properties of canola protein-based plastics and protein isolates modified using sds and sdbs

ORIGINAL ARTICLE

Properties of Canola Protein-based Plastics and ProteinIsolates Modified Using SDS and SDBS

Wajira A. R. Manamperi • Scott W. Pryor

Received: 18 March 2011 / Revised: 25 July 2011 / Accepted: 20 August 2011 / Published online: 6 September 2011

� AOCS 2011

Abstract Canola protein isolates were prepared from

canola meal flour using alkaline solubilization and acid

precipitation. The isolates were treated with sodium

dodecyl sulfate (SDS) and sodium dodecyl benzene sul-

fonate (SDBS) with concentrations between 1 and 5%.

Functional property analysis of modified isolates revealed

that water absorption and fat absorption increased by up to

115 and 78%, respectively, with increasing SDBS con-

centrations. Fat absorption of SDS-modified isolates

showed a similar trend to that of SDBS-modified isolates

(86% increase), but water absorption decreased (by 77%)

in the 5% SDS treatment. SDS and SDBS treatments

reduced the emulsifying activity of protein isolates by 34

and 30%, respectively. The denaturation effect of SDS and

SDBS treatments increased the surface hydrophobicity of

proteins and resulted in increased tensile strength (by 14

and 41%), tensile modulus (by 31 and 52%), and toughness

(by 44 and 64%) of plastic specimens prepared using

modified isolates. Only the 3% treatments of SDS and

SDBS increased the elongation of plastics (by 22 and 11%,

respectively) while other treatments did not show signifi-

cant differences from the plastics that used non-modified

isolates. The water absorption of plastics increased by 33%

with the 5% SDS treatment; the SDBS treatments showed a

9% decrease in water absorption for the 1% treatment but

no significant differences at higher concentrations. Dena-

turation by SDS and SDBS can be employed to alter

functional properties of canola protein isolates as well as

the mechanical properties of canola protein-based plastics.

Keywords Canola proteins � Protein modification �Denaturation � Functional properties � Bioplastics �Mechanical properties

Introduction

Plant-based proteins have emerged as raw materials for

various applications including food additives and industrial

products. When used as food additives, the amphiphilic

nature of most plant-based proteins can impose functional

characteristics such as water and fat absorption, emulsifi-

cation, foaming, and gelling in food systems [1]. Various

plant-based proteins such as soy, sunflower, and chick pea

have found applications as food additives [2, 3]. The suc-

cess of these ingredients largely depends on the charac-

teristics they impose on food systems.

Canola meal and isolate are comprised of two major

storage proteins, napin (albumin) and cruciferin (globulin)

that constitute 20 and 60%, respectively [4], of canola

proteins. The remaining proteins are a mixture of glute-

lins, prolamins, oleosins, and lipid transfer proteins.

Canola protein has a well-balanced amino acid profile that

is well-suited for food applications [5]. The most abun-

dant amino acid in canola proteins is glutamine, while

most of other essential amino acids such as threonine,

histidine, lysine, and sulfur-containing amino acids

(methionine and cysteine) exceed required levels specified

by the World Health Organization (WHO) for food

applications [6]. Like other proteins, the secondary

structure of canola proteins is a combination of a-helices

and b-sheets [7, 8]. A higher a-helix:b-sheet ratio is

preferred in food applications since it enhances the access

of proteins to digestive enzymes, thus increasing the

protein value [8].

W. A. R. Manamperi � S. W. Pryor (&)

Department of Agricultural and Biosystems Engineering

(Dept 7620), North Dakota State University,

PO Box 6050, Fargo, ND 58108-6050, USA

e-mail: [email protected]

123

J Am Oil Chem Soc (2012) 89:541–549

DOI 10.1007/s11746-011-1935-4

Canola proteins are polymeric materials that possess

some useful characteristics such as thermoplastic behavior

that are useful in industrial applications such as plastics and

composites. However, their usefulness is largely affected

by the water sensitivity of the final products [9]. Also, the

mechanical properties such as tensile strength, elongation,

and toughness are generally not comparable to petroleum-

based products [10–12]. Properties of protein-based plas-

tics can be enhanced through various means including

blending with synthetic polymers, incorporating plasticiz-

ers or cross-linkers, and modifying protein functionality

through controlled denaturation [13].

