Spices create aromatics so it is important when to add them. But what about salt? Is it critical if you add salt to the flavor base instead of adding it to the tomato paste, or adding it to the finished soup?
Salt is very soluble in water, and during the cooking process will tend to diffuse within the liquids of the food and permeate inside. Having a salty flavor throughout the food I find tends to help curb salt usage. A good example is pasta, where if you add salt you can achieve a salty taste for the pasta and largely decrease salt you add at the table. My grandmother always said add the salt while you are cooking so you don't have to add twice as much later.
Salt adds flavor and causes chemical changes in food. The sooner you add salt more time salt has to penetrate the food and the longer the chemical changes have to work. Whether that's desirable or not depends on the effect you want. Here are a few examples:
- When boiling potatoes if you add salt at the beginning of cooking the salt flavor will get through the potato, when you bite into it the center will be seasoned. If you add salt after cooking the inside of the potato will be bland and the outside salty. I always salt the water when I boil potatoes for this reason
- When sauteing zucchini I add some salt at the beginning of the cooking process as salt will draw moisture out, changing the texture of the flesh in a good way and making it cook a bit faster
- If you chop tomatoes and add salt to them they will shed a lot of moisture, which can sometimes be what you want
There are times when it doesn't really matter when you add the salt, for instance cooking pasta. You can add it 30 seconds before you take the pasta out and you won't tell any difference to taste or texture.
It's also important to remember not to oversalt at the beginning. You can put salt in but you can't take it out, so always put in less at the beginning than you think you need when making soups, stews, etc. Towards the end taste the food and add more salt if needed.