EFFECT OF OXIDATIVE COUPLING ON THE THERMAL-STABILITY OF TEXTURE AND CELL-WALL CHEMISTRY OF BEET BOOT (BETA-VULGARIS)

The purpose of this study was to investigate whether peroxidase-mediat ed oxidative coupling of ferulic acid in beet root cell walls could re duce thermally induced softening by enhancing the crosslinking of poly mers involved in cell adhesion. Beet root (Beta vulgaris, var. Detroit 2 crimson) tissue was incubated in the presence or absence of hydroge n peroxide (H2O2) for 18 h before cooking; Incubation in H2O2 increase d the time required for tissue to soften at 100 degrees C from an aver age of 130 min to 650 min and resulted in a significantly higher tensi le strength in heat-treated tissues (P < 0.05). This was accompanied b y a large decrease in esterified cis- and trans-ferulic acid and a 2-f old increase (approximately) in 5,5'-, 8-O-4'-, and 5,8'-(benzofuran f orm)-diferulic acid moieties. In addition, the yield of hot-water-solu ble wall polymers was much lower, consistent with increased cross-link ing of the pectic polysaccharides. However, the carbohydrate compositi on and degree of uronide methylesterification of cell walls from H2O2- treated tissues was similar to that of fresh tissue. Incubation of con trol samples in the absence of H2O2 had no effect on thermal softening or phenolic chemistry of the walls. These results indicate that the H 2O2-mediated changes in mechanical properties of the walls and the rat e of thermal softening result from enhanced phenolic cross-linking of pectic polymers involved in cell-cell adhesion, due to oxidative coupl ing of ferulic acid moieties.