Cheese Making and Cake Baking: The Chemistry of Cookery

25 September 2011
Presented by Ben Valsler, Chris Smith.

We've whipped up an appetising take on the science of food and cooking for you this week. With a main course of cookery in the kitchen served up by a cake-baking physicist followed by a microbiological look at the cheese board and then the bacterial basis of the Best Before Date for dessert, this three-course scientific combo is an absolute academic feast. Also on the menu this week, how scientists are using brain scanners to reconstruct the movies we see in the mind's eye and we ask whether Einstein was wrong as scientists report particles apparently moving faster-than-light...

In this episode

01:43 - DEET Scrambles Sense of Smell

DEET is one of the most common chemicals in insect repellents, but since its invention scientists have been unable to settle a decision about exactly how it works. Now, work published in...

DEET Scrambles Sense of Smell

DEET is one of the most common chemicals in insect repellents, but since its invention scientists have been unable to settle a decision about exactly how it works.  Now, work published in Nature suggests that DEET not only confuses scientists, but confuses insects too by scrambling the code they use to identify odours.

DEET, or N,N-diethyl-meta-toluamide, was developed by the US army after the 2nd world war.  It's very effective at preventing mosquitoes and other insects from biting, but research has never shown conclusively how it works; though we expect it is by either blocking the olfactory system from recognising a meal, or triggering an avoidance behaviour response.

A mosquito biting a fingerInsects like mosquitoes, and the intensely studied fruit fly Drosophila melanogaster, detect odours in the air through their antennae.  Each of the many receptors present consists of a protein called ORCO bound to any one of around 60 other proteins.  Odours are detected when they interact with these two proteins, locking together to form a complex and triggering activity in local nerves.  This activity can be either the excitement or inhibition of a nerve response, and it's the particular pattern of activity that encodes a smell.

Leslie Vosshall and colleagues at the Howard Hughes Medical Institute looked at the activity in certain fruit fly nerves and observed what happens in response to DEET alone, and in combination with a range of other odour molecules.

They found that the effect of DEET was not straightforward, but depends on the odour, receptor and concentration.  In some combinations, DEET suppressed a normally inhibitory response, increasing neural activity, while in others it reduced an excitatory reaction, reducing neural activity.  DEET alone actually elicited very little response.

The authors conclude that DEET acts as a molecular confusant, scrambling the normal odour code by changing the way the olfactory receptors react to a given chemical.  The insects are still capable of detecting odours, but can no longer work out what they are.

Understanding more about how DEET works could lead to the development of new repellents, and ultimately help to reduce transmission of insect borne diseases such as Malaria and Dengue fever.

04:33 - Predator-Prey role reversal: hunter becomes hunted

The hunter has become hunted. Israeli scientists have discovered a species of beetle larva that uses its antennae to attract amphibians much larger than itself before using razor-sharp jaws...

Predator-Prey role reversal: hunter becomes hunted

A beetle larva that waves its antennae to attract amphibians Toad and Epomis circumscriptusbefore grabbing them, drinking their body juices and reducing them to a dessicated husk has been discovered by scientists in Israel.

Small beetles and larvae make an ideal toad and frog-sized meal, and these amphibians are well-adapted to stealthily creeping up on them and using a lightning-fast flick of the tongue to turn them into lunch.

But in a rare example of role-reversal, predator has become prey. Writing in PLoS One, Tel-Aviv University researchers Gil Wizen and Avital Gasith have discovered that two species of Epomis ground beetle larvae found in the Middle East, E. circumscriptus and E. dejeani, exploit the very same movements that attract frogs and toads in the first place - by waving their antennae around - only to dodge the predators' attempts to eat them and then grab on to the amphibian themselves. The larvae are equipped with razor-sharp mouthparts, which they use to pierce the amphibian's skin and then drink the body fluids. In some cases they even consume the unfortunate victim's flesh.

The researchers began to study the Epomis beetles in 2007 after finding specimens of the larvae attached to amphibians in the wild. But how this status quo was reached - with what should be the hunter being consumed by the hunted - was not known.

To find out, in hundreds of experiments the research duo placed Epomis larvae mandible (thumbnail)one or more of the larvae into a tank together with a suitable amphibian study subject and videoed the results. The footage shows the larvae increasing the intensity of their antenna-waving activities as the potential victim approaches. When the amphibian makes a grab, the larva dodges the lunge before parrying with an attack of its own, in some cases grabbing the mouthparts and on other occasions latching on behind the head. With an hour or so the amphibian is dead.

The success rate of the strategy is staggering. Out of about 400 face-offs, the amphibians got the larvae into their mouths only 7 times. But even then only transiently: they quickly spat them out, at which point the larvae promptly turned around and made a succesful attack of their own. In one even more surprising experiment, a toad did swallow one of the larvae, which continued to writhe around in the animal's stomach for a further two hours before it was regurgitated. At this point, apparently unharmed, the larva then make a meal of the toad that had previously consumed it.

This kind of role-reversal is extremely rare. In only about 10 percent of predator-prey relationships in the animal kingdom does a smaller animal eat a bigger one, but these are all active attacks rather than a small creature luring its prey. Wizen and Gasith thing that it could have evolved as a means of anti-predator defence, which has subsequently become the creature's exclusive lifestyle. The amphibians may not have evolved to combat the threat since, compared with the majority of the their food, for which their hunting strategy is highly successful, these beetle larvae are relatively rare...

07:52 - Movies of the Mind

Successfully decoding and reconstructing of the visual images experienced by volunteers viewing a sequence of Hollywood movies may lead to communication with brain injured patients and even...

Movies of the Mind
with Jack Gallant, University of California, Berkeley

Emily Seward - Scientists have successfully decoded and reconstructed the visual images experienced by volunteers, viewing a sequence of Hollywood movies. Scanning the brain using functional magnetic resonance imaging, fMRI, they matched up how changes in the moving images correlated with changes in brain activity. They were then able to reconstruct the visual images experienced when viewing unseen movies. Published in Current Biology by Professor Jack Gallant and his colleagues from the University of California, Berkeley, this work may lead to communication with brain injured patients and even being able to watch your own dreams like a video.

