- “Morning sickness smell” ; diacetyl smell
The yoghurt-like odour arising when the growth of yeast in moromi is retarded, when lactic acid and other bacteria grow excessively, or the spoilage phenomenon called hi-ochi occurs . Its primary components are diacetyl, volatile acids and so on. It is a very similar smell to the characteristic hi-ochi ka smell associated with hi-ochi contamination, but, where hi-ochi ka is produced by the related lactic acid bacteria during storage or in product after packaging, tsuwari-ka is the result of abnormal bacterial growth during the brewing process itself.
When this smell is caused by acetic acid bacteria, it has their peculiar odour, one of the most detested to arise in sake moromi during brewing. However, near the beginning of kimoto-school yeast starter production, when the smell of koji gives way to a so-called “sweet smell” five or six days after mashing, tsuwari-ka may be faintly perceptible. This is the ideal timing to initiate the heating of the mash with the daki (containers of hot water) used for the purpose.
It is also sometimes said that this off-smell can arise when the fermentation of immature moromi is suddenly stopped by the addition of brewer’s alcohol, or if pressing takes place immediately after the alcohol addition.
Though the smell of diacetyl has traditionally been considered a flaw, it can be reminiscent of yoghurt under the right conditions, and so some think that it should be reassessed as a positive element of sake aromatics.