MODELS FOR THE INTERACTIONS BETWEEN PECTIN MOLECULES AND OTHER CELL-WALL CONSTITUENTS IN VEGETABLE TISSUES

In experiments to study the relationships between textural change and chemical changes in pectins of vegetables after different cooking treatments, pectins in the alcohol-insoluble solids (AIS) of the vegetables were fractionated into four groups according to their solubility in different extractants, and their yields were correlated with the firmness of the tissues. Based on the correlations observed for various vegetables, a simple model consisting of five types of basic pectin interactions in the tissues was proposed first. It includes S-type pectin extractable with cold water, A-type pectin extractable with cold chelating-agent solutions, B-type pectin extractable with hot water, C-type pectin extractable with hot chelating-agent solutions, and P-type pectin extractable with hot dilute acid or alkali solutions, among which C-type pectin was obtained by calculation rather than direct determination. It was found later that, in some cases, the amount of C-type pectin thus obtained came out as a negative value, which is apparently unreasonable. We therefore proposed, by speculation, an elaborate model consisting of twelve sub-types of complicated pectin interactions, which are based on the simple model, in order to explain the unreasonable situation resulting from the calculated value of C-type pectin content. Using snap-bean pods as sample, we now present several experiments to support the real existence of some of the complicated pectin interactions previously proposed. The supporting data came from the analysis and comparison of the amounts of bound calcium in the pectin fractions extracted and from the measurement and comparison of the size of pectin molecules extracted before and after further treatment by either heating or calcium removal.