WHAT IS WATER ACTIVITY AND WHY DOES IT MATTER?

Often we hear consumers kvetch about food being “full of preservatives.” Twelve-month shelf life? Must be preservatives. No refrigeration? Preservatives. Right? Well, no. As most food scientists would tell you, preservatives, as scary as they seem to consumers, just aren’t that powerful. They can help maintain flavor and color in a product, but to keep it safe from botulism and E. coli for a year--that all depends on water activity.

If you sell food that’s dried, cured, baked, or fried, reason stands that it’s a low water activity that prevents your shelf-stable product from being a breeding ground for fungus and bacteria.

water activity =

the vapor pressure of water bound in a food compared to the vapor pressure of distilled water

Water activity is the measure of the vapor pressure of the moisture inside of a food compared to the evaporative pressure of distilled water at the same temperature. A product with a water activity of 0.65 has 65% the vapor pressure of pure water. The higher the water activity, the freer the ability for the water to migrate through the cells of a food.

water activity

≠

moisture content

Why does that matter? Undesirable microorganisms such as bacteria, yeast, and mold--the ones that spoil food and are toxic to humans--need water to survive. If water activity is low, all the water in a food is bound up in such a way that a bacterial cell can’t absorb the water it needs to thrive and reproduce, rendering it harmless.

why does it matter?

low water activity means water is too bound up to migrate through cell walls, so harmful bacteria don’t have the moisture they would need to grow

Water activity is not the same as moisture. Moisture is the measure of the amount of water in a product by weight. Water activity, by contrast, is the measure of the availability of that moisture. Peanut butter, for example, has a moisture content of 15%. For every 100 grams of peanut butter, 15 grams of that is water. The water activity, however, is closer to 0.70, or 70%. Most fresh foods, including fruits and vegetables, have a water activity of 0.95 or above.

The U.S. Food and Drug Administration defines the safe upper limit for water activity in a shelf stable food at 0.85 (assuming it isn’t controlled for shelf stability in another manner, such as low pH or canning). At Natural Development we tend to hew closer to 0.65 and lower. That’s the sweet spot for nutrition bars and food bites. At any given moment you’ll find one of us in the lab bent over the water activity meter, fingers crossed, praying that the next sample round comes out under 0.65.