Food Engineering

Module 3. Food dehydration

Lesson 28
CONSTANT RATE PERIOD AND FALLING RATE PERIOD DEHYDRATION

28.1 Introduction

Drying process can be divided into two period

(a) Constant rate period

(b) Falling rate period

Fig. 28.1 General rate of drying curve for solid material

In a constant drying rate period, a material or mass of material contain so much water that liquid surface exists will dry in a manner comparable to an open faced body of water. Diffuion of moisture from within the droplet maintains saturated surface conditions and as long as these lasts, evaporation takes place at constant rate. When a solid is dried under constant drying conditions, the moisture content x­_t typically falls as shown in the graph A in the figure.

The graph is linear at first, then curves and eventually levels off. Graph B shows the drying rate : it is horizontal at first indicating that the drying rate is constant, then it curves downward and eventually falls. When the material has reached its equilibrium moisture content, reaches zero.

Constant rate drying period will proceed until free moisture appears from the surface, the moisture removal rate will then become progressively less. The moisture content at which the drying rate ceases to be constant is known as the critical moisture content. During the constant rate period, the moisture from interior migrates to the surface by various means and is vapourised. As the moisture content is lowered, the rate of migration to the surface is lowered. If drying occurs at too high temperatures, the surface forms the layer of closely packed shrunken cells which are sealed together. This presents a barrier to moisture migration and tends to keep the moisture sealed within. This condition is known as ‘case hardening’.

The constant rate period is characterized by a rate of drying independent of moisture content. During this period, the solid is so wet that a continous film of water exists over the entire drying surface, and this water acts as if solids were not there. As long as the rate of surface evaporation continues, the rate of drying is governed by equation:

The constant rate period ends when the migration rate of water from the interior of the surface becomes less than the rate of evaporation from the surface. The period subsequent to the critical point is called ‘the falling rate period’. Beyond this point, the surface temperature rises, and the drying rate falls off rapidly. The falling rate period take a far longer time than the constant rate period, even though the moisture removal may be much less. The drying rate approaches zero at some equilibrium moisture content.

Drying in falling rate period involves two processes:

a) movement of moisture within the material to the surface

b) removal of the moisture from the surface.

The method used to estimate drying rates and drying times in the falling rate period depends on whether the solid is porous or non porous. In a non porous material, once there is no superficial moisture, further drying can occur only at a rate governed by diffusion of internal moisture to the surface. In a porous material other mechanism appears, and drying may even takes place inside the solid instead of at the surface.