19 CFR 19.31: Bulk wheat of different classes and grades not to be commingled in storage.

All wheat shall be stored by class and grade according to the Official Grain Standards of the United States or the official standards of the Canadian Board of Grain Commissioners, in bins, compartments, or other enclosed spaces identified by clearly distinguishable insignia securely affixed thereto, so as to facilitate the maintenance of identity of the wheat. There shall be no mixing or commingling of different classes or grades of wheat in the same bin, battery of bins, or other bonded space. If the wheat is stored in bags or other transportation containers, such bags or containers shall be so marked and so placed in the warehouse that the identity of the wheat will not be lost while in storage, to permit easy access to all lots, and to facilitate inspecting, sampling, and the identification of each lot.