Total esterified polyuronide in plant tissue can be assayed by titrating the uronic acid liberated from the alcohol‐insoluble fractions of the tissue by alkaline hydrolysis. The total polyuronide can be determined in a similar manner if the carboxyl groups are initially esterified with methanolic hydrochloric acid. Comparison of the two results gives the degree of esterification of the total polyuronide. The divalent metals present in the same alcohol‐insoluble fractions are assumed to be bound to the uronide groups and their assay gives a measure of the extent of interchain cross‐linking by metal salt formation. Application of these procedures to samples from different parts of potato tubers showed that polyuronide generally had a low degree of esterification (about 40%) and was present in higher concentrations (about 4.6% a.i.s.) in the cortex and periderm than in the interior tissues (about 2.7% a.i.s.). The degree of crosslinking due to calcium and magnesium ions is lowest in the outer layers of the potato tuber and may be the reason that sloughing occurs in this area.
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Warren et al. (1973) studied this question.
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