Yeast biotechnology for nutritional improvements of fermented cereal-based foods - Phytate degradation and folate production

Sammanfattning: Anaemia is a serious public health problem in nearly all developing countries. It has negative consequences on cognitive and physical development of children, reduces work capacity in adults, and increases the risk of maternal and child mortality. Main risk factors include nutritional deficiency of iron, mainly due to poor absorption of iron from diets high in phytate, an anti-nutrient present in cereal-based diets; and consumption of a folate-deficient diet. Phytases, enzymes that degrade phytate, may be used in the preparation of such diets in order to improve the bioavailability of iron, and microorganisms present in fermented foods may produce folate. The work described in this thesis combines biotechnology with traditional processing methods in order to degrade phytate and increase folate content in African cereal-based foods. Yeast strains were isolated and identified from Tanzanian togwa, a natural fermented cereal-based food. The yeast strains were tested for phytase activity in yeast medium and further tested in a model togwa. In addition, selected yeast strains were screened for folate production. One yeast strain was finally selected for closer studies of production and secretion of phytase. Yeasts isolated from Tanzanian togwa were identified as Pichia burtonii, Issatchenkia orientalis/Candida krusei, Pichia anomala, Saccharomyces cerevisiae, Pichia guilliermondii, Hanseniaspora guilliermondii, Kluyveromyces marxianus, Candida glabrata and Pichia norvegensis. Strains of I. orientalis/C. krusei and H. guilliermondii exhibited extensive phytate degradation in a liquid phytase activity screening. The three strains with highest capacity to degrade phytate were investigated in a model togwa and fermentation with C. krusei TY13 or H. guilliermondii TY14 yielded a phytate reduction of 95% or more. Yeast fermentation increased the folate content in model togwa by 85-97%. Finally, the C. krusei TY13 strain was discovered to secrete phytase to the culture medium during growth. In conclusion, the results show that phytate present in model togwa can be substantially degraded by phytase-producing yeasts and that the folate content increased depending on the yeast strain used. Such yeast varieties have a potential to improve the mineral bioavailability and increase the folate content of fermented cereal-based foods prepared for use in developing countries.