Koji What?

This article discusses what koji is and what it contributes to sake.

The most mysterious ingredient use to make sake is koji. What is it? Why is it so important? What does it contribute to sake? Well, these are all important questions we will address here. Koji is a general term that is almost always used as a specific term by those talking about sake. In the general case koji is some kind of substrate with some kind of mold growing on it. How’s that for a technical description. Koji used for making sake is, in most cases, yellow koji consisting of  Aspergillus Oryzae growing on milled rice.

Yellow koji is also used for making Shochu, a distilled beverage, but has been mostly replaced by other forms of mold and substrate. The two most common molds now used for koji in the production of shochu are Aspergillus Kawachi (white) and Aspergillus Awamori (black). In the general case the substrates also very quite a bit. Substrates of buckwheat, sweet potato, barley and rice are common. Rice is always the substrate used for sake.

One other koji, Beni Koji, is worth mentioning. Beni Koji is better known as red yeast rice, which is Monascus purpureus mold growing on rice as the substrate.

The general term for the mold spores is koji-kin or tane-koji. Spores from any of the above molds can go by either of these names.

I mention that general case so that your thinking about koji will remain somewhat open. However, from this point on, I will limit my discussion to the more specific case of koji used in the brewing of sake.

To begin a batch of sake one of the first things that needs to be done is to make koji. Making koji is called seigiku and consists of preparing rice, inoculating the rice and cultivating the mold. The most common procedure is to steam the rice to produce an even consistency for the mold mycelium to grow into the center and around the rice. After steaming the rice is cooled to a good temperature for inoculation with koji-kin. Koji-kin sprinkled over and thoroughly mix with the prepared rice. After inoculation the rice is wrapped up to keep it warm and moist for close to 24 hours. Once this first half of the growth is passed the mycelia start to become visible on the rice. During the second half of the time the rice koji is mixed every couple of hours to keep it from overheating. The following picture illustrates what the koji looks like when it is done and ready for making sake:

Koji for brewing sake
Koji for brewing sake

OK, this is all well and good but why is koji important for brewing sake and what does it contribute? Well, koji is important because it contributes enzymes that are needed to break down rice starch into sugar and sugar is just what yeast need to produce alcohol and carbon dioxide. In a sense the koji and yeast are setting up a production line for the manufacturer of alcohol from starch. To be sure much more is created in this process and it all combines to make up sake. For example some of the enzymes produced by koji act on proteins rather than starch and transform them into the amino acid constituents.

Before we go too far, what is an enzyme? An enzyme is a protein that acts as a catalyst. That is a protein that performs some type of function that increases the rates of reactions. What enzymes do koji produce? Well, they produce many enzymes. A few of the known enzymes koji produce are:

–          Alpha-amylase – breaks polysaccharides (starches) at random location

–          Beta-amylase – snips off two glucose units (maltose) at a time

–          Protease’s – breaks proteins into smaller parts (amino acids, peptides)

–          Peptidases – breaks up polypeptides (chains of amino acids)

–          Sulfatases – breaks down sulphate esters to remove sulphates from substrates

In terms of the conversion of rice starch into sugar the alpha and beta-amylase are the enzymes to watch. They both work on the 1-4 link between glucose or maltose molecules. What? What is the 1-4 link between glucose molecules? Hmm. A little more background is needed.

Glucose is the simplest (tied with fructose) sugar molecule. Both glucose and fructose have the same formula C6H12O6 but have different ring structures. In the glucose diagram below the C at the right most edge is the first connection point (1). These points are counted moving in a clock wise manor through the Cs in the ring until the fifth C (5) with the sixth C hanging off the top (6). The ring is formed by the first (1) and fifth (5) connecting through an oxygen atom. Fructose is similar but has its second and fifth points connected through an oxygen atom to form the ring. This leaves both the first and sixth Cs hanging free. When two glucose molecules are joined to make a maltose molecule the first (1) position of one of the glucose is connected to the forth (4) position of the other glucose as shown in the diagram. It is this bond that these two amylase enzymes pull apart.

Glucose molecule
Glucose molecule
Maltose molecule
Maltose molecule

Now for the most part starches are long chains of sugars. Actually the shortest chains are sugars, those a little longer are dextrins and those longer still are starches. It is a little more complicated in that not all these structures a strain chains without branching but I will ignore this to keep things simple, Hmm, less complex. Dextrins make up much of the mouth feel of a drink.

