Studies on the refractive index of milk

Summary

The data collected and the limits of refractive index and constant of genuine milk worked out in the last two papers have been utilised in the attempt to detect different types of adulteration.

Samples of milk of different grades of refractive index and constant have been systematically watered and their rate of fall of the values determined. By viewing the two determinations in conjunction with each other it has been found possible to detect in average samples dilution with water up to a minimum of about 5–8 per cent.

Watered buffalo milk designed to pass off as cow milk reveals itself by the disturbed relationship between the refractive index and constant. While the refractive index of such a sample is normal for cow milk the refractive constant will be found to be usually too high for that value of the refractive index.

Added skim milk or removal of fat is detected by the lowered values of refractive constant, which in average samples is reduced below the minimum for pure milk at about a minimum of 20–25 per cent. addition. With samples of very high refractive constant, however, considerably larger dilutions (up to 50 per cent.) are possible.

Attempts at normalising the density of watered milk by adding sugar are detected up to about 10 per cent. added water in cow, and about 12–15 per cent. in case of buffalo milk samples.

Thus a simple determination of the refractive index and density of milk serves to detect a number of common types of adulteration of this product.