Available Moisture in Foods: What Is It Anyway?

Food preservation methods such as drying, freezing and adding salt or sugar work by lowering the available moisture in foods. Moisture in foods occurs in two forms: (1) water bound to ingredients in the food (proteins, salt, sugars), and (2) free or unbound water that is available for microbial growth. Water activity (Aw) describes water available for microbial growth and ranges from 0 (bone dry) to 1.0 (pure water). Water activity is controlled by removing water (drying or freezing), by adding salt (curing) or by adding sugar (jams, jellies). Water activity is a good predictor of food safety and of how long a food product will last on the shelf.
 
The relationship between total moisture and water activity is complex, is related to the relative humidity of the food and its water content and must be determined for each specific food. Foods with higher moisture content might be expected to have higher water activity than dry foods but the expectation is not necessarily correct. Products with the same water content may have very different water activities. For example, salami and cooked beef have similar total moisture (approximately 60%). However, the water activity of salami is 0.82 and that of cooked beef is about 0.98.

Total Moisture and Available Moisture of Common Food Products

Food

Total Moisture (%)

Available Moisture (Aw)

Peppers

92

0.99

Broccoli

91

0.99

Oranges

87

0.99

Apples

84

0.98

Chicken, raw

69

0.99

Beef, raw

73

0.99

Beef, cooked

60

0.98

Chicken, cooked

62

0.91 to 0.98

Bread, commercial

36

0.94 to 0.96

Beef jerky

23

<0.80

Jams/preserves

30

0.75 to 0.80

Cocoa

7 to 10

0.70

Peanut butter

2

0.70

Whole milk powder

7

0.70

Whole milk powder

2 to 3

0.03

Dried fruits

31

0.60 to 0.65

Source: http://www.gov.mb.ca/agriculture/food-safety/at-the-food-processor/water-content-water-activity.html#relationship

The majority of microorganisms that cause spoilage or foodborne disease grow well in foods with a water activity (Aw) of 0.91 to 0.99. Assuming other conditions are favorable, the organism that causes botulism, Clostridium botulinum, will grow and produce toxin in foods with a water activity (Aw) above 0.93. Staphylococcus aureus grows in foods containing high levels of salt at a water activity (Aw) above 0.85 but produces staphylococcal toxin in foods with an Aw of 0.93 or higher. The following table lists sample foods, their range of water activities and the microorganisms that can grow in them.

Water Activity of Some Foods and Microorganisms that Can Grow in Them

Aw

Microorganisms That Grow at or above This Aw

Typical Foods Containing This Aw or Higher Aw

>0.95

Spoilage bacteria, some yeasts, Bacillus, Clostridium perfringens

Fresh and canned fruits, vegetables, meat, fish, milk, cooked sausages, foods containing up to 7% salt, breads

0.91

Salmonella, Vibrios, Clostridium botulinum, some molds

Cheddar cheese, Swiss cheese, cured meat, fruit juice concentrates with 55% table sugar, foods with 12% salt

0.85

Most yeasts, Staphylococcus aureus

Fermented sausage, sponge cakes, dry cheese, foods containing 65% table sugar or 15% salt, margarine,

0.80

Most molds, bread yeasts

Jams, marmalades, fruit preserves, condensed milk, most fruit juice concentrates, syrup, flour, high-sugar cakes

0.75

Most salt-loving bacteria, molds that produce aflatoxins

Glace fruits, marzipan, marshmallows, ground black pepper, Parmesan cheese

0.70

Dry-tolerant molds

Peanut butter containing 15% total moisture; dry milk containing 8% total moisture

0.60

Some sugar-loving yeasts, a few dry-tolerant molds

Dried fruits containing 15 to 20% total moisture, caramel, toffee, honey

0.50

 

Noodles with 12% moisture, spices with 10% total moisture

0.40

 

Whole egg powder with 5% total moisture

0.03

 

Whole milk powder with 2 to 3% total moisture, dry soups

Table modified from  https://extension.psu.edu/food-safety-and-quality/home-food-safety

Water activity cannot be measured at home so it is important to following science-based directions for drying foods. Following science-based procedures reduces the available moisture (Aw) in foods to less than 0.60 so that microorganisms cannot grow. Storing properly dried foods in glass jars, metal cans or boxes with tightly fitted lids or moisture-vapor resistant freezer containers prevents them from re-absorbing water from the atmosphere.

Sources: