I live in a group house where we make our own yogurt once or twice a week. We've been doing this for many years, culturing fresh whole milk using a couple teaspoons of yogurt from the current batch to make the next batch.
I've read online and in books like The Art of Fermentation that commercial yogurts sold at grocery stores here in the United States can only be recultured a few times while heirloom cultures like the one we've been using can be recultured indefinitely and passed from one generation of humans to the next.
What is different about these two kinds of yogurt?
I will make streak plates from the two kinds of yogurt and examine the differences in colony appearance between the two kinds of yogurt.
2022-02-26: Formulate agar media suitable for growing yogurt cultures. Agar ends up layered and looks curdled after autoclaving.
2022-02-28: Retry making milk agar using recipe found on a school science fair resource website. Agar ends up layered, as if the milk curdled.
2022-02-30: (Novice Night at BosLab) 3rd try at making milk agar, this time successful, by NOT autoclaving the milk. Instead, in the sterile atmosphere of the PCR hood, I transferred skim milk from a fresh, newly opened carton in a sterile atmosphere to melted autoclaved agar, then heated in the microwave to thoroughly mix.