A difference in water activity means a difference in an energy level. With a high water activity, the water in the microbe wants to leave, which changes the trigger pressure within the microorganism. The microorganism will try to equal its energies with the water activity in the environment.
The microorganism will alter its membrane to reduce the water activity, thereby maintaining its trigger pressure. It might produce or transport amino acids or sugars in small quantities to try and reduce water activity. However, the water activity still won't match the environment, and the microorganism goes into stasis, or dormancy.
Common food pathogens
Food pathogens fall into two categories: foodborne intoxication and foodborne infection. A foodborne intoxication is when one ingests a toxin produced in the food and becomes sick. A foodborne infection is when the toxin grows in the GI tract. (Intoxication forms in the food, and infection forms in the gut.)
Staph aureus can grow with or without oxygen, has the lowest water activity limit, and is easily cross-contaminated. Therefore, finding Staph aureus on food processing equipment is a sign of poor sanitation. This pathogen growth can be easily prevented using cleaning, sanitizing, appropriate preparation, and minimizing cross-contamination.
Botulism is anaerobic and won't grow at pH levels lower than 4.6. Just three minutes of boiling can kill botulism. It has a higher water activity limit. Botulism exists in nature—soil, plants, water—and in improperly canned foods and low-acid foods—beets, green beans, baked potatoes wrapped in foil, smoked fish, herb-infused oil, and honey.
Salmonella spp. is the number-one reported pathogen. This microbe is a foodborne infection and is more common in the summer months. Salmonella can thrive in oxygen-rich or oxygen-depleted environments, can be killed by cooking, and is found in contaminated feces, drinking water, person-to-person contact, egg products, raw fruits and veggies, unpasteurized milk products, and even in products like flour and peanut butter.
Listeria can grow in refrigerated temperatures, can be killed by cooking and pasteurization, and has a water activity limit of 0.92. Listeria exists in uncooked meats, vegetables, unpasteurized milk, and some soft cheese. In low-temperature, low-oxygen environments, Listeria can grow if present.
Most E. coli strains are harmless and are essential to the intestinal tract. The pathogen strain can be killed through cooking or pasteurization. E. coli has a low infectious dose and is difficult to kill.
Bacillus cereus is anaerobic and has a short incubation time. This pathogen is often confused with the stomach flu.
Reducing water activity to prevent microbial growth