Cell wall modifications during cooking of potatoes and sweet potatoes

Sweet potato (Ipornoea batatas L) tissue, when cooked at 70 degrees C to ac tivate P-amylase and break down starch, takes on a distinctive firm, brittl e 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-este rifies pectic polysaccharides and protects them from thermal depolymerisati on. We therefore isolated cell walls from both potatoes and sweet potatoes cooked at 70 degrees C and 100 degrees C and determined the remaining degre e 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 softenin g, it is concluded that P-amylase is rapidly inactivated at that temperatur e and swollen starch distends and separates the cells, whereas the firm tex ture 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.