The director may make pertinent investigations which concern the aggregate amount of any product which is held in common or cold storage. In connection with any such investigation, the director may inspect only the pertinent books and records of common or cold storage warehouses for the purpose of determining and publishing aggregate amounts of any product which is held in storage, and may issue subpoenas for the attendance of witnesses and the production of pertinent books, papers, accounts, documents, and testimony in any such investigation.