Sujime (酢締め) is a traditional preparation method in which fish is first salted to draw out moisture (Shiojime) and then lightly marinated in vinegar. It is widely used as a sushi topping.
However, the techniques used for Sujime today differ significantly from those of the Edo period. Differences include the amount of time the fish is left with salt and the duration it is soaked in vinegar. Until around the 1980s, it was common practice to soak fish such as mackerel and horse mackerel in vinegar for an extended period. In modern times, thanks to advanced refrigeration technology, fish such as horse mackerel (aji) can now be eaten raw without concern. As a result, the time the fish is salted has been shortened to just a few minutes. Once moisture begins to appear on the surface, it is carefully rinsed off with water. Rather than soaking in vinegar, the fish is quickly dipped, more like a gentle rinse. The goal is to retain the natural flavor of the fish, requiring a delicate and refined touch.
Fish suitable for Sujime are those with a robust umami flavor and a distinctive aroma that can stand up to the acidity of vinegar. Typical examples include blue-backed fish like mackerel, Spanish mackerel, sardines, and gizzard shad. Depending on the case, white-fleshed fish such as barracuda, red seabream, and filefish may also be prepared as Sujime.
There are three main reasons why Sujime is used in sushi restaurants.
The first is preservation. Vinegar has strong antibacterial properties, and before the advent of refrigeration, Sujime was a way to prevent spoilage and extend the shelf life of fish. Vinegar kills bacteria on the surface of the fish, helping to keep it hygienic. Even today, in sushi where raw consumption is the norm, Sujime remains valuable for enhancing food safety.
Second, Sujime plays a key role in reducing unpleasant odors and refining flavor. Blue-backed fish are rich in fat, particularly unsaturated fatty acids found between the skin and flesh. These fats are prone to oxidation, which can cause a strong fishy smell. Additionally, the flesh of fish contains trimethylamine oxide, a naturally odorless compound that, over time and through bacterial action, turns into trimethylamine, which has a strong odor. Eventually, ammonia is also produced, intensifying the smell. Since Trimethylamine is alkaline, treating the fish with acidic vinegar neutralizes it, effectively reducing odor. A quick dip in vinegar removes unwanted smells and balances the umami and acidity, resulting in a more refined taste.
Finally, Sujime enhances both the flavor and texture of the fish.
When vinegar is added to fish, the proteins are denatured. The pH of fresh raw fish is around 6, but when it is into vinegar and the pH becomes more acidic. The gaps between the myofibrils decrease because of vinegar, tightening the flesh. This creates a texture and mouthfeel distinct from raw fish.
As the pH continues to drop below 4, the myofibril protein starts to dissolve in the acid, so the flesh becomes soft and mushy. But if the fish is salted and tightened before putting it in vinegar, the myofibrils do not dissolve and remain intact, so the flesh continues to tighten. This is why vinegared- marinated fish looks white.
Furthermore, the acidity of vinegar stimulates the taste buds on the tongue, creating a synergistic effect that brings out sweetness and umami. Even with a short vinegar marinade, a mature aroma and depth are added, giving blue-backed fish such as kohada and mackerel a deep, “mature flavor.”
Sujime is not merely a preservation technique; it is a skilled process that brings out the best qualities of the fish. It is one of the key steps in refining the sushi experience to perfection.