Lowering water activity with humectants: a step by step guide

Lowering water activity with humectants: a step by step guide

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. 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.

FormulationPredicted Water ActivityActual Water Activity
Normaln/a0.8230
With 0.193g glycerin added0.800.819
With 3.76g glycerin added0.750.754
With 5.96g glycerin added0.700.706

Predict final water activity right now

Want to see how the Moisture Analysis Toolkit works? Download a trial version and run some numbers to see how easy it is to predict final water activity using Grover or Norrish equations.

Water activity prediction equations can be a very powerful tool in formulation. For more information about these equations, contact support at METER Food.

Relevant publications

Brimelow, C. J. B. “A pragmatic approach to the development of new intermediate moisture foods.” In Properties of water in foods, pp. 405-419. Springer Netherlands, 1985. Article link.

Kapsalis, J. G., D. H. Ball, D. M. Alabran, and A. V. Cardello. “Polyglycerols and polyglycerol esters as potential water activity reducing agents. Chemistry and sensory analysis.” Properties of Water in Foods (1985): 481-496. Article link.

Labuza, T. P. “Water binding of humectants.” In Properties of water in foods, pp. 421-445. Springer Netherlands, 1985.. Article link.

Ledward, D. A. “Novel Intermediate Moisture Meat Products.” In Properties of Water in Foods, edited by D. Simatos & J. L. Multon, 447-463. Dordrecht: Springer Netherlands, 1985. Article link.

Linko, P., R. Kervinen, R. Karppinen, E-K. Rautalinna, and J. Vainionpää. “Extrusion cooking for cereal-based intermediate-moisture products.” In Properties of Water in Foods, pp. 465-479. Springer Netherlands, 1985. Article link.

Pomeranz, Yeshajahu. Functional properties of food components. Academic Press, 2012. Article link.

Sloan, A. Elizabeth, Dennis Schlueter, and Theodore P. Labuza. “Effect of sequence and method of addition of humectants and water on Aw lowering ability in an IMF system.” Journal of Food Science 42, no. 1 (1977): 94-96. Article link.

Sloan, A. Elizabeth, and Theodore P. Labuza. “Prediction of water activity lowering ability of food humectants at high aw.” Journal of Food Science 41, no. 3. 1976: 532-535. Article link.

Troller, John A., and J. H. B. Christian. Water activity and food. New York: Academic Press, 1978. Article link.

Norrish, R. S. “An equation for the activity coefficients and equilibrium relative humidities of water in confectionery syrups.” International Journal of Food Science & Technology 1, no. 1. 1966: 25-39. Article link.

MEASURE WATER ACTIVITY IN 5 MINUTES

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