OK, now we have a better understanding of the makeup of starch we can look at the difference in the way that alpha amylase works as compared to beta amylase. Beta amylase always works from one end of the starch, dextrin or sugar and pulls off one maltose molecule at a time (i.e., two glucose). Yeast has no trouble processing maltose sugar. Alpha amylase, in contrast, chooses a random 1-4 link to separate. This process results in two chains of arbitrary size; much of the time the resulting chains are not sugar and cannot be processed by yeast. Even when one, the other or both are sugars, many types of yeast have a hard time with more complex sugars.

If time were not an issue either alpha amylase or beta amylase could eventually break down all the starch (for the simple chains we are considering) to sugar. However, time is important, and these two enzymes working together can much more rapidly transform the starch to sugar.

Now what about those enzymes that work on proteins? Well, in many ways they are similar to the enzymes we have just been considering. The all work by breaking proteins apart at their peptide bond. Hmm, here again a little background will be helpful.

Proteins are very complex but for the most part, in this discussion, we will be able to ignore this fact because of a common feature of all amino acids. The formula for an amino acid is shown in the following figure. This figure shows two separate peptides in the first row. By combining the OH from the left peptide and the H from the right peptide we get a molecule of water and can then connect the remaining components to form the peptide bond at the C-N nodes; shown in the second row.

Peptide bond
Peptide bond

The R in the figure represents the remainder of the protein string. These strings, Rs, can be very complex in their own right. Combining these with several or even many other proteins, all connected through peptide bonds, can create extremely large and complex proteins.

As the enzymes work by pulling apart the peptide bond you might think that all of the enzymes work in the same way. They do not. As with the amylase enzymes, the enzymes that work on proteins attach to the protein or peptide in different ways. They connect like a lock and key with the enzyme taking hold of only those proteins that fit in the lock. For example the peptidases only work on smaller proteins while the protease works on large proteins.

While we try to eliminate most of the proteins from our rice by milling to higher levels we can’t get rid of them all. Also, koji is producing proteins. Yes, that’s right while we are milling the rice to remove fatty acids and proteins the koji is adding them. And, in fact, they are not all bad. However we do not want any large proteins, only those that are medium to small. Large proteins cannot be used by the yeast as they can use nothing more complex than amino acids to build up their cell walls. In addition large proteins contribute to chill haze. On the other hand medium to small protein add a fuller body and mouth feel. All this emphasizes the need for these enzymes to break down the proteins.

Woooh, I need a break. We have answered the questions:

–          What is koji?

–          Why koji is so important?

–          What does koji contribute to sake?

Is this everything we need to know? No, we still need to know how to create the koji we need for the sake we want to make. This is an area of even more mystery than the koji itself.

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6 thoughts on “Koji What?”

  1. Thanks Will. Can you tell me…is a Thai Hom Mali Jasmine rice co.sidered glutinous? Having a hortid time identifying in stores since they do not simply say glutinous. Any assistance appreciated. I have the Japanese rice component figured out I think…was told by several people near Knox to use sweet, sticky rice…sometimes called Sushi rice but do not use the white sushi rice, instead choose the cloudy sushi rice. Is that correct? Thanks again.

    1. Sara,

      Jasmine rice is a long grain rice while glutinous rice is a short grain rice, so they are quite different. Actually rice designated as sweet sticky rice is glutinous rice. This is not sushi rice, but sushi rice is close to sake rice.

      I have not heard of a difference between “white” sushi rice and “cloudy.”

      For your recipe you want the sweet sticky rice but for Japanese style sake you can use the sushi rice as being close enough.



  2. If I choose to assemble a recipe given to me by FOTF (friend of the family) and it uses steamed glutinous rice plus yeast balls and red yeast rice–both yeast finely pylverized; will koji spores form within my ferment simply because the red yeast rice is present with the rice? But I had also been informed that the process for making the yeast balls actually incorporates koji in them. So am I making sake or will I need to add an independent source of koji? Thank you, Sara. Direct email response also encouraged if possible.

    1. Sara,

      The recipe you have seems to be more for a Chinese “rice wine” rather than Japanese sake. Both have a long and honored tradition. Both use rice as the main source of starch which is converted into sugar by enzymes provided by, in part, Aspergillus Oryzae. In the case of sake, Aspergillus oryzae spores are cultivated on rice to make koji. Aspergillus oryzae is also found in the yeast balls (or most of them, there are several types).

      Beni koji or Red Yeast Rice is Monascus purpureus mold growing on rice as the substrate. This is rarely used in sake but is a key part of a recipe for a popular rice wine.

      The glutinous rice is also a difference between the two styles. The Chinese style uses glutinous rice where the Japanese style uses a short grain but non-glutinous rice.

      Hope this helps,


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