Cell wall modifications during cooking of potatoes and sweet potatoes

Sweet potato (Ipornoea batatas L) tissue, when cooked at 70 degrees C to activate P-amylase and break down starch, takes on a distinctive firm, brittle texture and does not show the cell separation that occurs in, for example, cooked potato (Solanum tuberosum L). Similar cooking conditions increase firmness in other plants by activating pectin methyl esterase which de-esterifies pectic polysaccharides and protects them from thermal depolymerisation. We therefore isolated cell walls from both potatoes and sweet potatoes cooked at 70 degrees C and 100 degrees C and determined the remaining degree of methyl esterification of their pectins. Pectins from both species were demethylated to a similar extent at: 70 degrees C and 100 degrees C. Since cooking sweet potato at 100 degrees C induced cell separation and softening, it is concluded that P-amylase is rapidly inactivated at that temperature and swollen starch distends and separates the cells, whereas the firm texture obtained by cooking that species at 70 degrees C is not the result of pectin demethylation but is caused by the breakdown of starch to oligomers that can escape from the cell, (C) 2000 Society of Chemical Industry.