2 Water and Food Properties
Understand how water’s molecular interactions, moisture content, and water activity affect food processing, safety, stability, and quality.
Water’s molecular interactions
A water molecule, , is polar: its oxygen end carries a partial negative charge, and its hydrogen ends carry partial positive charges. This polarity allows water molecules to form transient hydrogen bonds with one another and with suitable groups in proteins and carbohydrates.
These interactions help water dissolve salts and sugars and hydrate food macromolecules. Ice forms an open, hydrogen-bonded crystal structure; as a result, freezing can expand water-containing foods and damage delicate tissues.
Water in food structure
Water’s interactions with food components vary along a continuum. Some water associates closely with proteins, starches, and sugars, while other water is more mobile; these are not two perfectly separate kinds of water.
Water can hydrate ingredients, help proteins and starches develop their characteristic structures, and contribute to juiciness and tenderness. As a , water can also increase molecular mobility: greater moisture can make dry, glassy foods softer or stickier.
and
measures the total amount of water in a food. , written as , describes the availability of water for microbial growth and chemical reactions. The two measures are related but are not interchangeable.
is defined as the ratio of the water-vapor pressure above a food to the water-vapor pressure above pure water at the same temperature:
At equilibrium, equals the surrounding equilibrium relative humidity divided by . Its scale runs from near to , and pure water has an activity of .
How water availability affects stability
Foods with the same total water content can have different water activities because their ingredients bind or dissolve water differently. For example, sugar in jam lowers the availability of water compared with an unsweetened food containing a similar amount of moisture.
Water-activity measurements are useful for assessing product stability and setting processing targets. However, is an indicator rather than a complete account of stability: its relationship to chemical reaction rates depends on the food and the reaction involved.
and safety
Microorganisms need available water to grow. Lowering by drying, adding salt or sugar, or freezing can restrict growth, but reducing does not necessarily destroy microorganisms. Some molds and yeasts tolerate drier conditions than many bacteria.
must therefore be considered alongside other controls, including acidity, temperature, and hygienic processing. A single water-activity value is not a universal guarantee of food safety.
Water availability and mobility influence enzyme activity and chemical reactions that affect color, flavor, and nutrients. can help indicate , but how it relates to reaction rates varies with the food and the reaction.
Texture and physical quality
Water supports the crispness and turgor of fresh produce and contributes to softness and juiciness in many foods. Moisture gain can soften crisp products, while moisture loss can dry foods out.
Freezing can form ice crystals that disrupt food structure, and thawing may cause drip loss. In packaged foods, moisture can migrate from wetter regions to drier ones and change texture; for example, a crisp component can become soggy.
Managing water in foods
Processors manage water through drying, concentration, freezing, and adding ingredients such as salt, sugar, or other humectants. Packaging helps limit unwanted moisture gain or loss. These strategies can improve shelf life, but may also change texture and other quality attributes.
A preservation plan should account for the product’s formulation, , storage conditions, and other relevant safety factors. Managing water is therefore part of a broader approach to preserving both safety and quality.