The string is the tough fiber that runs along the two edges of a bean. To remove it, snap off the stem end by hand rather than cutting it with a knife, then pull the broken end down along the edge; the fiber comes away intact. Repeat on the other edge. Not every bean has strings: fresh, young ones often do not, while larger, older ones develop noticeably tougher fibers.
The string is a fibrous strip running along each of the bean pod’s two long edges. It is part of the structure that holds the pod together and becomes tougher as the bean matures. Cooking does not soften it; you can feel it in your mouth and it can get caught between your teeth. That is why it is removed during trimming. A string may be present on both edges, so checking only one side is often not enough. The string is even more noticeable on dry beans; on green beans, depending on the variety, it may not be present at all.
The method is simple and does not require a knife. Hold the stem end of the bean between your fingers and snap it gently; do not break the end off completely, but leave a thin connection. Then pull that end down along the edge, and the string comes away intact. Repeat the same motion on the other edge, starting at the blossom end. If you cut off the end with a knife, this connection breaks and you cannot grip the fiber. If the fiber breaks while you pull, the bean is very fresh and most likely has no string.
Not every bean has a string, and knowing the difference saves time. On fresh, thin, young beans, the string often has not developed at all; trimming the ends is enough for these. If the pod is large and bulging and the beans inside are clearly visible from the outside, it has aged and the string has grown tough. Test one bean and use the result to decide what to do with the batch. Also remove any beans that are spotted, dried out, or soft during trimming; do not put them in the pot.
The ends are a separate matter. Always cut or snap off the stem end because it is tough and woody. The blossom end, the thin tail, is harmless and can be left on if you like; trim it too if you want a neater appearance on the plate. You can also cut the ends with scissors. Line up the beans in a handful and cut them in a single motion to speed up the work noticeably. If you sort the trimmed beans by size, they will all cook in the same amount of time in the pot and some will not fall apart.
Wash the beans after trimming because dust gets on your hands and counter during the process. Drain them after washing; do not put them in the pan wet. Cutting them in half lengthwise or on a diagonal shortens the cooking time and helps the sauce cling. Trimmed beans keep for a few days in a sealed container in the refrigerator; if they will be stored for a long time, trim them just before cooking. To freeze beans, briefly blanch them after trimming, cool them, and then pack them in bags. Keep the blanching time short; the goal is not to cook the beans but only to stop enzyme activity.