go bad, sour
饐 centers on food spoiling: going bad, turning sour, or becoming inedible. The sourness is a natural result of that spoilage.
饐 combines the food radical 飠 with 壹, which likely contributes the sound and may relate to the idea of something being sealed or contained, leading to spoilage. The exact historical development is uncertain.
The food radical 飠 beside 壹, which looks like a container with a lid, suggests food left sealed too long: it goes bad and turns sour.
For イ, imagine the sour smell making you say 'Ee!' in disgust: 'Ee!' -> イ, as the food goes bad.
to go bad (of food); to spoil; to turn sour