PHYSICO-CHEMICAL AND PROTEIN QUALITY CHANGES IN CORN (ZEA MAYS) AND COWPEA (VIGNA UNGUICULATA) DURING STORAGE UNDER DIFFERENT CONDITIONS.

Abstract

This study was designed to investigate the changes in the protein quality and some physico-chemical properties of corn (Zea mays) and cowpea (V. unguiculata with respect to the storage time, temperature and water activity (AW). Whole flour of two cultivars each of corn and cowpea, 13 - 14% moisture, were stored in air-tight containers at 0°, 20° and 55°C. The protein quality was monitored by the protein digestibility (PD) and the changes in the physico-chemical properties by colour (Hunter L-values), non-enzymatic browning, pH, free amino nitrogen (FAN), soluble sugars (TSS) and titratable acidity (TA). The association between PD and non enzymatic browning was further investigated at 80°, 100° and 120°C, and 0.33, 0.55 and 0.75 water activity respectively. The changes in all the parameters increased with time. The rate constants increased with the storage temperature. The changes in the pH, TA, TSS and FAN correlated significantly (P< < 0.05) with the decrease in the protein digestibility of the samples at 20° and 55°C. The decrease in FAN and pH were most significantly associated with the decrease in PD in cowpea and corn respectively. The activation energy (Kcal/mole) for the decrease in PD increased from 5.79 to 17.75 for white corn, 8.2 to 11.6 for yellow corn, 5.5 to 8.0 for brown cowpea and 5.9 to 9.5 for white cowpea at Aw 0.33 to 0.75 respectively. Similarly, the activation energy for non-enzymatic browning ranged from 14.9 to 18.1 for yellow corn, 10.1 to 9.6 for white corn, 12.3 to 12.0 for white cowpea and 8.4 to 10.8 for brown cowpea at Aw 0.33 and 0.75 respectively. The PD of all the samples decreased as the non- enzymatic browning increased. The decrease in PD and increase in browning decreased as Aw tends to either zero or 1.0. Thus, the antinutritional effect of browning on protein digestibility is maximum at intermediate Aw. The decrease in PD after seven months was totally reversible in the samples stored at 0-20°c, but only partially reversible at 55°C. This coupled with the increase in the activation energy with increasing Aw demonstrated that the decrease in PD of corn and cowpea involves weak structural changes (reversible by boiling) followed by strong chemical changes. The changes in the TSS and FAN are compatible with non-enzymatic browning at high temperatures. Potential applications of the findings include the use of the temperature dependent rate equations to fix the shelf life of the samples, and the possibility of using FAN and pH as simple indices of protein quality deterioration in corn and cowpea during storage.