Don't worry too much, lid-on boilers. If you used extract, most of the DME will have been driven of when the extract was produced by the maltster. Your beers will be fine.
It's still a good idea to keep the lid off during the boil, especially if you upgrade to partial mashes or all-grain brewing.
I was leaving it on because I was afraid of contamination...even though the water was boiling. How about when cooling the wort, It's go faster with the lid off but the water is no longer boiling, do I need to worry about that?
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