The Essential Technology of the Kitchen
The thin edge of the wedge
If, like me, you are a fan of gadgets you have probably accumulated a number of peculiar devices in your kitchen drawers and cupboards. I have one drawer in particular that resists being opened due to all the kitchen technology crammed into it. Some choice items contained in this recalcitrant drawer include: the milk-foaming whizzy thing that was only used twice, the wine bottle vacuum pump for half-finished bottles, and the twice-as-fast mandolin that cuts both ways and slices your fingers twice as efficiently. A quick survey of all my gadgets reveals that they generally fall into one of two types: things for preparing food and machines for cooking food.
The food-cooking machines tend to be bigger and are geared to different methods of cooking that are, on the whole, only possible with these machines. So, the slow cooker contains a thermostat without which such prolonged cookery would be impossible, and the bread machine turns the production of a loaf into a ninety-second prep-and-ignore activity. The hot-air popcorn thing is mostly retained for the amusement value of seeing the kids trying to capture puffed corn as it flies violently out of the open mouth of the machine and ricochets around the kitchen.
However, when it comes to the food-preparation devices – the mandolins, peelers, crushers, dicers and chip-makers – I have a sneaking suspicion that every single one of these is redundant. With a bit of practice, all of these gadgets can be replaced by a really good knife. Surely, the knife is the ultimate in kitchen gadgets; an irreplaceable tool for the cook, and the most versatile.
I have a modest collection of kitchen knives. My current favourite knife is a wonderful Japanese-style Santoku with a cherry-wood handle. It holds a beautiful edge, cuts through anything like butter and suits my cutting style. But why does a knife cut in the first place? And can an understanding of this influence knife usage in the kitchen?
If you consider how a knife is used, it has two basic modes of operation. First, there is the classic chop, which entails a vertically straight down movement of the blade through the food. Secondly, you have the slice where the blade of the knife is drawn across and down at the same time as it cuts. While the chopping action is ideal for some things, like cheese and carrots, for others the slice is much easier than the chop. How can it be that the same knife cuts some items better when slicing than chopping?
As an extreme example, consider the painful yet all too common paper cut. A sheet of paper is quite useless when it comes to chopping your finger, but if you run your finger along the length it appears that it can readily slice into flesh.
The answer to this conundrum is all to do with shear and has been studied in some detail in the laboratory. The basic concept behind cutting anything is that you are producing a fracture and then forcing that fracture to propagate through the material being cut. Creating that initial fracture is the hardest part and, once made, the split can be pushed forwards through the material much more easily. All material, be it an apple, a chicken breast, a block of cheese or a lump of wood, has an inherent resistance to fractures. The molecules that make up the object are hanging on to each other and resisting the intrusion of the knife. Until, that is, the stress applied by the knife between the molecules gets to be greater than the force holding the molecules together. At that point, they snap apart and we have created a fracture. So, key to cutting is creating the initial fracture by increasing the stress between molecules.
This was wonderfully tested by a bunch of researchers at Harvard University back in 2012. They carefully measured the forces and stresses applied to a series of small blocks of agar jelly as they were cut with a tautly stretched, very thin wire. The force needed to create the critical level of stress in the jelly block when they tried chopping was more than twice needed for the slicing action. On a microscopic level, as the sharp edge slides across the object to be cut, it catches on it, effectively sticks to it and creates friction. This friction pulls the surface sideways, creating a shearing force as well as the downward force. Combined, these are enough to initiate a fracture and the cut can then propagate.
This is why paper, which cannot chop skin because the paper is all floppy, can still slice. If you slide your finger along the edge of a sheet of paper, the paper itself is pulled taught and acts as a knife blade. The very edge of the paper is rough and creates lots of friction and enough stress in your skin to start a fracture. Once begun the paper can then elongate this fracture, creating a cut. Interestingly, the reason paper cuts are so painful is due to the relative roughness of the edge of a sheet of paper when compared to a sharp knife. The paper edge creates a ragged tear in the skin, causing more tissue damage and more pain than a sharpened metal edge.
This helps us understand why the recommended way to use a knife is with a gentle forward motion along with the downward push. This way you are creating a slicing motion rather than a chop and the effort needed is much reduced. Why then do we still chop a carrot and a block of cheese? In the case of the cheese, the material is sufficiently soft that the blade easily pushes into the block and starts the fracture going. Carrots on the other hand are so brittle and their cells large enough that the blade of the knife can get the fracture started with little effort.
