Cut an apple in half and leave it for 24 hours. It starts to go bad. Make a cup of tea and leave it for 24 hours, it goes off.
After you’ve added liquid to your powdered Huel, milkshake mix, breakfast cereal, left your sausage sandwich out, cooked that chicken and left it for 24 hours it starts to break down.
Your Huel comes in a light proof sealed packet to make it last longer. Once you’ve taken it out and mixed it with water it’ll gradually start to break down, the vitamins will degrade, etc.