|KRALL, SANDRA - NCSU
Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/11/1998
Publication Date: N/A
Interpretive Summary: When we cook or can fruits and vegetables, they usually soften to some extent. Breakdown of the pectin structure in the cell walls is generally thought to be an important part of the reason softening occurs. However, plant cell walls are very complex structures, so it has been difficult to learn the specific chemical changes in pectin which cause softening. If we understood this process better, we would hope to improve our ability to control the texture of processed fruits and vegetables. This laboratory is concerned with keeping cucumbers, peppers, cabbage, and other commodities firm and crisp when they are fermented or pickled with acid. It has been assumed that the acid in these products could cause splitting of large pectin molecules into smaller pieces, and that this might be an important cause of softening in acid foods. This project looked at the effect of acid conditions and temperature on the rate pectin molecules split into smaller pieces. The major conclusion was that this type of chemical breakdown is too slow to cause the kind of softening we see when cucumber tissue is heated in acid. Therefore, this is not the most important reaction to cause softening of fruits and vegetables in acid conditions. This means that we have to look for other reactions of pectin or other cell wall components that cause plant tissues to soften when they are heated in acid.
Technical Abstract: Rates of pectin degradation were determined as a function of the degree of pectin methylation, pH, and temperature. Hydrolysis of pectin at pH 3, with the degree of methylation ranging from less than 5% to 70%, demonstrated that the rate decreased as the degree of methylation increased. This was opposite the effect of the degree of methylation on the rate of glycosidic bond degradation that occurs for the beta-elimination reaction. Degradation of polypectate and pectin with 35% and 70% methylation over the pH range of 2 to 6 indicated that acid hydrolysis was the dominant degradative reaction below pH 3.7, whereas the beta-elimination reaction was the major reaction at higher pH. The enthalpy and entropy of activation were determined for glycosidic bond hydrolysis of cucumber, apple, and citrus pectin samples in buffered 1.5 M NaCl at pH 3.0 by determining first-order rate constants over the range of 60 deg C to 100 deg C. The thermodynamic parameters obtained for pectin hydrolysis were very different from those obtained in similar conditions for non-enzymatic cucumber tissue softening. These results suggested that pectin hydrolysis is not the rate limiting step responsible for plant tissue softening in acid conditions. Also, 20 mM calcium ion had no significant effect upon the rate of pectin hydrolysis, which indicated that calcium does not inhibit tissue softening at low pH by inhibiting pectin hydrolysis.