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Detection of Adulteration of Milk by Lactometric and Freezing Point Methods^1,2

Dairy Industry Section, Iowa Agricultural Experiment Station, Ames

ABSTRACT

Lactometric and cryoscopic methods were studied relative to the adaptability and accuracy (of the lactometric procedures) for the determination of water added to milk.

The limiting formula: % added water =
was used in conjunction with the lactometric methods. SS₁ was calculated from the Babcock fat test by the Jacobson regression equation: SS₁ = 0.4F + 7.07; SS₂ was evaluated by the lactometric method for the sample under observation.

The seven common lactometric procedures employed yielded results that were low; the variations from the percentages of water actually added increased in non-linear fashion as the percentage of water added increased.

The modified Herrington formula, used with milk samples which were heated to 45° C, cooled to 30° C. and read at that temperature, yielded results that were slightly high for normal milk and decreased linearly as the percentage added water increased. This variation resulted from the fact that the fat percentage from which SS₁ was calculated was low in the watered milk.

As a result, the limiting formula was modified to yield the following formula:
which gave fairly satisfactory results when used in conjunction with the modified Herrington formula for the determination of SS₂.

With this procedure, 32 of 33 watered (4 to 15%) samples were diagnosed as watersed, the 33rd as suspected as watered, five of nine skimmed samples were designated skimmed or suspected as skimmed, two as normal and two as suspected as watered, and of six normal samples, four were designated as normal, one as suspected as skimmed and one as suspected as watered.

It is considered, therefore, that the procedure would serve well, for the milk supply studied, as a sorting method, that samples designated as having 5 per cent or more added water, would be watered 95 in 100 times and that the results are sufficiently promising to warrant other investigators checking the method in other milk sheds.

The Elsdon and Stubbs formula, for calculating added water from the freezing point depression of the milk, was found superior to that of the A.O.A.C. when the value 0.550° C. was employed as the average freezing point depression of milk. The data obtained indicate that the method will detect 3 per cent added water at the 99 per cent probability level and 2 per cent at the 95 per cent probability level.

¹ Journal paper no. J-1902 of the Iowa Agricultural Experiment Station, Project 1126.

² These data are part of a thesis submitted by the senior author for the degree Master of Science in Dairy Industry.

³ Current address: The Agricultural College of Norway, Dept. of Dairy Industry, Vollebekk, Norway.

⁴ Statistics section, Iowa Agricultural Experiment Station.