ADHESION AND GROWTH OF THERMORESISTANT STREPTOCOCCI ON STAINLESS-STEEL DURING HEAT-TREATMENT OF MILK

Streptococcus thermophilus adheres to plates of cheese-milk pasteurizers during the operating time. The adherence of these bacteria in the regenerator section of the heat exchanger is much larger than in the heating section. Calcium phosphate deposits are higher in the heating section than in the regenerator section. In a series of laboratory experiments with batch cultures of S. thermophilus B, adherence of this organism on stainless steel was determined after growth in the milk was nearly completed. S. thermophilus B adhered equally well to stainless steel in raw as in pasteurized milk cultures. Pre-coating of stainless steel with calcium phosphate reduced bacterial adhesion maximally by only 50%. Initiation of growth of S. thermophilus B was delayed in raw milk and not in pasteurized milk. The experimental set-up was thereafter changed in such a way that either raw or pasteurized milk containing high numbers of viable S. thermophilus B streamed in a continuous flow along stainless steel. In this way a better stimulation of the practical situation could be achieved. Now the initial adhesion of bacteria to stainless steel along which raw or pasteurized milk passed was approximately the same. During longer operating times, however, the population of bacteria on the steel in pasteurized milk increased much faster than on steel in raw milk. After 4 h of operating time stainless steel test plates contained 1.0 .times. 107 and 7.0 .times. 105 cfu[colony-forming units]/cm2, respectively. Apparently, raw milk contains growth-inhibiting compound(s), and by continuous feed of raw milk the effect of the inhibiting compound(s) is substantially increased. This results in a slow increase of the bacterial number on the surface. When pasteurized milk streams along the plates, bacterial growth is not inhibited, resulting in a fast increase.