THE ROLE OF PHYTASE AND LIGNIN IN DECORTICATED DRY BEAN (PHASEOLUS-VULGARIS) HARDENING DURING STORAGE

The lignification-like mechanism and that involving phytic acid degrad ation by phytase were evaluated in the hard-to-cook phenomenon of deco rticated Malawian white and red bean genotypes (Phaseolus vulgaris). S amples were stored under various temperatures (16C; 35C) a(w); (0. 55 a(w); 0. 85 a(w)) and time periods (4 and 8 months) compared to the co ntrol group (2C; 0.30 a(w); zero months). Phytase activities (Pi), phy tic acid, calcium and magnesium ions, water soluble pectic substances and lignin concentrations were determined spectrophotometrically. Elev ated phytase activities and slight, but nonsignificant increase in lig nin levels were produced in both bean genotypes maintained under adver se storage conditions for extended time periods. Positive correlations between phytase activities and cooked white bean hardness for the 4 m onth (r2 = 0. 844) (p less-than-or-equal-to 0. 01) and 8 month (r2 = 0 . 689) storage periods were found. Lignin content of red beans was sig nificantly (p less-than-or-equal-to 0. 01) correlated (r2 = 0. 669) wi th hardness for the extended (8 month) storage period. However, no cle ar relationships were found between lignin levels and the 4 month stor ed red (r2 = 0. 232) and white (r2 = 0.210) bean hardness, and between lignin concentrations and the 8 month (r2 = 0.232) stored white bean hardness. The mechanism involving phytic acid degradation appeared to be the dominant system influencing the hard-to-cook defect in the whit e, and to some extent the red bean genotypes for the storage period 0- 8 months.