The recommended cooking times often reflect the fact that beans and lentils used to be stored for long periods before being cooked. This made them very dry and hard. Packets now feature 'use-by' dates, so the dried beans and lentils may become tender in a shorter cooking time, especially if you follow these guidelines.
To low-boil beans, cook them at a boil that is low but continuous, not at a simmer. The loss of water in the saucepan through eveporation can be minimized by partially covering the pan with a lid, but as water is bing absorbed by the beans as well, you should be prepared to top up the level with boiling water when necessary.
The scum that rises to the surface during cooking is not dirt but a mixture of proteins and starches. It is best to skim it off, both for the sake of your pans and to prevent the liquid boiling over.
Do not salt dried beans during cooking, or cook them in a liquid containing salted meat for any longer than is necessary. This can toughen their skins and inhibit the swelling and softening of the beans. Acidity also impedes their cooking, so do not add tomato sauces or any other that are acidic until the beans are soft. Herbs and spices may be added early.
The acid to alkaline balance of local water can also affect cooking times. Keep testing the beans in the later stages of cooking.
** Asian Online Recipes