Water and its addition to or removal from agricultural products and materials is an extremely important topic in nearly all aspects of agriculture. The moisture contents of grain, feed, or hay to be bought or sold, of crops to be dried, or of meat and dairy products to be processed are but a few examples of products where moisture must be carefully managed. Moisture may be added to or removed from the product depending upon the desired final condition. Moisture is removed from products by drying. Drying usually is done to change the consistency or to extend the storage life of the product. For example, fruits and meats may be dried to change the way that they are handled, stored, and eaten. Grains and forages are dried to extend their storage life.
Some agricultural products, such as grains and forages, will dry naturally to equilibrium moisture content (the same as that of the environment) if left in the field. But with these and other products good management sometimes dictates that the crop be harvested at a wetter stage and dried artificially. Artificial drying is accomplished by causing natural or heated air to flow around and/or through the product. Artificially heated air is often used, because heating reduces the relative humidity of the air, increasing the amount of moisture each pound of air can absorb.
The management of a drying system requires the ability to be able to predict the amount of moisture that must be removed from the product. For other products, it is important to be able to predict the amount of water that must be added. The following discussion presents a method for determining the amount of moisture that must be added to or removed from biological products.
The moisture content of a given material is stated as a percent using either the wet-weight or the dry-weight basis. Because moisture content is expressed as a percent, we know that a ratio is involved. The difference between the wet-weight basis and the dry-weight basis is the value used in the denominator of the ratio. The wet-weight basis uses the weight of the product as it is received; the dry-weight basis uses the oven-dry weight (dry matter) of the product.
Study Figure 20.5. The total weight of the product is 4 pounds. When the amount of water is expressed on the dry-weight basis it is:
3 lb dry matter Expressed on the wet-weight basis, the amount of water is:
4 lb total
These relationships are explained in the following equations: Dry-weight basis:
where %MDB = Percent moisture, dry-weight basis; %MWB = Percent moisture, wet-weight basis; WW = Wet weight or weight of product before drying; DW = Oven-dry weight, or weight of product after drying; WW - DW = Weight of moisture removed.
It is impractical and often undesirable to remove all of the moisture from grain as well as many other products. Grain usually is considered to be dry when the moisture content is sufficiently low to discourage the growth of molds, enzymatic
action, and insects. This is usually about 12% moisture content, %MDB, depending upon the grain. Standards have been established to determine the heating time and temperature required to obtain an official oven dry sample of grain.
Either moisture content basis may be used with agricultural products; so to avoid confusion or misunderstanding, the basis being used should always be specified. This can be accomplished by writing the numerical value of the moisture content followed by %MDB or %MWB.
Problem: Express the moisture content on the wet-weight basis and the dry-weight basis for a product that weighs 150.0 pounds when wet, and after drying weighs 80.0 pounds.
Solution: Wet-weight basis:
= 46.666 ... or 46.7% On the wet-weight basis, the product is 46.7% moisture.
Dry weight basis:
On the dry-weight basis, the product is 87.5% moisture.
Notice that the only difference in the values used in the two equations is in the denominator of the ratio. For the DW basis the dry weight is used, and for the WW basis the wet weight is used.
One aspect of the DW basis is that the percentage of moisture can be greater than 100%. For example, if a product has a wet weight of 100 ounces and a dry weight of 40 ounces, the MWB is 60% and the MDB is 150%. This is why the standard moisture of many products is given on the wet-weight basis.
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