"main food" is correct and usable in written English.
You can use it when you want to refer to the primary food item or course in a meal. For example, "The main food at the dinner was roasted chicken."
These are his four main food groups".
Always in fridge: Ice cream is my main food source.
"My main food for running is rice," he said.
As their main food source was driven to near extinction, Plains peoples were forced onto reservations.
The researchers found that the elephants' main food sources changed over time.
Yet the pizza oven is the main food focus of Mr. Batali's Michigan life.
Climate change has steadily diminished the availability of Chinook salmon, the whales' main food source.
Pilot whales prefer deep water but come inshore to feed on squid, their main food.
Pigeons are the hawks' main food source in the New York area.
It was our main food source and livelihood and they took that away from us".
Scattered settlement and shifting agriculture predominate, with rice as the main food crop.
