soak it in hot water to soften it, and then wash it in water for later use.

Potato flour is a hard lump of starch when it is cold. After making potato vermicelli, it should be dried in the sun and then stored. Potato vermicelli can't be stored in a humid place, otherwise it will be hairy and moldy. Therefore, potato vermicelli should be soaked in hot water before cooking.

Extended information:

vermicelli (also known as vermicelli with thin stems) is a filamentous or strip-shaped vermicelli product made from sweet potatoes and potatoes. It is grayish white, yellow or yellowish brown in color, and can be divided into round vermicelli, fine vermicelli and wide vermicelli according to its shape. It has a smooth and tough taste and is produced all over China. The vermicelli contains nutrients such as carbohydrate, protein and nicotinic acid.

Generally, sweet potato vermicelli should be cooked in boiling water for 15 minutes to 3 minutes before eating, and the thicker vermicelli can last for 4 minutes. If you continue to cook, the noodles will be swollen and broken, and the water-soluble solids will be improved due to the increase of water absorption. If you continue to cook, the noodles will become small short knots, and eventually there will be a slight soup paste phenomenon due to the high temperature melting of soluble starch. Anyone who cooks for an hour or two without swelling and continuous noodles may add additives such as alum, and there will be quality hazards.

Reference:

vermicelli-Baidu Encyclopedia