What's the difference between heirloom and direct-set yogurt cultures?
It's simple. Heirlooms will produce expected results no matter how many times you reinoculate your culture. Direct set is a blend of species and eventually only the strongest strain will remain and it may not create anything you want to eat. I never bought cultures, I just buy plain or vanilla yogurt, eat it and use what's left behind to make another batch.

Remember the single most important part of yogurt making is hygiene. I can't stress this enough. Yogurt won't always tell you it's bad until it's too late and you eat it and get very very sick. Trust me on this from experience. With yogurt HYGIENE is the most important part of the process.!!!