Instant Genius - Food Science: The fascinating science of cheesemaking

Episode Date: March 24, 2025

Whether it’s the crowning glory on the top of a pizza Margherita, layered between two pieces of bread in a sandwich laced with pickle, or simply enjoyed by itself at the end of a meal, many of us adore cheese. But how exactly is it made, what gives it its distinct flavour and how can we make so many different varieties? In this episode, we speak to Bronwen Percival, a cheese buyer for Neal's Yard Dairy in London, co-founder of the website MicrobialFoods.org and author of the book Reinventing the Wheel. She tells us about the process that leads to milk becoming cheese, why we shouldn’t be afraid of mould and why some cheeses melt so beautifully to give what pizza fans call ‘the pull’. This episode is brought to you in association with EIT Food https://www.eitfood.eu/ Learn more about your ad choices. Visit podcastchoices.com/adchoices

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to learn more. Hello and welcome to Instant Genius, a bite-sized masterclass in podcast form. Every Monday and Friday, you'll hear world-leading scientists and experts talking about the most fascinating ideas in science and technology today. I'm Jason Goodyear, commissioning editor of BBC Science Focus. Whether it's the crowning glory on the top of a pizza margarita, layered between two pieces of bread in a sandwich laced with pickle, or simply enjoyed by itself at the end of a meal, many of us adore cheese.

But exactly how is it made? What gives it its distinct flavour? And how can we make so many different varieties? In this episode, brought to you in association with EIT food, we speak to Bronwyn Percival, a cheese buyer for Neal's Yard Dairy in London, co-founder of the website Microbial Foods.org, and author of the book, Reinventing the Wheel. She tells us about the process that leads to milk becoming cheese,

why we shouldn't be afraid of mould, and why some cheeses melt so beautifully to give what pizza fans call the pool. Welcome to the podcast. Thanks very much for joining us. It's great to be here. So today we're talking about cheese. So first off, everyone will know cheese is made of milk.

But how exactly does milk become cheese? Cheese is pretty incredible because essentially it's a technological process that capitalizes on the fact that milk was really designed to be digested by baby mammals in a particular way. And so when an animal is drinking its mother's milk, the first step of the digestion of that milk is the curdling of that milk into a solid within the stomach with this enzyme called rennet or chymocin. And essentially, that is important so that the liquid fraction of the milk that has all the sugars and the rapidly digestible portions can be absorbed straight away while the proteins and the fats, which require more time to digest, get condensed down into this sort of

a curdie mass that then can stay in the stomach for longer and move through the digestive system. And it's pretty amazing to think that actually that same process is essentially the process of cheese making. You add the enzyme, either an enzyme that's been extracted directly from the stomach of a baby cow or sheep or lamb or a sort of similar enzyme that's been produced in a laboratory that has essentially the same effect. And that allows you to turn this liquid milk, into a solid and then start to remove the moisture from that. And that has the advantage of making it much less perishable, but also when coupled with the process of fermentation,

much more shelf-stable too. So a lot of people say that cheese makers are sort of microbiologists. So what role do microbes play in the making of cheese? So you can make cheese with no microbes at all. It's possible to turn that milk into a curd without the action of microbes. And in fact, there's certain kinds of cheeses like Panir that are made simply using milk and acidity to curdle the milk proteins and create a solid substance that can be taken out. But for most of what we think of as sort of European aged cheeses, microbes are playing a really essential role in that not only is the moisture removed from the milk, but the milk sugars are also fermented in a way that increases. the acidity and actually prevent spoilage. The microbes are both driving the fermentation and

helping the cheese to be something that you can keep for anywhere from weeks to years, but they're also performing an important role in terms of flavor development during ripening. And so there are different sorts of microbial processes that are happening at the same time within cheese. So you've mentioned curds. So people will have heard of curds and weigh, but what exactly are they? So milk has a lot of moisture. It's almost, you know, sort of 90% of milk is water. But then within that, it's a very elegant packaging system for all of the nutrition that a young animal needs. So it's essentially you've got sugars, you've got fats, and you've got proteins.

