This thread is interesting as I have started using SPI and have the same questions. I used SPI at the recommended rate of 1.25 cups per 10 lb and found that the product to water ratio was 1:3 in a recipe I followed. The result was a firm sausage but was difficult to stuff which has led me to find out more on this product.
LEM products suggest using soy protein concentrate, SPC, with lean meat.
https://www.lemproducts.com/product/soy ... seasoningsIt's interesting that they only use the weight of meat in the use ratio of the product as does "The Sausagemaker" as well but make no mention of the product to water ratio or any need to rehydrate the product.
A search on the web gives more results-
SPI is a more refined product and can bind 5x its weight in water vs. 3x for SPC.
Soy protein isolate (80-90% protein), is a natural product that contains at least 80-90% protein and no other ingredients. It is made from de-fatted soy meal by removing most of the fats and carbohydrates. Therefore, soy protein isolate has a very neutral flavour compared to other soy products. As soy protein isolate is more refined, it costs slightly more than soy protein concentrate. Soy protein isolate can bind 5 parts of water. Soy isolates are excellent emulsifiers of fat and their ability to produce the real gel contributes to the increased firmness of the product. Isolates are added to add juiciness, cohesiveness, and viscosity to a variety of meat, seafood, and poultry products.
For making quality sausages the recommended mixing ratio is 1 part of soy protein isolate to 3.3 parts of water. SPI is chosen for delicate products that require superior flavor such as yoghurt, cheese, whole muscle foods and healthy drinks. Soy protein isolates sold by health products distributors online usually contain 92% of protein.
Hope this helps.
Herb