosity of a given solution and the concentration of the cellulosic

required to achieve this may be significant factors in the choice of product used, CMC is

offered in a range of viscosity grades. Typically, manufacturers of food-grade material offer

products giving 1% aqueous solution viscosities from 20 to 4000 mPa.

Solution viscosities are affected by temperature. As is generally true with cellulose ether

solutions (with the notable exceptions of MC and HPMC), the viscosity decreases with

As with most polyelectrolytes, the pH of the environment will also affect the viscosity of a

CMC solution, though in general little effect is observed between pH 5.0 and 9.0. Below pH

4.0 the free acid is substantially produced, which can cause precipitation of the polymer (see

The effect of solutes such as salts or polar nonsolvents on viscosity can be marked and is

dependent on order of addition. In general, if the solute is added to a prepared solution of

CMC then the viscosity drop is minimized compared to the situation in which the CMC is

added to the solute in solution. For brine this is probably due to an immediate ionic

interaction between the salt and the polymer carboxylate group, effectively shielding the

OO

O

O

OH

HH

HHH

H HH H H H

H HHH

HO

HO

HO

HO CH2

CH2

CH2

CH2

O

O

CH2

COO-Na+

COO-Na+

CH2

COO-Na+

OH

OH

O

OOH

n-2

FIGURE 5.4 Carboxymethylcellulose with a DS of 1.0.




increasing temperature (Figure 5.5) linearly on a semilog plot.

Figure 5.6). Above pH 10 viscosity decreases and cellulose degradation becomes important.

anionic charge and substantially inhibiting hydrogen bonding between the anionic site and

To avoid the problem of reduced viscosity in these situations, it is recommended that the

CMC be dispersed in water first and allowed to hydrate. Following polymer hydration, the

salt is added to the desired concentration. The probable reason this is effective is because as

the CMC hydrates, it collects water molecules around itself; these dipoles orient with respect

to the anionic carboxylate groups, partially satisfying and partially shielding them from the

cations that are added later. In general, monovalent salts form soluble salts of CMC;

therefore, the solution properties such as haze, viscosity, and clarity are relatively unaffected

CMC, and therefore the solution quality decreases. The effect of trivalent cations is generally

to precipitate the polymer. Examples of inorganic cations that are incompatible with CMC

solutions are aluminum, chromic, ferric, ferrous, silver, and zinc.

The interactions of this anionic polymer are not limited to inorganic salts. The carboxylate

can also interact with proteins, as long as the pH of the food system is greater than the

isoelectric point of the protein. In such a case, ionic interactions between the anionic CMC

chain and the cationic protein chains generate a higher viscosity than is otherwise expected.

displays very little interfacial activity except when ionically driven.

Another fundamental characteristic of many aqueous polymer solutions is pseudoplasti-

city: solutions that tend to lose viscosity with increasing shear rate are said to be pseudoplastic.

For CMC, pseudoplasticity generally decreases with increasing DS. At DS greater than or

10

1% 7HF

1% 9M31F

2% 7LF

2% 7MF

Temperature, �C

Vis

cosi

ty, m

Pa

s

100

1,000

10,000

20 40 60

FIGURE 5.5 Effect of temperature on viscosity of CMC solutions. Commercial grades of CMC

produced by Hercules.




water molecules (see Figure 5.7).

at low to moderate salt levels [cf. 81]. Divalent cations generally form less soluble salts of

This synergistic activity is well-known and described in the literature [3,45, cf. 24]. CMC

equal to 1.0, CMC forms solutions with very little interchain association and that exhibit little

or no hysteresis loop during increasing and decreasing shear.

Thixotropy is a time-dependent change in viscosity at constant shear rate. For CMC,

thixotropic rheology is a result of the interchain hydrogen bonding between adjacent areas of

relatively unsubstituted anhydroglucose units. In CMC, which has relatively large regions of

unsubstituted hydroxyl groups on the rings, there is a strong tendency for the chains to

2

2% 7 M

2% 9M31

1% 7H

100

500

1,000

5,000

4 6

pH

8 10 12

App

aren

t vis

cosi

ty, m

Pa

s

FIGURE 5.6 Effect of pH on solution viscosity of CMC.

20

0.04 0.10 0.20 0.40 1.00

40

60

80

100

200

300

Vis

cosi

ty, m

Pa

s

Molal concentration NaCl

Solute addedafter CMC

Solute addedbefore CMC

FIGURE 5.7 Effects of NaCl and order of dissolution on viscosity of 1% CMC solutions.




develop interchain hydrogen bonds, resulting in a gel-like network. Conditions that allow for

large regions of the chain to escape substitution favor products that will yield thixotropic

solutions. Generally, the tendency for this to happen is much greater at the lower DS.

Typically, a DS of 0.7 affords a substitution pattern heterogeneous enough to produce

carboxyl-rich and carboxyl-poor regions. As the substitution on the chain increases or

becomes more uniform, the tendency for interchain association to occur decreases. Commer-

cial manufactures take advantage of this to produce nonthixotropic CMC. Nonthixotropic

CMC is available today at DS as low as 0.7. This material has a high degree of uniformity in

substitution.

5.3.4 METHYLCELLULOSES

MCs comprise a family of cellulose ethers in which methyl substitution occurs with or without

additional functional substituents. Thus besides MC, this category includes methylhydroxy-

ethylcellulose (MHEC), which is allowed in the European Economic Community (EEC) (up

to 5% hydroxyethyl substitution permitted under the MC specification), and HPMC. MC, the

first cellulose ether to be made, was first described in 1905. Since then, intensive development

by The Dow Chemical Company, Hercules Inc., and Hoechst AG inter alia has led to the

availability of a wide range of the above cellulose ethers of food-grade quality. The MCs are

assayed by determining their substitution percentages, as described in the FCC [75].

The usefulness of these nonionic cellulose ethers is essentially based on four key attributes:

efficient thickening, surface activity, film-forming ability, and, probably of greatest interest to

the food technologist, the ability to form thermal gels that melt upon cooling.

