Nutritional properties of tempe fermented whole-grain barley and oats. Influence of processing conditions on the retention and availability of iron, starch and folates.

Doktorsavhandling, 2008

This thesis aimed at evaluating the tempe fermentation process as a means to develop nutritionally improved vegetarian products from barley and oats. The work was designed to assess relationships between selection of raw material and processing conditions in correlation to nutrient content and availability. Whole grains of high amylose/high ß-glucan barley and high ß-glucan oats were tempe fermented with Rhizopus oligosporus, in order to: reduce phytate content to increase iron absorption, obtain low glycemic responses, and increase formation and accessibility of folates. The barley and oat tempe were evaluated in human studies. Corresponding meals were also in vitro digested in order to evaluate the potential of estimating human nutrient bioavailability in a computer controlled gastrointestinal model. Pearling, soaking and returning of pearling fractions before tempe fermentation was found to degrade the phytate content by 97% and to increase human iron absorption from 3.0% to 5.5%. In vitro digestion of similar meals showed an increase in iron availability from 1.7% to 4.9%, indicating that the tendency of the iron absorption could be predicted. The glucose response was lower from the barley tempe while the insulin response was lowest after intake of oat tempe. The calculated glycemic and insulinemic indices (GI and II) were 30 and 55 from barley tempe and 63 and 21 from oat tempe, respectively. In vitro starch digestion rate of barley and oat tempe meals were consistent with the human glycemic response, and by combining the determination of in vitro starch hydrolysis with microscopy studies of starch granules, a possible correlation between structural changes during digestion and the rate of starch hydrolysis was observed. Furthermore, tempe fermentation of barley increased folate content by 2.5 times, and the in vitro folate accessibility was 46-86% in boiled barley tempe. This work shows the nutritional potential of tempe fermented whole-grain cereal products with a range of health benefits, such aslow glycemic responses and high availability of iron and folate. The work also shows that the in vitro gastrointestinal model is a possible tool to estimate the availability of important nutrients.