EFFECTS OF EXTRUSION ON THE DYE-BINDING, FLUORODINITROBENZENE-REACTIVE AND TOTAL LYSINE CONTENT OF SOYBEAN-MEAL AND PEAS

Commercial soyabean meal (hexane extracted and toasted) and peas were processed with a single- and twin-screw extruder, respectively, to tes t the effects of initial moisture level and die product temperature on the reactive lysine contents. For soyabean meal, extrusion temperatur es ranged from 90 to 140-degrees-C with moisture levels of -25-40%. Ex trusion of peas was carried out at temperatures ranging from 105 to 14 0-degrees-C at moisture levels of 15-30%. Extruded and non-extruded sa mples were analysed for total lysine, and for reactive lysine, determi ned by the fluorodinitrobenzene (FDNB) method and the dye-binding lysi ne (DBL) method. Extrusion of soyabean meal at moderate temperatures ( 140-degrees-C) and high moisture levels (27-40%) resulted in a reduct ion in FDNB-reactive lysine ranging from 10 to 14%. Increased moisture levels in the soyabean meal resulted in slightly lower FDNB-reactive lysine values; the major decrease was caused by the processing tempera ture. Extrusion of peas gave a 16% reduction in FDNB-reactive lysine o nly when the temperature was 140-degrees-C and the moisture level was low (15%). Although there was a marked reduction in reactive lysine un its, caused by the heat treatment, this was not detected by total lysi ne analysis, while DBL analysis gave a significant correlation (r = 0. 97; P < 0.001) with the FDNB-reactive lysine procedure for the extrude d soyabean meal but not for the extruded peas. For unextruded samples there was close agreement between the total and FDNB-reactive lysine c ontent. Overall, and based on the present findings, it is concluded th at the FDNB method is useful for measuring the reduction in nutritiona l value after extrusion of soyabean meal and peas whereas the rapid DB L procedure gives inconsistent results. The extrusion of soyabean meal and peas at moderate temperatures can lead to loss of reactive lysine units.