Structure of pectin in relation to abnormal hardness after cooking in pre-peeled, cool-stored potatoes

Unacceptable hard tissue can be detected in pre-peeled potatoes after cooking. Pectin methyl esterase (PME) has been suspected to facilitate formation of Ca-bridges between pectin molecules by deesterification of the uronic acids. But this has also been questioned, due to the low storage temperature of the peeled tubers, optimum of PME being 50–70 °C. Hardness in tubers was induced by several packaging methods. Annual and seasonal variations were observed. In the work, analysis of nonstarch polysaccharides (NSP) are considered representative of the pectin content. Analyses revealed an increase in insoluble NSP during storage after peeling and cooking, and a decrease in branching (arabinose, galactose) and degree of methyl esterification of total NSP, in both fresh and cooked tissue, dependant on storage period. Reduction of steric hindrance by debranching of the pectin molecules as well as a PME-induced increase of possible Ca-cross-linking sites may favor hardening of the tissue. The microscopy does not show any pectin changes until after cooking, but the chemical data do. This suggests that hardening, at least partly, is due to the removal of methyl ester groups, as well as galactose and arabinose in the side chains. The Ca-bridges can probably not be formed until sufficient calcium is released from the starch during gelatination.