You work for a milk manufacturing company. One day, you come into work to find that someone has mislabeled all the milk cartons. The company "big-wigs" are very upset. It is extremely important that the mistake be fixed. Your job as a food scientist is to test the milk and determine which milk is whole, skim, 1%, etc.

BACKGROUND INFORMATION:

Specific gravity (S.G.) is a physical measurement used to characterize certain foods. The specific gravity is defined as the ratio of the mass of a substance to the mass of an equal volume of water at 4C or another specified temperature. The specific gravity of a mixture is a function of its composition and can be used to determine the composition of liquids.

OBJECTIVE:

The student will determine the specific gravity of different types of milks using a hydrometer.

MATERIALS:

• Whole and skim milk
• Hydrometer

SAFETY AND DISPOSAL:

There are no real safety dangers for this laboratory.

TIME:

One class period

PROCEDURE:

1. Check the calibration temperature of the hydrometers. Bring the whole milk and skim milk to the same temperature at which the hydrometers are calibrated.
2. Transfer whole milk to a 500-ml cylinder. Carefully place the clean dry hydrometer into the milk. When the hydrometer comes to rest, read the scale keeping your eye level with the top surface of the milk.
3. Repeat for skim milk.

EXTENSIONS:

• Have students bring in other percentages of milk and compare their specific gravities.
• Have students bring in other liquid food types to determine the specific gravity and compare to the milk.
• Dilute milk samples with water and see where the students can detect the alterations.

QUESTIONS:

1. What factor(s) influence the difference in specific gravities of whole milk and skim milk?
   
   
   
2. How do specific gravity and density differ?
   
   
   
3. How are specific gravity and density similar?
   
   
   

FOR ADDITIONAL INFORMATION:

Dr. Navam Hettiarachchy
Marlene Janes, Microbiology, Food Science Department, U of A-Fayetteville,
mejanes@comp.uark.edu

REFERENCES:

Fennema, O.R. 1985. Food Chemistry. 2 ^nd Ed. Marcel Dekker, Inc., New York, NY.

Pomeranz, Y. and C.E. Meloan, 1987. Food Analysis: Theory and Practice . 2 ^nd Ed. AVI Publishing

Co. Inc., Westport, CT.

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