Discipline: quality; 

The report here is part of the Milk SA R & D programme to find solutions to the milk flocculation problem in the dairy industry. The investigation on the influence of psychrotrophic bacteria is by Prof Celia Hugo and postgraduate students at the University of the Free State. 

Background and objective: Psychrotrophic bacteria have the ability to proliferate at cold storage temperature of less than 7 oC. While growing, these bacteria produce proteolytic enzymes which destabilize casein. The commonly used psychrotrophic count method employs incubation at 7 oC for 10 days which means that by the time the results are known, the raw milk will already be spoiled. The research group, therefore, focussed on evaluating methods to rapidly and reliably detect psychrotrophic bacteria in raw milk. 

Results: The results from the project showed that the rapid qualitative Psychro-Fast test, which gives results within 48 hours, can be used to not only indicate the presence of psychrotrophic bacteria in raw milk, but the pink colour intensity can be used to indicate the degree of psychrotrophic bacterial contamination in raw milk. For the detection of proteolytic psychrotrophic counts, the standard method casein agar (SMCA) or skim milk agar (SMA) can be used, but the SMCA will give more accurate results. 

The study further indicated that lower counts of the total bacteria, total coliforms and the Pseudomonas count of raw milk resulted in milk with a better quality which in turn resulted in delayed flocculation. The statistical analysis indicated that the APC method and the Pseudomonas count may be used as reliable and rapid methods to indicate how fast the milk will flocculate. 

Lastly, the Gram-negative psychrotrophic bacteria were the prevalent bacteria at the time that flocculation of milk occurred, which indicated that these bacteria and their proteolytic enzymes were associated with milk flocculation. Pseudomonas spp., particularly Ps. fluorescens, were the prevalent genus at the time of flocculation.

Recommendations: The researchers made the following recommendations:

  • Since no standards exist in South Africa for the determination of psychrotrophic bacteria in raw milk, this study suggested that the following standards may be used for quality control in the dairy industry:   less than 5,000 cfu/ml when using the accelerated psychrotrophic count on standard plate count agar incubated at 18 oC for 48 hours; less than 1,000 cfu/ml when using the proteolytic psychrotrophic count on SMCA and less than 250 cfu/ml when using the proteolytic psychrotrophic count on SMA medium.

  • Flocculation may partially be controlled by following stringent hygiene practices starting at farm level until the end of production. Strict hygiene at farm level (e.g., no water left in milking machines, clean milk pipelines, clean udders and teats, adequate cleaning of dairy equipment surfaces for reception, transport and storage of milk, preventing biofilm formation etc) will ensure low proteolytic psychrotrophic counts.

  • Other measures that should also be considered, are cold storage of raw milk at 2 oC rather than at 4-7 oC and heat processing of raw milk at 65-69 oC for 15 seconds prior to pasteurization or UHT treatment.

  • The accelerated psychrotrophic count method on PCA, SMA or SMCA should be included in raw milk processing plants as a standard quality measuring method because of the high correlation of this method with the number of days to flocculation.