Journal of Scienti fic & Indus tri al Research Vol. 60, Jul y 200 I, pp 547-559

Hydrocolloid-based Packaging Films- Alternate to Synthetic Plastics Rudrapatnam N Tharanathan * and N Saroja

Department of Biochemistry & Nutrition , Centra l Food Tech nological Research Institute, Mysore 570 013. India

An alternate approach to non-bi odegradable synthetic plastics is packaging fi lms based on hydroco lloids and their deri va­tives. They are avai lable ~n plenty from highly renewable natural resources and thei r total biodegradability makes them ecofriendly. Blendmg as well as graft copolymeri zation of syntheti c monomers onto natural polymers a lTer addit ional means of preparin !! biodegradab le packaging fi lms. •

Introduction

Polysaccharides may be regarded as condensati on polymers of monosaccharides resulting in the forma­ti on of g lycos idic linkages by e limination of water. As components of a lmost all living organ isms, polysaccha­rides, also called hydrocoll oid s, are mos t abundant in the higher order of land plants and in seaweeds where they constitute approximately three-quarte rs of the dry weight. They perform diverse roles in the physiQlogy of plants, animals, and microorgani sms .1 Firstly, they serve as structural material s and as fuel reserves in plants. As sUiface materials, they parti a lly protec t ti s­sues from desiccation and as gums they are exuded from plants to seal and protect wounds. As thickeners, they serve a physical or mechanica l role in an imals and as specific substances, they are of importance in bl ood group specificity and in other immunological reacti ons.

Polysaccharides containing only one kind of po ly­merized sugar unit (homoglycans) are more abundant than those which contain two or more kinds of sugar units (he teroglycans). Structural polysaccharides, of which cellulose is the prime example, are a lmost a l­ways linear molecules, while those which serve prima­

rily as reserve foods are commonly branched or, in the case of starch , a mixture of linear and branched polysac­charides with the latte r predominatin g . In genera l, branched polysaccharides are eas ily soluble in water and have immense thickening properties. Linear molecul es, on the other hand, pack closely and form many inte r­molecular secondary valence attachments which make

*Author for correspondence

the structure strong, rig id and in so luble or at least diffi­cu ltl y soluble.

Starch , a polysaccharide o f repeating g lucose uni ts. is a mixture of two polymers , amylose and amylopectin (Figure I). Amylose is a predomin antly linear (to li ghtl y branched) po lymer, comprised of ( I - >4) aD-lin kages with number average molecul ar weight in the range of seve ral hundred thou sand s . Amylopectin is hi g hl y branched , with intermittent ( 1- >6) links; its mo lecul ar weight is of the order of several milli ons, and can be as great as 50 milli ons. The stmctural diffe rence in the two constituents has a considerabl e effect on the properti es

QH QIIO QHO

OH OH -o o o o-OII OH OH (a)

Qll QHO ~IIO OH OH -o o o OH OH H

QH Q4H OH O OH O -o o o o-OH OH OH

(b)

Figure !-Structural representati on of amy lose (a) and amy lopectin (b)

548 J SCI IND RES VOL 60 JULY 200 1

(a)

Eo\!'~ Eo\!' \~~~ OCH1 OCH1

(c)

OH

,E~~~Eo~ ~~~

OCH1 OCH,THCH1

OH

(b)

?H

'~ E~~~c:·E 0"' ~~~~

OCH><fHCH1 OCH1'fHCH1

OH ClH

(d)

0H,COON~aH 0

\ ~ 0 OOH 0\

lx;H,CUONa CH,COONa

Figure 2-Structure of cellulose, and (a) methylcellulose, (b) hydroxypropyl cellulose, (c) hydroxypropylmet hylcellulose, and (d) Na• salt of carboxymethyl cel lulose

of starch material s. Because of its branched structure, amy lopectin generally has inferior mechani cal proper­ties compared to amylose.

Starch in its unmodified form has limited usage in the food industry as it does not have functional properties needed by modern food processing indu stries 1• Aque­ous solutions o f amylose can be converted into transpar­ent films. But the films deve loped, though described to have mechanical properties similar to those of plastic films, have high permeability to oxygen and water at 25 °C and relative humidity between 25-83 per cent2

. Modi­fication of native starch by disruption of hydrogen bond­ing through chemical substitution results in a wide range of functional prope11ies. Hydroxypropy lated amylomaize sta rch has been used for preparing water so luble tran s­parent films '. The hydroxypropylation reduces the dry tensile strength of amylomaize starch film , but increases the bursting strength and elongation considerab ly. In­corporation of other ingredients such as waxes and plas-

ti cizers improves the mechani cal as we ll as barrie r prop­erties of these film s' .

