Amongst its many properties, a key aspect of salt is that it draws water out of cells. In the presence of salt, water moves across a cell membrane to try to equalize the concentration of salt on both sides of the membrane — this process is called osmosis. So when you add salt to a food, water moves towards the salt and the food dehydrates. This is why salt has been so useful historically as a food preservative. Once a certain stage of dehydration has been reached, the activity of the bacteria responsible for the rotting within the food is stopped.
Interestingly, this drying property of salt could jeopardize some of your dishes. Let’s take beans for instance: never add salt to your beans before cooking. Salt reacts with the seed coat, which forms a barrier that prevents absorption of liquids. For this reason, if you add salt to the soaking water or to the cooking liquid, the beans will never get softer, no matter how long you cook them.
On the other hand, adding salt to your beans once they have reached their best texture can be very useful: it will prevent them from disintegrating due to overcooking. During the cooking, you can test beans for doneness by pressing a few of them under a fork on a flat surface to see how easily they mash. Once the texture of the beans is just right for your taste, adding salt will allow you to re-heat them or cook them further with other ingredients without any change to their texture.
In fact, beans will react in the same way as with salt in the presence of any acid ingredient such as tomatoes, vinegar, lemon juice, or molasses, at any stage of the cooking process — the seed coat will toughen and they will resist further tenderization. Acid ingredients are a good alternative to salt in case your doctor recommends a low-salt diet.
submitted by N. Rémond, on behalf of the Food Issues Committee