5.3.4.1 Solution Rheology

The solution behavior of the nonionic MC family is markedly different from that of the ionic

CMC. The effect of pH is especially reduced, and the temperature-dependent rheology is

much more complex. When dissolved in water these gums give clear, smooth-flowing solu-

tions that are pseudoplastic and nonthixotropic [98,99]. The pseudoplastic behavior of MC is

a function of molecular weight, with MCs of higher molecular weight exhibiting greater

but is rather a factor of the molecular weight of the polymer. In the graph the four gums have

the same molecular weight but differing degrees of substitution.

The curves showing the effect of polymer concentration on solution viscosity are com-

5.3.4.2 Thermal Gelation

Aqueous solutions of MC and HPMC (at >1.5 wt%) form gels when heated, then on cooling

return to the solution state at their original viscosity. The temperature at which this gelation

begins and the texture of the gel formed are dependent on the type and substitution level of

the gum. For example, the commercially available MC and HPMC products have gel

temperatures ranging from 50 to 858C and gel strengths varying from firm to weak [100], as

with no anticipated loss of properties.

Thermal gelation is affected by a number of polymer-dependent characteristics. The most

important determinant of gel strength is the concentration of methyl groups and the methyl:

hydroxypropyl ratio. As the methyl concentration increases, the gel formed on heating




shown in Table 5.2. MC gel becomes fluid upon heating and the solution that eventually

pseudoplasticity as shown in Figure 5.8.

DS does not affect the rheology of MC and HPMC solutions as exemplified in Figure 5.9,

parable to those for CMC, with the curves for HPMC (Figure 5.10) illustrative of the class.

forms regains its initial viscosity (see Figure 5.11). This process may be repeated continuously

becomes firmer. Conversely, as the hydroxypropyl substitution increases, the gel becomes

softer. The most probable explanation involves dehydration followed by hydrophobic asso-

ciation of the chains. At temperatures greater than the thermal gel point (TGP), the vibra-

tional and rotational energies of the water molecules increase, exceeding the ability of

the weak hydrogen bonding to orient the dipolar water molecules around the polymer

chain. The energized water molecules then tend to disengage from the fragile envelope of

ordered water surrounding the chain. The dewatered hydrophobic polymer segments then

begin to associate with each other. As the temperature or the time at which high temperature

increases, the hydrophobic interactions increase in number, producing an ever-firmer gel due

to the increasing formation of cross-links.

10

100.1 1

100

100

25

400

1,500

4,000

100

1,000

1,000

10,000

App

aren

t vis

cosi

ty, m

Pa

s

Shear rate, Sec−1

FIGURE 5.8 Effect of shear rate on apparent viscosity of 2% solutions of methylcellulose. Note:

Numbers on curves indicate viscosity types.

1010

Shear rate, Sec−1

0.1 1

MC (4,000 mPa s)

HPMC (4,000 mPa s)

HPMC (4,000 mPa s)

HPMC (4,000 mPa s)

100

100

1,000

1,000

10,000

10,000

App

aren

t vis

cosi

ty, m

Pa

s

FIGURE 5.9 Effect of shear rate on apparent viscosity of solutions of HPMC with different degrees of

substitution.




The addition of hydroxypropyl groups to MC always tends to diminish the rigidity of the

gel and increase its critical thermal gelation temperature. Hydroxypropyl substituents are

more hydrophilic than methyl groups, and hence are better able to retain water of hydration

when exposed to heat. Because they hold on to their water more tightly, the temperature

needed to drive the substituent groups apart is correspondingly greater than with MC alone.

Furthermore, the equilibrium association between water and HP substituents tends to pro-

duce a more hydrophilic gel than is possible with MC.

10

400

0 1 2

Concentration, � mv−1

3 4 5 6 7

100

100

1,000

1,500

15,000

75,000

4,000

40,000

10,000

100,000

Vis

cosi

ty, m

Pa

s at

20�

C

FIGURE 5.10 Effect of concentration on viscosity of hydroxypropylmethylcellulose. Note: Numbers

on curves represent viscosity types.

TABLE 5.2Thermal Gelation Properties of MC and HPMC

Viscosity

Range (mPa)

Gel

Texture

Nominal

Gel Point (˚C)

Degree of Methyl

Substitution (DS)

Degree of Hydroxypropyl

Substitution (MS)

MCa 15–4000 Firm 50 1.6–1.8 —

HPMCb 3–4000 Semifirm 60 1.63–1.85 0.1–0.3

HPMCc 50–4000 Semifirm 65 1.0–1.8 0.1–0.2

HPMCd 3–100,000 Soft 85 1.1–1.4 0.1–0.3

aMethocel (trademark of Dow Chemical Company) A.bMethocel E.cMethocel F.dMethocel K.

Source: From Dow Chemical Co., Form No. 192-976-586, 1986.




The strength of the gel increases with increasing molecular weight until a maximum

strength is reached at about Mn 40,000. This corresponds to a 2% solution viscosity of

about 400 cps as determined by the ASTM Methods [101]. Further increases in molecular

weight do not increase the gel strength. In addition, the molecular weight has no effect on the

thermal gel temperature. This indicates that gel formation is essentially dependent on polymer

chemistry and thermal kinetics and not on inherent viscosity contributions due to molecular

weight [87].

A final point of interest in this discussion is the tendency of MC and HPMC to concen-

trate at air–water and oil–water interfaces. Like thermal gelation, this pronounced surfactant

behavior is the result of substitutional heterogeneity in the polymer. The concentrations at the

interface of dilute solutions of MC can be as high as several weight percent. At the interface of

dilute solutions, one expects and can find very resilient gels [102]. This can lead to, among

other things, foam stabilization, emulsion stability, and positive effects on crumb structure,

dough rising, and baking, etc. These topics are discussed later in the text.

