Psychrotrophic bacteria seem to be becoming the newest thorn in the side of the dairy industry. Scary bit is that we are helping them along.
“How?” do you ask?
Because of economic factors the pre-processing time of raw milk has been extended to an average of 4 days. This allows ample time for psychrotrophic, proteolytic bacteria to grow, and form heat stable enzymes. Although the psychrotrophic bacteria are easily killed off by normal pasteurization temperatures, the formed enzymes can survive UHT processes.
This causes multiple problems and we here at Elsenburg Dairy Lab have especially seen those problems on products like cottage cheese and UHT milk, more so over the past few years.
This is a group of bacteria that actually loves the cold chain, as it helps them outwit, outlast and outperform their competition. 😉
Nowadays psychotropic bacteria predominate in raw milk in the cold chain, especially Pseudomonas spp.
So much so that when the same milk is plated out and incubated at 7° C and at 30° C(as in Total Plate Count), the psychotropic growth will overtake the mesophilic growth, causing psychrotrophic counts to be higher than total plate counts.
I don’t think the methods of analysis are of much importance in this article, because there are many, ranging from microbial(determining the bacteria) and biochemical(determining the formed enzyme). Qualitative tests are of no use as we want to be able to enumerate the psychrotrophic bacteria.

The main concern I have is to raise awareness of a fast growing problem, that will only become more so in the future.

Possible solutions are the following:

1) Ask your lab to do a monthly psychrotrophic count on all farmers. This will identify
your high risk milk supply.2) Thermising the milk to 68° C before cooling on the farm. This would have to be done with a plate heat exchanger, but would be economical at this stage, because the milk is already at body temperature(37° C) and the treated milk can run against the incoming milk to heat it and to be cooled down itself. An ice bank would also be needed to finish the cooling process. The thermising process would obviously increase the overall quality of
the milk as well, without influencing heat stability to a large degree.
3) Very rapid cooling on the farm to 2° C, instead of the usual 4° C and keeping it there until processing.
4) Using a pre-cooler at the factory to cool the milk down quickly from the 5-7 °C that it arrives at, to 2 °C.

Kind regards,
Leon the Milkman