Jack - The goal of our laboratory is to build a computational model to describe how your brain processes visual information. And of course, in the real world when you're walking around, most visual information is dynamic. You see things moving, you move through the environment and so we want to be able to understand how the brain processes this dynamic information. We came up with a computational model that allows us to predict brain activity to new movies and they allow us to actually decode brain activity and sort of reconstruct a coarse representation of the movies you saw.

Emily - Their experiment involved two stages using three volunteers.

MRI machineJack - So this gave us a very long list of individual movies. They were short segments of movies, like Hollywood movie trailers, that were 10 to 20 seconds long, along with the brain activity, and that told us how the brain responded to individual shapes as they were moving through the displays.

Emily - As the scanner recorded their brain responses to the movie information, a computer program matched up how changes in the moving images were correlated with changes in the brain activity. Feeding this into a computational model enabled the researchers to create a dictionary that could be used to decode how the brain responds more generally to moving shapes. This can then be used to create predictions of what the brain is seeing.

Jack - In the second part of the experiment, we had people go back in the magnet and we showed them a different set of movies that they haven't seen before. We used the computational models to predict what movies they were most likely to have seen and then the computer basically tried to build a reconstruction of what they actually saw.

Emily - Based on what it had learned from the initial training sessions, the computer was asked to predict what the subjects had been watching. And then using a hundred clips selected from over 18 million seconds of video footage from YouTube, build a reconstruction of what it thought they had seen. Though blurred, the results are breath taking. But what could this method be used for?

Jack - The methods we came up with to solve this problem in vision are general. This is a vision experiment but you could apply very similar modelling framework to sort of a dynamic thought process. So if we want to build a communication device for example to communicate with stroke patients or people who have neurological diseases that cause them to be locked in so they can't communicate, having a method to decode dynamic brain activity would allow us to essentially communicate with those sorts of people. You can also imagine this would have interesting applications both for say, entertainment and therapy. If you can decode movies then in theory, you can decode say, dreams from the brain, and that would be kind of an interesting application.Hollywood

Emily - All these possibilities are still a little way in the future, but how can improving their algorithm help speed up the process?

Jack - Well the algorithm we have right now is limited, but it's limited in two fundamental ways and one kind of trivial way. The first is, the reconstructions that we have are essentially limited by computer power and disk space. We're reconstructing a movie that you saw using other movies that you didn't see strangely enough. The library of movies that we use to reconstruct what you saw actually affects the quality of reconstruction. So if we get more and more computer power then our reconstructions get better and better. At a more fundamental level, the reconstructions are limited by the quality of the models we have of the brain and as models of the brain get better and better then our ability to reconstruct brain activity and figure out what you saw get better and better.

Emily - But should you worry that people will be able to read your thoughts as you walk along the street?

Jack - So, it's natural for people to have concerns about the ethics of this process and about potentials for invasion of privacy, and I share those concerns. I think in the long run say, decades out, this kind of brain reading technology is going to face major ethical issues that are going to have to be addressed and overcome. In the short term, there's no danger of anyone having their brain read without their knowledge because it requires spending several hours in a very large MRI machine and anyone who was undergoing this procedure would know it.

Chris - Which is very reassuring. Jack Gallant ending that report by Emily Seward.

13:03 - Teaching T Cells to Tolerate Friends

Teaching the immune system to tolerate certain friendly bacteria is an important step towards gut health, and this week researchers in America have shed some light on how and where those...

Teaching T Cells to Tolerate Friends

Teaching the immune system to tolerate certain friendly bacteria is an important step towards gut health, and this week researchers in America have shed some light on how and where those lessons take place.

There is a well-known and understood way of training the immune system, which occurs in an organ called the thymus.  Special immune cells, known as T cells, are generated with a wide range of antigen receptor molecules on their surface that are used to recognise and attach to other molecules.  Some of these would interact with our own cells, leading to auto-immune diseases but in the thymus these receptors are checked and sorted, with the non self-reactive T cells maturing into T effector cells, to play a role in identifying and attacking infection.  The self-reactive T cells are either destroyed or matured into regulatory cells or Treg cells, which keep other components of the immune system in check. 

Working with mice, Chyi-Song Hsieh and colleagues at Washington University School of Medicine, showed that an encounter between immature T cells and commensal, or friendly, gut bacteria could also lead to the creation of Treg cells and, therefore, teach the immune system to hold fire.  This education happens directly at the site of the encounter - the gut.

Probiotic DrinkThe researchers noticed that Treg cells around the colon used different antigen receptors from those in other locations, and that these seemed to correlate with the gut bacteria themselves, interpreting the bacteria in a similar way to how Treg cells elsewhere interpret 'self'.

They then wanted to find out if these receptors were actually learned from the bacteria, and not just from the mouse itself.  To do so, they transferred the genes that code for these receptors into the bone marrow of modified mice that do not normally produce T cells.  This then caused them to produce immature T cells expressing the right receptors, but these cells didn't mature into Treg as expected.  They realised that as the modified mice had been delivered from elsewhere, they would have developed different gut bacteria, so only when they housed all the mice together, allowing them to share bacteria, did these Treg cells mature.  This showed that the presence of the bacteria themselves is essential for this immune training process to work.

Hsieh also noticed that in mice with colitis, an inflammatory condition of the bowel, these receptors were not present on Treg cells, but rather on effector T cells, which encourage an adverse reaction to the bacteria.  It's suggested that breakdown in this immune education process might similarly lead to ulcerative colitis and Crohn's disease in humans.

Although the exact mechanism for how the bacteria train the T cells is still unknown, this paper, published in Nature, marks an important step forward and the first in-vivo demonstration of T cell education outside the thymus, as well as suggesting new ways to approach the treatment of these diseases.

16:13 - Pitcher plant-inspired liquid repellent

Inspired by the insect-eating pitcher plant, scientists have created a material capable of repelling almost any liquid including blood and even crude oil...

