Development of reference samples for the calibration and quality control of somatic cell count using a Fossomatic instrument

Counting of somatic cells in milk has received considerable attention: as a means of detecting abnormal milk, whether due to mastitis or other causes; as a standard of milk quality; and in mastitis research. One method of analysis of the somatic cell count is by a fluoro-opto-electronic method, as is used in Fossomatic instruments. For quality-control procedures it is necessary to have samples with a constant reference value for the somatic cell count, and therefore the milk for these samples should be stabilized by chemical and/or heat treatment. The experiments described showed that in general heat treatment followed by addition of 0.1% potassium dichromate was superior to 0.1% bronopol and 0.1% potassium dichromate without heat treatment. Statistical analysis of 14 different batches of samples using the heat treatment and 0.1% potassium dichromate preservation showed that these samples were generally stable over the period of the experiment (10 weeks). The estimated decrease after 10 weeks was at most 2% and not significant (three exceptions are discussed). The results were obtained under well-defined conditions for the quality of the unpreserved milk (freshness), the preservation method and storage conditions of the samples. The results indicate that these samples can be used as reference samples for somatic cell counting by Fossomatic instruments. The samples can be very useful for calibration of Fossomatic instruments and in quality control of the laboratory.