Rapid estimation of psychrotrophic and proteolytic bacterial counts from total bacterial counts in cooled raw milk

Abstract

SummaryCooled raw bulk milk samples were examined for total bacterial count after 3 days at 30°C, and the numbers of psychrotrophs and proteolytic psychrotrophs after 10 days at 7°C. Positive correlations were found between the total bacterial count and the numbers of psychrotrophs, enumerated on standard plate count (SPC) agar (r = 0.88, n = 65), and between the numbers of psychrotrophs counted on SPC agar and on milk agar, respectively (r = 0.89, n = 59). Only 30.4% of the bacteria were psychrotrophs but the regression shows that the percentage of psychrotrophs increases when the total bacterial count rises. On average only small differences were found between the counts of psychrotrophs on SPC agar and milk agar, suggesting that in most samples the same organisms were probably enumerated on both media. The numbers of proteolytic bacteria were less well correlated with the numbers of psychrotrophs (r = 0.65, n = 56), as wide variations occurred between the samples. It is suggested that the total bacterial count at 30°C can be a useful estimate of the number of psychrotrophs.