What is Yeast?
Yeasts are single-cell organisms, called Saccharomyces cerevisiae, which need food, warmth, and moisture to thrive. It converts its food — sugar and starch — into carbon dioxide and alcohol through fermentation.
Over time, people have chosen the yeast that best fits their objective, whether for brewing beer, wine, or baking bread. The yeasts used for brewing sake were likely selected for their unique characteristics over many years. Sake yeast allows for fermenting at low temperatures, creating alcohol in high concentration levels of up to 20% and generating pleasant aromas.
The art of sake brewing is a joint effort of koji mould and yeast. On the one hand, koji contains enzymes that break down the starch present in the rice into glucose (fermentable sugars).
On the other hand, yeast takes that glucose and transforms it into alcohol, carbon dioxide, and other critical components of taste and aromas.
You might remember the famous saying, "first koji, second moto, third fermentation." This means that after koji, the moto (yeast starter) is the second most critical step in sake brewing that will determine the quality of sake. For this reason, since the 1890s, scientists have taken moromi (fermentation mash) from reputable breweries, extracting the yeast cells and performing fermentation tests to select only the very best yeast breeds. The yeasts chosen through this process are then made available and distributed by the Brewing Society of Japan to all breweries across the country.
Yeast development is also actively pursued at local prefectural brewing laboratories, industrial technology centres, breweries, and universities. Their objective is the brewing of "jizake", or locally brewed sake using rice, water and yeast of local origin - a growing trend in the world of sake.
A famous example of a prefecture-developed yeast is the Kumamoto-kobo, known as Yeast #9, discovered in Kumamoto Prefecture. It is one of the most widely used yeasts in the brewing of ginjo types. It allows long fermentation at low temperatures, creating rich aromas and mellow flavours.
The Importance Of Yeast In Sake Making.
(sake yeasts in capsules)
Different yeasts lead to different sakes. Therefore, brewers need to select the right yeast to achieve the specific aromas and flavours they are aiming for.
The Brewing Society of Japan has been distributing yeast that focuses on specific functions in recent years. This includes high ester yeast, which gives off a gorgeous fragrance and yeast that creates a highly acidic sake with malic acid as the main component.
Particularly with ginjo types, there is a trend toward relying on the work of the yeast to bring about gorgeous aromas.
In the brewing of ginjo sake, rice is polished down to 60% or less, resulting in diminished nutrients. On top of that, fermentation occurs at a low temperature of under 10°C. In these severe conditions, sake yeast produces fruity aromatic components characteristic of ginjo sake. Some clear examples of these components are ethyl caproate which has a fresh apple-like aroma, and isoamyl acetate, creating a sweet banana-like aroma. So far, only particular sake yeasts have been able to give off these classic and gorgeous ginjo-aromas, which explains the growing efforts of brewers to understand the work of this magical fungi in greater depth. A key element to perfecting their brew.
The Sorakami team has had the pleasure of visiting the "birthplace" of Yeast #7, one of the most famous sake yeasts, in Nagano at the Miyasaka brewery, responsible for the delicious Masumi sake brand. In 1946, Masumi won the top awards at the regional and national sake appraisals, which caught the attention of the brewing institute's yeast scientist, Dr Shoichi Yamada. Dr Yamada later visited the brewery and confirmed the presence of beautiful yeast in the fermentation tanks. "Brewing Association Yeast Number Seven" soon became the favourite of brewers across the nation and remained the world's most widely used sake yeast.
The original number seven had a brilliant and fruity ginjo fragrance. However, today's number seven is milder and strikes a better balance between aromas and flavour.
The Sorakami Team