And if you just had sugars and fats and proteins and mix them with water, what you would very quickly find is that you would get something that looks, a bit like a broken vinaigrette with all the fat floating at the surface and probably the protein sitting at the bottom and really a real mess. But milk has evolved to allow those components to be in this sort of emulsion together very stably. And essentially, when you use this redid enzyme to set that emulsion into a solid, you can start to draw out the moisture and you leave this lattice, this solid lattice of protein in which your globules of fat are trapped, and that is the curd.

And that really nutritive portion is what ultimately goes on to form the basis of the cheese. That's the curd. Obviously, there are hundreds and hundreds of types of cheese, but can we break them down into different categories? It's very difficult to categorize cheese. And I think this is something that has confounded generations of cheese competition judges because really cheeses exist, styles of cheese exist on a sort of spectrum. And where you draw the lines to kind of break that into categories is a kind of a movable feast, I suppose.

And so often, you know, people will classify it. And in fact, if you look at the food and agricultural organization, they've defined it according to how much bath there is in it and how much moisture there is in it. And if you fall on one side or the other of those different certain arbitrary lines, then you're either sort of hard, semi-hard, semi-soft or soft, or reduced fat or full-fat, and that's enough to classify cheeses. But as a consumer, that doesn't make for particularly interesting cheese shopping. And so other ways of categorizing cheeses that might seem a little bit more user-friendly

have to do with the way that they're ripened. Like you might have a cheese that's ripened from the inside out, like a blue cheese, where you've got mold growing within it and it has a very particular flavor profile associated with that versus a surface ripen cheese like a brie, which has a lot of the microbes for the ripening growing on the outside. I don't think that's entirely a great categorization either because it doesn't take into account the actual structure of the cheese itself. So if you were talking with a cheese maker, for example, you might talk about those different makes falling into different quadrants on the square where you're looking at the acidity and the

moisture level of those curds at the moment that they go into the mold. You mentioned that blue cheese. I mean, that's one of my favorites personally. But it's mold that makes it blue. And when we think of mold, we typically think it's something we shouldn't be eating. I mean, you know, whether we classify something as spoilage or whether we classify it as deliciousness is really a cultural construct. And provided it's not dangerous, everything is edible.

And so, I mean, I think this is. one of the reasons why cheese is an interesting sort of food is it sort of flirts with these ideas of what is edible, what is decay, what is disgusting, or what is delicious. And I think that microbial aspect of it has a big role to play in all of that. So yeah, lots of blue molds, I mean, are associated with decay, with, you know, things rotting on the forest floor or even more biological processes. And they're in many cases exactly the same microbes that are playing a part in that. But actually, one of the things, one of the things that those molds are doing, mold spongy bacteria are all very, they've got a lot of enzymes and they're really good at breaking things down.

That's what D.K. is. And in this case, if you create a pellet of curd, milk, proteins, and fats, the process of breaking that down with different microbes can yield all sorts of interesting small volatile molecules that have lots of pronounced flavors and are really delicious. So you mentioned there volatile molecules. A lot of people will say cheese has a distinctive smell. So what gives it that? Well, I suppose I should go on to say I think cheese has many different distinctive smells. And, you know, depending on, you know, a lot of cheesemakers might think of the process of making cheese as adding ingredients, adding microbes to milk as a sort of process.

And then what you add grows there and that gives you your flavor. But I like to think of cheese making more as building a house for microbes. And what you're doing as a cheese maker is creating this incredibly selective environment that picks the microbial winners. And in many cases, it's possible to make any sort of cheese with good raw milk without adding any microbes whatsoever by just taking that milk through a cheese making process that selectively creates a house where the right microbes are going to grow. And in fact, this is the way that cheese was made for, you know, thousands of years before anybody. knew, you know, the microbiology of cheese making or anything like that. Nobody was adding microbes.

They were building a house through processes that had been established through kind of almost infinite trial and error to create these things that turned into good and interesting foods. So what's creating those smells or flavors is really, you know, the microbes that flourish in a given environment as they start to break down those primarily proteins, but also when we're talking about molds, molds are quite lipolytic, so they break down a lot of fats and they can produce volatile molecules from those as well. And essentially, as those molecules go from, you know, as you're breaking down a protein, you have a long chain of amino acids and milk protein, which is called casing, and they don't taste particularly of anything. They're sort of milky and bland. And then as they

break down further through the action of these enzymes, you can often get, particularly in cows milk, quite bitter tasting peptides. And then as the cheese maturation process continues, you get amino acids, which can often have quite pronounced flavors, whether they're sweet or savory or, you know, any, any of these range of flavors. And then you can even break down those amino acids further into other compounds which might have like sensory properties, like histamines, which can be quite tingly or ammonia. So, you know, you're really looking at the process of breakdown in all of the, all of the little tiny molecules, which are, you know, mouths and noses have been adapted to be tuned into. And it can be really interesting and extremely varied.

