IRA FLATOW: This is Science Friday. I’m Ira Flatow. We’re talking cheese this hour. And for you cheese aficionados, you know that pairing cheese with the perfect wine or beer can really enhance the flavors of your fromage. A merlot brings out the mushroomy-ness of a nice gouda, or an amber ale with a sharp cheddar. I’m getting very hungry here. But if you’re a cheese geek and you really want to understand what makes those cheese tastes and textures you need to match your cheese with its microbes. Yes, microbes.

My next guest says that 10 billion– 10 billion microbial cells live on just the rind alone, talk about flavor enhancers. She’s even sequenced the colonies on the rind of 160 different cheeses. She’s here to walk us through the microbiology behind our favorite ones. Rachel Dutton is Assistant Professor of Molecular Biology, University of California in San Diego. Welcome to Science Friday.

RACHEL DUTTON: Hi, Ira. Thanks for having me.

IRA FLATOW: I’m not sure a lot of people realize that there are bacteria involved in all parts of cheese and cheese making. But you specifically focus on the rind. Why the rind?

RACHEL DUTTON: Yeah. Well, bacteria actually important for pretty much every delicious food that we eat. So whether we’re thinking about cheese or beer or wine, these microbes actually have an important role. And we’re really interested in cheese because it is an example of a simple ecosystem that we can really pull apart and put back together in the lab, and understand how microbes actually build communities and fight with each other, help each other, communicate. And the rind of cheese has this amazing biofilm like structure. So it’s actually a completely separate community of microbes that grows on the surface of cheese during the aging process.

IRA FLATOW: Our number, 844-724-8255. If you’d like to talk about microbes in the rind and cheese and whatever. And in the cheese making you purposely put the microbes to make the rind?

RACHEL DUTTON: So in some cheeses you do. So if you think about a brie or a camembert– those cheeses you actually add specific types of fungi and sometimes bacteria to make the rind of the cheese. So if you look at those cheeses you see this fluffy white surface on the cheese. And so that’s actually all microbes.

So the fluffiness is the penicillium camemberti, which as you can probably tell, was named after camembert cheese. And penicillium gives the cheese some of these sort of mushroomy flavors. And then you also have two other really interesting microbes. One is a fungus called geotrichum. And one is a bacterium called hafnia. And together they produce a lot of the sort of cauliflowery kind of smells and tastes that you get in these really ripe camemberts.

IRA FLATOW: That penicillin is it an antibiotic too when we eat it?

RACHEL DUTTON: It’s not. But it’s sort of a cousin of the fungus that produces the antibiotic penicillin.

IRA FLATOW: Now, there are bacteria inside the cheese versus the stuff that’s on the rind. Does that mean that the inside of the cheese is going to have a different flavor given by those microbes in the ones that are closer to the rind?

RACHEL DUTTON: Yeah, exactly. So there’s the lactic acid bacteria inside the cheese. So those are the species that were used to acidify the milk and produce the fresh curds. And so the closer you get to the rind, the more intense a flavor is often in the cheese. So if you, again, think about something like a brie you often have the cheese starting to get really gooey just underneath the rind so it’s ripening. And that’s because the activity of the microbes growing on the surface– they’re actually producing a lot of the molecules that cause the cheese to ripen. So it’s sort of ripening from the outside in.

IRA FLATOW: I know there are people who are afraid to eat the rind. Are they missing the whole part of the cheese, if you don’t eat the rind?

RACHEL DUTTON: Absolutely. Yeah, so those microbes are incredibly tasty. I encourage people to at least give it a nibble and see what they think.

IRA FLATOW: How many kinds of rinds are there?

RACHEL DUTTON: Well, if you think of different kinds of cheeses, there’s hundreds if not thousands of different cheeses. And we can kind of classify rinds into maybe three major groups. So we have the 3– so the first group would be these bloomy rind cheeses is what we call them. So the brie and camembert style ones.

And then we have the rinds, which during the aging process the cheese makers will actually wash the surface of the cheese with a salt water solution. And those create these really stinky funky flavors in the cheese. And so those are called the wash rinds.

And then the third variety of rind that we think about is the natural rind. So this is where you make the cheese so something like a cheddar and you put into a cave and you just let it be. Let the microbes colonize. Don’t really mess with it. And you have a completely different type of microbial community that forms on these three different types of rinds.

IRA FLATOW: Let’s go to the phones. Because lots of people love cheese and they are interested. First, let’s go to Moscow, Idaho. And Hannah, hi. Welcome to Science Friday.

HANNAH: Hey, Ira. Thanks.

IRA FLATOW: Go ahead. Sure.