5.3.4.3 Solute-Induced Gelation

In addition to thermal gelation, MC and HPMC also gel upon the addition of sufficient

coagulative cosolute. Solutes in this category are phosphate, sulfate, and carbonate salts. The

action of these salts tends to strip water molecules away from the polymer via disruption of

the hydrogen-bonding forces. This is analogous to the effect of thermal energy as a hydrogen

bond disruptor. The net result is that certain polymer segments have an insufficient attraction

to the electrolyte solvent, partially dehydrating the chain and allowing the formation of the

hydrophobic interactions.

A number of salts (e.g., trisodium polyphosphate and sodium sulfate) have been found to

be most effective for lowering the gelation temperature. MC is more sensitive in this respect

100 20 30 40 50 60 70

Vis

cosi

ty, m

Pa

s

Cooling

Heating

Incipient gelationtemperature

Temperature, �C

FIGURE 5.11 Effect of temperature on viscosity of methylcellulose (gelation of a 2% aqueous solution

of methylcellulose; heating rate 0.258C/min). (Methocel A-type material courtesy Dow Chemical Co.)




than are the various HPMCs. Firm gels can be formed at room temperature by adding 3%

trisodium polyphosphate to a 2% solution of MC. The use of salts to form gels with nonionic

cellulose ethers is presently investigated. The opportunities that room temperature gel for-

mation present to food formulators are obvious. A magnetic resonance probe has been

developed for the mapping of temperature profiles in gels [103].

5.3.5 ETHYLMETHYLCELLULOSE

Ethylmethylcellulose (EMC) is manufactured by only one supplier worldwide [11,104]. The

product has an ethyl DS of 0.3 and methyl DS of 0.7. Rheology, salt tolerance, pH stability,

etc. are in line with those of MC. Instead of gelling, however, EMC precipitates from aqueous

solution when heated above 608C; re-solution occurs on cooling.

5.3.6 HYDROXYPROPYLCELLULOSE

There are two manufacturers of HPC. Food-grade quality HPC is available in six viscosity

ranges, all at an MS of about 4 [11]. As is the case with MC and HPMC, this ether exhibits

considerable surface activity — for example a 0.1% aqueous solution of HPC has a surface

tension of 43.6 dyn/cm [105]. HPC differs from MC and HPMC in that it does not gel

thermally but, similarly to EMC, precipitates from aqueous solution above 458C. Addition-

ally, HPC shows a markedly lower tolerance to dissolved electrolytes; thus, while MC, CMC,

and HPMC are all soluble in 10% aqueous NaCl solution, HPC is insoluble. HPC has

long been known to show a greater degree of solubility than either MC or HPMC in polar

organic solvents [105]. HPC is assayed using the hydroxypropyl determination included in

the FCC [75].

5.4 APPLICATIONS IN FOODS

Cellulose and its physical and chemical derivatives have long been used in fabricating

formulated foods. The physically modified celluloses are useful in many products where

bulk properties are desirable [106]. This would include reduced- or low-calorie foods, flavor

oil imbibers, or flowable products such as artificial sweeteners and flavor packets. The use of

these cellulosics is generally due to their rheology, controlled water interaction, and textural

attributes, and not to solubility or other chemical properties. Hence, MCC and finely ground

cellulose perform a valuable bulking role in low-calorie foods. Five important roles for the

chemically modified cellulose derivatives in foods are the regulation of rheological properties,

emulsification, stabilization of foams, modification of ice crystal formation and growth, and

water-binding capacity.

The applicability of cellulose derivatives for specific food applications can be determined

from their physical and chemical properties. When a choice is to be made, a number of

parameters must be considered: (a) the chemical structure of the polymer; (b) the molecular

weight of the polymer; (c) the presence of other active ingredients in the food matrix; (d) the

processing operations to which the food will be subjected; and (e) the physical properties,

including fiber dimension of the polymer.

Arguably, the most important factor is the chemical structure of the cellulose derivative.

For physically modified celluloses [107], this generally refers to the crystalline or amorphous

nature of the product. For ethers, there are a number of substituent groups and a range of

substitution patterns allowed by regulation that affect the rheological and surface-active

properties of the derivative. For instance, addition of carboxylic acid moieties to the cellulose

chain increases the hydrophilicity, whereas addition of alkyl residues such as methyl or ethyl




increases the hydrophobicity of the polymer chain. By increasing hydrophilicity, the polymer

is better able to hydrate in the presence of other water-soluble species such as sugars. By

increasing the hydrophobicity, a polymer may be produced that is also a surfactant, thus

conferring on the chain a host of interesting physicochemical characteristics.

The molecular weight of the polymer is readily manifested in solution viscosity; as

molecular weight decreases, solution viscosity decreases. For many of the derivatives dis-

cussed herein, the most important property is viscosity; for some, however, the most important

characteristic is film-forming or surface activity. In such cases products of low molecular

weight might better serve the application.

The inclusion of other active ingredients in the product also plays a crucial role in food

formulation. The presence of proteins, simple carbohydrates, and some starches is important

in the choice of cellulose derivatives. For example, in products with high salt concentrations,

CMC does not build up viscosity to the same extent as when salt is absent, and therefore

more CMC is required to overcome the effect of salt. Similarly, because CMC is ionic, it can

interact with certain proteins. Interfacially active MC and its derivatives can also interact with

proteins through hydrophobic–hydrophilic mechanisms.

Transient environmental modifications such as changing temperature can have a pro-

found influence on gum selection. Because the solubilities of some cellulose ethers are affected

by temperature, systems can be designed to yield widely divergent rheological profiles under

different processing regimes. For instance, MC interacts strongly with other MC chains at

elevated temperatures. This can sometimes cause firming in a product upon heating.

Finally, the physical properties of the derivatives are important in the context of the food

product and its processing. In some cases, it is necessary to deliver the polymer in a powdered

form by dry blending. In others it may be more efficient to use a granular material.

In this section, we shall describe classes of food products and include formulations that

specifically reference the various cellulose derivatives and provide chemical and physical

explanations for their functions. Typical food applications for cellulose and its derivatives

are shown in Table 5.3.

