World-Class Tips For The Home Fermenter

27:42 minutes

someone bottling jars of pickled foods on a table
Credit: Shutterstock

logo that says 'best of 2019'If you’ve ever tried brewing your own beer or raising your own sourdough, then you’ve been acquainted with the process of fermentation. You also know that it’s not easy to get right. How do you control the growth of mold, yeast, or bacteria such that it creates a savory and delicious new flavor, and not a putrid mess on your kitchen counter? But, if you have the patience for trial and error (as well as a bit of luck), you now can seek the wisdom of world-class fermentation experts.

David Zilber is director of Fermentation at Noma, currently rated the second best restaurant in the world by the World’s 50 Best list. Noma has distinguished itself as a creative leader in fermented food, experimenting with dishes like lacto-fermented fruits and vegetables. And Zilber, along with Noma co-owner René Redzepi, tell their fermentation secrets and story of their “accidental success” in the new book The Noma Guide to Fermentation. So, once you’ve mastered beer and bread, why not try fermenting blueberries? Your taste buds will thank you.

Further Reading

  • Read an excerpt of The Noma Guide to Fermentation.

Segment Guests

David Zilber

David Zilber is the co-author of Foundations of Flavor: The Noma Guide to Fermentation (Artisan Books, 2018). He’s the Director of Fermentation at Noma in Cophenhagen, Denmark.

Segment Transcript

IRA FLATOW: This is Science Friday. I’m Ira Flatow. If you’ve ever tried brewing beer at home or baking your own sourdough or pickling cucumbers, then you are familiar with the process of fermentation, and you know that it’s not so easy to get it right.

How do you control the growth of mold or yeast to bacteria such that it creates a savory and delicious new flavor and not a putrid mess on your kitchen counter? It takes a little bit of trial and error, a little bit of luck, and a little help from some culinary experts. And that’s who we have with us today.

We want to know what is the strangest thing you’ve ever tried to ferment at home and how did it go for you. If you have a story about fermentation fail, something that didn’t work, something that worked great, well, like to hear that too. Give us a call. Our number 844-724-8255, 844-SCI-TALK, or tweet us @scifri.

One of the reasons I’m asking is that Noma Restaurant, currently rated number two in the world, has distinguished itself as a creative leader in fermented dishes, experimenting with things like lacto-fermented fruits and vegetables, and they’ve written down what they’ve learned in a new book, The Noma Guide to Fermentation. David Zilber, director of fermentation at Noma Restaurant, and author of the book The Noma Guide to Fermentation is here. David, welcome to Science Friday.

DAVID ZILBER: Hi, thanks for having me.

IRA FLATOW: There’s a photograph of a guy doing all this stuff in that book, I’m imagining. Is that you?

DAVID ZILBER: That is me. That is me, yes.

IRA FLATOW: Seems like you’re having a lot of fun with this.

DAVID ZILBER: Yeah, I mean, the photoshoots to make the book were a lot of fun. We had to make everything three or four times– for three or four times over, yeah.

IRA FLATOW: How did you get interested in fermentation?

DAVID ZILBER: Well, I’ve been cooking in professional kind of high-end kitchens for probably about 15 years, and the first restaurant I ever worked in was an Asian restaurant. And we did make our own kimchi there, and we cooked with lots of really amazing soy sauces imported from Japan. But I never really gave it second thought that these were anything more than just ingredients that you would grab off the shelf. But when I got to Noma in 2014, the chef at the restaurant, my boss, Rene Redzepi, kind of noticed that I had a knack for science.

People would just ask general questions like, why isn’t this cabbage the same as it was yesterday? Or how come the bone marrow wasn’t roasting just as well as it was last week? And I was usually the guy with a far too detailed answer. And I think he caught on that I had a knack for the sciences, and then he moved me into the fermentation lab. And I took over the reins after a couple of years.

IRA FLATOW: Well, you certainly have a geeky knack for it because you explain it all very, very well, and you certainly enjoy it. So there is a lot of science and chemistry going into fermentation. There is, but I will preface that statement and say that everything assigns them chemistry if you look closely enough.

