Koji: what it is and how to use it

Rice covered in a tamed mold that turns starch into sugar and protein into flavor. What it is, how to buy or grow it and what to make with it, with the figures from the guides.

8 min read Getting good Reviewed October 26, 2026 49 facts, each with its source

What koji is

Koji is cooked rice or barley on which a tamed mold, Aspergillus oryzae, has grown. Grown on rice it's called rice koji; on barley, barley koji¹. Japan has used it for centuries: koji mold was developed for sake brewing, back in the 8th century².

The word koji is used for three things, and it helps to keep them apart³:

The mold, called koji-kin in Japanese.
The spores for sowing, tane-koji. That's what you buy to make koji at home.
The grain with the mold, rice or barley koji. It's the ingredient that starts miso, soy sauce or amazake⁴.

What it does: enzymes

Koji isn't used for the mold itself but for what the mold leaves in the grain. As it grows, it releases enzymes. Amylases turn starch into sugars, and proteases turn proteins into amino acids and peptides, which give the savory umami taste⁵. Those enzymes are what give flavor to everything made with koji⁶.

That's why amazake tastes sweet with no added sugar: it's the rice starch turned into sugar. And that's why a miso with more koji comes out sweeter, and with less koji, saltier⁷.

It isn't a sour ferment

Sauerkraut and yogurt keep because they turn sour. Koji doesn't: its pH, and that of what's made with it, stays above 4.6⁸. pH won't tell you whether it's going well. Each koji ferment is protected by something else:

Ferment	What protects it
Koji	Bought spores, temperature and time, then the fridge
Miso and shio koji	Salt, which leaves very little free water for microbes⁹
Amazake	Heat, always above 50 °C (122 °F) while it's made¹⁰, then the fridge

Bought spores, always

Spores are bought from a supplier, so they're pure¹¹. Commercial koji strains don't produce aflatoxin, a toxin made by other molds of the same genus¹², and with them BCCDC doesn't consider aflatoxin a concern¹³. That assurance comes with the bought spores, not with any mold that happens to grow on the grain. That's why there are three things you don't do:

Start a batch with koji from the last one. It can carry wild molds that do produce toxins¹⁴.
Make your own spores. BCCDC, British Columbia's public health agency, doesn't recommend it even to food businesses¹⁵.
Grow another mold. Fervida covers only bought Aspergillus oryzae, and that's a house rule. The case of beni-koji, a rice fermented with a different mold, shows why. In Japan in 2024, a blue mold turned up at the factories making a beni-koji supplement¹⁶. Growing on the rice, it produced a substance¹⁷ that damages the kidneys¹⁸.

Keep spores dry and tightly closed. At 5–15 °C (41–59 °F) and low humidity they last a year¹⁹, or frozen at −20 °C (−4 °F) in a sealed container²⁰. In humidity they die fast²¹.

Bought or grown at home

To start, buy ready-made rice koji, fresh or dried. With it you can make shio koji, amazake or miso from day one.

Growing it at home takes two days and a probe thermometer pushed into the grain, which is a house rule. These are the figures from the guides:

When	What happens	What you do
Before	Soaking	If it's longer than 4 hours, in the fridge²²
Before	Steaming	1 hour, above 74 °C (165 °F)²³
Before	Cooling	From 60 °C (140 °F) to sowing temperature within 2 hours²⁴
Hour 0	Sowing	Spores on rice at 35 °C (95 °F) or below²⁵, covered with a cloth, at about 30 °C (86 °F)²⁶
First hours	It needs humidity	Close to 100% at first²⁷
Hour 10	It starts giving off heat²⁸	Check the grain every few hours
Hours 16–18	Grains stick together	First turn, to release the heat²⁹
Hours 20–24	A white fuzz appears³⁰	Keep watching
Hours 42–48	A white cake	Harvest³¹

The hard part is the heat the mold gives off. The grain has to stay below 40 °C (104 °F)³², because at 50 °C (122 °F) the mold dies³³, and above 35 °C (95 °F) the bacteria that form heat-resistant spores grow fast³⁴. If it went above 45 °C (113 °F) for more than an hour or reached 50 °C, it goes: that's a house rule.

