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