Water Activity and Bound Water Types in Food Quality

Functions and Activity of Water (H2O)

Water acts as a stabilizer of body temperature, transports nutrients and degradation products, and serves as a reagent in chemical reactions. It also acts as a stabilizer, forming polymers.

Connected Water Molecules

Water in contact with solutes and other aqueous constituents exhibits reduced mobility. This water freezes at approximately -40 ºC. The degree of H2O connection includes constitutional water, vicinal water, and multilayer water. This connected water is available for the growth of microorganisms and for enzymatic reactions.

Types of Bound Water in Food

• Constitutional Water: Represents a small fraction of the H2O present in food with high moisture content. It is water strongly linked to the aqueous constituents of the food, often through ionic bonds.
• Vicinal Water: Represents the H2O adjacent to the next layer of constitutional water. It occupies sites close to the most hydrophilic groups present in the aqueous constituents.
• Multilayer Water: Represents H2O linked with less intensity than vicinal water. It forms connections to the low aqueous constituents of the food. It has a slightly higher intensity of linkage compared to solutes, causing it to behave similarly to pure H2O.

Free Water

Free water is present in food and has the same properties as pure H2O. It is readily available for microbial growth and enzymatic reactions. In the food matrix, it flows freely and is released when the food is cut.

Maximum Water Connection (Trapping Capacity)

The matrix has the ability to trap large quantities of H2O molecules, thus preventing its exudation (syneresis). This water is easily removed during the drying process and converted into ice during freezing. This fraction constitutes the main portion of H2O in the food, directly influencing food quality.

Food Deterioration

Food deterioration results from microbial growth, enzymatic reactions, and chemical reactions.

Water Activity (Aw)

Water Activity (Aw) provides an estimate of the actual total water available. It is highly correlated with the speed of microbial growth and other deterioration reactions. Aw is a useful indicator for predicting the stability of a product and its microbiological safety.

Relative Humidity (RH) and Aw Relationship

Relative Humidity (RH) plays a role in the microflora that may develop in the food, as there is a tendency for the food to balance its moisture content with that of the environment. This can cause the migration of free H2O from the atmosphere into the food.

Aw composition is based on the default state, which is pure H2O, which has an Aw value of 1.0. In food, Aw will always be less than 1.0 because its constituents decrease the mobility of H2O.

Hysteresis

Hysteresis refers to the speed and quantity of H2O removed during desorption.