Residual Microorganisms in Cleaned-in-Place Systems for Handling Milk1

A model system was used to simulate commercial cleaned in-place (CIP) conditions. Alterations in the system were made to determine the effect on the microflora, as potential food product contaminants. By the use of Serratia marcescens as a tag, contamination was shown to be more complex than a simple residue from the daily operation. Also, the use of S. marcescens revealed that contamination did not move from outside surfaces of the equipment into the liquid food even with severely corroded equipment. Increasing corrosion provided greater harborage for microorganisms. The harborage could be limited somewhat by drying the surfaces, but a much greater reduction in harborage was obtained by storing the system filled with a solution at pH 11. With manipulations that reduced the total contaminants, there was an increase in the relative frequency of Gram-positive asporogenous rods, certain genera of which show promise as indicators of un-satisfactory equipment condition.