Water activity: a better moisture metric for dry and dehydrated products

Dry mixes are economical and convenient with increased shelf life, reduced packaging, decreased cost (via weight and/or volume reduction), and improved handling properties. Controlling water activity (aw) in a dry product maintains proper product structure, texture, stability, density, and rehydration properties.

Water activity affects the textural properties of dry, cereal-based foods and starch-based snack products.

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 are found where the product becomes unacceptable from a sensory standpoint. These fall into the water activity 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 the undesirable consequence of product loss by cracking and excessive breakage. See how to avoid this using phase diagrams and moisture sorption isotherms.

Controlling water activity slows chemical degradation

To preserve the initial quality as much as possible during dehydration and storage, the chemical and biochemical reactivity and stability must be considered. Water activity influences nonenzymatic browning, lipid oxidation, degradation of vitamins, enzymatic reactions, and protein denaturation. The likelihood of nonenzymatic browning increases with increasing water activity, reaching a maximum at a water activity range of 0.6 to 0.7. Generally, further decreases in water activity will hinder browning reactions. Lipid oxidation has a minimum in the intermediate water activity range and increases at both high and low water activity values, although due to different mechanisms. This type of degradation results in the formation of highly objectionable flavors and odors and the loss of fat soluble vitamins. Water soluble vitamin degradation in food systems increases with increasing water activity values (Kirk, 1981). Enzyme and protein stability is influenced significantly by water activity, due to their relatively fragile nature. Most enzymes and proteins must maintain conformation to remain active. Therefore, maintaining critical water activity levels to prevent or entice conformational changes is important to food quality. Most enzymatic reactions are slowed down at water activities below 0.8, but some reactions occur even at very low water activity values.

Prevent caking and clumping in powders

Knowledge of the water activity of powders as a function of moisture content and temperature is essential for the control of water content during processing, handling, packaging, and storage to prevent the deleterious phenomenon of caking, clumping, collapse, and stickiness. Caking is a deleterious phenomenon by which a low-moisture, free-flowing powder is transformed into lumps and eventually into an agglomerated solid, resulting in loss of functionality and lowered quality (Tsourouflis et al., 1976; Saltmarch and Labuza, 1980). This problem is ubiquitous in the food and pharmaceutical industries. Caking is water activity, time, and temperature dependent, and is related to the collapse phenomena of the powder under gravitational force (Chuy and Labuza, 1994). Stages in caking involve bridging, agglomeration, compaction, and liquefaction. Factors known to affect caking kinetics may be divided into those related to the powder itself (particle size distribution, hygroscopicity and charge of particles, state of the material, presence of impurities) and external factors such as temperature, relative humidity, and mechanical stress applied to the substance (Aguilera and del Valle, 1995; Peleg and Mannheim, 1977).

To maintain the proper flow properties of powders and prevent caking of powders, the following methods are available:

  • drying to low moisture content
  • treatment of the powders at low humidity atmospheres and packaging in high barrier packages
  • storage at low temperatures
  • in-package desiccation
  • agglomeration
  • addition of anticaking agents

Anticaking agents are food ingredients added to hygroscopic powders to improve their flowability and inhibit caking. Main food-grade, anti-caking agents are silicon dioxide, silicates and sterates, phosphates, and polysaccharides (Peleg and Hollenbach, 1984). An important class of anticaking agents are ones which can compete with the host powder for the available humidity due to their large water adsorptive capacity, thus reducing their hygroscopicity and the tendency to cake. Many of these ingredients are of porous nature and exert their protective action in foods first by preferentially adsorbing large amounts of water vapor onto specific sites with high binding energy; thus depressing water activity dramatically for relatively large moisture contents (Aguilera and Del Valle, 1995).

When all binding sites are occupied, pores start to fill and water activity corresponds to that of capillary water. Water activity is an important factor affecting the stability of dry and dehydrated products during storage. Controlling water activity in a dry or dehydrated product maintains proper product structure, texture, stability, density, and rehydration properties.

References

Aguilera, JoséM, JoséM del Valle, and Marcus Karel. “Caking phenomena in amorphous food powders.” Trends in Food Science & Technology 6, no. 5 (1995): 149-155. Article link.

Chuy Leticia E., and Labuza, Theodore P. “Caking and stickiness of dairy‐based food powders as related to glass transition.” Journal of Food Science 59, no. 1 (1994): 43-46. Article link.

JF III, Gregory, and J. R. Kirk. “The bioavailability of vitamin B6 in foods.” Nutrition reviews 39, no. 1 (1981): 1-8. 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.

Labuza, Theodore P. “The effect of water activity on reaction kinetics of food deterioration.” Food Technology 34, no. 4 (1980): 36-41. Article link.

Peleg, M., and C. H. Mannheim. “The mechanism of caking of powdered onion.” Journal of Food Processing and Preservation 1, no. 1 (1977): 3-11. Article link.

Peleg, Micha, and Ann M. Hollenbach. “Flow conditioners and anticaking agents.” Food Technology (USA) (1984). Article link.

Tsourouflis, Spyros, James M. Flink, and Marcus Karel. “Loss of structure in freeze‐dried carbohydrates solutions: effect of temperature, moisture content and composition.” Journal of the Science of Food and Agriculture 27, no. 6 (1976): 509-519. Article link.