Efficiency of Milk pasteurization

To determine the efficiency of milk pasteurization, several tests can be performed to check if harmful bacteria have been eliminated while beneficial enzymes remain intact. Here are some commonly used methods:

1. Phosphatase Test (Standard Test)

Purpose: Detects residual alkaline phosphatase enzyme, which is destroyed at proper pasteurization temperatures.

Materials Required:

• Milk sample
  
  

• Phosphatase reagent (para-nitrophenyl phosphate or Scharer’s reagent)
  
  

• Test tube
  
  

• Water bath
  
  

Procedure:

1. Take 5 mL of milk in a test tube.
   
   

2. Add phosphatase reagent to the milk.
   
   

3. Heat the test tube in a water bath at 37°C for 10 minutes.
   
   

4. Observe the color change.
   
   

Observation & Interpretation:

• If milk turns yellow, phosphatase is present → pasteurization was ineffective.
  
  

• If no color change, phosphatase is absent → proper pasteurization.
  
  

Pasteurized milk should NOT contain active phosphatase enzyme since it gets destroyed at 63°C for 30 min (LTLT) or 72°C for 15 sec (HTST).

2. Methylene Blue Reduction Test (MBRT)

Purpose: Determines microbial load in milk. If pasteurization is effective, bacteria count should be low.

Materials Required:

• Milk sample
  
  

• Methylene blue dye
  
  

• Test tube
  
  

• Water bath
  
  

Procedure:

1. Take 10 mL of milk in a test tube.
   
   

2. Add 1 mL of methylene blue solution.
   
   

3. Place the test tube in a water bath at 37°C.
   
   

4. Observe the time taken for the blue color to disappear.
   
   

Observation & Interpretation:

• If the color remains for more than 5 hours, milk is properly pasteurized (low bacterial count).
  
  

• If the color disappears quickly (<2 hours), pasteurization was ineffective (high bacterial count).
  
  

3. Coliform Test

Purpose: Detects coliform bacteria, which indicate improper pasteurization or post-pasteurization contamination.

Materials Required:

• MacConkey broth/lactose broth
  
  

• Durham tube
  
  

• Incubator (37°C)
  
  

Procedure:

1. Take lactose broth in a test tube with an inverted Durham tube.
   
   

2. Add 1 mL of milk sample.
   
   

3. Incubate at 37°C for 24 hours.
   
   

4. Check for gas formation in the Durham tube.
   
   

Observation & Interpretation:

• If gas is present, coliforms are present → pasteurization failed.
  
  

• If no gas is formed, milk is properly pasteurized.
  
  

4. Standard Plate Count (SPC)

Purpose: Determines the total number of viable bacteria in milk after pasteurization.

Procedure:

1. Dilute milk in sterile saline.
   
   

2. Plate on nutrient agar.
   
   

3. Incubate at 37°C for 24–48 hours.
   
   

4. Count bacterial colonies.
   
   

Interpretation:

• Properly pasteurized milk should have <30,000 CFU/mL.
  
  

• Higher counts suggest improper pasteurization or contamination.
  
  

Conclusion:

The phosphatase test is the most widely accepted test to check pasteurization efficiency. However, MBRT, coliform tests, and SPC provide additional confirmation of microbial quality.