Pitcher plant-inspired liquid repellent

Inspired by the insect-eating pitcher plant, scientists have created a Pithcer plantmaterial capable of repelling almost any liquid, including blood and even crude oil.

Dubbed "slippery liquid infused porous surfaces", or SLIPS for short, the new materials are inspired by a natural phenomenon. Carnivorous pitcher plants catch flies and other insects by crafting a specialised surface that traps a layer of water against itself. This repels the oil on the feet of visiting insects, causing them to slide helplessly into the plant's pitcher of digestive juices.

Harvard scientist Joanna Aizenberg and her colleagues have copied this design, creating a microporous surface made of Teflon to which a fluorine-rich organic inert liquid called Fluorinert FC-70 is added. The liquid spreads out across the surface, coating the Teflon, by capillary action and locks into place within the pores. But as it doesn't mix with oil or water-based substances, any liquids - and even ice - that land on the surface form droplets and slide off. And because liquids are incompressible, the new SLIPS-endowed surfaces remain functional even under very high pressures. The team successfully tested them to 767 atmospheres, which was the highest pressure they could generate in their laboratory.

Critically, previous attempts to build liquid-repelling, high-slip surfaces have been modelled on a different plant: the lotus leaf. This uses a system of tiny pillars to trap a layer of air against the leaf surface, causing it to shed water and dirt. Droplet of water on a leafBut the system is easily compromised by physical damage and liquids with low surface tension simply displace the air rendering the surface useless.

The pitcher-inspired SLIPS, on the other hand, being based on a trapped liquid rahter than air, do not suffer this drawback and are also far more resilient. In fact, when the surface is damaged it effectively "self-heals" when the liquid flows back into the abraded area.

Consequently, Aizenberg and her colleagues, who have described the work this week in the journal Nature, are optimistic that the surfaces will remain super-slippery and liquid repellent even under extreme conditions. Possible applications include uses in deep-sea drilling rigs, safer fuel transportation, anti-icing coatings and even anti-graffiti measures...

19:34 - Speedy Neutrinos, Hydrogen Cells and Promiscuous Females.

A roundup of the week's science news including neutrinos moving faster than light, anti-virals from sharks, limitless Hydrogen supplies and the benefits of female promiscuity...

Speedy Neutrinos, Hydrogen Cells and Promiscuous Females.
with James Gillies, CERN; Michael Zasloff, Georgetown University; Bruce Logan, Pennsylvania State University; Matthew Gage, University of East Anglia

Neutrinos faster than Light

Meera - Scientists might have found particles travelling
faster than the speed of light. The OPERA experiment sent a beam of subatomic neutrino particles from the CERN facility in Geneva to the Grand Sasso Laboratory 732 kilometres away. The neutrinos appear to have travelled 0.0025% faster than the speed of light, a finding which, if confirmed, could revolutionise modern physics, CERNS James Gillies comments on the discovery...

James -   Everybody in physics is going to be looking for an independent measurement before they decide whether this is true or whether it's not.  A lot of people seem to think that science is about proving things.  It's actually the opposite - science is about knocking things out.  It's about disproving stuff.  It happens all the time.  What's different this time is that the stakes are that much higher because the speed of light being a cosmic speed limit is one of the fundamental tenets of physics.  So if this turns out to be right, then there's some serious head scratching to be done.


Shark Antiviral

Smooth dogfish (Mustelus canis)Meera - A compound found in the liver of dogfish sharks could treat a range of human viruses.
The compound squalamine was found to be effective against diseases such as dengue, yellow fever and Hepatitus B and D in animal models by altering the environment needed for the viruses to survive inside a cell... as Michael Zasloff from Georgetown University explains...

Michael -   What squalamine has taught us is that by changing the characteristics of a cell or a tissue, it can render that cell or that tissue resistant to a virus.  So it's not targeting the virus, but basically changing the design, the internal architecture of the cell to make that cell or tissue inhospitable for viral replication or viral growth.


Limitless Hydrogen Cell

Meera - Water could be used to provide
limitless supplies of hydrogen. Microbial fuel cells harness the breakdown or organic matter by bacteria to produce Hydrogen, but electricity is needed to power the process. Until now this has been provided by fossil fuels but now Bruce Logan from Penn State University has developed a way using water.

Bruce -   What we figured out was that we could use the salinity difference between freshwater and saltwater.  This is a process called 'reverse electrodialysis' where if you have freshwater and saltwater next to each other, that can create energy.  It's like running uphill takes energy, running downhill doesn't take energy.  And now, you have a system where the electrical power isn't needed because the energy is being extracted from this salinity difference.


Promiscuity in inbred females

Flour beetlesMeera - And finally, female
promiscuity could be beneficial to a population. Working with flour beetles, Matthew Gage from the University of East Anglia found that females in small populations, where there's a higher risk of inbreeding, behave promiscuously to increase their chances of reproductive success...

Matthew -   Females were able to have their eggs fertilised by males that carry genes that had greater complementarity, and what that translated into was females gaining genetic benefits by mating with more males because they're able to somehow choose the right males or the right sperm to give great genetic benefits to their offspring, and so have higher offspring viability and leave more offspring in the next generation.

Meera - The team aim to explore further benefits of promiscuity in larger populations to explain why the behaviour is so widespread across the animal kingdom.

23:31 - Stonehenge Landscape Restoration Project - Planet Earth

When Turner and Constable first painted Stonehenge about two hundred years ago, the very famous monument was surrounded by species rich chalk grasslands. The Stonehenge Landscape...

