668. The influence of cream rising upon the activity of bacteria in heat-treated milk

Abstract

1. Criticism is directed towards the method of assessing cheese-starter activities, whereby pasteurized milk (vat milk) is inoculated with starter and its activity judged by the acidity developed after 6 hr. at 30° C.2. It has been found that certain starters are slow when subjected to this laboratory test, owing to the fact that they are agglutinated in such milk and consequently carried up into the cream layer, thus producing a lower acidity in the milk.3. Whilst such starters show higher activity in homogenized or autoclaved milk, it is suggested that starter activity tests should be carried out in the presence of agar in order to prevent cream rising and thus simulate conditions in the cheese vat. Agar is preferred to rennet for this purpose.4. It is emphasized that such apparently slow starters are not slow when used in the cheese vat.5. Evidence is given that keeping-quality tests on pasteurized and homogenized milks involving storage of the milk are also affected by the same factors, which may result in homogenized milk having a poorer keeping quality than pasteurized milk.