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Milk Quality Assessment��Milk quality is evaluated based on physical, chemical, microbiological, and sensory parameters.�

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1. Physical Tests

  • Colour & Appearance: Should be opaque white, free from dirt/sediment
  • Consistency (Viscosity): Normal milk is slightly viscous
  • Specific Gravity (Lactometer reading):
    • Normal range: 1.026–1.032
  • Sediment Test: Detects visible impurities

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Methods to Determine SNF

Calculation Method

Formula using fat content and specific gravity (lactometer reading):

Since lactometer is calibrated at 27°C, apply temperature correction to get Corrected Lactometer Reading (CLR)

CLR = LR ± (0.2 × ΔT)

Subtract if sample temp is below 27°C, Add if sample temp is above 27°C.

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ISI and Richmond formula are the same for 27 °C calibration

  • SNF %=CLR/4​+(0.25×Fat %)+0.44 ( more correct)
  • SNF %= (CLR / 4) + (Fat % x 0.21) +0.14

CIRC (Central Institute for Research on Cattle) formulas:

SNF = (CLR / 4) + (0.22 × Fat) + 0.12

NDDB field formula:

SNF = (CLR / 4) + 0.2 × Fat + 0.14

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  • Lactometer reading (LR) = 28
  • Temperature = 30°C
  • Correction:
  • 3°C above → add (0.2 × 3) = 0.6
  • CLR = 28 + 0.6 = 28.6
  • Fat % = 4%
  • Now,
  • SNF %=CLR/4​+(0.25×Fat %)+0.44

SNF=28.6/4+(0.25×4)+0.44

=7.15+1+0.44

=8.59%

Example Calculation

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2. Chemical Tests

  • Fat Content: (Gerber method / Babcock test)
  • SNF (Solids-Not-Fat)
  • Acidity: Normal: 0.14–0.16% lactic acid
  • pH: ~ 6.6–6.8

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Fat Content: (Gerber method)

  • Milk is treated with concentrated sulfuric acid (H₂SO₄) Acid dissolves: Proteins (casein)
  • Lactose
  • Fat is released and separated with the help of amyl alcohol Centrifugation isolates fat → measured in a butyrometer

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Apparatus Required

  • Gerber butyrometer (graduated)
  • Gerber centrifuge
  • Pipettes (10.75 ml for milk) Sulfuric acid (specific gravity ~1.82)
  • Amyl alcohol
  • Water bath (65°C)
  • Butyrometer
  • stopper

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Reagents

  • Sulfuric acid (H₂SO₄) → dissolves SNF
  • Amyl alcohol → prevents charring, improves fat separation

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Procedure (Step-by-Step)

Step 1: Add Sulfuric Acid

Take 10 ml sulfuric acid in the butyrometer

Tilt the butyrometer while adding (to avoid splashing)

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Step 2: Add Milk Sample

Add 10.75 ml milk carefully

Milk should form a separate layer over acid

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Step 3: Add Amyl Alcohol

Add 1 ml amyl alcohol

Helps clear fat column

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Step 4: Insert Stopper & Mix

Close with stopper

Mix thoroughly by shaking

Ensure complete digestion (no white particles)

Step 5: Centrifugation

Place in centrifuge

Spin for 5 minutes (≈1100–1200 rpm)

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Step 6: Water Bath

Transfer butyrometer to 65°C water bath

Keep for 5 minutes

Ensures clear fat column

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Step 7: Read Fat Percentage

Take reading directly from graduated scale

Read from:

Bottom of fat column

Top meniscus

Reading = % Fat in milk

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3. Microbiological Quality

  • Standard Plate Count (SPC) → total bacteria
  • MBRT (Methylene Blue Reduction Test)
  • Resazurin Test Pathogen detection (e.g., coliforms)

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Standard Plate Count (SPC) → total bacteria

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MBRT (Methylene Blue Reduction Test)

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4. Sensory Evaluation

Taste: Slightly sweet

Smell: Pleasant, no sour/rancid odor Texture: Smooth, no clots

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Common Milk Adulterants

Adulteration reduces quality and may be harmful. Below are major adulterants:

  1. Water

Most common adulterant Reduces nutritional value

Detection: Lactometer, freezing point test

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2. Starch

Added to increase thickness

Detection: Iodine test → blue color

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3. Urea

Increases SNF artificially Harmful to kidneys

Detection: Chemical test kits

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4. Detergents

Used in synthetic milk Causes foaming

Detection: Persistent foam, chemical test

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5. Formalin

Preservative to extend shelf life Highly toxic Detection: Chemical reaction test

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6. Neutralizers (e.g., sodium carbonate)

Added to reduce acidity Masks spoilage Detection: Rosalic acid test (pink color)

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Parameter

Normal Milk

Adulterated Milk

Density

1.026–1.032

Lower (water added)

Taste

Slightly sweet

Flat/abnormal

Foam

Minimal

Excess (detergent)

Chemical tests

Within limits

Deviations

Microbial load

Controlled

Often high

Summary Table

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Conclusion

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Milk quality assessment is essential to ensure:

  • Nutritional value
  • Consumer safety
  • Regulatory compliance

Adulteration not only lowers quality but also poses serious health risks, making routine testing critical in dairy industries.