Soy protein is made from soybeans, specifically from what remains after the beans are crushed, defatted, and processed to concentrate or isolate the protein. Whole soybeans contain 35 to 45% protein by weight, and commercial processing strips away most of the fat, fiber, and carbohydrates to leave behind a product that ranges from about 65% to over 90% protein depending on the type.
What’s in a Soybean
The starting material for all soy protein products is the whole soybean. On a dry weight basis, soybeans pack roughly 40 to 44% protein, 14 to 19% fat, and about 33% carbohydrates (including both sugars and starch). That protein-to-fat ratio is what makes soybeans uniquely suited for protein extraction compared to other legumes, which tend to have far less protein and fat.
The fat in soybeans is mostly unsaturated, dominated by linoleic and linolenic fatty acids. But for protein production, that fat is a problem. It has to be removed early in processing, usually before any protein concentration begins.
The Two Main Proteins Inside
Soy protein isn’t a single molecule. About 65 to 80% of it comes from two storage proteins that the soybean produces to fuel germination. The first, called glycinin, is the larger of the two and gives soy protein its gel-forming ability. The second, beta-conglycinin, is a smaller protein made up of three subunit types and accounts for roughly 25 to 45% of total seed protein. These two proteins behave differently when heated, dissolved, or mixed with other ingredients, which is why soy protein products vary so much in texture and function.
Both of these storage proteins also happen to be the main allergens in soy. The alpha subunit of beta-conglycinin, in particular, has been identified as a major trigger for soy allergies. All three subunits of beta-conglycinin show allergenic activity, as does glycinin itself.
How Soybeans Become Soy Protein
The first step in making any soy protein product is removing the oil. Soybeans are typically crushed and then washed with hexane, a chemical solvent that pulls out more than 95% of the crude oil. What’s left is called defatted soy meal or defatted flakes, and this is the foundation for everything from soy flour to protein isolates. Hexane is effective but considered toxic and highly flammable, so some manufacturers are moving toward solvent-free methods like cold pressing, though these leave significantly more residual oil in the meal (around 10% compared to 0.15% with hexane).
From defatted flakes, processing branches into three main product types:
- Soy flour is the simplest form. The defatted flakes are ground into powder, yielding a product that’s roughly 50% protein. It retains most of the fiber and carbohydrates from the original bean.
- Soy protein concentrate takes things further by washing away soluble sugars and flavor compounds. This is done through alcohol extraction, acid washing at a specific pH, or a moist heat process. All three methods work by making the protein insoluble so the carbohydrates can be rinsed away. The result is 66 to 70% protein on a dry basis.
- Soy protein isolate is the most refined version. Defatted flakes are dissolved in a mildly alkaline solution, which pulls the protein into liquid while leaving behind insoluble fiber and carbohydrates. The protein-rich liquid is then separated, acidified to cause the protein to clump together, and spray-dried. The final product is 90% protein or higher.
A Complete Protein, With a Caveat
Soy protein contains all nine essential amino acids your body can’t make on its own: leucine, isoleucine, valine, lysine, threonine, histidine, phenylalanine, methionine, and tryptophan. This makes it one of the few plant proteins that qualifies as “complete.” In soy flour, leucine and phenylalanine are the most abundant essential amino acids, at about 4.2 and 4.1 grams per 100 grams respectively, followed by isoleucine at 3.15 grams.
The caveat is that soy protein is relatively low in the sulfur-containing amino acids, particularly methionine. This is what keeps its quality score slightly below animal proteins. Using the newer DIAAS scoring method (which the FAO considers the gold standard for protein quality), soy products average a score of about 84 to 86 out of 100. By comparison, most animal proteins score above 100. A score above 75 qualifies for a “high quality” protein claim, and soy clears that bar comfortably. Using the older PDCAAS method, soy scores even higher, averaging around 92 when post-processing effects are excluded.
Isoflavones and Other Bioactive Compounds
Soy protein carries isoflavones, plant compounds that have a mild estrogen-like structure. The three main types are genistein, daidzein, and glycitein. How much ends up in the final product depends heavily on how it was processed. Soy protein isolate contains about 91 mg of total isoflavones per 100 grams. Water-washed soy protein concentrate is similar, at about 95 mg per 100 grams. But if the concentrate was made using alcohol extraction, isoflavone content drops dramatically to just 11.5 mg per 100 grams, because alcohol strips them out along with the sugars.
This is a meaningful difference if you’re choosing soy products with isoflavone intake in mind. Defatted soy flour, which undergoes less processing overall, retains the most at about 151 mg per 100 grams.
Anti-Nutrients and How Processing Handles Them
Raw soybeans contain several compounds that interfere with digestion. The most significant are trypsin inhibitors, which block enzymes your body uses to break down protein. Two types, called Kunitz trypsin inhibitor and Bowman-Birk inhibitor, together make up about 10% of the protein in raw soybean seeds. Soybeans also contain phytic acid (which binds minerals and reduces their absorption), lectins (which can damage the intestinal lining at high doses), and saponins.
Heat is the primary tool for deactivating these compounds. Temperatures above 95°C disrupt the structure of lectins and significantly reduce trypsin inhibitor activity. This is why commercially processed soy protein products, which go through multiple heating steps during defatting, concentration, and drying, have far lower anti-nutrient levels than raw beans. Fermentation and germination also reduce trypsin inhibitors by activating enzymes that break them down naturally, which is one reason traditional fermented soy foods like tempeh are so well tolerated.
Textured Soy Protein
Textured soy protein (often labeled TVP) is made by forcing soy protein through an extruder, a machine that applies intense heat and pressure. A typical formulation might blend soy protein isolate with wheat gluten and starch, then push it through a twin-screw extruder at temperatures between 130 and 150°C. The combination of heat, pressure, and shear force reorganizes the protein molecules into fibrous strands that mimic the chew of meat. The moisture content and die temperature during extrusion control whether the final product comes out as small granules, chunks, or larger strips. This is the same process behind most plant-based burger patties and meat substitutes on the market.