A sequential injection system for indirect spectrophotometric determination of lactic acid in yogurt and fermented mash samples

A suitable non-enzymatic method is presented as an alternative to the lactic acid determination in yogurt and fermented mash samples. The oxidative conversion of lactic acid by Ce4+ to CO2 was performed in a sequential injection system with a heating coil set at 45 °C. A gas diffusion unit was coupled to the flow system for promoting the permeation of CO2, which was collected into a bromothymol blue solution (pH 8.4), used as indicator solution for the spectrophotometric determination (619 nm). Simplicity in operation, low reagent consumption, low cost and ruggedness are some remarkable characteristics of the proposed system. Base line drift was < 0.005 h− 1. A linear range from 20.0 to 100.0 mg L− 1 lactic acid was obtained (r2 = 0.998), and the detection and quantification limits were estimated as 0.158 mg L− 1 and 1.6 mg L− 1, respectively. The sampling rate was 22 h− 1, with a consumption of 0.04 g Ce4+ per determination. Interferences of matrix components were not detected. Samples of yogurt and sugar cane fermented mash products were analyzed, and no significant difference at 95% confidence level was observed when comparing the proposed method with HPLC analysis.