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.
According to the Bureau of Education in the Philippines fresh tomatoes can be preserved in wood ash for up to three months. The information was shared in a newsletter for Pacific Islanders to share gardening and nutrition information suited to their local region.
Kikuyus in Central Kenya spray growing cereal with a water/wood ash mix to keep insects away. This technique is also used for long storage of cereals, by mixing the cereals (maize and beans) with ash
@rek I follow a homesteader who does something very similar for his grain! I think he mixes in Diatomaceous earth, then when he wants to eat the grain he pours it out in front of a fan to blow it out.
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