Water Activity 102: Microbial Growth
Webinar - June 18, 2019

Water Activity 102:
Microbial Growth

Everyone knows water activity is related to microbial growth. But how can you use that knowledge to your advantage in formulation, specification, production, and packaging? In this 30-minute webinar, learn:

  • what you need to know about how water activity predicts microbial growth
  • how to use specific organism aw limits relevant to your industry in setting your specs
  • how to use different formulation techniques (including humectants, films, coatings) to hit the water activity you need
  • why you should consider hurdle technology to address certain challenges

Presenter: Mary Galloway has been a lead scientist in the METER Food Research & Development lab for eight years. She specializes in using and testing instruments that measure water activity and its influence on physical properties. She has worked with many customers to solve their moisture-related product issues.

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Brad Newbold: Hello everyone and welcome to Water Activity 102: Microbial Growth. Today’s presentation will be 30 minutes, followed by 10 minutes of Q&A with Mary Galloway, application scientist here at METER Group. If you have a question for Mary, type it into the questions’ pane at any time during the webinar, and we’ll be keeping track of these to answer during the Q&A later. Please don’t be shy and submit those questions. We’ll also be sending out a link to the on-demand webinar as well as the slides for you to review as soon as they’re available. 

Without further ado, I’ll hand the microphone over to Mary Galloway. 

Mary Galloway: Hello everyone. Thank you for joining us today. We’re going to be talking about microbial growth. Today, we’re going to cover factors affecting microbial growth, how water activity controls it, government compliance, common food pathogens, how to formulate a specific water activity and hurdle technology. 

First, I want to talk about microbial failures, and in regard specifically to product recalls, it can be extremely expensive. The recalls can cost millions of dollars in product losses, operational delays, legal fees and medical claims. You have to get the food back. You have to dispose of it. You have to take care of the ill, and there’s also damage to your reputation. Future sales may suffer from loss of consumer confidence, or they could actually relate that failure to other products that you have, even though they were unrelated and there is a company damage to your reputation. 

Now, I have some pictures down here that show some of the things that actually currently are on a product recall starting this spring. In January this year, we had a Gold Medal Unbleached Flour, which was contaminated with salmonella, in April, organic peanut butter for Listeria. In the past, salmonella has also been an issue for that. In May, raw milk cheese for Listeria. Also, Aurora packing company in May had an issue with E. coli in their meats. In July of last year, Kraft Heinz had a cheese dip that had botulism. There are a lot of things that are touched with adverse microbial growth. 

What are the factors that affect microbial growth? We have food, acid, time and temperature, oxygen and moisture. Sometimes, you might have heard this as FAT TOM, which is a nice way to remember the different factors that affect microbial growth. We’ll go through these one by one. 

First, with food, we have nutritional composition that each one of these types of microbes need. For yeasts, they prefer foods that contain simple sugars. That make sense. That’s how we get nice, beautiful bread because the yeast eats the sugar in there and does just what we want it to, but it’s not always a good thing when we have yeast growing. Molds are capable of growth under difficult conditions, and we’ll see that as they are able to have the lowest water activity limit of other microbes, so they can really persevere under difficult situations. Also with acids, the same thing for molds. Bacterial pathogens, they pref protein-based foods. You’ll see that in our peanut butter, our milk, and our meat. Those happen a lot, all because those have a lot of proteins in them. Acids, we’re talking about pH here, so molds can grow at the lowest pH, so in more acidic conditions. So far as the progression between – molds, yeast, bacteria – molds can go the lowest, yeasts are in the middle, and bacteria are the highest. Interestingly, the bacterial pathogens will not grow at a pH below 4.6. This is going to be a really crucial factor. We’re going to learn more about that as the presentation goes along here.

Time – for a microbial growth, it’s an exponential growth. What that means is that if you have an initial population at time, let’s say one, and then by time two, it would be doubled. By time three, it would be four times, et cetera, and it would reach x4.3 billion by times seven. It’s really key to stop microbial growth in regards to time very early on before it gets out of control. Now, this growth will continue until it either runs out of food, runs out of oxygen, or there might be a competing species that’s also vying for the same nutrients and other things that that microorganism is looking for. 

