Best Test for Detecting Water Adulteration in Milk

Adulteration of milk with water is best judged by the determination of:

Correct Answer: Freezing point

The Freezing point depression test (Cryoscopy) using the Hortvet cryoscope is the best and most reliable method to detect water adulteration in milk. Pure milk has a constant freezing point of −0.535°C to −0.550°C (average −0.545°C). Addition of water raises (i.e., brings it closer to 0°C) this freezing point.

Why Freezing Point is the Best Test

• The freezing point of milk is the most constant physical property of milk.
• Addition of 1% water to milk raises its freezing point by 0.006°C.
• The test is highly sensitive and cannot be easily manipulated by adding other solutes (which would alter other properties).
• Note: Mastitis milk shows a normal freezing point (not depressed), because mastitis increases chloride which compensates for the drop in lactose.

Why Specific Gravity Alone is Not Best

• Specific gravity can be manipulated by adding skim milk (raises SG) along with water (lowers SG), masking adulteration.
• Freezing point is more specific and harder to falsify.

Instrument Used

• Hortvet cryoscope / Hortvet apparatus — measures the freezing point of milk.