Uni: a field guide to Japan's sea urchins

To most people outside Japan, uni is one thing: a soft, golden, faintly briny lobe on top of rice. In Japan it’s a whole category. The word uni (雲丹) covers several species, a dozen-plus regional grades, two completely different presentations, and a quality range that runs from supermarket trays to roe that costs more by weight than wagyu. Once you can tell them apart, the counter gets a lot more interesting.

What you’re eating, in every case, is the urchin’s gonads — five lobed strips inside the shell, present in both males and females. Species, diet, and water decide almost everything about how they taste.

The two that matter most

Most uni you’ll meet belongs to one of two families.

Bafun uni (馬糞ウニ — literally “horse-dung urchin,” for its squat shape and short spines) is the rich one. The roe runs deep orange to reddish-gold, and the flavor is concentrated, sweet, and almost custard-like. The benchmark is ezo-bafun-uni from Hokkaido: smaller, scarcer, and usually the pricier of the two.

Murasaki uni (紫ウニ — “purple urchin,” for its long dark spines) is the cleaner one. The roe is pale yellow, the lobes are larger, and the taste is lighter and more delicate — still sweet, but with a fresh, oceanic edge rather than the deep richness of bafun. Kita-murasaki-uni is the major northern variety.

Side by side they barely look related — one box golden-orange, the next pale lemon. Many chefs keep both and will tell you which is better that day.

Beyond the big two

Aka uni (赤ウニ, red urchin) is the southern prize, from Kyushu — Saga, Nagasaki, Karatsu. It’s intensely flavored, peaks in autumn, and is rare enough to be a genuine luxury.

Then there’s terroir. Hokkaido dominates: the Rishiri and Rebun islands, where urchins graze on prized konbu kelp, are spoken of the way wine people talk about grand cru. Down the coast, Sanriku (Iwate) urchins are also kelp-fed and excellent, and Aomori is another major source. As a rule, what the urchin eats — especially good kelp — is what you taste.

Grade, colour, and the alum question

Two things separate great uni from ordinary uni, and neither is the species.

First, presentation and grade. The everyday form is bara uni — loose roe in a jar or tub. The premium form is ita uni: whole lobes laid out unbroken on a small wooden tray, graded by colour, firmness, and how intact each piece is. Deep, even colour and clean edges command the highest prices.

Second — and this is the one worth learning — alum. Most uni is treated with potassium alum (myoban) to firm it up and keep it from dissolving in transit. It works, but it leaves a faint bitterness and metallic edge that many people mistake for “the taste of uni.” Look instead for muban (無みょうばん, alum-free) or shiomizu uni (塩水ウニ), packed in seawater. It’s sweeter, cleaner, and tastes purely of the sea — but it doesn’t keep, so it’s seasonal and costs more. If you’ve only had alum-treated uni and found it bitter, the alum-free kind is a revelation.

What’s actually on your rice

Plenty of cheap uni in Japan — and most of it abroad — is imported, often from Chile, Russia, or North America. It can be perfectly good, but it isn’t the same as in-season Hokkaido roe, and the price usually tells you which you’re getting. At a serious counter uni is served as gunkan (wrapped in a band of nori) or as a simple hand roll, where great roe needs nothing else.

The short version: if a chef offers you a choice, ask for both. A sweet, alum-free Hokkaido bafun and a clean northern murasaki, tasted back to back, is the whole point.