Once you have initiated the fracture, you then want a thin wedge of a blade to split that fracture and propagate it through the material, creating a cut. So, the knife actually needs to do two jobs. Conveniently for us, the best way to do this is to have a devilishly sharp edge on your blade. When looked at under a microscope, a sharp blade is not as smooth as it may seem. Instead, it consists of a series of ridges and furrows running up to the blade edge, creating what is to all intents and purposes a microscopically serrated edge. As this edge slides across food it catches, creates the needed friction to produce the shearing force that increases the stress that initiates a fracture. A blunted blade, on the other hand, has a rounded and smooth edge that slides, without catching, across food and does not start a cut so easily. Consequently, since you have no shearing force to help, you have to rely solely on the chopping action and need to apply much more force. Which is why blunt knives are more dangerous than sharp ones. All that extra force means you are more likely to slip and that’s how accidents happen.
Given the complexity of the task a blade is performing, with all the shearing forces and friction needed, it should come as no surprise that the manufacture of a knife is also a smidgeon complicated. To create a blade that can hold a sharp edge you want to use really hard steel. But on top of this you want the edge of the blade to be resistant to being worn down and for that you need a tough steel. Crucially for a knife, and any material scientists, hardness and toughness are not the same thing. Hardness is the ability of a material to resist being scratched or deformed by compression. Toughness is a measure of how well a material can absorb energy and deform without breaking, or, to put it another way, how well it copes with being bent. In a knife blade, you want your steel to be hard so that the edge stays there; in addition it should be tough so it doesn’t get worn down and the blade won’t snap the first time you flex it a bit. This is the tricky bit, as an increase in hardness usually reduces the toughness, and tough steel tends to be not so hard. Clearly, it’s a balancing act so knife manufacturers add carbon to the iron metal to create hard steel, tungsten and cobalt for toughness, and a spot of chromium to make it stainless and prevent rusting while they’re at it.
The final part of knife science I need to mention is the angle of your wedge. A standard Western- or Germanic-style knife blade will be sharpened so that the angle between the two sides of the blade is about 35 degrees. But the Japanese Santoku-style blades are much finer with a total angle of only about 25 degrees. The fineness of the blade makes a big difference to the edge you can put on a knife. Finer blades give a sharper edge and will thus cut easier and with less effort. So, why not make all blades as fine as possible? Well, this comes down to practicality and what the knife is being used for. Santoku blades, while being sharper, are more prone to being dented and bent in use and in storage. If you are using a 25-degree-angle blade and accidentally come across something hard in what you are cutting, like a bone for example, there is a good chance you will damage the blade. Similarly, if you want to keep your Santoku blade in good condition, don’t slip it in the kitchen drawer crammed with gadgets. Broader, 35-degree blades don’t suffer these problems, but will never take an edge quite like a Santoku.
Chop, chop, chop
What use is a wonderfully sharp and sleek kitchen knife without a chopping board? The board is the less glamorous but equally important part of this ubiquitous duo, yet even here there is hidden science for the unsuspecting.
The key issue when it comes to the design of a chopping board is hardness of the board material: its ability to resist being deformed by compression, or specifically its resistance to being cut. Too hard, and it will blunt your knives. Conversely, if it’s too soft the board would fall apart.
To get a sense of how hard is too hard and how soft is too soft, we need to quantify hardness. There are several ways to do this, but the simplest is to use the Mohs scale of hardness, created in 1812 by a German chap called Friedrich Mohs. The Mohs scale goes from 1 to 10 and was really created to quantify the hardness of minerals. In particular, any mineral with a higher rating on the scale was able to scratch those lower down. Diamonds are at the top of the scale with a 10 and they can scratch anything below them, such as quartz at 7 for example. Similarly, quartz will scratch gypsum since this is only 2 on the Mohs scale.
The steel used to make the blades of knives is in the order of 5 or 6 on the Mohs hardness scale. This means you should never use a chopping board harder than that. Note that both glass and granite kitchen countertops, which are primarily made of quartz, have a hardness of 6 and 7 respectively on the Mohs scale. Don’t chop onto glass or granite surfaces with your favourite knife, unless you also enjoy regular blade sharpening.
