Effect of heat-treatment, phytase, xylanase and soaking time on inositol phosphate degradation in vitro in wheat, soybean meal and rapeseed cake

Abstract

An in vitro method was used to evaluate the degradation of myo-inositol hexakisphosphate (InsP 6) in non-heat-treated wheat (NHW), heat-treated wheat (HW), soybean meal (SBM) or rapeseed cake (RSC) soaked separately or in combination. The feedstuffs were soaked in water (20 °C) and samples were collected after 2, 4, 8 and 24 h. The following hypotheses were tested: (1) phytase addition to soaked NHW or HW improves InsP 6 degradation, (2) phytase and xylanase addition to soaked NHW or HW improves InsP 6 degradation further compared solely with phytase, (3) phytase addition to SBM or RSC improves InsP 6 degradation, and (4) soaking of SBM or RSM together with NHW (ratio 1:1) stimulates InsP 6 degradation from SBM or RSC due to the activity of wheat phytases, whereby phytase addition results in no further InsP 6 degradation. Additionally, degradation of lower inositol phosphates ( myo-inositol pentakisphosphate– myo-inositol bisphosphate or InsP 5–InsP 2) was studied to verify if InsP 6 degradation leads to an accumulation of InsP 5–InsP 2. In NHW or HW addition of phytase alone or with xylanase had no significant effect on InsP 6 degradation. However, InsP 6 degradation was influenced by the interaction between heat-treatment and soaking time (P≤0.001). This was mainly due to a smaller proportion of non-degraded InsP 6-P at 24 h in HW compared with NHW (0.13 vs. 0.47) (P≤0.001) possibly caused by structural changes imposed by the heat-treatment. In SBM, RSC, SBM/NHW or RSC/NHW, the InsP 6 degradation was affected by the interaction between phytase addition and soaking time (P≤0.001) as phytase reduced the proportion of non-degraded InsP 6-P at 2, 4, 8 or 24 h. Soaking of NHW, SBM or RSC (without phytase) separately resulted in a limited InsP 6 degradation, whereas a pronounced InsP 6 degradation occurred when RSC or SBM were soaked together with NHW (without phytase) because wheat phytases degraded InsP 6 in SBM or RSC. However, addition of phytase to SBM/NHW or RSC/NHW increased further the degradation of InsP 6 indicating that endogenous wheat phytases were not sufficient. The formation of InsP 5–InsP 2-P was very small (40–180 μg/kg DM) during the degradation of InsP 6 indicating a rapid and almost complete degradation of these compounds. In conclusion, the effect of phytase on InsP 6 degradation during soaking depends on the feedstuff and processing of the feedstuff. InsP 6 degradation results in a negligible accumulation of InsP 5–InsP 2. Soaking of plant feedstuffs prior to feeding of pigs may improve P digestibility compared with dry feeding, but studies with pigs are required to clarify this.