"Between fresh and rotten, there's a creative space in which some of the most compelling flavors arise."

That creative space is fermentation, and exploring it is some of the best fun you can have in a kitchen. The quote comes from Sandor Katz, a master fermento, meaning one who ferments things, and author of The Art of Fermentation. He explains how to use culture, time and fat to control the yogurt-making process. [Read the yogurt excerpt from the book.]

Lynne Rossetto Kasper: Making yogurt is a pretty finicky process. Can you explain what the steps are and what's behind them?

Sandor Katz Sandor Katz (Sean Minteh)

Sandor Katz: I love to make yogurt, and I make yogurt pretty regularly at home. What makes yogurt tricky is that the bacteria that are involved are part of a group of bacteria known as thermophilic bacteria, meaning that they are at their most active at elevated temperatures. Yogurt requires some incubation, creating a warm temperature zone to support this thermophilic activity.

The way I do it, which is pretty simple, uses tools that most people have in their household. I take an insulated cooler and I preheat it with warm water; I bring it up to the target temperature, which is between 110 and 115 degrees Fahrenheit. Meanwhile, I take the milk that I want to turn into yogurt and I heat it up even warmer than that; usually my target is about 180 degrees Fahrenheit.

Basically this heating does a couple of different things. First of all, it kills any bacteria that are in the milk so that there's nothing to compete with the bacteria that you're introducing, which are the yogurt bacteria. Then the heating denatures the proteins, which enables them to be restructured by the bacterial fermentation. That's really the magic that reknits a liquid, milk, into a solid, which is what yogurt is. That's what makes it challenging; there's all this temperature manipulation involved.

I heat it up to 180 degrees Fahrenheit, cool it down to about 115 degrees Fahrenheit, then I introduce a small amount of the starter, which is just a mature batch of yogurt. I get it into the incubator and hold it between 110 and 115 degrees Fahrenheit. Four to 8 hours in that range is generally plenty of time to get the yogurt to set, become solid.

Making yogurt Making yogurt (Marisa | Food in Jars)

LRK: How do you maintain that temperature inside the cooler? Because the warm water you put in has been poured out, hasn't it?

SK: I usually actually leave the warm water in. I get it up to the top of that range, 115 degrees Fahrenheit, then I also preheat the jars that I add the milk and the yogurt culture to, and then I usually leave the thermometer in there. Then somewhere after a few hours, I'll just briefly open it up and look at the thermometer. If it seems like it needs warming up, I'll add a little bit of boiling water just to bring it back up a few degrees.

LRK: That's really pretty basic.

SK: Of course there are all sorts of dedicated devices, thermostatically controlled things. Historically people have often used the wood stove, maybe the oven with the pilot light, or even the illuminating light. There are lots of ways you can improvise a warm zone in your home, but that's the way that I found to have the most control over the process without special gadgetry.

LRK: I've made fresh cheese like ricotta and I'm convinced that when I heat the milk as you heat it for yogurt, if I do it too quickly, the cheese is grainy. It just isn't as creamy. But if I heat the milk slowly, there's a difference. Does that heating process make a difference?

SK: It's exactly as you've said. If you heat it up too quickly, you can get a granular texture to the yogurt. If you have the patience to just heat it slowly with a lot of stirring so that it doesn't burn, you'll get the most pleasing results.

LRK: What about the culture? I can use a yogurt -- it would be a plain yogurt -- that I buy in the store?

The Art of Fermentation The Art of Fermentation, excerpted here

SK: You could use any plain yogurt as a starter. The problem is that'll be great for one batch, it'll be pretty good for the second batch, but by the third, fourth and fifth generation, it will be less and less yogurt-like. After wondering about this paradox for years -- because how could a food have gotten passed down for generation after generation if it didn't really have the ability to effectively perpetuate itself -- I finally got ahold of what I would call an heirloom yogurt culture.

It is really different from the ones that are in use by commercial manufacturers. The commercial manufacturers are using starters based upon isolated bacterial strains, a couple of them added together. Whereas the traditional heirloom cultures were literally these evolved communities of bacteria that have a structure to them. Part of that structure is really a defense strategy to protect themselves as a community from other random bacteria in the environment.

These heirloom cultures, which are available online if you search for them, really can give you something that you can use over and over and over again. I would estimate that I've done about 60 generations from the Bulgarian heirloom culture that I've been using.

Microbiologists would call these empirical or undefined starters. Really the bias of microbiology in the field of fermentation is to look at the starters and try to exclude everything that doesn't seem specifically functional. The starters derived from selected strains -- they have just picked out a couple of strains that they deemed to be the ones that are responsible for the flavors and textures that we associate with yogurt. But by extracting them out of the rest of the broad community, you're removing the structure that enables them to be extremely resilient and consistent over the course of many generations.

LRK: Can you control the flavor and texture with the kind of culture that you use, or is it the amount of fat in the yogurt, or is it the time? Tell me how I can make the yogurt I'm dreaming of.

SK: It's everything you've just mentioned. The culture itself has implications for the flavor because cultures that evolved in different places have different microbial communities. Each community has certain unique aspects, which can contribute unique aspects to the flavor.

But time always plays a role because in any fermentation the acids accumulate over time. A longer fermentation always has the potential to be more acidic than a shorter fermentation. If you specifically prefer a sharper flavor, the solution would be to ferment it for a longer time. If you prefer a really mild flavor, then getting the yogurt to set in as short of time as possible would be desirable.

The type of milk certainly also has implications for the flavor and the texture. One technique that I learned from a woman from Turkey is if you want your yogurt to be thick, hold it at 180-degree heat for 15 minutes before you cool it down. With the evaporation that happens during that time, you'll be starting with a thicker milk and ending with a thicker yogurt.

Also you can strain the yogurt after it's fermented, that's how most of the Greek yogurts are made. You take a few layers of cheesecloth or any kind of clean cotton cloth and put the yogurt in it, then hang it, and you'll just have whey dripping out of that. You can make it a little bit thicker, or you can make it a lot thicker -- something like cream cheese or even a harder cheese than that -- all depending upon how long you leave it dripping.

LRK: How about making yogurt from non-dairy milks like almond milk, soy milk or rice milk. Is that possible?

SK: I have had the best results with soy milk. I have gotten the kind of solidification that you get from using milk. You could either use a starter from standard dairy yogurt, or you could go ahead and find a commercial soy yogurt and use that as a starter.

You can certainly make yogurt out of any of those other kinds of milks -- you'll get the acidification and the flavors of yogurt and a slight thickening. But I haven't had good success in getting the others to be solid, which is one of the characteristics that I think of first when I think of yogurt.

Lynne Rossetto Kasper
Lynne Rossetto Kasper has won numerous awards as host of The Splendid Table, including two James Beard Foundation Awards (1998, 2008) for Best National Radio Show on Food, five Clarion Awards (2007, 2008, 2009, 2010, 2014) from Women in Communication, and a Gracie Allen Award in 2000 for Best Syndicated Talk Show.