Food decays at 0.1% per turn, so if you bulk buy 3 mil food at a go, you are decaying 3000 food per turn while cashing, whereas if you only buy 1 mil food and cash 1/3 as many turns, and do this 3 times, you decay 0.1% less over 1 mil food, and 0.1% less over 2 mil food because those food weren't in your country in the first 1/3 and 2/3 of your turns respectively.

It's very minor, and typically not worth optimizing over.