Cereal- and Microbial Phytases. Phytate Degradation, Mineral Binding and Absorption
Cereal foods contain large amounts of phytate (myo-inositol hexaphosphate; IP6), which is the major storage form of phosphorus, in the plant. IP6 possesses a high potential for chelating minerals, such as Fe 2+, Zn2+, Mg2+ and Ca2+. This is of nutritional importance owing to the negative effect on the bioavailability of essential elements from the food. During food processing, IP6 can be completely or partly degraded. Thus, processed foods may contain a mixture of different inositol phosphates, with 6 or fewer phosphate groups, IP6-IP1.
This thesis deals with IP6, its degradation during breadmaking, the mineral binding capacity of its degradation products and the effect of cereal- and microbial phytases on IP6 hydrolysis and dietary iron absorption in humans.
The binding of metal ions (Cu2+, Zn2+ and Cd2+) to isolated fractions of inositol phosphates (IP6-IP3) was investigated at pH 3-7. All inositol phosphate fractions studied had a pronounced binding capacity at pH 5 - 7. This indicates that mineral complex formation with lower inositol phosphates (IP4, IP3) may occur in the duodenum, which is important from a nutritional point of view if these complexes inhibit mineral absorption. Phytases are phosphatases, capable of hydrolyzing IPV, thus producing inositol phosphates with a lower degree of phosphorylation. The effects of cereal (wheat) and microbial (Aspergillus niger and Saccaromyces cerevisiae) phytases on IP6 hydrolysis were studied in wheat during breadmaking. Activation of endogenous wheat phytase, the addition of Baker's yeast (S. cerevisiae) and, in particular, phytase from A. niger reduced IP6 during breadmaking.
A specific method for determining phytase activity by measuring the formation IP5 was developed. As different phytases produce different isomers of IP5, this method can also be used to distinguish between phytases of different origins.
Commercial strains of S. cerevisiae proved able to develop phytase activity, i.e. to grow on a synthetic medium with IP6 as the only source of phosphorus. Both strains studied expressed a metabolism well adapted for utilization of phosphorus from IP6. The phytases proved to be of the 3-phytase type, yielding DL-Ins(1,2,4,5,6)P5, DL-Ins(1,2,5,6)P4 and DL-Ins(1,2,6)P3 as degradation products.
The potential of wheat phytase and A. niger phytase to improve iron absorption from a meal with phytate was studied in humans using a radioisotope technique. When added to the meal at consumption, endogenous wheat phytase had no effect but A. niger phytase was found to markedly increase iron absorption.