Originally Posted by arfavreau
Adding the flesh of the lemon and especially the lemon juice will cut down on your head retention, this is caused by the acid in the fruit, 2 zested lemons should be good for 5 gallons.
Definitely the first I've heard about a ph drop killing head retention (edit: POST mash). I think we have all had noticeably tart beers with fine head. If you'd said it was something enzymatic I might believe it, but even then, you could drop it in at the end of the boil.
Shouldn't be any reason you can't add lemon like any other fruit, in boil or in secondary--except taste, which is up to you. Don't think I'd add more than half a lemon per 5g to a wheat beer, myself, until I had an idea of what that would do to the flavor. Most of the aroma is in the zest, but the juice is def more than just sugar water and acid.
BTW, as a cook, I will say: don't eat pith on purpose, but it is generally just something bitter you don't want to grate
into food because then it will be noticeable in small quantities. You can drop a zested lemon with the pith exposed into a boil or fermenter, it won't taint your beer. (Beer IS bitter!) Can also just cut the rest of the peel off at that point, though.