Yeah, well, that’s true. So let’s talk about what’s going on. What is going on in the fermentation process that makes– you start out with something, and you wind up with something else. What’s going on there?

DAVID ZILBER: Yes, that’s exactly it. Well, the most succinct way that I can define fermentation in layman’s terms is that it’s the transformation of one ingredient into another by way of a microbe. So if you imagine that you start out with cabbage and then you get lactic acid bacteria to grow in and alongside your cabbage, in two or three weeks’ time, you end up with sauerkraut. It’s not the same as it was going in. You’ve cultivated– cultured, really– this microorganism in your container with your cabbage, and lo and behold, a transformation is taking place.

IRA FLATOW: Well, for some people, though, the transformation just turns into rotten food.

DAVID ZILBER: This is true, yeah.

IRA FLATOW: So where is the fine line? And you say there is a fine line between rot and fermentation, and then you make the analogy to an actual line outside of a nightclub.

DAVID ZILBER: Yeah. Yeah, and the rest of the analogy is that as a fermenter, there’s actually three people in play in the definition of fermentation– the ingredients, the vegetables or the foodstuff, the microbes, but also the person who’s acting on that situation and actually willing the ferment into existence. So as a fermenter, you’re kind of like the bouncer outside of a nightclub. You’re the guy with the velvet rope, the big muscly dude, and you’re the one deciding who gets into the club and makes a great evening where everyone’s sipping champagne and beautiful people all around. And all the drunkards and rowdy boys stay outside.

So that velvet rope that you use as a fermenter– those are all sorts of control points, whether that be salt or access to oxygen or temperature or pH and acidity levels. These are all things that you have at your disposal as a fermenter to make sure that you’re actually fermenting and not rotting. Rot’s a club where everyone gets in. Fermentation is one where the party’s popping.

IRA FLATOW: Interesting. Let’s talk about lactic acid bacteria and lactic acid fermentation. Take us through that.

DAVID ZILBER: Well, fermentation– it’s one of the simplest fermentation processes you could undertake. By adding a little bit of salt to– let’s say, we’re taking a very simple ferment like sauerkraut. You have your cabbage. You shred it to rupture the cabbage cells, and it makes it easy for bacteria to get inside there.

Now lactic acid bacteria are all around us. They live on your skin. They’re on the fruits of vegetables. So they’re on the skins of fruits and vegetables. They’re basically ever-present in our environment.

And as you add salt to that shredded cabbage, you’re making sure that any malevolent microbes– things that might cause the mixture to rot– are kept at bay. Salt is a really great anti-microbial, but lactic acid bacteria have a little bit of resistance to it. They can tolerate salt up to a certain point. And you so you kind of clear the playing field for lactic acid bacteria to do their thing. They start consuming the carbohydrates and sugars in that cabbage.

And in doing so, they leave something else behind, and that’s something else is an exclusionary chemical. That’s lactic acid. It sours the mixture and then makes it even harder for different things to grow.

And over time, that fermentation process peters out. They consume as much sugar as they can. The page drops because of all of the lactic acid they’ve produced, and you have sour cabbage– literally translated from German sauerkraut.

IRA FLATOW: Interesting. I want to ask you a question that you ask yourself in the book. And what is it about cabbage that lends itself so well to fermentation?

DAVID ZILBER: You know, that’s a very good question. It comes up time and again in independent centers of agricultural production. But you go back through history, and the Koreans were doing it with kimchi. And people have been doing it in Europe for hundreds of years.

I think it is one of these hardy vegetables that’s harvested in the late summer and fall that was neither too sweet nor not sweet enough to ferment. And it just keeps really well. So it made sense that people would make sauerkraut or kimchi– sour cabbage in any form to have food for harsher times for winters and things like that.

IRA FLATOW: Right. Got a tweet from Maggie who says, what’s the difference between pickled and fermented?

DAVID ZILBER: So anyway, you break it down, a pickled product is fermented. Now, there’s two routes, typically. You can either do a quick pickle, which is making vinegar, and then boiling your vinegar with a bit of salt and sugar and spices and then pouring that over your vegetables, or you can sour your vegetables into a pickle. Now the difference is that there’s two different acids that play in there. With a quick pickle, with a vinegar pickle, you’re using acetic acid, but with a sour pickle, you’re using lactic acid.