There are several methods that can be employed to

denature the proteins [13]. The most common methods of

protein denaturation include exposure to high temperatures

or pressures, and interaction with acids, alkalis, organic

solvents, or detergents. Generally, excessive and uncon-

trolled denaturation of proteins is considered detrimental

since it changes the functional and physicochemical prop-

erties of proteins that results in loss of solubility. However,

partial and controlled denaturation can be used to enhance

the protein functionality [14].

Both SDS and SDBS are anionic detergents capable of

dissociating protein molecules and binding with them non-

covalently [15]. The interaction with SDS and SDBS

changes the secondary, tertiary, and quaternary structures

and conformation of proteins without breaking covalent

bonds. Denaturation is likely to decrease the a-helix:

b-sheet ratio, thus decreasing the nutritive value of canola

proteins [8]. Studies of denaturation effects on the tertiary

and quaternary structures of canola proteins are rare in the

literature [6]. However, denaturation by SDS and SDBS

molecules is likely to change both tertiary and quaternary

structures by disrupting hydrophobic and electrostatic

interactions between protein molecules making the protein

structure partially unfolded and flexible [16]. Unfolding of

proteins by anion binding takes place due to electrostatic

repulsion imposed by bound molecules (SDS and SDBS),

piercing of the protein’s apolar regions by the hydrocarbon

tails of bound molecules, and the changes in the protein-H?

equilibrium [17, 18].

Changes in protein structure and conformation can lead

to changes in the functional properties such as water

absorption, fat absorption, emulsification, foaming, and

gelation [14, 16]. Therefore, controlled denaturation is an

attractive means to obtain desired functional and physico-

chemical properties (depending on applications) through

structural changes in the protein molecules [9]. Denatur-

ation of proteins can also be used beneficially in protein-

based plastic preparation. The unfolded and flexible

peptide chains (with exposed hydrophobic residues)

resulting from partial denaturation can participate in more

entanglements and cross-linking during plastic processing

steps such as compounding extrusion, injection molding,

and compression molding [16]. This is expected to improve

the strength and toughness properties of plastics.

Studies of food and industrial applications of canola

proteins are rare in the literature. There is a significant

potential for canola protein-based products due to the

high protein content in the meal and its relatively low

cost. However, to be competitive with other products

canola protein-based products should be improved by

various means such as chemical modification of proteins

prior to their use in food and industrial applications. The

objective of this study was to investigate the effects of

SDS and SDBS denaturation of canola proteins on the

functional properties of protein isolates and the

mechanical and water absorption properties of protein-

based plastics.

Materials and Methods

Materials

Canola seeds (cv. Invigor 2573) were obtained from Cav-

alier County in northeastern North Dakota. A biodegrad-

able co-polyester PBI 001 was purchased from Natureplast

Inc. (Caen, France). SDS and SDBS were used as dena-

turants to modify proteins. Glycerol, polyvinylpyrrolidone

(PVP), and zinc sulfate (ZnSO4) were used without further

treatment as plasticizer, compatibilizer, and cross-linker,

respectively, in plastic preparation.

Preparation of Canola Meal Flour

Canola seeds were cleaned according to USDA-GIPSA

recommendations using a Carter-Day dockage tester

(Minneapolis, MN, USA) and hand sieves. Canola meal

was prepared by defatting the seeds by pressing twice

followed by solvent extraction using a Soxhlet apparatus.

Before screw-pressing, the moisture content of canola

seeds was determined gravimetrically at 120 �C using an

LJ16 moisture analyzer (Mettler Toledo Inc., Columbus,

OH, USA). To adjust the moisture content to 7% (wet

basis), distilled water was added and mixed uniformly with

seeds (in sealed plastic bags) and allowed to equilibrate

overnight. Seeds were then fed at 80 g/min to a model S

87G Komet screw press (Monchengladbach, Germany)

preheated to 70 �C. Screw rotation speed was kept at

24 rpm. The resulting canola meal was then ground using a

Retsch ZM1 mill (Brinkmann Instruments Inc.; Westbury,

NY, USA) and passed through a 25-mesh screen. Canola

meal flour was then solvent-extracted using hexane for

24 h and desolventized for 2 days in a fume hood at room

temperature.

