Structure and morphology of baked starch foams

Abstract

Baked starch foams were characterized by several physical methods in order to better understand the relationship between process parameters and starch foam structure. In this process, a thin-walled object such as a plate is formed by heating a starch batter inside a closed mould. Normal corn and potato starches are gelatinized by this treatment but some swollen granules remain. The foams have a dense outer skin and a less dense interior with large, mostly open cells. Overall foam density and strength increase with increasing starch concentration, molecular weight and amylose content. Foam flexibility tends to increase with decreasing density. Plates made from tuber starches such as potato have lower densities and higher flexibilities than those made from cereal starches such as corn. Starch foams are useful as disposable food packaging and serving articles which can be composted after use.

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.