OK I know loads of things that can totally ruin a brew and I'm really careful about sterilising and hygiene but I have had a couple of brews lately ( hedgerow fruit wines) that have an odd, slightly bitter undertaste. Not the sort of nasty taste that shows it's off but not quite right. Not so bad that I wouldn't drink it myself (!) but definitely wouldn't serve it to anyone else. Any ideas? This has happened with elderberry, hawthorn and gooseberry.