So I've been making yogurt for about two years now. It's been very successful, and I'm very pleased with the results, if for no other reason than the satisfaction of making it myself.
I've been using the Euro Cuisine 2 quart yogurt maker. So far, I've only had one "bad" batch - it was too liquidy - and it was completely my fault, I wasn't paying attention and let the milk cool far too much before putting it to incubate. Other than that, I've found that I can be very lax with the process - I use varying types of milk (1% to whole), varying amounts (1 Q to 2 Qs), go to anywhere between 185 and 200, can add varying amounts of "starter", and can let incubate anywhere from about 12 to 24 hours. While the finished product varies a little in taste and consistency, it's always (save once) very good yogurt.
For a "starter", I've been using about 1-3 tablespoons of either Dannon plain yogurt (I buy a quart occasionally if I run out of homemade and don't have time [or am too lazy] to make another batch), OR I use a couple tablespoons from the preceding homemade batch.
The Question: If I use the preceding homemade batch as the source for a starter, do I have to worry about about any "unfriendly" bacteria that may have taken up residence while that container has been opened a dozen times and been sitting in the fridge for up to 2 weeks. It seems that I might be taking yogurt containing bacteria that might be getting past its safe usability, and then heating it (and its bacteria) up to a perfect incubation temperature for not only the friendlies, but also the unfriendlies.
Please note - I'm am not a germ-a-phobe, and actually tend to be very lax in my approach to eating leftovers that might be past their prime. But in theory, you're re-using the same batch of yogurt potentially infinitely when using a starter in this method. Is there a food safety issue?
Can someone school me on this?