This is my first post here after spending at least a couple of years reading the Master’s columns and your posts with vivid interest. Alas, all the reading was not enough to free me of all ignorance, and therefore here I am, asking for your help.
My spouse and I like to prepare our own yogurt. We like it firm and creamy, and we used to get very good results by boiling the milk for 15 minutes and then letting it cool down to ~40C (yup, metric) before adding yogurt and starting the fermentation.
Now we are trying to achieve a similar result without boiling the milk, but so far we did not succeed.
While we were brainstorming the possible causes, Spouse said that on some facebook group (!) random people
without credentials where saying that both (1) adding too much yogurt to start the fermentation and (2) mixing too little or too much the milk after adding the yogurt could compromise the fermentation process.
Both assertions strike me as counterintuitive, and I am very tempted to call BS on them.
Concerning (1), I would imagine that changing the amount of yogurt would only affect the time that it takes for the little guys to digest all the lactose. One may need to adjust fermentation time accordingly, but the end result should be the same.
As for (2), I don’t think that one can mix it too much. It’s not like the bacteria get hurt or car sick if I swirl them around a bit longer or more decidedly. On the contrary, by mixing well one ensures that each and every little guy has some lactose close by to get the party started. As for not mixing enough, I see how that may cause the fermentation to start slower: if all the yogurt is in a clump, then only the surface of the clump would be active. Again, this should affect the duration of the process, but not the end result.
So what is the hard science here? I refuse to believe that anything posted on Facebook may possibly be true.
The boiling – or more properly scalding, you don’t need a rolling boil – is intended to kill any possible existing organisms that might interfere with the ones in the yogurt culture.
Even pasteurized milk is unlikely to be sterile once you’ve opened it.
It seems to me that’s more likely to be your problem, especially if that’s the one thing you’ve changed and your process used to work.
If you’d been doing an open active boil for 15 minutes, you were also boiling off some of the water and creating partially evaporated milk. I would think that would also change the texture of the eventual yogurt.
I don’t have the links handy, but when I researched making yogurt, I read that the boiling not only sterilizes the milk, but also denatures some of the proteins, and that is important for the yogurt to thicken properly.
Interesting. I haven’t made my own in quite a while, but I remember scalding it, not boiling it. Maybe scalding does the same thing? But it’s also true that I like what I think of as the old-fashioned thickness of yogurt, not the superthickness of at least some Greek yogurt or some yogurts with added thickeners.
I should have said scalding. I don’t boil for 15 minutes, I bring the milk up to “steam” in the instant pot for 1 minute.
We’ve been making about a quart of yogurt a week for more than a year, now, and it hasn’t yet failed to set. And yes, we are making traditional yogurt, not strained (“Greek”) yogurt. So it’s not super thick. But it holds its shape, and feels firm and creamy to me.
I believe the 40C (104F) plays a big part. I have tried both methods : boiling milk and cooling it to 40C and adding the seed yogurt and Using a electric yogurt maker that gently heats up refrigerator milk to about 40C and holds it constant at that temperature : both methods yield pretty comparable yogurts.
Reactions like these are exponential: the bacteria double every few x minutes. The x depends on temperature/ milk properties with a bigger play by temperature.
Indeed, I am sure that this contributes, as much as the removal of extra water and the production of cream.
(We are also using UHT milk which gave us nthe best results in the past, probably because it already has fewer bacteria than fresh milk.)
While I appreciate all the advice and shared experience, I was really looking for a sound explanation of the effect of the two factors that I mentioned in the post, namely the amount of yogurt to seed the fermentation and the intensity/duration of the stirring. QC seemed a better suited venue to get an answer to those questions.