Stonehenge Landscape Restoration Project - Planet Earth
with Chris Gingell, Grace Triston-Davies

Chris -  When Turner and Constable first painted Stonehenge about two hundred years ago, the very famous monument was surrounded by species rich chalk grasslands. However the need to grow food after World War II saw these grasslands turn into arable fields. But the landscape is changing once again.  In the year 2000, the National Trust, with help from local landowners and scientists from the University of Reading, began what is known as the Stonehenge Landscape Restoration project. It's aim is to restore the landscape surrounding the monument by recreating the chalk grasslands and reintroducing biodiversity to the area. Planet earth podcast presenter Sue Nelson went to one of the restored fields to meet two of the team, starting with the National Trust's countryside manager from Wiltshire, Chris Gingell. Stonehenge

Chris Gingell - Well, what we did in 2000 was to bring a small quantity of seed harvested from Salisbury Plain, the great ancient grasslands to the north of here, and to grow a crop in the field adjoining us there, Seven Barrows Field, with that naturally harvested grass.

Sue -   So the fields that we're by the edge of right here, with all this lovely swaying grassland, and I can see a few yellows of wild flowers and purples in the distance, this wasn't like this in 2000, ten years or so ago?

Chris - No, this was another field of winter wheat and so on, but in this field we grew the first stand of this flower-rich grassland and used this as the nursery site to harvest further seed. Every one to two years another large arable field has been converted from corn to grassland with seed, either harvested here or now the programme rolls on so that some of the fields that were laid down six or seven years ago are now in turn being harvested to spread the seed further around the site.

Sue - And in that field I found Grace Twiston-Davies from the University of Reading, about to perform a butterfly survey.

Grace - Basically when I do the survey I just need a survey sheet, so I can just write down any butterflies I see, and an identification sheet which just helps me with some of the female butterflies, some of the blues are quite difficult to identify so I have that as well, and a pen, and I have a stopwatch as well because I often do about a 30 minute survey.

Sue -   Talk me through exactly what you do...

Grace - We will walk in a straight line for about 100 metres, quite slowly, and we're looking either side of us, about 10 metres either side, and we will see what butterflies are in the transect.

Sue - Okay, let's walk about 10 metres or so through the grass.

Grace - I've seen some Meadow Browns. There's a few Meadow Browns fluttering about.

Sue -   Gosh, you must have extremely good eyesight.

race -   Yes.  It's the way they fly, they fly very distinctively, almost like they're on a piece of string, they kind of bob up and down. Oh, I can see a blue over there. It's probably a Common Blue.

Sue -   Oh yes, yes. Sorry, you do have to have good eyes for this one.

Grace - That's a male Common Blue there.

Sue - It's almost like it's ringed with pale white. So how many species of butterflies have your observed in this beautiful re-created chalk grassland?

Grace - Well last year when I did the surveys we saw over 20 different species, Common Blues, we saw Small Heaths, Small Tortoiseshells, Adonis Blues, Small Blues.

Sue - It's interesting, you mentioned Adonis Blues because they've not been as successful have they in terms of returning to the sites as other species of butterfly.

Grace - Yes that's because the Adonis Blue are mainly at the old chalk grassland fragments that are still on this landscape, so we only see them in the new restored fields when they're coming to look for nectar in the new flowers that are there, but they're quite specialised and they will only lay their eggs on Horseshoe Vetch and their caterpillars will only eat Horseshoe Vetch and that's a plant that's very restricted to these old fragments, but we're hoping that in the future that the Horseshoe Vetch will become established in the new restored field as we can expand their populations.

Sue - Right, now let's walk back through the grass towards Chris. Hi Chris, it's all gone really well. It was rather beautiful to see and it's nice to see you sheltering from the wind in your National Trust Land Rover. When, for you, will you consider this restoration project complete?

Chris - I suppose what we're doing is perhaps not something that in a sense has a completion but something which will be its own sustainable long-term future as a landscape and that really is a question of scale. I think we've all known for a very long time that our protected sites, little fragments of the countryside that have much of their biodiversity interest, many, many of them are too small or too isolated or too fragmented. One of the things which Grace's work will help us to understand here is just how extensive does this have to be to have that long term viability and for the free movement of plants and animals without so much intervention. What we've done of course in the last ten years is intervention, but for that grassland to persist with all its character and interest in the future.

Chris -   Chris Gingell from the National Trust and Grace Twiston-Davies from the University of Reading explaining the Stonehenge Landscape Restoration project to Sue Nelson.

29:08 - Baking a Cake

The science of cake baking...

Baking a Cake
with Amy Chesterton, Cambridge University

Ben -  This week, we're discussing the science of food so what better way than to explore the scientific basis for the perfect cake.  I'm joined by Amy Chesterton, a PhD student at Cambridge University.  So Amy, other than the very obvious reason of tastiness, why would a scientist care about cake?

Amy -   Well I work in a research group and we're interested in powders and pastes generally, especially those relevant to the food and pharmaceutical companies.  Cake is particularly scientifically interesting, firstly because the structure is caused by mixing in many ingredients and secondly, because of the changes that occurred during baking.

Ben -   Now I see that we have the ingredients out here already.  We have eggs, flour, and we have a mixture of butter and sugar.  That takes a while to beat so we've already started it, but why is that important?

 Amy -   Well the mixing of the fat and the sugar is important because the most important ingredient is air.  So we're beating the fat and the sugar together to incorporate lots and lots of bubbles.

Ben -   Okay, so as I've said before we're giving it a good go, let's just finish it off properly.  So I'm obviously putting lots of bubbles into this.  What's the next thing that needs to go in?

CakeAmy -   Well fat and sugar by itself doesn't make a cake because on heating, the fat will melt and the bubbles will escape to the atmosphere.  So now I'm adding the egg.  What we want to do is cover all of our fat covered bubbles in egg because the heat on baking will cause the eggs to solidify, the proteins to denature, and that will keep the air in the cake the cake structure up itself.

 Ben -   So mixing now isn't really to add any extra bubbles.  We've already got our bubbles in there.  Now we're just making sure that these bubbles are evenly coated with egg.  But if the bubbles are already in there from the fat and sugar, and now, we're coating them with the egg that will give it structure, what's the flour for?

Amy -   Well the flour also is an important structural aid together with the protein from the egg scaffolding. The starch within the flour will swirl and with the heat, it will gelatinise - create a gel - and together, create a very firm, nice, tender structure.

