Malting of Barley for healthy foods. Optimal conditions for phytate removal and preservation of ?-glucan, in vitro iron availability and degradation of phytate and ?-glucan during in vitro digestion
Doctoral thesis, 2005
A high concentration of phytate (myo-inositol hexaphosphate) in cereals is known to impair absorption of dietary minerals. The aim of this work was to optimize a malting procedure to degrade phytate and/or increase phytase activity in barley to improve iron availability and to preserve the content of β-glucan, since β-glucan is associated with health promoting properties and is therefore of nutritional interest. Moreover, the capacity of a high-phytase yeast strain to hydrolyze phytate was investigated in an in vitro digestion model.
The effect of different steeping and germination conditions was studied in a naked and a covered barley. Steeping at 48°C gave small changes in the content and molecular weight of β-glucan. By an addition of lactic acid (LA), this was combined with a strong phytate reduction (?70%) but a low phytase activity. Steeping at 15°C with LA rendered a higher phytase activity (>60% of raw material), a lower phytate reduction and a somewhat delayed the development of β-glucanase activity. With a subsequent soaking step at 48°C and porridge preparation, phytate was virtually completely removed. The molecular weight of β-glucan was slightly changed only in porridge made from malt prepared by steeping at 15°C with LA. Simulated digestion in a dynamic gastrointestinal model revealed a higher iron dialyzability of malt porridges (7.9 and 5.9%) compared to the reference porridge (0.4%). Accordingly, iron absorption by Caco-2 cells was higher from dialysates collected during digestion of barley malt porridges. In all porridges, β-glucan was continually degraded during digestion but was best preserved in the porridge made from malt prepared by steeping at 48°C with LA.
A high-phytase Saccharomyces cerevisiae strain was studied at a simulated gastric pH gradient and intestinal pH values with digestive enzymes using growth medium and wheat gruel as model meals. A cell density corresponding to 1 mg d.m. per ml resulted in a degradation of extracellular phytate up to 60% during the early gastric phase. The acid conditions in the late gastric phase limited further degradation in the intestinal phase.
The work shows the potential to improve iron availability in barley and still limit the degradation of polysaccharides during malting, soaking and digestion. It also shows that modified yeast is a promising gastrointestinal carrier of phytase activity.
in vitro digestion