Food Safety With Water Activity
This post, which is part of my broader framework on food safety,
is a deep-dive on how to prevent food spoilage and foodborne illness using water activity. Water activity is the science behind why you can preserve food by adding enough salt or sugar.
Disclaimer: This post was written while consulting reams of regulatory and industrial food safety documents, but I’m not an expert. If you disagree with anything, please comment with a source so I can research further. Thanks!
WHAT’S THE POINT?
Harmful bacteria, parasites, mold, and viruses might be in your food. To reduce the chances of food spoilage and foodborne illness caused by these pathogens, you need to both:
Kill any existing pathogens in your food. These pose the most immediate risks. You should kill them (or reduce them to safe levels) where possible.
Prevent any new pathogens from growing in your food. If you haven’t totally killed the existing pathogens in your food (or if your food is exposed to other pathogens after the fact), the few that remain will multiply over time. This is especially important for foods that you aren’t eating right away (e.g. food you are storing or cooking over multiple hours)
Water activity can’t kill existing pathogens in your food, but it can prevent new pathogens from growing. In that sense, water activity is an incredible preservative. In fact, you probably don’t go a day without using ingredients that were preserved with either salt and sugar using water activity.
This guide will start with a few simple rules then dive into details (so you know when it is safe to break the rules!).
THE CHEAT SHEET
About water activity
Water activity is how free the water in a food is to be utilized by other stuff. Picture water in a bowl as totally free, but water in a sponge as partially bound by the sponge. The water in preserved food is usually bound by either salt or sugar.
It’s possible to measure water activity with a water activity meter, but they are very expensive. The measurement is from 0 to 1, with low water activity measuring closer to 0 and high water activity measuring closer to 1. Most foods have a water activity above 0.95.
To kill existing pathogens
Water activity is not an effective means for killing harmful microorganisms or viruses. To kill existing pathogens in your food, you pretty much need to use temperature. To learn how, check out my deep-dive on temperature control here.
To prevent new pathogens from growing
Harmful microorganisms cannot grow with low water activity. Since microorganisms need water to function, when the water in food is bound to other ingredients (such as salt or sugar) in a way that microorganisms can’t utilize it, the microorganisms can’t function.
Generally, items with a water activity level of 0.85 or lower don’t require refrigeration. This is why you can store soy sauce (0.8) and peanut butter (0.7) at room temperature.
You can decrease the water activity of your food by dehydrating it or adding salt, sugar, or other humectants. Assuming you don’t have a water activity meter to do this accurately, I recommend:
Dehydrating it! Completely remove the water and then store it somewhere airtight. This is relevant for solid foods.
Making conservative assumptions. If you assume that your food has the same water activity as water itself, you can follow simple formulas (detailed below) to decrease water activity with salt or sugar. This is particularly relevant for sauces and jams that require salt or sugar anyway.
Following recipes from a trusted source. Many recipes (especially standard recipes for jam) have stood the test of time because they have adequately low water activity.
THE DEtails
About Water activity
A water activity meter
Water activity (aw) is a useful means for measuring whether the water in a food can be utilized by microorganisms. It’s defined as the partial vapor pressure of water in a food divided by the partial vapor pressure of pure water. It’s not the same as moisture content.
When the partial vapor pressure in your food is less than the partial vapor pressure of water, it’s because the water in your food is bound by hydrogen bonds, capillary and electric forces, and solutes. Water bound in this way requires more energy for microorganisms to utilize it compared to pure water (in the same way that water in a sponge requires more energy for us to utilize it compared to pure water).
Water activity can be measured with a machine: a water activity meter. Water activity meters produce an output that is a ratio between 1.0 (high water activity) and 0 (no water activity). A water activity of 1.0 means that the vapor pressure of the food is equal to the vapor pressure of pure water. A water activity of 0.8 means that the food has 80% of the vapor pressure of pure water.
Most foods have a water activity above 0.95. The water activity of a few common foods:
Fresh meat and fish: 0.99
Raw vegetables: 0.99
Raw fruits: 0.98
Salami: 0.82
Soy sauce: 0.8
Peanut butter: 0.7
To Kill Existing Pathogens
Low water activity doesn’t kill existing harmful bacteria. Instead, water activity merely slows their flow; as soon as harmful microorganisms in low water activity are returned to higher water activity water, they regain mobility.
