Relationship between Maceration and Pectic Change of Japanese Radish Roots during Cooking

Abstract

The relationship between the firming up of Japanese radish roots by preheating at 60 t or drying them at room temperature and pectic change was investigated. The results obtained were as follows :
1) The pectins of Japanese radish roots' tissues were extracted successively with diluted HCl, acetate buffer, and sodium hexametaphosphate solutions. The amount of HCl-soluble pectin (fraction A, the high-methoxyl pectin) decreased markedly, while that of acetate buffer-soluble pectin (fraction B, the low methoxyl pectin) increased significantly by preheating. A similar tendency was observed with the dried tissues. The degree of esterification and specific viscosity of both fraction pectins decreased by drying and preheating.
2) The activity of pectin methylesterase (PME) continued to be considerably active even after preheating or drying. There was also exo-polygalacturonase (PG) present in Japanese radish roots. The decrease of esterification degree was caused by PME, and that of specific viscosity was caused by PG.
3) The amount of pectins released into the cooking solutions was as follows : raw tissue >dried>preheated, respectively. The breakdown of pectins within the roots'raw tissues by β-elimination proceeded more rapidly than the others. A large amount of free methanol was released into the cooking solution during preheating.
The main factors which caused the firming of tissues within the Japanese radish roots through preheating or drying were : the demethoxylation of pectin, catalyzed by PME, inhibited the degradation of pectin caused by the β-elimination reaction during cooking, and as a result of this demethoxylation free carboxyl groups increased, which formed a rigid pectin gel with Ca²⁺and Mg²⁺.