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

The lignification-like mechanism and that involving phytic acid degradation by phytase were evaluated in the hard-to-cook phenomenon of decorticated Malawian white and red bean genotypes (Phaseolus vulgaris). Samples 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 control group (2C; 0.30 a(w); zero months). Phytase activities (Pi), phytic acid, calcium and magnesium ions, water soluble pectic substances and lignin concentrations were determined spectrophotometrically. Elevated phytase activities and slight, but nonsignificant increase in lignin levels were produced in both bean genotypes maintained under adverse storage conditions for extended time periods. Positive correlations between phytase activities and cooked white bean hardness for the 4 month (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 significantly (p less-than-or-equal-to 0. 01) correlated (r2 = 0. 669) with hardness for the extended (8 month) storage period. However, no clear relationships were found between lignin levels and the 4 month stored 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 white, and to some extent the red bean genotypes for the storage period 0-8 months.