Cellulose , a linear polyme r of ~-D·-g lucopy ran ose (Figure 2), is the most abundant of a ll naturally occur­rin g organic substances. It is the main constituent of the cell walls of land plants. Etherifi cation o f cellulose pro­vides a broad spectrum of products that include low­substituted a lkyl ethers, which are inso luble in water and organic so lvents. Alkyl ethers of intermediate substitu­tion are water soluble , for exam pl e hydroxy­propylcellulose, whereas hi ghl y sub ti tuted ethers such as ethyl and methy lcellul oses are solu ble in organic so l­vents . Methylcellulose gums, e ither a lone or in combi­nati on with starch or modified starch, are empl oyed as wall paper adhes ives4

. They are more res istant to micro­bi al growth than are the nati ve starc hes, and are often used as heavy-duty wall paper adhesives with papers that have low water permeability, such as viny l-coated pa­persr'. They can be used as adhesives for bondin g of leather and textiles, and to thicken a varie ty of adhe-

THARANATHAN & SAROJA: HYDROCOLLOID-BASED FILMS 549

R = -H

NH I R

R = -COCH3

NH I R

Chitos.'ln

Chitin

fi R

Carboxymethylchitosan

Figure 3-Structure of chitin , chitosan and carboxymethyl chitosan

sives based on aqueous polymer emulsion of pol y( vinyl

acetate) and acrylate ester copolymers. Excellent bind­

ing properties of methylcelluloses allow the ir use as a

granul ating agent for fe rtili zer compositions and as a

medium to bind fertili zer to seeds7. The use of methyl ­

ce llulose, particul a rl y th e hydroxya lk y l-modifi ed

methylce lluose gums as thickeners for latex pa ints has been a major application 4

Carboxymethylcellulose (CMC) is useful in applica­

tions in which hydrophilic colloids are indicated. It has

received considerable attenti on in the tex til e industry

because of its ready solubility and excellent film-form­

ing characteristics4. It serves as an ex trusion a id , acts as

a binder, helps to stabilize emul sion, and retards sugar

c rystal growth . Because of its uniform quality, CMC, which is insoluble in stomach acid but soluble in a lka­

line intestinal fluids, is a good enteric coating for pow­ders and tablets. It is a mild but effective bulk laxative .

It is used as a stabilizer for hand loti ons and vitamin-oil emulsions and is widely used as a binder in cosmetic

products. Chitin , a poly-~-( 14) linked N-acetyl-0-glucosamine

(Figure 3), is a biopolymer widely di stributed in nature . It is a polysaccharide o f considerable inte rest because of its abundance and unusual combinati on of properti es , which include toughness, biodegradability, and re lative inertness, all of which contribute to making chitin an

attractive specia lity materia l.5 However, because c hitin is insoluble in water and most ordinary so lvents, it can­

not readily be fabricated into useful mate rial s such as fibres or membranes, which has limited its use in many fields. The hydrophilic ity of chitin can be increased by

deacetylation of its N-acetyl glucosamine units in strong

Table !-Plasti c materia ls for packagin g

Plasti c materi als X 10'' l OllS

Total production of packaging materi als 10.0 Total plastics consumption 6.6 Pl as tic packaging (40 per cent food ,

60 per cent non-food) I .4 Polyolefins * 70 per cent Polystyrene* 15 per cen t PVC* I 0 per cent Household garbage 14.0* Plastic packaging materials in

household garbage. 0.7

*Disposal by incineration 33 per cent , and land fill 66 per cent

a lka li so lutions. Chitosan, a deri vat ive o f c hitin that has

been maximall y deacetylated, is readily soluble in d i­

lute acidic solutions and is eas ily fabricated into ge ls and films.

Plastic Packaging

Pl astic mate ri a ls have become an integral part of our

life because of the ir many des irable properties inc lud­ing durability and res istance to degradation (Tab le I )

These non-degradable plasti cs accumulate in the env i­ronment at a rate of about 25 milli on tons pe r yea r.r' 7

Plastics , the syntheti c po lymers that are large ly resis­tant to microbi a l degradati on and deterioration , are ac­cumulating in the environment in huge amounts. The ir persistence e ithe r as litte r or through landfill disposal of municipal solid wastes has prompted a rethinking on the ir

continuous use. The incorporat ion of natural polymers into pl astics not only helps in the ir di s integrati on but

also reduces the dependence on the dwindling and ex­

pensive petroleum-derived monomers . Graft copolymer­iza ti on of synthe ti c monomers such as ac ry loni tri le, a precursor of acrylic fibres and pl asti cs, onto sta rc h pro­vides an excelle nt method for preparing starch-graft co­

polymers, which on ex trusion form films useful for pack­aging applications . Starch is inex pensive, totall y bi ode­gradable and ava il ab le in large quantities.

Plastics in the household wastes (5 pe r cent) adds to an inc reas ing garbage mountain which is recognized as an ecological threat. Space for landfill s is limited and additional inc ineration capac ities require high cap ital in­vestments. Further, incineration yie lds C0

2 which adds

550 J SCI INO RES VOL 60 JULY 200 I

to the problem of green house effect and the liberated gases (NO, S02) and tox ic degradati on products like polycyclic hydrocarbons (PHCs), pose addi tional env i­ronmental problems. In recent years, there is a global awareness about the need to reduce the amount of pl as­tic was tes discarded in land fill s. Although improved ef­forts to recyc le di scarded pl astics woul d help accom­pli sh thi s goal, recyc ling would be neither prac tica l nor economical for certain end- use appli cati ons such as ag­ricul tural mulch film s, planting pots and ga rbage bags. For such appli cations pl astics are needed that will frau-

"' ment o r deg rad e into be ni gn by-products und er composting conditions. The different strategies empl oyed for plastic was te management are shown below:

I. Prevention a) Improved producti on processes b) Reuse of packag ing materials, e.g. bott les

2. Recycling (separati on, selec tion, cleaning, repro­cessing) a) Pure polymers b) Polymer mi xtures

3. Thermi c utili zation a) Incineration (heat of combusti on)

4. Disposal in landfill s/composting 5. Chemi cal utili zation

a) Hydrocracking (hydrocarbons) b) Hydrolys is (ac ids, amines, polyo lefin s) c) Pyrolys is (aromatic hydrocarbons)