5.4.1 FISH/MEAT

Protein-based foods frequently need stabilizers for increased shelf life during ambient or frozen

storage. These products generally have sufficient binding capacity to preserve structural

TABLE 5.3Commercial Food Uses for Cellulose and Its Derivatives

Cellulose/Cellulose Derivative Food Applications

Hydroxypropylcellulose Whipped toppings, mousses, extruded foods

Hydroxypropylmethylcellulose Whipped toppings, mousses, baked goods, bakery fillings, icings, fried foods,

sauces, dressings, frozen desserts, reduced-fat foods

Methylcellulose Sauces, soups, breads, tortillas, fried foods, restructured (matrix) foods,

reduced-fat foods, foams

Methylethylcellulose Whipped toppings, mousses, egg white substitute

Microcrystalline cellulose Dressings, sauces, baked goods, beverages, whipped toppings, reduced-fat foods

Powdered cellulose Breads, cookies, pastries, pasta, imitation cheeses, cereals, canned meats

Sodium carboxymethylcellulose Frozen desserts, baked goods, dressings, sauces, syrups, beverages,

extruded foods, animal foods, reduced-calorie foods




integrity during storage, but gums may be added to provide primary binding in special products

or secondary benefits for quality improvement.

Seafood processors can use cellulose derivatives to allow fabrication of novel, value-added

seafood products. Extruded shrimp or fish nuggets may be produced that take advantage of

fish pieces, shrimp bits, or whole shrimp that are too small to be otherwise useful. By

incorporating a small amount (0.6–2.0%) of MC, the products have good cold extrusion

and forming characteristics and also high-temperature stability. High-temperature stability is

crucial because these products are generally deep-fried and would lose their integrity in the

Unique red meat products can be formulated using cellulosics. For example, patties can be

formed from chopped corned beef, sauerkraut, and cheese using MC (0.6%) as a binder. This

formulation allows cold-forming of what is normally a nonbinding mixture, but also prevents

loss of product integrity during frying due to its thermally gelling nature.

Trends in industry toward reducing the fat content of meats to promote a more healthy

consumer diet have presented new opportunities for cellulosics. In particular, the film-

forming characteristic of the interfacially active MCs, combined with the variety of viscosities

that can be achieved with these products, offers the possibility of mimicking the texture of

lipids in these fat-reduced systems.

Batters for deep-frying are an important family of products associated with processed

meats. Cellulosics are an integral part of many batter formulations. These polymers contribute

viscosity and thermal gelation to improve processing control and batter quality. Cold viscos-

ity is a crucial quality-control indicator. Batters that are too fluid fail to enrobe a product

sufficiently; conversely, those that are too viscous cause inappropriately high coating levels. A

further benefit of using a thermally gelling cellulosic in a batter is the production of a

relatively oil-insoluble barrier during frying. By gelling, MC and HPMC generate a water-

holding gel that prevents oil ingress during frying, with a reduction of up to 50% oil

absorption in selected batters.

5.4.2 SAUCES, GRAVIES, SOUPS, AND SYRUPS

Sauces, gravies, soups, and syrups represent a broad range of fluid food products that are

generally stabilized using hydrocolloids, including cellulose derivatives. The physically modi-

fied celluloses can help to maintain structural integrity during freezing. In addition, they can

substantially reduce the caloric content of the food by replacing carbohydrates or fats.

Chemically modified derivatives are often used in these products to increase the efficiency

of water binding and reduce the problem of syneresis on thawing. They can provide fatlike

mouthfeel and viscosity in systems containing reduced levels of fats and oils. Additionally,

some cellulose ethers are used in conjunction with other stabilizers as emulsifiers in fluid

systems that contain fat. The ability of cellulose ethers such as HPC, HPMC, and MC to

accumulate at oil droplet interfaces and prevent oil droplet coalescence is important in many

of these products to prevent oiling-off during storage. In general, sauces and gravies also

include starches to provide bulk viscosity and desirable sensory characteristics.

Fruit fillings and table syrups, especially low- or reduced-calorie products, can make good

use of cellulosics. CMC is particularly effective in these applications as it is an efficient

thickener in systems where the concentration of soluble solids is quite high (45–60%) and,

like most chemically modified cellulosics, it produces transparent solutions, required in this

product category.

Sauces, gravies, fillings, and cream soups are similar in that most of them take advantage

of starches for viscosity control. However, a drawback to using starches exclusively for

rheology control is that they generally do not maintain viscosity over a wide temperature




absence of a thermally gelling binder [cf. 108].

range and many of them are not freeze–thaw stable. Cellulosics have long been used to boost

the performance of these products, especially those that are held for long periods at elevated

temperature as in a food-service operation or those that are frozen. Recent work in our

laboratory has shown that low molecular weight MC interacts with modified starches to yield

systems with rheological properties that are quite stable over wide temperature ranges. This is

in contrast to starches alone, which lose viscosity as temperature increases, and MC alone,

which gels at temperatures in excess of 508C. The specific nature of this interaction is not

known but may be due to intergranule bridging or hydrophobic interactions between the MC

and the modifying groups on the starch granules.

5.4.3 EMULSIONS

Two important emulsified food categories are salad dressings and whipped toppings, the

former oil-in-water emulsions, the latter, foams of oil-in-water emulsions. Both types require

certain fundamental chemical and physical properties that are obtainable by the use of cellulose

derivatives. Pourable salad dressings are typically oil-and-water emulsions with oil concen-

trations ranging from 10 to 50%. In these systems it is important to prevent the flocculation

and coalescence of oil droplets that would lead to rapid phase separation. Cellulose ethers

contribute to emulsion stability by concentrating at the oil–water interface, imposing a barrier

of hydrated polymer around each droplet.

In low-calorie salad dressings, emulating the mouthfeel of higher-oil dressings can be a

greater challenge than stabilizing the low level of oil that is present. The surface-active

cellulose derivatives can provide the film-forming property and slip that is characteristic of

oils, and can also be selected to optimize the viscosity needed to achieve the appropriate

texture in these products.

