This started as a question from a user of MakeYourOwnYogurt.com, and when I didn't know the answer, it turned into an experiment. If you are familiar with my homemade yogurt recipe, you know that it requires 2-3 Tbs of plain yogurt as a starter. If it is your first batch, I have you start with a quality yogurt with lots of active cultures (like Dannon), and for all batches thereafter, you can use your own. But this user was getting to the end of her current batch of yogurt, and was not going to be in town the following week to warrant making more. So she asked, can I freeze the 2-3 Tbs of yogurt, and will it work as a starter in the future?
It seemed perfectly logical, since you can buy freeze-dried yogurt cultures online and at health food stores, but I could not say for sure. So I took enough fresh yogurt to make a starter, and froze it in a 2 oz. plastic container. When I went to make my next batch of yogurt, I removed it from the freezer and put it into a slightly larger container filled with warm water (a water jacket). As I went about the rest of my preparation (sterilizing equipment, heating the milk, and then cooling the milk), it was defrosting to room temperature. Once my milk was at 110°F, I pitched my yogurt starter in, and mixed it up well. After waiting seven hours with it on the heating pad, I returned to a perfect pot of homemade yogurt!
So freezing some starter from your last batch is a great way to make sure you always have some on hand, and ensure you will never have to buy store-bought again. I imagine it will keep in the fridge for a few months, so make sure you always have a little, and you can make yogurt any time you like. Thanks again to my reader for this question. I had fun answering it, and learned a valuable, money-saving tip.