Complete Fermentation (完全醗酵)
Complete fermentation is one of the important notions in producing Japanese sake, and means that, in parallel multi-fermentation, yeast has almost completed decomposing sugar in Japanese sake moromi (raw unrefined sake), resulting in getting weak by itself and terminating fermentation activity, or that the yeast has thus far completed fermentation by itself.
Background
Japanese liquor is brewed in the same moromi through parallel multi-fermentation consisting of the saccharification which is conversion of starch to sugar by sake rice malt enzyme and of the decomposition which is the conversion of sugar to alcohol and carbon dioxide by sake mash (a large number of cultured yeast cells); and whether the latter process, namely the decomposition, progresses to the end or not determines roughly the mode of fermentation, complete or incomplete.
Mechanism of complete fermentation
If the fermenting moromi is left by itself, the decomposition mentioned above progresses steadily, but it never lasts indefinitely. It stops at a certain point.
There are the following two reasons:
The depletion of the sugar which is the source material to be decomposed. The amount of glucose sugar and other sugar (fermentable sugar) to be decomposed in the moromi is fixed at the beginning, and they are consumed as the decomposition proceeds. However, the sugar (unfermentable sugar) which is not used for fermentation is contained in the moromi and remains unchanged, and therefore even in case of complete fermentation, it does not happen that no sugar remains at all.
Death by alcohol contained in yeast itself
If the alcohol produced by decomposition by the yeast itself is too much, then the yeast cells are killed because of the high concentration of alcohol. Therefore, if the moromi is left by itself, the alcohol does not reach a concentration of equal to or greater than about 21%.
For the reason stated above, if the moromi is left by itself, fermentation activity is finite and stops at some point, and the fermentation left with the moromi by itself to reach this point is called complete fermentation.
Complete fermentation and quality of sake
The more complete the fermentation is, the more alcohol is produced, and so seisei-shu (product sake) naturally tends to be dry and the sake meter value (a numerical rating of a sake's sweetness or dryness) also tends to be + (dry). However, this is just a general trend (refer to the sake meter value) and it cannot be immediately concluded that the higher the sake meter value is, or more complete the fermentation is, the drier the sake is. Amino acid and many other factors affect the taste of sake in multilayered and stereoscopic ways.
In case of adding alcohol in producing sake, before the yeast cells have consumed their decomposing capability and died away, they are likely to be killed by externally added alcohol, and so the fermentation activity stops before completion, resulting in the incomplete fermentation.
A popular method is to add alcohol when the sake meter value reaches -5, and by so doing the proof becomes high instantly to decrease yeast cells and to stop the fermentation. As the relative amount of alcohol increases, the sake meter value gets higher. However, it is usual that the seisei-shu is felt sweet for the sake meter value because some amount of fermentable sugar remains unchanged in the moromi due to the incomplete fermentation.
In fact, some of chief sake brewers who like complete fermentation have an opinion that, in discussing the quality of fermentation, we should not focus only on parallel multi-fermentation, but we should consider the moromi as immature because the incomplete fermentation cuts off a part of process in which the ingredients such as protein and fat other than starch contained in rice are decomposed by Aspergillus oryzae and yeast, and then converted through complex processes to minor constituents to generate delicate flavors.
The difference of taste between the complete fermentation and incomplete fermentation can be in general more easily recognized when drunk hot rather than cold.