Modeling the thermal inactivation of phytase in steamed cowpea seeds

Cowpeas at moisture content of 20 g water/100 g were steamed at 100 or 121 °C for 0–900 s. Steamed seeds were milled whole or fractioned into three layers (inner, middle, outer) using a customized drill. Residual phytase activity was assayed by measuring the amount of inorganic phosphate produced using an external source of phytate. A combined kinetic and heat/mass transfer model was employed to predict phytase inactivation in the three layers. Seeds milled whole showed residual phytase activity between 59.5 and 63.4%, which was generally lower than the activity measured in the layers (60.3–81.4%) for the same treatment combination. Phytase was not uniformly inactivated throughout the seed but was spatially dependent. The measured residual activity in the outer layer (66%) was lower than in the middle (73%) and inner (75%) layers after 900 s steaming. The combined model predicted the spatial distribution of residual phytase activity at any given time. Predicted inactivation values were lower than experimental values. The average relative error for phytase inactivation was 5.8%, 12.3% and 14.2% respectively for outer, middle and inner layers. Predicted and measured residual activities were closer in the outer layer as compared to the inner and middle layers.