MICROBIAL MONITORING OF COMMERCIAL YOGHURTS

Abstract

Yoghurt is a fermented milk resultant of a mutualism microbia l interaction between the bacteria Streptococcus thermophilus and Lactobacillus delbrueckii subsp. Bulgaricus. For yoghurt quality assurance, the cell numbers of each microorganism, should be the same, therefore a relative cocci to bacilli ratio of 1:1. During the storage of yoghurts, post-acidification and modification in this ratio might occur. The objectives of this research were to evaluate yoghurts of four different manufacturers called A, B, C and D, in relation to the number and balance between coci i and bacilli during the storage and its relation with acidity and pH. In this way, yoghurts with up to 20 days of manufacturing (band A) and more than 20 days (band B) were evaluated. An imbalance in the lactobacilli numbers which was lower than that of st reptococci wa s verified and considered inadequate, in two out of four commercial brands. In yoghurts from the manufacturer A, there was a significant reduction in the number of lactobacilli from band A to band B leading to an increase in the relative ratio of cocii to bacilli. Acidity and pH of all yoghurt samples did not differ significantly from band A to band B. The acidity of yoghurts from manufacturer D revealed significantly higher (P< 0.05) than the others, but did not lead to an increased pH reduction. All samples attended the legislation in relation to total lactic acid bacteria counts