Electronic Counting of Somatic Cells in Milk

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) centrifuging 10ml of the diluted milk at a relative centrifugal force (RCF) of 1,020 for 15min at 4C, c) discarding the top 5ml of material (diluted milk and milk fat), and d) re-suspending the sedimented somatic cells by shaking in a vortex mixer. A Model B Coulter Counter fitted with a 100-μ-aperture tube was used to count the somatic cells in the diluted milk, with instrument threshold set to count all particles above 195 μ3 in volume (approximate diameter 7.2 μ). This setting was obtained by determining the best agreement of triplicate electronic counts with the direct microscopic method. Statistically designed experiments 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 α = 0.05. The linear correlation index was 0.997. When milk samples contained less than 300,000 somatic cells per milliliter, however, the electronic method gave significantly higher counts than did the microscopic procedure.