Dry mix salad dressings pose unique challenges to food formulators. In these commod-

ities, a dry mixture of stabilizers, spices, and flavors is packaged for consumer use. The

consumer is the end processor of the emulsion and requires an easily formulated and easily

processed product for domestic use. The use of HPMC and MC is advantageous because they

offer rapid hydration and emulsification under the relatively low shear conditions that are

encountered in home preparation. The rapid interfacial migration of cellulose ethers contrib-

utes to rapid stabilization of the emulsion and ease of use for the consumer.

Nondairy whipped toppings, whipped desserts, and mousses are foams of oil-in-water

emulsions. These products pose special concerns and constraints because the bubble wall is

thin, relatively weak, and unsupported. This is in contrast to the surface of an oil droplet in a

dressing, which is supported from inside by a mass of oil. In these systems, two requirements

exist. The first is a physical stabilization of the liquid within the interstitial regions of the

foam. The second is a strengthening of the cell wall, accomplished by the use of gums and

other ingredients in the mix.

For whipped toppings, structural integrity between the foam cell walls and in the inter-

stitial areas can be achieved by using a physically modified cellulose derivative such as MCC.

MFC was a unique physically modified cellulose derivative, which also provided stability in

whipped toppings; its current commercial status is unclear. The rheological role played by

MFC in these applications is that of imparting yield-point properties that result in structuring.

Cell wall integrity can be achieved by using cellulose ethers that accumulate at interfacial

surfaces. Examples often used in whipped toppings are HPC, MC, HPMC, and MEC, and

MC is used to stabilize foams [109]. In the case of the methyl derivatives, the polymer

concentrates at the interfaces and undergoes gelation, as the concentration at the interfacial

boundary is substantially higher than that of the bulk phase (usually less than 1%). This

surface gel confers stability on the bubble cell wall, resulting in a more resilient and stable




product. Furthermore, whipped products usually have an additional storage burden as they

must survive frozen storage. The use of chemically modified cellulose derivatives can reduce

syneresis, a severe quality defect, during repeated freeze–thaw cycling.

Cellulosics have been used frequently in emulsified food products. The most common

prepared emulsions are salad dressings, in which the two cellulosics most used are HPMC and

MCC. HPMC is incorporated in pourable and dry mix salad dressings for emulsification,

rheology control, and improved organoleptic properties. HPMC, since it is a surface-active

agent, migrates to the oil droplet–water interface and establishes a hydrated polymer barrier

at the interface. This prevents oil droplet coalescence and subsequent development of an oil

slick on the emulsion surface. In addition, since the additive accumulates at the droplet

interface and prevents large droplets from forming, the refractive index of the droplet changes

and the color is lighter than it would be otherwise. This is an advantage in creamy dressings.

Finally, HPMC increases the viscosity of the product and improves organoleptic properties,

ameliorating the otherwise ropy mouthfeel and gelatinous appearance of pourable dressings.

In contrast to their employment in pourable dressings as surfactants, cellulosics can also

be used in high-viscosity, spoonable dressings. MCC assists in the manufacture of full- and

reduced-calorie products by increasing the viscosity of the interstitial continuous phase and

contributing bulk to reduced-calorie formulations.

5.4.4 BAKED GOODS

There are a number of beneficial uses for cellulose and cellulose derivatives in baked products.

Breads require a certain strength-to-volume ratio to allow good formation, cell structure,

even texture, and high eating quality. Typical white pan breads achieve most of their volume

and textural quality from gluten, the most abundant protein in wheat flour. However,

variations in the quality and quantity of flours from various wheats affect product quality.

Additives have long been used to correct this.

High-fiber and variety of diet breads, buns, and rolls generally contain reduced quantities

of flour as a means of decreasing the caloric value; as a result, they require special reformu-

lation to provide loaf structure and baked quality comparable to products containing wheat

flour. High-fiber breads make use of several cellulose derivatives. Usually a physically

modified product such as MCC or cellulose flour is used as a partial replacement for wheat

flour and consequently some of the nutrient energy of the flour. However, physically modified

celluloses generally have little functionality in baked goods, and, therefore, certain compon-

ents must be modified to yield a product of suitable quality. Dough rheology is affected when

physically modified celluloses replace part of the flour because the water demand and mixing

qualities change. The baking qualities of reduced wheat flour breads are usually inferior and

must be compensated for by other functional ingredients.

The use of MC and HPMC returns to the bread the functionality lost due to the lowered

wheat flour concentration. These cellulose ethers provide an elastic mass during proofing and

baking that traps CO2 and allows the bread to rise and maintain adequate volume. Breads of

this type have been patented and are widely available in the U.S. retail trade.

In providing structure to breads and rolls, the thermally gelling derivatives are very useful.

Since MC and HPMC are interfacially active and form elastic gels at elevated temperatures,

they can impart added dough strength and uniformity through make-up, proofing, and

baking, and lead to an even, consistent crumb structure in the final baked product [110].

The properties of surface activity and gelation with heat also increase moisture retention and

tolerance to the changing environmental conditions of today’s automated bakeries. This

improves the production of bread from typical wheat flours as well as baked goods with

very low wheat protein levels or specialty breads made from other grains, such as rice,




sorghum, and barley, which contain no gluten [111,112] and to which as much as 3% of the

cellulosic may be added.

As well as acting as a loaf volume enhancer, cellulosics can be used as bulking agents in

the production of high-fiber breads. Since cellulose is nondigestible, the physically modified

derivatives can easily be used at substantial concentrations as a supplementary fiber source.

Sweet baked goods, such as cakes, can also be improved by the inclusion of cellulosics

albeit at lower levels. Like breads, cakes need elasticity and some structural integrity to trap

the gases produced for leavening action. Thermally gelling cellulosics can perform well in

achieving these requirements. Increased cake volumes have been demonstrated with MC and

HPMC as well as with CMC, with the CMC-induced volume generated through a viscosity

quality and cake heights have been observed with the use of HPMC and MC [114]. In the

sensory evaluations associated with this study, textures were identified as moist without

excessively chewy.

