Food Engineering

Lesson 26. THE STAGES OF DRYING

Module 3. Food dehydration

Lesson 26

THE STAGES OF DRYING

26.1 Introduction

The removal of moisture from a food product is one of the oldest preservation methods. By reduction of water content of food product to a very low levels the opportunity for microbial deterioration is eliminated and rates of other deterioration reactions are reduced significantly. In addition to preservation, dehydration reduces product weight and volume by significant amounts and improves efficiency of product transportation and storage. Often, the dehydration of a food product results in a product that is more convenient for consumer use.

26.2 Mechanism of Drying

Dehydration or drying of foods is a complex phenomenon involving momentum, heat and mass transfer, physical properties of the food, air and water vapour mixtures, and macro and microstructure of food product. There are many possible drying mechanisms, but those that control the drying of a particle product depend on its structure and the drying parameters – drying conditions, moisture content, dimensions, surface transfer rates and equilibrium moisture content. These mechanisms fall in to three classes: (i) evaporation from a free surface (ii) flow as a liquid in capillaries, and (iii) diffusion as a liquid or a vapour. The first mechanism follows the laws for heat and mass transfer for a moist product. The second mechanism becomes difficult to distinguish from diffusion when one sets the surface tension potential to be proportional to the logarithm of the moisture potential (or water activity). The third set of mechanism follows Fick’s second law of diffusion, which is analogous to Fourier’s law of heat transfer when the appropriate driving force is used.

All solid materials have a certain equilibrium moisture content when in contact with air at a particular temperature and humidity. The material will tend to lose or gain moisture over a period of time to attain this equilibrium value. Fig. 26.1 shows various moisture content regions of a food material such as bound, unbound and free.

In conventional drying the heating medium, generally air, comes into direct contact with the solid. Various oven, rotary, fluidized bed, spray and flash dryers are typical examples. In conduction drying, the heating medium is separated from the solid by a hot conducting surface. Examples are drum, cone, and trough dryers. In radiation dryers, the heat is transmitted as radiant energy. Some dryers also use microwave energy to dry food materials at atmospheric pressure or at vacuum.

When hot air is blown over a wet food, heat is transferred to the surface, and latent heat of vaporization causes water to evaporate. Water vapour diffuses through boundary film of air and is carried away by moving air as shown in the figure. This crates a region of lower water vapour pressure at the surface of the food and a water vapour pressure gradient is established from the moist interior of the food to the dry air. This gradient provides the ‘Driving force’ for water removal from the food. Water moves to the surface by following mechanisms.

1. Liquid movement by capillary forces.
2. Diffusion of liquids causes by difference in concentration of solutes in different regions of the food.
3. Diffusion of liquids, which are absorbed in layers at the surface of solid components of the food.
4. Water vapour diffusion in air spaces with in the food caused by vapour pressure gradients.

Foods are characterized as hygroscopic and non-hygroscopic. Hygroscopic foods are those in which the partial pressure of water vapour varies with the moisture contents. The difference is found by sorption isotherms. When food is placed into a dryer, there is short initial settling down period as surface heats-up to the wet bulb temperature. Drying then commences and, provided that water moves from the interior of the food at the same rate as it evaporates from the surface, the surface remains wet. This is known as constant rate period and continues until a certain critical moisture content is reached. In practice different areas of food surfaces dry out at different rates, and overall, the rate of drying declines gradually during the constant rate period. Thus the critical point is not fixed for a given food and depends on the amount of food in the dryer and the rate of drying. The three characteristics of air that are necessary for successful drying in the constant rate period are:

1. A moderately high dry bulb temperature

2. A low RH

3. A high air velocity

The boundary film of air surrounding the food acts as a barrier to the transfer of both heat and water vapour during drying. The thickness of the film is determined primarily by the air velocity. If air velocity is too low, water vapour leaves the surfaces of the food and increases the humidity of the surrounding air, to cause a reduction in water vapour pressure gradient and rate of drying.

When the moisture content of the food falls below the critical moisture content, the rate of drying slowly decreases until it approaches zero at equilibrium moisture content. This is known as falling rate period. Non-hygroscopic foods have single falling rate period whereas hygroscopic foods have two periods. In the first period the plane of evaporation moves inside the food, water diffused through dry solid to the drying air. It ends when plane of evaporation reaches the center of the food and the partial pressure of water falls below the saturated water vapour pressure. The second period occurs when the partial pressure of water is below the saturated vapour pressure as drying is by desorption.

During falling rate period the rate of water movement from the interior of the food to the surface falls below the rate at which water evaporates to the surrounding air. The surface therefore dries out. This is usually the longest period of drying operation.

The surface temperature of the food remains close to the wet bulb temperature of the drying air until the end of the constant rate period, due to cooling effects of evaporating water. During the falling rate period the amount of water evaporating from the surface gradually decreases but as the same amount of heat being supplied by the air the surface temperature rises until it reaches the dry bulb temperature of the drying air. Most heat damage to food occurs in the falling rate period.