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Method for Routine Determination of Lactose in Milk

Department of Dairy Science, Ontario Agricultural College, Guelph, Canada

ABSTRACT

Recoveries of added lactose in milk samples were significantly high using the polarimetric method of Grimbleby (2). Adjustments were made to the formula used for the calculations when it was found that lactose had a slightly higher than normal specific rotation in the protein-precipitating reagent, and that the serum pH was 0.2 unit lower than that found by Grimbleby. Factors used in correcting for fat and protein precipitate volumes were both increased, the former on theoretical grounds and the latter because of the lower pH, with the result that the recalculated average recovery was not significantly different from 100%. Filtering required 2 to 3 hr, but it could be shortened to less than 1 hr with no loss of accuracy by using a more dilute reagent containing slightly more phosphotungstic acid. Precipitate volumes were determined for 72 composite herd milk samples and, on the basis of these results, a new protein factor was obtained not appreciably different from the adjusted factor calculated for the Grimbleby method. Fat and protein factors were sufficiently similar to be combined. A modified procedure, using the more dilute reagent, and employing a simplified formula for the calculation of lactose, is suggested for routine analysis. Standard deviation between duplicates for the determination was ± 0.024% lactose. A mercuric chloride preservative could be used without affecting the results.