I often notice that my simple broth (well-cleaned chicken/animal flesh/bones; white onion; garlic; carrots; celery; herbs) is slightly tart to the taste. I'm sure I've noticed this when thawing or re-heating broth. I can't remember if it tastes this way directly out of the stock pot. I found another post on the topic
Which is full of old wives tales. But I'm craving some more sciencey explanation. Any ideas?