There’s a balloon in my flask!

☑ Ingredients are contained within airtight magical metal

☑ Ingredients never touch air when the contents are poured out

(Flask with partial fill)

(Flask filled up with fluid)

It is the air that contaminates the ingredients, and not being fully encased in magical metal. While you could try to put solid beads or marbles into the container for displacement, why not make the displacement a softer material: a balloon?

The flask is fully inflated when the balloon is flat. There is no air in the flask or the balloon. If you need 2.56 ounces of fluid out, you inflate 2.56 ounces of air into the balloon, which pushes 2.56 ounces of fluid out the spout. The air never touches the fluid, only the balloon does. Find whatever material does not spoil the fluid (like the marble), and that is the material for your balloon.

Filling the flask is just as easy.

When you remove air from the balloon inside the flask, the spout will suck in a fluid at the mouth.

Build the air cap out of your magical metal

This way the fluid is completely enclosed when you are not filling or pouring.