Imo-yōkan (Sweet Potato Bar Set by Its Own Starch)

Name: Imo-yōkan (Sweet Potato Bar Set by Its Own Starch)
Prep time: 20 min
Cook time: 35 min
Total time: 55 min
Yield: 8 servings
Cuisine: Japanese
Ingredients: 600 g sweet potato; 90 g sugar; 1 pinch salt; 2 tbsp water

A Japanese sweet made by sieving steamed sweet potato while hot, mixing in sugar, pressing it hard into a square mould and letting it set as it cools. It is called yōkan but has neither agar nor bean paste in it. What sets it is the sweet potato's own starch, and with far less sugar it does not keep: it is eaten within days.

Ingredients

8servings

sweet potato · a dry, floury variety, weighed unpeeled600 g
sugar · about a fifth of the steamed weight90 g
salt1 pinch
water · only if the paste is too stiff2 tbsp

Steps

Peel the sweet potatoes thickly, taking off the fibrous layer just under the skin as well, or the colour turns dull. Cut into 2cm slices and soak in cold water for 10 minutes, then drain.
⏲ 10 min
Steam over boiling water for 20-25 minutes, until a skewer goes in with no resistance.
⏲ 25 min
While it is still hot, press it through a fine sieve. Left to cool first, it goes through badly and leaves lumps.
Put the sieved sweet potato in a pan with the sugar and salt and stir over low heat for 3-5 minutes, until the sugar has dissolved and the paste pulls away from the bottom of the pan in one mass. If it is too stiff, add a little water.
⏲ 5 min
Line a 15x10cm mould with cling film and add the paste a little at a time, pressing it down hard with the back of a spoon so no gaps are left. Level the top and cover.
Let it cool at room temperature, then chill for 2 hours to set. Cut into 1.5cm slices. Keep it in the fridge and eat it within 3-4 days.
⏲ 120 min

Tips & Variations

Variations

Imo-yōkan with agar: some packaged imo-yōkan are set with agar for a smooth, glossy block. That is closer to what yōkan's Variations list describes when it says "Sweet potato instead of azuki. Chestnut, matcha and white bean versions work the same way". This recipe is the one ko.wikipedia describes, set with nothing but steamed sweet potato and sugar.
Purple sweet potato and matcha: the three in the photograph are, from the left, one with matcha mixed in, plain imo-yōkan, and one made from purple sweet potato. The matcha goes in with the sugar; the purple one is made exactly the same way.
With chestnut: roughly broken cooked chestnuts in the paste give it something to bite on. Press more carefully round the chestnuts as you fill the mould so no gaps are left beside them.
A sweet-potato town's speciality: ko.wikipedia also describes it as a noted product of places that grow a lot of sweet potato, such as Kawagoe in Saitama.

Tips

Nothing is added to set it. Yōkan is "The Japanese confection of sweet bean paste set with sugar and agar, firm enough to cut with a knife". Imo-yōkan has no agar, and the setting is done by the sweet potato itself. As mee rebus explains, "Sweet potato brings its starch already gelatinised inside intact cell walls". That starch, cooked during the steaming, firms again as the paste cools after being sieved hot and pressed into the mould — the same thing that makes a cold boiled potato firm. So the whole business of dissolving and boiling agar simply is not there.
Which is why the sweet potato matters. The setting comes from its starch, so it takes a dry, floury variety, high in starch and low in water, to set firm enough to slice. A moist, very sweet variety stays soft even when cold.
It does not keep, and that is the price. What sugar does credits the sugar for yōkan keeping a year without refrigeration, and says that "cutting the sugar does not just cut sweetness" — it removes the keeping. Imo-yōkan is the case where the sugar is cut on purpose. It leans on the sweet potato's own sweetness and takes sugar at only about a fifth of the steamed weight, so it is not a sweet to keep but one to eat from the fridge within days. It is one step further along the line yōkan's own Variations draw for mizu-yōkan: "More water and less agar, set much softer. Eaten cold in summer, and it keeps far less well".
Sieve it hot. Once it cools, that starch has already begun to firm and it will not go through the sieve well. Sieving straight out of the steamer is what gives the smooth grain.
Press it into the mould. This is not a paste that flows to fill the mould the way an agar mixture does. Without pressing, the gaps stay and the block crumbles when you cut it.
Peel thickly. The layer just under the skin is stringy and dull in colour; leave it and the yellow turns muddy and fibres catch in the grain.