Every Indian kitchen has rules. Don’t eat what was cooked yesterday. Don’t eat from someone else’s plate.
Chapter seventeen of the Gītā sorts food by what it does to the person who eats it. At 17.10 it lists the food dear to the dull and heavy mind, and the words could have come from any Nani: yāta-yāmam, food whose hour has passed. Gata-rasam, food that has lost its taste. Paryuṣitam, food kept overnight. Ucchiṣṭam, leftovers from another’s plate. Jootha.
She was not being fussy. She was quoting the Gītā.
Going deeper
Chapter 17 opens with a question from Arjuna: what becomes of people who worship with faith but without following the rules of the scriptures? Kṛṣṇa answers that faith comes in three kinds, and then shows each kind at work in everyday life, starting with food. At 17.8 he describes what the clear, steady mind is drawn to. At 17.9 come the sharp, sour and burning dishes of the restless mind. Then 17.10 gives the third list, the food loved by the dull and heavy mind: stale, tasteless, decayed, rejected and unclean. This is the verse behind every grandmother's rules about leftovers. It does not scold anyone. It says that what a person reaches for shows the state they are in. The very next verse, 17.11, carries the same sorting from the plate to worship. The whole passage sits in chapter 17, and what the Gītā says about faith gathers its other verses on trust.