I often make my own yogurt at home. I suggest you use it every time. Bacterial powder is better, because it is impossible to be clean and hygienic, and other miscellaneous bacteria. If you want to use old yogurt, it is usually reasonable to use 1/5. But old yogurt must be guaranteed. Clean and hygienic. Used to be made of old yogurt, which often turned red. They introduced that there are other miscellaneous bacteria entering.