A rapid screening method to evaluate acidifying activity by lactic acid bacteria

The determination of pH in fermented milk is an important parameter for monitoring the production of acid by lactic acid bacteria (LAB). In this work, a colorimetric method is proposed that allows a fast determination of LAB acidification ability by evaluation of pH drop in whey fermentation. The proposed method uses spectrophotometry to measure the pH change by bacteria and uses bromocresol purple as a pH indicator dye. The absorbance at 430 nm of a buffer solution with bromocresol purple was found to be correlated with pH values. This colorimetric assay was linear within the pH range of 4.6-7.0. Upon regression analysis, linear equation y = -0.1267× + 0.9196 was obtained having r² value of 0.9927. The assay was validated by the use of LAB fermentation in sweet whey and comparison to the values obtained by glass electrode/pH meter. Estimation of acidification activity of LAB in whey was found to be similar in both methods (r = 0.801, p > 0.05). The proposed procedure presents a viable alternative to the measurement of pH by the standard method and allows the simultaneous and fast screening of LAB acidifying activity.