So let's have a look at the different milks we can use to make cheese. I mean, you have cow's milk, goat's milk, use milk. I'm not aware if any others are used. I think people have tried with just about all the kinds of milks and with greater or lesser success. But yeah, no, you certainly can make cheese out of all those different kinds of milks, different species. if we just talk about cow, sheep, and goat, which are the most commonplace sorts of cheeses here in the UK, is, you know, they have different proportions of fat and protein and sugar. And as a result of that, they behave differently,

slightly differently in the cheese making process. And, I mean, I think a good example of sheep's milk, the solids within sheep's milk are way higher than in cow's milk or in goat's milk. And so as a result from a liter of sheep's milk, you'll get about twice as much cheese. but like basically the process is exactly the same and it's just the little tweaks as you drive that process that really differentiate them from one another.

And then finally I would say you're going to get different flavors from a sheep's milk than from a cow's milk or a goat's milk that are made in exactly the same way because those building blocks, particularly the fats, can be quite different. And you know, we talk about the goatey flavor of a goat's cheese or the kind of sheepy lanylin flavor of a sheep's cheese.

And a lot of that is simply that the, fatty acids within those milks are different from one another, and when they break down, they give those really characteristic tastes. So sticking with milk, how about pasteurization? You often see cheese labeled as unpasteurized or raw milk cheese. What's the difference there? So pasteurization is a really clever technology. Essentially, you know, cheese making is a fermentation process, and that means it's a process where you can grow microbes. In fact, it's a process that's designed to grow microbes. Pasterization is a process that treats that milk to a temperature, very briefly, that's adequate to destroy the most heat-tolerant pathogen that can be found

in that raw milk. And as a result of that, then you can add back the other microbes that you've killed off in the process, like those lactic acid bacteria or maybe some of the ripening bacteria, and go ahead and make cheese confident in the fact that up to that point, any contamination that occurred during milking or, you know, in the pipework up to the vat has been controlled for because those pathogens have been destroyed. So, you know, for the vast majority of the world's cheeses, pasteurization is an absolutely brilliant technique and it makes things a lot safer, particularly where cheeses are made of the mixed milk of many farms. You know, your milk is only as clean as your dirtiest supplier. And so it would be absolutely, you know, irresponsible to make

a raw milk cheese from milk that was pooled from distant sources where you didn't have that level of control. The problem with pasteurization is that in heat treating it to this level, which is 72 degrees for 15 seconds, is pretty hot. You're also destroying a lot of the potentially quite interesting microbes that are beneficial or add to that cheese making process. And the more interesting microbes you have, the more interesting enzymatic collection you have to do that ripening process and the more interesting complex or long flavor that you can get out of the cheese. So if you're comparing a pasteurized cheese with raw milk cheese and that raw milk cheese is made using well-produced, safely produced raw milk that has interesting microbes in it,

the raw milk cheese is going to be significantly more interesting. Yeah, no, I can go into, I can talk all day about that because I think there are a lot of very surprising things about the way in which even raw milk cheesemakers have started to conflate the idea of having clean and safe milk with having milk with a low microbial count, with a low bacterial count. And in the effort to make that milk seemingly safer and safer, they're resorting to more and more sort of ultra-hygienic practices with the aim of decreasing the number of organisms in the milk. The problem with that is that most of those organisms are really good organisms. And when you're getting to the point where your milk is free from bacteria,

you're essentially eliminating the whole reason for making your raw milk cheese in the first place. And so we're treading this very complicated line between making sure that the milk is free from pathogens. But at the same time, not completely destroying the microbial diversity that might be interesting from a flavor perspective, from a farming perspective, because, you know, I don't think anybody thinks now that blitzing everything with anti-microbials, including farm animals and farm environments and everything, is a particularly enlightened or good way of working. And in terms of just, you know, even when we think of things like gut diversity, part of the reasons why people search out raw milk cheeses is because they are an incredible source of microbial biodiversity, whether or not

science is caught up with the theory about exactly the mechanisms by which all of that diversity works in the interplay between the microbes in our foods and the microbes in our guts. We have a long way to go to understand all those mechanisms, but certainly what we have learned and, you know, with the help of a bunch of scientists working really in interesting ways on cheeses in France have established is that actually having a milk that's rich in interesting and good bacteria actually can provide protective effects against the growth of small amounts of pathogens should they occur, particularly just. depending on the type of cheese.

