Baking time increased from 140 s for trays containing no corn fiber to 190 s for trays containing 54.7% corn fiber (Table 2). The increase in baking time is probably due to the increase in batter needed to make a tray. This increase in batter would lead to more water having to be evaporated during baking. Baking time is also likely influenced by the amount of the corn fiber in the batter. Corn fiber is assumed to dry slower than the gelatinized porous foamed starch. Adding PVA to the batter slightly decreased the baking time needed to make a tray, except for trays containing equal amounts of corn fiber and starch. The increase in batter viscosity seen with increase in corn fiber also contributed to the increase in baking time. Increases in batter viscosity would slow the movement of water during drying part of the baking process. No differences were seen in the baking time for either of the PVAs used. Batters containing fiber with equal amounts of starch or less were able to be made into trays without cracks. Trays containing the greatest amount of fiber were very brittle and difficult to foam, probably because the high fiber content increases viscosity and resistance to expansion. Increasing fibers content of the batters increased batter viscosity and impeded the foaming process.
Thermogravimetric analysis was performed on raw materials and some representative trays to define thermal stability and to investigate interaction among the different components. Thermal characteristics are reported in Table 3. Fig. 1 shows the decomposition curves for PVA-PH, potato starch, and corn fiber. PVAs (PL, PH) and potato starch (St) have a similar decomposition onset temperature (290 °C), while corn fibers (CF) have a lower onset temperature (260 °C). Nevertheless, all these materials are suitable to be processed at 200 °C. After volatiles in the trays were lost, PVA and CF decomposition occurs in two steps while potato starch decomposition mostly occurs in one step with a decomposition peak placed at 303 °C. Trays composed of potato starch, PVA, and corn fiber show an intermediate onset temperature that attest to some interactions among the components. In particular for sample StPHCF100 (Fig. 2), the onset temperature is almost the same as potato starch and PH. The decomposition peak is placed at a temperature (319 °C) higher than CF alone (296 °C), and is the result of the overlaying of single decomposition peaks, thus suggesting positive interactions among the components. Tray decomposition starts at a temperature largely suitable for practical applications, such as fast food trays, also for trays with a large content of corn fibers.