Hydrothermal Processing of Cereals Optimal Conditions for Phytete Degradation and Myo-Inositol Formation. Effect on Phytase Activity and Mineral Absorption.
The aim of this thesis was to optimise a hydrothermal process to degrade phytate (myo-inositol hexaphosphate), increase the level of free myo-inositol and preserve phytase activity in cereals with the purpose to achieve products with improved mineral bioavailability. Phytate is known to impair mineral absorption and myo-inositol is considered to be a semi-essential nutrient.
The hydrothermal process comprised of alternating wet and dry steeping steps. Lactic acid solution of different concentrations was used in the wet steeps. The temperatures during the wet and dry steeps and the concentration of lactic acid used in the wet steeps were optimised to degrade phytate and increase the amount of free myo-inositol during the process. The first wet steep was in addition optimised to inactivate lipoxygenase and lipase, enzymes causing lipid oxidation that might shorten the shelf life of the products, while preserving the phytase activity. The microstructural changes of the phytate globoids in barley were also studied during the hydrothermal processing to evaluate where in the kernels phytate degrades. Evaluation of zinc and calcium absorption from the optimised hydrothermal barley product was performed in humans.
With the use of a central composite experimental design we have identified optimal conditions for phytate degradation and free myo-inositol formation; in barley, wheat, rye and rice phytate was reduced by 94-99.8% and in barley free myo-inositol increased from 0.56 to 2.45 µmol/g d.m. It was not possible to preserve the phytase activity in barley during hydrothermal processing. The microstructural studies of barley showed the phytate degradation to be highest in scutellum and less in the aleurone layer. We found soaking conditions that completely inactivated lipoxygenase, reduced the activity of lipase with > 80% while preserved > 60% of the phytase activity. Zinc absorption improved from 11% to 25% when barley was hydrothermally processed and served as porridge (p < 0.001) and from 15% to 23% when barley was malted and served as muesli (p < 0.05). Calcium absorption improved from 19% to 23% when barley was soaked for 60 min at 55°C in 1% (v/v) lactic acid and served as muesli (p < 0.05).