Ben -   Okay, well let's get the flour in there.  Still a little bit lumpy at the moment -  We need to make sure we mix that in properly.  I'll let you put that in.  How much of each ingredient did we actually start with?

Amy -   Well we're following the most basic cake recipe that's equal quantities of our four ingredients - fat, sugar, flour, and egg. It's the recipe that the Victoria sponge is based on.

Ben -   Now we're using an electric hand whisk which may be perceived as cheating, but really, it's very important to make sure that you get that air in there.  What sort of flour are you using, because I know that some flour will actually help give extra bubbles.

Amy -   We're using self-raising flour so there's added baking powder in there. On contact with the wet ingredients, that will release carbon dioxide and increase the size of our bubbles.

Ben -   You said increase the size of our bubbles.  Surely if we're creating carbon dioxide, that's going to give us new extra bubbles?

Amy -   Well actually, the creaming process is so important because the number of bubbles in our cake batter will be the number of bubbles in our final cake.  The carbon dioxide released by the baking powder can only increase the size of the bubbles and that's because the surface tension is too high for new bubbles to be created.

Ben -   We've got quite a nice, smooth paste here.  Now we are going to put this into little muffin tins and pop it in the oven.  What temperature does it need to be on and do you think we'll get to eat them before the show is over?

Amy -   Well yeah, we're going to heat at 180 degrees and because we're making cupcakes, they should be ready by the end of the show.

Ben -   Excellent!  So we're going to start spooning the mixture into our cupcakes now and then we'll pop them in the oven, and later on in the show, we'll come back to see how our cake is doing.


Ben -   My kitchen is now filling with smell of cake.  It's smelling delicious in here and the mixture that we poured in earlier that was fat, sugar, egg, flour, and importantly air is now starting to rise.  Amy, what's actually happening inside the oven?

Amy -   Well at the moment, we're at the early stages of baking, so heat is being transferred from the oven to the cake mixture and transforming it into something else.  We can see the cakes physically rising now and that's because the tiny bubbles that we incorporated earlier are starting to grow by three mechanisms.  We've got thermal expansion, because air expands when it's hot.  We've got water vapour which is starting to be produced, and also, carbon dioxide from the baking powder.

Ben -   So there's lots of different processes going on physically inside there, including tiny steam engines taking advantage of the expansion of water as it turns into vapour.  But what about the chemical changes that are happening, the changes to the proteins we were talking about earlier?

Amy -   Well, it depends on the exact temperature at the moment.  If we're above 60 degrees or so, we'll start to have starch granules expanding.  They'll eventually disrupt and form a gel.  The proteins will also denature at a similar temperature so we'll start to get the structure forming.

Ben -   So the structures are already forming.  It's growing.  It smells delicious.  Can I get this open and start tasting them now?

Amy -   At the moment, no.  All of our bubbles are still surrounded predominantly by liquid so, the cake shape and size that we can see is held up by pneumatic supports.  If we removed it from the heat, everything would cool down, the bubbles would contract because of the thermal expansion in reverse, and the water vapour would condense, and we would have very poor-looking cakes.

Ben -   So, is it safe to open it and have a look or is it best just to leave it alone until they're perfect?

Amy -   Again, because we don't want the batter to cool down at all, it's not a good idea to open the door or remove the cakes yet.  You'll just have to wait for later.

Ben -   So we'll have to wait a little bit longer before we get to taste the cake.  


Ben -   Well, it's now time to actually take it out of the oven.  Here's the moment of truth so we're going to see what the cakes look like.  Here they come.  Obviously, if you're doing this at home, be very careful that it's a bit hot.  Ooh!  They look delicious, that waft of smell that came out.  I'm just going to poke one - it's beautifully soft, spongy, and it's gone a gorgeous brown on the top.  Amy, what's happened now?

Amy -   Well at the end of the baking, we got Maillard reactions occurring.  That produces a nice brown colour and also the distinct cooked taste of the crust.  So the Maillard  reaction is actually a reaction between the protein or the amino acids, and the sugars producing hundreds of flavour compounds.

Ben -   Now this is the same reaction that we see when you fry chips or when you roast a chicken.  So, it's a set of reactions that produce this distinct colours, and that distinct flavour profile that gives you that gorgeous oven-cooked or roasted or fried flavour.

Amy -   Yeah, that's right and I'm just cutting into one now to have a look and we can see that on the inside, we've not got such a brown colour.  That's because the inside of the cake had quite a lot of moisture right until the end of baking.  So the Maillard reactions wouldn't really happen.  They occur at higher temperatures than the inside of the cake would've achieved, but the crust which heats up quicker will have dehydrated and Maillard reactions occurred.

Ben -   So that's why we get the crispy outer layer.  So if we'd left it in there for too long, then eventually would've all got to the right temperature, those reactions would've occurred all the way through and this would've turned into a stiff, crunchy, sort of biscuit rather than a cake.

Amy -   Yeah, although the crust would've got too hot at that point and turned black, dark and non-edible.

Ben -   Well that would've been an absolutely travesty.  Now we've taken it out and it hasn't collapsed.  So can we assume that the starch and the egg protein has locked in that structure for us?

Amy -   Yes, so it would've solidified towards the end of baking and that actually stops the bubbles from expanding.  They can't expand when the cake is solid.  So instead, you get the bubbles sort of popping and forming a continuous network, which is why when we open it, we can see this nice open structure, rather than individual small bubbles.

Ben -   So although it is a sponge cake, it doesn't look like a bathroom sponge which has those lots of little tiny holes.  But why would scientists need to know about how cake actually functions?  You said before that these pastes are interesting, but how can we apply the science of cake to other industry, and of course, how can we apply other science to make better cake?

Amy -   Well, the structure of our final cake here was completely determined by the batter we made it from, and that's true of products produced industrially in all sorts of sectors.  So, it's the relationship between the paste and the final product which is scientifically interesting and something which I'm interested in as a researcher.