So a vinegar pickle, you have to first make the vinegar, and that is the sugar of fruits first transformed into alcohol by yeast. And then another fermentation process happens. You have acetic acid bacteria– another ever-present bacteria that’s floating on dust in the air that’ll settle into an open bottle of wine and eventually sour it into vinegar. That gets poured over your vegetables, whether that’s carrots or radishes or cucumbers, and the pH drops so much so that it’s effectively preserved.

Lactic acid fermentation, the sour pickle, that’s the process I just described with sauerkraut, and you’re getting it all to happen at once. You’re getting those bacteria to grow in and around the vegetable you’re looking to ferment, and it sours the brine. It sours the plant matter itself. And in one shot, you have a pickle that you can keep in your fridge for months.

IRA FLATOW: I do that with my pickles. They last for a long time. Let’s go to the phones– Kathy in Orlando. Hi, Kathy.

KATHY: Hi, there.



KATHY: Hello.

IRA FLATOW: Hi, go ahead.

KATHY: Hi. Hello.


KATHY: This is Kathy.

IRA FLATOW: Go ahead, Kathy. Can you hear me?

KATHY: I sure can. Hey, we would make several crocks of sauerkraut in the fall, and the house either smelled wonderful or horrible depending on how much you like sauerkraut. But it was so much fun every day going down and snitching a little bit to see how the taste was different every single day.

And then later on when I read it learned about kimchi, I thought I could do that. I just got some of the Korean hot peppers spice and made that, and that was good. My friends that are Asian, a lot of them have a separate refrigerator so that it doesn’t make everything in the refrigerator smell like kimchi.

IRA FLATOW: Thank you for that call. And you talked, David, in your book about actually creating a homemade place to do the ferment.

DAVID ZILBER: Yeah, you can absolutely build fermentation chambers, or you if you have the space, you can make a little cellar. I mean, Korean grandmothers in South Korea, they have urns that they just bury into the soil in their backyard and actually helps to mediate the temperature to make sure that their cabbage doesn’t get too hot during the winters and stays nice and cool. But in building a fermentation chamber or a cellar or any place to really ferment– I mean, at Noma, we have a lab.

But it could just be a closet– what you’re really trying to do is create an environment. Just like there’s penguins in the Antarctic and birds of paradise in the tropics, every type of bacteria has an environment that’s best suited for its life to thrive. And as the fermenter, it’s really your job to try and make that environment is well suited to the microbe you’re trying to cultivate in whatever food you’re trying to ferment.

IRA FLATOW: Here’s a tweet that came in. Does fermentation always produce alcohol as a byproduct?

DAVID ZILBER: No, it does not. There are many different types of fermentation, and some types of fermentation have nothing to do with alcohol at all. Now, a biochemist might say, technically, that’s wrong because the very strict textbook definition of fermentation is the transformation of glucose into ethanol in an enzymatic pathway by yeast. But in the real world, in a much broader sense, as I said, there’s all sorts of different metabolites or byproducts that you end up with in fermentation. Sometimes, that’s sugar. Sometimes, it is MSG or the actual flavor of a umami. Sometimes, it’s alcohol. Other times, it’s acids. So there’s a whole flight of different end products in the world of fermentation. And the more you understand it, the more you can actually kind of paint with these flavors and really tweak the world of food to your will.

IRA FLATOW: I’m Ira Flatow. This is Science Friday from WNYC Studios, talking with David Zilber, author of The Noma Guide to Fermentation. He is also director of fermentation at the Noma Restaurant in Copenhagen. Beautifully illustrated book and one of the things I love about it is that you don’t take for granted that people know how to follow instructions. You actually have very detailed photos to show us how to do it correctly.

DAVID ZILBER: Yeah. And that and that’s important because you can read a recipe. Someone might talk about an experience they’ve had anecdotally, but there is something to actually seeing the texture of something, seeing how much pressure you need to squish a pea between your fingers to know that it’s perfectly done. And that’s something that we didn’t want to skimp on. We wanted people to be able to ferment successfully because there is a lot of hazy– there are a lot of hazy definitions out there and kind of folklore around this topic. And we just wanted to kind of clear the air and say, oh, this is how you can do it really well and really easily.