Next is temperature. There are three different temperature ranges for growth. We have the thermophile, which thrives at high temperature, so this would be more at hot springs, and you can see algae growing in that. We have the mesophile, which is the more moderate temperatures, and that would be more like body temperature, and the most common temperature zone for microbial growth. We have the psychrophile, which is at low temperatures – so refrigeration. Listeria, as an example, can grow in anaerobic conditions – refrigerated – so that Listeria is a real trouble at low temperatures. That’s why we have the food safety zone between 4-60C, which is about 40 to 140 degrees Fahrenheit, because that’s that moderate zone where a lot of microbes grow, but bacteria can be inactivated at elevated time and temperature combination. We’re talking about retort, which is why we have that. 

Next is oxygen. Sometimes this is also referred to as redox potential. We have aerobic which does require oxygen. We have anaerobic, which does not. We have facultative aerobic, which actually can switch between aerobic and anaerobic depending on the environment. You can see some examples I’ve listed for each one of those underneath there, and we have what we call microaerophiles, which do require oxygen but in smaller amounts than is actually available in the atmosphere. They wouldn’t go in full atmosphere but in restricted oxygen atmospheric conditions, then they would grow. 

Lastly, we’re going to talk about moisture. We do moisture in amount as the moisture content. We’re most specifically interested in the status or energy, which is the same as water activity. If you watched Water activity 101, then we know that that’s where the important part of water is for us, in the food system, because it tells us what water is able to do, what’s it available for. The higher water activity, the more it’s able to do if we had some moisture migration or in this case microbial growth. Pathogenic bacteria grow only in water activities above 0.85. Spoilage molds and yeasts grow at water activities above 0.7. We’ve seen that before, that molds can grow in a little harsher conditions than the pathogenic bacteria, but then there is no microbial growth at all for water activities below 0.60. 


Need to reduce water activity in your product? Learn about the unique relationship between the product’s water content and water activity using a moisture sorption isotherm.

How does it actually work? How does water activity control microbial growth? We’re going to talk about its mode of action, how it actually does that. Here in this slide, I have little beautiful ovals that are going to represent our microorganism. In here, we have – interior to the microbe – water activity of 0.95, and the environment that that microorganism is in is at 0.90. We know from thermodynamics, if we have a difference in water activity, we have a difference in energy level that it wants to go from high to low to even that out to equilibrate, so the water inside that microbe is going to want to leave. It does, and it moves out into the environment. What happens inside that cell is that trigger pressure is lost. That starts to stress that microorganism out. In response, the microorganism is going to try to stop that. How it does it is it’s going to try to equilibrate its own energies and water activities with the environment. You can see in the next little pathway there that the microbe tries to adapt by altering its membrane to reduce its water activity to maintain that trigger pressure. How it’ll do it is either they’ll produce or transport in small solutes to reduce the water activity. This could be amino acids, polyols, sugar, something like that. It’s trying to compensate for that loss of water out of it. You can see that it is able to drop it a bit, so it’s now at 0.93, but it’s still not matching the environment of 0.90. 

In the next little section here, we see that is not able to do any more. That’s as low as this microorganism can do. If it’s unable to reach that equilibration with water activity to its surroundings, then that microbe will remain in what we call a lag phase, where there’s no growth or it’ll begin to sporulate and go dormant. It’s just in stasis right now. It can’t do anything. It can’t grow. The water is not available for it to take in to start reproducing. It will stay like that until the environment changes. If it’s back in an environment that’s 0.93 or above, then it would be able to start growing, but at this point right now, it is in stasis. 

In the 1950s, W. Scott did some experiments on this idea of water activity in the different bacterium. What he did is he took different types of food and inoculated them with different bacteria. Here’s a list of the bacterium that he put in there, specific strains, and then he observed what happened. Did it grow? Did it not? 

You’ll see the whole top part of this table here is based on Staph aureus. At various levels, we have inhibition, and we also have growth in this toxin that’s formed. That’s the dangerous part of the microorganism and produces that toxin which is what makes us sick. I wanted to point out that precooked bacon towards the middle there, you can see at 0.86 that there is growth for that strain, but at 0.84 there is not. You’ll see that it’s inhibited. That’s a really small difference in water activity, but it illustrates – you can also see as you go up that list – that from anything above 0.85 or above is the difference between growth or inhibition. For staph, that’s true that if you can lower your water activity to below 0.85, you will not have any staph. As a matter of fact, you’ll also find out as we go along here that nothing, no pathogenic bacteria, will be able to grow. Staph is the hardest. It’s the most hardy. It’s the most adaptive of these pathogenic bacteria. 

