A primer on soyabean dry extrusion and its benefits

Soybeans are an important crop across the world and grown for a variety of agricultural and industrial uses. They are highly nutritious and comprise 34-36% protein, 18-20% oil and all essential amino acids. Though they are a great source of amino acids, in their raw form they are not fit for human consumption or as animal feed because they also contain anti-nutritional factors such as trypsin and chymotrypsin inhibitors, urease, phytohemagglutinins (lectins), allergenic factors and lipases and lipoxygenases that adversely affect digestive efficiency.

Soybean meal represents more than 50% of the total oil seed meal production. They can be made fit for human and animal consumption, through a variety of techniques at different cooking temperatures including roasting, expansion, micronization, wet or dry extrusion and solvent extraction.

Solvent extraction while the most effective method is also the most prohibitively expensive. The benefits of the high shear dry extrusion technique have proven to be the most effective method to process soybeans and reduce anti-nutritional factors while retaining maximum nutrition. Extrusion technology is also environmentally positive as compared to other cooking processes because it is energy efficient and has no waste streams that need treatment.

The advantages of Soybean extrusion

Extrusion frees the oil by rupturing plant cell walls, Oil in extruded full fat soybeans is stable and highly digestible. This method is easier and cheaper to add oil uniformly in the feed and production costs are much lower than solvent extracted soybean meal.

The benefits of Soybean extrusion

1. Increases amino acid digestibility and reduces bitter flavours
2. Modifies protein, eliminating several anti-nutritional factors, while changing the tertiary structure of protein, shortening protein hydrolysis time in the digestive tract and improving digestibility.
3. Improves gelatinisation of the starch and gives it strong water absorption and bonding properties.
4. Releases encapsulated fats and improves energy value.
5. Reduces FFA (Free fatty acid) content and deactivates lipase minimising oil and fat degradation while reduces rancidity of oil components during storage and transportation.
6. Increases palatability by reducing water and crude fibre content
7. Increases safety by eliminating maximum bacteria, muscardine and fungus in the raw material

The advantage of coupling dry extruders with expellers for mechanical oil extraction.
When coarsely ground soybeans are processed in a dry extruder, the material is discharged in a semi-fluid state at an extrusion temperature of 140-150°C. On cooling, the oil is absorbed back into the protein matrix and the material becomes a dry, mealy mass. To extract the oil, the material must be pressed immediately on extrusion as there is progressive loss of oil extraction efficiency on cooling. A single pass through the extruder and screw press results in 71.8% extraction of oil while the residual oil content in resulting press cake is 6–8%. Trypsin inhibitor activity in raw soybeans is reduced by 91% from heat exposure during extrusion and screw-pressing.

What is dry extruded soybean meal?

Meal from extrusion by friction heat and/or steaming of whole soybeans without removal of any component parts is known as DESBM or dry extruded soybean meal. It is sold based on its crude protein, fat and fibre content. DESBM is produced by mechanical friction creating a high temperature for a short time period. The temperature and the time spent at a given temperature directly affect the quality and nutritional value of the product. The extruded oil produced has several unique characteristics including uniquely stability, low levels of phosphatides, unsaponifiable matter, peroxide value and FFAs.

Applications of dry extruded soybean meal
Dry extruded soybean meal has a higher fat content than regular solvent-extracted soybean meal. It is commonly used in swine and livestock diets worldwide. It has elevated levels of lysine and tryptophan, amino acids that are deficient in other common energy sources such as corn. Soybean meal products contain high concentrations of protein, an excellent balance of amino acids and high-energy content, making them an excellent source of protein. Using soybean meal makes it possible simplify diets without other supplemental protein sources.

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