Cellulosics also play a role in improving the texture of fat-reduced snack cakes by

providing batter thickening, moisture retaining, and film-forming properties in these products

[115,116]. In addition to a functional contribution, physically modified cellulosics can be used

as a bulking agent for production of cakes with reduced nutrient energy. This application is

expected to grow dramatically as heat-stable, low-calorie sweetening compounds (see Ref.

glazes sometimes use cellulosics, especially CMC and HPMC, at low levels to improve texture

[118]; CMC has been demonstrated [90] to enhance flavor perception and have some effect on

sweetness. HPMC improves low shear flow properties in icings, facilitating spreading, pro-

viding favorable mouthfeel, and improving resistance to icing runoff.

5.4.5 FROZEN DESSERTS

Frozen desserts frequently include hydrocolloids such as cellulosics, gelatins, starches, and

carrageenans. The cellulosics are often used to control ice crystal growth and to modify

rheology [119]. A number of formulations are available that include a variety of cellulosic

materials. This is especially true for some whipped frozen products that take advantage not

only of the ability of a gum to modify rheology but also to entrain air.

The trend toward replacing fat in food systems has a strong presence in the frozen dessert

industry. Cellulose derivatives can produce textures in low-fat or fat-free frozen desserts that

are similar to the mouthfeel associated with products containing higher fat levels.

5.4.6 EMERGING TECHNOLOGIES: BARRIER FILMS

Barrier films, such as those described by Fennema and coworkers [120–123], take advantage

of cellulose derivatives as film substrates in complex systems for water vapor transmission

control, the concept being expanded, in principle, to reduce oil uptake [124–127]; these films

are especially formulated for frozen prepared foods to prevent migration of water from areas

of high relative humidity to areas of low relative humidity and to preserve the textural

qualities associated with fresh prepared products. In this technology, films are produced by

casting in successive stages. First a film of HPMC is formed; this is overlaid with a sprayed

coating of a triglyceride, and then coated with a thin layer of beeswax. These films when

finished are 2–5 mm thick, unnoticeable in a fabricated product, and are excellent water vapor

barriers at temperatures below the melting point of the wax and fat. However, a unique

property of these films is that at temperatures in excess of 658C, the hydrated HPMC film gels

inhibit moisture migration. This reduces such migration from areas of relatively high water




[117] ) are developed and formulated into processed baked goods. Finally, bakery icings and

contribution. In cake mixes formulated for microwave baking [cf. 113], improved eating

activity to areas of low water activity. As an illustration, the concept could be applied to

frozen pizza, with a film incorporated between the crust and the sauce and thus preventing

moisture migration during storage and home preparation. Other types of membrane, such as

developed [128].

5.5 NUTRITIONAL EFFECTS OF CELLULOSE DERIVATIVES

Any food component can affect the nutritional density or quality of the diet in a number of

ways. The most obvious is as a nutritious component of a food. An example of this is the use

of vitamins and minerals to fortify ready-to-eat cereals. A second way is as a functional

additive, without which food could not be produced, distributed, or accepted by consumers;

CMC added to ice cream improves the mouthfeel and reduces the growth of ice crystals

during frozen storage. A component could also have an impact on the nutrient density of

foods as a nonnutrient replacement, reducing the caloric load and making a wider variety of

products available to consumers; examples are the use of gums in the formulation of low-oil

salad dressings and cellulose in making low-calorie breads.

None of the cellulosics are of value as nutrient sources since humans lack the digestive

enzymes necessary to generate the b-1,4-linked glucose monomers. This fact is the basis for

much of the utility of these products in food, with cellulose and its derivatives playing

important functional and bulking agent roles. The use of CMC, for example, enables ice

cream to retain more of its initial quality during frozen storage than control samples. This

functional role is crucial to modern production and distribution systems and ultimately

permits more efficient use of raw materials, flexibility in handling and storage, and conser-

vation of food resources. Physically modified celluloses are especially important as bulking

agents in formulated foods, cellulose flours, and MCC are widely used as partial replacements

for flour and other nutrient materials in breads and some desserts.

5.5.1 DIETARY FIBER

Dietary fiber plays a significant role in gut physiology and nutrient absorption and has come

under very close scrutiny in recent years. In spite of its importance in our diet and for our

well-being, there is still much debate and confusion over the specific definition of dietary fiber,

fiber (crude) referred to the residue of plant material that was indigestible in acid and alkali.

This definition, based on a method of analysis, limited crude fiber to cellulose, hemicelluloses,

and lignin and is a dated term that has fallen out of favor because of its limited chemical

and inadequate biological usefulness. Today the term dietary fiber refers to a much broader

group of compounds. Most researchers and interested observers define dietary fiber as

ingested material that is resistant to digestion in the gastrointestinal tract of humans. The

components that make up dietary fiber are cellulose, hemicelluloses, lignins, pectins, gums,

mucilages, waxes, monopolysaccharides, and undigestible proteins. Chemically modified

cellulosics such as CMC, MC, HPMC, HPC, and EMC, being indigestible but soluble, fall

under the same umbrella [129–131].

There are two types of dietary fiber: soluble and insoluble. The insoluble materials form a

bulky mass and speed transit time through the gastrointestinal tract because of their bulk;

cellulose, hemicellulose, and lignin fall into this category, so that cellulose flour, MCC, and

MFC are included. Among the soluble dietary fibers are the pectins, gums, natural and

derived, and mucilages. All of these have the ability to hold water and thereby increase the

viscosity of the food bolus. Reference to gums as soluble dietary fiber was once confined to




which has come to represent many things to many people (see Chapter 18). Originally,

those formed by cross-linking cellulose with diepoxides (cf. HPC manufacture), have been

those of vegetable origin, such as gum Arabic, guar gum, and locust bean gum; today,

however, there is wide acceptance of the inclusion of chemically modified celluloses in this

category. All gums increase the water content of the stool, but may do so in different ways.