6. Degradable polymers a) Chemica l modi ficati on of class ical polymers b) Starch containing polymers (polyethylene,

polyethylene/ polyacrylate copolymers, polyv inylalcohol)

c) Thermoplas tic starches d) Biopolymers (polyhydroxybutyrate,

polylactic ac id)

Alternate Packaging Materials

Biodegradable polymers offer an interes ting alterna­tive to class ical non-degradable po lymeri c films, espe­ciall y for short-term usage such as agricultural mulches, beverage and fas t food packages, refuse and retail bags. After disposal in land fill s (compost), these materia ls are degraded by photodegradati on, chemodegradati on and biodegradati on or a combination of the three into envi­ron mentall y harmless substances or low molec ul ar we ight products which can be further metabo li zed by microorganisms. Biodegradable packaging materi Is can

[0 - J'-(CH2)n - C --]

I II R 0

- 100-30,000

Figure 4--Gencral st ru ctu re or polyhydroxyalkano ~tl C>

be broadl y classi fi ed into (a) Polyesters (biopolymers) : (b) Edibl e coatings and films; (c ) Starch as fill ers/ com­pos ites, and (d) Starch-graft-copo lymers.

a. Polyesters (bac terial polyhvdroxvalkonoates)

Po lyhydroxya lk anoa tes (PHA ) are po lyeste rs (Figure 4) synthes ized by numerous bac teri a as in trace l­lul ar carbon and energy storage compounds and acc u­mul ated as granul es in the cytop lasm of ce ll s.x Numer­ous bacteri a sy nthes ize PHAs as a sink for redunda nt reduci ng power under the conditi on of limitin a nu tri -

"' ents in the presence of excess carbon source. When the supply of limiting nutrient is restored, the PHA can be deg raded by intracellul ar depoly merases and subse­quently metabolized as carbon and energy source. The molecul ar weights of these polymers are in the range of 2 X I 0·' to 3 X I or· Da, depending on the microorgani sm and growth conditions.x

To date, most of th e stud ies on the ph ys ica l and therma l properties of bacterial PH As have been carried out with po ly(3-h ydrox ybu tyrate) and po I y(3-h yd rox ybu tyrate­co-3- hyd roxyvalerate). The P(3 HB) is I 00 per cent ste­reospec ific with all the asy mmetric carbon atoms in the D(-) configurati on. It is therefore h igh l. y crystal! in e. The degree of crystallinity ranges from 55 to 80 per cent. The glass transition temperature and the melting po int of P(3HB ) are approx imately 5 "C and 175 "C, respec­ti ve ly.'' The family of PHAs ex hibi t.- a wide variety of mechani cal properties, from hard crysta lline to e last ic, depending on the composition of monomer uni ts. To date, poly(3 HB-co-3HV) is the only copolymer that has be~ n

produced in large quantities .

Biodegradation o.f PHA

The most attrac tive fea ture of PH As is their co 111 plete biodegradab ili ty. A num ber of aerobic and anaerob ic PH A-degrad ing bacteri a and fun gi lr ve been iso lated fro m various environments which inc lude Acido\ •omx

foci/is, AspeJg illusfwn igatus from soil and A lcoligenes

THARAN ATH AN & SAROJA: HYDROCOLLOID-BASED FILMS 551

fa ecalis and Pseudomonas jluorescens from activated sludge. These microorgani sms excrete extracellular PH A depolymerases to degrade PHAs into water so luble monomers and oligomers, and later use them as a car­bon source. The PHA depolymerases have a hydropho­bic domain binding site to ad here to the surface and a catalytic domain containing the lipase specific sequence, Gl y-X

1-Ser-X

2-Gly 10

• The rate of biodegradati on is in­flu enced by a number of factors including the microbial population in a given environment, temperature and prop­erties of the plastic material to be degraded. It is shown that P(3HB-co-3HV) is completely degraded after 6, 75 and 350 weeks in anaerobic sewage, soil and sea water, respectively. Due to its complete biodegradation, PHAs fit perfectly well in the ecosystem. The PHAs have been drawing considerable industrial interest as candidates for biodegradable and/or biocompatible plastics for a wide range of applicati ons such as packaging film s; bags and containers; biodegradab le carriers for long-term dosage of drugs, medicines, insecti cides, herbicides; di sposable items such as razors, utensils; starting materi als for chiral compounds; surgica l pens, sutures, stap les and swabs, and wound dress ings. Many companies world- wide are developing products from PHAs.

