Food Safety Magazine, February/March 2013

PACKAGING(continued from page 34)food production, creating additionalvalues from biomass. Edible coatingsand films are not studied with the aim ofsubstituting traditional packaging materials.Due to their distinctive properties,they can be used to provide additionalfunctionalities to the food. Food preservationcan be therefore improved,and one may also reduce the cost andamount of traditional packaging used.The biopolymer used to develop ediblefilms and coatings is usually based onhydrocolloids, such as polysaccharideslike cellulose, starch, alginates, chitosan,gums, pectins and proteins, from vegetableor animal origin. Their functionalproperties can be used to modify the barrierto gases and moisture and, in moreadvanced developments, serve as carriersof food additives and nutrients. Blendsor composites with other additives andfillers are also developed to optimizethe barrier properties or to control therelease kinetics of substance. 8The most important property thesenatural biopolymers must possess is thepossibility of forming films with suitablemechanical and barrier properties. Thechemical, molecular and supramolecularstructures are, therefore, important featuresof natural biopolymers that permitdesign of the physicochemical propertiesof the films as well as the processingtechnology for the manufacture of theproducts.Cellulose and cellulose derivatives,obtained by chemical substitution ofsome hydroxyl groups along the chain,permit development of films with ionicproperties (carboxymethylcellulose) andnonionic cellulose ethers (methylcellulose,hydroxypropylcellulose andhydroxypropyl methylcellulose). Filmsmade by these derivatives are usuallyvery sensitive to water but resistant tooil and fat. They can be used to incorporatefunctional additives and antimicrobials,such as, for example, nisinor rosemary, and tea extract to reducelipid oxidative rancidity. Antimicrobialfilms based on starch and starch derivativescan be obtained by incorporatingpotassium sorbate or chitosan into thefilms. However, the presence of thesesubstances could modify the crystallinestructures of the films, thus reducing thebarrier to gases.Alginates, biomacromolecules extractedfrom brown seaweeds, are other interestingpolysaccharides due to their capabilityto react irreversibly with polyvalentmetal cations, in particular calcium ions,to produce water-insoluble polymers.For these classes ofpolysaccharide materials,the release ofpotassium sorbate orother antimicrobial additivesis controlled bymodulating the physicalproperties of thebiomacromolecularstructure, in particular,through control of thereticular density.Chitosan, extractedfrom the shells ofcrustaceans, is a highmolecularweight cationicpolysaccharide that is widely usedto make films with antibacterial and antifungalactivity. It has been extensivelyused to protect, improve quality and extendthe shelf life of fresh and processedfoods.Proteins represent another class ofbiomacromolecules employed for ediblefilms and coatings. They can haveimpressive barrier properties to oxygen,carbon dioxide, oil and fats. However,mechanical and water vapor barrier propertiesof films produced from these materialsare inferior to those of syntheticorigin. 9Both agro- and animal-based proteinssuch as wheat gluten, corn zein, soyprotein, whey proteins, casein, egg white,keratin, collagen, gelatin and myofibrillarproteins have been used to prepare filmsand edible coatings by using the solventcasting process. Very few publicationshave reported on the thermoplasticizationand the extrusion of these proteinsto produce films. The development ofextrusion-based technologies with goodreproducibility and control over themolecular architecture and spatial conformationof the natural macromolecule“The packagerepresents a protectionof the food againstnegative effectsfrom the externalenvironment.”is among the main scientific and technologicalchallenges to exploiting the use ofprotein-based films and coatings.While thermoplastic starches (TPS)have been widely studied and successfullyapplied in industry, including blendingTPS with other synthetic polymers,the thermoplasticization of proteins hasbeen reported only recently, and it hasbeen investigated ongluten, zein, soy, wheyand gelatine. 9Several factorsmust be taken intoaccount when choosingbetween differentpossible plasticizers forthe development ofthermoplastic proteins.The most commonlyused plasticizer is glycerol,which is misciblein most cases. Othersystems that have beeninvestigated includepolyfunctional alcohols, such as sorbitoland propylene glycol, as well as di- andtriethanolamine. Heat and shear stressescontribute to the unfolding of the proteinin the presence of the plasticizerduring the extrusion process. Proper processingconditions such as temperatureprofile, residence time and screw designare therefore necessary to supply theneeded mechanical and thermal energyto the proteins/plasticizer systems. Byoptimizing protein/plasticizer systemsand processing conditions, one couldobtain materials with rheological propertiessuitable for film-blowing technologies.10However, one should be aware thatthe use of extrusion-based processingcould affect the functionalities of antimicrobialcompounds embedded in thepolymeric matrix. These substances aregenerally heat sensitive and thermallyunstable; thus, they may become inactiveduring processing, mainly becauseof the high temperature, high shear ratesand high pressure an extruder can reach. 9ConclusionsThe ongoing scientific and techno-44 F o o d S a f e t y M a g a z i n e