One of the most common questions I get from users of my website has to do with reusing their current batch of yogurt to make their next batch. Being able to "reuse" or perpetuate the starter from one batch to the next can be a huge cost savings in making yogurt, completely eliminate packaging waste, and to gain even more control over the finished product as compared with starting from a new starter culture each time. But like a lot of my users, I have not had great success with keeping a starter going beyond four or five batches, before noticing a dropoff in quality of the finished yogurt. Until now that is.
As a quick review, the first time you make yogurt at home you need a starter and some milk. I recommend using 2-3 Tbs. of a high quality, store-bought, plain yogurt, such as Dannon or Stoneyfield. Alternatively, you can purchase freeze-dried cultures from health-food stores or online. Once you have made your first batch of yogurt, you can reserve 2-3 Tbs. of it to start your next batch, eliminating the need to buy more store-bought yogurt or cultures. Theoretically, this should work indefinitely, and in homes where yogurt making is a multi-generational tradition, it does. Yet, I have observed my yogurt starting to get less thick and less tangy by batch five, and have shied away from going beyond four batches for several years.
Back in June of this year, however, I decided to give perpetuating a culture another go. So I read up on yogurt cultures, and re-examined my process for defects. What I learned is that not all yogurt cultures are capable of continual perpetuation. Some cultures are "direct set", meaning that you use them once, and that is it. You need to buy new starter for each and every batch. Many freeze-dried cultures that accompany home yogurt makers are of this variety. Other cultures, however, are capable of being perpetuated indefinitely - under the right conditions. Two of the most common are almost always found together in good quality yogurt. They are Lactobacillus bulgaricus and Streptococcus thermophilus. They also happen to be the two strains found in Dannon plain yogurt, from which I started this latest experiment.
The other thing that I learned is that the longer your current batch of yogurt sticks around in the fridge, the less likely it is to be a good starter for the next batch. I sort of knew this intuitively, since we've all had yogurt go bad on us before we could finish it. But as long as the yogurt hadn't "turned", I figured it was okay. But if you think about the yogurt making process, the clock is ticking on those cultures for lack of anything to eat. Remember, the bacterial cultures in yogurt consume the lactose in the milk, converting it to lactic acid. This not only gives yogurt its distinctive tanginess, it also preserves the milk by creating an acid condition in which pathogenic bacteria will not grow. So your finished yogurt has much less lactose in it upon which the cultures can feed. Eventually, they run out of fuel entirely and die, and the bad bacteria moves in.
So how long is too long? I don't really know, and am open to comments from any biologists in the audience. But since June, we have been awash at our house in various forms of fresh berries followed by jams made from said berries. As a result, our yogurt consumption is way up. Our family of three has been ripping through a half gallon of yogurt in about seven to eight days. So the batch that will become the next starter is never more than 8 days old. However, we went out of town twice during this experiment, and I froze 2-3 Tbs. of the current batch, and picked it up anew upon our return. We will finish batch number seven this week (I know that's a whole lot of yogurt) and I will be making batch number eight this weekend. So far each batch has turned out just like the batch before it. Now I don't have commercial heating and freezing equipment in my home (I am sure you don't either) and there is certainly going to be variability in how cold it is in your fridge vs. mine. With that said, if you make a new batch - or freeze some starter from the current batch - within a seven or eight days, my results seem to indicate that you can keep these two strains going indefinitely. Time will tell of course, and I would not plan on freezing your cultures for more than a few weeks.
I will continue with this experiment for as long as I can, and provide updates in the form of comments to this thread. Please track my results and chime in with your own findings.