The natural gums, not based on b-1,4-glucan backbones, are all fermented to a substantial

degree, thereby losing their innate water-binding ability and promoting high bacterial cell

contents in the stool. This increased bacterial cell mass carries with it roughly 80% water and

is resistant to dehydration [130]. The water-soluble cellulosics, on the other hand, are resistant

to digestion and retain their molecular integrity and water-binding ability even in the colon.

The increased moisture content in the stool associated with derivatized cellulosics is therefore

to a large extent due to the water of hydration of the polymers. There is a beneficial dietary

effect in that the rate of diffusion of glucose is lessened in the presence of such viscous fibers

as MC [132,133].

5.5.2 WATER-HOLDING CAPACITY

Cellulose and its derivatives vary widely in their water-holding capacity. Cellulose and

physically modified celluloses generally tend to imbibe very little water because of their

relative insolubility and tendency to aggregate together through strong hydrogen bonding.

Chemically modified celluloses, on the other hand, are water soluble as a result of the

inclusion of substituent groups (methyl, carboxymethyl, or hydroxypropyl) and when in a

food product tend to retain high moisture contents in the stools after ingestion.

5.5.3 METABOLISM

The metabolism of cellulose and its derivatives has been studied extensively; as dietary

fiber components, they are very unreactive and nonfermentable. Marthinsen and Fleming

[134] in early work evaluated the response of rats to feeding with xylan, pectin, cellulose,

and corn bran by investigating the excretion of gases following administration of the

purified substances as dietary components. Breath gases were monitored to determine the

extent of fermentation occurring in the large intestine. Increased fermentation in the colon

was indicated by elevated gas excretion levels. The authors found that diets containing cellulose

and corn bran generally caused gas excretion levels that were not significantly different from

those of the fiber-free controls. This indicates the relative nonfermentability of cellulose [135]

and is supported by the work of Fleming and coworkers [136–138], who studied the effect of

fiber on fecal excretion of volatile fatty acids (VFA). The concentration of VFA in excreted

feces was found to be less for cellulose-containing diets than for the control diet. Higher levels

of VFAs or SCFAs (short-chain fatty acids) in the colon are associated with health benefits, and

MCFAs (medium-chain fatty acids) that are more readily absorbed by the colon may be of

The chemical derivatives of cellulose are also known to be safe for use in foods. Like

cellulose, they are indigestible. No significant radioactivity accumulates in the organs of rats

fed with radiolabeled MC, HPMC, HPC, and CMC, and no chronic or subchronic toxicity to

test animals fed up to several percent of the diet was noted. One beneficial effect of water-

soluble cellulosics in the diet is their very efficient water retention, promoting large, bulky

stools. It is not necessary to review the safety and toxicology of these products in this chapter;

our purpose to state that the cellulose derivatives allowed for food use have been reviewed

Canada, Australia, and many others.




assistance when small-bowel function is impaired [139,140] (see Chapter 18).

and approved by the food regulatory agencies in the United States, the countries of the EEC,

the interested reader may refer Refs. [141,142] for comprehensive reviews. It is sufficient for

5.6 CONCLUSION

Although cellulose is universally present in foodstuffs of plant origin and contributes to

human nutrition as a source of dietary fiber, relatively little is processed or submitted to

chemical modification in order to provide additives for use in the food industry. Powdered

a-cellulose and MCCs increase viscosity and provide bulk in the baking, dairy, and meat

industries, especially when fat reduction is required. The range of permitted ethers, of which

CMC in salt form predominates, adds the useful rheological properties of enhanced viscosity,

thixotropy, and pseudoplasticity, thereby extending the uses of cellulosics to include inter alia

confectionery and frozen desserts. The remarkable stability of cellulose and its derivatives

under physiological conditions, the diversity of chemical functionality of the polysaccharide,

and above all the abundance of the raw material will doubtless result in considerable

expansion of the technological development of food cellulosics.

The cultivation of transgenic plants is adding a new dimension to the production of

cellulose-derived substances aimed at improved technical performance in fibers, pulps, and

edible foodstuffs. Of passing interest to food aspects of cellulosics is the development of novel

derivatives to extend the important range of analytical uses of purified cellulose and of CMC

and diethylaminoethyl (DEAE)-cellulose, for example, as stationary phases in planar and

column chromatography [143,144], and for other purposes [145].

Technical appendices appended below offer information on the practical handling of

cellulosics and on the regulatory status of these products. The first edition of Food Polysac-

charides and Their Applications [11] gives more information on sources of supply, and includes

references to the earlier literature.

APPENDIX 1: DISPERSION–DISSOLUTION OF CELLULOSEAND CELLULOSE ETHERS

MICROFIBRILLATED CELLULOSE

ITT Rayonier Inc. has supplied MFC in three forms under the Nutracel trademark: a 4% paste

in water; a granular product 25% MFC, 75% water; and a powder 80% MFC, 50% sucrose. The

paste is readily dispersible in water or a number of water-miscible organic solvents. Medium

shear mixing is needed to obtain the maximum viscosity and water retention. The granulated

and powdered products require high shear mixing to allow the product to swell fully in aqueous

systems. Dry blending the MFC with other ingredients before adding to the wet mix is often

advantageous. The current commercial status of MFC is unclear.

MICROCRYSTALLINE CELLULOSE

MCC, as supplied by FMC Corporation under the Avicel and Micro-Quick trademarks, is

available in three basic forms: bulk dried, spray dried (both with the addition of CMC as a

dispersant), and dried with sweet whey. The bulk-dried material requires homogenization at

132 bar (2000 psi) after premixing to achieve full water uptake and viscosity. Spray-dried

material requires high-speed agitation, whereas the spray-dried whey-containing product

disperses and swells in water through the action of simple agitation.

