Why might some mussel shells not open when cooked?

Mussels that do not open when cooked may be a result of being harvested too early. When mussels are harvested before they have fully matured, they may not be able to open their shells properly during cooking. This immaturity affects their physiological responses and can result in a tighter closure of the shell, making it difficult for them to open upon reaching the requisite cooking temperature.

In contrast, proper harvesting times ensure that mussels have developed enough to respond normally to cooking. Factors such as improper storage or being left out too long may contribute to spoilage but do not directly relate to the maturity of the mussels. Cooking methods, like boiling too quickly, can impact the texture and doneness but would not typically affect whether the shell opens if the mussel is mature.