The water agar test: a new test to measure the bacteriological quality of cream

A new test, the water agar test, is described that gives a qualitative index of the presence of bacteria that indicate contamination of the cream, poor storage conditions or both of these factors. The method is simple and requires little equipment. The bacteria grow in a film of diluted cream adsorbed on the surface of a non-nutrient base. After incubation at 30 +/- 0-5 degrees C. for 18-20 hr., a proteolytic and mucoid colony count is obtained which has the same percentage coefficient of variance as a standard plate count. An examination of the effect of storage at different temperatures on the types of bacteria present in cream showed that of all the tests done initially, only the water agar test could predict subsequent bacterial growth with any consistency. The multiplication of presumptive coliform organisms occurred even at 3-5 degrees C. Irrespective of the colony count, the methylene blue reduction time was not shorter than 7 1/2 hr. unless the bacteria were in the logarithmic phase of growth when sampled. A survey was made of the bacterial flora of 188 retail samples of double cream of 15 different brands. The age of the samples varied from freshly separated cream to cream that had been kept in the shop for a day longer than that recommended for sale. The water agar test was compared with the colony count, the presumptive coliform test, a confirmatory coliform count in violet red-bile agar, a lipolytic colony count, a staphylococcal count and the methylene blue reduction test.