Mass Transport within Edible and Biodegradable Protein- Based Materials

1、Mass Transport within Edible and Biodegradable Protein- Based Materials 34 Mass Transport within Edible and Biodegradable Protein-Based Materials: Application to the Design of Active BiopackagingS. GuilbertA. RedlN. GontardCONTENTSAbstract34.1 Introduction34.2 Protein-Based Materials, Elaboration, a

2、nd Rheology34.3 Control of Solute Transport34.4 Control of Gas Transport34.5 ConclusionReferencesAbstractAdvantages, types, formation, and properties of agro-polymer based materials arepresented in this chapter, with emphasis for protein based materials. The structurationand rheological properties o

3、f the network depend on the material-forming technology(casting or thermoforming), the process variables (plasticizer, temperature, shearrate, etc.), and the usage conditions of the film (relative humidity, temperature).? 2002 by CRC Press LLC Some unique mass transport (gas selectivity, solute rele

4、ase) properties and theirability to be modified and controlled opens a new field of industrial application forthese macromolecules in the field of edible or biodegradable “active packaging.”34.1 INTRODUCTIONEdible and biodegradable packaging materials produced from agricultural originmacromolecules

5、offer numerous advantages (renewable and biodegradable materi-als) over conventional plastics.1,2 Applications of bioplastics can be classified inthree categories.1. Plastics to be composted or recycled (fields where reuse or fine recoveryare difficult)2. Plastics used in a natural environment (fiel

6、ds where recovery is not eco-nomically or practically feasible)3. Special plastics (fields with specific features were bioplastics possesspreferential properties)Different techniques1,2 using fully renewable agricultural raw material to make whatcan be called agro-polymers are summarized in Figure 3

7、4.1.Our studies demonstrated that proteins have interesting material-forming prop-erties. Homogeneous, transparent, strong, water resistant, highly permeable to watervapor, and highly gas (CO2/O2) selective protein-based materials have been obtained? 2002 by CRC Press LLCFigure 34.1 Different approa

8、ches in making bioplastics. either by “casting” techniques or by “thermoplastic processing.” Combination ofproteins with lipids, natural fibers, paper, or conventional plastic materials was foundto improve material water vapor barrier or mechanical properties. These propertiescan be exploited to for

9、m edible and/or biodegradable packaging that could providea supplementary and sometimes essential means for controlling physiological, micro-biological, and physicochemical changes in food products.3 Many protein materialscan be used: collagen, zein, wheat gluten, ovalbumin, soybean, casein, etc.4,5

10、 Wewill expand on the use of protein films as active layers (i.e., when the film itselfcontributes to the food preservation) with some examples of potential uses of thesebiomaterials (e.g., modified atmosphere packaging or controlled release of foodpreservatives). Figure 34.2 gives a schematic repre

11、sentation of food preservationwith edible/biodegradable films as active layers when the first mode of deteriorationresults from respiration, dehydration, or moisture uptake, or from surface microbialdevelopment or oxidation. The protective feature of the film is dependent on gasand water vapor barri

12、er properties, on modification of surface conditions, and on itsown antimicrobial properties.6,7Figure 34.2 Schematic representation of food preservation with or without edible filmsand coatings as active layers, when the first mode of deterioration results from respiration(a), from dehydration or m

13、oisture uptake (b), or from surface microbial development or? 2002 by CRC Press LLCoxidation (c). 34.2 PROTEIN-BASED MATERIALS, ELABORATION, AND RHEOLOGYProtein undergoes glass transition phenomenon,8 and the controlled presence ofwater or other plasticizers lowers the glass transition temperature b

14、elow the break-down temperature (molecular degradation). Proteins are thus allowed to be shapedby thermoplastic processes such as extrusion (Figure 34.3) as standard syntheticpolymers with similar transformation costs. The glass transition temperature of various proteins has been studied as afunctio