IRA FLATOW: Let’s go to Patrick in San Francisco. Welcome to Science Friday. Hi there, Patrick.

PATRICK: Hi. Hi, how are you guys?


PATRICK: Thanks for taking my call. First off, David, big fan your book– absolutely love it. I was here in San Francisco when you guys debuted it. It’s really amazing what you’ve done there. So thank you for that and congrats on number two best restaurant in the world. It’s just [INAUDIBLE].

IRA FLATOW: Patrick, I’m running out of time here, so you gotta come up with a question

PATRICK: Sorry, sorry. I guess do two questions. One is, what are you most excited about in fermentation? Are you guys going deeper into lacto-fermentation or any of those avenues that you’ve discovered or described in the book? Or is there a new type of fermentation that you’re venturing into? And then the second question is, as a citizen food scientist, what recommendations for somebody who is venturing into fermentation and going deep? What recommendations you have for somebody like myself?

IRA FLATOW: Let’s work backwards on that. What about the novice– The best way to start?

DAVID ZILBER: For the novice? Start with the things you like eating before you try and make something that you’ve never really had before. Before you try and get into the first half of the process of making the soy sauce, start with something that you really like eating.

If you love pickles on your hot dogs, make pixels for the first time. It’s really that easy. It’s something you can do on your kitchen counter, and you can watch it happen before your eyes. For a citizen scientist who wants to go a little bit deeper, I think it’s really fun to take up like craft brewing and really try and understand the world of yeast, which there are like tens of thousands of different varieties that all have these different flavor profiles.

And the coolest thing about fermenting at home and really getting into it and getting really nerdy with it is that you almost get to taste places on Earth in your own garage or in your own apartment. You can get yeast from Belgium and taste a piece of history because these yeast have been cultivated in the rafters of Abbys that the Belgian monks are famed for making their beers in. So it is really cool to get very nerdy and go really deep and taste the whole pantheon of flavors that the microbial world produces. And as for your first question about what I’m really excited about, to be honest, I’m really excited about the thing I haven’t found yet.

There’s a lot of things that people in the world of fermentation know really well. That’s because all of these ferments that we consume on the regular– whether it’s chocolate or coffee or pickles or wine– these are all very traditional products that have been passed down through generations for hundreds of years. That’s why we still make them today. But in the same way that that makes fermentation amazing, I also think about the way pharmaceutical companies send out teams of scientists into the Amazon jungle to find a rare type of mushroom that might produce some sort of miracle drug that will change the face of the pharmaceutical industry. I wish there was someone like that in the world of fermentation looking for that rare microbe that would produce a flavor that no one’s ever tasted yet.

IRA FLATOW: Really better know what they’re doing. We’re going to come back with David Zilber, author of The Noma Guide to Fermentation right after this break. Stay with us.

This is Science Friday. I’m Ira Flatow. We’ve been talking about the magic of fermentation and experimenting with it in your home kitchen with my guest, David Zilber, director of fermentation at Noma Restaurant in Copenhagen and he’s co-author of the new book, great book, The Noma Guide to Fermentation. Our number, 844-724-8255 and we have been taking questions not only online, but we also have a new app called the SciFri Vox Popp.

And you can join in the conversation for searching. You can search for that app. You can join in, get it SciFri Vox Pop– V as in Victor O-X-P-O-P-P. And you can download that and leave us a question as a listener from Japan did. And he had this to say about kombucha.

NOVA: I’m Nova from Japan. Kombucha is very familiar among the Japanese. It’s very useful not only drink, but also to use a lot of cooking. We also eat natto fermented soybeans. That’s very sticky.

IRA FLATOW: What exactly is kombucha, David? I have a little–

DAVID ZILBER: Kom– yeah?

IRA FLATOW: I actually have a little bit of it here in a cup, which smells a little vinegary. What is it?

