Preparation of a Positive Control Sample for Use in the Routine Analysis of Milk and Milk Products for Alkaline...

A method was developed for preparing filter paper impregnated with raw skim milk to serve as positive control samples during the routine analysis of milk and milk products for alkaline phosphatase. Whatman No. 40 filter paper circles (12.5-cm diameter) were dipped in raw skim milk standardized to known concentrations of alkaline phosphatase. Excess milk was removed by draining and blotting between folds of blotting paper. The filter papers were dried over silica gel in a desiccator under continuous vacuum for 5 to 6 days. Disks measuring 0.64-cm were punched out of the dried filter paper circles and stored in screw-cap test tubes at room temperature in the dark until use. The relationship between the alkaline phosphatase contents of milk and the filter paper disks was linearly correlated and characterized by the equation: [E_disk] = 0.0071 × [E_milk] + 0.41, and r = 0.98. Reproducibility of preparing impregnated filter paper circles showed coefficients of variation of 3.3 to 15.7%. Statistical analysis of the data relating alkaline phosphatase activity with days of storage by analysis of variance and regression analysis indicated significant differences in the slope of the regression lines at the a = 0.05 level. At the end of 406 to 599 days of storage, the estimated decrease in [E_disk] for significant samples ranged from 25.6 to 38.9%, with an average ± SD of 33.0 ± 4.4%. Data do show, however, that filter paper disks can be prepared to contain known concentrations of alkaline phosphatase and stored at room temperature for several months for use as positive control samples.