Limitations of Standard Processing

In recent years, large-scale solvent extraction facilities have replaced mechanical oil extraction equipment for processing soybeans. The solvent process, in which oil is leached from flakes using hexane, a petroleum product, can easily remove at least 99 percent of the available oil from soybeans. The protein meal by-product provides large quantities of cheap animal feed and the base for numerous food and industrial uses.

However, this technology has proved to be generally unsuitable for individual farmers and entrepreneurs in the United States and many smaller developing countries. A large solvent extraction plant costs approximately $20 million to build. Anything less than a daily volume of 200 tons of soybeans is considered uneconomical for even the smallest facility.

Mechanical Expeller Alternative

On the other hand, a mechanical screw press expeller costs from $5000 to $50,000, depending on its size, and can operate efficiently using the smaller quantities of soybeans available on individual farms and in developing countries. However, use of this alternative processing technique has been limited by several technical problems.

Because soybeans have a relatively low oil content compared to other sources such as peanuts, coconuts or palm kernels, conventional methods of expelling produce low oil yields. Running the soybeans through the expeller several times increases oil yields, but also causes overheating of the meal or cake, resulting in a brown color and scorched flavor. It also produces darkening and deterioration of the oil.

The Extruder as an Aid to Expelling

According to recent research at INTSOY, these problems can be overcome using relatively small-scale extrusion cooking equipment to condition the soybeans before expelling. The extruder produces heat by friction under pressure. A screw transports the ingredients through a series of restrictions within a cylindrical chamber, finally forcing the material through a die.

The extruder offers a convenient way of cooking the beans and breaking down the oil-bearing tissues in a fraction of the time required for conventional conditioning methods. The beans remain in the extruder system for less than 30 seconds at a temperature of approximately 275 degrees F. The short cooking time at high temperature is:

• Adequate to satisfactorily destroy anti-nutritional agents such as the trypsin inhibitor
• Not so long as to damage important nutritional components such as protein

INTSOY Research

The results of INTSOY research clearly indicate that expeller efficiency can be greatly enhanced by using hot extruded material in a nearly fluid state. High temperatures in the extruder release the oil, producing a semi-fluid state. At temperatures higher than 300 degrees F, the material tends to scorch.

At approximately 275 degrees F, there is a clear increase in the oil recovery rate when the extruded material is immediately fed into the expeller. As the extrudate cools, there is a drastic reduction in oil recovery regardless of the original extrusion temperature.

Advantages of Extrusion-Expelling

The results of combining the extruder and expeller into a single operation include:

• Oil yields approaching 75 percent with only a single pass through the expeller
• Production of a high quality, natural oil
• Large increases in the rated capacity of expellers
• Production of a partially defatted, protein-rich meal

Food Products from Extrusion-Expelling

A major objective of INTSOY's research effort is to produce low-fat cake or meal suitable for food uses. Results indicate that the combination of extrusion and expelling produces both a natural, edible oil and a high-quality meal with a color close to that of the raw material. The meal can be used in a number of food products including:

• A partially-defatted soy flour suitable for breads and tortillas
• Weaning foods
• Beverages
• Products for general protein fortification

Processing Advantages

This method also makes the milling process easier. For example, whole soybeans cannot be readily ground into flour by conventional milling equipment such as plate and hammer mills because of the high oil content, thereby limiting the production of full-fat soy flour in most developing countries. The partially defatted cake produced by the combination of extrusion and expelling grinds very well in conventional hammer mills.

This method further produces a high-quality, natural oil requiring little further processing. Oil from the expeller is:

• Passed while still hot through a screen to entrap the coarse sediments
• Allowed to stand overnight for cooling and sedimentation
• Decanted, leaving the sludge behind

The clear, light-colored oil is free of any off-flavor and is generally comparable to refined and partially hydrogenated oil in stability. It would be suitable for use in developing countries where there is little or no refining of vegetable oils used for human consumption. The oil also could be an important natural product for the U.S. health food industry.

Omega-3 in Soybean Oil

A number of recent reports indicate that dietary Omega-3 fatty acids have a beneficial effect on cardiovascular diseases. Raw soybean oil contains an average of 7 to 8 percent naturally-occurring Omega-3 in the form of alfalinolenic acid. In most cases, the Omega-3 is partially destroyed in during the normal refining process. The exact amount that is lost depends upon the degree of refining.

The highly stable oil resulting from extrusion-expelling retains virtually all the Omega-3 found in raw soybeans. The content in soybeans is much higher than in corn, coconuts, palm kernels and sunflowers, all of which have less than one percent.

The only common food crop that has more Omega-3 than soybeans is rapeseed, which has 10 percent. On the other hand, fish oil averages approximately 20 percent Omega-3.

Increasing Omega-3

University of Illinois germplasm experts already have identified several soybean varieties with Omega-3 contents in the 12-13 percent range. Through genetic engineering, it may be possible to develop new varieties with an Omega-3 content equal to or higher than that of fish oil.

The soybean oil from the extrusion-expelling process easily could be made into salad dressing and mayonnaise. Therefore, the relatively high Omega-3 content of soybeans creates tremendous potential for marketing the oil made using this new concept.

Benefits

This new combination of extrusion and expelling technology has tremendous potential for:

• Developing value-added products with high market potential as health foods
• Increasing decentralized processing of soybeans in regions where production is as small as 3,000 to 5,000 hectares
• Opening up soybean processing to many individual farmers and entrepreneurs interested in marketing high-quality, edible oil, along with animal feed and protein-rich soyfoods
• Promoting new economic activity which should result in both improved human nutrition and higher demand for soybeans and processed soy products