Introduction to Food Engineering

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Food Dehydration - Moisture Content, Drying Rates, and Energy Balance

Learning Outcomes

At the end of this module, you will be able to:

  1. convert moisture content values from one basis to another
  2. describe constant and falling rate periods during drying
  3. conduct a mass and energy balance to design a dryer
  4. describe sorption characteristics of a food

Food dehydration involves reducing water present in foods by converting liquid water into vapors. The progression of the dehydration process is followed by measuring the moisture content of the food sample. In this module, we consider different periods of drying, during which the rate of drying may change. In the first video, we will examine different ways to express moisture content, namely wet basis and dry basis.

During food dehydration, moisture removal may occur at different rates, namely, constant rate and falling rate. Typically, for highly moist foods, the constant rate period of drying precedes the falling rate period. In the next video, you will learn about different rate periods and how to express them on a graph.

In designing dryers for food dehydration, mass and energy balances provide us with information necessary to determine various operating conditions such as airflow rate, moist product feed rate into the dryer, and dried product rate at the exit of the dryer. In the next video, you will learn how to conduct mass and energy balances on a dryer to obtain design information.

During the storage of foods, moisture exchange between foods and the environment continues until the establishment of an equilibrium. The moisture exchange may occur when food is losing moisture to its environment, e.g., desorption, or when it is gaining moisture from the moist surroundings, e.g., adsorption. A sorption isotherm at a constant temperature describes the relationship between food and its environment. In the next video, you will learn about adsorption and desorption isotherm for foods.


In this module, you learned how to express moisture content of food either as a wet basis or dry basis, and how to convert values from one basis to another. The two drying rate periods, constant and falling rate, are commonly observed during drying. You learned how to conduct mass and energy balances on a dryer to obtain design-related information. Finally, you learned about moisture exchange between food and its surrounding during storage and how to express the sorption characteristics of foods.