Describe the different factors which increase the growth of microorganisms in milk. How microbial spoilage of milk can be controlled

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Factors Increasing the Growth of Microorganisms in Milk

Milk is a highly nutritious biological fluid, rich in proteins, fats, lactose, minerals, and water, making it an ideal medium for the growth of microorganisms. If not handled properly, milk gets contaminated easily and spoils quickly. The major factors that lead to microbial growth in milk can be classified into the following categories:

A. Environmental Factors

1. Temperature

  • Warm temperatures (30°C to 40°C) are highly favorable for microbial growth.
  • Most bacteria multiply rapidly in ambient Indian weather if milk is not chilled.
  • Psychrotrophic bacteria (cold-tolerant) can grow even at refrigerated temperatures, but at a slower rate.

2. Humidity

  • High humidity in the environment supports the survival and multiplication of airborne and surface bacteria.
  • Increases the chance of contamination during milking, storage, and transportation.

3. Dust and Airborne Contaminants

  • Air can carry dust, fungal spores, and bacterial cells, especially in unhygienic conditions.
  • Open handling of milk without proper lids increases contamination risk.

B. Milk Handling and Hygiene Practices

1. Unclean Milking Environment

  • Milking in dirty sheds, without proper cleaning of animal and utensils, causes heavy contamination of milk.

2. Dirty Udder and Teats

  • Mud, dung, or urine around the udder may introduce pathogenic bacteria into milk during milking.

3. Improper Cleaning of Utensils

  • If containers, cans, and milking machines are not properly cleaned and sanitized, they become a major source of bacteria.

4. Poor Personal Hygiene

  • Unwashed hands, long nails, and infected wounds on the milk handler can transfer harmful microbes into milk.

C. Time and Delay

1. Delay in Cooling or Processing

  • If milk is not cooled immediately to 4°C to 6°C, bacterial growth begins rapidly.
  • Every 20 minutes of delay at warm temperature can double the bacterial count.

2. Long Storage without Refrigeration

  • Storing milk at room temperature beyond 2 to 3 hours accelerates spoilage and souring due to lactic acid bacteria.

D. Health of the Animal

1. Diseased Animals

  • Animals suffering from mastitis or tuberculosis may shed pathogens like Streptococcus, Staphylococcus, Mycobacterium into the milk.

2. Use of Antibiotics and Contaminated Feed

  • Antibiotic residues and poor-quality feed can disturb the natural microflora and allow undesirable microbes to thrive.

E. Water Quality

  • Use of contaminated water for washing utensils or teats can introduce coliform bacteria and cause spoilage.
  • In some cases, water is deliberately added to increase volume, which reduces milk quality and stability.

How to Control Microbial Spoilage of Milk

Microbial spoilage of milk can lead to souring, curdling, off-flavors, and health hazards. Therefore, the following strategies must be adopted at every stage of milk handling:

1. Hygienic Milking Practices

  • Wash udder and teats with clean lukewarm water before milking.
  • Use sanitized milking equipment.
  • Ensure the hands of milkers are clean and dry.
  • Use a strip cup to check for mastitis (first few streams of milk discarded).

2. Clean and Sterile Equipment

  • Use stainless steel or food-grade plastic containers.
  • Wash all equipment with hot water and detergent, followed by sanitizer rinse (e.g., chlorine or iodine solution).
  • Avoid the use of rusted or cracked containers.

3. Immediate Cooling of Milk

  • Milk should be cooled to 4°C to 6°C immediately after milking.
  • Use Bulk Milk Coolers (BMCs) or place milk in ice-cooled containers.
  • Cooling reduces bacterial multiplication significantly.

4. Timely Transportation

  • Transport milk to chilling centres or plants as quickly as possible.
  • Use insulated tankers or ice-packed cans to maintain low temperature during transport.

5. Pasteurization

  • Pasteurization involves heating milk at 72°C for 15 seconds and then cooling rapidly.
  • It kills pathogenic and spoilage-causing microorganisms, while preserving taste and nutrients.

6. Boiling at Household Level

  • If pasteurization is not done, milk should be boiled at home before use.
  • Boiling destroys most of the bacteria but may affect flavor slightly.

7. Avoid Adulteration

  • Adding water, starch, detergent, or urea contaminates milk and encourages microbial growth.
  • Always source milk from trusted, hygienic suppliers or cooperatives.

8. Use of Preservatives (Only in Exceptional Cases)

  • Hydrogen peroxide or formalin may be used in lab settings or transport in emergencies, but they are not recommended for routine preservation due to health risks.

9. Animal Health and Feed Management

  • Keep animals healthy and disease-free through vaccination and veterinary care.
  • Provide clean feed and water, avoiding any fungal or bacterial contamination.

10. Regular Testing and Monitoring

  • Use platform tests like:
    • Methylene Blue Reduction Test (MBRT)
    • Resazurin test
    • Coliform count
    • Total Plate Count (TPC)
  • Monitor milk for bacterial load and spoilage indicators.

Conclusion

Milk is a highly perishable commodity, and microbial contamination not only reduces its quality but also poses a risk to public health. By understanding the factors promoting microbial growth and adopting scientific control measures, we can ensure that milk remains safe, wholesome, and suitable for various dairy products.

In India’s growing dairy sector, focusing on clean milk production and microbial control is essential for ensuring food safety, consumer confidence, and export readiness.

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