NRIB Sake Essentials

The NRIB released a series of short videos about sake on YouTube last year, called “Sake Essentials”, which I’ve just stumbled across while looking for something else. It’s nothing new for anyone who has been exposed to Japanese promotion of sake, but each video is 5 minutes or less, they’re produced very nicely and have decent English voiceover or subtitles. (I’ll refrain from commenting on the fluidity of the translation the voiceover/subtitling is based on.) 

• Episode 1: “History, Culture, Flavour“
• Episode 2: “Varieties“
• Episode 3: “Quality“
• Episode 4: “Food“
• All four episodes in one video
• NRIB channel (with a few other videos in Japanese, mostly showing lab testing)

But again, regardless of how it’s delivered, it’s nice to see an organisation like the NRIB doing some outward-facing communication.

And the “Food” video talks about how sake doesn’t just go with Japanese food, plus lots of ✨science✨, so they get extra points from me. (Although it’s also hard to figure out who would be interested in both basic pairing advice and the fact that iron and sulfurous acid in wine promote polyunsaturated fatty acid oxidation…)

The NRIB site also includes their infrequent newsletter (Japanese) with information on their research and training courses. The March 2022 issue features research by Atsuko Isogai, deputy head of the research division, on hineka and how to prevent it.

The article defines hineka as a smell of degradation produced when sake is stored at high temperatures for a long time. While sake can of course be protected by storing it at low temperatures, that can be difficult to ensure during transport (or indeed once it ends up overseas). With that in mind, NRIB looked into developing yeast that can produce sake less likely to generate hineka when stored at higher temperatures.

Hineka aroma itself is caused by a number of components, but the major one is dimethyl trisulfide (DMTS). Newly-brewed sake contains virtually no DMTS but it does contain a chemical precursor, so DMTS is formed during storage. But until recently it wasn’t clear what that precursor was.

Dr Isogai separated the components of sake by type, stored each type at high temperature, and further investigated those that produced DMTS. Repeating this process allowed her to isolate the precursor of DMTS in sake, which she named DMTS-P1. When the component was further analysed it turned out to be a novel compound similar to ones produced by the methionine regeneration pathway in yeast cells. DMTS-P1 was also found to increase during brewing, suggesting that it was produced by yeast during alcoholic fermentation.

Sake that contained large amounts of DMTS-P1 also tended to contain large amounts of DMTS after storage, and adding DMTS-P1 to sake also increased the amount of DMTS. So, Dr Isogai reasoned, reducing the amount of DMTS-P1 in sake would lead to less DMTS after storage. 

She knocked out methionine regeneration pathway genes out one by one, and used the resulting 13 yeast strains to brew sake. This identified two genes that, when knocked out, created a yeast that made sake with less DMTS-P1 and therefore eventually less DMTS.

However, although this research produced a yeast that produced sake with less DMTS, it’s still challenging to use genetically modified organisms. So the NRIB teamed up with Nihon Sakari to try to find a naturally occurring mutant yeast that had lost one or both of the genes pinpointed as producing DMTS-P1 – something that had a 1 in 100 million chance of occurring naturally. But as the pathway generating DMTS-P1 had been identified, they concentrated on yeast strains known to have deficiencies in their methionine pathways and eventually isolated a yeast that produces a fraction of the hineka precursor compared to normal sake yeast.

The Brewing Society of Japan commercialised the Kyokai Yeast mde-D1, a variant of the existing Kyokai Yeast #701 with the desired natural mutation, in January 2021. However, as you might expect from a yeast with a flaw in one of its metabolic pathways (similar to the yeast that produces red pigment) it does not ferment as strongly as normal yeast so much larger quantities of yeast are required as well as a slightly higher brewing temperature.