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Water Activity 301: Water Activity and Physical Properties

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Join Dr. Brady Carter for a more in-depth look at how to interpret moisture sorption isotherms and determine when physical changes will end shelf life.

This webinar operates under the assumption that all participants have a working knowledge of water activity. Therefore, the following content will explore how water activity impacts physical properties and how these relate to food temperature abuse. To learn the basics of water activity, how it’s measured, and how the measurements can be used, please refer to Water Activity 101: Mastering the Basics.

 

How to control water activity

Some of the most important questions in product formulation and physical stability (specifically relating to food temperature abuse) are how to control water activity and, if water activity levels are too high, how to lower them. The easiest way to reduce water activity is to dehydrate the product through baking, spray drying, or freeze-drying. Another way to lower water activity is to change the chemical interactions using humectants. The most common humectants used in product formulation and packaging are salt, sugar, and glycerol. When using humectants, be aware of how those additives will affect other physical properties of the product, like texture and taste.

Figure 1: The Ross Equation provides an excellent way to determine a product’s final water activity.
Figure 1: The Ross Equation provides an excellent way to determine a product’s final water activity.

An excellent way to determine a product's final water activity is to use the Ross equation, which takes the current water activity and multiplies that with the water activity of the humectant. This equation will save you time and help you avoid resorting to trial and error.

 

Considering multiple product components

Product formulation and packaging also must consider the water activity levels of different components within the product. One way to ensure better consistency is to require raw ingredient inspections to ensure that the different ingredients come in from your vendors with consistent water activity values.

Establish a critical water activity for the texture of your product, and remember that moisture migration is not controlled by moisture content. All components must have the same water activity level to prevent moisture migration within a product (and prevent food temperature abuse). Consider using edible barriers (like chocolate) or even separate packaging for elements with differing critical water activity levels. Establishing acceptable water activity levels and moisture content values will be vital when working with products containing multiple ingredients.

 

Evaluating for powders

Powders are challenging to evaluate for packaging because if water activity levels get too high – which can be caused by food temperature abuse during shipping, storage or production – the powders will cake and clump. 

The most important thing to consider when working with powders is glass transition. When powders are spray dried, they enter an amorphous state, which isn't technically solid. This glassy condition isn’t the most stable condition for powders, but it does allow for extended shelf life. By putting powders into this glassy state, we prevent caking and clumping.

Figure 2: This model helps learners visualize temperature abuse.
Figure 2: This model helps learners visualize temperature abuse.

Temperature is the most used method of achieving a phase transition, and the other common technique involves adjusting amounts of plasticizer. By increasing the plasticizer level, we can induce a phase change without changing the temperature.

Powders in a glassy state have much fewer points for water binding, which allows the critical water activity level to stay within the prescribed limits. However, once the product has reached its shelf life, the water activity will rise, creating more binding sites for water and increasing the probability of the powders caking and clumping. Measuring electrical properties can also be used to monitor phase transitions.

Figure 3: The DDI method for phase transitions shows how temperature and moisture content affect a food product.
Figure 3: The DDI method for phase transitions shows how temperature and moisture content affect a food product.

A linear relationship exists between water activity and temperature. High resolution dynamic dewpoint isotherms can help determine the physical stability of a product by pinpointing critical water activity levels where phase transitions will occur in both amorphous and crystallized states.

 

Packaging and physical stability

Conductance value and permeability of packaging (water vapor transmission rate, or WVTR) are crucial to consider when developing a product resistant to food temperature abuse. We want to consider the final water activity value, the current water activity, and the initial water activity level. Then we can solve for different factors to identify the best packaging material for a specific product. Make sure to test water activity in specific conditions, like temperature, ambient humidity, and atmospheric pressure, all of which can affect the product's water activity and shelf life.

Figure 4: Package permeability can have a significant effect on vapor transmission.
Figure 4: Package permeability can have a significant effect on vapor transmission.

When choosing and developing packaging, considering the WVTR will increase shelf life and decrease food temperature abuse. Please view the webinar recording for several real-world examples of applying these equations and calculations.

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