Abstract
Sweet potato slices were cooked for 5 min in distilled water, alum (aluminum potassium sulfate), AlCl3, NaCl and buffer solutions at several pH levels. Slices preheated (60°C)or pressurized(686MPa) were also boiled in water. The slices cooked at pH3∼pH4 were firmest among those in the range pH2∼pH12. When slices were cooked in various solutions at the same pH level (pH3.3), firmness was (greatest to least); cooked in alum buffer≥buffer>AlCl3>alum solutions. Firmness was (greatest to least); preheated>raw>ressurized>preheated then cooked>pressurized then cooked>cooked in alum solution>cooked in water>cooked in NaCl solution. A clear relationship was found between firmness and the amount of pectin remaining in tissues except slices cooked in alum solution. Scanning electron microscopy showed the middle lamella separated when cooked in water, NaCl, or buffer solutions of pH2 and pH8, but not when cooked in water after preheating or pressurization, in buffer solution (pH4), or solutions with aluminum ion. Starch grains were not swollen even after preheating for 2h at 60°C, but when pressurized for 1h at 686MPa, they were swollen and half-gelled. Acid, preheating, pressurization and Al3+ prevented softening of slices.
