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Mailbag: Soy Flour vs Soy Protein Isolate
08-May-2004

I am trying to find out what the difference between Soy Flour and Soy Protein Isolate is. I have seen both mentioned in recipes, but I know I don't like soy flour, and I don't know what soy protein isolate is.

If soy protein isolate is the same as soy flour, could I substitute something for it ?

Thanks,

Fran

Dear Fran,

This question comes up a lot, and it's an understandable source of confusion. Let's see if we can clear it up:

Soy flour is made from whole, ground-up soybeans. It comes in roasted and unroasted, full-fat and defatted versions. Each of these versions will have a slightly different nutritional profile, but a typical defatted soy flour will have about 11 grams of carbohydrate in a 30 gram (1/3 cup) serving; 5 of those grams are fiber. You'll also get about 11 grans of protein. Since soy flour is made from the whole bean, it has a distinct soy taste; it's a matter of personal preference whether or not you like it. Soy flour is used in traditional baking to add protein and lighten the texture of baked goods, and it serves the same purpose in low carb baking.

Soy protein isolate, on the other hand, starts out with defatted soybeans which are then further processed to remove everything that is not protein: the protein is isolated from the other components, yeilding soy protein isolate. Because the rest of bean is stripped away from the protein, the taste of soy protein isolate is much milder, very close to neutral. Many people who have a problem with the "soy-y" taste of soy flour have no problem at all with soy protein isolate. A typical 30 gram serving of soy protein isolate has zero grams of carbohydrates, and 25 grams of protein.

As for substitutes: soy flour has a different, more substantial texture than soy protein isolate. I have had good success with substituting vital wheat gluten flour (6 grams of carbs per 1/4 cup, so slightly higher in carbs), or a combination of vital wheat gluten and oat flours in recipes calling for soy flour. Oat flour is even higher in carbs, but used sparingly gives great texture. See Where to Shop for help with locating these items.

Substituting for soy protein isolate, the obvious candidate is whey protein isolate, which is protein that has been isolated from milk. However, while both of these are protein powders, they are not exactly interchangeable, cup-for-cup. I believe that soy protein isolate is a more dense powder, whereas whey protein isolate is a little more "fluffy". You may ending up using more whey protein isolate to get the same texture. You can start out with the measurement the recipe calls for, and then add an extra tablespoon or two of whey protein isolate if it seems too wet.

I personally use whey protein isolate exclusively, because I react badly to soy products. Current thinking on the use of soy suggests limiting your soy intake to one serving (say, about 20 grams of soy protein) per day, to avoid the possibility of the plant hormones in the soy interfering with your metabolism.

Hope this helps!

Regards,
Joan