The food is put in a container, covered in ash and let in a cool place.
Wood ash is known to stave off decomposition, keep insects away and prevent bad smells. It's useful as a drying agent, maybe it helps prolong the shelf life of the food by drawing out moisture.
Another theory I've read is that it creates a small amount of lye (hardwood ash + water drawn from the food = lye). Lye is used not to preserve, but to cure certain foods, like olives.
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