Harvest before the spores age and turn black, because they spoil the color of miso³⁵. Aspergillus oryzae spores are yellow-green to green³⁶. Fervida only accepts that pale green when it's even and from hour 40 on; any other color, or an ammonia smell, means throwing it all out (a house rule). Finished koji keeps a month in the fridge, or it can be frozen³⁷.

What to make with koji

Shio koji. Koji, salt and water, which at home mature for about a week³⁸. To keep, it needs about 12% salt³⁹, and Fervida works it out on the weight of everything in it⁴⁰. It's used instead of salt, to season and to marinate.
Shoyu koji. Koji with soy sauce instead of salt and water. It's used the same way.
Amazake. Rice, koji and water, held at 50–60 °C (122–140 °F)⁴¹. Fervida uses 55–60 °C (131–140 °F), the best temperature⁴². Never cooler, because food poisoning bacteria grow at around 40 °C (104 °F)⁴³. Strictly, it isn't a fermentation: the koji enzymes turn starch into sugar⁴⁴. It's high in glucose, so warn anyone with diabetes⁴⁵. And it goes in the fridge, because its pH can stay above 4.6⁴⁶.
Miso. Koji, cooked soybeans and salt, matured for weeks or months. It's all in "How to make miso at home."

Tempeh is a mold ferment too, but it uses a different one, Rhizopus, and no koji. It's in "How to make tempeh at home."

What you never do

Never corn or coconut, neither as the base nor mixed in with the grain. Bongkrekic acid can appear with them, and Singapore's food agency says not to ferment them at home⁴⁷. The toxin isn't removed by washing or cooking⁴⁸.
Never meat, fish or egg. Meat or egg koji is outside Fervida (a house rule).
Cooked grain with mold of another color isn't scraped: it's thrown out, because the mold reaches below the surface⁴⁹.

Where to start

Shio koji, with bought koji. It's the easiest recipe.
Shoyu koji and amazake, also with bought koji.
White miso, which matures in a few weeks.
Grow your own koji, once you know bought koji well.

Where every number comes from

Every number in the text leads to its figure: the exact sentence from the official guide it comes from, in its own language.

1

Grown on rice, it's rice koji; on barley, barley koji

“When grown on rice, barley, or wheat, starter is named rice koji or barley koji, etc.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
2

Koji mold was developed for sake brewing, in the 8th century

“Koji mould was developed in the sake brewing industry, back in the 8th century in Japan.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
3

The word koji names the mold, the spores for sowing and the grain with the mold

“The word koji is used to describe at least three different parts (ingredients) of the fermentation process: the mould, the starter culture, and the ingredient that forms the basis of miso, soy sauce, sake, amazake etc.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
4

Koji starts miso, soy sauce and amazake

“Koji is used in traditional Japanese recipes as fermentation starter for miso, soy sauce, amazake and other foods.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
5

Amylases make sugars; proteases make amino acids and umami flavor

“Amylases break down carbohydrates into sugars (glucose and oligosaccharides), and proteases break down proteins into amino acids and peptides, providing the savory umami taste”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
6

The mold's enzymes give koji foods their flavor

“It is the enzymes, secreted by the moulds, that create the flavours in koji fermented foods.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
7

More koji, sweeter; less koji, saltier

“Higher proportions of rice or barley koji create a sweeter taste, while lower proportions of koji produce saltiness.”

Journal of Fungi (MDPI), Kusumoto K.-I. et al. · Japanese Traditional Miso and Koji Making (2021) · read on October 25, 2026
8

It isn't an acid fermentation: pH stays above 4.6

“This is not an acidic fermentation. The final pH of koji and other koji products are above a pH of 4.6.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
9

Miso and shio koji leave very little free water for microbes

“Miso and salt koji (with added salt) should have aw of less than 0.85”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
10

Amazake kept above 50 °C so staphylococci don't grow

“Keep incubation temp >50oC to avoid growth of Staphylococcus.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
11

Spores always bought, so they're pure

“Starter culture must be purchased from commercial sources to ensure purity.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
12

Koji strains don't produce aflatoxin

“all brewing koji molds were confirmed to be non-productive of aflatoxin”

Journal of Fungi (MDPI), Yamashita H. · Koji Starter and Koji World in Japan (2021) · read on October 25, 2026
13

With bought spores, BCCDC doesn't consider aflatoxin a concern

“Aflatoxin is not a concern when commercially sourced Aspergillus starter culture (as described above)”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
14

Reusing koji can bring in wild molds with toxins

“Culture re-use (backslopping) is not recommended in these ferments as wild strains of Aspergillus that contain toxins may be introduced.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
15

Making your own spores isn't recommended

“We do not recommend any processor grow their own spores”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
16

A blue mold was found at the beni-koji factories

“Blue molds (Penicillium adametzioides) were collected at both Osaka and Wakayama factories.”

