Water activity in functional foods

Lycopene-rich tomatoes. CBD-enhanced desserts. Superfoods like blueberries and pomegranate seeds. Vitamin D-enriched milk. Functional foods come in many guises, but they share the same goal: to prevent disease and enhance health through our everyday diet.

Keep the benefits

All foods contain macro- and micro-nutrients that contribute to health. Functional foods contain something more: compounds that have a specific health benefit or disease-fighting function. And whether that compound is naturally occurring or has been added to a product, functional foods that are promoted for their benefits have a unique challenge: setting a shelf life that guarantees not only safety but effectiveness of the functional ingredient.

Protect the functional ingredient

Functional ingredients tend to be highly susceptible to degradation. Light, heat, moisture, and pH levels all impact degradation rates. When a functional ingredient is advertised as part of a  shelf-stable product, manufacturers need to understand the impact of pH and water activity on the potency of that ingredient over time. Product water activity is one of the important criteria to consider during formulation and manufacturing to ensure that the health benefits promised are actually delivered.

The impact of moisture

When functional ingredients are included in beverages, degradation can be rapid. For example, Figure 1 

shows the change in concentration of a vitamin-C fortified orange juice. Over the course of four weeks, the concentration decreases by as much as 50% (Nutraceutical Business Review, 2018). Many vitamins and probiotics are similarly affected when exposed to high-moisture environments (Turkmen, Priyashantha, and Jayarathna, 2019). 

It’s possible to slow down degradation by reducing the water activity (Aw) of the functional food. One way to do that is to produce a functional food at the monolayer value—the value at which a food product is most stable from a moisture perspective. But what exactly is the monolayer value, and is it something most manufacturers should be trying to reach?

Stability at the monolayer

The monolayer is a theoretical concept, one postulated by a trio of physicists (Stephen Brunauer, Paul Emmett, and Edward Teller) in 1938. The theory as it relates to a porous food medium is this: that as a completely dry material is hydrated, there will be a point at which water molecules coat the surface of every particle in the product one molecule thick. In theory, that is the most stable a product will be, when every particle is coated, and just barely coated, with water molecules. For products high in protein, a lot of water can be absorbed before the monolayer is reached, because protein has many folds and a lot of area to coat per unit mass. Crystalline sugar, on the other hand, is a simple cube, with almost no area per unit mass to coat.

Products will be able to absorb different amounts of water, but for most products, the first layer—the monolayer—is complete around a fairly consistent water activity: 0.3 aw. Some of the products typically found in this water activity range are breakfast cereals, flour, and pasta. Not surprisingly, these highly shelf-stable products have been the delivery method of choice for U.S. fortification programs since the 1940’s (Institute of Medicine (US) Committee, 2003). 

Delivering fresher texture

So why aren’t all functional foods produced at their monolayer value? Simply put, it’s because modern consumers want a different kind of functional food: something softer, fresher-tasting and more natural that is still ready-to-eat.  Manufacturers are creating shelf-stable functional foods at much higher water activity levels. Drying them down to their monolayer value would make them dry and unappealing. At these higher water activities, formulation becomes a complex balancing act to maximize the shelf life of the functional ingredient while keeping the product soft and fresh-tasting. 

Track degradation rates

Water activity plays an essential role in this balancing act. Formulators will be helped tremendously by mapping degradation rates of the functional ingredient to water activity. Though degradation rates are correlated to water activity, different ingredients have different relationships. Many vitamins, for example, will degrade faster as the water activity increases (see for example Lavelli, Zanoni and Zaniboni, 2007; Sablani, Al-Belushi, Al-Marhubi, and Al-Belushi, 2007).  Other ingredients, like probiotics, will have specific ranges where they reach maximum stability. Optimizing water activity can extend shelf life from days to months.

Optimize shelf life

The more natural the functional food, the bigger role water activity may have to play. In dried fruits, for example, water activity of the functional product affects not only the shelf life of the functional ingredient, but impacts susceptibility to mold growth and quality attributes like texture. When a functional food is a combination of two or more natural ingredients, moisture migration also comes into play. 

Regardless of the specific product, the beneficial effect of any shelf-stable functional food can only be maintained at its highest level by understanding and applying basic principles of water activity. 

References

Nutraceutical Business Review. “Degradation of Vitamins, Probiotics and Other Active Ingredients Caused by Exposure to Heat, Water and Sunlight.” August 7, 2018. 

Turkmen, Nazli, Hasitha Priyashantha, and Shishanthi Jayarathna. “Challenges in Probiotic Dairy-Based Beverages.” New Food Magazine, October 26, 2019. https://www.newfoodmagazine.com/article/97303/challenges-in-probiotic-d….

Institute of Medicine (US) Committee on Use of Dietary Reference Intakes in Nutrition Labeling. “Dietary Reference Intakes: Guiding Principles for Nutrition Labeling and Fortification. Washington (DC): National Academies Press (US); 2003.

Shyam S. Sablani, K. Al-Belushi, I. Al-Marhubi & R. Al-Belushi (2007) Evaluating Stability of Vitamin C in Fortified Formula Using Water Activity and Glass Transition, International Journal of Food Properties, 10:1, 61-71, DOI: 10.1080/10942910600717284

Lavelli, Vera, Bruno Zanoni, and Anna Zaniboni. “Effect of water activity on carotenoid degradation in dehydrated carrots.” Food Chemistry, Volume 104, Issue 4: 2007. Pages 1705-1711.