Milk quality testing is essential to ensure safety, nutritional value, and compliance with standards, particularly in India, where adulteration and contamination are significant concerns. Tests for milk quality can be classified based on their purpose, such as detecting adulteration, assessing microbial safety, evaluating chemical composition, or determining physical properties. Below is a comprehensive classification of milk quality tests, categorized by their objectives and types, with relevance to the Indian context (e.g., FSSAI standards).
Classification of Milk Quality Tests
1. Organoleptic Tests
These tests evaluate the sensory attributes of milk using human senses to detect basic quality issues.
Purpose: Assess appearance, smell, taste, and texture to identify spoilage or abnormalities.
Examples:
Visual Inspection: Checks for color (e.g., creamy white for cow milk), presence of clots, or foreign particles.
Odor Test: Detects off-odors (e.g., sour, rancid, or chemical smells) indicating spoilage or adulteration.
Taste Test: Identifies abnormal flavors (e.g., bitterness due to bacterial growth).
Relevance in India: Used at milk collection centers for initial screening but limited by subjectivity.
2. Physical Tests
These tests measure physical properties of milk to assess quality and detect dilution or adulteration.
Purpose: Evaluate density, temperature, and other physical characteristics.
Examples:
Lactometer Test: Measures specific gravity (density) to detect water adulteration (normal range: 1.028–1.032 g/cm³ for cow milk).
Freezing Point Depression Test: Detects added water by measuring the freezing point (normal: -0.520 to -0.550°C).
Temperature Check: Ensures milk is stored at safe temperatures (below 4°C) to prevent spoilage.
Clot-on-Boiling Test: Checks for heat stability and acidity; clotting indicates high acidity or microbial activity.
Relevance in India: Lactometer tests are widely used in cooperatives (e.g., Amul) to detect water adulteration, a common issue.
3. Chemical/Compositional Tests
These tests analyze the chemical composition of milk to ensure nutritional quality and detect adulterants or contaminants.
Purpose: Measure fat, solids-not-fat (SNF), protein, lactose, and detect chemical adulterants or residues.
Subcategories:
Proximate Analysis:
Fat Content Test: Uses Gerber or Babcock methods to measure fat percentage (FSSAI standard: min. 3.5% for cow milk).
SNF Test: Determines solids-not-fat (e.g., proteins, lactose) using gravimetric methods or lactometers (FSSAI: min. 8.5% for cow milk).
Protein Content: Kjeldahl method or infrared spectroscopy to measure protein levels.
Lactose Content: Polarimetry or enzymatic methods to quantify lactose.
Adulterant Detection:
Urea Test: Detects added urea using p-dimethylaminobenzaldehyde (DMAB) reagent (color change indicates presence).
Starch Test: Iodine test to detect starch adulteration (blue color confirms starch).
Sugar/Sucrose Test: Resorcinol test for added sugars (red color indicates presence).
Neutralizer Test: Rosalic acid test to detect sodium hydroxide or carbonates used to mask acidity.
Detergent Test: Methylene blue or turbidity tests to identify detergent residues.
Formalin Test: Hehner’s test for formalin preservation (violet ring formation).
Hydrogen Peroxide Test: Paraphenylenediamine test for peroxide additives.
Contaminant Analysis:
Aflatoxin M1 Test: ELISA or HPLC to detect aflatoxin M1 from contaminated feed (FSSAI limit: 0.5 µg/kg).
Antibiotic Residue Test: Charm ROSA or LC-MS to detect residues like tetracycline or beta-lactams.
Pesticide Residue Test: Gas chromatography-mass spectrometry (GC-MS) for organophosphates or organochlorines.
Heavy Metal Test: Atomic absorption spectroscopy for lead, cadmium, or arsenic.
Relevance in India: FSSAI’s 2018 Milk Survey highlighted the need for tests like aflatoxin M1 and antibiotic residue analysis due to non-compliance. Gerber method is common in dairy plants for fat testing.
4. Microbiological Tests
These tests assess microbial load to ensure milk safety and shelf-life.
Purpose: Detect pathogens, spoilage organisms, or total bacterial count.
Examples:
Total Plate Count (TPC): Measures total viable bacteria (FSSAI limit for pasteurized milk: <30,000 CFU/mL).
Coliform Count: Detects fecal contamination using violet red bile agar (FSSAI limit: <10 CFU/mL for pasteurized milk).
Pathogen Detection:
E. coli Test: Selective media (e.g., MacConkey agar) or PCR for Escherichia coli.
Salmonella Test: Enrichment culture or ELISA for Salmonella spp.
Listeria Test: Selective media or PCR for Listeria monocytogenes.
Staphylococcus aureus Test: Baird-Parker agar for coagulase-positive staphylococci.
Somatic Cell Count (SCC): Measures white blood cells to detect mastitis in cows (high SCC indicates infection).
Methylene Blue Reduction Test (MBRT): Assesses microbial load by measuring dye reduction time (faster reduction indicates higher bacterial activity).
Relevance in India: High microbial loads due to poor cold chain infrastructure necessitate TPC and coliform tests. Mastitis detection via SCC is critical for farmer-level quality control.
5. Biochemical Tests
These tests evaluate milk’s biochemical properties to assess freshness, spoilage, or processing quality.
Purpose: Measure acidity, enzyme activity, or spoilage indicators.
Examples:
Titratable Acidity Test: Measures lactic acid content to assess freshness (normal: 0.13–0.16% lactic acid).
pH Test: Determines milk acidity (normal: 6.6–6.8); lower pH indicates bacterial fermentation.
Phosphatase Test: Checks pasteurization efficiency by detecting alkaline phosphatase (inactive in properly pasteurized milk).
Peroxidase Test: Verifies high-temperature pasteurization (e.g., UHT milk).
Alcohol Test: Assesses milk stability for processing; precipitation indicates high acidity or salt imbalance.
Relevance in India: Phosphatase tests are mandatory under FSSAI to ensure proper pasteurization, addressing public health concerns.
6. Instrumental/Analytical Tests
These advanced tests use specialized equipment for precise quality assessment, often for regulatory compliance or export.
Purpose: Provide accurate, automated analysis of composition, contaminants, or authenticity.
Examples:
Infrared Spectroscopy (e.g., MilkoScan): Simultaneously measures fat, protein, lactose, and SNF content.
High-Performance Liquid Chromatography (HPLC): Detects aflatoxins, antibiotics, or vitamins.
Gas Chromatography-Mass Spectrometry (GC-MS): Identifies pesticide or volatile compound residues.
Polymerase Chain Reaction (PCR): Detects specific pathogens or species authenticity (e.g., cow vs. buffalo milk).
Electronic Nose/Tongue: Detects off-flavors or adulterants through chemical profiling.
Relevance in India: Used in large dairy plants (e.g., Mother Dairy) and FSSAI-accredited labs for export compliance and contaminant testing.
7. Shelf-Life and Stability Tests
These tests assess milk’s durability and quality during storage and transport.
Purpose: Ensure milk remains safe and palatable until consumption.
Examples:
Keeping Quality Test: Incubates milk at controlled temperatures to predict shelf-life.
Mosely Test: Measures microbial growth in pasteurized milk over time.
Rancidity Test: Detects lipid oxidation (e.g., peroxide value) affecting flavor.
Relevance in India: Critical due to limited cold chain infrastructure, especially in rural areas.