As charcoal itself tends to absorb odors, it is often caused by odor absorbed by charcoal during storage and discharged into sake when charcoal is put into sake.
炭自体が臭気を吸収しやすいので、保存中に炭が外部から吸収した臭い成分を、酒に投入されたときに酒の中へ放出して生じることも多い。