What I did also want to point out is he put this in milk, cream filling, eggs, meat, cheese, beef, bacon, all of these different things are inoculated with staph, but it didn’t matter. The water activity limit stayed the same. That cut off is poor bacteria or bacterium, but not food matrix related, which is really important. You can use this in any of the industries. 

Here’s the list where those lie for each one of them. We can see at the top here we have botulism, E. coli is listed here, there’s salmonella, Listeria, and then at the very bottom, you can see that Staph aureus, the aerobic version of that, is that 0.86. There are no more pathogenic bacteria that will grow below that level. If you have ever seen that – about the 0.85 – for water activity, that is why, because nothing else can adapt to a water activity lower than that. Staph is the hardiest, the most adaptive as the case may be. 

This is a table we have for not only those pathogenic bacteria but also molds and spoilage, and where no microbial growth is. I wanted to point those out. Also, you can see where foods that are generally in that similar range to those microorganisms. We have at 0.85 and up: that’s where all the potentially hazardous foods lie, above that level. If we go down to 0.7, so between 0.85 and 0.7, that’s where you’ll find the yeast and molds will be, and those are our spoilage ones, but there aren’t any spoilage molds below 0.7. You see this funny little section with the osmophilic yeast right there, and there are a few molds but they don’t produce spoilage. Then under below 0.6, you’ll get no microbial growth at all. Nothing will grow below that. 

Water activity is a critical parameter for compliance, and it can be used to justify a limited microbial testing which is very important. The FDA has it in their definition of potentially hazardous food. They also have it with FSMA, the Food Safety Modernization Act, both in the HARPC, which is a risk based approach, and in HACCP as a critical control point. We have the 21CFR 110 for good manufacturing practices. USDA also has that as a critical point and good manufacturing practices, and pharma, you can also find that in USP 1121, and also the new one that’s coming, but it won’t be out until next year, 922 as well. The last one, if you’re familiar with ICH, it’s part of the decision tree for assessing hazards. This is the International Conference on Harmonization. Now, these are just a few places where you can find water activities specifically listed as a critical control point, or it mentions how it can be used to justify the limited microbial testing, but this is not an exhaustive list. You can still find it more. This is just a taste. 

Now, I’d like to talk about common food pathogens. We’re going to go through them one by one. There’s two differentiations between the pathogens. We have the foodborne intoxication: those are caused by actually ingesting a toxin. The toxin is produced in the food, and then you ingest it, and then you get very sick. There are some examples of what would be foodborne intoxication. Then the second type is foodborne infection. These are caused by ingesting the pathogenic microorganism, and then it gets into your GI tract and then it starts to grow. Intoxication is formed in the food, and infection is formed, essentially, in the gut. That toxin is formed in your gut. 

Let’s start with staph aureus. This one is facultative. If you remember, that means that it can grow in both situations. It can grow without air, without oxygen or with oxygen. This is a concern for pharmaceutical companies because they have creams and things, and you’ve got people who are immune compromised. Staph is always a concern for them. It can be destroyed by heat treatment and nearly all sanitizing agents, which is very good. It has the lowest water activity limit, so it’s at 0.85, the lowest of all the pathogens. Sources: we find it on your skin and sores, hair, in your nasal passages, in your nose. On food, you can have that with hand contact with food. Then that food does not require any additional cooking. You have a lot of cross contamination issues with that in salads, filled bakery goods and sandwiches. The interesting thing is if you find staph aureus on food processing equipment, it is generally an indication of poor sanitation. This one can be taken care of quite easily if you’re careful with cleaning and prep and try to minimize that cross contamination issue. 

Next, we’re going to talk about botulism. It’s anaerobic. It will not grow in a pH below 4.6. It just needs three minutes of boiling to destroy. Its water activity limit is a little higher at 0.94. Where you’re going to find botulism is nature, soil, water, plants. In foods, it’s improperly canned foods, especially the low acid foods, so beets, green beans, baked potatoes wrapped in foil. In smoked fish you have the herb infused oil where you have the herbs that will contain the botulism on there, and then it’s infused in oil, and now you’ve got an anaerobic condition. Honey can cause children, more specifically infants, to have infant botulism, so that’s why they have the recommendation to not feed your child honey until they’re a year old. Botulism, it’s unusual in that it’s anaerobic. Some of them are not, but that’s where it can be a real difficulty because once you remove the air and you have a higher pH, if you don’t do the retort, then you can have botulism be an issue. 

