Kinetics of protein quality change in extruded cowpea:corn flour under varied steady-state storage conditions
An examination of the affects of varied steady-state storage conditions on the protein quality of a low-cost extruder cooker (LEC) processed 70:30 (w/w %) blend of cowpea (Vigna sinensis) and corn flour was carried out. Samples were equilibrated at water activity (a(,w)) values of 0.44, 0.55, and 0.65 with the appropriate saturated salt solutions, and they were stored at 25, 35, 45 or 55(DEGREES)C for 15-21 week storage periods. Losses of 1-fluoro-2,4-dinitrobenzene (FDNB) reactive lysine, and pigment production due to non-enzymatic browning were followed to determine kinetic parameters of protein quality change. In addition, end-point changes in in vitro protein digestibility due to the storage conditions were determined. First order reaction rate constants for FDNB-reactive lysine loss ranged from 8.9 x 10('-3) wks('-1) for storage at 25(DEGREES)C and a(,w) of 0.55 to 25.9 x 10('-3) wks('-1) for storage at 55(DEGREES)C and a(,w) of 0.65. Energies of activation (E(,a)) were 4.4, 4.9, and 6.0 kcal/mole for a(,w)'s of 0.44, 0.55, and 0.65, respectively. Q(,10) values ranged from 1.0 to 1.5;Change in temperature demonstrated a greater impact of kinetic parameters in this food system than did change in a(,w). Parameters of temperature dependence for the FDNB-lysine loss reaction were low in this food system. Measureable non-enzymatic browning pigment was found only in samples stored at the highest temperature and a(,w) (55(DEGREES)C and 0.65) used in the experiment;Potential implications for storage and processing of food products of this type in order to minimize protein nutritional quality changes due to Maillard browning reactions are discussed.