DAVID ZILBER: Yeah, kombucha is a sweet and sour microbial tonic. I guess you could call it. But folklore goes back to an ancient Korean physician that would travel around Asia. Again, I don’t even know when in history this would have taken place.

But this physician would brew this drink and kind of heal people with it. Kombucha is basically sweetened tea that is then fermented in a symbiotic way by yeast, which converts the sugar into alcohol and then acetic acid bacteria that convert that alcohol immediately into acetic acid, the acid that you taste in vinegar, like white vinegar. Now if you drink kombucha you buy it off the store shelf, sometimes you might taste really vinegary. And that’s probably because, well, in my opinion, it’s overfermented.

The thing that you have to understand about fermentation is that fermentation is cooking. It’s just cooking that happens much more slowly. So just in the same way that you can overcook a piece of chicken by roasting it in a pan for too long, you can also overferment something like a combustion and make it too sour by letting it just ride out on your kitchen counter for three weeks instead of two. And sometimes, if you taste combustion you’re like oh it’s a little hard to get down, try making it yourself with some of the guidelines in the book, and you might find that it’s actually really, really pleasant to drink.

IRA FLATOW: I’m going to try coffee–

DAVID ZILBER: We make coffee kombucha.

IRA FLATOW: Yeah, that’s the one I saw.

DAVID ZILBER: Yeah, we make coffee kombucha.

IRA FLATOW: I want to try that one– coffee kombucha.

DAVID ZILBER: It’s so good. Mmm.


IRA FLATOW: Summertime drink?

DAVID ZILBER: We serve it to our guests in the summer as a chilled kind of after-coffee drink for the meal. And people love it. I drink it everyday instead of hot coffee in the restaurant. I’m a little addicted to it.

IRA FLATOW: The kombucha bottle– it says organic and raw probiotics. It’s good for you. Are there live probiotics in kombucha?

DAVID ZILBER: There are. There can be. Kombucha can be pasteurized just like milk can be pasteurized or canned goods can be pasteurized. You can heat it and kill everything in it not really affect the taste that much.

If they say that there are live cultures in it, it means that it was fermented, and nothing was really done to it after it was put into a bottle. Now there’s a lot of conflicting information about kombucha out there, and I’ve read a lot of pretty hardcore studies that say, well, a lot of this is a bit bunk. But at the end of the day, I’d probably say the drink kombucha is better for you than drinking a can of Coca-Cola– so yeah.

DAVID ZILBER: Let me go to the phones because we have so many people before we run out of time. Let me take a couple of calls. I’m going to go to Robert in Cleveland. Hi, Robert.

ROBERT: Hello.

IRA FLATOW: Hi there, go ahead.


ROBERT: Couple of questions real quick here– I’ve made sauerkraut, and sometimes I’ve made sauerkraut with some other vegetables mixed in. And what I would do is I put it in a large jar and put a big cabbage leaf on the top and a stone on top of that to hold it down below the brine. And sometimes I’d get a significant amount of mold on top of that, which I would just take that top leaf off and throw it away with the mold. Another time, it’s not so much.

And I’m just wondering is the variable the temperature that it’s sitting around in? Usually, it’s in room temperature for a couple of weeks. But also just fermented foods in general– are they really good for you or is that just how they used to preserve food and we just keep doing it?

IRA FLATOW: Good questions.

DAVID ZILBER: Yeah, as for the mold question, now that is something that you are constantly trying to fight back when you’re trying to ferment, especially lacto-ferment in something like a crock. There are so many variables that go into making a successful ferment. How clean was your vessel before you put the food in there? How clean were your hands or your utensils?

How much salt did you use? How old was the cabbage you were even trying to ferment in the first place? Every little detail is basically another variable in the equation that leads to fermented product being amazing or terrible, and it is a little bit like chaos. It’s a little bit like a butterfly flapping its wings in Thailand and causing a tornado in Ohio.

But with lots of practice, you’ll begin to understand that OK, if it was 30 degrees that day, maybe things were getting a little bit too active. Maybe the fermentation was happening a little bit too quickly. Maybe I opened it a couple of times more than I should have, and it was open to the air instead of being covered.

