Question

What is water activity? What is sorption isotherm? What are the applications of these 2 concepts in the food industry? Give an example for each.

Answer 1

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Answer 2

Water activity

Definition: Water activity ($a_w$) refers to the state of water in food, that is, the degree to which water is combined with food ingredients or the degree of freedom. It indicates the degree to which water in food can be used by microorganisms, and is numerically equal to the ratio of the water vapor pressure of food to the pure water vapor pressure at the same temperature.

- **Application in the food industry**

- **Controlling microbial growth**: The growth and reproduction of microorganisms requires a certain water activity. Generally, the water activity required for bacterial growth is above 0.90, the water activity required for mold growth is around 0.80, and the water activity required for yeast growth is around 0.88. By reducing the water activity of food, the growth of microorganisms can be inhibited and the shelf life of food can be extended. For example, controlling the water activity of biscuits between 0.2 and 0.4 can effectively prevent the growth of microorganisms and keep the quality of biscuits for a longer period of time.

- **Maintaining food quality**: Water activity has an important influence on the quality characteristics of food such as texture, flavor and color. For example, in the processing of preserved fruits, by controlling the appropriate water activity, the soft taste of preserved fruits can be maintained, and it can also be prevented from mold and deterioration due to excessive water.

Adsorption isotherm

- **Definition**: The adsorption isotherm is the relationship curve between the moisture content and water activity of food at a constant temperature. It reflects the ability of food to adsorb or desorb moisture at different water activities.

- **Application in the food industry**

- **Determine the packaging conditions of food**: According to the adsorption isotherm, the hygroscopic characteristics of food under different environmental humidity can be understood, so as to select the appropriate packaging materials and packaging methods. For example, for milk powder that is easy to absorb moisture, its adsorption isotherm shows that when the environmental humidity is high, the milk powder is easy to absorb moisture and cause agglomeration and deterioration. Therefore, packaging materials with good moisture resistance, such as aluminum foil composite bags, can be selected to prevent milk powder from absorbing moisture.

- **Optimize food processing technology**: In food processing such as drying and concentration, the adsorption isotherm can help determine the best process parameters. For example, when drying vegetables, according to its adsorption isotherm, the temperature and humidity during the drying process can be controlled so that the vegetables can maintain good quality and nutrients while reaching the expected water activity.