Ben -   So that might include - obviously in our case, it's cake - but that might include how you mix and manufacture say, a pharmaceutical product, because again you need to make sure that the final product is very well understood, very homogenous, and almost identical from one batch to the next.  So it's understanding these processes, be they in drugs or in delicious cake that really is what scientists are looking in to.

Amy -   Yes.  It's useful for quality control and also for development of new products.

Ben - Well thank you ever so much.  That's Amy Chesterton from Cambridge University.

34:11 - The Microbiology of Cheese

Professor Martin Adams talks through how cheddar and stilton cheeses are made.

The Microbiology of Cheese
with Martin Adams, University of Surrey

Chris -  First of all, can we just run through the basic process that enables us to make cheese.  How does it get made?

Martin -  It starts with a lactic acid bacterial culture that is added to the milk and once that lactic acid culture starts working, it converts the milk sugar lactose to lactic acid and then an enzyme preparation called rennet is added which causes the casein in the milk, the milk protein, to coagulate and so the whole of the milk forms a solid coagulant.  All the while, the bacteria carry on producing acid from the lactose in the milk, the coagulant is then cut and when it's cut, it starts to separate into curds and whey - the liquid phase. Then it goes through various sorts of manipulations to separate the curd from the whey, and you end up with the curd which goes to make the cheese.  It's essentially those simple processes by slight variations in them. I forgot to mention at the very end of course, the curds are sorted.  And those are the of basic steps of lactic fermentation, addition of the enzyme, the curd cutting, the removal of whey and sorting, just by slight variations in that, you can make this huge array of different cheeses.

Cheese CurdsChris -  So the bacteria that are added right at the beginning that do this souring process, the lactic acid bacteria, they're presumably quite a special strain of bacteria that are used to do that?

Martin -  Yes and normally, in the case of cheddar cheese making, an organism called Lactococcus lactis and, traditionally, these would've been naturally present in the dairy environment but of course nowadays they need to be much more assured of control of these processes and so, they add a starter culture as of Lactococcus lactis that is produced industrially, and those bacteria are the primary microflora.

Chris -  Just opening this one here.  So I've got a cheddar here.  It's a very firm cheese, but still nonetheless quite springy and moist.  It's quite tasty.  It's got a sharp taste to it.  What actually goes into making say, a cheddar like this?  How would you do that?

Martin -  Well the first stage, the lactic acid production by the primary starter culture is really just the start of it,  you end up with a curd that has been salted and when you add the salt, the starter culture stops growing because it's inhibited by the salt. But what happens in cheddar cheese, at it matures or ripens, you get a secondary microflora, which are known as nonstarter lactic acid bacteria, and these have not been added they're just naturally present in the milk or in the environment and they get into the cheese, and these grow up to levels about 10 million per gram in the course of a few weeks of ripening.  It's a combination of proteolytic enzymes and lipolytic enzymes that are still present from the starter organism (although the starter organism can't grow any longer, its enzymes can still be active).  There's also a residual proteolytic activity from the rennet that's been added, and there's enzyme activities from these non-starter lactic acid bacteria.  And so, particularly in the case of cheddar, they break down the proteins to produce peptides that give that sort of meaty, savoury, salty flavour to the product.  As the cheese matures, that flavour becomes stronger.

Chris -  That's a very high bacterial density, isn't it?  Does that then mean that quite a significant weight of the cheese that you're eating does contain viable bacteria.  If I took my block of cheddar here and I actually did a gram stain for microorganisms under a microscope would I see viable organisms in there?

Martin -  Yes,  you should be able to see the non-starter bacteria.  Normally, they're rod-shaped bacteria.  The starter used in cheddar cheese making, Lactococcus lactis, is a spherical bacterium whereas the most common non-starter lactic acid bacteria are Lactobacillus casei and Lactobacillus paracasei.  So they'd be quite evident.

Chris -  So does it make a difference if you use pasteurised milk which has effectively been rendered sterile then?  You would presumably have to put those back in?

Martin -  Well you don't, that's the interesting thing.  Certainly, people claim that if you use unpasteurised milk, you get a much more flavoursome cheese because you don't destroy the natural flora of the milk.  But even if you pasteurise the milk, some of these bacteria might survive pasteurisation, but others may just be present in the environment and they grow as biofilms on equipments and so on,  so they just naturally contaminate the environment, but in a good way.

stilton blue cheeseChris -  That's convenient.  Now the other cheese I've got here is a bit whiffy and this one is a blue cheese.  This is very different.  If I take a slice through this one, I see that this is A) Absolutely riddled with holes and B)  A lot of those holes are full of what looks like the same stuff that grows on your loaf when it goes mouldy.

Martin -  The first stage in making a stilton cheese would be similar to cheddar in that they would either start a culture which  produces lactic acid from the lactose in the milk, but they would also add mould spores.  In blue cheese, the mould is Penicillium roqueforti. Moulds need air to grow, so they make the cheese in a way to have a fairly open texture with some sort of cracks in it to have some air there.  They also stick needles through the blocks of cheese, and if you look very carefully at a substantial-sized piece of stilton, you'll see the tracks of these holes where they put needles through to allow air into the cheese, to allow the mould's spores to germinate and the mould mycelium to grow throughout the cheese.  The blue colour you actually see is the mould's spores when the mould has sporelated.

Chris -  And what about the flavour?  Is that imparted by the growth of that mould inside the cheese?

Martin -  Yes, very much so.  The mould is much more metabolically active, and produces lipases and proteases, and produces a whole range of breakdown products from the components in the cheese - things like methyl ketones, lactones, fatty acids, and so on - all of which give the characteristic flavour to that cheese.

45:11 - Best Before Dates

What are the risks of food? Are 'best before' dates actually useful?