15、n of water content and various other plasticizers.911 The general behavior ofthe glass transition temperature broadly follows the CouchmannKarasz relation.Comparing the plasticizer efficiency on a weight basis, a similar plasticization wasobtained with plasticizers of different molecular structure (

16、hydroxyl or aminogroups). The plasticizing efficiency (i.e., decrease of Tg) at equal molar content isgenerally proportional to the molecular weight and inversely proportional to thepercent of hydrophilic groups of the plasticizer. Migration rate of the plasticizers inthe polymer is related to their

17、 physicochemical characteristics. In this study, it wasassumed that polar substances interacted with readily accessible polar amino acids,whereas amphiphilic ones interacted with nonpolar zones that are, in most proteins,buried and accessible with difficulty.11 The rheological behavior of a gluten m

18、aterialprepared in a two-blade counter-rotating batch has been studied by Redl et al.12,13(Figure 34.4). During batch mixing, the evolution of torque is characterized by alag phase, followed by an exponential increase up to a maximum and thus a con-tinuous decrease. Viscous heat dissipation is very

19、important and leads to an increaseof temperature of about 60C. Maximum torque is obtained for a stable level ofspecific energy (500600 kJ/kg) and temperature (5060C) independent of process-ing conditions.? 2002 by CRC Press LLCFigure 34.3 General mechanisms of formation of protein-based agromaterial

20、. In the first stage of material structuration (Figure 34.4), plasticization occurs.The reason for the drastic change at maximum torque and 5060C may be anexposure of hydrophobic sites, reinforcing a gel-like behavior. Thereafter, the net-work structure might be stabilized with covalent cross links,

21、 probably via disulphidebonds through free-radical reaction mechanisms, leading to the observed increaseof molecular size. This structuration mechanism is probably a temperature-controlledphenomenon, dependent on mechanical energy input, but quite independent of mix-ing conditions and glycerol conte

22、nt. The influence of temperature and plasticizer content can be roughly determinedwith time/temperature and time/plasticizer shifts. A general expression was proposedby Redl et al.12 for describing the viscous behavior of a gluten/glycerol mix, whichallows the modeling of the flow behavior in a twin

23、-screw extruder.Extrusion of wheat proteins plasticized with glycerol appeared to be feasible insteady-state conditions. However, an important problem encountered within theextrusion process was the high viscosity of the “proteic melt” and its reactivity athigh temperatures. Therefore, the possible

24、processing conditions were restricted ata given glycerol content (35% w/w), and this window would be even smaller orvanish at a lower plasticizer content. Best results were obtained at low temperature(60C) and low specific energy input, which led to smooth surfaced materials witha molecular weight d

25、istribution close to native gluten. Evolution of molecular sizeFigure 34.4 Evolution of torque, temperature, storage modulus G at 1 rad/s (?), the slopeof the storage modulus n (?), and the high-molecular-weight fraction Fi (?) with reducedmixing time (t/tpeak) (80C, 65% gluten, 45% glycerol, 30 rpm

26、).? 2002 by CRC Press LLCdistribution along the screw shows a slight depolymerization in high shear rate zones, but polymerization is globally dominant and positively related to specific mechanicalenergy input. The mechanical behavior of the protein material is network-like. This behaviorhas been ch

27、aracterized and modeled for wheat gluten (mainly composed of gluteninsand gliadins) by using ColeCole distributions.13The molecular weight between network strands or entanglement couplings asderived from the rheological properties, corresponds to 13 times the gluteninsubunit and suggests that the bu

28、ilding block of the network is the glutenin subunit.Changes in molecular size of proteins during extrusion have been measured bychromatography and appear to be correlated to the molecular size between networkstrands, as derived from the rheological properties of the obtained materials. Increas-ing n

29、etwork structure appeared to be induced by the severity of the thermomechanicaltreatment, as indicated by specific mechanical energy and the maximum temperaturereached. Evolution of molecular size distribution along the screw shows a slightdepolymerization in high shear rate zones, but polymerizatio

