Learn the science behind:
Ever wondered why you can’t store a peach at room temperature for several days, but why a peach flavored candy or peach jam will stay good for years? Or why fresh chilis may turn moldy in the fridge, whereas dried chilis stay free of mold for years? Why fresh goat’s cheese turns moldy in a matter of days, whereas uncut parmesan cheese can be stored for years?
Water, or more precisely, available water, plays a crucial role in each of these scenarios. That available water is described using the water activity and as such, water activity is a great predictor as to whether microorganisms will be able to spoil your food!
What is water activity?
The water activity of a food, often abbreviated with aw, is a measure for the amount of ‘available’ water in a food. The water activity is expressed in a value ranging from 0 to 1. Pure water has a value of one, all water is available. A product with no available water at all (virtually non-existent in food) has a water activity of zero. For more details we discuss the theory of water activity in greater detail here.
Water activity is different than water content. Water content describes how much water is physically present in a product whereas water activity describes how much if that water is ‘free’. Only ‘free’ water can participate in chemical reactions, migrate throughout the product, or be used by microorganisms!
Water activity & micro organisms
Microorganisms, such as yeast, molds, and bacteria, grow all around us. Our food is covered in them as well. Most microorganisms are harmless, or even good for us. They might help prove a bread dough, brew beer or create yogurt. Others aren’t so good though, they spoil food or can even make us sick.
In order for microorganisms to grow and survive, they need a source of food and water. Most foods contain plenty of molecules such as starches, proteins, and sugars that can serve as food for microorganisms to grow on. Despite there being enough food, microorganisms will not be able to grow though if there is insufficient available water.
You can use the water activity value of the food to determine whether there is enough available water for your microorganism of concern to grow. Once the water activity is below the minimum value for a specific microorganism, it won’t be able to grow anymore. Some microorganisms need a lot of free water, whereas others need considerably less. Keep in mind that this does not mean the microorganism dies at this low water activity. It may survive under these conditions, and start to grow again once there is again enough water.
Scientists have determined the minimum required aw-value for most microorganisms relevant to food.
The aw at which no microorganisms grow (<0,6)
At an aw-value smaller than 0,6, virtually no microorganism will be able to grow. There just isn’t enough water. Crackers, dried pasta, dried spices, and flour are examples of foods that can have such a low water activity. Because of their low water activity, spoilage by microorganisms isn’t a concern as long as they are stored properly.
Preventing growth of bacteria (<0,75)
Most molds can handle lower moisture environments than bacteria. If the water activity is above 0.6, but below 0.75, molds will be able to grow whereas bacteria can’t yet.
Whereas molds and yeasts do visually spoil your food, most of them aren’t harmful as long as they aren’t able to produce toxins (mycotoxins). And even though a lot of molds and yeasts can grow at a water activity value lower than 0.75, a lot of these can’t produce these toxins at such a low water activity.
Peanut butter and dried fruits fall within this category, as do honey, nuts, and some jams.
How to lower the water activity to prevent microbial growth
Before the availability of fridges or widespread cooling, which greatly slows down growth of microorganisms, people needed to find other ways to preserve foods. This was mostly done by lowering the water activity of foods.
Salting meats for instance, greatly lowers their water activity and inhibits the growth of harmful microorganisms. It’s also why butter was salted, helping to extend its shelf life. Salt ‘binds’ the water, making it less accessible.
Another commonly used method is to add a lot of sugar to foods. Sugar also ‘binds’ water and makes it less available. Making jams out of fruit was a great way to preserve fruits throughout the year. Gingerbread (Dutch ontbijtkoek) also uses this strategy.
Alternatively, you can dry foods. By simply decreasing the overall amount of water in the food, you lower the water activity of that food. Drying fruits, vegetables, and meats are all very effective ways to lower the water activity and extend the shelf life of the food by preventing growth of microorganisms!
Combining strategies to improve shelf life
So does that mean that foods with a high water activity cannot be stored safely? No, luckily it does not!
Water activity is just one of many parameters that determines how and if a food spoils. For instance, the pH-value (measure for acidity) also impacts the growth of microorganisms. It’s why a sour fruit jam with the same water activity as a non-sour fruit jam has a far longer shelf life.
Aside from pH, you can also use temperature to control the growth of microorganisms. Most microorganisms prefer warm, room temperature or slightly warmer temperatures to grow. Storing food in the fridge, or even freezer, completely stops the growth of some microorganisms or slows it down considerably. A heat treatment, such as pasteurization, can kill microorganisms. If they’re dead they can’t grow anymore, although you’ll have to be very careful not to reintroduce microorganisms after such as heat treatment.
Disclaimer: This post is written for informational purposes only and should not be used to determine your food safety procedures or shelf life. For proper consultation, contact a (local) professional or expert within the field who understands and evaluates your specific circumstances.
J. Kramer, Lebensmittelmikrobiologie, 5. Auflage, 2007
Labcell, Water activity and growth of microorganisms in food, link
M. Shafiur Rahmann, Handbook of food preservation, 2007, 2nd edition, p. 467, link
Safefood 360, Water activity (aw) in foods, 2014, link