How do manufacturers keep intermediate moisture foods microbially safe during a lengthy shelf-life? The answer is often humectants—ingredients such as glycerine, sugar, and different types of salts. Humectants lower the water activity (aw) of a product because water preferentially binds to them. Each humectant has its own unique ability to lower water activity depending on its chemical makeup.
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The amount or concentration of the humectants you add determines how much the aw decreases.
Which humectant should you use?
As a general rule, the lower the molecular weight of the humectant, the more powerful its water binding capacity. For example, a simple sugar like sucrose is more effective than a starch, whose interlocked glucose molecules limit available water-binding sites. For this reason, fiber, gums, and waxes are not effective humectants.
When choosing a humectant, it’s important to consider unintended effects. For instance, when protein is present in a cookie dough, if you lower the water activity to 0.7 using a reducing sugar, you enter the ideal range for Maillard browning reactions. The lowered water activity keeps the dough microbially safe, but the product turns brown after only a few hours.
Humectants can also introduce undesirable flavors into a product. Glycerin is an advantageous humectant because it’s flavorless except at high concentrations. It’s also completely miscible in water and won’t precipitate out in solid form when a product’s water activity changes. Salt or sugar can be problematic because, in addition to adding flavor, they form solids if the concentration reaches the saturation point in the product. To address these different challenges, many manufacturers use a combination of humectants.
Basic steps to lower water activity
Product developers can use the following steps to figure out how much and what type of humectant or combination of humectants will help them achieve a desired aw.
- Identify the current water activity of the product
- Decide what water activity you want to achieve
- Select the candidate humectants
- Determine the reactivity factors of each humectant
- Predict water activity change through calculations (described below)
- Use moisture sorption isotherms to improve your ability to determine an optimal combination through modeling
How to predict a change in water activity
You can predict how much a humectant changes a product’s water activity by using either the Norrish equation or the Grover equation. These equations have scientifically determined constants and coefficients that you can use in your predictions. (For more details on the science, see the list of relevant publications.)
The Moisture Analysis Toolkit is a software program that has these equations built in. Its prediction tool uses data specific to your product to predict the quantity of humectant or combination of humectants needed to achieve a desired water activity. The following chart illustrates how well the prediction tool worked when we added glycerine to syrup.
|Formulation||Predicted Water Activity||Actual Water Activity|
|With 0.193g glycerin added||0.80||0.819|
|With 3.76g glycerin added||0.75||0.754|
|With 5.96g glycerin added||0.70||0.706|