Let's get a little geeky.
A key ingredient in Sake production is koji, which is basically a culture created by growing different fungi on grains or legumes in a warm and humid place. The term "koji" is derived from "kabi-tachi" which means "bloom of mold." Koji is used across Asia, though it is known by different names such as qu (pronounced “chew”) in China and nurukgyun in Korea. Koji produces a wide variety of fermented foods and alcohols, from soy sauce to shochu. In Japan, the specific fungus used in koji for Sake production is Aspergillus oryzae, while a number of other fungi. such as Rhizopus oryzae, may also used in other Asian countries.
Like Sake itself, koji was invented in China, and is at least 2300 years old with the first written mention of koji occurring around 300 B.C. Koji came to Japan at least as early as the 8th century, and likely earlier. However, there were differences in their approach as the Japanese primarily relied on the fungus Aspergillus oryzae and usually propagated it on steamed, rather than raw, grains. Initially, this fungus originated in the air and it was a time later that producers started saving koji from previous batches and using it to start new batches. The Japanese fungus was originally called Eurotium oryzae but was later changed, in 1884, to Aspergillus oryzae. Interestingly, in 2006, the Brewing Society of Japan named aspergillus as the kokkin, national fungus, of Japan.
With Sake, and at its most basic level, koji serves to break down the starches in rice into sugars so that yeast can turn that sugar into alcohol. It also serves to help form amino acids, which are important to the taste of Sake. In some respects, koji acts like malt in beer brewing, but there are significant differences as well.
In making Sake, how is seigiku, koji-making, conducted? There are actually numerous ways that this can be done, and each brewery may perform it differently, even if only slightly. However, I'll provide a basic overview of the process with the understanding that this is a complex and diverse activity, and that I am omitting numerous details.
Steamed rice is brought to a koji-muro, a special, heated room where the koji will be created. The steamed rice will be spread out and then mold spores, known as koji-kin or tane-koji, will be spread across the rice. For Sake, ki-koji, a yellow mold is used while in comparison white koji is commonly used to make shochu and black koji is commonly used for awamori. Over the course of about two days, the koji-kin will germinate and spread over all the rice, creating kome-koji, molded rice, which looks like it has been frosted.
Rhizopus oryzae is another fungus, like Aspergillus, which is used in other parts of Asia to make koji, though the Japan largely have ignored it throughout the centuries. The Japanese found that it wasn't conducive to making koji for Sake because it doesn't work well with steamed rice. However, that hasn't stopped a few, innovative Japanese from trying to devise ways to make Rhizopus work in Sake brewing. Why would they try this? Because Rhizopus may have a significant effect on the creation of amino acids, which can affect the taste of Sake.
Sake lover Gordon Heady recently posted about his encounter with an intriguing innovation in Sake production. Yukae Sato, a 22 year-old woman, has invented, patent pending, a method of creating koji with Rhizopus oryzae on steamed rice. Ms. Sato learned about this idea during her time at the University of Kitakyushu. There are still few details available concerning the exact procedure, likely due to patent issues, but we know that a Sake was brewed where 20% of the koji was from Rhizopus. The resultant Sake, a Junmai named Hibikini no Mori, is commercially available and Gordon reports that it is delicious. The Rhizopus appears to have boosted the levels of succinic and lactic acid, and may have boosted the levels of other amino acids too.
The use of Rhizopus in making koji for Sake actually isn't new, and its history extends back over 30 years. In September 1981, four Japanese inventors filed for a patent "Process For Preparation Of Japanese Sake Using Koji (Rice Malt) Which Is Made By Propagating Rhizopus On Raw Rice." I wasn't able to find much information about this patent, but it is clear they used raw rice rather than steamed rice, differentiating it from Ms. Sato's pending patent which uses steamed rice.
Later, in 1988, a study was published, in three parts, in the Journal of the Agricultural Chemical Society of Japan discussing a series of experiments in Sake brewing using seven different strains of Rhizopus on steamed rice. The study concluded that "The genus Rhizopus grew well in polished rice steeped in a particular solution of amino acids (Ala, Glu, Lys, Tyr, Val) and its steamed rice." They also stated that "Organic acids, especially fumaric, citric and malic acids, were present in the “peculiar koji” at higher levels than in ordinary koji." So, they too used steamed rice but I am unsure whether they actually sought a patent or not for their processes. The results though clearly showed elevated amino acid levels.
Despite this prior history, I've never previously heard of any commercially available Sake that was produced using Rhizopus. The Hibikini no Mori may thus be the first commercial Sake using Rhizopus, making it an intriguing new innovation. Ms. Sato's koji making process could be an important step forward in a fascinating and compelling new style of Sake. It is an important step as well for women in the Sake industry, who for many centuries who were not permitted even entrance into a Sake brewery. I look forward to learning more about Ms.Sato and her Rhizopus process, and hope to one day taste the resulting Sake.