Why It Works
- Mixing a slurry of potato starch into hot boiled potatoes gelatinizes the starch, producing a cohesive dough that can be shaped and cut to size.
- Shallow frying further gelatinizes the shaped dough’s interior while crisping and puffing the exterior, resulting in a crispy shell with chewy insides.
- Thickening the sauce with potato starch yields a glossy glaze that lacquers the mochi.
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.