Science Basics for Food Safety and Quality

All raw food items contain microorganisms. These microorganisms can eventually lead to food spoilage or even foodborne illness. In order to prevent food from spoiling or causing illness, we use different methods of food preservation to control or destroy the growth of microorganisms associated with food. By understanding how food microorganisms grow and survive, we can use this information to more effectively preserve food and increase the safety of the foods we eat.

Each year, nearly 1 in 6 Americans suffer from foodborne illness. These illnesses cause approximately 128,000 hospitalizations and 3000 deaths. Most foodborne illness can be prevented through proper food processing and handling techniques. Most food regulations are focused on keeping the food supply safe, and most of the requirements put in place are established with food safety, rather than quality, in mind. However, with a sound understanding of how food microorganisms grow and survive, the food processor can control or destroy microorganisms and even improve food quality.

Microscopic visuals
Microscopic images of A) Bacteria B) Yeast C) Mold and D) Virus particles

Microorganisms that are the primary concern to food processors include bacteria, yeasts, molds, and viruses. Bacteria are associated with both food spoilage and foodborne illness. Yeasts and molds are most often associated with food spoilage. Viruses are a food safety issue.

Microorganisms that cause food spoilage and degrade food quality are not the same microorganisms that cause foodborne illness. You cannot look at a food and know that it is not safe to eat. Food spoilage microorganisms cause food to look, smell, taste, and/or feel different, but will not cause illness. Foodborne pathogens will not alter the look, taste, smell, or feel of a food, but WILL cause illness. Therefore, it is of utmost importance that food is properly processed and preserved to prevent foodborne illness, because there are no obvious signs that foods are not safe to eat.

Not all food microorganisms are bad. In fact, they can be beneficial. Microorganisms are used in the food industry to make bread rise, ferment beer and wine, and to make pickles and sauerkraut. Many yogurts and dairy products contain probiotics, which are bacteria that promote healthy digestion.  

Knowing that there are microorganisms that are desirable in food, as well as those that cause spoilage and possibly even illness, it is important as a food processor to understand the conditions that affect how these microorganisms grow and survive. An understanding of the growth requirements of the different types of microorganisms will allow the food processor to better control or eliminate microorganisms in food. The following factors will influence the growth of bacteria: food, acidity, temperature, time, oxygen, and moisture.

Bacteria must have access to nutrients in order to survive and grow. Often they find their source of nutrients in the same place that we do: in our food. Bacteria need sugars, proteins, and some other materials such as calcium and phosphate to survive. Since removing the food from our food product would leave us empty handed, we must look to other ways to control microorganisms.


Much like there is a scale to measure how hot or cold a substance is (temperature), there is also a scale to measure the acidity of a product: pH. Acidity is perhaps the most important component of food preservation that can be manipulated in food. The pH scale ranges from 0 to 14. Acidity increases as pH values decrease. Below pH 7, foods are considered acidic. At pH of 7, the acidity is neutral. Above 7, foods are considered basic. Almost all food falls into the acidic category (pH below 7). Egg whites are one of the few foods that fall into the basic category (pH ~ 8).

Beyond acidic and basic, we classify foods as acid or low acid. As mentioned above, almost all foods are acidic. But, in order for it to be considered an acid food, the pH must be 4.6 or below. These foods have enough acidity to prevent pathogenic microorganisms from growing. Acid foods include most fruits and fruit juices, vinegar, jams, jellies, and honey. Products may be naturally high in acid or you might acidify products by adding acid (e.g., vinegar, wine, Worcestershire sauce, or lemon juice) to lower the pH.  Pickling foods (by adding acid or through natural fermentation) lowers the pH enough to discourage many microorganisms. Low acid foods have a pH of above 4.6. These foods do not have enough acidity to prevent the growth of pathogenic organisms. Examples of low-acid foods include beans, vegetables, meat, and milk.

