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Joshinko (上新粉) is a Japanese rice flour. Joshinko is made from milled short grain rice has been washed, dried, and ground down into flour while mochiko and shiratamako are both made from glutinous rice (mochigome). The rice is milled first, then washed, soaked, drained, ground very finely in water, and dried.
Joshsinko is usually used for making dango, Kashiwa Mochi, and Zenzai (Oshiruko). Since it doesn’t include glutinous rice, it can be chewy and doughy in texture compared to Shiratamako (glutinous rice flour).
Source: this Japanese link
Recipes Using Joshinko