What happens if you leave a dry cracker lying around on the kitchen counter? And what happens to a freshly baked bread? What will happen to a tomato? The dry cracker might have become a little less crunchy, but will otherwise be fine. Your bread might have actually started moulding, but the tomato will surely have moulds growing on it after several days.
Now what about a moist fresh goat’s cheese and a parmezan cheese? The goat’s cheese will have moulds growing for sure, whereas the parmezan cheese might well be kept for a lot more days without any growth of micro organisms!
Whereas a lot of factors play a very important role in spoilage of food through micro organisms, water activity definitely is one of them. A product with a high water activity (bread or that fresh goat’s cheese) will spoil a lot more easily than one with a very low water activity (that cracker or dry parmezan cheese). In this post we will be diving into that relationship between water activity & growth of micro organisms.
In several other posts we’ve discussed this question in a lot more detail. Summarized, the water activity of a food is a measure for the amount of ‘available’ water in a food. Available water can be used for chemical reactions, but also by micro organisms for instance. The water activity does not say how much water in total sits in the food. Foods with the same water activity may have a very different water content.
Every food has a value for water activity and this value always lies between 0 and 1. A very high value indicates a high water activity, thus quite a lot of available water, whereas a low value indicates little available water.
Side note: The aw-value is an abbreviation for the water activity value of a food. This aw-value can often be found in literature sources.
Water activity & micro organisms
Micro organisms grow all around us and also sit on our foods. Most micro organisms are harmless, or even good for us. They might help proof a bread dough, brew beer or they might be healthy for us. Others aren’t so good for us and can make us sick. Both types of micro organisms can sit on our food.
In order for micro organisms to grow and survive on our food, they need food (often sugars), some sort of gas (some need oxygen, others don’t) and water (as we discussed when introducing micro organisms to you). All micro organisms need at least a bit of water. The amount of water they need though can differ quite a bit. Some micro organisms need quite a lot to survive, whereas others only need very little.
When it comes to water in food and micro organisms the water activity value of a food is used to indicate whether there’s enough water available for certain micro organisms to grow on. Researchers have identified the water activity values that micro organisms need to survive. Generally, the lower the water activity, the lower the amount of micro organism types than can grow on food.
The aw that stops virtually all microbial growth (<0,6)
At an aw-value smaller than 0,6 virtually no micro organisms will be able to grow on a food. When a product is dried than that, it can be kept forever (with regards to micro organisms) as long as the water activity remains low enough. Examples of foods with such a low water activity are dried pasta, peanut butter, flour or dehydrated soups (source). As you see, these are typically quite dry foods, most foods will not belong to this category, and thus growth of micro organisms is possible.
Stopping growth of pathogenic micro organisms (<0,9)
Luckily, the micro organisms that can make use really sick don’t need such a low aw-value for growth to be stopped. At an aw-value lower than 0,9 most bacteria will not be able to grow in food anymore. These include Salmonella, L. monocytogenes, Clostridium and Bacillus. Foods that would belong in that category of just below 0,9 are products such as salami, sweetened condensed milk and maple syrup. Do note though, that the actual water activity of a food greatly depends on the recipe and production process, so might not be the same for different recipes.
So products in this range tend to be pretty safe, in the sense that most bacteria that make you sick will not be able to grow anymore. However, there are still plenty micro organisms that can grow in these, especially yeasts and moulds (hence the mould growth on maple syrup).
Water activity with some growth only (<0,8)
So at a water activity value below 0,9 most pathogenic bacteria will not be able to growth anymore. These foods are often considered less risky by food authorities. However, they can still spoil through growth of micro organisms. Mostly moulds and yeasts can still grow and if these are able to produce toxins, the food can still be unsafe to eat. A benefit though, is that the growth of these moulds and yeasts is often visible since hairy structures start forming on the food.
Only when the water activity is reduced down further, below 0,8 does the growth of most moulds and yeasts stop. Remember, there are still some moulds, bacteria and yeasts that can grow now, these have adjusted themselves to cope with a lot aw-value.
Examples of foods that have a water activity higher than 0,6 but lower than 0,8 are dried fruits, honey, nuts and some jams. These foods can be stored pretty easily and the chances of micro organisms growing are quite small, especially when stored well. That said, there are still micro organisms that are able to grow on these products.
Water activity & other types of spoilage
Lowering water activity can be very beneficial for the shelf life of foods. However, lowering water activity will also change other properties of your food. It might become very dry. In reality, any fresh food such as milk, bread, meat, fruit and vegetables has a water activity well above 0,9, closer to 0,95-0,98 and will those spoil faster.
Adjusting water activity will also influence other processes in foods. We won’t zoom in on those here, but the speed of enzymatic reactions or other browning reactions is influenced by the water activity as is the oxidation of fat. Sometimes a lower water activity can actually speed up these reactions, you might not actually want this! Shelf life of food thus remains quite complex, and proper validation is often required, as we discussed here.
Handbook of food preservation, p. 467
Lebensmittelmikrobiologie, J. Kramer, 5. Auflage, 2007
Further reading on food safety & water activity
Disclaimer: this post is written for informational purposes only and should not be used to determine your food production procedures. For proper consultation, contact a (local) professional or expert within the field.