Soybean meal was once merely the by-product of the soybean oil processing industry. Today it is a major protein source in livestock feeding. There are two main types of meal produced in the United States: 44% protein solvent extracted and 48% protein dehulled solvent-extracted meal. Dehulled, solvent extracted meal can be fed to all classes of livestock and poultry. Solvent-extracted meal is better utilized by animals with mature gastrointestinal tracts and the ability to digest higher concentrations of fiber. Good-quality soybean meal should be light tan to light brown in color, with a fresh odor that is not sour, musty, or burned. The taste should be bland.
The process of extracting oil from beans, thus producing soybean meal, is as follows:
1. Raw beans are cleaned and dried. Hulls are separated.
2. Beans are passed through cracking rolls and then heated to 165° F to 175° F for 10 minutes.
3. Beans are flaked and passed through the extraction tower. Solvents such as hexane or other hydrocarbons are used to extract the oil from the flakes. The oil level is reduced to less than 1%. Temperature is 113° F.
4. The extracted flakes are dried at 208° F for 10 minutes, toasted at 220° F for 90 minutes, and ground into meal. This meal is generally 48 to 50% protein. Hulls are then added back to produce 44% protein meal.
Good-quality soybean meal should be light tan to light brown in color, with a fresh odor that is not sour, musty, or burned. The taste should be bland and free of any burned taste. The texture should be homogeneous and free-flowing, without coarse particles or excessive fines. Bulk density should be 36 to 40 lb (16 to 18 kg) per cubic foot. A moisture level of 12% or less improves flowability and reduces incidence of mycotic growth.
The final product must not contain more than 3.5% crude fiber. It may contain an inert nontoxic conditioning agent, either nutritive or nonnutritive, to reduce caking and improve flowability. The conditioning agent may not exceed 0.5%, and the name must be shown on the label as an added ingredient. Frequently calcium carbonate is used as the conditioning agent.
Soybean meal must be properly cooked during processing to provide optimum protein nutrition for animals. The aim of heat processing is to reduce digestive enzyme (protease) inhibitors to acceptable levels, not to totally destroy them. Underheating soybean meal fails to reduce the effects of growth inhibitors, resulting in low protein efficiency as well as reduced growth and feed efficiency in monogastric and young ruminant animals. It has been shown in turkey poults that underheated soybean meal greatly increases the need for vitamin D to prevent rickets. Overheating soybean meal tends to inactivate or destroy essential amino acids such as lysine, cystine, methionine, and possibly others.
Overprocessed soybean meal is seldom a problem with commercial vendors because of the energy required to process soybean meal. However, soybeans processed on the farm or at smaller crushing plants are likely to be over- or underprocessed if the operator does not carefully monitor the temperature.
The two main assays for determining soybean meal quality are urease activity and protein solubility. Feed manufacturers tend to prefer the urease method because it is easy, quick, and relatively inexpensive. It indirectly determines the amount of inactivation of trypsin inhibitor by measuring urease activity, an enzyme present in soybeans similarly destroyed by heating. A major limitation of the urease assay is that it cannot measure overheated soybean meal. Protein solubility can measure both over- and underprocessed meal, but the assay is more complex and requires an analytical laboratory. Numerous studies have shown decreases in weight gain and feed efficiency when bean meal has been overprocessed.
Soybean meal is usually considered strictly as a protein source; however, there is a significant amount of carbohydrate in soybeans. After the oil is removed, the remaining constituents are predominantly protein, ash, and carbohydrate. The carbohydrate fraction is approximately 42%. Because hulls are removed during processing, and by definition dehulled meal can have a maximum of 3.5% fiber, that leaves 38 to 40% available carbohydrates. These are primarily oligosaccharides or the complex sugars raffinose and stachyose.