Evaluation of the effect of malt, wheat and barley extracts on the viability of potentially probiotic lactic acid bacteria under acidic conditions.

In this work, the effect of cereal extracts, used as delivery vehicles for potentially probiotic lactic acid bacteria (LAB), on the acid tolerance of the cells was evaluated under conditions that simulate the gastric tract. More specifically, the effect of malt, barley and wheat extracts on the viability of Lactobacillus plantarum, Lactobacillus acidophilus and Lactobacillus reuteri during exposure for 4 h in a phosphate buffer acidified at pH 2.5 was investigated. In the absence of cereal extracts all strains demonstrated a significant reduction in their cell population, particularly L. plantarum. The viability of L. plantarum was improved by approximately 4 log(10) cycles in the presence of malt and 3 log(10) cycles in the presence of wheat and barley. The survival of L. acidophilus and L. reuteri was increased by more than 1.5 and 0.7 log(10) cycle, respectively, upon addition of cereal extracts. In order to evaluate the contribution of the cereal constituents on cell survival, the individual effect of glucose, maltose and free amino nitrogen (FAN), which were added at concentrations that correlated to the reducing sugar and FAN content of the cereal extracts, was examined. The viability of L. plantarum was progressively improved as the maltose or glucose concentration increased; an increase by approximately 2 log(10) cycles was observed in the presence of 8.33 g/l sugar. The survival of L. acidophilus increased by more than 1 log(10) cycle, even at very low concentrations of maltose and glucose (e.g., 0.67 g/l), while L. reuteri stability was enhanced in the presence of maltose but no appreciable effect was demonstrated in the presence of glucose. Sugar analysis indicated that glycolysis was inhibited in all cases. Addition of tryptone and yeast extract, used as sources of FAN, enhanced L. acidophilus acid tolerance, but did not affect L. reuteri and L. plantarum. The results presented in this study indicate that malt, wheat and barley extracts exhibit a significant protective effect on the viability of L. plantarum, L. acidophilus and L. reuteri under acidic conditions, which could be mainly attributed to the amount of sugar present in the cereal extracts.