Preparation of a proteinaceous material suited for use in milk substitutes by treatment of crude vegetable proteins such as soy flour, faba bean flour and the like with an SPS-ase preparation. In addition to conversion of the vegetable protein, the treatment hydrolyzes the soluble polysaccharide content in the crude vegetable protein into predominantly mono- and di-saccharides, including dissolving +hydrolyzing previously insoluble polysaccharides.