What Factors Determine Microbial Growth?

A constant 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 what factors determine microbial growth. This knowledge will help you develop strategies to control them. An easy way to remember the key conditions that affect 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 a small amount of food left on equipment, especially in the hard-to-clean areas will provide enough nutrients to sustain bacteria. Over time, poor cleaning can result in some bacteria forming biofilms on equipment. Once these are established, they are very hard to be removed.

Acidity

All bacteria have an optimum pH at which they thrive. Acidic environments inhibit bacterial growth and cause them to be more susceptible to thermal kill. Foods can be acidified by adding various food-safe acids such as ascorbic, citric and acetic acid (vinegar). If the pH of the food product is above 4.6 and below 7.0, it’s classified as “low acid” (note that low acid does not mean low pH). Low acid foods will need other interventions to control bacteria such as additional heat and hermetically sealed packages. A pH of 4.6 and below will inhibit Clostridium botulinum and most other pathogens. Foods to which acid has been added to reduce the pH below 4.6 are called acidified foods. Yeasts and molds favor an acidic environment compared to bacteria and have a much wider pH tolerance range (2 to 11).

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 has four phases. These include a lag, log, stationary, and death phase. During the lag phase, there is little growth. They use this time to get adjusted to the environment, just like how we take time to wake up in the morning if you want to look at it that way. However, once they get adjusted and the right conditions are available such as breakfast, juice, and warmth…………; well, I mean food, moisture, and the right temperature, they accelerate through a log phase of rapid growth.

Your plan should be to prevent bacteria from getting into this growth phase. The way to do that is to control the conditions that would facilitate this growth burst. For example, as mentioned earlier, you can keep food away from them by cleaning and properly sanitizing equipment.

The stationary phase is a phase where there is no further growth since the bacteria are dying at the same rate that new ones are being formed. During this phase, there is a lot of competition for nutrients. In order to survive, some bacteria will produce spores. You can think of spores as “seed capsules” that the bacteria produce in order to shelter themselves. Spores are much more resistant to harsh environments e.g. heat, acid, toxins, and dehydration, than the normal vegetative form of the bacteria.

In the death phase, more bacteria are dying than new ones are being formed. Therefore, their total population begins to decline. Look at it this way, the bacteria are actually dying in their own “filth”, i.e. their byproducts (alcohols, acids, toxins, etc.) that accumulate to toxic concentrations.

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. Psychrophiles: Bacteria that prefer refrigeration temperatures of 15^oC or lower
2. Pychrotrophs: Bacteria that prefer cool temperatures between 15 and 20 oC
3. Mesophiles: Bacteria that prefer temperatures that fall within the 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 (40 ^oF) and 60 ^oC (140 ^oF). 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 amount of water in food may be reported as moisture or water activity. Water activity is a more accurate indicator of potential bacteria growth than moisture. That is because what’s important in the shelf life of food is not the total moisture, but how much free or active water is present in the food material. Only the free water will be accessible to microorganisms.

Water activity ranges from zero to one, where a water activity of one has the maximum amount of free water possible. The water activity value will dictate what type of organisms are likely to grow in your food. The water activity graph below illustrates this. For example, bacteria will start to grow when the water activity is at 0.9 or above.