Is a separate expiation (kaffarah) for an oath required for each type of food one swore to abandon and felt distress upon eating, or is a single expiation sufficient, given that some have already been expiated? And does an oath formed in the mind take effect?

Whoever doubts an oath does not incur expiation, because the default is the acquittal of responsibility. The mere intention is not sufficient for an oath to be binding according to the majority of scholars; rather, it must be uttered. And whoever swears not to eat in a specific container or situation, owes one expiation if he breaks his oath. It is advised not to resort to excessive swearing.