Uni (うに), or sea urchin, is a staple ingredient served at high end sushi restaurants around the world. The bright yellowish-orange part of the sea urchin that is actually consumed are the gonads (reproductive organs) of the sea urchin. The taste of uni is actually highly variable depending on the diet, season and location from which the uni is sourced. This article will be covering the various differences between different sea urchins commonly sourced for sushi.
The term in season here would mean when the gonads of the sea urchin are at their best to be ready. This is quantified using three main criteria:
- The grain density of the gonads
- How plump the bright the veins are (and indication of freshness)
- And the richness and vibrancy of their colour
Sea Urchin is typically in season at the height of their feeding season when most of the food they collect is converted to energy stores in their gonads in preparation for spawning. Immediately after spawning, most of these valuable nutrients would have been passed down onto their offspring and thus these sea urchins’ gonads would be small and taste extremely bitter. Some sea urchins might not even have any visible gonads.
Bafun Uni (バフに) Hemicentrotus pulcherrimus;
Murasaki Uni (ムラサキウニ) Heliocidaris crassispina
Honshu, being Japan’s largest island, produces the most sea urchin. The 2 main types of sea urchin found here are Bafun Uni and Murasaki Uni. These sea urchins are typically considered middle of the range in terms of quality and price.
Murasaki Uni is the most common sea urchin found throughout Japan with its main season being in August. In a way you could say that Murasaki Uni sets the baseline of what you would expect sea urchin to taste like. A box of good quality Murasaki Uni typically retails for around 2500 yen a box and are a bright yellow color, plump with a nice light , mildly sweet and salty flavour with very little fishiness in the after notes. These sea urchins feed on Wakame (ワカメ, Undaria pinnatifida), all along the coast of Japan, which contributes to their characteristic taste. Wakame is a kind of seaweed typically served in miso soup in Japan.
After Murasaki Uni, Bafun Uni is Japan’s second most common sea urchin. It is in season in October and is typically harvest from deeper waters compared to that of Murarsaki Uni. This gives the sea urchin a more intense and deep flavour but also stronger bitter notes if low quality. It is also darker orange in colour compared to the bright yellow of Murasaki Uni.
Ezo Bafun Uni (エゾバフに) Strongylocentrotus intermedius
Kita Murasaki Uni (キタムラサキウニ) Heliocidaris crassispina
The island of Hokkaido of course, is where the very best sea urchin from Japan is harvested. This is because instead of Wakame, sea urchin here feed on the Kombu (kelp) that grows plentifully in the cold and nutrient rich waters around the island. The two main sea urchins found here are the Ezo Bafun Uni and Kita Murasaki Uni.
Kita Murasaki Uni is known as Japan’s top quality sea urchin, with it being extremely plump and large in size (around the size of your thumb), whilst being extremely creamy and having a complex depth of flavour. It is famous for having the sweetest taste of all sea urchin. These sea urchins can fetch up to 10000¥ per box at the market.
Comparatively, Ezo Bafun Uni (Sea Chestnut) is famous for being the creamiest of all uni even if it is less sweet. It also better retains it shape during transportation, which makes it a more popular export to other parts of Japan.
In term of seasonality, both these sea urchins at actually in season at the same time twice a year around the Island. Sea urchins from the east side of Hokkaido facing the Pacific Ocean are in season during the winter while sea urchin on the west side facing the Sea of Japan is in season during the summer.
Interestingly, before the days of plane transport of uni from Hokkaido to the rest of Japan, high quality uni was hard to obtain from Hokkaido as they would typically start to melt before they could arrive. In order to bring down the cost of transportation, companies would hire retired National Rail workers to carry the uni to Tokyo on trains as they were able to travel anywhere in Japan for free with their rail pass.
Aka Uni (赤うに) Pseudocentrotus depressus
Japan’s most southern island of Kyushu also harvests a large amount of sea urchin thought it is rarely exported out of Kyushu island itself. The main prefectures from which sea urchin is sourced are Saga (佐賀県), Kagoshima (鹿児島県) and Kumamoto (熊本県). Whilst Murasaki Uni found throughout Japan is also found here, Kyushu is famous for Aka Uni, or Red Sea Urchin. As the water here in not as cold a northern Japan, the sea urchins grow to much smaller sizes. They also are unable to feed on Kombu (kelp) and thus mainly feed on Wakame, baby shrimp and small fishes. From personal experience, sea urchin here tends to taste much stronger and bad quality sea urchin can have an extremely bitter and fishy taste. However, it is not impossible to obtain good quality sea urchin from here, just that quality in this case would be defined by strong and bold savoury notes instead of creamy and sweet notes.
Purple Sea Urchin, Strongylocentrotus purpuratus
Long Spine Red Sea Urchin, Strongylocentrotus franciscanus
Spiny Sea Urchin, Strongylocentrus droebachienisis
American sea urchin is probably the main competitor of that of Japanese sea urchin and in my opinion is actually pretty much on par in terms of quality. This therefore means that they provide much greater value for what you are paying for if you buy American sea urchins outside of Japan.
The size of sea urchin in American is much larger of that compared to other countries (some people say it’s because of the higher sulphur content). Interestingly enough, the sea urchin from America also feed on seaweed but not on the same kind of that in Japan.
South America, Chile/Mexico:
Another large scale exporter of Uni is South America, especially from Chile in particular. This being said, sea urchin that originates from Chile (erizo rojo) tends to have a very strong shellfish taste and regarded by many as inferior in quality. In terms of being used in the local cuisine, it is normally added to cooked dishes instead of served raw.
Europe (Including Russia):
Sea urchin from Russia is actually very often important into Japan and can be easily found in markets sold at a cheaper price. This was especially true at the Sagana Machi at Tsuruga city (the biggest fish market close to the Sea of Japan). Although normally thought of as inferior in quality, Russia actually does harvest the same species of sea urchin as those found in Hokkaido such as Ezo Bafun Uni. These sea urchins are mainly sold straight away at markets to high end restaurants which makes only the lower quality sea urchins able to be found at local markets.
Denmark, Faroe Islands:
On a little side note, whilst working at Noma in Copenhagen, I was lucky enough to try sea urchins harvested from the Faroe Islands up north in Denmark. These sea urchins were unlike any that I had tasted before. Whilst not being the creamiest, they definitely had by far the best taste in terms of complexity and after taste. This of course wasn’t a fair comparison and my perception of their taste might have been influenced by other environmental factors but if given the chance you should try it!
In terms of processing, most uni sold in wooden boxes in Japan are treated using alum, a chemical that maintains the firmness in texture of the uni. However, as it losses it’s freshness it tends to leave an extremely undesirably metallic taste which accentuate the fishy taste of uni and can be quite horrible. This is actually noted on the box in Japanese as myoban (ミョウバン). A more advantage technique that is employed now is storing the Uni in a brine that has been deoxygenated and enriched with nitrogen, which slows the degradation of quality considerably without introducing any off flavours.