However, there are two drawbacks to the commercial use ofP(3HB). Firstly, P(3 HB) has a po01· melt stability, it decomposes at approximately 200 "C, wh ich is close to its melting temperature. Secondl y, P(3 HB ) becomes brittle over a peri od of several days upon storage under ambient condition s.

h. Edible Coatings and Films

I. From Proteins

The film-forming ability of several proteinaceous sub­stances has been utilized in many industri al applications. A number of proteins, both of plant and animal ori gin , have received attenti on fo r production of films and coat­ings. These proteins are corn zein , wheat gluten, soya protein, peanut protein, keratin , co llagen, gelatin , casein , and milk whey protein s. The use of protein-based coat­ings on fresh produce of hi gh moisture content has been restricted due to its limited water vapour resistance. Corn ze in coat ings on tomatoes showed 11 shelf-life ex tension by six days, as ev idenced by sensory eva luation. Casein and acety lated monog lyceride emulsion coati ngs have been used in contro lling moisture loss and ox idative browning in cut , peeled app le pieces 12

• Corn ze in-based edible coatings extended the she! f-li I e of nuts by retard-

Table 2- Tensi lc strength and elongati on at break of protein-based edible films

Fil m Tensile Elongat ion at st rength break

(MPa) (per cent )

Wheat gluten-lacti c acid ( I : I ) 0.0 1 75

Wheat gluten-lacti c ac id ( I : I ) 0.02 63 Wheat gluten-soya

protein-glycerin ( 1.75:0.75: I ) 4.4 233

Corn zein-glycerin (2.9: I ) 3.'J

Soya protein-glycerin ( 1.7: 1) 4.3 78

ing rancidity, stal ing and sogg iness and prolonged the shelf-life of coated chocolate cu bes, donuts and fi g bars 13

.

Glutenins are the high molecul ar weight protein s in the wheat gluten complex that are primaril y responsible for dough viscoelasticity. It is believed that glu tenins result from cross-linking of pol ypeptide subunits. Wheat gluten films can be produced by deposition and subse­quent drying of wheat gluten di spersions in aqueous etha­nol. Alkaline or ac idic conditi ons are required for the fo rm ati on of homogeneous film-forming so luti ons. 14

Upon casting, di sulphide bridges are reformed , lin king together polypept ide chain s, to yield a firm structure. Re-oxidation in air and sulphh ydryl-di su lphide inter­change reactions are the mec hani sms th at contribute to reformation of disulphide bonds.1

"

The brittl eness of these films results from ex tensive intermolecular assoc iations. The plastici zer additi on to wheat gluten so lution brings about modifi ca ti on in fi I m fl ex ibility (Tab le 2). Plastici zer molecules mediate be­tween polypeptide chains, disrupting some of these as­sociations and decreas ing the rigidity of the film struc­ture. The advantages of protein film s are excellent oxy­gen and carbon dioxide barrier at low relati ve humidi­ties, whereas their res istance to water vapour transmis­sion is rather limited. Their good gas barrier propert ies are drastica ll y reduced in hi gh humidity environments because protein s are susceptible to moisture absorption and swelling. Improvement in res istance to water vapour remains one of the main challenges fo r protein fi lm s. Several chemica l and physica l treatments (i.e. , tanni ng treatment with aldehyde, treatment at the protein iso­electric point) show some effec ti veness in promotin g cross- I inking and hardenin g of the protein structure and also improving film barri er and mechani ca l properti es

The di sadvantage of edible protein-based films and coati ngs is the potential a ll ergenic responses to spec ific

552 J SCII ND RES YOL 60 JULY 200 1

protein sources by some indi viduals, as a llergy to food prote in s is linked to several di seases. 16 Inc idence of g lu­ten intolerance is known whi ch is c haracte ri zed by nu­

trient malabsorpti on as a consequence of g luten-d pen­

dent damage to the mucosa of the small intes tine .17

2. From Lipids and Resins

L ipid-based coatings are ma inl y used to preve nt

weight loss, slow down ae robic resp irati on and to im­prove appearance by prov iding g lossy characteri sti cs.

They are a lso useful in reducing sUiface abras ion during handling of fruits and vegetables and as ca rri e rs of fun ­

g ic ides, anti oxidants, antimi crobi a ls and orowth reou la-o b

tors. T he edible lipid-based coatings inc lude mainl y neu-

tral lipids of g lycerides and waxes which are este rs of

long chain monohydric a lcohols and fatty ac ids. Waxes

such as carnauba, beeswax, paraffin , ri ce bran wax and

cande lill a a re reported to be used in combin ati on with

res in s or polysaccharides to coat fresh fruit s and v g­etables.1 x Beeswax and vegetable o i I have shown ex ten­

sion of she lf-life of stored ra isins. 1'J

The disadvantage o f lipid and res in-based coati ngs is

that they show poor fl ex ibility and high degree of cohe­siveness. Thi s is minimized by the addition o f pl asti c iz­

ers suc h as monoglycerides, phospho lipid s and es te r

de rivati ves of g lycerol? 1

3. From Polysaccharides

The deve lo pment o f coatin gs fro m wa te r so lu b le

polysaccharides has brought a surge of new types of coat­ings for extending the she I f-1 i fe of fruits and vegetabl es,

because o f se lecti ve permeability of these polymers to C0

2 and 0

2. Polysaccharide-based coatings are thu s uti­

li zed to modify the atmosphere, the reby reduc ing fruit

and vegetable respirati on. They are of significant im­portance to the food industry because they are abundan tly

available, usua lly are of low cost, and are non-tox ic.

Wate r so lubl e po lysaccharides are long chain po ly­

mers that dissolve or d isperse in water to give a th icken­ing or viscos ity building effec t. Cellul ose is in solubl e in water clue to the high level of int ramolecul ar hyd rogen

bonding in the po lymer, whereas ethers ofcellulos such as carboxymethy lce llul ose (CMC), hyd roxyp ropy l-cel­lul ose (HPC ) a nd hydroxy pro py lme th y lc e llul ose (HPMC) are water soluble and are good film formers. T hey are capabl e of y ie lding tough-fl ex ible and trans­

parent film s ow ing to linear struc ture of the polymer backbone . The films are so luble in wate r and re:istant

to fa ts and o il s. M ethylce llul ose be ing the least hydro-

philic of the cellul ose e thers, produces film s that have re lat ive ly hig h wate r vapour pe rmeabili ty.

