Edible moisture barriers limits

Figure 23.3. Promises and limits of edible moisture barriers.

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-... 

Starch can also be directly used to form edible or biodegradable films. Different sources of starch, high amylose starch, and modified starch (Wolff et al, 1951 Roth and Mehltretter, 1967 Mali and Grossmann, 2003 Mali et al 2006) have been used to form self-supporting films by casting from aqueous solution. These films appear to have moderate oxygen barrier properties but poor moisture barrier and mechanical properties, which limit their commercial applications. 

Chitosan, a natural polymer derived from chitin, has application in edible coatings or films for packaging foodstuffs, but its use is limited by its high sensitivity to moisture. Its ability to form a water barrier can be improved by blending with a more hydrophobic polymer also derived from natural resources, such as PLA. Blends of starch with PCL can be reinforced with nanoscale platelets derived from smectic clay to significantly increase film ductility and barrier properties. 

Chitosan is a water-insoluble, nontoxic, edible, biodegradable polymer (polysaccharide) that is obtained commercially from chitin by alkaline deacetylation [103]. Chitosan is the second most abundant biopolymer in nature after cellulose. Since chitosan is a polycationic polymer, its high sensitivity to moisture limits its applications. One way to overcome this drawback is to blend the material with humidity resistant polymers such has PLA. Suyatma et al. [104] combined hydrophilic chitosan with hydrophobic PLA (92% L-lactide and 8% mesolactide, Mw = 49,000 Da) by solution and film mixing, resulting in improved water barrier properties and decreased water sensitivity of the chitosan films. However, testing of mechanical and thermal properties revealed that chitosan and PLA blends are incompatible. 

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