Relationships between the textural changes and the contents of calcium, magnesium ions, and non-freezing water in the alcohol-insoluble solids of snap bean pods during cooking processes

The alcohol-insoluble solids (AIS) of snap bean (Phaseolus vulgaris L.) pods after different cooking treatments were used as testing materials. The contents of calcium, magnesium ions and non-freezing water, which were analysed by differential scanning calorimetry, in the AIS were determined and compared with the firmness of the bean pod tissues after different cooking treatments. It was observed that the contents of calcium and magnesium ions in the AIS of the bean pods after direct cooking were lower than those of fresh and precooked bean pods, and were also lower than those of the bean pods after precooking followed by cooking. The contents of these metal ions were significantly and positively correlated to the tissue firmness (P < 0.01). The degree of esterification (DE) of pectin in the tissue after precooking was 39.3%. The DE of of pectin in the tissue after precooking followed by cooking was 39.9%. These DE values were significantly lower (P < 0.05) than that in the fresh tissue (46.0%) or that in the tissue after direct cooking (46.0%). Moreover, the amounts of non-freezing water in the AIS of fresh and precooked bean pods were larger than those of the bean pods after precooking followed by cooking and direct cooking. This also showed a significant and positive correlation to the tissue firmness (P < 0.05). It was apparent that the linkages between the pectin molecules in the tissues of snap bean pods after precooking were changed. This change was caused by the action of pectinesterase in de-esterification of pectin molecules and subsequent formation of calcium or magnesium bridges between the free carboxyl groups of adjacent pectin molecules, which resulted in increases in the amounts of nonfreezing water in the AIS and in the tissue firmness. These results can be taken as further supporting evidence for the theory of the firming effect of precooking treatment of vegetables.