Edible Films
Attila E. Pavlath
Western Regional Reseaech Center
U.S. Department of Agriculture
Albany, California

In the fast moving modern life, many people do not want to spend the time in the kitchen paring, coring, slicing and/or dicing fruits and vegetables, but at the same time they do not want to use canned foods, they uant to have the taste and aroma of fresh commodities. There is a similar requirement in hospital, industrial and school cafeterias for another reason. Generally these cafeterias serve 300-500 persons for which amount the manual labor for these processes is too expensive, and a machine for such a small volume is not cost effcient. The best solution for this problem is the ]arge scale processing of these produces at some centrally located area and shipping the products to the consumer. Considering the time required for packaging, shipping, distribution and a short period of safety factors for shelf-life, one can estimate a period of 15-20 days between the start of processing and the time when the processed produce will reach the table of the consumer. Unfortunately, the removal of the natural outer skin from agricultural products immediately starts various physical and biochemical processes which result in the loss of flavor, taste, freshness and consumer acceptance. An economic method is needed to protect the product in this period of 15-20 days.

When the natural outer protection of agricultural products is removed, the loss of water accelerates, air oxygen will start various enzymatic processes resulting in brown or black spots, volatile flavor components will be rapidly lost and microorganisms will attack the exposed surface. These undesired activities could be slowed down by storing the processed pieces under special atmosphere during transportation, e.g., high humidity, nitrogen atmosphere,refrigeration etc., but this would be economically prohibitive for most produces. If the surface could be uniformly covered by a material which act as a barrier at least to water and oxygen, the problems could be diminished provided that the material is safely edible and, therefore, it would not have to be removed before consumption. While there are many materials which have the right transmission characteristics, the edibility requirements considerably limits the choices. The components of any usable coating must be edible and any consecutive treatment must exclude any major chemical reactions which might modify edibility.
There are three major classes of materials which separately or in combination may provide the basis for edible films: proteins, carbohydrates and fats. Unfortunately each of them has certain limitations.

1. Proteins. Most of them will form structurally stable films which will stick to the hydrophilic surface of processed agricultural commodities. Howe~er, they do not pro~ide any considerable protection against the diffusion of water and oxygen.

2. Carbohydrates. Starch, pectin or alginates are highly hydrophilic compounds, therefore, the diffusion of water through carbohydrate films is quite considerable. On the other hand, they will easily adhere to cut surfaces of fruits or vegetables and they represent good barriers to oxygen.

3. Fats. Fats have been used on whole sgricultural products preventing the loss of water, but they do not stick to cut surfaces. Stand-alone films are excellent water barriers.

It is not surprising that alone neither of these materials provide the desirable protection for lightly processed agricultural products, but it was found that their combination in the right ratio can be very efficient. A composition of l% alginic acid, 10% casein and 15% Myvacet (an FDA approved artificial fatty ester of glycerol) decreased the water losses from apple pieces by almost five times. However, the elimination of any of the components resulted in an unacceptedly high weight loss.

This combination might appear unexplainable at the first glance. As rightly expected, fat will slow down the diffusion of water. At the same time, as indicated above, carbohydrates are quite hydrophilic and, therefore, they should increase the water loss. However, in the absence of alginic acid, the coating did not provide protection. The most probable explanation is that the carbohydrate acts as a binding agent between the cut surface and the fat to assure adherence. Without such binding force the fat can not provide a uniform protection.

If we accept this as a major requirement, it is easier to explain the unexpected effect of casein. Inspite of the fact that casein does not provide any protection against the migration of water, the presence of casein is vitally important in the carbohydrate-fat mixture. The casein, as most proteins, acts as an emulsifier and assures an adequate mixing of the two other components.

While the alginic acid/caseinlMyvacet mixture provides protection for a processed piece of appe for up to three days, the quality of the coating is highly dependent on the degree of emulsification of the coating mixture. It is not only dependent on the temperature-- best results were obtained at 55-60 °C-- but various other factors,e.g. origin of the casein will have considerable effect on the stability of the emulsion. This can be especially an important factor at commercial scale. A mixture of carbohydrate and fats without the need of emulsification is expected to give longer and more consistent protection.

At the first glance, a clear solution of carbohydrates and fatty materials seems to be impossible. However, the situation is not hopeless. Carbohydrates could be esterified with fatty acids, though their edibility might have to be studied more extensively to obtain approval by the Food and Drug Administration. There is an easier combination, possibility utilizing simple acid-base reaction.

Chitin is a poly-acetaminohexose which is the major component of the shells of various crustaceous sea-life, such as lobster, crab or shrimp. This forms a large surplus which presently has no essential utilization. Chitin can be easily deacetylated to chitosan without breaking up the polymeric chain. While chitosan is not soluble in water, it can be disolved in formic or acetic acid to form a 1% solution. Mixing the solution with various higher boiling acids results in a chitosan-fatty acid salt.

The water transmission of films prepared from such mixtures would depend on the hydrophobicity of the fatty chain. One would expect that the longer the chain the more resistance would the film represent to the diffusion of water. Surprisingly, however, this is only true until the length of the lauric acid. The water transmission not only does not decrease with longer chains, but it even increases. The effect is not characteristic to the C12 chain, since methyl laureate, under similar conditions does not provide any protection. Newest data shows that the ionic bonding is needed because when the amino group of the chitosan is laurylated with lauric anhydride the film does not represent as good barrier to water as the chitosan-lauric acid complex. The necessity of an ionic reaction is also indicated by Fig.7 which shows a steady decrease of water transmission with the addition of increasing amount of lauric acid until it reaches the 0.95% concentration which represents the 1:1 ratio of amino and carboxyl groups. Increasing the amount of lauric acid beyond this ratio does not affect the water transmission.

The first indication that the chitosan-lauric acid complex represents a completely different structure from other chitosan-fatty acid complexes was obtained from scanning electron microscope studies. Chitosan films revealed a structure with number of irregular large holes. Similar results were obtained for chitosan-palmitic acid, chitosan-stearic acid and chitosan-methyl laureate films. The chitosan-lauric acid mixture, however, provided an essentially hole-free structure with regularly spaced layers.

The best explanation for the specialty of the chitosan-lauric acid mixture was obtained from computerized molecular modeling. While the chitosan itself was found to form in its lowest energy level a flat structure with large enough holes between strands for the easy migration of water molecules, the structure forming by the addition of fatty acid prefers a helical structure. The fatty chains are located in the inside of the helix, filling its center more and more with increasing length of the fatty chain.While the hole is decreasing, a maximum is reached around the C12 chain when the increasing length forces the chain more toward the outside of the helix and not only the hole inside of the helix starts to increase, but the holes between parallel strands will become larger.

Various combinations of proteins, carbohydrates and fats form films which can provide protection against water losses, but it is still not enough. Computerized molecular modeling can pinpoint promising structures and this allows the design of better molecules. These combination can be used not only as coatings, but they also have good probability for other applications, such as the bases for biodegradable wrapping materials.