I have always wondered this. Is it because any extra sugars or flavors will affect it somehow? I want to duplicate Zoi's honey greek yogurt. I have tried just regular yogurt cultures but I always get a sour taste that I don't like, and I am not incubating too long. It's been a while so I don't remember the exact details, but I got frustrated and gave up. I wanna give it another shot. I really would like to be able to make a sweet creamy honey greek yogurt like theirs. So the reason I ask is - in case it has to do with the specific cultures they have, would there be a problem if I used some of the honey yogurt to start mine? I've tried their plain and it still had that sour flavor.