וְעַתָּה֙ יֵרֶ֣א פַרְעֹ֔ה אִ֖ישׁ נָב֣וֹן וְחָכָ֑ם וִישִׁיתֵ֖הוּ עַל־אֶ֥רֶץ מִצְרָֽיִם׃
Now therefore let Pharaoh look out a man discreet and wise, and set him over the land of Egypt.
Ramban on Bereishis 41:33, citing Bereshit Rabbah 90:5. The cited Midrash supplies the preservation method for the grain; Ramban explicitly brings it into his explanation of Yosef’s “understanding and wise” administrator.
Bereshit Rabbah explains that the stored grain was preserved by adding suitable material from the locality. Ramban cites this Midrash while explaining why Yosef required an especially wise administrator.
What Is Learned: The “wise” qualification is practical as well as intellectual: the administrator must know how to preserve the stored produce through the coming famine.