The high qual ity film forming harac te ri sti cs of HPC

have been applied to retard mo isture abso rpti on and the development of ox idati ve rancid ity and spoil age in nut­megs, coated nuts and candies 21 B il ayer films, composed

ofHPMC and so lid lipid suc h as beeswax, paraffin , hy­drogenated palm o il , o r stea ri c ac id y ie lded wa te r vapour

pe rmeabilit ies that were lower than that of low dens ity pol yethylene.22 A bil ayer film cons isting of steari c-pa lm­itic ac ids and HPMC showed mois ture transfe r from hioh

. b

mOis ture food (to mato paste) to low moi ·tu re food . For-

mu lations co nsistin g o f M C, HPMC and HPC ( late r

named Nature Seal) res ulted in a de lay of ripening and

brown ing and inc rease in vo latile flavour components

of f resh commodities such as mangoes and banana .""

T he use of an aqueous slurry of an amy lose- ri ch starch

ethe r in ge latini zed fo rm as a protecti ve coating fo r foods

is known. A gluten-dextrin coati ng has been used to coat dry roasted peanuts pri or to app licat ion of sa lt. Low­methoxy l pectin ate as a coatin g age nt i. used for ce rt ai n foods as it g ives an att racti ve , non -sticky surface to foods?' C arrageenan which is a complex mi xture of sev­

era l polysacchari des is used as food coatings to enh ance the stability aga inst the growth of surface microorgan­isms.24 A carrageenan-based coati ng app lied on cut grape fru it halves resulted in less shri nkage and deteri orat ion

of taste after two weeks o f storage at room temperature. 24

Chitosan-based Films

C hitin on treatment with alka lies g ives chitosan, a het­

erogeneous substance in vari ous stages of deacetylati on

and depo lymeri zati on. C hitosa n appli catio ns inc lude coatings , fl occu lat ing agents, and ingred ients fo r foods

and feeds. C hitosan can form a sem ipermeable coati ng which can modi fy the interna l at mosphere, the reby de­lay ing ri pening and decreas ing transpirat ion in frui ts and vegetables .' It can inhib it the growth o f fun gi and phy­

topathogens 25

A meth od fo r preparing chi tosan de ri vati ves with a wide variety of ag ricul tura l and indu str ia l applications has been deve lo pe d . N , 0-Cttrb oxy m e th y lchitos an (NOCC) is wa ter so luble, bi odegradable and fo rms se­

lec ti ve ly pe rmeab le no n-tox ic f ilms. Nutri save , a n NOCC-based form ul ation was re ported to have some success as a post-harvest edibl e coat ing fo r fresh fru its ."6

THARANATHAN & SAROJA : HYDROCOLLOID-BASED FILMS

(c) Starch as Filler/Composites

Physical incorporation (as blends) of starch as a bio­

degradable e lement in classical pol ymers has beens re­ported. fi Starch (6-20 per cent) is incorporated, e.g. into

polyethylene matrix without any chemica l interaction. In these films starch degradation is caused by the attack of microbial enzymes and thu s the pl astic film becomes

porous and susceptible to furt her oxidative degradation. The amount of hydrophilic starch and hydrophobic poly­ethy lene determines the property and performance of these films. The starch/polyethylene blend film s in the

ra tio I : I s howed properties suc h as anti bloc kin g

behaviour, printability and water vapour pe rmeability but on the other hand, the ir mechanical properti es were con­

siderably reduced . In the case of composites with starch, the starch con­

tent is as high as 50 per cent by weight and it form s a continuous phase with the hydroph ob ic syntheti c poly­

mers. The latte r a re non-toxic and o f comparative ly low

molecu lar wei gh t (500-5000 Da). The monomers used are hydrophilic and are abl e to create strong ph ys ica l

inte racti ons by direc t che mi cal linkages with starch. Starch/polyvinyl alcohol composite films as biodegrad­

able agricultural mulches have been described. 11 The

water absorpti on behav iour of such compos ite films de­

pends very much on the concentration of additives and

process conditi ons. The ir mechanical properties are be­tween that of LOPE and HOPE, as far as e lasti c ity modu­lu s, shear modulus and e longation at break are concerned

(Table 3).fi

Burial tests, C02

development and oxygen consump­

ti on by microorgani sms showed that the bi odegradab il ­ity of these composite mate rial s li e in between non-de­

gradable polyethylene and fully degradab le paper.

d. Starch-gmft-copolymers

The incorporati on of sta rch into p lasti cs to enhance

the ir fragmentation and degradability in the environment

Table 3- Mechanical properti es of starch composite films

Elas ti city modulus

(kg/cm1)

Shear modulus

(kg/cm2)

Elongation at break

(per cent )

Starch composite

1000-8000

I 00-250

20-300

LOPE HOPE

1000-2800 4200-12000

50-160 220-380

90-1000 20- 130

Starch-OR ---• ... Starch-0 + CH2=CH(CN)

Starch-O-CH2 I

NC-CH-CH=CH-CN

Scheme I I

has generated conside rabl e inte rest. Starch is an inex­

pensive, tota lly bi odegradabl e mate rial and is ava il ab le in large quantities from certa in crops ( i.e . corn , wheat

and tubers). Rep lacement of petroleum-based plastics

with starch is a lso attractive from the standpo int o f con­serving our costl y pet rochemical resources. Two bas ic

approaches are be in g pursued in the direction of bi ode­gradable pl ast ics, viz.

