Vacuum-sealed glass jars are used for tomato sauces, pickles, tuna, and similar products. Creamy spreads are never vacuum-sealed, not even Nutella. That’s why they don’t make the “clack” sound when you open the jar. Why? Because they are anhydrous (they contain no water), so they don’t have shelf-life issues. By contrast, products that contain water, such as tomato purée, require a vacuum seal created by pasteurization. When we boil jars of homemade passata, we are pasteurizing them, which creates the vacuum and when we open them, we hear the “clack.”