Salmonella – salmonella is the number one for most reported cases. This is now into food infection, where you have to ingest it and then it grows in your gut. It’s more common in the summer months, and that is because it’s warmer weather and we have more active animal life. It is also facultative. We’ve seen that before, where it can go either in an oxygen rich or oxygen depleted environment. It’s also killed by cooking and pasteurization, and the water activity limit for this one is 0.95. The sources are contamination by feces, contaminated drinking water, person to person contact. In foods, we have inadequately cooked poultry and poultry products, eggs and egg products, raw fruits, vegetables, unpasteurized milk and milk products like raw milk cheese, which there’s a recall for that as well, flour which there is also for that as well for salmonella right now, and peanut butter, which we’ve seen in the past. For flour, how that could be contaminated is it actually could be contaminated at the processing facility if it’s not kept clean or there was something else there to infect it with salmonella, or it could be in the field. You could actually be growing the wheat and if, say the fertilizer was contaminated with salmonella or something like that, then it can even be on the grain before it’s even come into process. For peanut butter, most commonly, there’s birds around the processing plant that can contaminate with salmonella. Generally, salmonella is killed during roasting, but if the exposure happens after the roasting or as again introduced after the roasting, then that salmonella is viable. For salmonella, the important thing to know is that it won’t proliferate at a lower water activity, below 0.85. As a matter of fact, we call this – peanut butter and flour are part of it – the low moisture your food group, so they generally have a low water activity, quite a bit lower than 0.85. If they are contaminated with salmonella at that point, they’re harmless. They can’t grow, right? We already talked about that, but the problem is is they’re additives to other things. When you add flour to make a batter or a peanut butter into something, now you’ve introduced it to a high water activity environment, and they will start to grow. That’s where the problems start happening. 

This is actually a big concern. How do you inoculate for salmonella on flour without changing the properties of that ingredient, well, flour or peanut butter or something else? There are studies done, Dr. Bradley Marks was actually working on getting a database for the Michigan State University on low moisture foods and salmonella if you were interested in looking at that or contributing to that. There’s also some articles. It’s definitely a big concern of how we can combat salmonella and E. coli as well, salmonella in particular, to mitigate those problems so we don’t find that when people start using them in their own food products that they get contaminated. 

Now, Listeria. Listeria is also facultative. It can grow in refrigerated temperatures, which can be a real problem and unusual for most of our microbes that we’re talking about today. It can also be killed by cooking and pasteurization, and it has a water activity limit of 0.92, so a little bit lower, a little more hardy than some of the other ones we’ve seen. Sources for this are soil, water and animals carrying that bacterium. In foods, we see it in uncooked meats like raw hamburger, vegetables, unpasteurized milk and cheese, and cooked or processed foods. Certain soft cheeses, we talked about there was one for Heinz for last year, processed and ready to eat meats and smoked seafood. This is also been a factor for hotdogs, so a similar thing: if you have a low oxygen environment, refrigerate it. Listeria can grow if it’s present in that food. The thing about Listeria which is good to point out, is that at least 90% of people who get Listeria infections are in a high risk group like pregnant women, older adults, people with weakened immune systems. Healthy children and adults occasionally do get infected with Listeria, but they rarely become seriously ill, which is important to point out. 

Next, we’re going to talk about E. coli, also facultative. Most strains of E. coli are actually harmless and important in the digestional tract. It actually is used quite a bit in the pharmaceutical industry to make other drugs and things. It’s very effective that way. However, this particular strain that we’re talking about here is the nasty one that does make people sick. It can be cooked and pasteurized to kill it. Some packagers of lettuce can use or have used a chlorine wash that is partially effective so it does help. It’s not a for sure kill to that E .coli, so it’s always important to cook your food well, to wash your raw ingredients well, to refrigerate and or defrost correctly, and watch for cross-contamination. E. coli is relatively dangerous and that you can have a low infectious dose but it’s relatively difficult to kill. We’ll see this in intestines of birds and animals. In food, we have ground beef, raw milk and milk products, raw fruit and vegetables like the greens, like the lettuce. Cross-contamination is a real issue. We’ve seen that a lot in the news. Last year, if you remember, all of the lettuce out of Arizona last April had E. coli and they had to throw all of that away. I’m not sure they even found what was causing that. I was looking recently and I didn’t see if they’d actually identified where it was. Right now, like I mentioned before, Aurora packing company, they have a recall for E. coli as well. 

