If you have taken sake courses 10 years ago, I would suggest you do it again today. Of course, most of the basic knowledge and principles remain the same, but trends and technologies have indeed advanced and changed with time.
Genshu refers to sake that maintains the original alcohol content obtained by fermentation, no water is added after fermentation and before pressing. Under traditional brewing process, genshu usually contains 18% – 20% alcohol by volume (ABV). But in recent years, the alcohol content of genshu has been declining.
The reason why conventional genshu is high in alcohol is that traditional yeast has strong fermentation power. Without any adjustments, they are able to produce sake with 22% ABV straight away. However, it will no longer be an enjoyment to drink such a powerful sake. Therefore, brewers will dilute genshu with water to around 16% ABV, and this is also an economical approach to produce sake. In this way, rice can be dissolved as much as possible, and it will become alcohol after being saccharified. The lesser sake kasu left after pressing, the fewer the wastage.
It is not difficult to find genshu with less than 15% ABV in current sake market. How is this done? In fact, the techniques behind are easy to understand. First, to use new yeasts, like 1501, 1801, etc, which have relatively weaker fermentation ability. The fermentation process is carried out at a very low temperature (below 8°C) to limit the reproduction of yeast. By changing external conditions, the amount of potential alcohol is controlled.
Second, to reduce the amount of rice-koji in the early stage of the koji making process. Rice-koji creates the enzymes needed to break starch down into sugar. Yeast then consumes the sugar and produce alcohol. So when the production of sugar is limited, the potential alcohol level can also be reduced.
An even more direct way is to add water during fermentation process (adding water before the completion of fermentation also meets the definition of genshu). While the space for yeast is expanded, they will also feel stressed and affect alcohol production.
This way of making genshu in a Ginjo approach, will result a sake with good balance of aromas and flavours. Because of the low proportion of alcohol production, less amount of finished sake and higher sake kasu leftover, it is considered as an extravagant brewing method.
However, in order to provide good sake to drinkers, breweries spare no expense in production. I dare not to say all genshu will become low in alcohol, but the popularity of Aramasa and Senkin confirms this direction. My favourite “Yuki no Bousha (雪の茅舎)” is not only the sake with no water added, but also no stirring process, and no filtration is undergone. Their “three no-s” has indeed established a superb technical indicator for the sake world.
Based in Hong Kong / Official trainer of Dassai / Experienced sake educator (WSET, SSI) / JSA Sake Diploma / International sake judge