Milk Quality Assessment & Adulterants

Milk Quality Assessment & Adulterants

Milk quality assessment is a critical process to ensure that milk is safe, nutritious, and meets regulatory and consumer standards. It involves evaluating physical, chemical, microbiological, and sensory properties to confirm purity and detect adulteration. Adulterants are substances intentionally added to milk to increase volume, mask inferiority, or reduce costs, compromising its quality and safety. Below is a detailed discussion of milk quality assessment methods and common adulterants, with a focus on the Indian context as of March 22, 2025, where applicable.

Milk Quality Assessment

Objectives

• Ensure safety (pathogen-free, no harmful residues).

• Verify nutritional content (fat, SNF, protein).

• Detect adulteration or contamination.

• Assess shelf life and sensory appeal.

Methods of Assessment

1. Physical Assessment

• Appearance:

  • Normal: White to slightly yellowish (cow/buffalo milk), uniform.

  • Abnormal: Clots, discoloration (e.g., blue from Pseudomonas), sediment.

• Odor:

  • Normal: Mild, sweet, fresh.

  • Abnormal: Sour (LAB fermentation), rancid (lipolysis), chemical (adulterants).

• Taste:

  • Normal: Slightly sweet, creamy.

  • Abnormal: Bitter (proteolysis), salty (mastitis), soapy (detergents).

• Density:

  • Measured with lactometer (specific gravity: 1.028-1.032 g/mL at 20°C).

  • Low density indicates water addition; high density suggests added solids.

2. Chemical Assessment

• Fat Content:

  • Method: Gerber method (sulfuric acid separates fat), Rose-Gottlieb (solvent extraction), or infrared spectroscopy.

  • Standard: Varies by type (e.g., 3.0% for toned, 6.0% for full cream per FSSAI).

• Solids-Not-Fat (SNF):

  • Method: Gravimetric (drying), lactometer + fat test calculation (SNF = Total Solids - Fat).

  • Standard: 8.5-9.0% (FSSAI minimum).

• Protein:

  • Method: Kjeldahl (nitrogen content), dye-binding (e.g., Amido Black).

  • Normal: ~3.2-3.5% in cow milk.

• Lactose:

  • Method: Polarimetry, enzymatic kits.

  • Normal: ~4.6-4.9%.

• pH:

  • Normal: 6.6-6.8 (fresh milk).

  • Abnormal: <6.5 (souring), >7.0 (mastitis or alkali adulteration).

• Freezing Point:

  • Method: Cryoscopy (-0.530 to -0.550°C for cow milk).

  • Purpose: Detects water addition (raises freezing point).

• Acidity:

  • Method: Titration with NaOH (as lactic acid %).

  • Normal: 0.13-0.15%; >0.2% indicates spoilage.

3. Microbiological Assessment

• Total Bacterial Count (TBC):

  • Method: Standard Plate Count (SPC).

  • Standard: <100,000 CFU/mL (raw milk for processing, FSSAI); <20,000 CFU/mL (pasteurized).

• Coliform Count:

  • Method: Violet Red Bile Agar.

  • Standard: <10 CFU/mL (indicates fecal contamination).

• Pathogens:

  • Method: Selective media (e.g., Listeria on Oxford agar, Salmonella on XLD).

  • Standard: Absent in 25 mL.

• Somatic Cell Count (SCC):

  • Method: Microscopy, automated counters (e.g., Fossomatic).

  • Standard: <200,000/mL (healthy udder); <750,000/mL (legal limit, FSSAI).

  • Purpose: Indicates mastitis.

4. Adulterant Detection

• Water:

  • Test: Lactometer, freezing point depression.

  • Indicator: Low SNF, raised freezing point.

• Urea:

  • Test: Para-dimethylaminobenzaldehyde (DMAB) test (yellow color).

  • Indicator: >0.07% (normal milk urea ~0.02-0.05%).

• Detergents:

  • Test: Phenolphthalein test (pink in alkali).

  • Indicator: pH >7.0, soapy taste.

• Starch:

  • Test: Iodine test (blue-black color).

  • Indicator: Presence confirms adulteration.

• Sugar/Salt:

  • Test: Resorcinol (sugar, red color), silver nitrate (salt, white precipitate).

  • Indicator: Masks low SNF, detected chemically.

• Formalin/H₂O₂:

  • Test: Hehner’s test (formalin), potassium iodide (H₂O₂, brown color).

  • Indicator: Preservatives; illegal in milk.

• Melamine:

  • Test: HPLC, ELISA.

  • Indicator: Boosts apparent protein; rare in India post-2008 scandal.

• Antibiotic Residues:

  • Test: Delvotest, ELISA.

  • Standard: Below MRLs (e.g., penicillin <4 µg/kg, FSSAI).

5. Sensory Assessment

• Trained Panels: Evaluate taste, smell, texture against standards.

• Purpose: Quick screening; subjective but effective for gross defects.

6. Advanced Techniques

• Infrared Spectroscopy: Rapid fat, protein, lactose analysis.

• PCR: Identifies pathogens or species (e.g., cow vs. buffalo).

• Chromatography: Detects chemical adulterants (e.g., urea, melamine).

Adulterants in Milk

Adulteration is a widespread issue, especially in India, where ~41% of milk samples failed quality tests (FSSAI, 2019 survey). Adulterants compromise safety and nutrition.

Common Adulterants

1. Water:

   • Purpose: Increases volume, reduces cost.

   • Impact: Lowers fat/SNF, dilutes nutrients; detectable by lactometer/freezing point.

   • Prevalence: Most common globally.

2. Urea:

   • Purpose: Boosts apparent SNF/protein (nitrogen content in Kjeldahl test).

   • Impact: Toxic at high levels (>0.1%), causes kidney strain.

   • Source: Fertilizer-grade urea.

3. Detergents/Soap:

   • Purpose: Masks water addition, mimics fat foaminess.

   • Impact: Gastrointestinal issues, chemical residues.

   • Detection: Alkalinity (pH >7), soapy taste.

4. Starch:

   • Purpose: Increases SNF, thickens diluted milk.

   • Impact: Adds empty calories, indigestible in excess.

   • Source: Potato, corn starch.

5. Sugar/Salt:

   • Purpose: Masks dilution, adjusts taste/density.

   • Impact: Alters flavor, increases sodium/sugar intake.

   • Detection: Chemical tests (resorcinol, AgNO₃).

6. Formalin/Hydrogen Peroxide:

   • Purpose: Preservatives to extend shelf life.

   • Impact: Formalin is carcinogenic; H₂O₂ causes oxidative stress.

   • Detection: Specific chemical tests.

7. Skim Milk Powder:

   • Purpose: Increases SNF in watered milk.

   • Impact: Legal if declared (reconstituted milk); fraudulent if undeclared.

   • Detection: Difficult without isotopic analysis.

8. Vegetable Oils:

   • Purpose: Mimics fat content.

   • Impact: Alters fatty acid profile, reduces nutritional value.

   • Detection: Gas chromatography (non-milk fatty acids).

9. Melamine:

   • Purpose: Falsely elevates protein readings.

   • Impact: Kidney damage (e.g., 2008 China scandal); rare in India.

   • Detection: HPLC, mass spectrometry.

10. Synthetic Milk:

    • Composition: Water, urea, detergents, vegetable oil, sugar.

    • Purpose: Cheap substitute for real milk.

    • Impact: Toxic, devoid of milk nutrients.

    • Detection: Comprehensive testing (pH, fat type, urea).