My friend, who has a severe sulfite allergy, wanted to try some of my beer. I had to refuse because I wasn't 100% sure that there weren't sulfites in it.
How are sulfites introduced to beer? I was under the impression that they are usually added as a separate preservative compound. What preservatives (that could reasonably be used in homebrewing) might contain sulfites?
Is there any chance that other ingredients used (in a simple all-grain, minimal- or no-flavoring brew) could introduce sulfites into a beer?