Studies on the refractive index of milk

Summary

A simple method of determining the refractive index of milk with the Abbè refractometer has been devised. About 10 c.c. of sample in a Gerber butyrometer is centrifuged for 5 minutes in an ordinary milk centrifuge. A few c.c. of the defatted milk is carefully collected without disturbing the fat layer and tested for R.I. It takes less than 30 minutes to test the R.I. of a dozen samples in this fashion ; and these values represent, unlike those of milk-sera, the true refractive index of milk. From the density and R.I. the refractive constant, K, has been calculated.

The R.I. and K of more than 200 samples each of cow and buffalo milk have been tested over a period of 8 months. The limits of R.I. for cow milk lie between 1·3449 and 1·4480, the mode being 1·3463, and for buffalo milk between 1·3461 and 1·3500, the mode being 1·3480. K is distinct and lies within much narrower limits for each type of milk—cow milk 0·2065-0·2075, and buffalo milk 0·2076-0·2088. These limits, unlike those of R.I., are independent of the solids-not-fat content of milk. From the available data it is concluded that samples of cow milk with R.I. < 1·3449 and K < 0·2065, and buffalo milk with R.I. < 1·3461 and K < 0·2076 can be considered to be adulterated.