I . Blends of starch with other polymers, especiall y

biodegradable ones, are compounded and fo rmed into films or injec tion mo lded into artic les , and

2. Starches grafted with the rmop lasti c c ha ins are sy n­

thesized and formed into films o r inj ection molded

items.

Grafting a syntheti c pol ymer to a natura l polysaccha­

ride is a way of creating large mo lecul es that have some of the characteristics of each indi vidual po lymer. Graft­ing is initiated by generating one or more free radi ca ls

on the polysaccharide chain and a ll ow ing them to react w ith polymeri zable monomers th at w ill const itute the grafted chain. Thus, if a polysaccharide is reacted, for exampl e, with hi gh energy rad iat ion, such as X-rays and

ultrav io let radiati on, free radicals may be c reated in vari­ous ways but principa ll y throug h hydrogen atom e li mi­nati on (Sche me I ). More ofte n, free radical generators such as ferrou s ion or ee ri e ammonium nitrate, a long with hydrogen peroxide, is used. After formati on, the free radical is ava il ab le to initi ate a free radi ca l poly­meri zation as illu strated by the following path way for reac ti on of starch w ith acrylonitrile.

554 J SCI IND RES VO L 60 JULY 200 1

Scheme 2

A lthough not a ll radicals produced on a po lysacc ha­ride cha in react w ith monomer to initi ate growth of a

synthetic po lymer cha in , po lymeri zation effi c ie ncy ca n

be fa irly high, often exceeding 50 per cent . Many diffe r­

ent monomers subjected to free rad ia l po lymeri zation have been tri ed successfull y, w ith those commonly used

be ing vinyl acetate, acry lamide and methyl methac ry late.

Ev idence for the formati on of free radi cals in graft co­polymerization reactions has been obta ined by e lectron

spin resonance spectroscopy (ESR). The react ion of eerie

io n in aqueous so luti ons with ce llul ose c leaves the

anhydrog lucose ring between c2 and c, with the fo rma­tion of short- li ved radical on C, and ox idati on ofC to a

- ·' red uc ing group (Sc heme 2). G rafting occurs by reaction

of the radica l at C2

with monome r. Radi ca l terminati on occurs by reaction of C

2 w ith Ce4+ to y ie ld Ce3+ and ox i­

dation of C, to a reduc ing group . The ro le of carbons C, and C

3 in the reacti on could be reversed Y -

Oxidati ve depolymeri zati on of cellul ose a lso occurs

and could lead to short-li ved intennediate homopolymeri­

zati on. In case whe re an inte rmedi ate radi cal is not formed , the reactions could occur to form the initi ating

mac rocellul os ic radicals, which were fo ll owed by ES R

spectroscopy. Graft copo lymeri sa ti on of methy l-meth­

ac rylate (M A) onto e ithe r granul ar or ge latini zed starch is reported2x using eerie ammonium nitrate as init iato r.

Homogeneous graft copo lymeri za ti on of methy l-meth­acry late onto ethylce llulose using radica l initi ators such

as ammonium persulphate, potass ium persulph ate and be nzoy l p e ro xid e w as c arri ed o ut in be n ze ne/

dimeth ylsulphox ide ( I: I , v/v) mi xed solven t system 2'1

Biodegradation of Starch-graft-copolymers

A mylo lyti c bacteria have been iso lated from a wide ranae of environments and studied fo r starch-pl astic bio-o degradation. Bacteri a have been se lected fo r thei r ab il-ity to utili ze starch in S-g-copo lymers as the so le carbon source in liquid culture media . In an effort to understand the mechani sms o f microbi a l degradati on o f starch-con-

raining pl asti cs, severa l s tarch-hydro lys ing bacteria l iso­

lates have been examined . One of these isolates , a con­sortium of bacteria des ignated as LD67 , degraded up to 80 per cent of sta rch in starch-po lyerhy le ne--ethy lene­co-acry lic ac id (S-PE-EAA) pl as ti c film (orig ina ll y 40

per cent starch by we ight) in 60 days in a liquid cultu re med ium, leaving behind the non-starch componen ts of

the film non-degraded .311 The loss of starch from the film

was accompanied by concomitant loss in weight and ten­s ile strengt h of the film , w hi ch may contribute to furt her

degradation of the film by mechanical forces. Labora­tory studies w ith hig hl y amylolyti c Arthrobacter sp ., revealed that in a liquid culture mediu m where sta rch

conta ining films were the so le carbon source, bacteria

readily metabo lized starch. Progressive removal o f starch fro m the S-PE-EAA fil m by the bacteri a over 56 clays

o f ex posure was de monstrated by e lect ron m ic roscopy. ·111

It was noted that bacte ri a binds s igni f icantly more in­

tensely to sta rch- g-po ly(methac ry late) pl as ti c f ilm than to S- l?E-EAA film , w hereas starch is more read il y hy­

dro lyzed in the latter. Thi s suggests that adhes ion of bac­te ria to th e film is no t a n ade qu a te indi ca ti o n of degradab ility of sta rch w ithin the film.