30、n is globally dominantand positively related to specific mechanical energy input. Gliadins might react ascross-linking agents.34.3 CONTROL OF SOLUTE TRANSPORTAside the mechanical properties of “agro-plastics,” the control of mass transferproperties may be important, especially for applications that

31、are designated to be incontact with food. For such applications, the migration of plasticizers or additivesinto the food bulk is generally undesired, unless a controlled release of a chosenadditive is wanted (“active packaging”). The improvement of food microbial stabilitycan be obtained by using ac

32、tive layers such as surface retention agents to limit foodadditives (particularly antioxygen and antifungic agents) diffusion in the foodcore.6,7,1416 Maintaining a locally high effective concentration of preservative mayallow to a considerable extent reduction of its total amount for the same effec

33、t(Figure 34.5). It is then important to be able to predict and control surface preser-vative migration. The mathematical theory of diffusion in isotropic substances isbased on the hypothesis that the rate of transfer of a diffusing substance throughunit area of a section is proportional to the conce

34、ntration gradient normal to thesection, i.e., Ficks first law. One method to quantify the diffusion coefficient is tostudy the desorption of a solute from a thin layer of sorbic into a stirred liquidmedium.In the case of Fickian diffusion, the initial desorption is linear versus the squareroot of ti

35、me and a diffusion coefficient can be derived.17 As for the rheologicalproperties, morphological or physicochemical changes of the matrix are reflected inthe migration properties of the diffusant. The study of the influence of variablessuch as temperature and composition of the matrix can therefore

36、be of preciousinterest for structural insight. Fickian diffusion has been identified as the predominantrelease mechanism of sorbic acid desorption from thin protein layers. The diffusioncoefficient of sorbic acid in a gluten based film was found to be 7.6 1012 m2/s? 2002 by CRC Press LLC(Table 34.1)

37、. Addition of lipidic components led to a 2050% reduction in sorbic acid diffusivity but remained far from the diffusivity in the corresponding pure lipidicfilms (2.4 1016 m2/s and 2.7 1013 m2/s for beeswax and acetylated monoglycerides,respectively). The effect of temperature could be described by

38、an Arrhenius-typelaw with activation energies ranging from 30.0 to 39.8 kJ/mole. An explicit numerical method for the modeling of the migration of sorbic acidfrom a surface layer into food is proposed, allowing the tracing of the evolution ofTABLE 34.1Diffusion Coefficient of Sorbic Acid from Gluten

39、 and LipidBased FilmsT = 20C D 1012 (m2/s) T = 10CD 1012 (m2/s)T = 4CD 1012 (m2/s)Gluten 7.5 4.1 3.1Gluten-beeswax 5.6 3.0 2.2Gluten-acetylated monoglycerides 3.2 2.2 1.6Acetylated monoglycerides 0.27 Beeswax 0.00024 Figure 34.5 Evolution of storage G (?) and loss G () modulus and of complex viscosi

40、ty* () with pulsation. Lines represent the fit of ColeCole functions. (N = 100 rpm, Q = 4.8kg/h; Tr = 80C); adapted from Reference 13 with permission from Cereal Chemistry.? 2002 by CRC Press LLCthe concentration profiles of sorbic acid within the surface layer and the food. The model was validated

41、with experimental data obtained with wheat gluten and beeswaxfilms placed on agar-agar gels as model foods. Modeling showed that, in the caseof a gluten film, the surface concentration drops below 10% of the initial value after1 h whereas, in the case of a beeswax film, surface concentration remains

42、 above75% after one week. Simulation lead to the conclusion that, to achieve a significantsurface retention, sorbic acid diffusivity in the edible surface layer has to be lessthan 1015 m2/s.Microbiological analysis has confirmed the efficiency of preservative retentionwithin surface coatings. The an

43、timicrobial effect of various gluten/sorbic acid basedfilms was evaluated against Penicillium notatum. The study was conducted at twotemperatures on an acid model food. Simple gluten-based films had no fungicidaleffect; however, the addition of lipidic compounds (e.g., datem or beeswax) to thesefilm

