OK, so what exactly are we trying to do when making koji? Well, to examine this we need to consider the role koji plays in Sake Brewing. In sake brewing we use koji to provide a wide variety of products. These include products that provide flavor and aroma elements as well as enzymes which degrade proteins and starches into smaller component parts. For example proteins are disassembled into peptides and amino acids while starches are converted into smaller starches, dextrins and sugars.
Rice starts out with 7% to 8% protein, but much of this is milled away, so we do not focus on koji’s production of enzymes to break down this protein. Rice starch is our main focus and needs to be broken down as effectively as possible into sugars. Koji produces alpha, beta and gama amylase. Depending on how we culture the koji, we can emphasize protein or starch degrading enzymes. High temperature cultivation, 98°F to 110°F, lead to the production of saccharification enzymes whereas lower temperature cultivation, from 98°F down to 68°F, emphasizes protein degrading enzymes. So to make good koji for sake brewing we must culture the koji at high temperatures.
Another important aspect of cultivation is to ensure that koji mycelia reach deep into the grain. When the mycelia work hard to bore into the grain more saccharification enzymes are produced. The primary factor that can prevent the mycelia from boring into the grain is when the grain is too moist and the fungi can get all the moisture it needs from the surface. Under these conditions the fungi will produce a small amount of enzymes which will go to work right away in the moist environment on the surface producing sugar. With the readily available sugar for the fungus there is no need for it to push into the grain or produce more enzymes. Koji produced under these conditions and with these results is called Nurihaze koji.
Tsukihaze koji or koji with the mycelia reaching deep into the grain is produced with rice that has about 38% moisture after steaming and has been cooled such that the surface of each grain is relatively dry. Too dry and the koji-kin (spores) will not stick properly but beyond that it should be dry. Then, cultivating with moist air for the first day enables the koji to get a good start. The lower the seimai-buai, milling rate, of the rice the less humid the air should be. For example, a brown rice would be safer in a more humid environment. On the second day, moving to dryer conditions helps the koji mycelia to move into the grain.
A koji that is between Nurihaze and Tsukihaze koji is Sohaze koji which is more like Tsukihaze in that the entire surface of the grain is fully covered but unlike Tsukihaze the mycelia do not penetrate very far into the grain. For this reason, Sohaze koji has less saccharification power and is used in circumstances where there is less need to produce sugars; for example, the moto.
Of the three amylase enzymes produced alpha and gama are the most prevalent but because of the low pH of the mash alpha amylase has a very low activity level. Alpha amylase is well suited for a pH of 5.5 but a sake mash will be closer to a pH of 3. Gama amylase on the other hand thrives at a pH of 3 and produces glucose. The very low temperature of the mash inhibits all of the enzymes which slows the rate sugar production to a trickle. The slow rate of sugar being added to the mash helps the yeast stay healthy longer than in most fermentation where there is a high concentration of sugar at the start. A high concentration of sugar increases the osmotic stress on the yeast cell walls. This increased osmotic pressure causes the yeast to shut down sooner than they might with lower levels. It is, this characteristic as the sake brewing process that allows sake to reach the high alcohol levels it does.
So producing a koji with lots of saccharification enzymes which will provide long slow conversions of starches to sugars so that the yeast can convert the sugars to alcohol without becoming overly stressed by the sugar level and allowing the yeast to produce the highest level of alcohol they can is what we are after when we make koji. (breath….. (:-) )