Enzymatic colorimetric method for turn-on determination of l-lactic acid through indicator displacement assay

l-Lactic acid is a natural α-hydroxy carboxylic acid and is commonly used as an addictive. Quantitation of l-lactic acid is indispensable in food and cosmetic industries. An enzymatic colorimetric method was developed for the determination of l-lactic acid by competitive indicator displacement assay. Boric acid inhibited the colorimetric reaction of l-3,4-dihydroxyphenylalanine (l-DOPA) catalyzed by tyrosinase. l-Lactic acid competitively displaced and released l-DOPA bound with boric acid to serve as substrate, and thus restored the tyrosinase activity. Recovery of color reaction could be spectrophotometrically determined at 475 nm and was proportional to the amount of l-lactic acid. A calibration curve between l-lactic acid concentration and recovery of absorbance were built. The concentration range of the l-lactic acid was 0.25-2.25 mM. The limit of detection (LOD) and the limit of quantification (LOQ) for l-lactic acid was estimated to be 0.05 mM and 0.16 mM, respectively. The method achieved turn-on and visual sensing with good precision, accuracy, specificity, and robustness. The assay method exhibited a promising prospect to determine the content of l-lactic acid in foods and cosmetics.