Instead, the wise chef will use either wooden or plastic chopping boards. But which is the best? There is a long-running debate between professional chefs, food technologists and microbiologists as to what sort of board is the most practical, the most enduring or the most hygienic. It quickly becomes complicated by a multitude of confounding factors. For example, I have been reliably informed by a professional chef that cutting for long periods of time on anything other than wood leads to a sore arm. Conversely, many domestic users of chopping boards prefer plastic because they don’t have dedicated cleaning staff, and the board can be chucked in the dishwasher. Then again, some people claim that the natural phenolic compounds in wooden boards actively kill off bacteria lingering on the surface. Which leads me nicely onto one of the most crucial aspects of chopping board science: hygiene.
Since you are invariably placing raw food on the board, the potential for bacteria to remain behind and contaminate the next thing on the board is a real risk. Clearly the most obvious thing to do is to follow the lead of all commercial kitchens, which use a separate chopping board for items such as raw meat that contain the highest potential for harbouring nasty bacteria, including salmonella.
In an effort to go beyond anecdotal arguments, several scientific studies have been carried out, including one I was involved in for a TV series that I presented. In a rare example of properly controlled TV science, the tests were carried out by an accredited laboratory of UK government scientists based in Glasgow. We started out with a big pile of new and used chopping boards, some made of wood and some from plastic. First up, to give us a uniform hygienic baseline, the boards were all identically sterilized. We then contaminated sections of each board with solutions containing a known number of bacteria. The boards were air-dried and then sampled over the course of twenty-four hours. The number of bacteria in each sample was then worked out by laboriously smearing a bit of each sample on a Petri dish, leaving it to mature and then manually counting the bacterial colonies that had grown up.
The aim of this part of the test was to simulate putting something such as raw chicken onto the board, then failing to clean it properly – perhaps giving it a perfunctory wipe – and using the board again some time later. We were deliberately seeing if we could test the idea that wooden boards were in some way anti-bacterial. Would the wood kill off more bacteria than the plastic? To the disappointment of the director on the day, the answer was no; in fact, it made no difference at all what the board was made of or how old the board was. Uncleaned boards retained a disturbingly large number of bacteria.
So, what about if you actually do what you are supposed to do and clean your board after you have used it? Once more, we set to with our chopping boards, but this time after they had been inoculated with bacteria, we gave them a thorough scrubbing with hot soapy water. The boards were tested for bacteria one last time and once again there was a resounding failure to find any significant difference.
From a television perspective, this was a bit of a disaster. We had set up the huge scientific test, explained all the complicated procedures and our result was a thoroughly disappointing lack of difference. Which was a little surprising and flew in face of several of the previous studies made on chopping boards.
From a scientific perspective, what this indicates is that if there is a difference between wood and plastic chopping boards, it is marginal and probably more influenced by the exact cleaning protocol used rather than the chopping boards themselves. If this is the case, the implication for the home cook or even professional chef is that you should use whatever board you fancy. If you want something that can go in the dishwasher, go plastic, but if you prefer the feel or the aesthetics of wood, go with that instead.
However, all studies agree on one thing: if the surface of your board gets really hacked up with deep grooves, then it becomes a serious health hazard as no matter how much you scrub, it will never come clean and bacteria will fester in the grooves. And another thing, if your wooden board starts splitting then you won’t just be harbouring bacteria, but chunks of food as well. I would also advise against using bamboo for chopping boards. While it may look and feel like wood, bamboo is actually a grass, and grass is particularly good at producing little shards of silica called phytoliths in its stems. And silica is harder than steel, so a bamboo chopping board will blunt your knives just like glass. As you can see, the business of choosing a chopping board is a complicated task.
What about ceramic?
Now that I’ve introduced some of the issues facing chopping-board choice, it’s worth mentioning the newest technology to hit the kitchen cutting scene. It’s now possible to make a knife blade from a ceramic. While this conjures images of a porcelain knife blade, which we can all agree would be rubbish, the material in question is much more high-tech. The blade of a ceramic knife is an otherworldly substance: very hard and light, almost translucent and with a razor-sharp edge. The blade is made from zirconium dioxide, or zirconia; the same stuff used to make the cubic zirconia gemstones found in jewellery on late-night shopping channels.
