Unit Operations in Food Processing - R. L. Earle

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Contents

Introduction

Material and energy

balances

Fluid-flow theory

Fluid-flow applications

Heat-transfer theory

applications

Drying

Evaporation

Contact-equilibrium

separation processes

separations

Mixing

Appendices

Index to Figures

Index to Examples

References

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CHAPTER 7
DRYING (cont'd)

EQUILIBRIUM MOISTURE CONTENT

The equilibrium vapour pressure above a food is determined not only by the temperature but also by the water content of the food, by the way in which the water is bound in the food, and by the presence of any constituents soluble in water. Under a given vapour pressure of water in the surrounding air, a food attains a moisture content in equilibrium with its surroundings when there is no exchange of water between the food and its surroundings.
This is called its equilibrium moisture content.

It is possible, therefore, to plot the equilibrium vapour pressure against moisture content or to plot the relative humidity of the air in equilibrium with the food against moisture content of the food. Often, instead of the relative humidity, the water activity of the food surface is used. This is the ratio of the partial pressure of water in the food to the vapour pressure of water at the same temperature.
The equilibrium curves obtained vary with different types of foodstuffs and examples are shown in Fig. 7.4.

Figure 7.4 Equilibrium moisture contents

Thus, for the potato starch as shown in Fig. 7.4, at a temperature of 25°C in an atmosphere of relative humidity 30% (giving a water activity of 0.3), the equilibrium moisture content is seen to be 0.1 kg water/kg dry potato. It would not be possible to dry potato starch below 10% using an air dryer with air at 25°C and relative humidity 30 %. It will be noted from the shape of the curve that above a certain relative humidity, about 80% in the case of potato starch, the equilibrium content increases very rapidly with increase in relative humidity. There are marked differences between foods, both in shape of the curves and in the amount of water present at any relative humidity and temperature, in the range of relative humidity between 0 and 65 %. The sigmoid (S--shaped) character of the curve is most pronounced, and the moisture content at low humidities is greatest, for food whose dry solids are high in protein, starch, or other high molecular weight polymers. They are low for foods high in soluble solids. Fats and crystalline salts and sugars, in general absorb negligible amounts of water when the RH is low or moderate. Sugars in the amorphous form absorb more than in the crystalline form.

Drying > AIR DRYING

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Unit Operations in Food Processing. Copyright © 1983, R. L. Earle. :: Published by NZIFST (Inc.)