One kernel of corn and one grain of rice is how those things naturally occur, without being ground up into much smaller pieces, kind of a completely different category, but like this is entirely a silly hypothetical thought experiment so you can set whatever parameters for "single object of food" to whatever you want, you could consider a molecule of soup if you go far enough down the chain

I think if you can count one kernel of popcorn as a food item then you can count one grain of rice, in which case absolutely no contest, rice is eaten every day by like half the world population and also intentionally thrown on the ground a lot at weddings.