How should I store the sachet and how long does it keep after opening?

Store unopened sachets in a cupboard away from heat, steam, and direct sunlight — typical room temperature (below 25°C) and low humidity are fine. The 30 g sachet is designed for single use: once you tear it open, the fine starch powder rapidly absorbs ambient moisture, which causes clumping and uneven dissolution. If you must split a sachet, fold the top tightly, seal with a clip, and use the remainder within a day or two; however, the texture and thickening power may degrade. Do not refrigerate the dry powder, as condensation when removing it will accelerate clumping. Prepared kissel should be consumed warm; if left to cool completely it will thicken further into a gel, which is safe to eat but not the intended drinking consistency. Discard any prepared drink left at room temperature for more than two hours.