Milk Processing Operations

Milk processing operations transform raw milk into safe, standardized, and value-added products suitable for consumption or further use in the dairy industry. These operations enhance shelf life, ensure safety, improve quality, and cater to diverse market demands (e.g., liquid milk, cheese, butter). Below is a detailed discussion of the key milk processing operations, their mechanisms, equipment, and significance.

Overview

• Definition: A series of physical, thermal, and mechanical treatments applied to raw milk.

• Objectives:

  • Eliminate pathogens and spoilage organisms.

  • Standardize composition (fat, SNF).

  • Extend shelf life and enhance sensory properties.

• Scope: Applies to market milk (pasteurized, UHT) and raw material for dairy products.

Key Milk Processing Operations

1. Collection and Reception

• Process:

  • Raw milk collected from farms in cans or bulk tanks, cooled to <4°C.

  • Received at the plant, weighed, and filtered to remove debris (hair, dirt).

• Quality Check:

  • Organoleptic (odor, color), TBC (<100,000 CFU/mL), SCC (<750,000/mL), fat/SNF, adulterants.

• Equipment: Filters, pumps, refrigerated tanks.

• Purpose: Ensures clean starting material.

2. Cooling

• Process: Milk chilled to <4°C immediately after milking and during storage/transport.

• Mechanism: Slows microbial growth (e.g., Pseudomonas, LAB) and enzymatic activity (lipase).

• Equipment: Bulk cooling tanks, plate coolers.

• Significance: Preserves quality until processing; critical for raw milk.

3. Filtration/Clarification

• Process:

  • Filtration: Passes milk through fine mesh or cloth to remove coarse impurities.

  • Clarification: Centrifugal clarifiers spin at 6,000-7,000 RPM to remove finer particles (somatic cells, bacteria, sediment).

• Equipment: Filters, clarifiers (e.g., disc-stack centrifuges).

• Purpose: Improves clarity, reduces microbial load slightly.

4. Standardization

• Process: Adjusts fat and SNF to meet product specifications (e.g., 3.0% fat, 8.5% SNF for toned milk).

• Mechanism:

  • Cream separated via centrifuge (5,000-10,000 RPM).

  • Skim milk or water added to whole milk; cream reintroduced as needed.

• Equipment: Cream separators, mixing tanks.

• Significance: Ensures consistency for market milk variants (full cream, skimmed).

5. Heat Treatment

Heat treatment is the cornerstone of milk safety and shelf life extension.

1. Pasteurization:

   • Process:

     • Batch (LTLT): 63°C for 30 minutes.

     • HTST: 72°C for 15 seconds.

   • Mechanism: Kills vegetative pathogens (Listeria, Salmonella) and most spoilage bacteria.

   • Equipment: Plate heat exchangers (PHE), holding tubes, vats.

   • Shelf Life: 2-15 days refrigerated.

2. Ultra-High Temperature (UHT):

   • Process: 135-150°C for 2-5 seconds (direct steam or indirect heating).

   • Mechanism: Sterilizes milk, including spores (Bacillus).

   • Equipment: UHT plants (e.g., Tetra Pak systems).

   • Shelf Life: 3-6 months ambient.

3. Sterilization:

   • Process: >100°C (e.g., 121°C for 15-20 minutes) post-packaging.

   • Mechanism: Complete sterility; less common today.

   • Equipment: Retorts, autoclaves.

   • Shelf Life: >6 months ambient.

6. Homogenization

• Process: Milk forced through tiny nozzles at 15-25 MPa, reducing fat globule size (<1 µm).

• Mechanism: Prevents cream separation by breaking fat globules and coating them with protein.

• Equipment: Homogenizers (single- or two-stage).

• Significance: Improves texture (smoothness), stability, and whiteness; optional for some products.

7. Separation

• Process: Centrifugal force separates milk into cream (fat-rich) and skim milk (low-fat).

• Mechanism: Density difference (fat: ~0.9 g/mL, skim: ~1.03 g/mL) exploited at 5,000-10,000 RPM.

• Equipment: Cream separators.

• Purpose: Produces skim milk, cream for butter, or adjusts fat content.

8. Fortification (Optional)

• Process: Addition of vitamins (A: 200-250 IU/100 mL, D: 40-100 IU/100 mL) or minerals.

• Mechanism: Blended post-heat treatment using premixes.

• Equipment: Mixing tanks, dosing pumps.

• Significance: Enhances nutrition (e.g., vitamin D in India).

9. Flavoring (Optional)

• Process: Flavors (e.g., chocolate) and sweeteners added post-pasteurization.

• Equipment: Blending tanks.

• Significance: Targets niche markets (e.g., children).

10. Packaging

• Process:

  • Pasteurized: Filled into sanitized pouches, bottles, or cartons at <10°C.

  • UHT: Aseptically packed in sterile multilayer cartons (e.g., Tetra Pak).

• Equipment: Filling machines, aseptic packagers.

• Purpose: Protects milk, extends shelf life, ensures convenience.

11. Quality Control

• Tests:

  • Microbial: TBC (<20,000 CFU/mL post-pasteurization), coliforms (<10 CFU/mL), pathogens (absent).

  • Chemical: Fat/SNF, pH (6.6-6.8), adulterants (e.g., urea, water).

  • Physical: Taste, odor, no clots.

• Equipment: Lactometers, IR spectroscopy, microbial incubators.

12. Storage and Distribution

• Pasteurized: Stored/distributed at <4°C (cold chain).

• UHT/Sterilized: Ambient storage, no refrigeration needed.

• Equipment: Refrigerated trucks, warehouses.

Additional Operations for Dairy Products

• Fermentation: LAB (e.g., Lactococcus) added for yogurt, cheese; controlled pH drop.

• Concentration: Evaporation or ultrafiltration to produce condensed milk.

• Drying: Spray drying for milk powder.

• Churning: Cream processed into butter.