A survey of sources of heat-resistant bacteria in milk with particular reference to psychrotrophic spore-forming bacteria.

Samples of winter bedding and summer pasture, rinses of teats and milking equipment, and milk samples from bulk tanks from 4 farms were analysed bacteriologically for total count (TC), thermoduric count, total spore count (TSC) and psychrotrophic spore count (PSC). In bedding and pasture samples the TSC comprised the main component of the thermoduric flora but was less than 0.1% of the TC. The PSC formed approximately 1% of the TSC in winter bedding and approximately 23% of the TSC in pasture. For pasture, the PSC:TSC ratio was highest for the upper layer of soil and lower for the grass and deeper layer of soil. The TSC formed 5% of the thermoduric flora in rinses of milking equipment and only a very low PSC was obtained. The TSC of farm bulk tank milks was higher in the winter than the summer but the PSC was similar in both seasons. This was due to an increase of approximately 9 times in the PSC:TSC ratio for milks in summer compared with winter. The PSC in raw and commercially pasteurized milks from a local creamery showed a similar seasonal trend.