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. It was found that 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%. It was shown that 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 simulation 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 × 107 and 7.0 × 105 cfu/cm2, respectively. It is suggested that 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.

Growth of Thermoresistant Streptococci and Deposition of Milk Constituents on Plates of Heat Exchangers During Long Operating Times.

Adhesion of the thermoresistant bacterium Streptococcus thermophilus to the surface of a heat exchanger and deposition of milk constituents during long operating times were investigated. Experiments were carried out on a pilot plant pasteurizer with raw whole and preheated skim milk. Adsorption of calcium, phosphorus and proteins was studied using chemical analysis, scanning electron microscopy and X-ray microanalysis. With increasing operating times the amount of deposits increased gradually on the raw milk side of the regenerative section and in the heating section, whereas on the pasteurized side of the regenerative section no detectable deposits were formed. The bacteria adhering to the plates of the heat exchanger were sampled with a swab technique. The bacteria adhered mainly to the plates in the pasteurized section. Electron micrographs of sample plates showed that the bacteria seemed to adhere directly to the metal surface, without calcium phosphate acting as an intermediary.