542 J Am Oil Chem Soc (2012) 89:541–549

123

Preparation of Protein Isolates

Screw-pressed and milled canola meal flour (100 g) was

dissolved in 400 mL of distilled water. The pH of the

solution was adjusted to 12 using 6 N sodium hydroxide

(NaOH) and stirred for 1 h to solubilize the proteins. The

solution was centrifuged at 5,0009g for 30 min to obtain

the protein-rich supernatant, which was filtered through

cheese cloth followed by Whatman 41 filter papers. The

pellet, consisting of fiber and other solids, was discarded.

To precipitate the proteins from the supernatant, 6 N

hydrochloric acid was added drop-wise to bring the pH of

the solution to 5.0. The precipitated proteins were recov-

ered by centrifugation at 5,0009g for 30 min and collected

in plastic bottles for freeze-drying. Freeze-dried canola

meal protein isolates contained approximately 88% pro-

teins as determined by the Kjeldahl method using a con-

version factor of 6.25 [19].

Protein Denaturation by SDS and SDBS

The SDS and SDBS solutions (1, 3, and 5 wt%) were

prepared by dissolving SDS and SDBS powders in distilled

water. Canola protein isolates (25 g) were dissolved in

200 mL of SDS and SDBS solutions and reacted for 6 h

while stirring continuously using a magnetic stirrer. The

solutions were then freeze-dried and powdered to obtain

SDS- and SDBS-modified protein isolates.

Functional Properties of Native and Denatured

Protein Isolates

The water holding capacity was measured for native and

denatured protein isolates according to the methods repor-

ted by Ghodsvali et al. [20] with minor modifications.

Protein isolate samples (2 g) were dissolved in 16 mL of

distilled water and placed in 50-mL centrifuge tubes. The

tubes were vortexed for 30 s every 10 min for 1 h. The

tubes were then centrifuged at 2,0009g for 15 min and

drained for 30 min by inverting the tube. The final weight of

each sample was measured and water holding capacity was

calculated as the percentage increase of sample weight.

To measure the fat holding capacity of native and

denatured protein isolates, 2 g of protein isolates were

mixed with 12 mL of canola oil in 50-mL centrifuge tubes.

The tubes were vortexed for 30 s every 5 min for 30 min

and centrifuged at 1,6009g for 25 min. Oil was then

decanted and drained by inverting for 1 h. Oil absorption

was expressed as the percentage increase of sample weight.

To measure the emulsifying activity, 1.4 g of native or

denatured protein isolates were homogenized for 30 s in

20 mL of distilled water, followed by adding 10 mL of

canola oil and homogenizing again for 30 s. Another 10 mL

of canola oil was then added and homogenized for 90 s. The

emulsion was centrifuged at 1,1009g for 5 min at 25 �C

and the volume of the emulsified layer was measured. The

emulsifying activity was expressed by calculating the per-

centage volume of the emulsified layer after centrifugation

to the emulsion volume before centrifugation. Three repli-

cates were analyzed for each functional property test.

Preparation of Plastic Specimens

Standard and denatured protein isolates were used to pre-

pare plastic specimens according to the method described

by Mungara et al. [21] with modifications. A base formula

was used as follows: canola protein isolates (35 parts),

plasticizer (15 parts), synthetic co-polyester (40 parts),

compatibilizer (2 parts), water (7 parts), and cross-linker

(1 part) were mixed mechanically until a homogeneous

blend was obtained. Glycerol was used as a plasticizer and

PVP was used as the compatibilizer to mediate the interaction

between protein isolates and synthetic co-polyester. Zinc

sulfate was used as a cross-linker. The blend was allowed to

equilibrate for at least 24 h before further processing.

The formulation was fed to a Leistritz Micro-18/GL-

40D co-rotating twin-screw extruder (American Leistritz

Extruder Corp.; Somerville, NJ, USA) for extrusion com-

pounding. The temperature profile for the extruder was

maintained at 95, 116, 126, 136, 136, and 141 �C from

feeder to die. The extruded canola protein blends were then

pelletized. The pellet moisture content was analyzed as

described above and adjusted to 7% (wet basis) by mixing

with distilled water and allowing to equilibrate overnight.

