Protein water barrier properties

The most superficial layer of skin is the stratum comeum (SC), which consists of terminally differentiated keratinocytes (comeocytes) that originate from actively proliferating keratinocytes in lower epidermis (basale, spinosum, and granulosum cells), and contain a lamellar lipid layer secreted from lamellar bodies (Fig. 7a). Flydration of the SC is an important determinant of skin appearance and physical properties, and depends on a number of factors including the external humidity, and its structure, lipid/protein composition, barrier properties, and concentration of water-retaining osmolytes (natural moisturizing factors, NMFs) including free amino acids, ions, and other small solutes. 

These properties can be modified to resemble those of polyethylene films by adding lipid compounds (beeswax, paraffin, etc.) to the film formulation (36,37), As already noted for the mechanical properties, water barrier properties are highly dependent on the temperature and relative humidity of the protein material and decline suddenly during the glass transition phase 22),... 

P., and Chiralt, A. (2009) Effect of oleic acid-beeswax mixtures on mechanical, optical and water barrier properties of soy protein isolate based films. J. Food Eng., 91, 509-515. 

This type of material, based on hydrocolloids is generally not very resistant to water and their moisture barriers properties are poor. In some cases, water solubility or sensitivity to water is a functional advantage, e.g. for die formulation of soluble sachets to carry chemicals such as fertilisers or pesticides. For die majority of uses, the improvement of water resistance and water barrier properties is of first importance. Chemical modification of biopolymers and development of specific additives (cross linking agents or plasticisers) adapted to the polymer structure are then proposed. Regarding these developments, protein rich materials which have a non monotonous complex structure with very large potential fimctional properties are promising (Cuq et al, 1998). 

Formulation additives used in topical drug or pesticide formulations can alter the stratum comeum barrier. Surfactants are least likely to be absorbed, but they can alter the lipid pathway by fluidization and delipidization of lipids, and proteins within the keratinocytes can become denatured. This is mostly likely associated with formulations containing anionic surfactants than non-ionic surfactants. Similar effects can be observed with solvents. Solvents can partition into the intercellular lipids, thereby changing membrane lipophilicity and barrier properties in the following order ether/acetone > DMSO > ethanol > water. Higher alcohols and oils do not damage the skin, but they can act as a depot for lipophilic drugs on the skin surface. The presence of water in several of these formulations can hydrate the skin. Skin occlusion with fabric or transdermal patches, creams, and ointments can increase epidermal hydration, which can increase permeability. 

Several studies reviewed formulations, barrier properties and possible application of edible protein-based films (Table 23.3) (Gennadios et al. 1994 Krochta and Me Hugh 1997 Torres 1994). Overall, similarly to polysaccharide films, proteins exhibit relatively low moisture barrier properties, two to four times lower than conventional polymeric packaging materials (McHugh and Krochta 1994d). The limited resistance of protein films to water vapour transmission is attributed to their substantial hydro-philicity and to the amounts of plasticizers, such as glycerol and sorbitol, incorpo-... 

Park, H.J., and Chinnan, M.S. (1995). Gas and water vapor barrier properties of edible films from protein and cellulosic materials. J. Food Eng. 25,497-507. 

Barrier Property—Water Vapor Permeability (WVP) Protein hlms are excellent gas barriers but poor moisture barriers because of their hydrophilic nature. Mechanical properhes and gas permeability depend on the relative humidity (1). 

From studies of lipid-water mixtures and isolated membranes the general functional features of the bilayer are known barrier properties, lateral diffusion, acyl chain disorder and protein association. To vmderstand the mechanisms behind a wide spectrum of membrane functions, a detailed picture at the level of local curvature is needed. Examples are fusion processes, cooperativity in receptor/ligand binding or transport through the bilayer of the proteins that are constantly synthesised for export from the endoplasmic reticulum. Some preliminary discussions of the possibilities of curved, rather than flat, membremes follow. 

Among the films invesfigafed, edible films based on proteins showed the best mechanical properties. However, their barrier properties are variable (Kester and Fennema, 1986). The increase of cohesion between protein pol) eptide chains was thought to be effective toward the improvement of the barrier properties of fhe films. For insfance, fhe presence of calcium was reported to decrease the water permeability of caseinate-based film (Avena-Bustillos and Krochta, 1993). 

However, one of the main drawbacks of these protein materials is their high sensitivity to water. As interaction of a protein film with water is a sum of complex phenomena, different parameters can be taken into account. The water vapor barrier properties and the plasticizing effect of water in the film (with its important influence on mechanical properties) belong to the most studied characteristics, probably because they can easily be linked to macroscopic measurements. 

Other biopolymers useful for synthesis of nanocomposites include (i) gelatin—a water-soluble protein obtained by extracting collagen liom animal skin and bones and thermal denaturation. (ii) PHB—a natural product of biosynthesis performed by bacteria in nature, (iii) Chitosan—a natural polymer widely found in exoskeletons of crustaceans and insects, as well as in the cell walls of microorganisms (Maiti et al. 2003 Zheng et al. 2002 Takegawa et al. 2010). Moreover, the mechanical and water vapor barrier properties of chitosan-based nanocomposites with cellulose nanofibers could be enhanced. 

A downside to the sustainable and extensive use of soy protein-based materials is their intrinsic reactivity and thus lower inertia when compared to most conventional petrochemical-based plastics. They are known to be sensitive to microbial spoilage and also to water due to hydrophilic nature of many amino acids constituting their primary structure and to the substantial amount of hydrophilic plasticizer required to impart thermo-processability and film flexibility. As a consequence, their mechanical properties and water vapor barrier properties in high moisture conditions are poor compared to synthetic films such as low-density polyethylene. 

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