Drying Rate Equation

During the process of drying, the drying rate is classified into two types, namely the constant drying rate period, and the falling drying rate period (Defraeye and Radu, 2017).

Constant Drying Rate Period

In this drying rate period, moisture evaporation occurs on the surface of a material. This process is like the evaporation of water from the free-water surface. The process depends on the surrounding conditions (Kant et al., 2016). After the constant drying rate is maintained for a certain span of time, the falling drying rate begins until the crop attains equilibrium moisture content. This procedure of drying is practical for hygroscopic materials. However, for non-hygroscopic products, drying means the constant drying rate only and no falling drying rate.

The drying rate depends on a range of conditions such as the difference in vapor pressure between the wet surface and the drying surface, the surface area, the drying air velocity, and the mass transfer coefficient. Figure 2.5 shows the variation of the drying rate with free moisture. There are three major sections, namely the heating zone, the constant drying rate and the falling drying rate. In the graph, points A to В represent the heating zone of the material, points В to C represents the constant drying rate, C to D is the first falling rate, and D to E the second falling rate.

Falling Drying Rate Period

The falling drying rate is the next step after constant drying rate. The process begins when the product moisture content becomes less than the critical moisture content.

This process is also further divided into two sections, namely the first drying rate period, and the second drying rate period. The first drying rate

FIGURE 2.5

Variation of drying rate along with free moisture.

FIGURE 2.6

A schematic presentation of various food graphs (Lamidi et al., 2019).

period relates to unsaturated surface drying, and the second drying rate period takes place with inner moisture transfer to the outer surface. This happens through the process of diffusion and whole process is a slow one. This is the most critical factor in the drying process (Gunathilake et al., 2018). However, initial moisture content is also an important factor in the drying process. Figure 2.6 presents various food graphs showing percentage moisture content, drying rate, and product temperature.