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Use water activity to get—and maintain—the right texture

Use water activity to get—and maintain—the right texture

Many candies have a distinctive texture–chewy, crunchy, smooth, grainy, sticky, or soft. Confectionery flavors are similarly distinctive. These two things come together to form the customer experience. Moisture plays a key role in the texture of confectionery products. In fact, there is an ideal range of water activity—a sweet spot—that maximizes texture and quality. Getting the moisture measurement right is critical for nearly every product. Water activity is the most powerful way to measure and monitor moisture.

Water activity makes choosing the right specifications easy

Water activity affects the physical properties of foods. Foods with a high water activity have a texture that is moist, juicy, tender, and chewy. When the water activity of these products is lowered, undesirable textural attributes, such as hardness, dryness, staleness, and toughness occur. Low water activity products have texture attributes described as crisp and crunchy, while these products at higher water activity levels change to a soggy texture.  Because water activity is closely correlated to product texture, it’s easy to set specifications that relate to important texture characteristics.

Critical water activities determine where products become unacceptable

There are two ways texture can go wrong: it can be wrong as soon as your product is made, or it can go wrong over time as your product sits on the shelf. If a chewy chocolate chip cookie rolls off the production line already crispy, it’s likely too much water has been taken out in the baking process.  Another complication occurs if products undergo phase change during storage.  These changes, including glass transition and crystallization, make smooth products into grainy ones, or soft products into hardened bricks.

Crackers, potato chips, puffed corn curls, and popcorn each lose their sensory crispness with increasing water activity. The crispness intensity and overall hedonic texture of dry snack food products are a function of water activity (Katz and Labuza, 1981). Critical water activities illuminate where the product becomes unacceptable from a sensory standpoint. These fall into the range where amorphous to crystalline transformations occur in simple sugar food systems and mobilization of soluble food constituents begins. Excessive and rapid drying or moisture reabsorption by a glassy material can cause product loss by cracking and excessive breakage.

 

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.

Water activity: a simple way to prevent texture problems

Water activity plays a key role in preventing such issues.  First, sensory tests can be used to define water activity levels that are acceptable during a manufacturing process.  Once specifications are set, monitoring them is easy: just use a water activity meter on each lot or batch.  Using a Vapor Sorption Analyzer to identify problems before they start is even more powerful.  A moisture sorption isotherm can illuminate what water activity levels are critical for texture changes.

Isotherms pinpoint critical water activities

Typically, a critical water activity is found through an extensive texture study, but there is an easier way. High resolution dynamic isotherm curves (see Figure 1) have been shown to identify critical water activity values (RHc) that pinpoint where correct texture is lost by sharp inflections in the moisture adsorption curve.

Figure 1. Each product has its own unique isotherm curve due to different interactions (colligative, capillary, and surface effects) between the water and the solid components at different moisture contents

One study performed by researchers at Washington State University tried to determine if the dynamic isotherm curves for low water activity snack cookies could be used to identify a critical water activity and if this water activity could be used as a texture stability indicator.

Researchers found that the effect of water activity on crispness was more important than temperature and that the critical water activity did indeed identify the initial point at which the correct texture was lost.  This means that using a critical water activity allows for extreme accuracy in determining the exact point of texture loss–important because an RHc can be obtained with much less labor and time than a texture study. Additionally, in products sold by weight, knowing the exact amount of water influencing textural stability directly impacts the bottom line.

Resources

Carter, Brady Paul. Investigations into practical applications for the critical water activity from dynamic dew point isotherms. Washington State University, 2015. Article link.

Katz, E. E., and T. P. Labuza. “Effect of water activity on the sensory crispness and mechanical deformation of snack food products.” Journal of Food Science 46, no. 2 (1981): 403-409. Article link.

Find critical water activity limits

How to use isotherms to discover where your product is most stable and find the points beyond which it loses its desired texture.