Bacillus cereus is anaerobic. It multiplies very quickly at room temperature, and its incubation time is minutes to hours, but it doesn’t last very long. This is mostly confused with the 24-hour stomach flu. If you’ve come down with a bug that only lasts a day, you probably got this guy, and gave yourself some nice food poisoning. The sources for this are mammals, shellfish, and contaminated water. In foods, it’s raw and undercooked poultry, raw milk and milk products, and for starchy foods like rice, sauces, soups, they’ve been left in the danger zone for more than two hours, and now, this can grow and give people food poisoning. 

Campylobacter is the last one we’re going to talk about today. It’s microaerophilic, where it has to have a low oxygen, less than atmospheric. It also can be killed by cooking and pasteurization. It is the number one cause of bacterial diarrhea. This is also known as traveler’s diarrhea. If you ever get this when you’ve been traveling abroad, this is probably what you’ve been infected with. The problem with it is that it can cause bigger issues in the future like IBS, GBS, or arthritis, but the thing is it’s generally isolated to a person or a group, so it’s not extremely widespread. Its water activity limit is extremely high at 0.99, highest out of all of them. You’re gonna find this in the gut of mammals, shellfish, and contaminated water. In foods, we have raw, undercooked poultry and raw milk and milk products. 

If we know that water activity, if we can reduce that, will change what microbes we’re going to grow, how do you actually do that? How can you formulate for water activity? First, you can dehydrate the product. That is the ancient way of preserving food is just to dry it out. You lower the water activity. Microbes can’t grow when it stays for a long time. You can have edible films or coatings that would be to prevent or limit the moisture migration. 

Moisture migration is caused by differences in water activities. If you can limit that shift in moisture change from one component to the next, then you can limit where that can grow.


By using water activity, you can predict moisture migration and ensure that your product is safe, palatable, and shelf-stable.

You can introduce anti-caking agents. What these do, they absorb excess moisture. You’ll see that in table salt will have it. There’s a little powder in there that they put in that will absorb the moisture. It’s not part of the salt and salt crystal, but it is added with it. That absorbs the moisture before the salt can absorb the moisture. 

Actually, one of the most useful ways to lower water activity is adding a humectant. These can be added singularly or in groups, so they can have additive properties. Salt is a very effective humectant. Adding a humectant means that it’s going to lower the water activity, also tends to increase the moisture content that a product can hold because we’re binding the water so that it can be used anywhere else. We’re chemically binding it. It lowers the water activity, but it also increases the overall moisture in your product. Then we have sugar which is also very effective at that. Glycols like propylene glycol and things like that are good humectants, amino acids, and polymers. 

Polymers are interesting. They’re not quite as effective. These last ones here are not quite as effective as sugars and salts and things and some of the glycols, because starches are really long. What happens is they have water binding sites on them, but because they’re so long, they wrap around to each other and hide those binding sites. Even though they have the capability, they just don’t have them available, so they’re not as effective as some of the things, but they do have some good humectant properties. 

Then lastly, we have acids. There are some limits to using humectants to lower your water activity. I’ve gone over a few of them as we talked about the list prior. Solubility, there is a solubility limit as an example for salt, which is 0.75, so you could add salt to your product, but it won’t lower it below 0.75 because that salt is actually going to start to crystallize out. 

Also, molecular weight like I was talking about before with the starch, it’s just really long chains and so they wrap along themselves and so they have a limited amount that they can actually bond with. 

Organoleptic, as an example, if you were to use salt to its maximum effect, you probably wouldn’t want to because it would make your product so salty, no one could eat it. 

Crystallization in storage, that could be salt, again, about what I talked about, the same idea but sugar has the same issue as well. If you try to use it below 0.84-0.85. That can crystallize out as well unless you have something else in there with it. 

