On Folates in Yeasts -Exploring Yeasts as Vehicles for Biofortification of Folates in Food

Sammanfattning: High intake of the B vitamin folate during pregnancy is known to decrease the risk for development of neural tube defects. In addition, a good folate status may prevent the progression of several diseases such as megaloblastic anaemia, cardiovascular disease, cancer and Alzheimer’s disease. However, reaching sufficient amounts via the diet only is complicated and new strategies must be developed to increase the mean folate intake. The aim of this thesis is to explore the use of yeasts for biofortification of folates in food. Yeasts synthesise folates de novo and may constitute potent vehicles for natural folates. Current work includes investigations of folate content and composition in yeasts as well as studies of the impact of growth rate and chemical environments on folate levels in Saccharomyces cerevisiae (Baker’s yeast). The applicability of the results was then investigated in two food model systems (bread and a cereal-based fermented porridge called togwa) to demonstrate the practical relevance. In a screening of 52 yeast strains, the folate content ranged from 40 to 145 µg per gram dry matter of yeast, showing that inherent variations in yeast folate levels exist. Several S. cerevisiae strains showed at least a two-fold higher folate content than a commercial Baker’s strain, indicating possibilities to increase folates in fermented foods simply by choosing the proper strain. With regard to cultivation conditions, folates in S. cerevisiae were demonstrated to increase linearly with growth rate in a defined medium, showing that high growth rate, i.e. respiro-fermentative metabolism, is most favourable for high folate content. Cultivation in a rich medium yielded cells containing lower amounts of folate compared to cells grown in a poor chemical environment even though the specific growth rate was higher in the rich medium. The results obtained show that the growth rate per se could not predict intracellular folate concentrations and that the chemical environment has a substantial impact on intracellular folates. By applying strain selection and cultivation at optimal conditions prior to baking we were able to increase folates five-fold in white wheat bread compared to bread leavened with commercial Baker’s yeast. In addition, folates increased at most 23-fold during yeast fermentations in a cereal-based porridge (togwa) compared to togwa raw material. Final folate levels were shown to be highly dependent on strain and cultivation time. The results demonstrate that yeasts constitute flexible and controllable vehicles for biofortification of folates in fermented food.