Minimising the between-sample variance in colony counts on foods

The objective of this study was to assess whether it is possible to minimise the variance in colony counts on replicate target samples of foods by aseptic compositing of the samples or by increasing the quantity of sample examined. The results show that compositing reduces the overall variance, and hence the standard deviation, to very low levels, although in some cases the overall variance remains relatively high, reflecting the heterogeneous distribution of microorganisms in the foods. Increasing the weight of target sample examined (e.g. from 10 g to 100 g) had a pronounced effect on the mean log10 colony count and significantly reduced the variance of the mean. The results are discussed in relation to the quantity of sample that is recommended for examination in international and other standards.