The rotting is part of the Dark One's touch. Corruption, death, and so on. Rand, as the champion of the Creator represents life. The food in the sacks didn't spoil when they were opened, they were always rotten. The food in the other sacks didn't "unspoil" from Rand's presence, they were always good. Of course the outcome depends on Rand's state and presence, but he doesn't turn bad food into good, that would be unnatural. The state of the food cannot be known until the sacks are opened, Rand's presence simply did what his presence has done since the beginning of the series, since The Dragon Reborn, at least: cause an extremely unlikely outcome to be true. Only now, thanks to his position as Light's champion, that result is one that favours life.

This only works because the state of the food was theoretically unknown, so while it was rational of Iralin to suspect that the sacks not opened would be rotten as well, he didn't know. Rand could not have caused an impossible scenario to be true, only a highly implausible one.