Moisture Content
Moisture content is an important parameter to determine for a given substance before designing a method to process it. For example, to most efficiently dry a material, the amount of moisture removed should be known to be able to discontinue operation when drying is complete. There are several methods for initially determining the moisture content of a material; these will be discussed shortly. But first, let us review the two basic ways to express the moisture content of a material: dry-basis moisture content and wet-basis moisture content.
Wet-Basis Moisture Content
The wet-basis moisture content expresses the ratio of moisture mass to the total mass of the substance. The ratio is represented by the following equation:
where
Mwb = wet-basis moisture content,
mH2O = mass of moisture in kg, and
mdm = mass of dry matter in kg.
Dry-Basis Moisture Content
The dry-basis moisture content expresses the ratio of the moisture mass present in the material to the mass of the dry matter and is represented by the equation:
where
Mdb = dry-basis moisture content,
mH2O = mass of moisture in kg, and
mdm = mass of dry matter in kg.
Example 1. Drying Grain (adapted from Henderson, et al., 1997)
Find the mass of moisture that would be removed when drying grain from 28% wet-basis
moisture content to 12% wet-basis moisture content. The initial total mass of the grain is
800 kg. What is the final total mass of the grain?
Solution:
Given: Initial and final moisture contents, initial total mass of grain
Find: Mass of moisture removed, final total mass of grain.
Example 2. Increasing Moisture ContentConsider 500 kg of wheat bran with a dry-basis moisture content of 2%. How much water needs to be added to the wheat bran to increase the moisture content to 15% wet-basis?
Solution:
Moisture Content Measurement
The equilibrium relative humidity (ERH) of a substance is a state at which the substance neither gains nor loses moisture. relative humidity at which the substance is in equilibrium is simply determined and by knowing a The relationship between ERH and moisture content allows estimates of material moisture content.
Water activity aw, which was defined and discussed in the previous chapter, is the same as the equilibrium moisture content, in that aw x 100 = %ERH. Temperature, relative humidity, previous moisture intake and release, and the physical properties of the substance affect water activity. Often for a given temperature, water activity is plotted against the dry-basis moisture content. This plot is called moisture sorption or desorption isotherms based on whether the substance is being dried or wetted. For many substances there is significant hysteresis between the sorption and desorption isotherms, which is thought to be caused by the substance previously taking in and releasing moisture in different ways. For example, during its original hydration, a substance will hold more moisture than if it is dried and then re-hydrated due to the manner in which the cells react to the lack of moisture while the substance is dry.
There are several equations that have been developed to plot the relationship between either the water activity or the equilibrium relative humidity versus the moisture content. One of the more widely used equations for biological materials is the Henderson equation:
where
rh = relative humidity expressed in decimal form
T = absolute temperature in degrees Kelvin
Me = dry-basis equilibrium moisture content expressed as a percent
c and n = constants based on the material
As an example, Figure 1 shows an example of a desorption isotherm for sorghum constructed from the Henderson equation.
Figure 1. Desorption Isotherm for Sorghum based on Henderson Equation. (Adapted from Henderson et al. 1997.)