Until now, there has been a common belief that food with water activity below 0.6aw is safe, can be easily processed and comes with a very long shelf life. Recent studies have clearly shown that this assumption is not correct. Moreover, most companies are missing optimisation opportunities in the manufacturing process and thereby produce at higher costs than necessary.

 

The stability and shelf life of a product can be preserved by controlling its water activity below a critical threshold. This can be achieved by either removing water or making it unavailable to microbes and chemical reactions. You might wonder how this can be done. Well, we’ve been doing this for centuries, even before the concept of water activity was discovered.

 

The most common and easiest method of controlling water activity is dehydration or drying. Drying removes water and thus reduces water activity, resulting in increased shelf life. Examples include dried fruits and smoked meats. Dry spices like red pepper also have lower water activity and can be stored for longer periods. Drying can be done through various methods such as sun drying, mechanical drying, freeze-drying, and so on.

 

Another common and traditional method for controlling water activity is by adding salts. We preserve vegetables in brine or fruits in sugar syrup. The practice of preserving food with salt and sugar has been used since ancient times. But why do salts preserve food? The answer is that salts reduce the water activity of the product. Added salts (such as salt or sugar) bind with water molecules, making it inaccessible to microbes.

 

A reduction of 0.005 in water activity, from 0.955 to 0.95, reduces the intracellular water content by about 50% and decreases the cell volume by approximately 45%. This demonstrates the sensitivity of microorganisms to even small changes in water activity.

 

Jam and jelly are excellent examples of this technique. A combination of these two methods works even better. We’ve been preserving food using these techniques for several decades, and now we understand that the primary reason for preservation is the control of water activity.

 

Thus, having the correct knowledge of the concepts affecting water activity in the food industry helps us make informed decisions about preservation methods, packaging materials, shelf life, and ultimately designing a safe, high-quality product.