So like everything, it's far more complicated than any of us knows. But one of the things that's the most amazing is that as we learn more through this really sophisticated science, what we see is that the techniques and methods used by our great grandparents, working, making cheese with no idea of what a microbe was whatsoever, were actually quite clever at making something that was probably both safe and delicious. Peak pollination season, and my business is scaling fast. To keep the nectar flowing, I need a phone plan with top priority data speed. That's why I chose GoogleFi Wireless.

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Another of my personal favourites is a really mature cheddar, you know, the deep rich flavour. So what happens when we age a cheese? So cheese maturation can take many for us. And if we're talking about cheese is made on an industrial scale, often the most important aspects of that cheese maturation are going to be not losing any weight and getting it to mature as quickly as possible. So those cheeses might be formed into blocks and wrapped in plastic so that the moisture can't escape.

And then often when the cheeses are being made, they'll have extra enzymes added that will allow the development of like powerful good flavors quite quickly in that process. So you can turn it from milk into cash as quickly as possible. And it works. You know, that that's the same essential process that's going on when you create, say, a really amazing clothbound cheddar that will mature for two years before it's really in its palm. I think the difference when you're looking at your raw milk cheddar is, you know, primarily that breakdown process, that enzymatic process isn't happening from the rind in,

when you talk about these big hard cheeses. What it's doing is taking the enzymes from the bacteria in the original raw milk, but also in the starter cultures, the lactic acid bacteria that have been added to that milk, and which have grown to really, really high levels, you know, in the billions per gram during the cheese making process when that milk is incurred as being fermented. And it's just a very, very slow process of first those microorganisms,

finding themselves suddenly in this, you know, having gone from being in milk to going to this very dry, almost desert-like environment of the inside of this hard cheese, where there's not a lot of moisture, where there's a fair amount of salt, where there's a lot of acidity, it's fairly uncomfortable. And over time, they will die off and they will, you know, lice or open up and empty all of their enzymes into that curd. And those enzymes will just go to work, digesting the fats and primarily the proteins in that curd and developing those flavors. But that because it's such a low moisture environment, because it's, you know, it's a very different environment.

than like a soft gooey cheese where there's a lot more water activity, there's a lot more opportunity for those enzymes than those processes to happen, like to work quickly. It just goes about its way very, very slowly. First, you can feel the proteins breaking down, going from sort of rubbery and curdy and bouncy into something that's much more kind of supple and digested and kind of creamy feeling. And then the flavors develop at the same time. And, you know, with my work, I do a lot of visiting farms and tasting cheeses, tasting those cheddars, that we might be selling it a year and a half for two years when they're nine or ten months old. And part of that process is really being able to taste this and project,

hmm, where is this going? Is the structure right to support further aging? And are there any weird flavors creeping in with that enzymatic breakdown that we are going to find are really disgusting by the time it gets the age that we want to sell it? So you say there that some cheeses are aged for several years. I mean, not that I've ever done this because it's too delicious and I have no self-control. But what happens when you leave a piece of cheese in the fridge and it spoils? Well, again, spoilage is in the eye of the beholder. And I think cheese is very different from some of these other foods that, you know, if you put a piece of ham in the fridge and forgot about it for three weeks and it had microbes growing all over the outside of it, I think everybody would

pretty much agree that, you know, that's food waste right there, but you're not going to want to go in there and eat it. Whereas with the cheese, you know, if a piece of cheese sits around in the back of your fridge, it will probably start to grow a rind, even if it never had a rind in the first place. You know, you see wisps of mold, little dots of blue mold and so forth, and those are just the equivalent processed cheese ripening. Some microbes have found their way there, and they're, you know, sending their enzymes in to digest it and get some nourishment from that cheese. The thing about cheese is, you know, for the most part, you can cut that, like the molds that grow on dairy products are very, very rare.

