- “Wood-like” smell; aldehyde smell
Kiga yo-shu (written with characters indicating a “smell like wood”), though similar to the smell of grass, straw and kiga (“wood smell”), is the smell of acetaldehyde, and is also called arudehido shu (long “u”; aldehyde smell).
Even if wooden containers are never used during brewing or storage, if acetaldehyde (an intermediate by-product of alcoholic fermentation by yeast) is produced in quantities greater than around 80ppm in moromi, an odour similar to kiga may be detected. Acetaldehyde formation in moromi is likely to occur if brewer’s alcohol is added to a young mash still containing large amounts of pyruvic acid (pilbin san), an intermediate product of fermentation.
The smell of acetaldehyde was long considered a defect, but it is also reminiscent of the aroma of green apples. For that reason, some consider it should be re-assessed when properly handled to be thought of, not as a flaw, but as a positive character element of sake aroma.