How water activity relates to microbial growth

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.

What is water activity?

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).

making jam: fig and grape jam in bowls
Jam, it has a water activity low enough to prevent growth of a lot of micro organisms, but moulds and yeasts can still grow.

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.

Sources

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.

11 comments

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  • Hi. My question is related to vapour pressure of a substance like milk where along with water there are proteins, fats and other components. As mentioned in the depression in freezing point video, these solute molecules will also occupy the surface along with water thus reducing the amount of water on the surface. This should in turn make it difficult for water molecules with enough kinetic energy to escape and reduce the vapour pressure. This will also reduce the water activity as lesser water is available on the surface.
    Won’t there be a significant difference in the vapour pressure of milk and water? How is it that that the activity still remains in the range of 0.95-0.98?
    Is my understanding of vapour pressure and water activity correct in this case?

    • Hi Shriya,

      Thank you for your question! I think you understand the concepts well and understand your confusion.The reason that the water activity of milk is still very high, is that you will need a lot more solute molecules in there to bring down the water activity. Milk is still over 85% water. This link shows that to bring the water activity of a solution of only water and sugar down below 0.95, you need as much as 40w% sucrose! This is also why fresh fruits (which actually contain quite a bit of sugar) still have such a high water activity, you really need a lot less free water molecules to bring it down.
      A quick addition as well, the fats don’t impact the water activity (since it doesn’t dissolve in the water).
      Also, the vapor pressure of milk and water are very similar according to the Engineering Toolbox.

      Hope that helps!

      Did you also see our other post on water activity?

  • Hi thanks for the detailed reply. I just had one quick question.
    Since fats don’t dissolve in water they don’t affect the water activity because they don’t become an integral part of the system. However, most fats being lighter will float on the water medium or solvent medium right? So won’t they form like a barrier for the solvent molecules to vaporise? Won’t that in turn produce resistance and decrease the vapour pressure and hence the water activity as well?
    Or is it that since again the percentage of fat in the overall system is a lot lesser when compared to the solvent that it doesn’t affect it as much.

    • Hi Shriya,

      I understand where you come from, nice thinking! But it’s not entirely correct :-).

      Let’s take a cookie as an example. The cookie will have a certain water activity, determined by the moisture content and presence of sugar, salt, etc. This cookie can now be coated in a full layer of chocolate. Since chocolate contains a lot of fat, this is essentially a fat barrier around the cookie.
      You are correct that the fat will prevent exchange of moisture between the cookie and the environment. However, the water activity of the cookie itself stays the same. If micro o organisms could grow in there before, they probably still can (assuming they don’t need oxygen for instance). The cookie itself isn’t changed. What the chocolate layer does prevent is drying out of the cookie or absorption of extra water by the cookie. So it actually helps it maintain its water activity.
      I understand why you’re confused about the vapor pressure, however, in these case you have to look at the individual components to determine the water activity, even though the shelf life of a food will depend on the whole thing together. That’s one of the reasons food science tends to get complicated in real life :-)!

  • Hi, I understand that moisture or water content doesnt relate to aw directly, my question is, for a certain product, is there a way to reduce the aw by heating or drying?
    e.g.: If a product is with 0.1% water content and a test shows aw at 0.8. Can we dry the product further to less than 0.1%, will the aw be reduced relatively? Or, if we don’t use sugar or salt or any ingredients to bond the water, is there a way that we can reduce the aw by any kind of activity?
    Thanks a lot!

    • Hi Jason,

      Even though they are not linked directly, reducing the amount of water while not changing anything else will definitely reduce the water activity. Therefore, drying will definitely lower the water activity. It might take some effort to bring it down to the value you want to.
      Hope that helps!

  • What works best to assure baked goods freshness (preservative) and in what quantity it should be added when making bread,cakes and pastries

    • Hi Adris,

      This is a very complicated but great question! It is very hard to preserve bread well for long periods of time without also affecting the texture of the bread. For instance, a really good French baguette is known for its short shelf life. Within a day the whole baguette has turned rock hard. Baguettes that stay good longer are generally of less quality and don’t have that great contrast between very crispy outside and soft inside. What you may hear from this, most baked goods (unless there’s some sort of a filling that’s not fully cooked) don’t spoil because of micro organisms. Most spoil within days because they get old and stale.

      That said, there are things to be done, for instance:
      – Use sourdough instead of regular dough, that tends to help the dough stay soft.
      – Use enzymes that help break down some of those starches which cause retrogradation.
      – Various additives can be added, to name a few glycerol (to keep it soft), calcium propionate (this one does prevent growth of micro organisms), phosphates or calcium stearoyl-2-lactylate. With these additives, keep in mind the legislation in your country. Not every ingredient is allowed and also within bakery there might be differences between categories (e.g. bread vs cookies).

      If you would like some inspiration for your specific product you may want to buy some products in the supermarket that are similar which stay good for several days and have a look at their ingredient list. Send me an e-mail if you’d like help understanding terminology.

      Hope that helps!

  • We are dehydrating tomatoes sliced and also fruit leather.
    We just have purchased a water activity tester. What is the recommended number for stopping any microbes for both items. Our tomatoes have tested at .62 our fruit roll ups are below that except the straight apple one.
    Thanks
    Bernice Neff
    Glenwood Valley Farms Ltd.
    Langley BC Canada

    • Hi Bernice,

      Thanks for coming by! Buying a water activity tester is a great idea for these types of projects. It is a great way to control quality over time and ensure your products are consistent. Do take care that you packed your products shortly after dehydration and use packaging material that keeps moisture out. Leaving them open and exposed to the air may increase the water activity again (especially on humid days). If you treat these dehydrated products well they should easily keep for months if not longer.

      That said, your water activity is in a good range. There is virtually no micro organism that can still grow below a water activity of 0.6 so any value below that is a safe choice. Also, since your fruits will from themselves be slightly acidic they tend to spoil less quickly. The 0.62 value of your tomatoes is just a little high, no bacteria will be able to grow anymore but there are some moulds and yeasts that may still do so. If I were you I would try to go below 0.6. What I would always recommend is that of the first few batches you make you pack the samples and keep a few for yourself to do a simple shelf life study. Store them in the way your customers would and check at least half way your shelf life as well as three quarters and at the end. Not only will it tell you whether they spoil it will also teach you whether they discolour (some fruit ones will likely turn browner) and whether the texture is still the way you want it to be. Good luck!

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