The reason why Lm increases so much more on addition of PVOH for waxy starch than normal cornstarch is unknown, but may reflect differing compatibilities of the polymers involved. Previous work has shown that amylopectin is less compatible with poly(ethylene-co-vinyl alcohol) than amylose (Simmons and Thomas, 1995 and Simmons and Thomas, 1998). Removal of starch from starch/PVOH 80/20 foam plates by microbial enzymes leaves a continuous, hollow framework of PVOH (Shogren et al., 1998a). Thus, the higher flexibility of waxy starch/PVOH foams may result from a morphology in which the stronger PVOH occupies the continuous phase and swollen starch granules serve as a filler. The thin cell walls in these foams are probably also an important factor.
This flexibility is critical for applications such as a flexible hinge region of a clamshell container that must flex without breaking. As a test of the suitability of the above formulations for use as sandwich containers, a hamburger and bun was placed between two foam plates consisting of waxy potato starch/PVOH/fiber/MSC 70/20/10/2 and microwaved for 30 s. The internal temperature of the hamburger reached 75 °C. After 10 min, the bottom plate was slightly soft under the hamburger, but as a whole, the container was still rigid and did not deform significantly. The weights of the plates increased from 19.4 to 22.2 g after the hamburger was removed.
In conclusion, the flexibility of starch foam plates can be improved and baking times decreased by incorporating chemically or genetically modified starches into the foam formulations. Strength, especially at low and high humidities can be improved by addition of fibers and water resistance is improved by adding monostearyl citrate. Although the plates we prepared are not adequate for applications such as hot drink containers, the range of uses are extended to applications where moist food contact occurs for short periods. The eventual goal is to have as short baking times as possible and no secondary coating steps to reduce process costs.
The reason why Lm increases so much more on addition of PVOH for waxy starch than normal cornstarch is unknown, but may reflect differing compatibilities of the polymers involved. Previous work has shown that amylopectin is less compatible with poly(ethylene-co-vinyl alcohol) than amylose (Simmons and Thomas, 1995 and Simmons and Thomas, 1998). Removal of starch from starch/PVOH 80/20 foam plates by microbial enzymes leaves a continuous, hollow framework of PVOH (Shogren et al., 1998a). Thus, the higher flexibility of waxy starch/PVOH foams may result from a morphology in which the stronger PVOH occupies the continuous phase and swollen starch granules serve as a filler. The thin cell walls in these foams are probably also an important factor.This flexibility is critical for applications such as a flexible hinge region of a clamshell container that must flex without breaking. As a test of the suitability of the above formulations for use as sandwich containers, a hamburger and bun was placed between two foam plates consisting of waxy potato starch/PVOH/fiber/MSC 70/20/10/2 and microwaved for 30 s. The internal temperature of the hamburger reached 75 °C. After 10 min, the bottom plate was slightly soft under the hamburger, but as a whole, the container was still rigid and did not deform significantly. The weights of the plates increased from 19.4 to 22.2 g after the hamburger was removed.In conclusion, the flexibility of starch foam plates can be improved and baking times decreased by incorporating chemically or genetically modified starches into the foam formulations. Strength, especially at low and high humidities can be improved by addition of fibers and water resistance is improved by adding monostearyl citrate. Although the plates we prepared are not adequate for applications such as hot drink containers, the range of uses are extended to applications where moist food contact occurs for short periods. The eventual goal is to have as short baking times as possible and no secondary coating steps to reduce process costs.
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