Sake Brewing Science Vol. 4 - Sake Yeast

I feel like 99% of sake brewing uses yeast known as "sake yeast."
It is commonly referred to as "Kyokai yeast" (Brewing Society of Japan yeast).

Wine sometimes uses natural yeast and sometimes uses cultured "wine yeast." Beer sometimes uses what is known as "brewer's yeast," sometimes wine yeast, and sometimes even sake yeast.

What makes them different?
There are many elements that make sake what it is, such as the taste of the rice and the water, but to what extent does "sake yeast" influence it?

What is sake yeast in the first place?

There is a complex research background here; it was once called Saccharomyces sake, but today it is generally considered the same as common yeast, S. cerevisiae (though, naturally, some studies argue against this).

However, while it may be the same species as wine or brewer's yeast, it has developed "sake-like" characteristics through a long history of repeated selection and experimentation.

What are the characteristic features of sake yeast?

1. High alcohol tolerance
2. Ability to ferment at low temperatures
3. High capacity for ester production at low temperatures
   
4. Relatively high lactic acid production

In particular, elements 1 and 3 can be considered the factors that define sake.
While some wine yeasts also have high alcohol tolerance, what is noteworthy is the high production volume of ethyl caproate and isoamyl acetate, which are known as Ginjo aroma.

If low temperatures produce ethyl caproate, what about lager beer?

As I have mentioned several times, sake yeast produces ethyl caproate during low-temperature fermentation.
This is because, at low temperatures, yeast increases the production of caproic acid to enhance the fluidity of its cell membrane (essentially creating more lipids to make the cell membrane smoother). Excessive caproic acid, combined with enzymes that promote esterification with ethanol, results in an increase in ethyl caproate.

If this is a characteristic of yeast in general, why doesn't lager beer, which is also fermented at low temperatures, produce that Ginjo aroma?

In reality, lager beer uses a different yeast with a different genetic makeup called S. pastorianus, but the story is not that simple.
Like sake yeast, lager yeast produces caproic acid at low temperatures. However, instead of using this caproic acid for ester production, it uses it in a process called β-oxidation to generate energy.

While sake yeast endures the cold by "putting on fat," lager yeast is more like someone working hard to exercise and stay warm when it’s cold.

As a result, even with low-temperature fermentation, lager yeast does not produce large amounts of caproic acid, and consequently, no ethyl caproate is produced.

Incidentally, as for wine yeast, while some white wines ferment at lower temperatures, even then it is only around 10°C. Sake and lager yeasts work well even at 8–10°C. If wine yeast had greater low-temperature tolerance, there is a possibility that it could produce ethyl caproate as well.