The effect of storage temperature and duration on the microbial quality of bulk tank milk

Discipline: contamination; Keywords: raw milk, milk storage, storage temperature, total bacteria count, psychrotrophic bacteria.

Milk collected from bulk tanks are usually collected within 24 hours. However, circumstances may require that milk needs to be stored on-farm for longer periods, maybe exceeding 48 hours. So, the question then is how will the quality of the milk be affected? In this scenario it is accepted that the milk enters the bulk tank according to all specifications of clean milk. The work of Dr A. O’Connell and co-workers published in the Journal of Dairy Science, Volume 99 of 2016, pages 3367-3374, with the title: The effect of storage temperature and duration on the microbial quality of bulk tank milk, aimed to shed some light.

The objective of their study done in Ireland was to investigate the effects of storage temperature and duration on microbial quality of bulk tank milk when fresh milk is added to the bulk tank twice daily. Bulk tank milk stored at three temperatures was sampled at 24-hour intervals during storage periods of 0 to 96 hours. The samples were analyzed for total bacterial count (TBC), psychrotrophic bacterial count (PBC), laboratory pasteurization count (LPC), psychrotrophic-thermoduric bacterial count (PBC-LPC), proteolytic bacterial count, lipolytic bacterial count, presumptive Bacillus cereus, sulfite-reducing Clostridia (SRC) and somatic cell count (SCC). The bulk tank milk temperature was set at each of three temperatures (2°C, 4°C and 6°C) in each of three tanks on two occasions during two 6-week periods. Period 1 was undertaken in August and September, when all cows were in mid lactation, and Period 2 was undertaken in October and November, when all cows were in late lactation.

None of the bulk tank bacterial counts except the proteolytic count were affected by lactation period. The proteolytic bacterial count was greater in Period 2 than in Period 1. The TBC and PBC of milk stored at 6°C increased as storage duration increased. The TBC did not increase with increasing storage duration when milk was stored at 2°C or 4°C, but the PBC of milk stored at 4°C increased significantly between 0 and 96 hours. The numbers of proteolytic and lipolytic bacteria, LPC, or PBC-LPC were not affected by temperature or duration of storage. Presumptive B. cereus was detected in 10% of all samples taken over the two 6-week periods, with similar proportions observed in both periods. A greater incidence of SRC was observed in Period 2 (20%) compared with Period 1 (3%).

In general, the results suggest that milk produced on-farm with minimal initial bacterial contamination can be successfully stored at 2°C and 4°C for up to 96 hours with little effect on its microbial quality.