I would advise not writing them an angry letter. Write them, but do so with courtesy. I was on the other side of the customer service equasion for several years, and I can tell you that angry rants about products are not handled well by the majority of employees.
When you contact any company with a problem, start out on a good foot; write a well-thought-out note listing the facts, stating your disappointment, and saying what you expect. Avoid inflammatory remarks, name-calling, and generalization. It doesn't hurt to say something like "normally, your frozen entrees are great and I buy a lot of them, but...". End it with something like "I know your company stands behind its products and I know you'll make this right."
That will net you better, and likely faster, results. As the saying goes, you catch more flies with sugar than vinegar.