Changing sugar to acid, a flaw.

A taste fault in the coffee beans producing a highly displeasing sour sensation on the tongue. The result of enzyme activity in the green coffee beans changing the sugars to acids in the drying process during harvesting.

An undesirable flavor resulting from a chemical change by yeast or enzymes in the green coffee, similar to those occurring when wine changes to vinegar.

having undergone fermentation; "hard cider"

An aroma reminiscent of alcoholic fermentations.