At its simplest, to make a ceramic knife you take powdered zirconia, press it into a knife shape and then heat it up to fuse the powder together. Which makes it all sound like a project for a science fair. The reality is that you need pressures in the order of 900 atmospheres, or one tonne for every square centimetre (14,200 psi), and a temperature of 1,400 ºC (2,550 ºF). At this pressure and temperature the fine zirconia powder fuses together to form a solid. The process is properly known as sintering and is the same process that takes snow and turns it into an icy glacier. Once your blade is sintered and sharpened it’s ready to be put through its paces. A ceramic blade has a big advantage over steel blades as the sintered zirconia has a hardness of 8.5 on the Mohs scale, which makes it harder than steel or glass or pretty much anything naturally occurring except for diamond. This means that it holds an edge far longer than a steel blade – ten times as long according to one manufacturer.
So, clearly we should all be using ceramic blades and chuck out all the rubbish steel. Well, not so fast there. The very hardness that makes the blade so enduring is a problem. To sharpen any blade, you need to use something harder than the blade, and that means a diamond-dust coated tool for a ceramic blade. It’s also much trickier to sharpen a ceramic blade so manufacturers either recommend you send the blade back to them for sharpening, or just throw it away and treat it as a consumable item. That extreme hardness will also cause problems with your chopping board. A ceramic blade will slice up any surface you cut on, leaving marks on glass and even granite worktops.
On top of that, there is another major issue with ceramic blades. As with so many things in life, the extraordinary hardness of zirconia comes with a compromise. As the hardness goes up so the toughness goes down, and this is where we once again come up against those cheeky material scientists and their very specific use of common words. As we have seen, toughness is the ability of a substance to absorb energy and not fracture. Steel is pretty tough and if you try to bend a steel knife it will flex and return to its original shape. Put more energy in and it will eventually give way, bending and changing shape. Ceramics, zirconia included, are not very tough and if you try to flex or bend a thin sheet of ceramic it will crack. If your lovely ceramic blade hits a bone or an unexpected hard bit and you twist the knife, a little bit of your super sharp and hard edge will snap off. Worse still, drop the knife on the floor or sling it carelessly into a drawer with lots of other utensils and there is a good chance it will snap in two.
Ceramic blades are currently viewed with some suspicion by cooking professionals. They are undeniably sharp and remain so, even without regular honing, but their fragility makes them less useful as a multi-purpose tool. You will notice I say currently, as there is a constant stream of new ceramic formulations being developed that provide new material properties. But it is unlikely that a ceramic blade will ever beat a steel one on toughness and hardness. If you have a ceramic blade, save it for those delicate tasks at which it will excel. And try not to drop it.
Equally complicated and confusing can be the task of working out how to supply the heat needed to cook the food you have so carefully chopped and sliced and peeled. There is a huge panoply of ways to add heat to food, from the simplest barbecue, through grills, fryers, slow cookers, ovens, microwaves, induction hobs and the newest, hippest, most scientific way: the sous vide cooker.
All of these heating devices and machines are trying to do one thing: change the temperature of the food you are cooking. Now, I appreciate that this may be the most stupidly obvious statement anywhere in this book, but bear with me for a moment. The act of cooking anything is about changing the temperature of the food so that one of a variety of biochemical reactions can take place. Which biochemical reaction you are trying to make happen depends on exactly what you are cooking and what you are trying to achieve in the way of flavour and texture. There are really only three categories of food that you may be playing about with: sugar, starch and protein. The first two of these I’ll get to in other chapters of this book, but I wanted to discuss protein now as this is where the most interesting new technological developments are being made. I have not included fats in the above list because, while the melting temperature is important, you are rarely trying to chemically change the fats through the process of cooking.
So, consider a lump of protein that you are trying to cook. This may be a steak, a piece of fish or even an egg. The end result is that you are trying to take the protein molecules from their normal or native state to a heat-changed form known as denatured. To understand this, we need to remind ourselves of some basic protein science. All proteins are made up of chains of a family of chemicals called amino acids. Key to the whole family is the presence of at least one nitrogen atom per amino acid and there are generally only twenty different types of amino acids in proteins. The order of amino acids in the protein chain is what makes proteins different from one another. So, the protein called ovalbumin, which makes up the bulk of an egg white, has a chain of 385 amino acids always in a particular order. On the other hand, 55 per cent of all muscle fibres in something like a steak are made up of a protein called myosin, which has some 2,000 amino acids in their own unique pattern. It is the order of amino acids that gives each protein its function, but also determines how the protein folds itself up. Since many amino acids will create bonds to other amino acids, any chain of these chemicals will spontaneously fold itself up and create a blob, whose shape is also determined by the order of amino acids. The native state of any protein is this folded-up blobby shape. But that’s not what we eat once the protein has been cooked.
