Effect of Batter Solids and Starch Type on the Structure of Baked Starch Foams

Cereal Chem. 76(5):682–687 The effects of starch type on the properties of baked starch foams were investigated. Starch types used for baking were normal corn, normal potato, waxy corn, high-amylopectin potato, wheat, and tapioca. Solids content of the starch batters used to bake foam trays ranged from 25 to 45%. Processing parameters and physical properties of the foams were examined. Starch-foamed trays were formed by heating a starch batter inside a closed mold. Scanning electron micrographs showed that the thin-walled foamed trays have a dense outer skin and a less dense interior with large cells. The weight of the foamed trays and density of the foam depended on the amount of batter cooked inside the mold, the percent solids of the batter, and the type of starch used. The high-amylopectin starches made the lightest trays, while the normal cereal starches made the heaviest trays. Baking time depended on percent solids of the batter, the batter volume added to the mold, and starch type. The normal cereal starches had the longest baking times and the high-amylopectin starches had the shortest baking times Strength and flexibility of the trays are correlated with tray weight and foam density. Heavier trays had greater strength and less flexibility than did lighter trays. Physical properties of the trays can be tailored to meet specific criteria by changing the starch type used and the batter solids. Currently, there is a great deal of interest in the manufacture of single-use articles such as plates, packaging foams, cups, and containers from biodegradable materials (Doane et al 1992). Although many biodegradable synthetic and natural materials have excellent mechanical properties, they are currently quite expensive ($4–10/kg) when compared with nonbiodegradable polymers currently on the market ($0.80–1.20/kg for polyethylene and polystyrene). Because of its low initial cost, thermoplastic starch is receiving a great deal of attention as a possible ingredient for disposable items. Starch, alone or with other ingredients, is now being used to manufacture packing peanuts (P. D. Tatarka, unpublished data). These packaging foams have excellent properties and now have captured 20– 25% of the market. Starch foams are generally manufactured with an extruder (Harper 1981, Harper and Tribelhorn 1992). Foaming of the thermoplastic starch by extrusion is achieved as the starch melt exits the extruder die. Superheated water flashes off in the form of steam and acts as a blowing agent for the starch or starch-based material. Even though these starch foams have excellent properties, they are difficult to shape into items such as cups, plates, bowls, and clam shells. There is another process for making starch foams wherein the items are both foamed and shaped in the same step during the manufacturing process (Tiefenbacher 1993, Haas et al 1994, Shogren et al 1997). The process, which is closely related to the baking of waffles and wafer cookies, is capable of manufacturing many different thin-walled materials. A batter of ungelatinized starch, alone or with other ingredients and water, is injected into heated molds. The temperatures of the heated molds are substantially higher than the gelatinization temperature of starch, usually in the range of 145– 225°C. During the heating of the batter, water vapor is released which both foams the batter and allows the foamed article to be dry when it exits the mold. Baking times vary with the ingredients (40–230 sec). The relationship between structure, morphology, and mechanical properties of these foams has been described (Shogren et al 1998a,b). These studies also investigated the morphology of the foams as they pertain to corn, wheat, tapioca, and potato starches. This article describes how changing the starch batter solids level affects the properties of the foam trays made from starches from different sources. MATERIALS AND METHODS Materials Normal potato starch was purchased from Avebe (Veendam, Holland). Amylopectin potato starch was a gift from Lyckeby Stärkelsen (Kristianstad, Sweden). Normal corn starch (Buffalo 1304) was purchased from CPC International (Englewood Cliffs, NJ). Amylopectin (waxy) corn starch (Amioca) was purchased from National Starch and Chemical Co. (Bridgewater, NJ). Tapioca starch was purchased from A. E. Staley Co. (Decator, IL). Wheat starch was purchased from Sigma Chemical Co. (St. Louis, MO). Guar gum and magnesium stearate were reagent-grade and purchased from Sigma. Pasting Profile Determination The pasting viscosity and temperature-time profile of the starches were determined using a Rapid ViscoAnalyser (model RVA-4, Foss North America, Inc., Eden Prairie, MN). The standard heating profile of the RVA software (Thermocline for Windows, ver. 2.0, Newport Scientific Pty. Ltd., Warriewood, NSW, Australia) was used to produce pasting curves based on 4 g (14% mb) of starch. The heating profile started at 50°C with stirring speed at 960 rpm for 10 sec, then stirring speed was reduced to 160 rpm for the duration of the procedure. Heating continued from 50 to 95°C in 3.7 min and was held at 95°C for 2.5 min. Then cooling commenced to 50°C in 3.8 min. Amylose Determination Amylose content of the starches were determined as described by Knutson (1986). Amylose content reported is the average of two analyses. Baking Starch, guar (1%, starch weight basis), and magnesium stearate (2%, starch weight basis) were mixed dry using a standard mixer (model KSM5, Kitchen Aide, St. Joseph, MI) with a wire whisk attachment. The material was dry-mixed on a low setting until the guar was dispersed into the starch. Guar was added to increase batter viscosity so the starch did not settle out of the batter. Magnesium stearate was added as a release agent so the trays would not stick to the mold. A batter was formed by adding water to the 1 Plant Polymer Research Unit, USDA, ARS, NCAUR, Peoria, IL. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. 2 Corresponding author. E-mail: [email protected] 3 Franz Haas Machinery of America, Richmond, VA Publication no. C-1999-0802-01R. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. American Association of Cereal Chemists, Inc., 1999.