Time Temperature

Bacteria grow through binary fission, meaning that they replicate by dividing in two. Most bacteria replicate, or divide in two, every 15 to 20 minutes under ideal conditions. Ideal conditions for pathogenic bacteria are between 41°F and 135°F. This range is often referred to as the “Temperature Danger Zone”. Pathogenic bacteria can proliferate quickly in the Temperature Danger Zone, to the point where they can cause foodborne illness if allowed to grow for long enough time. Therefore, it is important to hold potentially dangerous food below 41°F (refrigeration or freezing) or above 135°F (hot-holding), to keep it out of the Temperature Danger Zone. If a potentially dangerous food is exposed to the Temperature Danger Zone, it may only be there for 4 hours or less. Once it has been exposed to the Temperature Danger Zone for more than 4 hours, it must be thrown out.

Different microorganisms have different oxygen requirements to survive and grow. One group needs the same amount of oxygen that is present in the atmosphere. These are known as aerobic microorganisms. Aerobic microorganisms commonly include yeasts, molds, and spoilage bacteria. Some microorganisms need oxygen to be absent in order to grow. These microorganisms are known as anaerobes. Anaerobic microorganisms can grow when oxygen is removed, like in vacuum packaged foods, or in canned foods. Clostridium botulinum, the bacteria that causes the foodborne illness botulism, is an anaerobic bacteria. Therefore if you have a situation where your food has no oxygen present (canned or vacuum packaged foods), other factors need to be in place to prevent the growth of Clostridium botulinum. A third category of microorganisms are known as facultative anaerobes, which are capable of growth in the presence or absence of oxygen. Most foodborne pathogens are facultative anaerobes. Since aerobic microorganisms need oxygen to grow, their growth can be stopped or delayed by reducing oxygen levels. This is often employed to delay food spoilage. However, reducing oxygen levels may create environments ideal for pathogenic bacteria. Therefore, other growth conditions need to be controlled if oxygen is removed or reduced.

In foods, the moisture content is typically measured using water activity (aw). Water activity is a measurement of the amount of free water that is available in a food for microorganisms to use for growth. In food, water can interact with other components of food, such as salt and sugar. When water binds to these components, it is no longer available for microorganisms to use for growth. Water activity is measured on a scale of 0.0 to 1.0. Pure water has an aw of 1.0. Pathogenic bacteria cannot grow when aw is 0.85 or below. Therefore, aw can be used to preserve foods. There are several ways to lower aw in food. One way is to add salt or sugar to bind water. Another way is to remove water, as is often done with dehydration. Once the aw of a food is brought to 0.85 or below, the food is considered shelf stable and may be stored at room temperature. Water activity is often the method used to keep foods such as jams, jellies, bread, and cookies safe and shelf stable.

awMicroorganisms that can grow
0.95Salmonella, E. coli, Bachillus cereus
0.90C. botulinum, Listeria monocytogenes
0.87Yeasts, Staphylococcus aureus
0.80Molds, Saccharomyces spp.
0.60Some yeasts and molds

Now that you have a basic understanding of how the conditions in food can affect the way microorganisms grow in food, you can manipulate your food product to create a safe product. The main methods of preservation for shelf-stable foods are controlling the water activity or lowering the pH. Factors are often combined, like lowering pH and using refrigeration. Understanding how food supports the growth of microorganisms can help improve both food safety and food quality.

The term pH refers to acidity of a product.  Much like there is a scale to measure heat (temperature), there is also a scale to measure acidity in food. The pH scale ranges from 0 to 14. A value of 7 is assigned to something whose acidity is neutral. Any values below 7 are considered acidic, while values larger than 7 are basic (or alkaline).

In scientific terms, the pH scale actually measures the concentration of positively-charged hydrogen ions [H+] and negatively-charged hydroxyl ions [-OH] that are present in a sample. When the concentration of hydrogen ions increases, so does the acidity. pH is measured using a logarithmic scale, meaning that each change in a whole number represents a tenfold change in [H+] concentration. For example, a sample with a pH of 3.0 has ten times the concentration of [H+] ions as a sample with a pH of 4.0, and a one-hundred times the concentration of [H+] than a sample with a pH of 5.0. Therefore, small changes in pH measurements actually represent large changes in [H+] concentration (and therefore, acidity). For this reason, it is important to get accurate pH readings of any food product tested.