As you gradually warm the protein, the heat energy begins to shake the blobby molecules and eventually breaks all the bonds between amino acids. This is the point at which the protein denatures. It uncoils itself from its balled-up shape and turns into a freely wiggling bit of spaghetti shape. What then invariably happens is all those wiggly spaghetti molecules stick to each other. Once it has denatured, the overall texture and colour of the lump of protein changes and we would consider it cooked and more easily digested by us. This is the crucial bit for cooks: the temperature at which a protein goes from native to denatured depends on the bonds inside it and is thus unique to the type of protein. Which is why you need less heat to cook fish than you do to cook meat. Myosin from a salmon is a little bit different to myosin protein from a cow. They both do the same job within the animal, but subtle amino-acid differences mean that salmon myosin starts to denature at 40 ºC (104 ºF) while beef steak myosin starts at 50 ºC (122 ºF).
Understanding the physics of how changes in temperature affect our food is one thing, but what about the science of how you go about doing this? The direct application of heat requires some sort of saucepan or frying pan in which to heat your food. It might seem that this would be a relatively straightforward process, but if you venture into a shop looking to buy such an item you are confronted with a bewildering array of choices. Once you remove cosmetic details, the real issue is what do you want your pan to be made from? You can choose from steel, aluminium, copper, cast iron or even layered combinations of these materials. As with knives (see here), the choice comes down to the physical properties of each material in question. In this case one of the key properties is the ability of different metals to transfer heat, and the scientific term for this is thermal conductivity.
Not all metals conduct heat as well as others. Copper is one of the best but it is somewhat surprising that stainless steel is a really poor conductor of heat. Heat conductivity is really important in a pan as the source of heat is often not even across the bottom of the pan. Gas hobs in particular apply a ring of heat with an unheated spot in the middle. If you make a pan out of a highly heat conductive material, like copper, the heat quickly disperses across the entire base giving a uniform heating surface. On the other hand, a pan made from stainless steel, especially if it’s thin steel, won’t give an even heat and, in extreme circumstances, will have hot spots that burn your food. It would seem that copper is the best material to make pans from, but pure copper is very rarely used for a few reasons: it’s expensive, it tarnishes easily and in acid conditions it dissolves into food to toxic levels, so it can’t be used with things like tomatoes or lemons. Pure copper has only one real niche application and that is in bowls for beating eggs (see here).
The next best material we have in terms of ability to conduct heat is aluminium and you do find many pans made from this, but it is not a perfect solution. While it does make for very lightweight pans, once again it will react with acid foods. In this case, it’s not a toxicity issue but the dissolved aluminium can turn your food an unappetizing grey colour. Aluminium does have one special advantage that makes it a popular choice with chefs, and that is its ability to hold onto more heat than an equivalent weight of copper. This property is known as specific heat capacity and is measured as the amount of energy used to heat one kilogram of material by 1 ºC. Aluminium has nearly three times the specific heat capacity of copper, which means it heats up more slowly, but it also cools down more slowly. This property makes it ideal for frying pans. If you are trying to rapidly cook a piece of meat, an aluminium pan will cool down more slowly and you can more effectively sear the meat, creating delicious Maillard reaction products (see here).
Carrying on down the line of thermal conductivity, and just a bit better than stainless steel, is cast iron. But as anyone who has owned a cast-iron pan knows, the issue here is rust. If your pan is not properly dried after washing it is going to rust, which, if not cleaned off, will ruin the food you next cook in it.
Finally, we get to the worst thermal conductor, stainless steel, which is somewhat paradoxically also the most used material for saucepans and frying pans. In the end, it is the convenience of stainless steel that trumps all other materials. It does not tarnish, needs no special treatment and is much tougher and thus less likely to scratch or dent in use. It is also the only material commonly used for pans that is magnetic, which is a big consideration as modern induction hobs only work with magnetic materials. An aluminium pan is useless on an induction hob.
Fortunately, material science can come to the rescue of the cook wanting to have both the thermal properties of aluminium or copper and the durability of steel. Many pans are now made with more than one type of metal. The simplest way this is done is with what are known as copper-clad pans. Manufacturers take a sheet of steel, a sheet of copper and then another sheet of either steel or occasionally aluminium. This sandwich of metal sheets, with copper in the middle, is run through a very hot system of rollers that squeezes and fuses the sheets together into one. Pans made from these sheets have steel on the outside not only for durability but also so that they work on inductions hobs. The copper sandwich filling helps spread the heat around. On the inner surface is either another layer of steel or, in frying pans, aluminium for its specific heat capacity.
