A recent conversation came up about stock. My sister listens to a radio cooking program with Melinda Lee, and apparently Ms. Lee said that stock should be simmered with the lid off, or else you run the risk of "souring the stock". I've made plenty of stock over the years, and don't really understand what she's talking about. Any thoughts or experience on what this "sour the stock" phenomenon is ?
I've generally simmered with the lid off, btw, but not necessarily always, so I'm interested in thoughts on this generally.