Pellets were then fed to a Technoplas SIM 5080 injection

molder (Technoplas, Inc., Australia) to form dog-bone-

shaped tensile test specimens according to the ASTM

standard D638 [22]. Injection molder temperature profile

was maintained at 116, 124, 127, 132, and 124 �C from

feeder to nozzle.

Mechanical Properties

Mechanical properties (tensile strength, tensile modulus,

elongation, and toughness) of injection molded specimens

were analyzed using a model 5567 Instron Universal

Testing Machine (Instron Corporation; Canton, MA, USA)

according to the ASTM D638 testing method [22]. Five

replicates of each treatment were analyzed for tensile tests.

Fracture Morphology

Low temperature impact performance was assessed by

examining the freeze-fractured tensile test specimen sur-

face morphology. Specimens were fractured after cooling

under liquid nitrogen and morphology was analyzed using

J Am Oil Chem Soc (2012) 89:541–549 543

123

a JEOL JSM-6490lv scanning electron microscope (SEM)

(JEOL Ltd; Tokyo, Japan). Fractured specimens were cut

and mounted on aluminum stubs and surfaces were coated

with gold under vacuum. The electron micrographs were

taken at 15 KV.

Water Absorption of Plastics

Plastic specimens (prepared according to ASTM D638)

were used for the water absorption test. Water absorption

of specimens was determined according to the ASTM

D570 testing method with minor modifications [23]. Plastic

specimens were conditioned in a convection oven at 50 �C

for 24 h and cooled to room temperature in a desiccator

and weighed. Specimens were then immersed in distilled

water for 24 h, and weighed after removing surface water

using paper towels. The samples were then reconditioned

following the same initial drying method and the final

weights were recorded to account for the loss of water-

soluble matter during soaking. Water absorption was cal-

culated as a percentage weight increase of the samples after

accounting for the loss of water-soluble matter.

Statistical Analysis

Mechanical and functional properties were analyzed with

one-way analysis of variance. Multiple comparisons of

means were done using a Tukey’s test controlling the

overall confidence level at 95%. Statistical analyses were

done using Minitab software (Minitab Inc., State College,

PA, USA).

Results and Discussion

Functional Properties of Isolates

The water absorption of the protein isolates modified using

SDS and SDBS treatments with 1, 3, and 5% concentra-

tions are compared with that of standard protein isolates in

Fig. 1. The water absorption of SDS-modified isolates

decreased with the SDS concentration with a 77% reduc-

tion seen in the 5% SDS-treated isolates. However, the

SDBS treatments increased the water absorption of isolates

with the increasing concentration showing a 115% increase

in the 5% SDBS-treated isolates. This can be explained by

the denaturation effects of SDBS treatments. Higher SDBS

concentrations are expected to increase the degree of

denaturation of protein isolates. Higher denaturation

unfolds protein molecules [17] and enhances the physical

entrapment of water. This phenomenon was reflected in

water absorption of protein isolates, showing an increasing

trend with the increasing SDBS concentration.

The differences in structure and polarity of SDS and

SDBS molecules could have caused the differences in

protein water absorption properties. At low concentrations,

both SDS and SDBS exist only in the form of monomers

[24]. Therefore, similar water absorption was observed in

both SDS and SDBS treated isolates at low concentrations.

However, at high concentrations, SDS and SDBS have

different micellar formations because of their different

structures [24]. When coming into contact with water, the

benzene ring in SDBS allows the formation of p–p bonds

leading to a negative charge delocalization, which is not

seen in SDS molecules. The binding-induced changes in

the protein–hydrogen equilibrium can result in the different

water absorption properties of SDS and SDBS treated

protein isolates [17, 18]. The water-absorption property

differences may be a useful factor in deciding which type

of denaturant to use for a given application. Generally,

higher protein water absorption is desired in food appli-

cations of protein isolates.