You can increase your reactivity to have a browning reaction. If you have what we call a reducing sugar in there and you’ve used it a lot, it can actually cause a browning reaction. 

Toxicity – if you use propylene glycol, that does a good job of lowering water activity and increasing your moisture in your product, but there is a limit. One, if you use quite a bit, it also has the organoleptic property of being bitter, but it actually can get to a toxic level if you use too much. 

Those are some considerations when you’re using humectants as additives, but what you can do to mitigate some of those things is do combinations. You could add some of one, some of another, a little more sugar, a little salt, maybe a little preservative or a glycerin or something like that. 

The last thing I want to talk about is hurdle technology. Hurdle technology combines preservation techniques to establish a series of preservative factors or hurdles that the microorganism in question are unable to overcome or jump over. Some of these hurdles might be temperature, like refrigeration, reducing the water activity, increasing the acid, reducing the oxygen, adding a preservative or other things. The thing about this is that these factors can be combined to have  their effects be additive. They can be added together, and that will make the environment where the microbes won’t be able to grow. This is pretty powerful stuff because you can combine things to reduce that microbe’s ability to overcome those hurdles. 

Here is a demonstration of the hurdle effect and then what each of these hurdles are. If you look at one, two and five, you’ll see that this particular microbe was able to overcome everything. The last thing they have to do is they have to add a preservative. That’s what it’s going to take to stop that microbial growth. If you look at example three, we have a temperature there, but it’s actually controlled by water activity. In this sense, we can just drop the water activity. That would be enough to stop microbial growth, and we’re done. Four, the preservative is not enough, and so it’s able to clear all of the hurdles that we put on there. Maybe we’re limited by the product that we wanted, the flavor or some kind of property that won’t allow us to change anything else, and so it just needs to be refrigerated. As an example of a packaged meat or cheese o something like that, we have to maintain the integrity of the product. In this sense, we just need to refrigerate it. Six is the last one here. We can see that, excuse me, we have the temperature was able to clear the water activity just barely, but then pH was able to control the microbial growth for this example number six. 

I want to talk about that a little bit more, about this interaction between water activity and pH. This is a table from the US food code. What it shows is the interaction between pH and water activity to control the spores in the food. Particularly, heat-treated foods that are packaged afterwards. You’ll notice that we have across the top of the pH values, and going down the left are the water activity values. Inside the table, we have non PHF, which means not potentially hazardous food, or it doesn’t need to be time temperature controlled for food safety. You’ll notice that when we have a higher pH, then our water activity has to be lower. If you look here, it says greater than 5.6 pH; the water activity needs to be below 0.92. Now, as you move to the left on pH, which means you’re decreasing the pH, you’ll notice that our allowable water activity limit is actually going up, so we’re able to raise that to 0.95. 

This next example is similar to the first one except for this time, the food is not heat-treated or its heat-treated but not packaged, so little lighter conditions in the treatment of the food. You can see the same setup here. We have pH across the top, and then the water activity values are on the left. We’ll see here that at greater than 0.56, you have to have a water activity below 0.88, but as we drop our pH, our water activity was able to increase. If you go all the way over, if we’re able to drop it below 4.2, we can have a water activity of up to 0.92. 

This is really powerful stuff because if you have a sauce, and especially let’s say a ketchup or something like that, if could drop the pH, and generally those types of sauces do with the vinegar in them, then you’re actually going to have a higher water activity allowed for that product because the acid is increased. 

In conclusion, we have the factors that affect microbial growth, which is food, acid, time, temperature, oxygen, and moisture, or you can refer to that as FAT TOM, which I think is funny but effective. Microbial growth can be controlled by water activity regardless of food matrix. You can’t say that by some of the other things. If you think about food that you eat, meat, if you’re going to cook a turkey or beef, steak, or pork, especially poultry and pork, there is a minimum time and temperature, but they’re different between each one of them. It does matter which food matrix that is in to control microbial growth, but that is not the case with water activity. It’s also a critical control parameter for government compliance and can be used to justify limited microbial testing. That’s really huge too, because since it doesn’t matter if you’re able to keep your water activity below .85, it doesn’t matter what microbes. You just know that they know that they’re not going to grow, and it’s very difficult to try to test for every kind of microbe possible in your food, as you can see from when we were talking about before about the specifics, the different common food pathogens. There is a lot of industries that overlap for different microbes. The interaction between water activity and pH is the only combination that is specifically outlined in the FDA food code. I think that is it for me. Let’s see. Do we have any questions? 

