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

Abstract

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.