The alternative to clad copper are copper-core pans, which are usually more expensive as the process is more difficult to do and uses more metal. These have a base made from a flat disc of copper encased in aluminium, which is then encased in steel. The resulting thick base disk is fixed to the bottom of a steel pan, which now has the advantages of copper’s high heat conductivity, aluminium’s high ability to hold onto heat and steel’s durability.
You now have a frying pan or a saucepan with the perfect combination of materials to give a uniform and long-lasting heat, but when you cook with it food sticks to the surface. Chemically, what happens here is that proteins and sometimes sugars are reacting with the surface molecules of metal in the pan. This happens with copper, aluminium and steel pans, and the simplest way to stop this is to stir the food, keeping it moving and not giving the chemical bonds time to form. Failing that, coating the metal with something less reactive will also prevent sticking, and the most common non-stick coating used is Teflon.
Invented by accident in 1938 by an American chemist called Roy Plunkett, Teflon or polytetrafluoroethylene (abbreviated to PTFE) is a long carbon molecule made unreactive by the addition of lots of fluorine atoms. The problem is how you stick something very non-sticky like PTFE to the surface of a frying pan. Chemical methods have been used to do this, but the chemicals involved are unpleasant and toxic. Instead, these days the pan to be coated is first sandblasted to create an incredibly rough metal surface. When liquid PTFE is then applied, it flows into all the rough nooks and crannies made by the sandblasting. As the now smooth surface hardens, it is physically bonded to the underlying material. The PTFE grabs onto and holds all the little bumps and lumps of metal. Once the coating is securely stuck down, the secret to the success of PTFE is the wall of fluorine atoms at the surface. Fluorine bonds incredibly strongly to the carbon in PTFE and, once bonded, cannot bond to anything else. Consequently, the food in the frying pan has nothing to attach itself to and won’t stick.
There is a lower-tech version of Teflon that is the saving grace for cast-iron pans. To make a cast-iron pan non-stick, or to season it, you need to first coat it in a thin layer of oil and then bake it in a very hot oven (260 ºC or 500 ºF) for about an hour. This intense heat breaks down the oil into small two- or three-carbon atom units. As you then cool the pan, these units link together forming hugely long carbon chain molecules. These long carbon chains act like PTFE, coating the underlying metal and preventing food from making chemical bonds with it. It doesn’t have the unreactive fluorine coating but it has the advantage of being scratch resistant and easy to reapply.
When you come to choose a frying pan, be it a new one from a store or maybe just one from the selection in your cupboard, take a moment to consider the science. Critical to what makes one pan better than the other is the material science of the component metals, and also the chemistry of the surface layer. Get this correct and you have a perfect cooking surface every time.
Under vacuum
Of all the ways you can cook protein, the sous vide method relies more than any on the digital precision of a temperature probe. First, you place your food in a plastic bag, then suck all the air from the bag with a vacuum, seal the bag and finally place it in a digitally temperature-controlled water bath. The name sous vide comes from the French and means ‘under vacuum’.
You may be thinking at this point that this is just a fancy and overly complicated way of poaching something. You may have a point, but there are two things that set sous vide cooking apart from simple poaching. Firstly, the food is sealed in a bag containing no air. Any flavour or moisture from the food stays in the food and is not wafted away in the poaching water. The same is true of added spices and herbs you put in the bag before sealing it. Furthermore, the absence of any air in the bag stops spoilage from oxidation and if the cooking temperature is high enough the contents are effectively sterilized by the process, so the food can be stored in the bag.
The second big benefit of sous vide cooking is that the temperature of the water bath is never boiling; in fact, it is rarely used above 80 ºC (176 ºF) and more often it is set to somewhere around 60 ºC (140 ºF). It is the temperature of the water bath that is critical to the mouth-watering results of sous vide cookery. The water bath temperature is usually controlled to within a fraction of a degree. It’s not a complicated machine as it consists only of a heating element controlled by an ever-so-handy digital thermometer probe. You set the exact temperature you want: the probe monitors it and turns the heater on and off accordingly.