TO PREVENT NEW PATHOGENS FROM GROWING
Generally, foods with a water activity level of 0.85 or lower don’t require refrigeration. Mold growth is still possible (though difficult) at water activity levels as low as 0.6, but the FDA only enforces 0.85.
I’ve struggled to find the FDA-approved water activity levels for refrigerated foods. But some documents, and my assessment of particular harmful pathogens (e.g. clostridium botulinum, listeria monocytogenes) seem to indicate that water activity levels lower than 0.93 would be safe for extended refrigerated storage.
Decreasing water activity
There are many ways to decrease the water activity of your food.
Adding salt: Salt is the primary reason why soy sauce is self-stable. The amount of salt necessary to preserve a given food is dependent on the food’s starting water activity.
Adding sugar: Sugar is the primary reason why candies or low-acid jams are shelf-stable. The amount of sugar necessary to preserve a given food is dependent on the food’s starting water activity.
Adding other humectants: Specialized products can decrease water activity without adding saltiness or intense sweetness. Shelf-stable ganache, for example, often utilizes one or more of glucose syrup, sorbitol, dextrose, or glycerol to reduce water activity without making it too sweet.
Freezing: Water activity decreases as temperature decreases. Freezing foods (especially below –4ºC) will prevent future microorganism growth.
Removing water content: Completely removing moisture (e.g. via dehydration or freeze-drying) is a straightforward means to reduce water activity.
Achieving low water activity without a water activity meter
Water activity meters are very expensive, but there are multiple methods for safely reducing water activity without a water activity meter.
Follow a recipe from a trusted source. Trusted sources uses salts or sugars in excess of what is required to adequately reduce water activity.
Make conservative assumptions:
For liquids and sauces, you can make the conservative assumption that your liquid has the same water activity as water itself. Then you can follow well-documented water activity charts for water-based solutions. Dip lower than these suggestions only to the degree you are confident that your liquid has a meaningfully lower starting water activity than water.
11% salt is enough to reduce water’s water activity to below 0.93. Best for liquids stored in the fridge.
20% salt is enough to reduce water’s water activity to below 0.85, the FDA-approved level to avoid refrigeration.
50% sugar is enough to reduce water’s water activity to below 0.92, the amount necessary to prevent the growth of botulism.
67% sugar is enough to reduce water’s water activity to below 0.85, the amount necessary to prevent the vast majority of bacteria from growing. This is why you can keep a rich simple syrup in the fridge for months longer than a traditional simple syrup.
Combinations of salt and sugar are used in many consumer packaged goods, but combinations are best measured with a water activity meter.
For charcuterie, you can make conservative assumptions based on water loss. This is a whole separate topic that I’m not going to cover on this site.
For shelf-stable ganache, I’ve seen recommendations of around 30% sugar (where sugar could be sucrose or another applicable humectant). Learn more here.
Combine ingredients that already have low water activity. You can consult water activity charts before combining ingredients. For example, there’s no reason why you shouldn’t be able to combine two low water activity foods (like peanut butter and soy sauce) to create a low water activity condiment.
If you want to minimize the salt or sugar added to your food, you probably need an expensive water activity meter to get it right. You could also submit your food for a test at a local lab.
WAYS TO BREAK THE RULES
Don’t want to add enough salt or sugar to your food to achieve a low water activity? There are other ways to prevent new bacteria from forming.
Avoid the temperature danger zone. You don’t need to worry about acidity if you are following the temperature food safety guide.
Decrease the pH to 4.6 or lower. High acid foods are protected from new bacteria growth.
Use a combination of water activity and acidity. Foods with pH of 5.0 are shelf stable when water activity is 0.9 or lower. See table 4 in this document for more interaction effects.
FAQs
How much salt should I add to my sauce to make it safe? Without measuring the water activity or acidity of your sauce before salt is added, the only way to provide an accurate recommendation is to assume that your sauce has the same water activity as water itself. Doing so reveals:
11% salt is enough to store your sauce in your refrigerator.
20% salt is enough to store your sauce at room temperature.
