Factors Affecting Microbial Growth

A continual challenge that we have in the food industry is keeping microbial contamination and growth under control. Therefore, it’s important to have an understanding of the conditions that aid microbial growth. This knowledge will help you develop strategies to control them. An easy way to remember the key  conditions that affects the growth of microbes is the acronym FAT-TOM. It stands for food, acidity, time, temperature, oxygen, and moisture. Let’s take a look at each of these conditions.

Food

Food is a nutrient and energy source for microbes. Therefore it is essential to clean and sanitize all food contact surfaces thoroughly. Even small amount of food left on equipment, especially in the hard-to-clean areas will provide enough nutrients to sustain bacteria. Overtime, poor cleaning can result in some bacteria forming biofilms on equipment. Once these are established, they are very hard to be removed.

Acidity

All microorganisms have an optimum pH at which they thrive. Bacteria are generally unable to grow in acidic environments below a pH of 4.6 while yeast and molds have a wide tolerance range being able to grow at a pH of 2 and above. Foods acidified below 4.6 are called acidified foods. An example would be ketchup. Due to the inability for bacteria to grow at low pH, acidified foods require less heat to sterilize. Foods with pH above 4.6 such as canned potatoes, canned carrots and canned beans are called low-acid foods.  They require very high temperatures to cook and must be hermetically sealed for long term storage.

Time

Like all living systems, microbes including bacteria need time to grow. Bacteria multiply by a process called binary fission where they divide approximately every 20 minutes, as long as they have access to the right conditions. Their growth cycle have four phases. These include a lag, log, stationary and a death phase.  These phases are characterized by no growth as the bacteria is getting used to the environment (lag phase), rapid growth (log phase), no net growth as the number produced is equal to the number that die (stationary phase), and negative growth as the environmental conditions become too toxic for growth and as nutrients are depleted (death phase). Your plan should be to prevent bacteria from entering into the growth phase. You can do this by reducing the time that the bacteria is exposed to the required conditions for growth. For example, presence of food on equipment, and exposure to optimum conditions of pH, temperature, oxygen and moisture.

Temperature

Bacteria have different optimal growth temperatures. Some like it cold, most like it warm, and a few like it hot. Here is how they are classified by temperature:

1. Psycrophiles: Bacteria that prefer the refrigeration temperatures of 15^oC or lower
2. Pychrotrophs: Bacteria that prefer cool temperatures between 15 and 20 ^oC
3. Mesophiles: Bacteria that prefer temperature that fall within room and body temperature range i.e. 20 to 45 ^oC
4. Thermophiles: Bacteria that prefer hot environments i.e. greater than 45 ^oC

Try to remember these categories, but if not, do know that in order to prevent pathogens from becoming a problem, you must keep them out of the danger zone. The danger zone is the temperature between 5 ^oC and 60 ^oC. The general rule is that you must “keep cold foods cold and hot foods hot”.

Oxygen

Bacteria can also be classified based on how much they tolerate oxygen. Here are the three general categories to know:  

1. Aerobes: Bacteria that need oxygen to grow
2. Anaerobes: Bacteria that cannot grow when oxygen is present
3. Facultative anaerobes: Bacteria that will generally need oxygen to grow but if it is not present, they find a way to survive without it

Vacuum packing food is a way to remove oxygen and prevent aerobic bacteria from growing. Another strategy to accomplish this is to replace air with another type of gas, for example, nitrogen or carbon dioxide. This is called modified atmospheric packaging (MAP).

Moisture

All living systems need water to survive. Without water, chemical reactions come to a stop and cells die. The total amount of water in food is reported as moisture while the amount of available water is reported as water activity. Available water simply means water that is available for chemical reactions and microbial growth. Not all the water in food is available since some are bound by food components such as sugars, lipids and proteins. Bound water is not involved in chemical reactions that lead to spoilage nor does it provide moisture to microbes. Therefore, the lower the water activity the longer the shelf life of the food.  The graph below shows different effects of water activity. Note that water activity ranges from 0 to 1 where 1 is the highest. Pure water would have a water activity of 1. From the graph below you can see that molds are the most tolerable to dry conditions (low water activity) followed by yeast and then bacteria which are the least tolerant. Therefore bacteria is unlikely to grow in dried flours, spices and dehydrated fruits but molds may grow.  The graph also that enzymatic activity gradually increases from a low water activity to high, therefore metabolic reactions leading to spoilage of food will increase, the greater the quantity of free water present. Adjusting to a lower water activity to extend shelf life can be done by food dehydration or by adding salts or sugars. For example, adding curing salts to meats is a way of reducing its water activity.