The assay is run at pH 2.0. Any proteins soluble at this pH will not be digested or included in the measurement.
The protease enzymes will not interact with or digest the protein if the protein is not in solution. The digestion is performed at pH to 2 to help facilitate solubility. Many plant based products (including legumes) have been tested as standards using the kit procedure for the kit and generated excellent correlation compared to literature values for rat in-vivo digestibility.