Effects of dry matter content on trypsin inhibitors and urease activity in heat treated soya beans fed to weaned piglets

A nutrition study was conducted to evaluate the growth response of weaned piglets fed diets containing soya beans that had been processed into protein supplements at two different levels of dry matter (DM) and temperature. Four diets contained protein supplements prepared from whole full-fat soya beans equilibrated at 800 or 900 g kg−1 DM prior to being heated to 110 or 125°C. An additional diet contained a protein supplement prepared from raw whole full-fat soya beans at 900 g kg−1 DM; i.e. an unheated soya bean protein supplement. The experimental control diet was supplemented with a solvent extracted commercially processed soya bean meal (900 g kg−1 DM) containing 480 g crude protein kg−1. Soya beans at 900 g kg−1 DM prior to heat treatment at 110°C produced protein supplements, after heat treatment, that had higher residual levels of trypsin inhibitors and urease activity than measured in soya beans at 800 g kg−1 DM prior to the same heat treatment. The moisture content of soya beans prior to heat treatment affected the level of heat necessary to lower values for trypsin inhibitors and urease activity. Soya beans at 800 g kg−1 DM prior to heating at 110°C, produced a protein supplement with similar residual concentrations of trypsin inhibitors and urease activity to soya beans at 900 g kg−1 DM prior to heating at 125°C. This observation suggested that the soya beans with higher moisture content required lower heat energy to inactivate trypsin inhibitors and urease. The pen unit response of piglets fed the diet containing the soya bean protein supplement prepared from soybeans processed at 900 g kg−1 DM and heated to 110°C was not improved when compared to piglets fed the diet containing the unheated soya bean protein supplement. Soya beans at 800 or 900 g kg−1 DM prior to heating to 110°C, or soybeans at 900 g kg−1 DM heated to 125°C, produced protein supplements that were inadequately heat processed as indicated by the values for residual trypsin inhibitors and urease activity, and, the depressed pen unit response of piglets when compared to those fed the control diet. In contrast, piglets fed the protein supplement prepared from soya beans at 800 g kg−1 DM prior to heating to 125°C, displayed an average daily feed intake and feed-to-gain ratio that did not differ significantly from piglets fed the control diet. These data indicate that when whole full-fat soya beans were processed by the dry roasting method, their initial DM content of 800 or 900 g kg−1 affected the processing temperature necessary to denature or otherwise inactivate inherent trypsin inhibitors and urease activity.