Absorption-spectrophotometric determination of DNA (deoxyribonucleic acid) in milk ad modum Feulgen. Accuracy of analysis, specificity, correlation to content of cells.

The Feulgen reaction is examined by absorption photometric measurements at 565 nm with correction for varying background absorption at 485 nm. This correction is done to improve the accuracy of measuring. Examinations of the accuracy of analysis for the Feulgen reaction and the direct microscopic cell counts show that the former is of the same order, when the cell count is made at a working factor of 17,000. The standard deviations expressed in percentage of the reaction value are greater the lower the reaction is, whereas the absolute standard deviation is reduced. It has been demonstrated that addition of a few (< 5) µg of DNA per ml milk can give Feulgen reaction. The coefficient of correlation between log added amount of DNA and log Feulgen reaction is 0.98. The coefficient of correlation between log Feulgen reaction and log cell content depends on the accuracy at which both determinations are made. In a determination of cell content at a working factor of 550, and several determinations of the Feulgen reaction the coefficient of correlation (r) is found to be 0.98. A cell content determination at a working factor of 20,000 and a single determination of Feulgen reaction on each milk sample yields r = 0.83. An examination of foremilk samples of the same cell content reveals on an average the highest reaction in mastitis-affected quarters which have been infected within the last 4 weeks. Quarters that have been infected for more than 4 weeks, on an average show the second highest reaction, whereas quarters of physiological cell number show the lowest reaction. The DNA-content in most cases is found to be too high compared with the number of cells counted microscopically. The DNA-content in centrifugated cells corresponds to the one calculated theoretically. The surplus DNA-content can be demonstrated in the cell-free skim milk fraction, probably originating from destroyed cells. The studies performed suggest that determination of the content of 2-deoxyribose in milk by means of the Feulgen reaction is a more correct measure of the cell content in the milk than is the microscopic cell count. Studies are being continued for illustration of these conditions.