Firming of cooked sweet potatoes as affected by alum treatment

Sweet potato slices were cooked for 5 min in distilled water, alum (aluminu m potassium sulfate), AlCl3 and several pH level buffer solutions. Slices p reheated (60 degrees C) in these solutions were boiled in water. The slices cooked at about pH 4 were firmest among those in the range pH 2-pH 12. Fir mness was (greatest to least); preheated in alum > preheated in water > coo ked in buffer (pH 3.5) > AlCl3 (pH 3.5) > alum (pH 3.5) > water. Preheating , Al3+ and acid prevented softening of slices. Acid-treated slices were fir mer than Al3+ treated slices. Alum contains K+ and S0(4)(-2), which acceler ate solubilization of pectin, thus slices cooked in alum were softer than t hose in AlCl3. Scanning electron microscopy showed the middle lamella separ ated when cooked in water but not when cooked in other solutions.