Best Before Dates
with Dr Nick Brown, Addenbrookes Hospital

Chris - Today, we're looking at the science of food.  One very important issue about food is safety.  There are millions of people every year ending up locked to loo seats for longer than they'd like - try saying that when you've had a few - just because of something that they ate.  But what are the risks and how can we reduce them?  And are 'best before' dates actually helpful?  To help us look into this is Dr. Nick Brown.  He's a Consultant Medical Microbiologist at Addenbrookes Hospital.  So tell us first of all, when food goes off, what actually is happening?

Nick -   This is a natural process. If we start by looking at things that haven't got 'best before' dates, for example things not from the supermarket like fruits, vegetables and so on, we know that if we keep them for a long period of time, they start to deteriorate.  They either go squidgy, or they change colour, or sometimes they smell horrible. Certainly, the texture and taste can be affected as well.  We've heard already about natural bacteria in cheeses.  Well, a lot of other foods have natural bacteria too and this is a process that is occurring all the time - the process of decay or food spoilage.

Chris -   So I think it's an important distinction between food that's spoiled and breaks down and just turns mushy but isn't necessarily bad for you, and food that has got things that could make you quite unwell.

Nick -   Absolutely.  The classical things that can harm you of course are meat products.  Often, these are contaminated with food poisoning organisms such as Campylobacter or Salmonella. If you do not cook these foods properly then you can be affected by the illnesses that these organisms cause.

Chris -   But where did that food get those particular organisms in the first place?

Nick -   Well they can get them from a variety of places.  We know that many foods contain organisms already.  We've heard about cheese, from milk for example, and from the environment. Often meat products are contaminated with organisms that are acquired from the organisms, the animals themselves.  So the animal has the organism for example in its gut, and then when we prepare the meat, the meat is contaminated.  Another way of course they can get contaminated is by virtue of the processing itself.  So if you have a product, for example a cold meat, a prepared meat that you're going to eat. If it's stored in an inappropriate way, next to raw meat, then it can be contaminated by cross contamination.

Chris -   I did read somewhere that when chickens are prepared for example, although only a small fraction of chickens may carry Salmonella naturally in their gut at a reasonable level, because of the way they're prepared, many of the carcasses are put through a vat of ice cold water to rapidly chill them down after they've been plucked.  And so, if one of them has some Salmonella in it and on it, it then contaminates the water which means that pretty much all of them come out with at least some Salmonella.

Nick -   That would be an example of the way in which a process can contaminate a wide range of different products, yes.

Chris -   Now what about actually when you've cooked some meat?  So we cook the said chicken.  I can eat it cold after I've cooked it and let it cool, but what about reheating it, because there seems to be a lot of confusion around that.  People often say, "Well it's cooked.  You shouldn't reheat it or if you do reheat it, you've got to reheat it properly."  What's the actual bottom line on this?

Use by dateNick -   Well of course when you cook food,  most of the microorganisms that cause food poisoning are very sensitive to heat, so the numbers are reduced very rapidly. But on storage, even if there are one or two left behind, then they can replicate very quickly and so, over time the product can almost become recontaminated if you like.  So that is a danger if you don't heat it properly thoroughly before eating it.

Chris -   In other words, you would heat it but insufficiently to destroy the organisms, but sufficiently to warm them up so they grow faster.  They then turn a non-infectious dose into an infectious dose, and you then catch whatever they've got to give away.

Nick -   Absolutely, yes. And some organisms have a different mechanism.  They can produce toxins that can cause food poisoning as well.

Chris -   How would that work then?

Nick -   For example, Staphylococcus aureus, an organism that's on many of our skins, can contaminate dairy products. If this then is given the opportunity to replicate, it can produce a variety of different toxins that can act on the gut, and usually causes upper gut problems, particularly nausea and vomiting rather than diarrhoea itself.

Chris -   And when you reheat the food, does it matter how hot you make it? The bacteria break down but the toxin doesn't necessarily?

Nick -   Absolutely.  Some of these toxins are heat stable so they're not broken down again.

Chris -   And what about the dreaded 'best before' date then?  Is this is a safe guard?  How do supermarkets and vendors work out, "Well, we're going to put this particular date on an item of food" and that means it's safe and then it goes past midnight, and it's the day after the 'best before' date, now it's not?

Nick -   There's actually quite a lot of confusion about this and partly that's because there are a lot of different ways that food can be labelled.  Food can have 'sell by' dates, 'display until' dates, and these are largely for the use of the shop itself, and relate to quality of food. Then you've got 'best before' dates, and importantly, 'use before' dates and I think that these two are different.  'Best before' dates as it suggests, implies that the food will start to deteriorate and won't be as good after the 'best before' date, but is not actually dangerous.  Whereas the 'use before' date really does say that you should be consuming the product before the end of that and often, there is evidence to show that it does deteriorate and become - not always dangerous, but it has the potential to be dangerous if you eat it afterwards.

Chris -   So the bottom line is, let's use the 'use by' date, but we could disregard the 'best before' and still probably get away with it okay.

Nick -   Still get away with it, but it might not be as good as it was before.

56:24 - Is smell a good way to tell if food is safe?

Is smell a good way to tell if food is safe?

Nick - It can be, but I think there's another important point there as well. You can often throw food away before the sell by date - just because it's got a date on it, it doesn't necessarily mean it's safe up until that point if hasn't been stored properly. But yes, smell can be an indicator that the spoilage process is starting, though it's one of several things that you might use.

57:27 - Does blue cheese alter antibiotic resistance?

Does blue cheese alter antibiotic resistance?

Martin - It's not a problem because the mould in cheese making doesn't produce Penicillin, it's actually a different species that's used to make Penicillin.Chris - Well that's reassuring. So the species that's used to make blue cheese is Penicillium roqueforti?Martin - Yes.Chris - And it's related but you couldn't make antibiotics with it?Martin - No. It's Penicillium chrysogenum and P. notatum that produce Penicillin, the antibiotic.Chris - Okay, so if I scrape all the fluff off my bread, I can't argue that I'm going to be pathologically abiotic and noninfected with things because it won't make any antibiotic molecules?Martin - No.

58:13 - Why does mould change colour?

Why does mould change colour?

