Structure and morphology of baked starch foams
Introduction
The use of starch as a biodegradable base material for disposable articles such as plates, utensils and bags is desirable as an environmentally friendly alternative to the present use of non-degradable petroleum-based plastics such as polyethylene and polystyrene1, 2, 3, 4, 5, 6, 7. Although the latter can be recycled, this is often difficult and expensive due to contamination of plastic articles with food and collection costs. Totally biodegradable starch-based plastics can be composted into a useful mulch rather than being entombed in landfills. In addition, starch is a widely available biopolymer and, at $0.25–0.60 kg−1, is less expensive than polyethylene and polystyrene ($0.80–1.50 kg−1).
Expanded starch-based foams have been prepared since the 1970s by extrusion of grains or starch with water8, 9, 10, 11. High temperatures and shear within the extruder barrel cause the starch–water mixture to melt into a plastic-like consistency. Water serves as the blowing agent as it expands upon exiting the extruder. For example, expanded snack foods and cereals are prepared in this manner. More recently, starch foam packing `peanuts' and sheeting have been prepared using a similar process and have begun to penetrate the market formerly occupied entirely by expanded polystyrene foam packing12, 13, 14. It is, however, difficult to prepare shaped objects such as a plate from starch foam because the heat required to soften the foam and mould it into the desired shape will cause the starch to lose moisture and raise its glass transition temperature to levels above the decomposition temperature (∼200°C)[15].
Recently, a new process for preparing shaped foam articles from starch has been described6, 7. This involves baking a starch–water batter in a hot mould. A starch-formed article is fashioned as the starch gelatinizes, expands and dries. In this report, baked starch foams were characterized by several physical methods in order to better understand how variables such as amylose content, starch concentration, starch type, molecular weight and temperature influence starch foam structure and properties. The effects of humidity and time on the mechanical properties of baked starch foams were also studied.
Section snippets
Materials
Normal corn starch (27% amylose) was Buffalo 3401 from CPC International, Englewood Cliffs, NJ and had a moisture content of 10–12%. Waxy maize starch (18% moisture) was Amioca from National Starch and Chemical, Bridgewater, NJ. High amylose corn starches (12% moisture) containing 50 and 70% amylose were Amaizo 5 and Amylomaize VII from American Maize-Products, Hammond, IN. Wheat starch (16.6% moisture) was AYTEX-P3201-8 from Ogilvie Mills, Minnetonka, MN. Tapioca starch (18.6% moisture) was
Results and discussion
The process of foaming a starch batter inside a heated, closed mould can be divided into several steps. First, the temperature of the starch–water mixture inside the mould rises above the starch gelatinization temperature or to the boiling point. Next, the starch gelatinizes and becomes a thick paste at which point the steam trapped in the paste expands causing the paste to fill the mould and begin to exit the vents around the edge of the mould. As this happens, a small amount of pressure (∼1
Acknowledgements
The technical assistance of Elizabeth Bissett and Lee Baker is gratefully acknowledged. This work was conducted under Cooperative Research and Development Agreement (CRADA) No. 58-3K95-M-228 with Franz Haas Machinery of America.
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- 1
Bradley University, Peoria, IL 61604(current address)
- 2
Franz Haas Machinery of America, Richmond, VA 23231
- 3
Franz Haas Machinery of America, Richmond, VA 23231