Handling, Moisture Management, and Storage of Biological Products

Introduction  The term biological products describe all of the food, feed, and fiber produced by agriculture.  These products include everything from fruits and vegetables to grain, hay, and cotton.  Although the diversity of agricultural production is too broad to be totally covered, this chapter will discuss some of the principles involved in the handling, drying, and storage of these products.

Handling  Because of differences in shape, size, and consistency, each product must have a handling system capable of moving that specific product.  The designer of a handling system also must consider product perishability and the desired form of the finished product.  A harvester designed to harvest tomatoes for the fresh vegetable market will be different from one designed to harvest tomatoes used for catsup.  Grains were one of the first products to be mechanically moved because they flow by gravity, are small, and have a relatively hard outer coat.  These characteristics allow them to be moved by different mechanical devices.

Augers  Augers are available in two types, Archimedean screw and belt.  Of the two, the screw type auger is the most popular in agriculture.  An auger is like a bolt, but instead of threads it uses flights that turn inside a tube.  Another name for an auger is a screw conveyor.  As the auger rotates, the flights move the product through the tube similarly to the way that threads move a nut on a bolt.  Augers are available in several diameters and are capable of handling many different types of products.  Augers have the advantage of requiring less horsepower per bushel and having fewer mechanical parts than pneumatic systems; but their disadvantages include the danger of the exposed auger at the inlet, and their requirement of more space because the inlet is some distance from the outlet.

Augers

 Augers are selected on the desired capacity (bu/hr) and length requirements.  The length is usually predetermined by the distance needed to move the grain, if horizontal, or by the discharge height and angle if the auger is at an angle.  The selection criteria are primarily based on the auger diameter and speed because the capacity increases as the speed increases.  Table 1 and Table 2 contain typical values for two sizes of screw augers and two different crops.  This type of information can be used to make decisions in managing a grain handling system, such as determining the size of auger required to convey grain at a given rate (bu/hr).

TABLE 1. Screw auger capacity handling dry corn (12 in exposure). TABLE 2. Screw auger capacity handling dry soybeans (12 inch exposure).

Exercise 1: What is the minimum size of auger that can be used to convey dry corn at the rate of 500 bushels per hour when the auger is inclined 45 ◦ ? Solution: The minimum size of auger is 6 inches

Exercise 2: How much horsepower (including drive train) is required to operate a 100-ft, 6-inch auger, installed at 45◦, conveying 690 bushels of soybeans per hour?

Using the Table above, the power requirement is 1.2 hp/10 ft. It is important to read the note at the bottom of Table % must be added for drive train losses. Therefore: Solution: The horsepower required by the auger, including the drive train, is 13 hp.

Pneumatic Conveyors  Pneumatic conveyors are used to move grain and other products using air.  Pneumatic conveyors are more flexible than augers because the duct does not need to be in a straight line. They are self-cleaning, and do not have an exposed auger at the inlet.  They do require more horsepower per bushel and are noisier than augers.  Three types of pneumatic conveyors are used: positive pressure (push units), negative pressure (vacuum), and a combination of negative and positive pressure.  In a positive pressure unit, a blower supplies the pressure, and the product enters the air stream through a rotary air lock. The material then is blown through the duct.

Positive pressure pneumatic system

 In a negative pressure unit, the material is vacuumed up by the inflow of air and then separated from the air in a cyclone separator.  The material collects in the bottom of the separator where it can be released with a gate valve or a rotary lock.  The air continues on to the pump and out through a filter into the atmosphere. Negative pressure pneumatic system

Sizing a Pneumatic System The capacity and the horsepower requirements of pneumatic conveyors depend on eight factors: 1. The horizontal distance that the material is moved. 2. The diameter of the pipe. 3. The vertical distance that the material is moved. 4. The number of bends in the pipe. 5. The elevation above sea level. 6. The temperature of the outside air. 7. The type of material being conveyed 8. The moisture content of the material.

Moisture Management  Water addition or removal from agricultural products and materials is an extremely important 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 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.  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.  The management of a drying system requires the ability to predict the amount of moisture that must be removed from the product.  For other products, it is important to predict the amount of water that must be added.  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.

Dry-weight basis: Wet-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

Express the moisture content on the wet-weight basis and the dry- weight basis for a product that weighs pounds when wet, and after drying weighs 80.0 pounds. Exercise 3:

Solution: Wet-weight basis: 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

 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.