Editors note: This publication contains a lightly edited transcript of the question and answer portion of the corresponding lecture.

Header image: The local baker since 1885 by the Europeana Foundation (CC BY-SA 4.0)

Q + A :

Question: We have lots of questions on dehydration, specifically. One that came up a few times was at what stage to start the dehydration. Is that a ripe levain? A mature levain?

That's a very good question. What is important here is basically stopping it before we have too much fermentation activity. So what I like to do is, if I feed it, I would start dehydrating it 8 hours after I fed it, because it'll be between that young and ripe phase, so the yeast is still starting to work. We want to make sure that we don't utilize all of that fermentation power for no reason. So we want to stop that as early as possible, but with some initial activity in the back-end.

Question: And then what temperature? You said you dehydrate the levain at a low temperature, but is there a specific?

Yeah, so there's a couple of things you can do here. If you have a dehydrator—they're not very expensive and they're multipurpose, so you can use it for many things—I put it at the lowest setting, which is around 36 degrees Celsius. In Fahrenheit, it's around 95 degrees. So the lowest temperature.

If you don't have a dehydrator, don't use an oven unless it has a pilot. That would be the only way to do it, but I don't know how many oven still have pilots. The other alternative is to put a fan blowing directly on top of the starter. The thinner you spread it, the better. It'll dry faster the thinner it is.

Question: Once you have it in its pulverized form, how long can that last?

In the fridge? In our tests, it lasted six months in refrigeration. Don't freeze it. You might be tempted to freeze it, but again, yeast and freezing—it's never a good idea. We want to keep those microbes around.

Question: Going back to the stages of a levain, are there pH differences between a ripe, young, and mature starter?

Yes, absolutely. When you just fed your levain, the pH is at around 6, 5.5. But as it starts to ferment, the lactic acid bacteria is going to be producing more acid. So as it starts to ferment the acidity is going to increase.

The lowest pH that we've recorded is 3.6, I believe, or 3.7. After that, there's just no fermentation activity. So you want to use it anywhere between 4.1 and 4.4.

Question: Stephen and Clara are asking about flavors in your sourdough. Do you add flavors into it? And then Steven was waiting for the chocolate and cherry sourdough recipe as well.

The chocolate and cherry sourdough recipe is available on our website. I don't like saying what's my favorite recipe in the book, but it's in the top 10. I could eat an entire loaf of the chocolate cherry sourdough. I mean, it's amazing.

And could you flavor your sourdough? I mean, you can flavor it with other flours. I suppose you could add tea and so forth, but it's better to add that to the dough. Tea can have some effect on the dough that will actually slow down fermentation activity. Or if you add coffee, coffee is alkaline, so it's going to affect fermentation as well.

If I'm going to add flavors, I'd rather add them to the dough. And I'd rather keep my sourdough starter—my levain—as pure as possible with just water and flour. You can do different mixes of flours. You can get creative with that. It's fine. But I would add my more pronounced flavors in the mixing stage.