CELLULOSE ETHERS (CMC, EMC, HPC, HPMC, MC)

Food-grade cellulose ethers have a tendency to lump if incorrectly added to water or aqueous

solutions. The key to successful solution preparations is to disperse the cellulose ether




particles, then hydrate them in an even manner leading to lump-free, clear solutions. Four

basic methods are used:

1. Vortexing: The cellulose ether is added into the vortex of rapidly stirred water. A

balance has to be struck here between a rate of addition slow enough to allow the

particles to separate and wet-out, yet fast enough to prevent interference in this process

by viscosity build-up. Method 1 is most suitable for CMC and HPC.

2. Nonsolvent dispersion: The cellulose ether is dispersed in a water-miscible solvent.

Water is added (or vice versa) with stirring. The mixture will gradually clear and a

solution will be obtained. Method 2 is most suitable for HPMC and MC.

3. Dry blending: The cellulose ether is blended with a nonpolymeric, dry material at a

ratio approximately 1 part cellulosic to 7 parts of other ingredient. The mixture can

then be added to or mixed with water. Method 3 is suitable for all cellulosics.

4. Hot–cold aqueous dispersion: A number of cellulose ethers are not soluble in hot water.

Approximately one third of the total water is heated to approximately 908C for MC

and HPMC, 708C for EMC, or 508C for HPC. The cellulose ether is added with

stirring, then the balance of cold water is added — stirring is continued until a solution

is obtained. Method 4 is most suitable for MC, EMC, HPC, and HPMC.

The above four methods are generalizations, as certain cellulose ethers are more or less

soluble, depending on, for instance, DS for CMC and the ratio of hydroxypropoxyl to

methoxyl substitution for HPMC.

The manufacturers of celluloses and cellulose ethers produce a wide variety of literature

covering the properties of their products and copies are readily available.

APPENDIX 2: REGULATORY STATUS OF CELLULOSE AND CELLULOSEETHERS IN FOOD APPLICATIONS

A full discussion of this subject is beyond the scope of this chapter. Companies intending to

use cellulose or its approved derivatives are urged to check that, first, the product is allowed

within the foodstuff group in question and that, second, the grade of product to be used meets

in full the criteria of purity set either internationally or nationally. As a source of sound

advice and data, the authors highly recommend the excellent service provided to members by

the Leatherhead Food Research Association, Leatherhead, U.K., on regulatory issues and

other subjects throughout the spectrum of food science.

EUROPE — THE EUROPEAN ECONOMIC COMMUNITY

The EEC is striving to harmonize food regulations throughout its member states. Essentially

the EEC Scientific Committee for Food reviews the data or the various additives for food and

then assigns to them an E number and sets purity criteria. Member states then decide whether

to allow the use of these E derivatives and set limits on use quantity and application.

Confusingly, some additives only have numbers, but are under review for E status. It is a

real dichotomy within certain European states that E-numbered compounds, reviewed as to

their suitability for food use, are often viewed by the public and in particular by a number of

pressure groups as potentially harmful. The opposite opinion is often held about generally

impure and often totally untested natural ingredients, usually sold under the guise of health

foods.




Celluloses and cellulose ethers fall under regulations stemming from EEC directive 74/329/

EEC and its amendments — covering emulsifiers, stabilizers, thickeners, and gelling agents.

Cellulose and cellulose ethers have the following E numbers:

E460 (i) Microcrystalline cellulose

E460 (ii) Powdered cellulose

E461 Methylcellulose

E463 Hydroxypropylcellulose

E464 Hydroxypropylmethylcellulose

E465 Ethylmethylcellulose

E466 Carboxymethylcellulose

NON-EEC COUNTRIES

Each of these has its own food regulations. In general, however, cellulose and its derivatives

are widely accepted. Those products having wide regulatory approval (FDA, USP, EEC,

etc.), such as MC and CMC, tend to be more frequently used.

United States

Food additives are regulated in the United States by the Food and Drug Administration and

the U.S. Department of Agriculture (USDA). Of the chemical derivatives, only MC and

sodium CMC are generally recognized as safe (GRAS). The specific approval is listed in the

U.S. Code of Federal Regulations (CFR) Title 21; MC under 21 CFR 182.1480 and CMC

under 21 CFR 182.1745. The other cellulose derivatives of importance — HPC, HPMC,

MEC, and EC — are approved under Part 172 of Title 21 of the CFR. The compounds and

their specific approvals are HPC, 172.870; HPMC, 172.874; MEC, 172.872; and EC, 172.868.

There are other specific approvals for cellulose derivatives such as those that define their use

in Adhesives and Coatings for Food Use (21 CFR 175.300) or specific food products such as

Artificially Sweetened Fruit Jellies (21 CFR 150.141).

The USDA has regulatory authority over meat products in the United States. The

approvals for cellulosics in meats are different than those for other food products. For red

meats, sodium CMC is approved as an extender or stabilizer in baked pies and MC is

approved as an extender or stabilizer in meat and vegetable patties, both according to

9 CFR 318.7. For poultry products, both CMC and MC are approved as extenders and

stabilizers according to 9 CFR 381.147. Finally, in addition to these approvals for CMC and

MC, HPMC is listed in the USDA Standards and Labeling policy book as an appropriate

ingredient when used in a manner consistent with the policy book’s regulations.

Canada

In Canada, CMC, MC, HPMC and HPC, and MEC are listed on the Food and Drugs Act

and Regulations as food additives that may be used as emulsifying, gelling, stabilizing, and

thickening agents.

Australia

In Australia, sodium CMC, HPMC, and MC have been approved for inclusion in the Food

Standards Regulations under Section A10, group 1, and referencing modifying agents.

Japan

In Japan, MC and sodium and calcium CMC have approval for food use. The materials must

meet specifications for the Japanese Pharmacopeia or the Japanese Food Codex.




Other Areas

For all other areas not listed specifically, the formulator should determine the approvals and

regulated limits for various cellulosic derivatives for foods.

ACKNOWLEDGMENT

A.M. Stephen provided the new material for the second edition.

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