Many hold all unflavored beer is kosher without a hechsher. However, CRC says that beer from microbreweries needs a hechsher. This is confusing because I don’t know how small a company needs to be to be considered a microbrewery.
Star K says only flavored or home- or pub-brewed beer needs a hechsher. So according to them, even unflavored microbrews should be OK without hechshers.
I try to avoid unflavored microbrews if I see on the shelf that that same company also produces flavored beer with problematic or clearly treif ingredients (like grape juice or oysters — yes, such strange things exist) but that wouldn’t seem to be necessary per the Star K.