Procedure for Enumeration of Microorganisms in Raw Milk

January 11, 2024 by
Procedure for Enumeration of Microorganisms in Raw Milk
Kadesha Dawkins

Objective: The primary aim is to quantify the microorganisms present in raw milk, especially focusing on potentially harmful pathogens that may pose health risks to consumers.

Microbial Co​ncerns

 The introduction highlights various pathogenic microorganisms that can contaminate milk, even when the animal doesn’t display any visible signs of infection or illness. These include Salmonella, Escherichia coli O157:H7, Listeria monocytogenes, Staphylococcus aureus, Yersinia enterocolitica, Bacillus cereus, Clostridium botulinum, Mycobacterium bovis, Brucella abortus, and Brucella melitensis.

Risk Associated with Raw Milk

 Consumption of raw milk carries a significantly higher risk of causing adverse health effects in consumers compared to treated or pasteurized milk. Hence, the need for various sterilization processes like UHT, usual sterilization, or pasteurization to mitigate these risks.


  • Sample Collection and Preparation

  • Collect representative samples of raw milk from different sources or batches.
  • Ensure thorough mixing to obtain a composite sample that accurately represents the milk's microbial content.

  • Plate-Loop Technique

    • Use a standardized loop instrument to apply measured volumes of the milk sample onto sterile agar plates.
    • This technique enables consistent and controlled inoculation of the agar plates with the milk sample.
  • Incubation

    • Incubate the inoculated agar plates at a controlled temperature of 30ºC for a specified duration.
    • This temperature is conducive to the growth of various microorganisms commonly found in milk.
  • Colony Observation and Counting

    • Post-incubation, visually inspect the agar plates for distinct colonies that have developed.
    • Count and record the number of visible colonies on each plate.
    • Focus on plates with colony counts within the specified range of 10 - 300 colonies for accurate enumeration.
  • Manual Enumeration and Calculation

    • Perform the colony count manually, employing aids like colony counters for accuracy.
    • Calculate the average colony count per specified volume of milk sampled based on the data obtained from all plates.
  • Data Interpretation and Analysis

    • Analyze the compiled colony count data to evaluate the microbial load within the raw milk samples.
    • Compare the obtained counts against regulatory standards or industry-established thresholds to determine milk quality and safety.

This standardized enumeration procedure is pivotal for evaluating the microbial content in raw milk. It supports risk assessment, quality control, and ensures consumer safety within the dairy industry.

PCA Milk Agar 1033

Methylene Blue 5058

Nutrient Agar 1060 

Oxidase Reagent 6007

BCP Glucose Agar 1320

MRS broth 1215  

M17 Broth1318

MRS Broth1431


REST Client (Huachao Mao)