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How did Amazonian peoples make poisonous bitter cassava safe to eat?

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Map of South America and the Caribbean showing the distribution of Aboriginal cultural groups, with regions color-coded by cultural type including Andean, Central American, tropical forest, and various nomadic cultures such as aquatic nomads, forest hunters and gatherers, and shellfish gatherers. Major group names are labeled across the continent and surrounding islands. — South American forest Indian Distribution of aboriginal South American and circum-Caribbean cultural groups.

To make bitter cassava safe for consumption, indigenous peoples in South America developed a sophisticated refining process to remove cyanogenic glucosides, which produce toxic hydrocyanic acid.1

Because bitter varieties contain high levels of these harmful compounds, they require extensive preparation compared to "sweet" varieties. The process typically involves the following steps:1

Grating: The tubers are grated or crushed to break down the plant tissue.12
Pressing: The resulting pulp is pressed to extract the liquid containing the toxins.1
Leaching and Fermentation: The material may undergo long soaking times and fermentation to further neutralize the chemicals.1
Exposure: The cassava paste is often exposed to the air for several hours to allow the volatile poisons to dissipate.1
Heating: Finally, the processed starch is heated, often to create flour or cakes.1

Improper preparation is dangerous, as it can lead to acute cyanide intoxication or long-term health issues such as ataxic neuropathy. Through these traditional methods, indigenous communities successfully transformed a potentially lethal plant into a vital dietary staple.12