Three cheers for cheese... and microbes!

15 January 2019

Interview with

Bronwen Percival, Neal’s Yard Dairy

Cheese

Blocks of cheese

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We've got cows covered! But, apart from methane, what else comes out of a cow? Milk, of course; and what does milk make? Well the answer is one of our favourite treats! Hannah and Adam from the team gave us a quick-fire introduction to how cheese is made, an art that goes back over 7000 years. Plus Chris Smith was joined in the studio with Bronwen Percival; she’s the Cheese Buyer at Neal’s Yard Dairy in London and is also co-author of the book Reinventing the Wheel - Milk, Microbes and the Fight for Real Cheese.

Hannah - The first cheese was probably made by accident.

Adam -  Historians believe that people discovered the cheesey potential of milk thousands of years ago when they were storing milk in the stomach of calves.  After leaving it there for too long the liquid milk started to clump together.

Hannah - Luckily, cheesemaking has come a long way since then. There are now hundreds of different types from creamy camembert to tangy cheddar. But how are they made?

Adam - The main ingredient for making cheese is milk and for most cheese this means cow’s milk.

Hannah - Although, goat, sheep and even buffalo milk are used too. The first stage of transforming liquid milk into cheese is called souring.

Adam - This is when bacteria - either those naturally found in raw milk or added to pasteurized milk - digest the lactose sugar present in milk and convert it to lactic acid, making the milk sour.

Hannah - This acidic environment helps with the next stage - coagulation - where moisture is squeezed out. Milk is clotted under the influence of an enzyme called chymosin - also known to cheesemakers as rennet.

Adam - Rennet causes proteins in the milk to clump together, which starts to separate out from the watery whey as solid curds.

Hannah - These curds are a 3-dimensional net of tiny, sticky proteins that trap fat globules and bacteria.

Adam - Once the curds have set they are cut into pieces and the water drained away…

Hannah - Depending on the exact process used - and there are a lot of potential combinations of time, temperature, cutting, stirring and pressing that can be inflicted on a vat of curd - the same milk can be transformed into all sorts of different cheeses.

Adam - The curds are mixed with salt, pressed into moulds and left to age and mature.

Hannah - As the cheese ages - the salt, moisture, temperature, acidity and nutrients available in the curds determine which microbes grow and thrive...

Adam - Microbial enzymes break down tasteless fats and proteins into small volatile molecules creating the tastes and smells of the cheeses we love…

Chris - Don't we just! Thank you to Hannah and Adam for that. And now with us to taste our way through the microbiology of cheese is Bronwen Percival, she's the cheese buyer at Neal's Yard Dairy in London. She's also the co-author of the book Reinventing the Wheel - milk, microbes and the fight for real cheese. And very pleased to say she has what looks like a delicious place of cheese in front of her. What have you brought in?

Bronwen  - I have brought you two different goats cheeses from a farm in Staffordshire called Highfields farm dairy. And the thing that makes them really special is that they are made without added microbes, expressing just the microbial communities in that farm's milk, but actually doing it in two completely different ways.

Chris - So when you say just the microbes that are there, so you take the milk from the animal and the microbes that are naturally in that milk are then used to create the process that we've just heard about in order to produce these changes and these flavours?

Bronwen - That's correct. And if you look back about 120 years, nobody was adding microbes to the milk that they were using to make cheese.

Milk is sterile when it leaves the udder of a healthy animal but then it has many many opportunities to be inoculated with naturally occurring microbes from the environment and that could be from the udder, from the skin of the teat -

Chris - from the hands of the farmer?

Bronwen - Absolutely

There are a lot of interesting lactic acid bacteria, the bacteria that then ferment the milk turn the lactose into the lactic acid, that lies at the very centre of the cheese making process, but there are also lots of interesting ripening bacteria that are responsible for a lot of the flavour development as the cheese matures, and many of those ripening bacteria are actually skin bacteria.

Chris - So basically the environment where a lot of milk is collected, stored and cheese is made is gonna become overtime a complete bacterial banquet going on, there’s going to be loads of those microbes. And so a cheese making area is gonna become better as time goes on, there will be  a big enrichment for the right sorts of bacteria. And funghi.

Bronwen - Absolutely. And one of the really interesting things is looking at which microbes dominate in different areas of the farm and then how those manifest themselves on the outside or the inside of the cheese. And so the microbes that are dominating at Highfields farm dairy where these cheeses that we're tasting are from, they might be lactococcus lactis just like you could see at a different farm but chances are good that they’re different strains of the same species of microbe and that those are going to give those cheeses specific different flavors.

