Instrumental assessment of milk quality by the color of reductase test with resazurin
At this paper the objective quality control of milk is under consideration. In particular a feasibility of transition from its organoleptic analysis in accordance with the requirements of the GOST 32901-2014 to the quantitative assessment is examined. The results of experimental studies of the changes in the optical properties of a reductase milk sample with resazurin during the deterioration of milk quality are shown. These results were obtained with the help of spectrophotometric method of measurements followed by calculation of color parameters of milks sample in accordance with CIE (International Commission of Illumination) recommendations. The investigations were carried out on the working solution of the resazurin sodium salt, pure milk, as well as mixtures of milk with mentioned working solution. The resultant spectral transmission characteristics are used to calculate chromaticity coordinates of analyzed samples for E light source illumination, as well as to calculate color coordinates of these samples in color system sRGB with D65 light source illumination (for adequate visualization of analysis results on computer monitors). At the first day of experimental studies chromaticity coordinates of mixture of milk with working solution were x=0,36506 and y=0,28075. These values match the violet color of mixture and the first quality grade of milk (milk with approximate quantity of bacteria up to 500 thousand in 1 sm<sup>3</sup>). At the last day of experimental studies chromaticity coordinates of mixture of milk with working solution were x=0,4082 and y=0,33833. These values match the pink color of mixture and the third quality grade of milk (milk with approximate quantity of bacteria up to 20 million in 1 sm<sup>3</sup>). Besides, points on the chromaticity diagram characterized colors of pure milk and mixtures of milk with working solution at different days of experiment and at different time of experiment line up in some sequence. Thereby, it is shown that the proposed approach makes it possible to uniquely determine the quality of milk, and it can be used to solve the problem of laboratory quality control of milk. In conclusion, general recommendations concerning spectrophotometer and the radiation source are given for practical implementation of the proposed method.