BN: All right, thanks Mary. That was a ton of great information. Thank you very much. We appreciate that. It does look like we’re a little short on time, so I think what we’re going to do, because we have a lot of great questions out there and we want to be able to get to them and give them their due time, what we’re gonna do is we’re going to collect all the questions. If you have any questions right now, please put them, type them into the questions pane. We’re going to collect all the questions that have come through. I think we’re gonna write up a written up Q&A that we’ll be able to send out, we’ll be able to email out to everybody who has attended today’s webinar here. What we’re gonna do, we’re just going to advance to this next references slide because there have been a lot of people that have been interested in some of the information you’ve been giving out. Again, thank you all you attendees today for joining us today. We hope you enjoyed this discussion as much as we did. Again, we’ll be sending out an email in the next couple of days with links to this recording as well as the slides from this presentation. Stay tuned for future METER Food webinars and have a great day. 


Not a “quick mode” reading, but a direct measure of water activity to 0.003. No calibration, no approximation


What is the difference between moisture content and water activity?   

Moisture content is strictly the amount of water in a system.  Water activity is the energy status of water in a system and determines what that water is capable of doing.  Water activity, and not moisture content, is the driving force for microbial growth, moisture migration, chemical, physical, and enzymatic reactions. METER has produced a webinar that further discusses this topic, called Water Activity 101 and is available on our website.

How does water activity help in controlling the growth of Mycotoxins?  

Water activity controls the production of mycotoxins in the same way as other microorganisms– by forcing the fungi that produce the toxins into a dormant or spore-forming state.  Mycotoxin-producing fungi are more adaptive and have lower water activity limits than bacteria or yeasts, around 0.70-0.80. Here is an article from the Journal of Food Protection that gives specific aw limits for the formation of various mycotoxins. 

What is the recommended water activity for green coffee?

It depends on the qualities and shelf life of the bean you’d like to maintain.  In general, to maintain quality and longer stability, the water activity should be between 0.45-0.55 aw.  However, there is more to the correlation between green coffee and water activity.  To better understand this relationship, please access an informative webinar, The Role of Water Activity in Coffee Quality featuring Meter’s Research Scientist Wendy Ortman and Ian Fretheim from Café Imports, available on the METER Group website.

My water activity is 0.645 and Staph was found in the sample. The APC was 11,000. Is this safe?  

Reduced water activity is not a kill step. However, although lowering water activity does not kill microbes, it does inhibit their growth. If your product can maintain a aw below 0.85, then any Staph that is present in the product will become dormant, so it will not grow and produce toxins. It would be termed safe, but not sterile.

It is hard to say if an Aerobic Plate Count (APC) of 11,000 is safe, since they do not directly correlate to the presence of pathogens or toxins. Interpretation of the APC results must take into consideration knowledge of the product and whether a high APC is expected.

Do you know what temperature kills Staph?

It isn’t just temperature that kills pathogenic bacteria, but how long it is kept at that temperature.  As an example, Staphylococcus aureus isolates from dairy products did not grow after pasteurization at 60 C for 60 min, 70 C for 50 min, or 80 C for 20 min. For further information, see the publication from the AIP Conference Proceedings 2017.

How does Salmonella survive in a low water activity environment like flour?

Since reduced water activity does not kill microbes (it only inhibits their growth), possible pathogenic bacteria like Salmonella can be present on or in food stuffs. If the aw level can be maintained below 0.85 for all pathogens and 0.95 for Salmonella specifically, the bacteria will not produce harmful toxins.  This is true for any food matrix. The problem comes into play when those infected foodstuffs are placed into an environment above what is safe. For example, flour and peanuts can be contaminated with Salmonella in the field or processing plant. The Salmonella bacteria can stay dormant at their native aw levels which are below 0.85, and then be added to a dough or batter with a much higher moisture level.  Now they are in a high aw environment, and the bacteria become active again, and therefore those toxins can form. Cooking would kill the toxins, but batters and doughs aren’t always cooked before consuming.

