Bioprocessing of Oats Influence on Phytate Hydrolysis and Mineral Bioavailability

Doctoral thesis, 1994

The effects of bioprocessing on enzymatic phytate hydrolysis in oats were studied during malting, soaking and breadmaking to elucidate the reasons for poor phytase activity in oats and determine the optimal conditions for phytate hydrolysis. Malting of oats in a pilot plant was studied to investigate whether the malting process developed in the laboratory could be used in large scale preparation of oats with reduced phytate content. The effects of malting on zinc and iron absorption from single meals were examined in humans using a radioisotope technique. The nutritional significance of cereal processing and high dietary phytate intake was also investigated. Optimal conditions for oat phytase was found at pH 4 - 4.5 and 38°C, which differs from those of wheat, rye and barley. Combined malting and soaking reduced the phytate content of oats to very low levels (99% reduction). It was demonstrated that the malting process developed in the laboratory was also applicable on a large scale. In the pilot plant, malting of oats was performed at 15°C for 5 days, which were the conditions found to be optimal for achieving satisfactory phytate reduction. The intracellular location of oat storage proteins is different from that of other cereals, and interactions between phytate and proteins may partly explain the resistance of oat phytate to phytase attack. It is possible that the increased phytate hydrolysis following germination is the result of structural and chemical changes occurring in the oat seed during steeping and germination. Fluorescence microscopy was used to study the microstructure of malted and ungerminated oats. The micrographs indicated differences in phytate location and structure of malted oats when compared with ungerminated oats. The addition of 10% sour dough during rye breadmaking increased the phytate hydrolysis to 97% at pH 4.6 in doughs and bread. The combination of scalding and adding 20 and 30% rye sour dough to breads in which 30% of the wheat flour had been exchanged for oat flour resulted in a phytate reduction of 95-96% at pH values between 4.3 and 4.6 in doughs and bread. The absorption of zinc from oat porridge prepared using malted and soaked oats (phytate content reduced by 77%) doubled the amount of zinc absorbed over the absorption from oat porridge made with untreated oats. The iron absorption was 47% higher from malted oat porridge than from the control porridge. The iron stores in a group of pregnant Pakistani women were significantly lower than in a group of pregnant Norwegian women. The dietary intake of heme and nonheme iron as well as factors known to inhibit iron absorption, except for phytate, did not differ between the groups. The dietary phytate intake was considerably higher in the Pakistani group, owing to the consumption of unleavened Chapati bread, thus suggesting the importance of using efficient food processes to improve the iron bioavailability in cereals.