HANNAH: OK. So I had a question. My mom growing up whenever we got mold or fungus or any fuzzies on cheese in our home or in our fridge, she would always cut that part off and then we could use it. She would always say oh, it’s just mold. It’s not going to hurt you. Because she would cut it off. I didn’t know where that came from. Does that come from the cheese itself or would that happen in the packaging facility? Or where would that come from?

IRA FLATOW: Good question.

RACHEL DUTTON: Yeah so there’s mold all around us. And there’s sort of wild species of mold that are often closely related to the ones we find in cheeses that are just in your home, say floating around in your kitchen. And so once you slice into the cheese you create this open environment that any micro floating around might want to grow on.

So often when you cut into the cheese and you put in your refrigerator, some of those microbes might start growing on there. And they actually like cool temperatures like your refrigerator. So it’s sort of like a cheese cave. And so that’s probably where they’re coming from.

IRA FLATOW: Is it safe to eat that mold that’s growing on there?

RACHEL DUTTON: I probably wouldn’t recommend eating it. But yeah if you sort of cut it away that seems to be fine.

IRA FLATOW: Let’s go to another phone call. Let’s go to Molly in Graton, Massachusetts. Hi, Molly.

MOLLY: Hi. Thanks for taking my call. So my question is that I have passionately fallen in love with goat cheese over the past five or ten years, the brie goat cheese, the cheddar goat cheese. What is that flavor? I’ve almost stayed away from cow cheese all together.

RACHEL DUTTON: Yeah, so goat milk– so most cheese is made from one of three different kinds of milk. So you have cow, goat and sheep. And they have some similarities, but each of them have different properties that are interesting in cheese making. And goat milk has a lot of these special fatty acids that have these interesting flavor properties. So you have things that are specific to goat milk that have these spicy, tangy flavors that you don’t find in cow or sheep milk.

IRA FLATOW: Quite interesting. Do you have a favorite cheese, Molly? Goat cheese? Molly might be gone.

MOLLY: I mentioned [INAUDIBLE]. But I really love the brie and the cheddar. I like the hard cheeses better than the soft cheeses.

IRA FLATOW: Thank you for calling. So Rachel, what makes a hard cheese versus a soft cheese? What’s going on there that turns its milk soft or keeps it hard?

RACHEL DUTTON: Yeah. So a lot of that has to do with what the cheese maker’s doing to the milk and the curds when they first make the cheese. So if you heat the curds to a higher temperature that will help get rid of moisture. If you press the curds once you’ve made the first wheel, that will also get rid of moisture. So the more moisture you remove at the beginning of the cheese making process the firmer the cheese will be, and the longer you can age that cheese.

So things like the cloth bound cheddars you can age them for a year or more. And that’s because they have a low moisture, which actually slows down the growth of microbes. So you can kind of control how quickly the microbes grow and how quickly your cheese ripens by how much moisture is in the cheese.

IRA FLATOW: I know that’s the veins running through gorgonzola whatever– were they put in there or did the cheese make them?

RACHEL DUTTON: Yeah. So those are really fascinating. So those are due to a microbe. It’s a fungus. Again, it’s a penicillium species. But this penicillium is called penicillin roquefort. So named after roquefort cheese in France. And this particular species of penicillium produces this blue pigment. And it only grows in low oxygen environments.

So when cheese makers make this cheese they actually take the spores of the fungus and mix it into the milk. So it’s all over throughout the milk and throughout the cheese. And then after they’ve made the wheel of cheese they actually take a metal spike and punch holes into the cheese to make these little cracks throughout the wheel.

And oxygen can get into these cracks and that little bit of oxygen is what stimulates the growth of penicillin roquefort. And it starts to grow. It produces these beautiful blue green pigments as it’s growing. And the other interesting thing it does is not just producing this color, it’s actually doing a lot to the flavor of these cheeses.

So any time you eat a blue cheese you recognize that you’ve eaten a blue cheese. Right? It’s a very distinctive flavor. And that’s because this mold penicillium roquefort is breaking down the fat in milk. And it’s releasing these free fatty acids. So some of free fatty acids have the spicy, peppery flavor that you associate with blue cheese. So the more blue mold you have in the cheese the spicier the cheese will be. And then it actually can break those down further and produce these volatile molecules, so very small molecules that we can smell.

And so there’s one of these molecules that it produces called 2-heptanone. And that’s one of the key aromas of blue cheese. So when you’re smelling blue cheese you’re actually smelling the byproduct of this fungus as it’s growing.

IRA FLATOW: Wow. That is complex stuff going on. And I understand that you have even found the reen marine bacteria on some cheeses. How did it get there?

