The effect of different pre-cooling rates and cold storage on milk microbiological quality and composition.

Discipline: management; Key words: milk pre-cooling, milk microbiological quality, energy, milk storage.  

Immediate cooling after milking and storage at refrigeration temperature are crucial to reduce bacterial growth. Milk leaves the udder at approximately 35°C, which is a favourable temperature for bacterial growth. Thus, the microbial load could increase rapidly if milk is maintained at that temperature. Cooling milk rapidly to below 6°C is necessary to avoid multiplication of micro-organisms, especially psychrotrophs, which can grow at refrigeration temperatures, although their optimum (>15 ^oC) and maximum growth (20 ^oC) temperatures are much higher. Thus, pre-cooling of milk (before it enters the bulk tank) could further reduce bacterial growth rate. A further possible benefit of pre-cooling milk is the reduction of on-farm energy costs. To study these influences further Dr L.F. Paludetti and co-workers designed an experiment, the results of which were published in the Journal of Dairy Science, Volume 101 of 2018, page 1921 to 1929. The objective of their study was to measure the effect of different cooling rates before the milk entered the bulk tank, on the microbiological load and composition of the milk, as well as on energy usage. The title of their paper was: The effect of different pre-cooling rates and cold storage on milk microbiological quality and composition. 

Three milk pre-cooling treatments were applied before the milk, which was of good quality, was channelled into three identical bulk milk tanks: no plate cooler (NP), single stage plate cooler (SP) and double-stage plate cooler (DP). The pre-cooling treatments cooled the milk to about 32.0°C, 17.0°C and 6.0°C, respectively. Milk was added to the bulk tank twice daily for 72 hours and the tank refrigeration temperature was set at 3°C. The blend temperature within each bulk tank was reduced after each milking event as the volume of milk at 3°C increased simultaneously. 

The bacterial counts of the milk volumes pre-cooled at the different rates did not differ significantly at 0 hours of storage or at 24-hour intervals thereafter. After 72 hours of storage, the total bacterial count of the NP milk was about 3.90 log10 cfu per mL, whereas that of the pre-cooled milk volumes were 3.77 (SP) and 3.71 (DP) log10 cfu per mL; the apparent differences were not statistically significant. The constant storage temperature (3°C) over 72 hours helped to reduce the bacterial growth rates in the milk; consequently, the milk composition was not affected and minimal, if any, proteolysis occurred. The DP treatment had the highest energy consumption (17.6 Wh per litre), followed by the NP (16.8 Wh per litre) and the SP (10.6 Wh per litre) treatments.  

Conclusions and recommendations: Since the microbiological load of the milk pre-cooled at different rates did not differ statistically at 0 hours or over the 72 hours of storage, there was no significant difference between the pre-cooling treatments. Also, no relevant variations were observed in milk composition, and no measurable enzymatic activity was observed, possibly because of the good initial microbiological quality of the milk. This suggests that the bacterial count and composition of milk are minimally affected when milk is stored at 3°C for 72 hours whether the milk is pre-cooled or not, provided that the  milk entering the tank has good initial microbiological quality. Regarding energy usage, the SP treatment required less energy than the other treatments to maintain an equivalent microbiological load. Considering that the milk volumes undergoing the SP and DP treatments had the lowest bacterial counts over 72 hours of storage, it may be beneficial and economical to incorporate the DP system on farms that already use an ice bank bulk milk tank and the SP system on other farms. Pre-cooling good quality milk with an SP or DP system and subsequent storage at 3°C for 72 hours should therefore maintain good microbiological and compositional quality of milk with reduced energy consumption.