So, let’s say you are cooking a piece of fillet steak. You set your sous vide water bath to 57 ºC (135 ºF), pop the steak in a bag, vacuum seal it and bung it in the water. Now, very slowly, taking about an hour, the temperature of the meat comes up to the temperature of the water bath, 57 ºC (135 ºF). At this temperature, most but not all of the different types of protein molecules in steak will denature. The myosin that makes up the bulk of the steak will be denatured, making the meat tender and not tough. Another protein called myoglobin, which gives the meat its red colour, will be just starting to denature so the meat won’t be blood-red but pinkish. However, the actin protein will still be in its native form, which is good as when this denatures the meat turns tough and tastes less juicy. The whole block of meat, from the outside to the very centre, will be at exactly 57 ºC (135 ºF) and consequently a perfectly cooked medium-rare. If you want your steak rare, the temperature you need is 49 ºC (120 ºF), well below the temperature that myoglobin denatures. For medium, it’s 60 ºC (140 ºF) and the myoglobin has completely denatured. And if you want to ruin the steak, at least in my opinion, a temperature of 74 ºC (165 ºF) will denature all the protein including the actin and give you well done.
This is the genius of the sous vide method of cooking and why it is the most scientific way to cook. By knowing at what temperature the different proteins in your food become cooked, you can achieve precise, reproducible levels of cooking. To take another example: the humble egg that so many of us struggle to cook the way we like it. Part of the problem is that we all like our eggs cooked differently. Are you a runny-yolk person and, if so, are you willing to put up with a bit of unset egg white? Or is the thought of floppy egg white so vile you take the egg the whole way and want the yolk just set, but not overset when it turns crumbly. The other issue with cooking eggs is that not all eggs are equal: they come in different sizes, varying freshness and the temperature before you start cooking makes a difference. When you plop an egg into boiling water, the outside is immediately at 100 ºC (212 ºF), which is a temperature that will denature all the proteins in the egg, setting the white and the yolk and even beginning to release some of the sulphur compounds in the proteins. Clearly the key to cooking the perfect egg in a pan of boiling water is timing it just right, taking into account size, freshness and starting temperature. Not so if you use a sous vide cooker.
Firstly, it should be noted that eggs are perfect for sous vide as they already come in a handy sealed package and need no vacuum treatment. The albumin or white portion of the egg is made up of a number of proteins, most of which cook or denature between 61 and 65 ºC (142 and 149 ºF). The yolk protein denatures and turns solid between 65 and 70 ºC (149 and 158 ºF). Based on this information you can now make the perfect sous vide cooked eggs. For just-cooked white and completely runny yolk set you water bath to 63 ºC (145 ºF). If you prefer a firmer white and a mostly runny yolk, it is 66 ºC (151 ºF), and for a just-set yolk turn it up to 70 ºC (158 ºF).
The beauty of sous vide cooking is that the temperature at which a protein from a particular animal denatures or cooks is always exactly the same. Knowing this, you can take the guesswork out of cooking the food the way you want it, and get reproducible results every time. So, why don’t we all have sous vide cookers? Well, you actually need two pieces of kit, the water bath and the vacuum-sealing system, both of which tend to be bulky and expensive. On top of that, sous vide cooking gives a different type of result. A steak cooked sous vide may be perfectly medium-rare, but will have no delicious browned surface. The yummy outside of a steak is caused by the Maillard reaction that happens at 154 ºC (309 ºF) (see here). Finally, it takes a lot longer when you are cooking at such low temperatures for the heat to fully penetrate your food. While sous vide is in theory a brilliant way to cook and has its place, I can’t see my family waiting an hour in the morning for a perfectly runny egg.
Under pressure
So if the sous vide method is the most scientific way to cook food really slowly, then how does a science geek go about cooking food fast? You may be expecting me to start on with the microwave oven at this point, but for me the pressure cooker wins the geek credentials. The story of its discovery is also rather intriguing.
Towards the end of the seventeenth century, a Frenchman called Denis Papin was working as an assistant to the Curator of Experiments at the Royal Society in London, the oldest scientific society still in existence. The curator was a somewhat irascible man called Robert Hooke who cast a huge shadow across the scientific landscape. Papin presumably had some leeway with his work as in 1679 he demonstrated to the assembled Royal Society dignitaries his ‘New Digester or Engine for Softening Bones’, or the pressure cooker to you and me. The device consisted of a metal pot, without a handle and a lid that could be screwed down to create an airtight seal. Crucially, he had also invented the safety valve that used a lever and a weight to prevent steam escaping through a hole in the pot’s lid until the correct pressure was achieved.