harmful, like extremely rarely. And they wouldn't grow in your refrigerator anyway. They require high temperatures and long amounts of time. And, you know, it would be very rare to find a mold growing on a cheese that would make you sick. And so because we know that, you can just kind of cut around it and go on, you know, the flavor might have changed a little bit from the enzymes of those molds, particularly if you're cutting very close to where that rind had formed. But it's not going to hurt you. The other thing is if you leave the cheese in the back of the refrigerator and sometimes, you know, my refrigerator is full of cheese aging along. What I often find is that the biggest problem with that cheese is that it's dried out in the really dry environment of the fridge

and that that's caused it to crack or get a little bit waxy. And again, that can be salvaged by cutting the dried part off. But that's not a microbial process. I would just say that's a damaged process. So I love a cheese toasty and you'll often hear the cheese monger say when you're buying a cheese. This is a great melting cheese. So why do some cheeses melt more than others. Ah, see, this is a, this is a simple question that actually, when you look at the sort of physical chemistry of cheese curds is actually really complicated. But essentially, that meltability has to do with two things.

It has to do with how much the proteins themselves have been digested and broken down, because you need those proteins to be more or less, you know, not too broken down, to be able to keep that stringy texture that's going to allow you to kind of get what my four-year-old son calls the pole that really long, stringy, melty, deliciousness that everybody really like. So the proteins can't be too broken down. And it might be one reason why you may have seen, like, if you're trying to make a toasty with a really, really mature cheddar, it doesn't have that same pole. Those proteins have been digested at the point where they just kind of, they kind of break and the oil comes out of them. and they can still taste really good and it's still quite rich,

but you're never going to, you're never going to melt like a super matured cheddar and find that it's going to give you those long, stretchy pizza strings of cheese. So that's one aspect of it. The other aspect of it is how the cheese is made and like how much calcium remains within the protein, like the way the protein is packaged so that it doesn't fall out of solution and milk

involves it being packed together with a lot of calcium phosphate. And when cheese is made, if you have wet curds in an acid environment, that calcium, it's almost like the mortar between bricks and it dissolves out of the curd. And that calcium is actually really, really important for that ability of a curd to stretch. So if you have a cheese where it's very, very high moisture and it's draining, you know, it's wet and high moisture, that cheese is unlikely to be something that's ever going to have that great pull. Whereas if you have a cheese where you're getting the moisture out,

before the acidity develops, then you haven't lost that kind of calcium mortar within the protein structure, and that's much, much more prone to giving you that silky, stretchy texture. So how about vegan cheese then? I mean, is there really such a thing? You know, it's interesting. Of course there is. You can go to the supermarket today and buy vegan cheese. There's different kinds of approaches to making vegan cheese, which I think are all really interesting. There are a lot of people who take, you know, nuts and rehydrate them and then drain the liquid fraction off of that and ferment them. So, you know, nut-based cheeses. I mean, we could get into a long discussion about whether to call them cheeses. But for the sake of,

for the sake of our conversation, let's call them cheeses. And then there's some really interesting sort of startup biotech firms that are working on essentially creating vegan cheeses based on, you know, recombinant protein producing technology. So, essentially, Essentially, you would ferment using a microbial host, a nature-identical casing molecule, just like you would find in milk. And then that would be the basis for the structure of your cheese. The problem with that is that it's more than just protein. You have to figure out how to get fat into that. And again, the fat within milk is just exquisitely, beautifully packaged to fit within a curd structure.

And when you start to try to emulsify plant fat into a protein network, things get super complicated. So I know there are lots of people working in this space because I think being able to make something that was really kind of tasty and had the capacity to break down and produce all those volatile aromas and everything could be really, really interesting. And in fact, you know, I'm totally o'fay with the fact that animal agriculture needs to justify its existence in the world. You know, we can't just float by on the fact that, oh, it, you know, it tastes good and therefore it's fine. Like there are extractive ways to farm animals. and there are regenerative or sustainable or responsible ways to farm animals.

But the truth is that regardless, you know, we should probably be as a society eating less animal protein. And when we do eat animal protein, it should be of the absolute highest quality. And it should be expensive because that's the real cost of producing it to a standard that we'd all be happy with. And so I think some of these ideas about, you know, cheese substitutes that might have a lighter footprint and be able to reproduce cheaply and substitute for, you know, animal-based cheeses on things like, you know, commodity pizzas and so forth.

