Complementary use of alkaline and acidic detergents within one system for soak cleaning was examined. A cleaning simulator was used to determine the changes in soil and bacterial numbers on stainless steel and rubber surfaces over 28 soiling and washing cycles (each of 12 h) with four cleaning systems. The soiling milk was inoculated with 5 × 105 Streptococcus faecalis and 5 × 105 Escherichia coli colony forming units/ml. In one system the soiling milk was followed by a wash with a chlorinated - alkaline detergent (20 C) and two rinses of cold water (20 C) and an intercycle period of 9.5 h. The second system was the same as the first, except an acidic detergent (20 C) was substituted for the final rinse every 9th cycle. In the third system, the same acidic detergent was used in every cycle, and in the fourth, an iodophor was used as the final solution in every cycle. The experiment demonstrated that the complementary use of alkaline and acidic detergents is ineffective in the soak-cleaning situation as acidic detergents aggravate accumulation of soil under these circumstances. Although the periodic application of a sanitizer did not influence soil accumulation, it transiently affected numbers and types of microorganisms on the surfaces. Under these conditions, the efficiency in bacterial control of the four systems was related not to the detergency, but to the performance of the sanitizer used in the system. The two substrates varied greatly in the absolute and relative amounts of soil which were deposited upon them - these differences affecting the numbers of bacteria which the surfaces supported. Neither S. faecalis nor E. coli grew during the intercycle period of any system (9.5 h, RH 90%, 30 C).
1Present address: Dairy Research Centre, P.O. Box 217, Richmond, N.S.W. 2753, Australia.