Microbiological quality of raw milk attributable to prolonged refrigeration conditions.

Discipline: contamination; Key words: raw milk quality, refrigerated storage, bacterial counts, proteolysis, psychrotrophic bacteria.

Raw milk in circumstances where it is used should be refrigerated for effective storage. However, with indifferent control of temperature on farm or in informal processing environments, coupled with less than optimal hygiene, one expects the microbiological quality of the raw milk to be compromised. A study in India by Dr N.R. Vithanage and colleagues, with relevance to the informal sector in South Africa, investigated the effect of different refrigeration conditions on the microbiological quality of raw milk from three dairy farms with different initial microbial counts. They published their results in the Journal of Dairy Research, Volume 84 of 2017, page 92 to 101. The title of their paper is: Microbiological quality of raw milk attributable to prolonged refrigeration conditions.

The researchers tested different refrigeration temperatures – 2, 4, 6, 8, 10 and 12⁰C - and measured bacterial counts, protease activity, proteolysis and microbial diversity in the raw milk during storage. In addition, the effect of combining heating, simulating pasteurization at 75⁰C for 15 seconds, with refrigeration on controlling the contaminating micro-organisms was investigated.

The results showed that all samples had increasing bacterial counts, protease activity and proteolysis due to temperature, time and initial bacterial count; the increase being significant above 6⁰C. With extended storage these trends were also evident at 4⁰C. Both protease activity and proteolysis were strongly correlated with the psychrotrophic proteolytic count at about 4⁰C and the thermoduric psychrotrophic bacterial count at above about 8⁰C, compared to the total plate count and the psychrotrophic bacterial count, which are often used as the industry standard. Significant increases in protease activity and proteolysis were observed when the psychrotrophic proteolytic count and the thermoduric psychrotrophic count reached respectively 5 x 10⁴ cfu per ml and 1 x 10⁴ cfu per ml. These levels were defined as respectively storage life for quality and storage life for safety. The storage conditions furthermore affected microbial diversity significantly, where Pseudomonas fluorescens and Bacillus cereus became the dominant isolates. With deep cooling (2⁰C) though, and the combination of heating and refrigeration (about 4⁰C), both storage life for quality and storage life for safety were significantly extended.