By Hannah Turner, Montana State University Barley, Malt & Brewing Quality Lab

An old tradition that is starting to be adopted in a new way by forward-thinking American brewers is koji processing, a practice widely found in Asian cultures. Utilized for centuries (originally discovered in Japan’s Muromachi Period of 1136-1573), koji is commonly used to modify grain for use in sake, soy sauce, miso, and shoyu (Japanese soy sauce). The process utilizes fungi (commonly Aspergillus oryzae) to modify grain prior to fermentation and the product is a loosely comparable alternative to traditional Western malt. Koji “malts,” having a lower enzymatic profile, would be used similarly to other adjunct grains in a brewers grist bill. 

Like malting, koji processing promotes the breakdown of grain structures and the development of the enzymes required for starch to sugar breakdown, a key aspect of fermentation. Koji can be applied to many different types of grain and other starch sources, but commonly barley, rice, and soybeans are used. Many tend to associate koji with sake, and akin to sake, Koji malts can evoke desirable flavors well outside the realm of malts, such as apple, tropical fruit, and citrus! These flavors come about in part from the fungi’s production of citric, succinic, and kojic acids.

Koji processing starts by washing the grain, then allowing it to soak and pick up moisture. In the case of barley, either hull-less or pearled options would be used rather than the hulled malt barley commonly known to brewers. Working without a hull allows the fungi better access to the grain’s central starch reserve. Post washing the grain is steam cooked, gelatinizing the hydrated starch and achieving sterilization prior to adding the fungi. This ensures only the targeted koji fungus is growing in the coming fermentation step. 

Next, the grains are sprinkled with koji starter and kept in warm, high humidity conditions for roughly a day, allowing the fungi to begin the transformation. During incubation, the grains may be agitated to break up the koji a bit. Once completed the grain can be used directly or dried to preserve the final product.

Examples of how you might find a brewer has taken advantage of koji include sour beers where adding koji to the mash, say for example in a Berliner Weisse, will dramatically cut the time needed to properly kettle sour a beer. An inclusion rate of 25 percent koji can cut down the souring time from a weekend-long stew to souring within a typical 1 hour mash time. In addition to bespoke flavor, koji has other potential benefits, for example, Kojic acid is noted for its inhibition of oxidation, which might suggest some utility in counteracting astringency that arises from oxidation. Additionally, koji fermented grains are touted for having health benefits such as digestion aid via improvement of intestinal flora, acting positively for skin conditions, and maintaining body immunity. Who could complain about a cold beer that may have some health benefits?!

Through the first half of 2022, we have been hosting visiting Ph.D. candidate Aline Brito, who is investigating several koji strains. Aline is working to develop the ideal processing conditions, and assess what impact this has on quality, and next summer we will brew some beverages for flavor assessment. Brito joins us from Brazil, where she began her Ph.D. research at the Universidade Estadual de Campinas. Although she was able to generate initial protocols for the processing in South America, an initial period has been devoted to translating those techniques to our local equipment. She is also working to determine the best parameters for fermenting koji in combination with Buzz barley, the most recently released malt variety from our Montana Breeding Program. Read on to learn more about the specifics of her project.

The Grain

Buzz barley is a spring two-row type that was released from the MSU in 2019. Buzz incorporates characteristics that allow it to maintain stable plumps and low protein under dryland growing conditions. These traits particularly shined in last year’s extreme drought conditions, allowing this variety to maintain malting quality while much of the US malt barley crop ended up as feed due to poor conditions. Although Buzz is a traditional hulled type, Brito’s work is making use of grain that was pearled by Montana Milling in Great Falls, removing ~10-15 percent of the outer grain surface. Also known as “dehulling,” this process sands or polishes off the outer seed layers, allowing the koji fungi better access to the inner starchy endosperm of the seed. We are excited to see what comes of koji processing in combination with Buzz as, in blind sensory trials, we have found that brewers tend to prefer Buzz over other varieties! More info can be found on Buzz in our Barley Variety Dictionary.

The Equipment

Koji processing starts with washing and steeping to allow the grain to soak up moisture. Brito is testing three lengths of steep time: 6, 7, and 8 hours. This is followed by steam treating, allowing the grain starches to cook (makes them more accessible to enzymes), and also sterilizing the grain to ensure only the desired/introduced fungi strains will be present. We are able to achieve these steps at a research scale in a pressure cooker. 

Following this, the grain is moved to glass Ball canning jars, inoculated with fungi, and allowed to ferment for varying treatments of 40, 44, or 48 hours in a commercial bread proofer. The temperature is maintained at roughly 86-90ᵒF for the first day and rises to approximately 93-97ᵒF on the second day. The last stage is kilning, for which we make use of our micro malting machines, similar to what a craft maltster might use for the production of traditional malt. 

Brito is experimenting with both base style (low/slow) drying profiles, as well as more flavor/color forward methods such as the production of caramel malts. In the caramel malt processing, we make use of a sous vide water bath at 149ᵒF (same as a common brewer’s mash temperature), which allows some starch to sugar conversion within the grain and prior to kilning.

The Fungi

Brito is working with two specific strains of fungi: the traditional koji form Aspergillus oryzae, and Aspergillus luchiensis, which is part of the black Aspergilli and is an important industrial workhorse used in Japan to produce a distilled alcoholic beverage, awamori. Brito’s initial trials in Brazil yielded two widely varying specialty products. The A. oryzae had comparable extract and enzymatic content to that of traditional malts but with higher levels of viscous cell wall remaining and evoking floral flavors of jasmine tea, while A. luchiensis had low extract and was highly acidic with high protein modification and provided more of a sourdough flavor profile.

The Experiment

Brito’s project has two objectives: 

1) Evaluate how changes in koji-barley fermentation, steeping, and fermentation time influence koji metabolites and malt quality 

2) Determine the end quality and sensory profiles of beers brewed with the products of her work 

She is currently finalizing equipment parameters and will soon be generating koji malts to be evaluated via hot steep sensory (think of this as making koji tea) as well as simplified analytical evaluations such as extract and enzyme measures. From these results, she will select the best conditions and conduct metabolomic analysis to determine how chemical profiles are affected by the treatments. The chemical profiles will be paired with sensory tastings and later this summer we will brew with the two most successful products.  

This project is a fun and exciting way for us to explore the art of developing culinary products for the brewing industry and we are thrilled to learn and collaborate with Brito. 

Get in touch with the MSU Barley, Malt & Brewing Quality Lab if you have any questions or are interested in learning more about the project.

A similar version of this article was previously published in the Montana Craft Beer Connection, Spring 2022 – Issue #12 (pg. 21) and on the MSU Barley, Malt & Brewing Quality Lab website.