The Lactobacillus amylovorus bacte rium , iso lated from corn waste fe rmentation, secreted amylase that rap­

id ly degraded starch granules in starc h-graft f ilms . S tud­ies o n m ic ro b ia l surface int e rac ti o ns be twee n L. amy!ovo ru s a nd g ra nular s ta rc h in d ica te d th a t co loni sat ion by these bacteri a was important for deg ra­dati on .-11 At very low concentrations o f starch, onl y su r­

face sta rch will be access ible to direct at tack by mic ro­organ isms. The rate and ex tent of starch remova l from starch plasti c composites by severa l commerc ia ll y ava il ­ab le amylases were measured in cell -free systems. Amy­

lases deri ved from ani mals, pl ants and microbia l sources a ll hydro lysed starch rapidly. Pu lverised S-PE-EAA p las­tic from inject ion mo lded specimens was 40-60 per cent

hydro lysed by these enzy mes within severa l days. 31

Biodegradati on of S-g-PM A was studied us ing three fungal suspens ions such as Aspergillus niger, Penic·i/­liumfimiculosum and Tricodemw vi ride (Table 4) 3 ~ T he per cent deg rada tion/starch uti I izati on was determined by loss in tensile strength and scanni ng e lectron mi cros­copy. W ith starch as a readily avai lab le carbon source, exce ll ent growth and sporul ati on were observed in fi ve

days with A. niger, whereas good g rowth but less sporu­

lation were noted with P.jimiculoswn and T viride.

THARANATHAN & SAROJA: HYDROCOLLOID-BAS ED FILMS 555

DMBQ

0

DMHQ

OH

Mycelial reductase

0

Figure 5-Proposed pathway for extracellular Fe 1' redu cti on and Hp1 production by G.!rahet tllt

Biodegradation of the Synthetic Moiety of' Starch Composites

The ability of lignin-degrading microorganisms to at­tack starch-polyethylene was investigated in pure shake flask cultures. The known lignin-degrading bacteria Streptomyces viridosporus and fungus Phanerochaete chrysosporium were used by following reduction in per cent elongation and molecular weight distribution. It was found that ligninase of S. viridosporus and peroxidase of P. chrysosporium were involved in polyethylene bio­degradation.7 But it was noted that these microorgan­isms were unable to utilize starch, which was a compo­nent of the degradable plastic film . In another report , it was shown that a brown rot fungus Gloephyllum trabeum used an extracellular oxidative system to degrade a re­calcitrant polymer, polyethylene g lycol (PEG). An ex­tracellular metabolite , 2, 5-dimethoxy-1 , 4-benzo­quinone, produced by the fungus was reduced to 2, 5-dimethoxy-hydroxybenzoquinone along with reduction of Fe3+ to Fe2+ and with concomitant production of H

20

2

(Figure 5). These results provided ev ide nce that G. trabeum used a hydroquinone-driven Fenton reaction to cleave PEG. 33 Further invest igations led to the conclu-

s ion that G. trabeum also cleaved polyethylene oxide (PEO) rapidly by an endo route . 13C-NMR analysis of unlabelled and perdeuterated PEOs recovered from G.trabeum cultures showed that a major route for depo-

Table 4- Loss in weight and tensile strength or biodegraded S-g-PMA

Copolymer'' Inocu lum WeightTensi le

loss, per cent strength . Mpa

A P.funiculosum 37 .5 4.14

A A. niger 40.0 8.4 1

A T.viride 16.9 7.52

A None" 10.2 10.10

A Control (100.0) 20.44

8 P.funiculomiiJ 12.9 5.93

8 A. niger 16.0 5.]4

8 T.viride 12.4 5.65

8 None" 1.0 8.03 8 Control ( 100.0) I J.R2

a. Copolymer A contains 50 per cent starch , copo lymer 8 40 per cent starch. b. Incubated for 22 days at 25 "C without inoculum.

556 J SCI IND RES VOL 60 JULY 200 1

I

1---..o~o~o~o':/ ~ 1---..o~oy-----.o . ./'..___o':/ ~1---..o~oH "

PEO 1 OJH H a ~f~O"J ~ 0

t·[HJ

1---..o~o~~o':! 1---..o~or~~ ~ 1---.. c 0 a

1<0, kMo'' "" O·• '"'"" ~~0)'

IM",I1 A-..o~0ro~~ ~ A-..0~o'C'~o':!

Figure 6--Preclictcd pathway of PEO scission after hydrogen abstraction. M" indicates a transition meta l ion.

lymerization was ox idative C-C bond cleavage, a reac­tion for hydrogen abstraction from a PEO meth ylene group by a radical ox idant, which cleaves PEO rapidl y via ~-scission reaction. 34 In another report , the cleavage of labe lled polyethylene ox ide, [ '~C] PEO, was clemon­strated.3'i The MW distributi on of ['~C] PEO in C. trabeum cu ltures spread uniform ly to lower va lues as degradation progressed. It was establi shed that depo ly­merization followed an end o rather than an exo rou te, as there was no free '~C released from PEO. Two likely routes (Figure 6) deduced for endo PEO oxidation by C. trabeum were oxygen inserti on between ca rbon and hy­drogen in a methylene group and hydrogen abstracti on from a methylene and carbon. Identification of new end groups in degraded PEO showed that C. trahe11111 pro­duced a strong extracellul ar ox idant that leads to exten­sive PEO depolymerization by abstrac ting hydrogens from the intern al methylene groups.-'·' The role of ox i­dant required for this reacti on is played by H

