I have a mixture that I make up in powder form and can be stored dry until mixed with water and heated. It is starch, sugar,yeast,agar and preservative. When required I mix with water and heat up. It makes 3 small containers and remains like firm jelly so you can tip the container up. The preservative stops it going mouldy and stinky. I use old yeast that is no longer active (cheap) as a source of nutrient. The source of starch and sugar does not matter too much.