Ministry of Health, Labour and Welfare (MHLW), National Institute of Health Sciences · Responses to the events associated with Beni-koji containing foods produced by Kobayashi Pharmaceutical Co., Ltd. (18 September 2024) · read on October 25, 2026
17

That mold made the toxin while growing on the rice

“puberulic acid was produced by P. adametzioides grown on rice”

Ministry of Health, Labour and Welfare (MHLW), National Institute of Health Sciences · Responses to the events associated with Beni-koji containing foods produced by Kobayashi Pharmaceutical Co., Ltd. (18 September 2024) · read on October 25, 2026
18

The toxin damages the kidneys

“It was determined that puberulic acid causes renal dysfunction while Compound Y or Z does not.”

Ministry of Health, Labour and Welfare (MHLW), National Institute of Health Sciences · Responses to the events associated with Beni-koji containing foods produced by Kobayashi Pharmaceutical Co., Ltd. (18 September 2024) · read on October 25, 2026
19

Dry spores, at 5–15 °C and low humidity, last a year

“When dried koji starter is stored at a temperature of 5–15 °C and a humidity of under 50%, the germination rate is stable for about one year.”

Journal of Fungi (MDPI), Yamashita H. · Koji Starter and Koji World in Japan (2021) · read on October 25, 2026
20

Or frozen at −20 °C in a sealed container

“it is easy and effective to store at minus 20 °C in a sealed container.”

Journal of Fungi (MDPI), Yamashita H. · Koji Starter and Koji World in Japan (2021) · read on October 25, 2026
21

In humidity, spores die fast

“dried spores, like seeds of plants, easily absorb moisture and die rapidly under humid conditions.”

Journal of Fungi (MDPI), Yamashita H. · Koji Starter and Koji World in Japan (2021) · read on October 25, 2026
22

Soaking over 4 hours, in the fridge

“Soaking of rice, soy beans and other substrates in excess of four hours should occur in refrigerated conditions or in”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
23

Rice steamed 1 hour above 74 °C

“1 hr (rice) above minimum temp. 74oC.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
24

From 60 °C to sowing temperature within 2 hours

“Cooling from 60oC to target inoculation temperature should occur within two hours.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
25

Spores go on rice at 35 °C or below

“Inoculation at or below 35oC, optimal growth in range of 35 to 38oC.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
26

Covered with a cloth at about 30 °C

“covered with a food safe cloth, and incubated at approximately 30oC”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
27

Humidity near 100% at first

“Humidity of near 100% initially to allow mould growth.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
28

The grain starts giving off heat around hour 10

“During fermentation heat begins to develop approximately 10 hr into the process.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
29

Mixed by hand between hours 16 and 18

“In manual methods, koji is mixed by hand at 16 to 18 hr to release heat and to separate rice grains to prevent clumping.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
30

Around 20–24 hours, white fuzz on the surface

“At 20–24 h, white hyphae slightly cover the material surface, and koji temperature may rise to over 40 °C in strains with strong growth.”

Journal of Fungi (MDPI), Yamashita H. · Koji Starter and Koji World in Japan (2021) · read on October 25, 2026
31

42 to 48 hours in all

“It takes a total of 42 to 48 h to produce rice koji.”

Journal of Fungi (MDPI), Kusumoto K.-I. et al. · Japanese Traditional Miso and Koji Making (2021) · read on October 25, 2026
32

The grain below 40 °C

“Keep koji starter cooled to below 40oC for optimal growth”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
33

At 50 °C or more, the mold dies

“for cooling as temperatures of 50oC and higher will kill the Aspergillus mould”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
34

Above 35 °C, spore-forming bacteria grow fast

“The count of heat-resistant spores increases rapidly in the case of temperatures above 35 °C, and they remain in foods, even in the presence of high salt, so special attention is required.”

