spoiled aquafaba

How Water Activity Relates to Microbial Growth

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!

ah excellent fermented sausage
Drying sausage, thus lowering the water activity, helps extend its shelf life

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.

mold on green chutney
The water activity of this coriander chutney was too high: molds thrived!

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 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 does honey, nuts and some jams.

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.

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.

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.

lemon
A lemon might have a high water activity, but a very low pH-value, which helps extend the shelf life of the juice (once it’s out of the lemon).

Sources

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

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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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