Pasteurized milk is valued for its extended shelf life and safety due to a heating process called pasteurization. This process eliminates harmful microorganisms but does not create a sterile product, meaning some bacteria remain. This article will explore why certain bacteria are present, identify these microorganisms, and discuss their effects.
Understanding Pasteurization and Bacterial Survival
The goal of pasteurization is to destroy pathogenic bacteria and reduce organisms that cause spoilage. This heat treatment, typically to at least 72°C (161°F) for 15 seconds, makes milk safer and extends its usability. As a heat treatment rather than sterilization, not all microorganisms are eliminated, as some bacteria are naturally resistant to these temperatures.
These heat-resistant microbes are known as thermoduric bacteria. Another category of survivors includes bacterial spores, which are dormant structures produced by bacteria like Bacillus species. These spores are resistant to heat and can later germinate into active bacteria when conditions become favorable.
Bacteria can also be introduced after pasteurization, known as post-pasteurization contamination (PPC). This occurs if milk contacts non-sterile surfaces like pipes or packaging materials. Psychrotrophic, or cold-tolerant, bacteria are often introduced this way and are a primary cause of spoilage in refrigerated milk.
Common Bacteria Found Post-Pasteurization
One significant group of bacteria is psychrotrophic, defined by their ability to grow at cold temperatures like those in a refrigerator. Species of Pseudomonas are a major concern in this group and are often introduced through post-pasteurization contamination. These organisms produce heat-stable enzymes that break down milk components, leading to spoilage.
Another group consists of spore-forming bacteria, particularly from the Bacillus and Paenibacillus genera. Their spores can withstand pasteurization temperatures and germinate once the milk is cooled and stored. While many are thermoduric, some are also psychrotrophic, enabling them to grow slowly during refrigerated storage and cause spoilage.
Lactic acid bacteria (LAB) can also be present in pasteurized milk. Some hardy strains can endure the heating process, while others may be introduced via post-pasteurization contamination. These bacteria are known for fermenting lactose into lactic acid, which causes milk to develop a sour taste as it spoils.
Bacterial Impact on Milk Spoilage
The presence and growth of bacteria are the direct cause of milk spoilage, characterized by changes in smell, taste, and texture. The breakdown of milk fats and proteins by enzymes from bacteria like Pseudomonas produces volatile compounds. These compounds are responsible for a range of off-flavors, from bitter and soapy to fruity or putrid.
Changes in texture are also an indicator of spoilage. The production of acid by lactic acid bacteria lowers the pH of the milk, causing milk proteins to clump together in a process known as curdling. Other bacteria can produce substances that result in a slimy or ropy consistency.
The rate of spoilage is highly dependent on storage temperature. While psychrotrophic bacteria can grow at refrigeration temperatures, their growth is much faster at warmer temperatures. Keeping milk consistently cold, ideally between 34-38°F, significantly slows bacterial multiplication and the spoilage process.
Health Implications of Bacteria in Pasteurized Milk
For commercially produced milk that has been properly handled, the bacteria found are spoilage organisms rather than pathogens. These bacteria, such as Pseudomonas, Bacillus, and lactic acid bacteria, are not considered harmful to human health. Consuming milk with low levels of these organisms is unlikely to cause illness, though the flavor and smell may be unpleasant.
When milk becomes heavily spoiled, the bacterial count can be extremely high. Ingesting such a product could potentially lead to mild gastrointestinal discomfort in some individuals. This is a result of the high load of spoilage organisms, not dangerous pathogens.
The risk of illness from pasteurized milk arises if the pasteurization process fails or if significant contamination with pathogens occurs after treatment. Such events are rare in modern dairy processing due to strict safety and quality control protocols. Consumers can ensure safety by proper refrigeration and adherence to the “use-by” date, which minimizes the growth of any surviving spoilage bacteria.