I Need the Science-Homemade Yogurt

I used the same yogurt maker and starter brand for a bazillion years. The temp was about 110f. The maker began to overheat, so I bought one that has a water bath feature, uses much lower temperature (97, 100, or 104.) Used the exact same method and quantities of milk, powdered milk and starter. Several batches were just awful. When I finally tried using about ⅔ the amount of starter, I ended up with the best batch of yogurt ever. So..why does the lower temp and a lot more time need less starter?



Lori T. March 2, 2020
Simply put, the biocultures responsible for making your tasty yogurt actually prefer temperatures closer to natural body temp- that is right around 98.6-99F. You end up with a smoother and more stable yogurt. The downside to the lower temp is that it takes longer to convert the sugars in the milk to make it into yogurt. Higher temps, around 104F, make yogurt faster- but the trade off is you get a less smooth product with more whey release, and sometimes a sharper taste. To some extent it is also a result of which type or types of bacterial cultures are present in your starter culture as well. Some of those are known to make a tarter end product than others. As for why you needed slightly less starter for the lower temperature- some of your bacteria were just getting heat damaged and unable to reproduce as well. Your own body responds to bacterial or viral infections by raising your internal temperature for this exact reason. Body cells are adapted to work well within a larger spread of temperature- where the temperature tolerance of bacteria tends to be much a much smaller range. You feel like doo-doo at 102, and near dead at 104. Your yogurt bacteria feels pretty much the same way. Since you have hit on a combo of cultures and temperature that suit your taste, just take care of that culture. It would be worth it to start a specific culture to freeze in ice cube size lots- and to use one of those to start a batch. You can safely reuse part of an old batch of yogurt to inoculate up to a dozen or so batches after that. Then you want to start with a new frozen cube, because foreign bacteria and natural mutation can alter your culture over time if you continue to reuse it over and over. Though you think it's a straight duplication process, it's not quite. Your frozen reserve helps you out there. When you get down to one or two frozen cubes- repeat the culture and freeze process to have a new batch on hand to use in the future.
Nan March 3, 2020
How long for the yogurt at the lower body temp? I use an Instant Pot and leave it for 9 hours
Lori T. March 4, 2020
How long the yogurt requires for incubation depends on what you want the finished product to be like, so far as tartness or firmness of set, as well as the type of bacterial cultures you are using to inoculate the milk. At or near body temperature, yogurt will ordinarily take between 9-12 hours to set. What counts most is that you are satisfied with your end product, regardless of how quickly or how long it takes to achieve that. If you like your current method, and your culture makes a yogurt you thing tastes good- stick with that. It only becomes a problem if you don't like the product you get, and want to figure out what you could do to try to change that.
HalfPint March 2, 2020
Regardless of quantity, starter is needed to grow the right bacteria for yogurt. Longer incubation and lower temperature means that you can use less starter (i.e. bacteria) because you are allowing more time for the bacteria colony to multiply and grow. Longer time also allows for flavor development. So that is the trade off. Less start = better tasting yogurt, but longer incubation. More starter = less time but a blander yogurt.
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