A high-throughput spectroscopic method for pH determination in lactic acid bacteria screening

In this study, a colorimetric method was developed to enable the rapid determination of lactic acid bacteria (LAB) acidification ability by assessing the pH drop during juice fermentation. The proposed method used spectrophotometry to track pH variations induced by LAB, utilizing bromophenol blue as a pH indicator. This colorimetric method demonstrated linearity within the pH range of 2.50–5.70. Optimal performance was achieved with 5.0 % bromophenol blue, yielding a linear equation of y = 1.25x-0.78, with an R² value of 0.99 upon regression analysis. The method's high accuracy, repeatability and reproducibility were validated through comparisons of pH values obtained using different matrices with those measured using glass electrodes/pH meters. Several Lactiplantibacillus plantarum strains with excellent acid-producing properties were identified through this method. Specifically, the growth of L20, L2 and L6 in MRS medium with a glucose concentration of 200 g/L for 24 h led to a pH decrease of 1.00 unit, while the growth of L20 and L21 in MRS with an ethanol concentration of 12 % (v/v) for 24 h resulted in a similar pH decrease. Strains L12, L14, and L18 could lower the pH of banana and lychee juices by more than 0.30 units at both 25 and 36°C. These results highlight the effectiveness of the method for rapidly screening LAB acidifying activity.