So there’s lots of variables. But I would say that if you’re having a lot of trouble with mold, just up the salt percentage by a couple of percent. You’ll make for a slightly saltier sauerkraut, but it’ll actually help to keep those microbes at bay–

IRA FLATOW: You talk about something called koji in your work. You say we find koji indistinguishable from magic. Experience koji’s brilliance for yourself. You simply need to pop some in your mouth and taste it. What is koji and why do I need to have it?

DAVID ZILBER: It’s the biggest microbe you’ve never heard of. Koji is responsible for everything tasty that comes out of East Asia from China to Korea to Vietnam to especially Japan. It is a mold– a helpful mold called Aspergillus oryzae. It is responsible for turning the starches in rice and barley and all sorts of grains into sugar and turning the proteins in those same grains into the flavor of umami. It’s responsible for soy sauce, for suck–, for rice wine vinegar, for miso, and it can be used in all sorts of novel and inventive ways as well.

But you never see it as the finished product because it usually is kind of the first step in that process. I liken it to the step of malting barley when you make beer or whiskey. That’s basically how ancient Asian civilizations came about that process of turning grains into something sweet that you could then ferment with yeast. They found a mold instead of finding the process of malting, and it is absolutely remarkable for the flavors that it brings to the table itself.

IRA FLATOW: All right, tell me how I would get it or make it or what would be a good way to start?

DAVID ZILBER: Well, there’s a line that I say when people always ask like, where can I eat where how do I just start growing koji? All life comes from life. All cells come from cells. At the end of the day, everything living on Earth today has been an unbroken chain of succession for 3 and 1/2 billion years, and koji is no exception. If your kid wants a golden retriever puppy for Christmas, you have to go and find a golden retriever mom.

And it’s the same for koji– you’re going to have to find a koji breeder and actually get some spores from it. We buy ours from a laboratory in Japan. And we have it shipped over to Copenhagen. But that’s one of the funnest parts about fermenting is that once you start making fermenter friends, people are just sharing cultures and having a good time, and you get to taste a little bit of someplace else.

IRA FLATOW: And you have to keep the cultures alive or else they’ll die out?

DAVID ZILBER: Koji can coast for a while. When you throw it into its next-generation, when you get it to produce basically offspring in the form of spores, those spores can actually be stable for six months to a year or even longer in the freezer. So once you produce the spores, you can hold onto them and then just keep them on hand and then make your actual koji by growing this mold on your barley or on your rice whenever you see fit. And it grows pretty quickly too. It only takes 48 hours to completely do its job.

IRA FLATOW: So you can get cozy with your koji

DAVID ZILBER: We actually tuck ours in with a blanket at Noma. So yeah, we do get cozy with koji.

IRA FLATOW: I can’t top that, and we’ve run out of time. David Zilber, director of fermentation at Noma Restaurant in Copenhagen. And if you want to see all these great recipes well-illustrated, it’s a wonderful big book. It’s The Noma Guide to Fermentation. It starts out with the really simple. You can just ferment some plums or blueberries. It goes up all the way up to koji and other kinds of stuff. It’s a terrific book. Thank you, David, for taking time to be with us today.

DAVID ZILBER: Thank you so much for having me.

Copyright © 2019 Science Friday Initiative. All rights reserved. Science Friday transcripts are produced on a tight deadline by 3Play Media. Fidelity to the original aired/published audio or video file might vary, and text might be updated or amended in the future. For the authoritative record of Science Friday’s programming, please visit the original aired/published recording. For terms of use and more information, visit our policies pages at http://www.sciencefriday.com/about/policies/

Meet the Producers and Host

About Katie Feather

Katie Feather is a former SciFri producer and the proud mother of two cats, Charleigh and Sadie.

About Ira Flatow

Ira Flatow is the host and executive producer of Science FridayHis green thumb has revived many an office plant at death’s door.

Explore More

Talk To Science Friday Using The VoxPop App

We’ve launched a new way for you to talk to us all week long: the SciFri VoxPop app.

Read More

The Eerie Glow Of Blue Ghost Fireflies

From afar, these fireflies glow blue-white instead of flashing yellow-green.

Read More