Is it possible to predict the microbiological quality of goat milk by physicochemical parameters?

Abstract

This study evaluated the impact of microbial growth (at 7 and 25 °C) on methylene blue and resazurin reduction and on the physicochemical parameters of goat milk. It was also evaluated the impact of milk freezing-thawing on these same parameters. The Lactobacillus (Lb) showed the highest growth rate and the time required for total bacteria count (TBC) to reach the limit recommended by Brazilian and European legislation was 4 and 5 days at 7 °C and 7 and 8.4 h at 25 °C, respectively. In addition, boiling instability was observed after 22 h at 25 °C (TBC = 1.9•1010 CFU mL−1), showing high thermal stability. Among the studied physicochemical parameters, only the methylene blue and resazurin tests showed significant changes (reduction from ~11 to 10h to ~7-6h) when TBC reached the limit of Brazilian legislation. For the limit of European legislation, dye reduction tests showed a reduction of 60%, while only slight changes (<20%) were observed for the other evaluated parameters. For frozen milk, no difference in the physicochemical parameters were observed after thawing, although an increase in Lb count (~0.8 log) was found at 7 °C. The results indicated that the dye reduction test, especially resazurin, was the best option to indirectly assess the quality of goat milk.