After the tapioca is boiled, spread it in a large mixing bowl to cool down completely. Grind yeast biscuit into a fine powder in a mini electric food processor. Sprinkle the powdered yeast over the tapioca.
Transfer the tapioca to a ceramic casserole. Sprinkle sugar over, then pour in the water. Cover the casserole with a lid and place in a cool place in the kitchen, away from sunlight. Leave undisturbed for the tapioca to ferment for about 2 days. Once the tapioca has fermented, transfer the tapioca to a clean dry glass jar and chill in the refrigerator. Serve the fermented tapioca after a spicy hot meal.