At the highest SDS and SDBS concentrations, the fat

absorption of protein isolates increased by 86 and 78%,

respectively (Fig. 2). This demonstrates the increasing

0

50

100

150

200

250

300

350

400

450

500

1% 3% 5% Standard Isolate

Wat

er a

bsor

ptio

n of

isol

ates

(%

)

SDS SDBS

Fig. 1 Water absorption of canola protein isolates with and without

modification using SDS and SDBS. Error bars represent sample

standard deviation

0

20

40

60

80

100

120

140

160

180

200


Fat a

bsor

ptio

n (%

)

SDS SDBS

Fig. 2 Fat absorption of canola protein isolates with and without

modification using SDS and SDBS. Error bars represent sample

standard deviation

544 J Am Oil Chem Soc (2012) 89:541–549

123

denaturation effect of SDS and SDBS with the increasing

concentrations. Unfolding of protein molecules due to

denaturation helps to increase physical entrapment of oil

[25, 26]. Also the capacity of oil molecules to bind with

nonpolar side groups of proteins is enhanced by the

denaturation of proteins [27]. All the SDBS treatments and

3 and 5% SDS treatments showed significantly higher fat

absorption compared to that of standard isolates. SDS-

treated proteins absorbed more fat at higher treatment

concentrations compared to SDBS-treated protein isolates.

The size of the SDBS molecule (C18H29NaO3S) is bigger

than that of the SDS molecule (C12H25NaO4S); hence the

number of SDS molecules present is 21% higher at a given

mass concentration [17]. This enhances the efficiency of

denaturation by unfolding a higher number of protein

molecules and enabling increased physical entrapment of

oil molecules (fat absorption) in SDS-treated proteins

compared to SDBS-treated isolates (Fig. 2). Similar to

water absorption, the higher fat absorption is generally

desired in food applications of proteins. The results show

that both SDS and SDBS treatments with higher concen-

trations are capable of enhancing the fat absorption of

protein isolates.

The emulsifying activity of the modified proteins and

the standard protein isolates are shown in Fig. 3. The

denaturation of proteins resulting from all the SDS and

SDBS treatments significantly decreased the emulsifying

activity of protein isolates. Emulsifying properties are

mostly determined by the surface charge and surface

hydrophobicity of proteins. The increased unfolding leads

to an increase in viscosity and intermolecular interactions

that may hinder the repulsive effects of increased anionic

charge [28]. Other studies have shown that denaturation by

detergents is not as effective as thermal denaturation in

changing surface related properties such as emulsification

and foaming [14].

The variations of the glass transition temperatures of the

protein isolates with various treatments are shown in

Table 1. All the treatments decreased the glass transition

temperature by several degrees compared to the unmodi-

fied protein isolates. This can be attributed to denaturation

effect as well as the attachment of side groups in to the

protein chains, enabling polymer structure to possess more

free space and flexibility in molecular movement. Bulky

groups in SDBS were more effective than that of SDS in

this regard, which could be seen by the lower glass tran-

sition temperatures of SDBS-modified proteins.

Properties of Plastics

Denaturation of protein isolates by SDS and SDBS treat-

ments increased the tensile strength of the plastics prepared

using these modified proteins by up to 14 and 41%,

respectively, compared to that of plastics prepared with

standard protein isolates (Fig. 4). However, there was no

significant concentration-dependent response with respect

to tensile strength.

The increase in tensile strength could be attributed to the

increase of surface hydrophobicity resulting from dena-

turation [14]. Denaturation exposes the hydrophobic resi-

dues buried in the core of protein molecules at the

molecular surface. These hydrophobic groups can

0

10

20

30

40

50

60

70

80

90

100


Em

ulsi

fyin

g ac

tivity

(%

)

SDS SDBS

Fig. 3 Emulsifying activity of canola protein isolates with and

without modification using SDS and SDBS. Error bars represent

sample standard deviation

Table 1 Glass transition temperature of canola protein isolates with

and without modification using sodium dodecyl sulfate (SDS) and

sodium dodecyl benzene sulfonate (SDBS)

Modification Concentration (%) Glass transition

temperature (�C)

Standard – 56

SDS 1 53

SDS 3 54

SDS 5 54

SDBS 1 50

SDBS 3 51

SDBS 5 48

0

2

4

6

8

10

12

14

16


Tens

ile s

tren

gth

(MPa

)

SDS SDBS

Fig. 4 Tensile strength of plastics prepared from canola protein

isolates with and without modification using SDS and SDBS. Errorbars represent sample standard deviation

J Am Oil Chem Soc (2012) 89:541–549 545

123

participate in more hydrophobic interactions between pro-

tein molecules as well as with synthetic polymers during

compounding extrusion and injection molding processes.

