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Mostly Soy Protein Concentrate or Soy Protein Isolate are used when making sausage or sausage like items... if protein is being used as a binder over things like Non Fat Dried Milk (NFDM).
I don't think you will have much luck buying flavorless Whey Protein Isolate.
To answer your question though, you can find Whey Protein Isolate on sports nutrition and supplement websites, stores, or on Amazon. Whey Protein Isolate is mainly used by people who are working out or are athletes to assist in muscle building, strength training, weight loss, and nutritional requirements needing to be met to prepare your body for some kind of change you are wanting to make to it.
Soy Protein Isolate (or concentrate) is MUCH cheaper than Whey per pound.
Again if you are needing a binder you have some options other than protein :)
I'm not sure what the advantage of whey is over the others but I know a disadvantage is that is is much more expensive than the other options and good luck finding a flavorless version of it.
I'm lactose intolerant and can't even mess around with milk, ice cream, and I don't want to chance what will happen with NFDM.
I don't have soy allergies and I have soy protein isolate on hand for supplement and meal replacement shakes so I have gone with it as a binder to this point but I haven't used a lot of it. It usually comes in flavorless profiles so that is also a bonus. Also it is about half as expensive as whey isolate.
Hopefully others will chime in with more pros/cons for each.
Unflavored protein is easy to find on Amazon. I personally prefer animal sourced protein over plants, but that’s my bias. What ratio do you use when using soy/whey protein for a binder? 5%? I’ve only used milk powder in my sausage up to this point.