Analysis of the fluctuating microbial counts in commercial raw milk - A case study

Microbial counts of raw milk from eight commercial dairy farms recorded ove r a period of about 2 years were obtained from a processing plant. All the counts formed irregular fluctuating series whose statistical properties wer e examined. In all eight cases the autocorrelation function indicated that the counts were random and had no underlying periodicity. This suggested th at the fluctuations, at least to some extent, reflected the combined effect of numerous factors, some unknown, and that these factors operate randomly and independently. In five out of the eight cases the fluctuating pattern of the record could be described by a probabilistic model based on the assu mption that the counts have a lognormal distribution. The validity of the m odel was confirmed by comparing the calculated frequencies of counts exceed ing preselected levels with those actually observed in fresh data. The mode l enabled ranking the five sources on the basis of anticipated frequencies of excessive counts rather than on the basis of mean performance. Two seque nces had clearly identified regions with different statistical characterist ics. They were interpreted as a reflection of changes in the sanitary condi tions at the corresponding farms. In one case, both the overall magnitudes and the fluctuation amplitude of the counts progressively decreased, a situ ation for which the described model as formulated was inapplicable.