The Mainichi Shinbun reports on the Nishibori brewery in Oyama city, Ibaraki Prefecture, using acrylic "barrels" (sakadaru, 酒樽) for brewing. The made-to-order tanks are normally used in aquariums, but make it easy to see how rice is transformed into sake when used for brewing.
The Ibaraki Prefecture Brewers Association commented that they had never heard of completely transparent tanks, and a patent application has been filed. The fifth brewery owner, Nishibori Kazuo, expects the tanks will make it easier to observe the state of the fermenting mash and therefore contribute to improved brewing techniques.
Sake brewing normally uses enamelled tanks or wooden barrels, which only allow observation from above, but clear tanks also allow viewing from the side. Nishibori Kazuo came up with the idea while on a study tour of an aquarium in Tokyo. He first looked into modifying their existing brewing tanks, but the company he spoke to weren't willing to take on a project that had never been done before. In the end, he wound up talking to the manufacturer of the tanks used in Hiroshima Aquarium.
The tanks are made of acrylic and measure 120 cm across and 150 cm deep, with 2 cm thick walls. Kōji rice, steamed rice, water and yeast are added to create the mash that will produce sake. Bubbles released by the fermentation process and convection currents can be observed during the approximately one month it takes for the sake to brew.
The tanks have been used twice, with the resulting sake sold as Mongai Fushutsu CLEAR BREW (門外不出CLEAR BREW, meaning a jealously guarded treasure or something not allowed outside the house). Nishibori Kazuo's eldest son, Tetsuya, left his job at a big city IT company last year to return to the brewery. He streamed footage from the tank, and the brewery also held an educational open day where people could come and observe the fermentation in the clear tank.
One reason my interest was drawn to this story is that I saw barrels with a clear cut-out on the (very brief!) tour of the Gekkeikan model brewery in Fushimi, Kyoto. Nothing was said about it, but you could at least see the volume in the tank and the colour.