I just dethawed a frozen lentil soup which was poorly seasoned before it went into the freezer. I added salt to the other half which I ate fresh and made a mental note of seasoning the frozen batch once thawed. Once I thawed it, I corrected the seasoning and observed there was this unpleasant metallic flavor to the soup. I can't put a point on it, but it tasted "off".
I considered for a moment: what if extreme temperatures sap the freshness out of food by reducing the acidity of the fresh product? Lo, when I gave it a slight splash of sherry vinegar, all was right again. However, I love sherry vinegar and use it often in any salad or soup to sour it up to my taste.
My question is: does freezing reduce acidity in products? Is acidity what we perceive to be the "fresh" flavor of things not previously frozen or boiled to oblivion?