New Products From Traditional Crops

New Uses for Starch

by William M. Doane, Plant Polymer Research Leader, National Center for Agricultural Utilization Research, ARS, USDA, Peoria, IL.

Interest in natural products as annually renewable raw materials for industry has greatly intensified, especially during the past two decades.

Much of this interest can be attributed to the oil embargo of the early 1970's. However, another reason for this interest is the increased abundance resulting from agricultural production that has more than satisfied available markets, and that has generated an oversupply of many commodities. Thus, there is increased interest in using such commodities as raw materials for industry, either to develop new products or to replace those products now made from nonrenewable sources such as petroleum.

Corn is one of the commodities produced in excess of available markets, and since the corn kernel consists of about 70 percent starch, it is one of the natural materials that is receiving considerable attention as a renewable resource.

Starch is one of the most abundant materials produced in nature, is easily recovered from plant organs holding it, is relatively low in cost, and is readily converted chemically, physically, and biologically into useful chemicals now derived from petroleum. More than 200 billion pounds of petroleum chemicals are produced annually in the United States.

Starch: Occurrence and Availability

Although starch occurs in many plant tissues, commercially it is mostly recovered from seeds, roots, and tubers. In the United States, cereal grains, such as corn and wheat, provide the major source of starch. The average yearly U.S. corn crop contains about 300 billion pounds of starch, with only about 15 percent of the crop being processed to separate the starch or starch-protein (flour) component from the corn kernels. The corn processing industry is expanding, doubling the amount processed during the past decade, and has both the interest and the capability to further expand as market opportunities increase. Of course, increased processing translates into more jobs and an improved economy.

Starch has played an important role in the human diet since before recorded history, and it also has a long history of use in nonfood (industrial) applications such as papermaking and adhesives. Today, 5 billion pounds of starch or flour are provided in the United States for industrial (nonfood) uses. About 3.5 billion pounds of the 5 billion are used in the paper, paperboard, and related industries, where starch serves a variety of adhesive functions. In addition, the starch in nearly 400 million bushels of corn was converted into ethanol in 1991.

Now, new technologies based on starch and new products derived from starch are emerging. Four of the most promising of these technologies are starch for biodegradable plastics, biopolymer plastics, water absorbent polymer (Super Slurper), and starch medium for encapsulation. These developments, described in this chapter, hold promise for vast new markets for starch and for consumer products that are more environmentally acceptable than many currently in use.

Starch: Role in Biodegradable Plastics

In the mid- 1980's many popular and scientific articles began to appear on the need to develop biodegradable polymers to replace plastics. Plastics, the major nonenergy product of petroleum chemicals, are considered to be nonbiodegradable, or at best only slowly degradable over many years. This, coupled with the amount of plastics produced and ending up as litter or in landfills, has led to a push toward developing plastics from natural materials that would biodegrade. In the United States, about 60 billion pounds of petroleum-derived plastics are produced each year. Municipal solid waste contains 5-7 percent by weight (17-25 percent by volume) of plastic, largely from packaging materials.

Considerable local and national legislative activity, in the United States and abroad, has caused much research and development to be devoted to biodegradable replacements for currently used plastics. In some instances, certain uses for plastics have been banned and other uses are being phased out over the next few years. The types of uses given most attention are those resulting in quick disposal after one-time use. Packaging films, shopping and produce bags, egg cartons, bottles, and fast-food service items are among the many plastic items targeted for replacement.

The Marine Plastic Pollution Research and Control Act of 1987 (Public Law 100-200, December 29, 1987) requires the U.S. Navy to cease disposing of nonbiodegradable plastics at sea by the end of 1993. This act has brought researchers together from academia, industry, and Government to develop plastics based on natural materials that will biodegrade in the sea into harmless components.

Plastic materials containing starch as a filler and biodegradable additive are now on the market. Polyethylene (PE) garbage and compost bags containing small amounts (generally less than 10 percent) of granular starch are now produced commercially by coextrusion of polyethylene and starch. coextrusion, a process widely used in the food, feed, and plastic industries, allows thorough mixing and heating in a screwlike device to produce a very uniform product. Often the starch is pretreated to reduce moisture content and improve adhesion with the polyethylene to enhance strength properties of the bags. Other additives during extrusion include agents that assist in breakdown of the polyethylene to increase its degradability.

Corn is the major source of starch in the United States. Starch-based products produced by NOVAMONT North America, Inc., range from plastic bags and flatware to golf tees.

Ken Hammond/USDA 92BWO806-17A

Today, we are in an early stage of a promising new technology for producing a broad spectrum of plastic products containing high levels of starch, with some consisting entirely of starch. This technology can be traced back to the 1970's, when researchers at USDA's Agricultural Research Service (ARS), National Center for Agricultural Utilization Research (NCAUR) at Peoria, IL, studied the extrusion of starch under various conditions to produce plastic films. Agricultural mulch film, containing up to 50 percent starch, was one of the resulting products.

In late 1990, an expanded (foam) packaging material containing 95 percent starch entered the marketplace, suitable as a biodegradable replacement for styrofoam "peanuts." This product made by extrusion has resilience and compressibility properties similar to those of Styrofoam, and it readily dissolves when placed in water. In 1991, companies began offering commercial quantities of starch compositions prepared by extrusion processing for use in making films and injection-molded articles. It is expected that the 1990's will see greatly expanding markets for these novel starch-based plastics.

Biopolymer Plastics via Starch Fermentation

Fermentation of starch or starch-derived sugars has long been practiced to produce a variety of alcohols, polyols, aldehydes, ketones, and acids, chemicals used in foods and beverages, paints, solvents, plastics, cosmetics, and many other products. One of the acids, lactic acid, has received considerable attention as the basis of biodegradable plastics with a host of potential industrial applications. Lactic acid is converted into a plastic by polymerization, a process by which individual units of lactic acid are joined together like links of a chain to form a long chain. Because of the high cost associated with producing and recovering lactic acid, the use of this biopolymer plastic has been restricted mostly to the medical field, where it's used in biocompatible, biodegradable, reabsorbing sutures and prosthetic devices. There is a potential market of millions of pounds yearly for commodity plastics, as research leads to improved preparation and recovery of lactic acid in the fermentation process. Companies in the United States and abroad are now pursuing this promising market.

Another biopolymer plastic known as polyhydroxybutyrate (PHB) or poly-hydroxybutyrate-co-valerate (PHBV), also made by fermenting starch, is now being used to make bottles for oil and shampoo. A 1988 product bulletin of a company producing PHBV lists several applications for PHBV in such areas as medical implants, plastic dinnerware, bottles, films for packaging, and media for slow release delivery of medicinals. The biodegradability of PHB and PHBV polymers has drawn attention to these natural polymers, as interest in replacements for nonbiodegradable polymers has grown. The polymers, which have good shelf stability, undergo microbial degradation when buried in soil.