Today I made a quart of yogurt using the slimy batch (see yogurt bloopers) from 11/22/09 as the starter. I was surprised by the results. The texture remains smooth and slightly slimy, but the flavor has a lot more tang.
This batch of yogurt did incubate for a little more than eight hours, and the additional time may have effected the tang. There is something about the tangy-ness that seems to be consistent with other 2nd-plus generations of yogurt.
Sandor Katz addressed this exact issue in a lecture that I recently attended. He wasn’t sure what the microbial cause was of the increased tang and decreased density of 2nd-plus generations of yogurt, but he had two contradictory theories. His first theory is that the starter culture collects additional strains of bacteria as it ages and gets exposed to the air, this increases heterogeneity of the microbes, and causes the flavor and texture to change, or some might say, to degrade.The second theory Katz had was that as the culture ages, certain types of bacteria get crowded out as other types dominate. For example, the bacteria that contributes to the thickening of yogurt is Streptococcus Thermophilus. When yogurt that is made with older generations of culture seems thinner than earlier generations of that same culture, perhaps the S. Thermophilus is dying off as other cultures multiply and consume the existing resources. In this second postulation, the bacteria strains are actually becoming less diverse or more homogeneous, and perhaps this “purification” of the bacterial strains contributes to a more tangy, thinner product.
Katz also reminded us that we have been raised on yogurt that is cultured by bacteria that is produced in a lab. The thick, mild, creamy product to which we are accustomed, in no way resembles its cousin from pre-industrial America or the yogurt consumed by most people in other parts of the world. He had a couple of suggestions to address this issue. One was to refresh the yogurt culture when the product is no longer to one’s liking by getting a fresh yogurt off the shelf (watch those ingredients, though!) or by buying a powdered yogurt culture. I think some people refresh the culture after four or five generations as a matter of course. Katz’s second recommendation was that we work towards a new paradigm of what yogurt really is and wean ourselves off of the misconception of what yogurt has become through commercial and scientific manipulation.
I have not had the opportunity to exhaust a yogurt culture because I am still constantly experimenting and adapting my method and ingredients. I also live with three other people who occasionally eat up the last of the available yogurt before I can save a little as a starter.
I am not going to continue culturing the slimy yogurt from which I made today’s 2nd generation batch. As I made the experimental quart, I also made three quarts from a brand of commercial yogurt I hadn’t previously tried as a starter, and I really, really liked the product. I think it is my favorite homemade yogurt so far! I may try to exhaust that culture and see how long that takes. In the meantime, my cat seems to really like the slimy yogurt, so she can have that quart to her heart’s content. I am looking forward to many bowls of my recent success.