The domestication of soybeans can be traced back thousands of years to China in the 11th century BCE. Though they were believed to have been introduced to the North American colonies about a decade before the American Revolutionary War, the term “soybeans” only came into use in the United States during the early 19th century. Soybeans were mainly used for animal feed rather than harvested for their seeds before World War 2 when interrupted supply chains made soybeans a source of oil and meal.

Soybeans, when properly treated, can be made into valuable ingredients for many different food products. Processing soybeans involves cleaning, drying, cracking, and dehulling the beans before further processing occurs. The soybeans are often made into flakes, which can then be made into tofu, soymilk, or other soy-based foods. Though there are multiple ways to process soybeans, the soy extrusion process offers numerous benefits over other types of processing.

Methods of Soybean Processing 

Though soybeans contain all the essential amino acids and are high in protein, the human body can’t absorb these nutrients from raw soybeans. To access these nutrients and aid digestion, soybeans must be processed. Soybeans can be processed in several ways, requiring heating soybeans to various temperatures.

Methods by which soybeans are heated and processed include: 

  • Cooking raw soybeans is a simple process that entails immersing them in water and allowing them to cook for up to two hours, after which they’re either dried mechanically or spread out to dry.
  • Dry soy extrusion involves pressing soybeans through a narrow opening under high pressure and heating them to 302-320°F (150-160°C) for up to two and a half minutes.
  • Expanders use temperatures usually ranging between 212-284°F (100-140°C), utilizing pressure, steam and water to expand soy material, a less energy-intensive process than soy extrusion.
  • Flaking involves injecting steam under low pressure into a conditioner, then forcing the full-fat soybeans through rollers until they flake.
  • Jet-sploding involves moving beans through a pre-heated air stream between 284-599°F (140-315°C), cooking the intercellular water until they swell and explode like popcorn.
  • Micronization is a process that utilizes radiated dry heat that warms soybeans to around 356-428°F (180-220°C); as this heat penetrates the interior, the temperature within the beans increases rapidly, altering their molecular structure.
  • Roasting soybeans to about 221°F (105°C) via electricity or gas flames for about 20 seconds reduces moisture by 30 percent without breaking down cellular structure, though soybeans require further milling prior to being used for food.
  • Wet soy extrusion involves heating soybeans to 275-284°F (135-140°C) in a process similar to dry soy extrusion, though it uses steam and a mechanical extruder.

Any of these processing methods helps decrease the factors that inhibit nutrients from being digested, though some are more efficient than others. Soy extrusion processes that utilize higher temperatures for shorter periods are the most effective, with dry soy extrusion the most effective way to retain nutrients and improve digestibility.

Other Benefits of Soy Extrusion

Besides resolving anti-nutritional factors inherent in raw soy, extrusion offers multiple other benefits.

Additional advantages of soy extrusion include the following: 

  • Alters the natural qualities of soy proteins to make them easier to digest while removing anti-nutritional properties.
  • Augments nutrition of lower-quality material, reducing costs.
  • Decreases natural bitterness.
  • Improves palatability by reducing water and fiber content.
  • Increases shelf life and stability by neutralizing fat enzymes
  • Lowers bacterial content and fungal diseases to improve the quality of soy-based animal feed.
  • Ruptures cell walls to free oil from the seeds, improving energy value.

Processing soybeans via high-shear dry extrusion results in a particularly high-end product.

Okara from Processed Soy

Also known as soy pulp, okara is an insoluble byproduct from soymilk or tofu preparation. Though used for various traditional foods in Asian countries, it’s often just considered a waste product in North America. Yet, for every pound of soymilk or tofu made, a pound of okara is generated, which results in significant wastage when not utilized.

Add to this the substantial volume of oils and proteins okara contains, and it makes sense to use this material productively. It contains about 50 percent dietary fibers, 25 percent proteins and 10 percent lipids, and other nutrients; it can be further processed to make several useful products. Studies have even considered its possible use in certain foods as a dietary supplement or replacement ingredient.

Due to its high moisture content, however, okara spoils quickly. For this reason, it must be dried as soon as possible after soybeans have been processed. Though there are several means to treat okara further, soy extrusion offers an economically and environmentally viable means to use it as animal feed, as well as for human consumption or to make a variety of other soy-based products.

Soy extrusion utilizing okara requires a drying process to control moisture content before introducing it to the extruder. Without drying the okara before soy extrusion, producers can lose as much as 10 percent of the starting material weight. Most of this extruded okara is turned into animal feed.

The Soy Extrusion Process for Full-Fat Soybeans

Soy extrusion can also be used on full-fat soybeans, with much of the resulting extruded soy used for animal feed. Extruding full-fat soybeans combine pressure, grinding, crushing, and mechanical treatment at high temperatures. Soy extrusion results in beans high in nutrients and low in anti-nutritional properties while transforming protein structures and gelatinizing starches.

The dry soy extrusion process involves the following: 

  • Raw soybeans are fed into a hopper and the main body of the grinder via a rotating screw.
  • Soybeans are reduced in a high-temperature, high-pressure environment to break down oilseed cellular structures.
  • Oil released during this process drains into a separate container, from where it is funneled into an expeller and moved through a screening tank.
  • A choking device forces the product through an adjustable ring opening at one end of the main body.
  • Shear forces increase against the choking device’s surface area, tearing the soybean cell structure apart to release oil.
  • At this point, the material reaches its maximum temperature.
  • This results in water within the beans turning rapidly to gas from liquid at the point where the extruder discharges.
  • The gasified water escapes from the solid material, reducing its moisture content to below 6 percent.
  • High temperatures and pressures kill microorganisms and remove other harmful substances.

This process's short time ensures minimal protein degradation within the soy material. This material can then be fed into the expeller to recover optimal oil.

Rotordryer for Soy Extrusion

Durable and reliable, the Rotordryer is an ideal piece of equipment for soy extrusion. It combines the Rotormill with a heat source, creating a rotating flash drying system. Causing moisture to evaporate rapidly, the Rotordryer enables processing without degradation of high-moisture soy material, providing a high throughput of biomass material like okara.

Some of the prime benefits of Rotordryer include the following: 

  • Doesn’t degrade the product during processing.
  • Dries moisture-laden products uniformly, quickly, and effectively.
  • Integrates easily into existing systems.
  • Provides both grinding and flash drying in a single process.
  • Requires no screens due to long gap design.

The Rotordryer grinds and dries wet material introduced into the dryer’s main body via a feeding system. Here, a grinding rotor diffuses wet material until it’s very fine, then exposes it to heated, temperature-controlled air from the heater until it becomes fluidized. The exposure to heat breaks down the wet material, expanding the material’s particles’ surface area and causing moisture to evaporate instantly.

To learn more about how Prater's Rotordryer and other processing equipment, contact us today.