Chris - Now if you pasteurized milk does that not destroy all the microbes and would that mean that that milk would be useless for cheese making, you'd have to add microbes back in if you had pasteurized the milk?

Bronwen - So pasteurization is heat treating the milk so it takes out a wide variety of the microbes in the milk. It doesn't kill all of the microbes in the milk though. There are thermoduric bacteria that survive pasteurization and many of the ripening bacteria you find on the outside of certain washed rind cheeses for example are thermoduric  and can have come from the original raw milk.

So it's not to say that it wipes the microbial slate clean, but it definitely decreases the diversity of what's going on in there.

Chris - Well I'm being assailed with these lovely smells that are coming off this plate. Talk me through these two cheeses one of them is a much harder looking cheese and the other one is a round, sausage shaped piece of cheese, so what are the two? How do they differ?

Bronwen - So the hard cheese is a fairly new cheese it's called Highfields and Joe and Amy the cheese makers decided that they wanted to start making it because they were really interested as English cheesemakers in making a typically English style of cheese.

Chris - It's delicious. I’ll tell Joe right now. It's just lovely cheese. Excellent. Hang on Georgia says she wants some.

Georgia - Yes please!

Bronwen - British cheeses really like to have milk that's stored overnight at a fairly warm temperature to allow those native microbes to start multiplying and growing within the milk. We call this pre-ripening and it's very interesting that that is also a technique that's shared by the other style cheese on this plate which we also associate much more with farms in France. So those sausage seep cheese is called Innes Log, quite fresh, quite acidic a little bit of a funky rind, it looks a bit brainy on the outside, and so by taking the same raw material this milk and then putting it through two separate processes you're able to select for different sets of bacteria which then grow within those different substrates and create cheeses with utterly different flavours.

Chris - So basically you're selecting strongly selecting by varying the environments in which you make the cheese and the techniques you used to make the cheese, for slightly different populations of microbes. And it's that shift in microbial population that means that you get slightly different biochemistries, altering and what's the starting material but also adding their own particular flavor fingerprint.

Bronwen -  Exactly. So in just the same way as if you're setting out to make a batch of sauerkraut you need to provide a salty environment and you need to provide an anaerobic environment to select for those microbes.

Georgia - This might be a dumb question but Swiss cheese with all the holes in. Do microbes do that as well?

Bronwen -  They certainly do. The holes in Swiss cheese are caused by a bacterium called propionibacterium and they produce gas during the ripening process and you get those really beautiful round holes.

There are also other microbes that produce gas during the maturation process that can cause defects and sometimes you'll see those styles of cheeses, instead of having holes actually having great big chasms and cracks in the middle of them. And those are caused by clostridium bacteria which can produce hydrogen gas and also butyric acid  -

Chris - Smells like vomit!

Bronwen - It does, it’s the principle flavour component of vomit and you can imagine that the cheeses that have these microbes growing in them during their maturation, they taste like sick and they're completely unsellable not to mention that they have massive cracks in them!

Chris - One quick question that occurs me, talking about selection of bacteria. Why is there such a contrast between cheese that say a cow’s milk cheese and cheese which is say a goat's or sheep's milk cheese? You can tell what you're eating by the flavour profile. Is that because the starting composition of the milk is different or is it because the microbial spectrum is different or is it both?

Bronwen - I would say it's probably both. Cow’s milk has different fatty acid composition than goat's milk and sheep's milk and with goat's milk you can get these fatty acids caproic acid, caprylic acid all from capra which is goat, which have that really goaty taste. If you have really fresh goat's milk where the fats haven't yet started to break down it doesn't taste goaty at all, but the moment you start getting that breakdown you get really goaty flavors and so yes there is a lot to do with the initial composition of of the milk.

Chris - Are people sciencing the hell out of this food? In other words are people now using the power of microbiology and our understanding of biotechnology to spike the mixture with some magic microbes that could make even more exciting flavours, textures, types of cheese?

Bronwen - Yes people have been working to try to develop cultures of bacteria that will bring out specific flavours for years and years and there are already many of these designer cultures on the market.

At the same time it's a really exciting time for farmhouse cheesemakers because there are many scientists out there microbiologists who are looking for model systems to study microbial community formation and they've found that actually unlike the gut microbiota which need very very anaerobic conditions that are difficult to maintain in a laboratory, cheese is a system which all you need is a wine cooler and you can grow your own cheese rinds and look at them in great detail, and start to use them to tease apart some of these basic principles of microbial competition, communication and so on.

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