Bacteria are present in nearly every environment, including the foods we consume. While some cause illness, many are harmless, and some provide significant benefits. Milk, a highly nutritious liquid, naturally contains a diverse array of these microscopic organisms. This article explores the types of bacteria found in milk, distinguishing between those that pose risks and those used for beneficial purposes.
Bacteria in Raw Milk
Raw milk naturally harbors various bacteria. These originate from several sources, including the cow’s udder, the surrounding farm environment, milking equipment, and human handling during collection. A healthy cow’s udder contributes a relatively low number of bacteria.
However, conditions like mastitis, an udder infection, can significantly increase bacterial shedding into the milk, potentially introducing millions of bacteria per milliliter. Other common bacteria found in raw milk include Lactic Acid Bacteria (LAB), which contribute to spoilage. Psychrotrophic bacteria are also present and can multiply even under refrigeration, leading to spoilage over time. High counts of bacteria like coliforms can indicate environmental contamination, such as manure on udders or milking equipment.
Ensuring Milk Safety Through Processing
Controlling bacterial populations in milk is achieved primarily through processing methods, with pasteurization being the most widespread. This heat treatment aims to eliminate harmful bacteria and extend milk’s shelf life while largely preserving its nutritional value. Pasteurization involves heating milk to specific temperatures for precise durations, followed by rapid cooling.
One common method is High-Temperature Short-Time (HTST) pasteurization, also known as flash pasteurization, where milk is heated to at least 71.7°C (161°F) for 15 seconds. Another method is Ultra-High Temperature (UHT) processing, which heats milk to even higher temperatures, around 135°C to 150°C (275°F to 302°F), for 1 to 2 seconds. UHT-treated milk, when packaged in properly sealed containers, can be shelf-stable for several months without refrigeration until opened. After pasteurization, refrigeration and proper hygiene during handling and storage are important to prevent the regrowth of any surviving spoilage bacteria and maintain safety.
The Role of Bacteria in Fermented Dairy
Specific bacteria are intentionally introduced into milk to create fermented dairy products. This process, known as fermentation, relies on “starter cultures” or beneficial microorganisms like Lactobacillus, Streptococcus, and Bifidobacterium species. These bacteria convert lactose, the natural sugar in milk, into lactic acid.
The production of lactic acid lowers the milk’s pH, causing it to thicken and develop distinct flavors and textures. This acidification also acts as a natural preservative, inhibiting the growth of spoilage microorganisms. Common examples include yogurt and cheese, where various lactic acid bacteria and sometimes molds contribute to unique characteristics. Some fermented dairy products also contain probiotics, which are live microorganisms that may offer health benefits.
Understanding Harmful Milk Bacteria
While many bacteria are beneficial or harmless, certain pathogenic bacteria can contaminate milk and pose serious health risks. These pathogens can cause foodborne illnesses, particularly in raw, unpasteurized milk. Common pathogens include Salmonella, Listeria monocytogenes, Escherichia coli O157:H7, and Campylobacter.
These bacteria can enter milk through various routes, such as fecal contamination from the animal, unsanitary milking practices, or improper handling. Consuming contaminated milk can lead to severe symptoms like vomiting, diarrhea, abdominal pain, and fever. Illnesses can range from mild discomfort to life-threatening conditions such as Guillain-Barré syndrome, hemolytic uremic syndrome (which can cause kidney failure), or even miscarriage in pregnant women due to Listeria infections. Pasteurization remains a widely recommended public health measure to eliminate these threats and ensure milk safety.