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Correction for Volume of Precipitate in the Polarimetric Determination of Lactose in Milk

Dairy Chemical Laboratory, Ohio Agricultural Experiment Station, Wooster, Ohio

ABSTRACT

It has been shown that in case of 100 representative samples studied the average volume of the fat and protein in the 66 grams of milk used as a sample was 5.19 cc. against a correction of 2.6 cc. allowed in the official method of the association of Official Agricultural Chemists. In every sample, the volume as actually calculated exceeded the correction called for in the official method by amounts ranging from 0.6 to 5.53 cc. The actual maximum, minimum and average discrepancy in the final lactose reading would have been respectively 0.28, 0.03, and 0.13 per cent. The actual computed volume of fat and protein is also shown to be greater than the correction called for by the Official method in each group of 10 groups aggregating 543 samples reported by Haecker and in each group except one of 10 groups totaling 807 samples previously reported by the writer.

A suggestion is made for the use of the fat content of the milk as a basis for determining the proper correction of volume when other data are not available. It is shown that the degree of accuracy obtainable by this simpler method is not as great as that obtainable by the use of the formula first discussed, but is much greater than that secured by the use of the official method in its present form.