RACHEL DUTTON: Yeah. We don’t know. So on these washed rind cheeses that I mentioned, so where the cheese makers will wash the surface with a salty solution of water, we actually find many organisms, many bacteria that we normally would find in the ocean, which is sort of perplexing. And so we still don’t know exactly where they’re coming from. But we think it’s probably from the sea salt that’s being used in cheese making, or it could be due to the fact that cheese itself is a very salty environment. And these microbes are used to growing in salty environments. So they just happen to find an environment that they really love.

IRA FLATOW: Yeah. Well speaking of loving, I want to remind our listeners we also have a nacho cheese recipe on our website if you want to try that. It’s at sciencefriday.com/nachocheese to go along with all the other cheese stuff we have going on on our website there. And a quick reminder that this is Science Friday from PRI, Public Radio International, talking with Rachel Dutton, Assistant Professor of Molecular Biology UC San Diego. Do you ever get tired of talking about cheese?

RACHEL DUTTON: No. I feel like I have the best job in the world. I get to talk about cheese. I get to study cheese in my lab. It’s wonderful.

IRA FLATOW: I asked our last guest for a favorite cheese. And it was like what’s your favorite son or daughter? It’s impossible. Dare I ask that of you?

RACHEL DUTTON: Yeah. I get asked that a lot. One of things I love the most about cheese is how much diversity there is. So you can go to a cheese shop and there’s hundreds of varieties and every time you can experience a new set of flavors. One of my favorite cheeses is a cheese that we actually study a lot in my lab.

We collect samples from all over the world. And a cheese maker that we work with a lot is Jasper Hill Farm in Vermont. And they make a blue cheese called bayley hazen blue which I love. And it’s sort of our lab rat. We’re very interested in the microbes there. And it’s just a delicious cheese besides being very interesting from a scientific perspective.

IRA FLATOW: Yeah. I’d go get some of that to research too. Let’s go to Portland Oregon with Michael. Hi, Michael.

MICHAEL: Hi, there. My question is about storage of cheese. I’ve always been taught to wrap it in plastic to keep the oxygen out. And then I just was in a fancy kitchen store that had something called a cheese ball where it said air was important to be around the cheese when you’re storing it.

RACHEL DUTTON: Yeah. So because cheese is alive and the microbes in it, if you want it to keep those microbes alive and happy, you want to allow them to have a little bit of oxygen. So if you wrap it in plastic wrap then you’re suffocating the microbes on the cheese.

IRA FLATOW: That’s kind of interesting. Thanks for the call. That brings up the question, can every cheese batch or however you call it be the same? If there are microbes and bacteria and different kinds of stuff floating in the air, isn’t every– if it’s the wheel or whatever– what do you call a batch of cheese, Rachel? A batch of cheese.

RACHEL DUTTON: A batch is used.

IRA FLATOW: And they can all be different? Really?

RACHEL DUTTON: Yeah. So you can have differences depending on the season, where the milk is coming from. But it’s sort of amazing to me how reproducible cheese can be. And so that’s part of why we’re interested in studying these microbial communities that form on the surface. Because you can think about it as humans over thousands of years have figured out ways to very precisely manipulate how microbial communities form.

And we know exactly the conditions to create on the cheese. And those conditions will allow the growth of a certain type of cheese rind versus another. So you can have subtle variations. But if you zoom out a little bit it’s kind of amazing how reproducible they are.

IRA FLATOW: All right. Here’s my last question is, what is your holy grail for cheese? What do you need to know?

RACHEL DUTTON: Yeah. Well, we’re really trying to understand how a microbial community is organized. So what I would love to be able to do is figure out how all these microbes are communicating with each other. Are they fighting, are they helping? So what are all the molecules that are being shared amongst the community and how are the species interacting?

IRA FLATOW: Yeah. With all these microbes, do we know how it might interact with our own microbiome? All these cheeses you’re eating?

RACHEL DUTTON: Yeah. So we actually had a study a couple years ago where we fed human subjects cheese and followed the microbes. And actually many of the microbes can survive the gastrointestinal tract. So we’re becoming more and more interested in the interaction of microbes from fermented foods with the human gut microbiome.

IRA FLATOW: Rachel, you’re welcome back any time to talk about cheese.

RACHEL DUTTON: Thanks so much for having me.

IRA FLATOW: You’re welcome. Rachel Dutton, the Assistant Professor of Molecular Biology at UC San Diego. And you can check out our full melting pot of cheese stories– get it– at the cheese spotlight at sciencefriday.com/cheese.

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