Specific Gravity of Milk

The specific gravity of milk is a physical property that measures its density relative to water, providing insights into its composition, quality, and potential adulteration. It’s an important parameter in milk quality analysis, often used to detect added water, fat removal, or changes in solids content. Here’s a detailed explanation:

Definition

• Specific Gravity (SG): The ratio of the density of milk to the density of water at a specified temperature (water’s density = 1 g/cm³ at 4°C).

• Formula:
  SG = (Density of milk) / (Density of water).

Typical Values

• Whole Cow’s Milk: 1.028-1.035 (average ~1.032) at 20°C (68°F).

• Skimmed Milk: Slightly higher, ~1.035-1.037, due to fat removal increasing the relative proportion of denser solids.

• Range Variation: Depends on fat content, solids-not-fat (SNF), and temperature.

Factors Affecting Specific Gravity

1. Fat Content:

   • Milk fat (density ~0.93 g/cm³) is less dense than water, so higher fat lowers SG.

   • Example: Cream (high fat) has SG ~1.008-1.015, while skim milk (low fat) is higher.

2. Solids-Not-Fat (SNF):

   • Proteins (~1.34 g/cm³), lactose (~1.54 g/cm³), and minerals (~2.5-3.0 g/cm³) are denser than water, increasing SG.

   • Higher SNF (e.g., in buffalo milk) raises SG compared to cow’s milk.

3. Temperature:

   • SG decreases as temperature rises due to thermal expansion.

   • Standard measurement is often at 15.5°C (60°F) or 20°C (68°F) for consistency.

4. Adulteration:

   • Added water (SG = 1.0) lowers SG toward 1.0.

   • Added solids (e.g., sugar, salt) increase SG.

5. Breed and Diet:

   • Breeds like Jersey (higher fat) have slightly lower SG than Holstein.

   • Diet affecting SNF (e.g., high-protein feed) can subtly raise SG.

Measurement Methods

1. Hydrometer (Lactometer):

   • Tool: A calibrated float (lactometer) marked with SG units (often 1.000-1.040).

   • Procedure:

     • Fill a cylinder with milk (~250 mL) at a standard temperature (e.g., 20°C).

     • Gently lower the lactometer into the milk and let it stabilize.

     • Read the SG where the stem intersects the milk surface.

     • Adjust for temperature if needed (e.g., add 0.0002 per °C above 20°C).

   • Use: Common in field testing; quick and inexpensive.

2. Pycnometer:

   • Tool: A small glass bottle with a precise volume.

   • Procedure:

     • Weigh the empty pycnometer, fill with milk, and weigh again.

     • Compare to the weight of water in the same volume.

   • Use: Lab-based, more accurate but time-consuming.

3. Densitometer:

   • Tool: Electronic device measuring oscillation frequency of a milk-filled tube.

   • Use: Modern, precise, used in large-scale dairy labs.

Interpretation

• Normal Range (1.028-1.035): Indicates typical composition (e.g., 3.5% fat, 8.5-9% SNF).

• Below 1.028: Suggests added water or low solids (e.g., diluted milk might read 1.015-1.025).

• Above 1.035: Suggests fat removal (skimmed milk) or added solids (e.g., adulteration with salt or sugar).

• Extreme Deviations: Rarely natural; often indicate tampering or spoilage (e.g., fermentation reducing SG slightly).

Applications

• Adulteration Detection:

  • Combined with freezing point tests to confirm water addition (SG alone isn’t conclusive).

• Quality Control:

  • Ensures consistency in milk batches for processing (e.g., cheese yield depends on SNF).

• Payment Systems:

  • Farmers may be paid based on solids content, indirectly tied to SG.

• Processing:

  • Low SG milk (high fat) suits cream production; high SG (low fat) suits skim products.

Correlation with Composition

• Clerget Formula (Approximate):
  SG = 1.000 + (0.0039 × Fat%) + (0.0056 × SNF%).

  • Example: Milk with 3.5% fat, 8.7% SNF:
    SG = 1.000 + (0.0039 × 3.5) + (0.0056 × 8.7) ≈ 1.032.

• Limitations: Formula assumes standard densities; variations occur with breed or feed.

Standards and Context

• U.S. PMO: No strict SG limit, but used in adulteration checks.

• India (FSSAI): Minimum SG of 1.028 for cow’s milk to ensure authenticity.

• Practical Use: More common in regions with adulteration risks (e.g., water addition in developing countries).

Practical Example

• Scenario: Lactometer reading at 20°C = 1.030.

  • Interpretation: Normal whole milk, likely 3-4% fat, 8-9% SNF—accepted.

  • Alternative: Reading 1.020—suspected water addition, flagged for freezing point test.