Cook macaroni according to package directions.
Combine lemon juice and baking soda and allow it to fizz. It will expand so use a container with some height. Once it's settled add to a pot with milk and bring to a light simmer.
Stir in cheese and turn heat off. Blend on high until smooth and season with mustard powder and salt, to taste.
Toss with noodles and serve right away.
