Factors that affect microbial growth, and consequently how long they can be stored, can be separated into to groups:
The main factors that control microorganism growth are: nutrients, temperature, pH, water activity and atmosphere.
Let’s look at these factors separately.
Bacterial need nutrients to grow, just like all organisms. Since most foods contain some type of nutrients, foods support the growth of microorganisms.
This is a measurement of the acidic or basic nature of food. A low pH value (0 to less than 7) is considered acidic, whereas a high pH value (greater than 7 to 14) is basic. A pH value of 7.0 is neutral (such as that of distilled water).
This is a scientific term that simply means the amount of free water that is in a food. Free means water that is not bound to other substances in a food – this free water is available to microorganisms to help them grow in food. Foods last longer (meaning they don’t support fast microbial growth) if there is less free water.
Let’s look at some examples of what is meant by free water.
However, we can also reduce the amount of free water in other ways – ways that people have used for centuries to preserve food.
Foods that have little acid (low acid food) and a lot of water are named Potentially Hazardous Foods (PHFs). Because PHF easily allow microorganisms to grow, they are the foods that are most inspected by government regulatory agencies.
These are factors that are external to a food. Temperature and atmosphere are the best examples, although humidity is a related factor.
Temperature is the one single factor that we have the most control over. Luckily, the vast majority of microorganisms that make us sick, only grow over a temperature range of 40°F -140°F – this why this temperature range is named The Danger Zone.
One important rule of thumb – the higher the temperature – the faster bacteria, mold and yeast grow. This means that foods don’t last as long (both safety and quality are affected) at higher temperatures, but much longer at low temperatures.
A good rule of thumb is that foods should not be stored longer than 2 hours when they are in The Danger Zone.
You may have noticed that more and more refrigerated foods in the store are in plastic packages. For example, nearly all pre-cut salads come in packages – puffed with a gas. Packaging creates an atmosphere around the food that can have a remarkable effect on preventing spoilage.
Packaging atmospheres are classified as 3 types:
Without any packaging, fresh foods spoil very quickly. This is because the level of oxygen in the atmosphere promotes the growth of the worst types of bacteria, especially ones called Pseudomonas. These are the bacteria that make foods feel slimy.
Modified atmosphere packaging (MAP) is where food is placed inside a plastic bag that contains a mixture of gases, normally combinations of carbon dioxide and nitrogen, but without oxygen. Carbon dioxide is very effective at stopping the worst spoilage microorganisms from growing. Nitrogen is there as a filler and has little effect.
Vacuum packaging is where there the food is placed inside a package and then all of the air is removed using a vacuum. The net effect is that oxygen is removed and spoilage is reduced. This method is commonly used for refrigerated meats, fish and some vegetable products like guacamole.
Question: Do some microorganisms grow better on certain foods?
Molds grow on and spoil drier foods such as bread and hard cheese. Because they also better tolerate acid, they can spoil acidic fruits such as strawberries and peaches.
Bacteria and yeast require a lot of water to grow. As a result, they grow quickly in foods such as milk, meat, poultry and seafood.