Effects of Various Salts on the Softening of Cooked Japanese Radish Roots and on the Fine Structure of the Parenchyma Cell Wall after Cooking

Abstract

The effects of salts on the softening of cooked vegetables were investigated, and the softening mechanism was elucidated by means of light microscopy and quick-freeze, deep-etching electron microscopy. Disks cut from the xylary tissue of Japanese radish roots were cooked for 30 min in 0.2 M solutions of nine salts (acetates, sulfates and chlorides of Na⁺, Mg²⁺ and Ca²⁺, respectively). The firmness of the disks varied after cooking in both kinds of cations and anions, the degree of firmness scaled from highest to lowest being as follows : Ca²⁺>Mg²⁺>Na⁺, and chlo-rides>sulfates>acetates. The presence of both Ca²⁺ and Cl^- in the cooking solution increased the firmness. In the tissue of the greatly softened samples, separation of the middle lamella was distinctly observed, and the fine granular substances cementing cellulose microfibrils were markedly decreased in the primary cell wall. In the walls treated with CaCl₂, however, the granular substances were preserved well, which suppressed softening of the tissues. Ca²⁺ and Mg²⁺ displayed a different effect on the softening of the vegetables and on the fine structure of the cell walls.