Development of birch (Betula pendula Roth.) sap based probiotic fermented beverage

Birch sap is traditionally used as a refreshing beverage in the springtime in northern Europe. The aim of this study was to determine the suitability of birch sap for the growth of potentially probiotic Lactobacillus reuteri strains in order to develop a non-dairy functional beverage. All L. reuteri strains used in the study grew well in birch sap. pH values fell from an initial pH 6 to pH 4.20-3.18 characteristic for fermented products. Total acidity up to 36 T° and sufficient cell count was reached (6.79 cfu/ml). Glucose and fructose supplementation as well as their combination at a concentration of 0.5-1% did not significantly improve the growth of L. reuteri. Supplementation with 0.5-2% sucrose and a 2% glucose-fructose combination had a notable effect, although the latter had less effect than the former. Given that the viable cell count is the most important parameter of probiotic products, supplementation with sucrose was chosen as the best way to improve the substrate. The addition of sucrose stimulated biomass formation and improved acidification power, with the best results for sucrose 0.5-2%. Several other food grade supplements were evaluated to improve the growth of L. reuteri strains in 1% sucrose-supplemented birch sap. The best results were achieved using peppermint and malt extract supplements, which clearly indicate that L. reuteri growth in birch sap is limited not only by the availability of carbon but also by the availability of other growth factors present in the supplements used.