Microorganisms grow only when environmental and food conditions permit. Important factors include temperature, time, acidity (pH), available water (water activity), nutrients and the surrounding atmosphere. Understanding these factors helps explain why refrigeration, drying, acidification, salting and other preservation methods can slow microbial growth.

Temperature is one of the most important factors consumers can control. Refrigeration slows the growth of many microorganisms but does not kill most of them. Keep refrigerators at 40°F (4°C) or below and freezers at 0°F (-18°C) or below. Freezing stops microbial growth while food remains frozen, but microorganisms that survive can become active again after thawing.

Water activity describes the water in a food that is available for microbial growth. Drying food, or binding water with ingredients such as salt or sugar, can reduce water activity and inhibit many microorganisms. Acidity is also important: many disease-causing bacteria grow poorly in sufficiently acidic foods, although yeasts, molds and some acid-tolerant microorganisms may still grow.

Time matters as well as temperature. Perishable foods should not remain at room temperature for extended periods. Refrigerate or freeze them within 2 hours, or within 1 hour when exposed to temperatures above 90°F (32°C). Divide large quantities of leftovers into shallow containers so they cool more rapidly.

Prevent cross-contamination during both storage and preparation. Keep raw meat, poultry, seafood and eggs separate from foods that will be eaten without further cooking. Store raw animal foods in sealed containers or packages that prevent juices from dripping onto other foods. Wash hands, cutting boards, utensils and work surfaces thoroughly after contact with raw foods.

Thaw frozen foods safely in the refrigerator, in cold water, or in the microwave. Do not thaw perishable food on the kitchen counter. Food thawed in cold water or in the microwave should be cooked promptly.

RECOMMENDED RESOURCES

Refrigerator Thermometers — Cold Facts about Food Safety — FDA

FDA guidance on refrigerator and freezer temperatures, safe storage, cross-contamination and the clean-separate-cook-chill approach.

How Do You Thaw Food Safely? — USDA Food Safety and Inspection Service

Current USDA guidance identifying refrigerator, cold-water and microwave thawing as safe methods and explaining why countertop thawing is unsafe.

Cold Food Storage Chart — FoodSafety.gov

Recommended refrigerator and freezer storage times for a wide range of foods.

FoodKeeper — FoodSafety.gov

Searchable storage information designed to help consumers maintain food safety and quality and reduce unnecessary waste.

Basics of Handling Food Safely (US Department of Agriculture-Food Safety Inspection Service)

Factors that affect food shelf-life

Factors that affect microbial growth, and consequently how long they can be stored, can be separated into to groups:

• Factors in the food (intrinsic) – ones that we have very little control over, and
• Factors outside of the food (extrinsic) – ones that we can control

The main factors that control microorganism growth are: nutrients, temperature, pH, water activity and atmosphere.

Let’s look at these factors separately.

Intrinsic factors

Nutrients

Bacterial need nutrients to grow, just like all organisms. Since most foods contain some type of nutrients, foods support the growth of microorganisms.

 pH

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).

Water activity

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.

• The water activity of distilled water = 1.
• The water activity of a something that is completely dry = 0.

However, we can also reduce the amount of free water in other ways – ways that people have used for centuries to preserve food.

• If you have a cup of pure water, then of it is free water. However, if you pour salt into the cup of water, the water molecules will bind to the salt molecules. Only water molecules that aren’t bound to sugar are available to support microbial growth –the more salt that is added, the less free water and the less spoilage. This is the principle behind salt preservation of food, like salted country ham.
• Sugar does the same thing as salt, meaning it binds tightly to water molecules.  This is the principle of sugar-curing food, like jams.

 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.

Extrinsic factors

These are factors that are external to a food. Temperature and atmosphere are the best examples, although humidity is a related factor.

Temperature

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.

Atmosphere

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:

• Aerobic (normal air)
• Modified atmosphere (specific mixtures of gases)
• Vacuum packaging (no oxygen)

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?

Answer: Yes.

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.