Martin - Well the blue colour of the cheese is due to the spores produced by the fungus, and Penicillium roqueforti grows at very low oxygen concentrations, but it needs higher oxygen concentrations to sporulate. It may be that the mould had grown throughout the cheese and made the cheese, but it was the exposure to more air that caused it to sporulate and get the blue colour.

59:00 - What happens to cake that's been frozen for 11 years?

What happens to cake that's been frozen for 11 years?

Amy - Well in terms of the microbiology, I'm not really an expert, but in terms of how the cake will be now compared to when it was first frozen I think that the problem could be the development of large ice crystals over time. You see, small ice crystals are okay, but over time they can grow as the temperature in the freezer fluctuates and the crystals melt and re-freeze. This can dry the cake by removing all its moisture, and the crystals can get so big that they cut into the cake, affecting the structure.Chris - So it doesn't taste as good.Amy - I would expect not!

59:54 - Is is safe to eat food that's soon after its 'Sell by' date?

Is is safe to eat food that's soon after its 'Sell by' date?

Nick - I think it is. Obviously many of the listeners will be aware that there's been quite a discussion over the last couple of weeks about 'sell by' dates and whether we should have them or not, and indeed a proposal that we should get rid of them. I think by and large, they're a mechanism for the sellers to use stock rotation to make sure their produce is as it should be, rather than specifically being anything to do with food safety.

60:43 - Which moulds are dangerous?

Which moulds are dangerous?

Martin - Moulds that are a problem are those that produce mycotoxins. Things like Aspergillus flavus produces afflotoxin. This commonly occurs on nuts in particular, but generally speaking, that's the principal threat from mould - the production of toxins.

61:24 - Why don't animals and birds get food poisoning?

Why don't animals and birds get food poisoning?

Nick - I guess sometimes they might, but they just don't tell us! I think there are some important differences between animals and ourselves. Many animals, of course, live on rotting carcases, and are designed to be able to cope with that. One of the main protections we've got against food poisoning is the acid in our stomachs, and some animals are able to ingest food poisoning organisms and destroy them within their stomachs before they get down to the gut. Then there are other animals that we know carry food poisoning organisms naturally, for example chickens carry Campylobacter and Salmonellas, Cattle carry E. coli 157, and they don't seem to be affected by these organisms, presumably because they don't have the right receptors in their gut for the organism to bind to and cause disease.

Chris - So the bottom line is, those animals in nature that are not equipped with the ability to cook food or keep it clean are therefore endowed with better immunity, better resistance mechanisms, or just don't actually respond to whatever toxin the organism uses to make us ill.

Nick - Absolutely.

62:50 - Is eating what the locals eat a safe strategy to adopt when travelling?

Is eating what the locals eat a safe strategy to adopt when travelling?

Nick - I don't think the strategy in itself fends off food poisoning. The most important thing is the precautions that are taken to make sure the food is safe and that it's prepared properly. One common saying for travellers abroad is that whenever you have any food, make sure that you wash it, peel it, and cook it. And I think the same principles apply wherever you're eating. Chris - The whole idea about people who live in an area, having the local bacteria which in some way protect them better, is that actually scientifically sound - that argument? Nick - I'm not aware of any evidence to support that. Some people anecdotally, I know, suggest that it's true, but I'm not aware of any evidence to support that.

63:59 - How can we store and preserve food safely?

How can we store and preserve food safely?

Nick - Most of the way that we ensure that food is safe in this respect is to make sure that the process that we go through is appropriate, so that we've cooked things for the appropriate time or we've added the right ingredients or that it's reached the right temperature, for example. And then afterwards, once we've preserved it, we should make sure that it appears as it should do. So for tinning for example, if the tin is blown or damaged in any other way, then clearly the product inside it might be inappropriate to eat. It's the same with jams and other preserves, if they clearly do not look or taste right then something might be wrong.

65:00 - What's actually happening when you smoke and cure meat to the microbes?

What's actually happening when you smoke and cure meat to the microbes?

Nick - Basically, these are historical processes that people have done for centuries to try and make foods last longer and basically, what you're doing is making conditions within the meat less tolerable for the bacteria that are going to spoil it. So, making it either very salty or in the basis of smoke, you're changing the conditions of the meat such that the bacteria can't proliferate in the same way as they would do normally.

65:40 - Why do some foods complement each other so well?

Why do the tastes of some foods complement each other so well? For example, cheese and wine? What is the chemical reaction that takes place to make it so palatable?

Why do some foods complement each other so well?

We posed this question to Dr. Marcia Pelchat from the Monell Chemical Senses Centre in Philadelphia...

Marcia - The main reason that some foods are considered to go together better than others is culture. In each culture, we're used to certain pairings and unaccustomed to others. A good example is in the United States, we're used to putting sweet sauces on our meat - things like barbecue sauce and ketchup, whereas the French don't consider that to be such a good combination. But there are some important scientific principles that can explain classically good combinations like wine and cheese. There's a saying for wine merchants, "Buy with apples and sell with cheese" and what this means is that wine is at its worst when consumed with apples and at its best with the cheese. One principle is that salt, which of course is found in cheese, is a very good bitterness inhibitor and when wine is consumed with salt, some of the bitterness in say, a big tannic red wine is suppressed and this reveals some of the sweetness and people tend to like it better. Another important principle is taste adaptation and this is the idea that when you eat a lot of a particular taste, you temporarily become less sensitive to that taste. One classic bad wine and food pairing is a big red wine and dessert. What happens is when you consume a sweet food, you become less sensitive to sweetness and this reveals the bitterness and the tannins in the wine. So that's why wine and cheese tend to go together. Sarah - So food pairings are a trade-off between what society tells us should go together and the way in which certain flavours contrast with others. So a sweet food might reveal and enhance the bitterness in another while a sour food can make its complement taste sweeter. On the forum, Techmind expressed how personal tastes can differ from classical opinion and Griselda listed the delightful combination from the back of a Crisp Packet. Clearly, citric acid and disodium inosinate are an excellent pairing.

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