Professional chefs use sodium citrate to prevent melting cheese from curdling. You can make sodium citrate at home for a fraction of the cost by using lemon juice and baking soda.
When you combine lemon juice with baking soda, the ingredients will froth, and you'll have sodium citrate. You can incorporate it into cheese sauces without the cheese coagulating.
If you're using weaker acids like citrus fruit juices or vinegar and opt for citric acid, it needs to be dissolved in water before adding other liquids to the recipe.
Heat the liquids to a simmer and gradually whisk in cheese until the sauce is smooth. The sauce will thicken as it cools but should remain pourable at room temperature.