The plastic specimens that used SDBS-modified pro-

teins showed a higher tensile strength than those that used

SDS-modified proteins. This could be attributed to the fact

that SDBS possesses a more hydrophobic side chain

compared to SDS resulting in increased hydrophobicity and

more hydrophobic interactions within the polymer system

[17]. In our previous studies, significant correlations

between tensile strength, toughness, and water resistance

were observed in protein-based plastics [29].

Similar to tensile strength, all modification treatments

increased the tensile modulus of plastics (Fig. 5) but there

was not a strong concentration-dependent response. The

samples freeze-fractured and analyzed by SEM revealed

that plastics prepared with modified protein isolates

showed more brittle low-temperature failure compared to

plastics prepared with unmodified protein isolates (Fig. 6).

Generally the structures of SDS and SDBS-treated protein

plastics did not vary discernibly with the concentration.

Below the glass transition temperature, plastics usually

0

50

100

150

200

250

300

350


Tens

ile m

odul

us (

MPa

)

SDS SDBS

Fig. 5 Tensile modulus of plastics prepared from canola protein


Fig. 6 Freeze-fractured surfaces of plastics prepared from canola protein isolates with and without modification using SDS and SDBS: a 1%

SDS, b 3% SDS, c 5% SDS, d 1% SDBS, e 3% SDBS, f 5% SDBS and g unmodified canola protein isolates

546 J Am Oil Chem Soc (2012) 89:541–549

123

show a brittle type of failure [30]. Figure 6g shows white

spikes on the fracture surface indicating that crazing pre-

ceded actual fracture failure in the unmodified protein

plastic specimens. During the tensile tests, more crazing

was also visible throughout the cross sections of the plastic

specimens prepared with standard protein isolates com-

pared to the specimens prepared with modified protein

isolates. These macro-scale features in failure showed that

failure mode of modified protein-based plastics were more

brittle than that of the standard protein-based plastics.

Other reports showed that compression-molded soy

protein-based plastics (prepared without additional plasti-

cizers such as glycerol) showed decreasing tensile modulus

with increasing SDS concentrations [13, 18]. This was

attributed to the plasticizing effect of the SDS molecules,

which improved ductility of the modified plastics. Also, the

permanently attached plasticizers incorporated by chemical

modification methods extend the shelf life of protein-based

plastics [9]. However, these soy protein-based plastics had

higher modulus values (1.25 GPa) than those for canola

protein-based plastics shown here (300 MPa). Low mod-

ulus values in these plastic specimens can be attributed to

the presence of external plasticizers (glycerol).

The elongation of plastic specimens prepared using

modified and standard protein isolates are shown in Fig. 7.

Only the 3%-SDS and 3%-SDBS treatments showed sig-

nificant increases in elongation (22 and 11%, respectively)

over the plastics prepared from standard isolates. Similar to

other mechanical properties, elongation did not show any

concentration-dependent response.

The toughness of the protein-based plastics increased

from all SDS and SDBS treatments (except 5%-SDS)

compared to the standard isolates (Fig. 8). This indicates

that insertion of SDS and SDBS molecules between protein

chains increased free space in the polymer system and

increased the ability to absorb more energy before failure

[31]. The SDBS treatments showed no significant

concentration-dependent response while the 3%-SDS

treatment showed the highest increase in toughness among

SDS treatments. Other reports, however, showed that

toughness and other properties such as modulus and tensile

strength generally increased with the increasing concen-

tration of SDS with soy protein plastics [13, 18].

Water absorption of plastics prepared using standard and

denatured protein isolates is shown in Fig. 9. Water

absorption of plastics increased with the increasing con-

centrations of SDS and SDBS, but SDBS-treated plastics

showed a much lower response than SDS-treated plastics.