Can microorganisms become resistant to reduced water activity?

The lowering of water activity has been used in food preservation since early man, albeit unknowingly. Preservation techniques which use reduced aw like cooking, dehydration, salting, or curing in honey or sugar have been used since ancient times. 

Water activity controls microbial growth by causing osmotic stress and loss of turgor pressure inside the cell membrane.  To compensate, the microbe will change its metabolic process to reduce its own aw by condensing solutes.  There is a limit to how well a microbe can do this and that is what determines the aw limit for each species of microbe. The survival mechanism for microorganisms is to stop growing, go dormant, or begin sporulation under this type of stress. Since this response still serves to keep the microorganism viable for future generations, there does not seem to be an evolutionary need for microorganisms to adapt to lower water activity limits in order to survive. The fact that water activity has continued to be a successful means of controlling microbial proliferation for most of recorded history seems to support this conclusion.  

How does peanut butter obtain microbial growth if it’s before its shelf stable date? And when mixed with other dry ingredients, does it affect the activity of the peanut butter?

Shelf life can end for many reasons. Microbial growth, rancidity, change in texture, loss of vitamins, or browning are some examples of what are termed modes of failure.  Microbes can be present in peanut butter, but will not be able to grow if the aw is below 0.70.  There are two possible reasons why microbial growth can occur before the shelf stable date.  One is that the date was incorrect and was using the wrong metric to calculate the shelf life and should have been using microbial growth. The second (and more likely) reason is that the peanut butter experienced a temperature increase during storage.  Increases in temperature increase aw , therefore if there was an increase in temperature, the aw could have risen above 0.70,  resulting in microbial growth.

The mixing of any ingredients affects the aw of all the individual components.  If mixing a high aw ingredient with a lower aw ingredient, their water activities will equilibrate somewhere in the middle, depending on their mass fractions. 

How is water activity affected in hydrophobic foods like oils?  Do the same aw levels apply in these products?

The water activities of oils are typically low.  However, when in the presence of higher humidities, the water molecules will adhere to the outside of the oil and will reach the same aw as the environment.  Therefore the same aw limits do still apply to these products and all other food matrices. Microbial growth can be a real problem in herb-infused oil, for example. Herbs and other fresh produce have very high water activities, and can also have spoilage microorganisms on them prior to infusion. Together, these factors can create a potentially hazardous condition around the herbs. Botulism has been known to flourish under these conditions, since oil also creates an anaerobic environment.

If a product has a water activity of .80 will bacteria ever be able to grow? 

At a water activity of 0.80, no pathogenic bacteria can grow.  However, there is still a risk of mold and yeast growth, unless other means are taken to limit microbial growth.

Does water activity allow you to define shelf life without specific shelf life testing?

Water activity can be used to determine at what point a product becomes undesirable and its shelf life ends. However, it is always advisable to perform specific shelf life testing that focuses on the particular mode of failure, which could be microbial growth or another factor.  There are equations that will take into account aw, ambient humidity, temperature, packaging, etc to predict the shelf life of a product, but those results should always be confirmed with actual testing.  However, shelf life testing can be done at elevated temperatures and humidities to accelerate results.


Using water activity to generate all the data you need to predict your product’s shelf life - all from an experiment anyone, even a small startup, can afford to run.

Will sugar alcohols like sorbitol increase mold growth?

No, sugar alcohols will not increase mold growth.  Many sugar alcohols are quite adept humectants and will lower water activity in foods.  As a matter of fact, many are more effective per gram at lowering water activity than sucrose.

Can steam sterilization reduce water activity?

Steam sterilization is a good kill step for microbes, however, introducing water into a system generally raises the water activity.  Two exceptions to this rule are the formation of waters of hydration or the crystallization process where the water activity actually decreases.  

Is water activity is major for liquids only? What about dry powders like plants, leaf powder, etc.? 

Actually, water activity is most relevant for non-liquid substances, particularly those that would fall in the Low Moisture Foods category that have water activities less than 0.85.  It is an extremely powerful metric for determining susceptibility of microbial growth, changes in texture (especially caking and clumping for powders), oxidation, browning, and chemical reactions. Below is a graphic illustrating the relationship between the rates of some of these reactions and water activity.

Figure 1. The relationship between the rates of certain reactions and water activity.



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