In general, the pH of a product will determine which microorganisms are capable of growing in it. Most microorganisms are able to survive and grow in pH environments between 4.6 and 9. Most food items are naturally acidic, meaning their pH values are less than 7. As the pH values decrease (become more acidic), the microorganisms have a more difficult time surviving and growing. Therefore, the acidity of a food product is often used as a means of preservation and a way to keep food safe for consumption.

There is one important pH value to know in regards to food safety, and that value is pH 4.6. At a pH of 4.6, Clostridium botulinum, the bacteria that causes the deadly disease botulism, is prevented from growing and forming deadly toxin. The pH value 4.6 is used to place foods into different categories based on their food safety risks:

When a food item is comprised solely or mainly of ingredients with a pH of 4.6 or lower, we refer to those food items as an acid food. Acid foods include apples, oranges, and lemons.

Acid or Acid food + Acid or Acid food = Acid food (pH naturally ≤ 4.6)

When the pH of a food item is greater than 4.6, that food is referred to as a low-acid food. Low acid foods include most vegetables and meats.

Low acid food + Acid, Acid food, or Low Acid food = pH > 4.6 = Low acid food

When food items contain some ingredients whose pH is greater than 4.6, but the overall pH of the food is 4.6 or less, that food item is called an acidified food. Acidified foods have a pH of 4.6 or less due to the addition of acid or acid foods to low acid ingredients. Examples of acidified foods are “pickled” products such as pickled beets, cocktail onions, and cherry peppers.

Low acid food + Acid or Acid food = pH ≤ 4.6 = Acidified food

Note: Low acid foods contain lower amounts of acid, and have higher pH values.

Image of gooseberries in various states of aw: Fresh gooseberries, dehydrated gooseberries, and gooseberry marmalade


Water activity (aw) refers to the amount of free water that is available in food for microbial growth. It is a measurement of the water that is not bound to components in the food, and therefore available for microbial growth. All microorganisms have a level of water activity that they prefer to grow within, and have lower limits as to how “dry” a food can be in order for them to grow. For that reason, water activity is often used as a way to preserve foods and gain a longer shelf-life.

Water activity is measured in values from 0.0 to 1.0. Water has an aw of 1.0. Most food falls within a water activity range of 0.2 to 0.99. The lower the aw value, the more “dry” a food item is considered. However, water activity should not be regarded as moisture content. The scientific definition of water activity refers to its measurement in equilibrium relative humidity. You can think of water activity as the humidity of a food.  

Table 1. Water activity ranges of common foods and the microorganisms of concern in those aw ranges
 Microorganisms of concernFoods in this aw range
0.95-0.97+--Fresh meat, fruit, vegetables, canned fruits, canned vegetables, cooked sausage
0.90-0.94+++Some cheese, cured meat (ham), evaporated milk
0.87-0.89+-+Sweetened condensed milk, aged cheeses, dried meats, bacon, chocolate syrup, fondant
0.71-0.79-+-Jam, marmalade, marzipan, molasses, dried figs
0.60-0.70-++Dried fruit, corn syrup, marshmallow, chewing gum
0.00-0.60---Caramels, toffee, honey, cocoa, crackers, dry mixes, boiled sweets, milk powder

A variety of microorganisms can grow in food products, and each microorganism can survive in different water activity ranges. Bacteria require the highest amount of free water to grow, and can be found in products with aw as low as 0.75, but most are inhibited at aw below 0.91. One pathogenic bacteria is able to grow at aw as low as 0.86, so foods that depend on water activity as a means of preservation should have an aw of 0.85 or less. Some yeasts and molds are able to grow on food items with aw as low as 0.60. Below 0.60, yeasts, molds, and bacteria will not proliferate.

The water activity of a food product can be altered in a variety of ways.

  • Salt and/or sugar can be added. Both salt and sugar work by binding with free water in the food product, making that water no longer available to microorganisms for growth. Salt is more effective at binding water than sugar, but the influence of taste should be considered before choosing to alter your recipe.
  • Water activity may also be decreased by removing water from the food product. This can be done during cooking, by “cooking down” your recipe for a longer amount of time and allowing more water to evaporate.
  • Water can also be removed post-cooking through dehydration. This method is often used to preserve fruits and meats. Both work by removing free water, reducing the amount that is available for microorganisms to grow.