Harmful Microorganisms Found in Milk

Milk is a white, nutrient-rich liquid secreted by the mammary glands of female mammals to nourish their young. It serves as the primary source of nutrition during the early stages of life, containing an ideal balance of macronutrients (carbohydrates, proteins, and fats) and micronutrients (vitamins and minerals) essential for growth and development. The principal carbohydrate in milk is lactose, a disaccharide composed of glucose and galactose, which provides energy and aids in calcium absorption. The main protein fractions are casein (about 80%) and whey proteins such as β-lactoglobulin and α-lactalbumin, which contribute to tissue building and immune function. Milk lipids are present as emulsified fat globules surrounded by a phospholipid membrane, supplying essential fatty acids and fat-soluble vitamins like A, D, E, and K.

In addition to these beneficial components, raw (untreated) milk can harbor various microorganisms, including lactic acid bacteria, spoilage organisms, and pathogenic species such as Escherichia coli, Salmonella spp., Listeria monocytogenes, and Mycobacterium bovis. These pathogens may enter milk through contamination during milking, handling, or storage, and can cause foodborne illnesses if the milk is consumed without pasteurization or adequate heat treatment. Pasteurization, typically heating milk to 72 °C for 15 seconds (high-temperature, short-time method) is a critical step to inactivate harmful microbes while retaining most of milk’s nutritional value.

Milk is a sterile secretion when it is synthesized and stored inside the healthy mammary gland (udder) of dairy animals. Under normal conditions, it contains no bacteria due to the protective barrier of the teat canal and the presence of natural antimicrobial agents such as lactoferrin, lysozyme, and immunoglobulins. However, microbial contamination can occur even before the milk leaves the udder if the animal is suffering from intramammary infections like mastitis, which allow pathogenic organisms to enter the milk directly (source: University of Guelph).

Beyond the udder, the risk of contamination increases significantly during various stages such as milking, handling, transferring, and storage. Bacteria can enter from multiple sources, contaminated milking equipment, unclean storage containers, handlers’ hands, or even airborne particles in the milking environment. Pathogens of concern include Escherichia coli, Listeria monocytogenes, Salmonella spp., and Staphylococcus aureus, all of which can survive and multiply if milk is not promptly cooled to below 4 °C.

Improper storage conditions not only encourage microbial proliferation but can also lead to enzymatic degradation and spoilage. Psychrotrophic bacteria, for example, can thrive at refrigeration temperatures and produce heat-stable enzymes that persist even after pasteurization, impacting milk flavor and quality.

Because of these factors, maintaining strict hygiene from udder to consumer is critical. This involves proper sanitation of equipment, immediate cooling, and adherence to safe handling practices to prevent foodborne illnesses and preserve milk’s nutritional value.

Harmful microorganisms found in milk include Escherichia coli, Salmonella, Listeria monocytogenes, and Mycobacterium bovis, which can cause serious foodborne illnesses.

Major Harmful Microorganisms found in Milk

Raw milk is a major source of harmful pathogens. This milk is completely untreated which means it is unpasteurized and there is the presence of harmful bacteria in the milk. At the time of handing, packaging too, milk can get in contact with such pathogens which can contaminate it and can lead to its spoilage. Hence, it is also harmful in terms of the economy. Following is the table which contains all the major bacteria and other harmful microorganisms found in milk, their diseases, and their sources.

Sources

Yersinia enterocolitica

Milk can be made pathogen-free through several practices, with pasteurization being one of the most effective and widely used methods. Pasteurization involves heating the milk to a specific temperature for a defined period, followed by rapid cooling. This process is designed to destroy pathogenic microorganisms such as Mycobacterium bovis (responsible for bovine tuberculosis), Listeria monocytogenes, Salmonella spp., and Escherichia coli O157:H7, while minimizing changes to the milk’s nutritional and sensory qualities.

There are different pasteurization techniques, including:

• Low-Temperature Long-Time (LTLT): Heating milk to about 63 °C (145 °F) for 30 minutes.

• High-Temperature Short-Time (HTST): Heating to 72 °C (161 °F) for at least 15 seconds, which is the most common in commercial dairy processing.

• Ultra-High Temperature (UHT): Heating to 135 °C (275 °F) for 2–5 seconds, resulting in milk that can be stored unopened for several months without refrigeration.

By effectively inactivating harmful bacteria, pasteurization greatly reduces the risk of milkborne illnesses without significantly affecting its vitamin and protein content.

How Heat Destroys Bacteria at the Microbiological Level

When bacteria are exposed to sufficiently high temperatures, several destructive processes occur simultaneously, targeting essential components of their cellular structure and metabolism.

1. Protein (Enzyme) Denaturation

   • Most bacterial enzymes are globular proteins with complex three-dimensional structures maintained by hydrogen bonds, ionic bonds, hydrophobic interactions, and disulfide bridges.

   • Heat increases the kinetic energy of molecules, causing these bonds to break.

   • Once the tertiary and quaternary structures of these proteins unravel (denature), the active sites deform, rendering enzymes non-functional.

   • Without functional enzymes, bacteria cannot perform critical life processes such as DNA replication, transcription, translation, and metabolic reactions.

2. Cell Membrane Damage

   • The bacterial plasma membrane is a lipid bilayer made of phospholipids interspersed with proteins.

   • Heat disrupts hydrophobic interactions within the membrane, increasing fluidity to the point of compromising structural integrity.

   • As the membrane loses its semi-permeable properties, it becomes leaky, allowing uncontrolled flow of ions, nutrients, and water.

   • This osmotic imbalance leads to cell lysis or collapse of metabolic gradients essential for ATP synthesis.

3. Nucleic Acid Damage

   • Elevated temperatures can cause irreversible damage to bacterial DNA and RNA by breaking hydrogen bonds between base pairs, leading to strand separation or fragmentation.

   • Heat also accelerates depurination (loss of purine bases), impairing genetic stability and stopping replication.

4. Thermal Death Point & Time

   • Each bacterial species has a thermal death point (TDP) — the lowest temperature at which all cells in a suspension are killed within 10 minutes.

   • They also have a thermal death time (TDT) — the minimum time required to kill all cells at a given temperature.

   • For example: Escherichia coli may be destroyed at 70°C within seconds, whereas heat-resistant spores of Bacillus or Clostridium require higher temperatures (121°C for 15 minutes under steam pressure in an autoclave).

5. Synergistic Effects

   • Heat often works in combination with dehydration during dry heat sterilization, which removes water essential for biochemical reactions, further inhibiting bacterial survival.

   • In moist heat (e.g., boiling, autoclaving), denaturation is accelerated because water molecules facilitate disruption of hydrogen bonds in proteins.