20

2, which

is produced by extracellul ar oxidoreductase of the brown rot fungi. 3c'

Production of Fre e Radica ls hv th e A ct ion of

Ligninase with H20

1

Resting ligninase enzyme reacts with H10

1 to produce

the two-electron oxid ised intermed iate, compound I, which oxidises li gnin substrate (RH) to yield the one­e lectron ox idi zed intermediate, compound II and a sub­strate radi ca l. Compound II returns to restin g enzy me by ox idi si ng a second substrate molecule. The free radi­ca l (R) (Figu re 7) can undergo a vari ety or reacti ons

mainly C-C bond cleavage, hydroxylation, pheno l dimer­iza ti on and demethylation.:17 The chem istry of li gninase­catalyzed oxidation is very simi lar to on electron chemi­cal (Fenton) oxidation . The product profil es, stoichi om­et ri es and kinetics for many of the lignin-! ike substrates are consistent with cation radi cal chemistry.>x If the re­sults with the model compounds are ex trapolated to ot her aliphatic polymers, one can envision depolymerizati on occu rrin g throug h radica l-med iat ed c leavage of C-C bonds . The mechanism of lignin biod gradati on is thus viewed as a process directed more by substrate chemis­

try t an by enzyme chem istry.

Oxidative and Microbial Effects in the Degradation c~{ Polyetl!vlene

The bioconvers ion of 14C presen t in HDPE film to resp iratory '~C02 durin g aerated cu ltiva ti on with soi l or with a fun gus Fusariwn redo/ens showed a weight loss of about 0.16 per cent. On ge l permeation chromato­graphic ana lys is, it was found that the long polyo lefin chain s were degraded. It was also found that the autooxidati ve-progressive ag ing of the film was una vo id­able, which had a cumulative el'fect on biodegradat ion Y1

The enzymatic degradation of a long, strai ght olefin chain is dependent on the c leavag or C-C bond by an endoenzyme such as n-alkane C-C endohyd rolase. which does not seem to occur in nature. '" Consequentl y. the po lythenes can never stimulate the pmcluction of a long­chain sp litting degraclati ve enzy me by ' inducti on· . Such an inducti on is possi ble exc lusively in a specifi c case

THARANATHA & SAROJA: HYDROCOLLOID-BASED FILMS 557

Compound U ~ Cnmrnumll R RH

Figure 7-Production of free radicals by the acti on or li gni nase

when the genetic information [codon] is present in the cell s of an attacking organism. A recent report on the induc ti on of a kind of ox idi zing enzy me in Pseudomo­nas sp. by poly(v inyl a lcoho l) shows that the induced

enzyme resulted in the producti on ofH,O, , followed by the appearance of methyl ketones. 41

- -

In view of the current industria l and commercial in­

terests in the bas ic molecular mechani sms o f degrada­tion, it may be sa id that mo lecules above mol wt I 000 are inert to microbial utili zati on. The long chain po ly­mers are initi ally converted to short chain fragments by aging which is mere ly an autocatalytic pathway. The cyc les of both aging and biodegradation are complemen­tary or rather synergistic w ith each other.

Considerable work has been done in author 's lab over the past several years on the biodegradation o f starch-g­polyacry lonitrile (S-g-PAN), which can be used as a pack­aging mate ial, by Bacillus cereus . Ev idence has been provided for the bi odegradation of both starch and po ly­acrylonitrile chains by different enzymatic pathways 4~ The initi a l depolymerizati on o f PAN chains occurred by the ex trace llular peroxidase, whose acti vity was max i­mum afte r -3 h. and the re leased free ac rylonitril e res i­dues were subsequently converted into acry lamide and ac rylic ac id by the intrace llul a r nitrile hydratase and amidase, respecti ve ly. Afte r 48 h of g ro wth ne ither acrylamide nor ac rylic ac id could be detected indicating the ir complete utilization. Extracellular a-amylase ac­ti vity was maximum after 8 h o f growth and degraded the sta rch moiety to low molecular weight dextrin-type

products.

In vitro amylo lyti c degradation of S-g-PAN showed 55 and 50 per cent a - and ~- amy l o l yses values, whereas

seque nti a l degradati on with a-a mylase fo ll owed by g lucoamylase showed 70 per cent hydro lys is. HPLC of the maltooli gosaccharide profile showed o li gome rs up to a degree of polymeri zati on. Treatment of the grafted starch hydrolysates w ith Bacillus cereus cell s showed the presence of very low molecular weight PAN cha ins grafted on to maltooli gomers. By size exc lusion chro­matography it was noti ced th at the amylose component of starch undergoes graft copo lymeri zation 43

A sensiti ve reverse phase HPLC method fo r simulta­neous de te rmin ati on o f ac ry lonitril e, ac rylamide and acrylic acid was developed to monitor the ir presence (as res idual mon omers) in grafted copo lymers. The method was useful to assess the products of biodegras:J ati on of these and other industri al copo lymers 4~

A simple co lorimetric assay based on the determina­ti o n o f ac ry lamide fo rmed by the acti o n of nitril e hydratase on ac rylonitrile has been reported by Saroja et a /. 4-'

Acknowledgements

The authors thank Mr A B Vi shu Kumar for he lp in typesetting the fi gures . One o f the authors (NS) thanks the Counc il of Sc ientific and Indu stri a l Research. New Delhi , for the award of a Senior Research Fellowship .

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