Earth-friendly wrappers

Resource, May 1998 by Dawson, Paul L

Films made of soy and corn protein are one food packaging option

About five years ago, the Food Science and Chemical Engineering departments at Clemson University set out to combine food protein with nylon to produce a biodegradable packaging material.

They have since produced films that could someday create new uses for agricultural products and reduce packaging waste and disposal costs.

The research began as a South Carolina "seed" project funded by the state and the experiment station at Clemson. Collaborative work later expanded to include other units on and off campus. The core team is composed of four researchers, their research associates, and graduate students from the two departments.

This group includes Clemson professors James C. Acton and Paul L. Dawson of the Food Science Department and Doug H. Hirt and Amod A. Ogale of the Chemical Engineering Department.

The theory behind the studies is that a film will form when similar amino groups in nylon and food grade proteins interact. The approach uses heat and pressure to form films rather than an organic solvent or pH adjustment followed by casting. Casting films involves evaporation of an organic solvent, such as ethanol in the case of corn zein, to form a porous material.

Why develop these protein biopolymers?

One reason is to reduce waste by using renewable resources. The U.S. Environmental Protection Agency (EPA) reports that in 1993 the United States generated about 8 million tons of plastic packaging. This represented 11.7% of the total packaging produced and 4% of the solid municipal waste. Americans generated another nearly 4 million tons of nondurable goods including plastic plates, cups, trash bags and disposable diapers.

The biodegradability of proteins makes them attractive as prospective raw packaging material components. Because they are derived from plants, proteins are a renewable resource, unlike petroleumbased products.

Protein biodegradation requires depolymerization followed by mineralization of the packaging material, In 1994, a researcher discovered that corn protein and dialdehyde starch degrades up to 63% after 180 days under composting conditions. This proves that proteins are good candidates for biodegradable polymers.

The research objective

The team's goal was to develop a heat-pressed film that could be used in a heat extrusion system without a casting process. The heat-pressed system would mimic the traditional film industry's largescale production of polymer plastics.

Nylon and protein were chosen as components due to their similar chemical structure - a common peptide bond that joins repeating units within their structures in the following sequence:

Also, the researchers needed to find a food protein with a similar melting temperature to a nylon's. Thermogravimetric analysis determined which nylons and proteins might be good matches. The researchers expected their chosen nylon to have a low melting point when compared to most other nylons. What they did not know was whether the proteins would melt, or burn, before forming a film.

Ogale discovered that the proteins would melt, but also that matching a nylon source would be difficult. He found that a protein taken from beef bone formed the best match when mixed with nylon 6. Using the food protein only, he met success and the team experienced their first major breakthrough.

Since then, they discovered that other food proteins, including a soy protein isolate and corn zein, will form films. The team expanded the research to:

include adding compounds antimicrobial and others - to the protein films;

measure and alter the film physical properties through additives and processing treatments;

determine the bioavailability of the films through in vivo and in vitro studies.

Qualitative comparisons have been made between cast protein films and heat-pressed films using scanning electron microscopes. Using heat to form films with corn zein or soy protein produces a more homogenous structure with fewer voids seen when magnified 1,000 times.

Combining food-grade biocides with protein films is an attractive alternative for a packaging material to preserve food. Lysozyme, a natural enzyme found in egg white, and nisin, a bacteriocidal protein produced by lactic acid bacteria, have been mixed with protein before a film forms. The produced film contains properties that kill potentially harmful bacteria on food products. Adding lysozyme and nisin to soy protein before forming films can increase inhibition of Lactobacillus planta rum.

However, these biocides can be inactivated with high heat. The researchers at Clemson were pleased to discover that the thermal processes they used for developing heat-pressed films were not hot enough to inactivate the antimicrobial agents.

Testing also aimed to determine whether a protein's digestibility is affected by thermal film formation. These tests have been done in vitro, using a digestive cocktail and gel electrophoresis, and in vivo rat feeding studies to determine Protein Efficiency Ratio and Nitrogen Protein Ratio. Results show that the film formation does not affect protein digestibility. However, some conformation changes occur within the protein.