44、s delayed fungi growth. Sorbic acid retention by all films was highly dependenton temperature. At 4C, gluten-based films containing sorbic acid delayed the Pen-icillium notatum growth for four days, while no effect was observed at 30C. Thegluten/lipid-based films showed a strong sorbic acid retentio

45、n and a marked fungi-cidal effect, either at 30 or 4C, delaying the Penicillium notatum growth for morethan 21 days.34.4 CONTROL OF GAS TRANSPORTThe development of biopackaging films with selective gas permeability (oxygen,carbon dioxide, ethylene) that allow control of respiration exchange seems ve

46、rypromising for achieving a “modified atmosphere” effect in fresh “l(fā)iving” productssuch as fruit, vegetables, and cheese microflora. An improvement of the storagepotential of these products, as schematized in Figure 34.6, could then be expected.Films formed with wheat gluten have particularly good o

47、xygen and carbondioxide barrier properties under low-moisture conditions. Oxygen permeability ofwheat gluten film was found to be 800 times lower than low-density polyethyleneand twice lower than polyamide 6, a well known high oxygen barrier polymer.Increased aw promotes both gas diffusivity (due to

48、 the increased mobility ofhydrophilic macromolecule chains) and gas solubility (due to the water swelling ofthe matrix), leading to a sharp increase in gas permeability. With carbon dioxide,the sharp increase of permeability is more important than with oxygen permeability. The selectivity coefficien

49、t between carbon dioxide and oxygen (defined as theratio of the respective permeabilities of both gases) is sensitive to moisture variations.The selective coefficient of edible gluten films18,19 varies from 4.0 at aw = 0.30 to25 at aw = 0.95, whereas the selectivity coefficient for synthetic polymer

50、s remainsrelatively constant, at 4 to 6. This could be explained by the differences in watersolubility of these gases (i.e., carbon dioxide is very soluble) but also to specificinteractions between carbon dioxide and the water plasticized proteic matrix, asschematized in Figure 34.7.18The exceptiona

51、l selectivity of wheat gluten films can be qualified as sorptionselectivity (while, for main conventional selective films, the mechanism of selectivity? 2002 by CRC Press LLCof diffusion is different). Figure 34.6 Theoretical evolution of concentration profile of sorbic acid in wheat gluten filmplac

52、ed on the model food as a function of time. Calculated values are obtained using DFilm =7.5 1012 m2/s, Dmodel food = 9 1010 m2/s. (C sorbic acid concentration at time t, C0 initialconcentration of sorbic acid in film).Figure 34.7 Evolution of O2 and CO2 permeability in gluten films as a function of

53、temperature? 2002 by CRC Press LLCand relative humidity. Resulting effect on gas selectivity.Some selective gluten-based films have been shown to lead to the creation oforiginal atmospheres when used to wrap fresh vegetables. The evolution of atmo-sphere composition around fresh mushrooms placed in

54、a glass jar covered withvarious films was studied. With a wheat gluten film, atmosphere composition risesto an equilibrium of 23% CO2 and 23% O2, i.e., a modified atmosphere within thesemi-permeable package is produced after application. This high in situ selectivityleads to a modified atmosphere th

55、at is favorable to the mushrooms overall quality.34.5 CONCLUSIONThe studies presented here have demonstrated a number of characteristics of protein-based materials that make them suitable for the formation of different types of edibleor biodegradable packaging. Their ability to be modified and contr

56、olled, as well astheir compatibility with paper, natural fibers, or even conventional (modified) plasticsto form composite material, open new applications in the field of active edible/bio-degradable wrappings or coatings.REFERENCES1. Gontard, N. and S. Guilbert. 1994. “Bio-Packaging: Technology and Properties ofEdible and/or Biodegradable Material of Agricultural Origin,” in Food Packaging andPreservation, M. Mathlouthi, ed. Blackie Academic & Professional, pp. 159181. 2. Guilbert, S. 1999. “Biomateria