One of my favorite winter pastimes growing up was to snuggle under the futon covering a kotatsu (heated table), doing my homework or watching TV, as I methodically worked my way through a big bowl of Satsuma mikan, the little oval-shaped oranges that are known as clementines or tangerines in the West. Few things are as cheering as the bright colors, refreshing fragrance and sweet-sour flavors of citrus fruit on a dull winter’s day, and there are many Japanese varieties that may not be that familiar to you.

It’s not certain when citrus farming started in Japan, but they are almost certainly one of the oldest types of fruit to be cultivated domestically. They are grown almost all over the country, but they especially thrive in the warm and humid coastal regions. Today the biggest citrus-growing areas include Wakayama, Shizuoka, Tokushima, Kochi, Oita, Miyazaki and Ehime prefectures, depending on the variety. The name Satsuma mikan refers to the old Satsuma region, the western part of present-day Kagoshima Prefecture, which is also still a citrus-growing region.

Domestically grown citrus can be divided into two types: those that are sweet, juicy and easy to peel; and tart and bitter varieties with thick, robust and flavorful peels.

The former kind include the tangerine or clementine types mentioned above (citrus unshiu), as well as recent developments such as the bumpy, sweet, juicy and very popular dekopon (generically known as shiranuhi), which were first hybridized in Japan in 1972. These are best indulged in as is, and are an invaluable source of vitamin C in the winter months.

The latter kind are not really edible as is, but are indispensable in washoku (traditional Japanese cuisine). The juice from any of these fruits is used in ponzu, a sour condiment that’s indispensable for warming nabe (hot pots) in the wintertime. The main types of cooking citrus are:

Sudachi: small, green and extremely sour. Its fragrance is considered an ideal foil for the rich pungency of matsutake mushrooms, so you will usually see one included in a packet of these wildly expensive mushrooms.

Kabosu: another extremely sour citrus fruit that is usually used while still green, although it turns a pale yellow when ripe.

Daidai: has a unique characteristic in that if the fruit are left on the tree they turn green again, and can stay alive for years, or generations — hence the name, which means “many generations.” Daidai are prized as highly as decorative trees as they are for their edible qualities.

But the queen of sour, bitter Japanese citrus is yuzu — a very trendy ingredient these days in high-end cuisine worldwide. What makes yuzu so distinctive above all the sour-bitter types of citrus is the highly fragrant peel of the ripe, yellow-orange fruit, which is rather like a cross between grapefruit, lemon and lime. The bitter-citrus flavor of the peel is indescribably addictive.

Yuzu peel is used as a garnish on soups and simmered dishes, and is the critical part of yuzu koshō, a heady condiment made with yuzu peel, juice and sanshō peppers. Yuzu-yu, a hot fragrant bath with yuzu fruit floating in it, has been enjoyed since the 18th century, and the seeds used to be soaked in shōchū to create a natural skin-softening conditioner.

The recipe accompanying this article is for marmalade made with whole ripe yellow yuzu, which are in season now. This fragrant marmalade can be used as a spread on bread or in tarts, as well as a condiment or hidden flavoring in all kinds of dishes. Jars of this marmalade make great gifts.

Makiko Itoh is the author of “The Just Bento Cookbook” (Kodansha USA). She writes about bentō lunches at www.justbento.com and about Japanese cooking and more at www.justhungry.com.

Recipe: yuzu marmalade

Ripe yellow yuzu — 1 kg (8-9 fruits)

Granulated sugar — 500 to 600 g, about half the weight of the fruit

1. Wash and halve the fruit. Scoop out the pulp and set aside. Scoop out the rest of the insides, wrap in a clean kitchen towel and squeeze tightly to extract as much juice as possible. Reserve the seeds too.

2. Put the juice in a non-reactive (stainless steel or enameled) pan with 150 g of sugar. Heat while stirring to dissolve the sugar, then simmer for 20 minutes over low heat. Reserve.

3. Finely shred the peel, and simmer in water for 40 minutes. Drain, and soak in plenty of cold water overnight. Change the water four or five times if you can.

4. Put the drained peel and reserved pulp in a pan with water to cover. Wrap the seeds in a piece of gauze or cheesecloth (or a fillable tea bag) and add to the pan. Simmer over low heat for 45 minutes. Add the remaining sugar in two or three batches. Add the strained juice from step 2, and continue simmering until a spoonful of the mixture stiffens to a consistency similar to jam. Remove the seeds. Store in sterilized jars.

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