Electronic Counting of Somatic Cells in Milk

Abstract

Several procedures were developed and evaluated to find a satisfactory method for electronic counting of somatic cells in milk. Best results were obtained when milk was prepared for counting by a) diluting 1:50 with 0.85% saline, b) eentrifuging 10 ml of the diluted milk at a relative centrifugal force (RCF) of 1,020 for 15 nlin at 4 C, c) discarding the top 5 ml of material (diluted milk and milk fat), and d) resuspending the sedimented somatic cells by shaking in a vortex mixer. A Model B Coulter Counter fitted with a 100-g-aperture tube was used to count the somatic cells in the diluted milk, with instrument threshold set to count all particles above 195 g~ in volume (approximate diameter 7.2 g). This setting was obtained by determining the best agreement of triplicate electronic counts with the direet microscopie method. Statistically designed experimeuts on 111 samples of milk counted in duplicate by both the electronic and microscopic methods showed that when milk samples had a somatic cell count of 300,000 per milliliter or over, counts by the two methods were not different at a = 0.05. The linear correlation index was 0.997. When milk samples contained less than 300,000 somatic cells per nfilliliter, however, the electronic method gave significantly higher counts than did the microseopic procedure.