Agemochi is a type of fried mochi that you can find at grocery stores and convenience stores in Japan. While agemochi is typically made using joshinko, a type of Japanese rice flour, this recipe is made almost entirely with potatoes and potato starch.
Potato starch has powerful binding and gelling properties, which we can take advantage of by understanding gelatinization, the process that occurs when starch granules hydrate and swell as they’re heated. In this recipe, vigorously boiling russet potatoes gelatinizes their native starches, and then mashing the potatoes while hot into a slurry of potato starch and dashi partially gelatinizes the slurry, which helps to bind the potato into a cohesive dough that can be shaped by hand and cut to any size.
Gelatinization is also responsible for the chewy interior of the finished snacks, and for the rigid, brittle network with a porous, open structure that forms on their surface as they dehydrate while frying. Basically, gelatinization is behind everything that makes these snacks texturally interesting: Puffed and crispy on the outside and slightly chewy on the inside.
Potato starch works particularly well for this application because its rate of gelatinization is faster than other starches, like tapioca starch or wheat starch, and its rate of retrogradation* is slower, so the snacks won’t stale as quickly.
*Retrogradation is the re-association of starch molecules during cooling. A good example is stale rice in the fridge.
To bring it all home, I glaze the hot fried mochi in a savory-sweet tare made from soy sauce and mirin; it’s even thickened with a small amount of potato starch to give it an attractive sheen. And, of course, an agemochi look alike wouldn’t be complete without a thoughtfully placed strip of nori running down the middle. When you put it all together, what you’ve got amounts to a very satisfying, crispy-chewy, intensely savory hash brown.
From a purely technical perspective, this recipe illustrates three possible textures you can produce with potato starch: Crispy, chewy, and glossy. At its heart, it’s a study of the ways starch can be manipulated by adjusting the timing, the proportion of ingredients, and the amount of heat and hydration. But in the end, all that aspirational, fancy-pants chef speak doesn’t really matter. This snack is tasty, pure and simple.
For the Mochi: Rinse potatoes in a large saucepan under cold water to remove debris; drain, return to pot, and cover with 2 inches of water. Bring to a boil over high heat, reduce heat to low, and simmer until potatoes can be easily pierced by a knife and begin to flake, about 20 minutes. Drain potatoes thoroughly.
In a large mixing bowl, whisk together potato starch, water, and dashi until smooth, then add the drained potatoes. Using potato masher, mash potatoes until a smooth, lump-free dough begins to form, about 2 minutes, then transfer to a clean, unfloured work surface. Alternatively, use a ricer or food mill to purée the drained potatoes directly into the bowl, and mix with a spatula to form a smooth dough, before turning onto a clean work surface.
Gently knead, using a bench scraper in one hand to help gather dough, until dough no longer sticks to hands and is malleable and cohesive, about 30 seconds. Return dough to bowl, cover loosely with plastic wrap, and let cool until lukewarm and firmed slightly, at least 30 minutes and up to 2 hours.
Line a rimmed baking sheet with parchment paper and spray with nonstick cooking spray. Lightly spray work surface as well. Transfer dough to prepared work surface and gently press into an even 9- by 10-inch rectangle, measuring approximately 1/2 inch thick. Use bench scraper to press and square sides to help form the shape.
Using a bench knife, cut dough into twelve 3- by 2 1/2-inch portions. If pieces are not perfectly square, trim with a knife, if desired. Using the bench scraper, carefully transfer squares to the prepared baking sheet. Refrigerate uncovered until cooled and firm, at least 30 minutes and up to 2 hours.
For the Sauce: Meanwhile, in a medium saucepan, whisk together mirin, water, soy sauce, sugar, and potato starch. Add garlic, place saucepan over medium heat, bring mixture to boil, and cook, stirring occasionally, until bubbling subsides and mixture is thickened, glossy, and coats the back of a spoon, 4 to 6 minutes. Transfer mixture to a heatproof bowl and cool to room temperature (discard garlic). Set glaze aside until needed.
To Fry and Finish: Set a wire rack in a rimmed baking sheet. Heat oil in a 12-inch cast iron skillet to 325℉ (190℃). Carefully lower 6 potato squares into the oil and cook, adjusting heat to maintain a temperature between 275℉ and 325℉ (135-160℃), until first side is light brown, 2 to 4 minutes. Using a fish spatula, carefully flip potato squares and cook on the second side 2 to 4 minutes more. Continue flipping and cooking every 2 to 4 minutes, until mochi are deep golden brown all over, 8 to 10 minutes total. Transfer mochi to wire rack and let cool slightly. Repeat with remaining portions.
Lightly dip one side of mochi in sauce to coat, letting excess drip off. If desired, wrap each mochi with nori, and place seam-side down. Serve right away.
Potato Masher, ricer, or food mill; digital thermometer; 12-inch cast iron skillet.
Do not use waxy potatoes like Red Bliss or Yukon Gold varieties; the texture will be gummier and less appealing. This recipe works best with starchy potatoes such as Russets.
Be sure to use unmodified potato starch such as Bob’s Red Mill Premium Quality Unmodified Potato Starch. Using modified potato starch may produce a gummier, unpleasant texture.
You can substitute freshly prepared cold dashi for the instant dashi and cold water in the recipe if desired.
Make-Ahead and Storage
Uncooked mochi can be stored, covered, in the refrigerator prior to frying for up to 3 days. Uncooked mochi can also be frozen for up to 4 weeks by freezing in a single layer on a baking sheet, then transferring to a zipper-lock freezer bag. To cook frozen mochi, let mochi thaw on rimmed baking sheet until soft but cool before frying (frying time may be a few minutes longer).