Abstract: The instant application provides a method for screening batches of soy hydrolysate for a desired amount of a component thereof, such as ornithine or putrescine, and selecting only those batches of soy hydrolysate that have a desired amount of such component. The present disclosure also sets forth methods for culturing cells in media supplemented with selected batches of soy to produce more consistent, high quality lots of a protein of interest. Further, the present disclosure provides a plurality of protein preparations that have each been produced by culturing cells in media supplemented with separate batches of soy hydrolysate containing a desired amount of ornithine or putrescine, whereby each batch of protein produced exhibits improved quality of the protein of interest or amount of quality protein produced.