Bacteria are microscopic organisms found naturally in diverse environments, including on and around food. While some contribute to food production, others cause spoilage or, as pathogenic bacteria, can cause illness when consumed. Understanding how bacteria grow on food is important for ensuring food safety.
The Favorable Conditions for Growth
Bacteria require specific conditions to grow and multiply on food, often summarized by the acronym FAT TOM: Food, Acidity, Time, Temperature, Oxygen, and Moisture. Food provides the necessary nutrients for bacteria to thrive, with protein-rich items like meat, milk, and eggs being particularly susceptible. Bacteria generally grow best in foods that are slightly acidic to neutral, typically with a pH range of 4.6 to 7.5. Highly acidic foods, such as citrus fruits or vinegar, tend to inhibit bacterial growth.
Time is a significant factor; bacteria multiply rapidly under suitable conditions. Temperature is important; bacteria multiply most rapidly within a “danger zone” (40°F to 140°F / 5°C to 60°C). Within this range, bacteria can double in number in as little as 20 minutes. Keeping foods refrigerated at or below 40°F (4°C) or heated to 140°F (60°C) or above helps to slow or prevent bacterial growth.
Oxygen requirements vary among bacteria; some, like most foodborne pathogens, need oxygen to grow (aerobic), while others can only grow in its absence (anaerobic), such as Clostridium botulinum. Some bacteria are facultative, meaning they can grow with or without oxygen. Moisture, or water activity (aw), is important, as bacteria need available water for metabolic processes. Most bacteria require a water activity above 0.90 or 0.91, meaning dry foods or those with high sugar or salt content (which bind water) generally inhibit their growth.
The Bacterial Multiplication Process
When conditions are favorable, bacteria reproduce through binary fission, a form of asexual reproduction. A single bacterium grows, replicates its genetic material, and then divides into two identical “daughter” cells. Each new cell is a complete organism capable of further division.
This division leads to exponential growth, where the bacterial population doubles with each generation. For instance, if one bacterium divides every 20 minutes, it can lead to a rapid increase in numbers. After just one hour, a single bacterium could become 8 cells, and within a few hours, the population can reach millions.
Common Sources of Contamination
Bacteria can contaminate food from various sources. Raw ingredients often contain bacteria; for example, raw meat, poultry, seafood, and produce from soil can harbor them. The surrounding environment (soil, water, air, dust) can also introduce bacteria.
People are a frequent source, transferring bacteria from hands, clothing, or respiratory droplets during food handling. Poor personal hygiene, like not washing hands thoroughly, facilitates this transfer. Cross-contamination occurs when bacteria are unintentionally moved from one food or surface to another. This occurs when raw foods contact cooked or ready-to-eat foods, or when the same cutting boards or utensils are used for different food types without proper cleaning. Pests like rodents and insects can also carry and transfer bacteria to food.
Impacts of Bacterial Proliferation
Once bacteria proliferate on food, they can lead to two main types of impacts: food spoilage and foodborne illness. Food spoilage makes food undesirable due to changes in appearance, smell, texture, and taste. Spoilage bacteria produce waste products causing unpleasant odors, sliminess, discoloration, or altered tastes. While spoiled food is generally not eaten, these bacteria often do not cause illness.
Foodborne illness, commonly known as food poisoning, occurs when pathogenic bacteria multiply to dangerous levels or produce toxins in food. Unlike spoilage bacteria, pathogenic bacteria often do not cause noticeable changes in the food’s odor, taste, or appearance, making contaminated food seem normal. Symptoms of foodborne illness can vary but commonly include nausea, vomiting, diarrhea, and fever. The onset of symptoms can range from minutes to several days after consuming contaminated food, depending on the specific pathogen or toxin involved.