Is rice protein concentrate derived from cooked rice? If so, rice protein concentrate is potentially bishul akum and should not be a group one.
Promotional material for one manufacturer (Organic Rice Protein, Inc.) describes the process below. The reader will have to examine the sentence slowly, then we’ll go over it carefully. This is a worthwhile exercise because the fundamentals of the process is common to many starch processing facilities:
Rice protein concentrate…is basically the concentrated protein fraction of the rice grain and is produced from the rice milling process. It relates to the manufacturing of the rice syrup where natural enzymatic process transforms the complex carbohydrates of rice into a simple sugar. The leftover of this process is recognised as organic rice protein, which is filtered and dried at low temperatures. The resulting powder is referred to as Rice Protein Concentrate.
Rice syrup (like corn syrup) is produced in two stages: liquefaction and saccharification (which makes the syrup sweet).
Rice is liquefied when rice is cooked and, simultaneously, treated with enzymes. The physical change from solid to liquid is the result of long chain starches (the complex carbohydrates referred to in the paragraph above) breaking down to shorter-chain starches (think of a chain of thousands of repeating units being broken down to chains of hundreds of repeating units). Enzymes are responsible for the breakdown of these chains. The enzymes work effectively in the context of heat, which is why the rice is simultaneously cooked.
The rice liquid, or slurry, is still not particularly sweet at this stage. Additional enzymes continue to break down some of these medium-sized molecules to much smaller molecules. These smaller molecules, called glucose and maltose, are made of one or two repeating units (called mono- or disaccharides, respectively). These are the simple sugars referred to in the excerpt above.
Rice protein is filtered out of the rice syrup, and is a powder.
Back to the question: rice syrup is not derived from fully-cooked rice; rather, it is the result of liquefaction of rice grains, and the liquid is not olah al shulchan melachim. Therefore, rice protein concentrate, which is made from rice syrup, poses no problem of bishul akum.
The use of enzymes creates a new question, also related to the group one status of rice protein concentrate: what enzymes are used in liquefaction and then in saccharification? Can we assume they are kosher?
The enzymes used for these processes are kosher and we are not familiar with non-kosher enzymes that are capable of liquefaction or saccharification of starches. However, one of the enzymes used in the saccharification process is derived from soaked barley. Its use in the production of rice syrup, the main ingredient in rice milk, must be considered before owning or drinking some rice milk products on Pesach.