“Stuffy smell”; isovaleraldehyde smell

An unpleasant smell arising when unpasteurized sake is stored at room temperature. Its root components have been identified as  isovaleraldehyde ,  ethyl isovalerate  and  iso-varelaldehyde diethyl acetal . The first of these is the primary cause of mure-ka, with the other two contributing compound aromas. Isovaleraldehyde is produced during maturation by the enzymatic oxidization of isoamyl alcohol.

The following prevention strategies can be given.

  1. Storage at sub-zero temperatures.
  2. Removal of enzymes by ultrafiltration and ori-sage (fining).
  3. Yeast selection (avoiding yeasts which produce isoamyl alcohol in large quantities).