Factors Influencing Phytate Degradation in Piglets. Feed phytate behaviour and degradation by microbial phytases

Doctoral thesis, 2009

Phytate, the main storage form of phosphorus (P) in plant feedstuffs, is indigestible for monogastric animals, and phytases are therefore used as feed additives to increase the digestibility of phytate-P and limit the inclusion of inorganic P. However, new and improved phytases are needed to obtain better phytate digestibility and allow for lower inclusions of inorganic P. The overall aim was to identify factors influencing phytate degradation in piglets as a means to develop improved phytases. The main part of the work therefore focussed on studying the in vitro and in vivo behaviour of feed phytate (soy/maize) and its degradation by microbial phytases (P. lycii and an experimental bacterial phytase). Pelleting and extrusion cooking only slightly affected the composition of inositol phosphates in feedstuffs. Phytate in soybean and maize meal precipitated with protein at pH below 4.0 and with minerals at pH above 4.5, and calcium critically increased the formation of phytate-mineral complexes. Both phytases demonstrated mainly 6-phytase activity when using Na-phytate as substrate in a simple buffered assay, but they showed different degradation pathways. Interestingly the bacterial phytase degraded InsP6 to InsP4 faster than the P. lycii phytase. During simulated digestion of a soybean-maize meal blend both phytases degraded phytate efficiently. The main difference also in this system was that the bacterial phytase was faster than the P. lycii phytase at degrading InsP6 to InsP4. In piglets, the bacterial phytase demonstrated double phytate degrading activity per unit of enzyme compared to the P. lycii phytase mainly due to superior survival during gastric digestion. Higher solubility of InsP4-InsP3 compared to InsP6-InsP5, together with the fact that InsP3-InsP5 was degraded in the small intestine, indicated that the bacterial phytase may also have an advantage of quickly producing InsP4. A high rate of gastric emptying was found to be limiting for phytate degradation in vivo, but supplying 50,000 FTU kg-1 DM partly overcame this limitation. Phosphorus was absorbed in the small intestine, but potential absorption of inositol phosphates measured as plasma InsP level could not be detected. Overall this work shows that in the search for new phytases, it is important to evaluate phytate degradation at the right conditions. In vitro assays should therefore carefully be adjusted with respect to e.g. substrate, pH, digestive proteases and mineral and protein content. Gastric stability and the speed at which certain inositol phosphates are degraded were found to be important factors for the efficacy of a phytase in vivo. However, a high rate of gastric emptying and the dosage of phytase seem to be the main determining factors for phytate degradation in piglets.