Temperature step changes: a novel approach to control biofilms of Streptococcus thermophilus in a pilot plant-scale cheese-milk pasteurisation plant

A pilot plant-scale cheese-milk pasteurisation plant was designed and constructed to study the development of biofilms of Streptococcus thermophilus during pasteurisation of milk, and to investigate methods for preventing this growth from occurring. Under base run conditions, S. thermophilus grew on surfaces in the cool-down sections of the pilot plant, between 50 and 35 °C (bulk milk temperature), and could be detected in the product stream after 8–10 h production, reaching levels of 106 CFU ml−1 after 16 h. Thermoduric bacteria also grew in the heating sections of the pilot plant, although to a lesser extent, as did non-thermoduric bacteria that originated in the raw milk. The novel application of temperature step changes, implemented periodically to the growth region of S. thermophilus, successfully controlled the development of biofilms of these organisms. The growth of bacteria on the heating side of the pilot plant was also prevented by the implementation of these same step changes. The optimum step change conditions required to achieve a 20-h production run without detectable growth of S. thermophilus comprised a step change to 55 °C, applied for 10 min, with a 60-min interval between step changes.