Super interesting. So, yeah, those are the two main ways that I know of that people are thinking about making these sort of non-animal cheeses. So we've talked about a lot of science and processes there. So let's get on to the big question. What should we look for when we're tasting a cheese? I mean, I think I'd even back up one step and say, how you buy your cheese. is going to be very important to the cheese that you end up with.

And, you know, of course I would say this because I work for a company that buys and sells cheese in cheese shops. But I think, you know, one of the things to recognize, particularly when you're operating within the artisan sphere or the specialist cheese sphere, for lack of a better word, is this idea that each batch is going to be subtly different, partly because the milk that's going into it is changing across the days and seasons, but also because it's made by a person. And you're going to get slightly different, you know, you're building slightly different houses for those microbes every single day. And one of the things that customers and

suppliers really need to be geared up to do is to embrace that batch variation and to buy cheese in an environment where you have the opportunity to taste before you buy. I think it's a really important part of making sure that you're actually a buying the cheese that you like the most, but working within this sort of commodity that actually doesn't behave like a commodity at all. So tasting is really, really important. What should you taste for when? you're buying a cheese, I mean, that ultimately, again, comes down to the definition of, like, personal taste and what you like. For me, personally, I think I like cheeses where I can taste the milk that went into them. And when I say that, I mean that they're complex and that there

lots and lots of different flavors that are going on and that they're long-lasting. And I think, again, when you're buying an environment where you can be having a conversation with somebody, you can ask questions about like how, you know, where was this cheese made? Where, you know, what sort of farming system did it come from? And like, does the cheese make really show the milk? Or are we tasting things like adjunct cultures, which, you know, are these cultures of bacteria that are selected for making very, like, they could be savory flavors. They could be sweet flavors. You know, the crystals that you can find in a lot of those chattas that you can find quite cheaply in the supermarket are something that you can dial in.

They are a function of the way that certain strains of lactic acid bacteria process lactose. And so, you know, if you're going to be trying to find an interesting cheese and if you're going to be spending a lot of money on that cheese, for me, I would want to be buying a cheese where people aren't relying on cheats that would allow a factory working at, you know, a thousand times the scale to get exactly the same result and flavor. The rational thing is just to buy the cheaper cheese. So one final question then. How about putting together a cheese board?

Is there a certain order we should go with? It's really funny. Everybody always asks questions about the rules of how to put together the cheeseboard or the rules of what cheese to taste first or whatever. And, you know, I suppose there are out there in the world a few absolute thermonuclear cheeses that will destroy your palate and not allow you to taste anything for the rest of your tasting session. But I would say in general, those are few and far between. and they also aren't the kind of cheeses I would want to put on my cheese board personally in the first place. So I think the first rule is to relax and choose cheeses that you like. But beyond that, it's all personal expression.

You know, often people will say choose cheeses from multiple different styles so you can get some diversity. And absolutely, if you're looking for a diverse flavor experience and you could do four completely different kinds of cheeses. But some of the most fun cheese boards that I've experienced have been ones where, oh, Here are three different Wensleydale's. Here are three different cheddars. Exploring the sort of nuance between things that are alike but different can be just as interesting. And I guess a lot depends on the sort of social situation where you're having them or if you're just having that cheeseboard for your own lunch or whatever.

But don't feel that there are any hard and fast rules that you have to stick to. Thank you for listening to this episode of Instant Genius. Brought to you from the team behind BBC Science Focus. That was Bronwyn Percival. If you liked what you just heard, then please do consider subscribing to Instant Genius on your preferred podcast platform. If you'd like to see our presenters and guests in person, then also check out our YouTube channel at Science Focus. The current issue of BBC Science Focus magazine is out now. Pick up a copy wherever you buy your favourite magazines or downloaders on your app store of choice.

You can also find us on Apple News or online at sciencefocus.com. This podcast is sponsored by name, audio and focal. The texture and emotional depth of music can be lost through digital sources or poor signal. Name Audio believes you can have digital precision with analog warmth. Alongside French acoustic specialist vocal, Name creates high-end audio systems combining innovation with craftsmanship, so you can listen to music, just as the artist intended. Discover more at name audio.com.

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