In academic terms, “fermentation” is a reaction that uses no oxygen to break down carbohydrates to obtain energy. On the other hand, the reaction that uses oxygen to break down organic matter to obtain energy is called “respiration”.
In more familiar terms, microorganisms such as lactic acid bacteria, koji-mold, and yeast break down organic compounds such as carbohydrates and proteins to produce a variety of by-products to obtain energy in their own life activities. Among these, “fermentation” is a phenomenon in which beneficial substances are produced for humans, while “putrefaction” is a phenomenon in which harmful substances are produced for humans. Food spoilage can be detected by the five senses, such as smell, appearance, and taste.
To better understand the differences among fermentation, putrefaction, and maturing, let’s look at some examples.
Fermentation can be seen in the production of foods like yogurt, miso, soy sauce, and kimchi. In these cases, microorganisms such as lactic acid bacteria, koji mold, or yeast break down sugars or proteins and produce substances like organic acids, alcohol, and amino acids, which contribute to flavor and preservation. These changes are considered beneficial and intentional.
Putrefaction, on the other hand, occurs when undesirable bacteria break down proteins and other compounds, usually in improperly stored food. For example, spoiled meat or fish that emits a rotten odor is a typical case of putrefaction. It produces substances like ammonia, hydrogen sulfide, and other toxins that can be harmful to humans.
Maturing refers to a slow transformation process that occurs over time, usually after fermentation or with the help of enzymes present in the food itself. For instance, dry-aged beef becomes more tender and flavorful as natural enzymes break down muscle fibers. Similarly, aged cheese like cheddar or parmesan develops deeper taste and aroma over months or even years. In the world of sushi, fish such as hirame (Bastard halibut) or kuro maguro (Bluefin tuna) are sometimes aged for several days to enhance umami and texture.
What is the difference between fermentation and maturing?
While “fermentation” and “putrefaction” are caused by microorganisms, “maturing” is a process in which the food itself is transformed by enzymes and other substances to produce something beneficial to humans. Or, “maturing” is the process of improving the flavor and quality of food by allowing it to rest under controlled temperature and humidity after fermentation is complete. Maturing is said to be beneficial to humans because it changes the texture and taste of the fish, making it tastier.
In case you are wondering, “enzyme” is mainly composed of protein, which promotes chemical reactions such as digestion, absorption, and metabolism that are necessary for all living things, including humans, animals, and plants, to survive. It is said that there are approximately 5,000 enzymes in our body, but each enzyme is a specialist that performs only one function and is largely divided into “Digestive enzymes” and “Metabolic enzymes.
How these processes affect Nigiri sushi
These processes—fermentation, putrefaction, and maturing—each have a distinct role in the world of nigiri sushi. Fermentation is used in components such as vinegared rice (sushi-meshi), where rice is seasoned with vinegar, salt, and sugar—often derived from fermented ingredients like sake lees or rice vinegar—to enhance preservation and flavor balance. Some traditional sushi also uses fermented fish, such as narezushi, which is a historical form of sushi predating modern nigiri.
Putrefaction, of course, is carefully avoided in sushi preparation, as freshness is critical. Skilled sushi chefs are trained to detect the earliest signs of spoilage by sight, smell, and texture to ensure only safe and high-quality ingredients are served.
Maturing, on the other hand, is often deliberately applied to improve the taste and mouthfeel of certain types of fish. For example, makogarei (Marbled flounder) or kohada (gizzard shad) may be aged for several days under precise temperature and humidity control. This process allows enzymes in the fish to break down proteins into amino acids such as glutamic acid, enhancing the umami flavor—a concept deeply valued in Japanese cuisine. In essence, controlled maturing in sushi brings out the hidden depth of flavor while maintaining delicate balance.
We hope this information will be helpful.
Revision date: July 10, 2025
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Ikezukuri means to take a fish that’s swimming in a tank at the restaurant, and immediately making sashimi after killing it, while the meat is still super fresh. In this state, there is no Inosinic acid, but there is quite a show with movement still in the fish and this preparation method gives the best-tasting texture.
Nojime* means using a massive amount of ice to suddenly reduce the temperature, resulting in the death of the fish, either at the fishing site or at the local market. This method is used for small, cheaper mass-market fish that are caught in large amounts. Nojime starts with a reverse calculation for flavor from the day after fishing, but if the process isn’t thorough or there are any deficiencies, the fish won’t stay as fresh, so detailed care must be taken in temperature management.
Ikejime is used so that the peak flavor will be reached during afternoon and evening business hours. The spinal cord of the live fish is cut and spinal fluid drained at the early morning market. This result is a firmness from the remaining ATP, and delicious flavor from the Inosinic acid that is generated as time passes. After some time has passed, even fish for which Ikejime is applied, can reach the same state as Hamajime if used after being refrigerated for one to two days, in order to maximize the Inosinic acid generated. Of course, sushi chefs find their own balance of firmness in meat or added flavor, and incorporate this balance for the optimum combination with their shari in each piece of sushi.
The fish used in sushi is generally salted or soaked in vinegar then matured for several days while the umami Inosinic acid component increases. This is called “Jukusei” (aging). Sushi made with toppings that have been aged in this way is called “Jukusei sushi”. The aging period depends on the type, individual size and origin of each fish, and some are even aged for over four weeks. However, the preparations are not only difficult and time-consuming, but the discolored parts and inedible parts must also be trimmed, so these toppings tend to be expensive. If gone too far, the Inosinic acid converts to hypoxanthine and rots. The ability to make this judgment is important. In the end, Jukusei is an evolved version of the culture of “maturing toppings” which existed in Edo-style Sushi.
In order to mature seafood, after completing advance preparations (removing the head and internal organs then washing thoroughly; all blood must be removed), more than adequate considerations must be made for the fat content of the fish and management of the bodily fluids. Specifically, this includes processes like dry-aging at a low temperature, removing moisture using salt, utilizing enzymes and fermentation, wet aging by putting the item in a vacuum pack, and wrapping in aging sheets, which were developed thanks to 
There is a part on the tuna belly called “Sunazuri (gizzards)” or “Zuri” . Normally “Jabara,” with the diagonal white lines is the king of tuna, but the fatty tuna is spoiled if the white lines are left in your mouth. Also, on the dorsal side there is a part that produces chutoro called wakaremi.
Zuke is one of the traditional Edo-style sushi methods. It is said that it was started in the Edo period to stop tuna from rotting when there were large amounts of the fish in the market. Now that there has been advances in refrigeration technology, it’s no longer necessary, but maturing the fish gives it a completely different taste and brings out its umami. Zuke is divided into two broad methods. Here we describe the characteristics of each.