Grains, particularly wheat is one of the most important necessity for human beings. When gold and silver coins or currency did not exist in ancient times, people used to trade these though barter. Therefore, when Prophet (peace be upon him) prohibited loan/credit in silver and gold and their exchange of homogeneous with excess or premium and it was considered as Riba being used as currency their quality was irrelevant because of their intrinsic value at that time, similar to the gold and silver the Holy prophet SAW prohibited exchange on in equal quantity of some other commodities like weat /grain, barley, dates, salt, as they were used as medium of exchange in the ancient era of barter and were considered as gold and silver and the qualities also were considered irrelevant while exchange homogeneously in excess was also considered as RIBA. Because they could serve as a medium of exchange, such as gold, silver.