Journal of Fungi (MDPI), Yamashita H. · Koji Starter and Koji World in Japan (2021) · read on October 25, 2026
35

Spores shouldn't age and turn black

“It is not desirable for spores to age and turn black, as this will discolour miso.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
36

Aspergillus oryzae spores are yellow-green to green

“The spores of A. oryzae and A. sojae are yellow-green to green, but their color slightly changes depending on the type of medium and culture time.”

Journal of Fungi (MDPI), Yamashita H. · Koji Starter and Koji World in Japan (2021) · read on October 25, 2026
37

Koji keeps a month in the fridge, or frozen

“The starter may be kept refrigerated for approximately one month, or frozen.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
38

At home, shio koji matures for about a week

“塩麹は家庭において，乾燥麹に食塩と水を加え，約 1 週”

日本調理科学会誌 (Enguchi N., Yukawa N., Nakanishi Y.) · 自家製塩麹のカゼイン分解活性 (Caseinolytic Activity of Homemade Shio-Koji, 2016) · read on October 25, 2026
39

Shio koji keeps with about 12% salt

“塩麹は保存に耐えうる 12％前後の食塩”

日本調理科学会誌 (Enguchi N., Yukawa N., Nakanishi Y.) · 自家製塩麹のカゼイン分解活性 (Caseinolytic Activity of Homemade Shio-Koji, 2016) · read on October 25, 2026
40

The percentage is worked out on the weight of all ingredients

“Calculations of percent amounts to add are based on the weight of all ingredients.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 2: Starter cultures and fermented food standards (August 2024, updated December 2024) · read on October 25, 2026
41

Amazake is held at 50–60 °C

“the rice-koji is mixed with water and placed in a tank set at 50–60 °C”

Journal of Fungi (MDPI), Kurahashi A. · Ingredients, Functionality, and Safety of the Japanese Traditional Sweet Drink Amazake (2021) · read on October 25, 2026
42

The best temperature is 55–60 °C

“したものであるが,その際の至適温度は55〜60℃ とされ”

日本衛生学雑誌 (Japanese Journal of Hygiene), Ui S. · 甘酒を原因食とする食中毒に関する基礎的研究第III報甘酒と食中毒細菌との関連について (Fundamental Studies on the Food Poisoning by Amasake. III. Growth of Bacteria in Amasake, 1959) · read on October 25, 2026
43

Food poisoning bacteria grow around 40 °C

“as food poisoning bacteria can grow at approximately 40 °C, the saccharification temperature of koji amazake should be kept at 50–60 °C.”

Journal of Fungi (MDPI), Kurahashi A. · Ingredients, Functionality, and Safety of the Japanese Traditional Sweet Drink Amazake (2021) · read on October 25, 2026
44

Amazake isn't a fermentation: the enzymes make sugar

“The second part of this process is saccarification or creation of sugars, using the enzymes in the koji, which is not a fermentation process.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
45

Lots of glucose: a warning for people with diabetes

“High levels of glucose sugar are present. Ingredient warnings are recommended for diabetics.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
46

Its pH can be above 4.6, so it goes in the fridge

“the pH can be above 4.6, requiring refrigeration of this product.”

BC Centre for Disease Control, Environmental Health Services · Safety of Fermented Foods, Section 3.10: Koji foods: miso, soy sauce and amazake (September 2023, updated December 2024) · read on October 25, 2026
47

Don't ferment corn or coconut at home

“Avoid fermenting corn or coconut milk/pulp products at home as dedicated facilities and/or processes under strict hygiene practices are required to mitigate the risk of Bongkrekic acid contamination.”

Singapore Food Agency · Bongkrekic Acid: Safety of Fermented Corn and Coconut Products · read on October 25, 2026
48

Bongkrekic acid isn't removed by washing or cooking

“Bongkrekic acid cannot be destroyed by washing or cooking.”

Singapore Food Agency · Bongkrekic Acid: Safety of Fermented Corn and Coconut Products · read on October 25, 2026
49

Moldy cooked grain is thrown out

“Cooked grain and pasta Discard Foods with high moisture content can be contaminated below the surface.”

USDA Food Safety and Inspection Service · Molds on Food: Are They Dangerous? · read on October 20, 2026

Keep reading

How to make miso at home How to make tempeh at home Lactic acid bacteria, yeasts and molds Cultures: getting, keeping and sharing them Temperature and time Fermenting with care: botulism and how to avoid it Cooking with ferments