Bok Choy stalks that are pure white, not rusty, and firm. Look for leaves that are dark green and are not wilted. Avoid any stalks that are broken, wilted, brown spotted leaves, limp stalks and any with discoloration.
To prepare full-size bok choy, first halve the head lengthwise and cut out the tough core at the base. Next, soak the halves in cool water. Make sure to swish them around a few times. Rubbing the leaves together will help remove the grit and sand that can get lodged close to the core. Pat dry.
Bok choy will keep for 3-5 days unwashed in a tightly sealed plastic bag in the refrigerator.