Heat processing of soybean kernel and its effect on lysine availability and protein solubility

Soybean kernels of cultivars Bosa and ZPS 015 were used in the experiment. The contents of available lysine as well as water and salt soluble proteins, were analysed in fresh soybean kernels, soybean products made after the processes of dry extrusion, micronisation, microwave toasting and autoclaving. Utilizing a technological procedure of processing, kernels were exposed to temperatures from 57 to 150°C. The duration of exposure of the soybean kernels to the increased temperatures, ranged from 25-30 seconds in dry extrusion to 30 minutes in autoclaving. All treatments were subjected to different sources of heat, causing different thermodynamic processes to take place in kernels and change their chemical composition; i.e. nutritive quality. The content of water and salt soluble proteins decreased under the influence of higher temperatures in the course of all treatments of processing. The drop of solubility already was drastically effected by temperatures of 100°C in dry extrusion, while there was a gradual decrease in other treatments. The content of available lysine was determined by the modified Carpenter methods with DNFB. The processes of micronisation and microwave toasting showed the greatest effect on the reduction of lysine availability. Dry extrusion and autoclaving, performed within closed systems — in which the increased moisture content has a special effect — resulted in significantly smaller changes of the available lysine content.