Soy protein-based plastics have also shown decreased

water resistance with increasing SDS concentrations [13,

18]. The net charge of protein-based plastics is increased

due to the interaction of hydrophobic protein side chains

with the hydrophobic moieties of SDS; this may contribute

to the increased water absorption seen with the increasing

SDS and SDBS concentrations [15, 18, 28].

The impact of SDS and SDBS modification on the water

absorption of protein isolates did not correlate well with the

water absorption of plastics prepared with those isolates.

0

5

10

15

20

25


Elo

ngat

ion

(%)

SDS SDBS

Fig. 7 Elongation of plastics prepared from canola protein isolates

with and without modification using SDS and SDBS. Error barsrepresent sample standard deviation

0

1

2

3

4

5

6


Toug

hnes

s (J

)

SDS SDBS

Fig. 8 Toughness of plastics prepared from canola protein isolates

with and without modification using SDS and SDBS. Error barsrepresent sample standard deviation

0

1

2

3

4

5

6

7

8

9

10


Plas

tic w

ater

abs

orpt

ion

(%)

SDS SDBS

Fig. 9 Water absorption of plastics prepared from canola protein


J Am Oil Chem Soc (2012) 89:541–549 547

123

Similar lack of correlation in these properties was observed

in our previous studies [29]. It was evident that different

governing factors are involved in water absorption of

protein isolates and plastics. Water absorption of protein

isolates is influenced by changes to the secondary, tertiary,

and quaternary structures of proteins and takes place pri-

marily as physical entrapment of water molecules between

polymer chains. However, plastic water absorption is

governed by various factors including the extent of poly-

mer interactions, porosity and density, homogeneity,

intermolecular space, cross-link density, and hydropho-

bicity of the polymer matrix [32, 33]. These factors can be

controlled by pretreating proteins (e.g. denaturation, acyl-

ation), using various additives (e.g. plasticizers, compati-

bilizers), and adjusting parameters for production processes

(e.g. extrusion, injection molding) [7, 8, 16, 33, 34]. Lower

water absorption is desired in most applications of protein-

based plastics. Therefore, using lower concentrations of

both SDS and SDBS treatments would be beneficial for the

applications of canola protein-based plastics.

Conclusions

The functional properties of protein isolates were affected

by both SDS and SDBS modification. SDBS was gener-

ally more effective than SDS in denaturing protein mol-

ecules as was reflected in several properties of protein

isolates as well as in plastics. Water absorption of isolates

was enhanced by SDBS treatments with higher concen-

trations (C3%), while SDS treatments reduced water

absorption. Both SDS and SDBS treatments increased the

fat absorption of isolates. These treatments could be

beneficial for food applications that require changes to fat

or water absorption properties. Emulsifying activity of

proteins was suppressed by both SDS and SDBS treat-

ments indicating that these treatments are detrimental for

applications that require functionality such as emulsifica-

tion and foaming.

Denaturation of proteins increased the tensile strength

and toughness of plastics; these are beneficial properties for

the structural applications of protein-based plastics. How-

ever, there was no concentration-dependent response for

either of these properties. Plastic elongation showed min-

imal impact from treatments with either SDS or SDBS;

only the 3% treatments of SDS and SDBS increased the

elongation of protein-based plastics. All the treatments

increased modulus indicating a reduction in the ductility of

the plastics. Reduced ductility is often a disadvantage in

overmolding applications, but higher modulus is essential

in many other engineering thermoplastic applications. Both

SDS and SDBS treatments increased water absorption with

increasing concentration. Water sensitivity is detrimental

for outdoor applications of canola protein-based plastics;

hence moderate treatments of SDS and SDBS are recom-

mended for achieving better mechanical and water resis-

tance properties of canola protein-based plastics.

Acknowledgments We acknowledge the technical support given by

Michael A. Fuqua, Jaidev Sehrawat, Dr. Zisheng Liu, Gloria Nygard,

and Marsha Kapphahn. Also we thank Mr. Tom Borgen (Langdon,

ND) for providing canola seed. This research is supported by the

USDA/CSREES NRI program through grant 2008-35504-18667.

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