As part of his demonstration he placed within his pot all manner of cheap cuts of meat and a little water. In no time at all he had produced a delicious, tender and succulent stew to the delight of the assembled scientists. In 1681, he published his culinary experiments and invention in a small pamphlet in which he explained how the pressure cooker could, among other things, be used to feed the poor with nutritious gravy made from the cheap meat nobody else wanted, as well as rabbit. He seemed to spend a lot of time cooking rabbit in his New Digester. Sadly, the Royal Society didn’t really pay much attention to Papin’s invention and it seems to have been viewed more as an academic curiosity.
At some point in the next 200 years, the pressure cooker moved from the hands of academics to the everyday cook. What history does not record is exactly how or when this took place. We know that in 1864, Georg Gutbrod of Stuttgart in Germany had a secret process to make cast-iron pressure cookers, coated in tin. His cookers were deemed to be superior to others available at the time, which clearly implies that the devices had already been in general use for quite a while. What marks these early devices from the modern variety is that they all look like pieces of industrial kit; they were incredibly heavy, had enormously thick walls and the lid was fastened with great big screws and clamps. Which probably goes some way to explain why they never became particularly popular. Then in 1938 an American chap called Alfred Vischer revealed a new pressure-cooker design that looked and handled much like a regular saucepan. Since then the design of the pressure cooker has remained essentially the same.
A Papin pressure cooker (left) with screw-down lid and lever safety valve, and a modern pressure cooker (right).
The clever bit about a pressure cooker is that it makes the water inside the cooker boil at a temperature greater than 100 ºC (212 ºF). When a pressure cooker is up to steam, the water inside is boiling at around 120 ºC (248 ºF). Which might be surprising, as everyone knows that water boils at 100 ºC (212 ºF). But this only applies at standard atmospheric pressure, which is the average air pressure at sea level (101,325 Pascals or about 14.70 psi). To explain this, let’s look at why a liquid boils at all.
Liquid water is made up of a bunch of water molecules all jiggling about. The molecules are not completely free to move anywhere they want as they are all holding onto each other. They are not holding on to each other as tightly as molecules in ice, which is why liquid water can flow and slosh about. I said that the water molecules are all jiggling and this is due to the heat energy they have. More heat equals more jiggling, but not all molecules have exactly the same amount of heat energy. Most will have an average amount but some will have less and some will have more. When one of those juiced up, high-energy molecules is at the surface of the water it may be able to break free from its neighbours. It needs to overcome not only the clutches of the other water molecules but the molecules of gas above the liquid trying to push it back in. So far, so good, as this explains why a puddle of water will eventually dry up without boiling. It slowly evaporates as the high-energy molecules break free. Now, imagine what happens when you heat the water up. You put more and more energy into the molecules and they jiggle faster and faster. When you hit 100 ºC (212 ºF) at standard atmospheric pressure, most of the molecules now have enough energy to not only break free from their neighbours, but push past all of the molecules in the gas above the water. In fact, at this point you start to see bubbles spontaneously forming inside the liquid, which then grow as more liquid rushes into the gaseous form.
But now imagine that the pressure of the air above the water is higher. For a water molecule to escape the liquid it needs to push past more gas molecules, which makes it harder and requires more heat energy. So, the boiling point goes up. If you increase the pressure to twice standard atmospheric pressure (about 203,000 Pascals or 30 psi), the boiling point of water rises to 120 ºC (248 ºF). Which is precisely what happens inside a pressure cooker. As the water begins to boil at 100 ºC (212 ºF), the water vapour or gas produced has nowhere to escape to, so it pushes the pressure up. In turn, this increases the boiling point and you continue heating and the water begins to boil again, and the pressure goes up and so on. Eventually Papin’s safety valve comes into play at about double atmospheric pressure and the pressure stabilizes.
So, the physics of pressure and boiling point give us an extra 20 ºC (36 ºF) for cooking with, which does not seem very impressive. Until, that is, we consider the Arrhenius equation from 1889 that says if you increase the temperature by 10 ºC (18 ºF) a chemical reaction will proceed at twice the rate. So, the temperature inside a pressure cooker will cook food about four times faster than plain old boiling. Which is why, with all that delicious science at play, the pressure cooker wins the geek award for fast cooking.
There are a few other benefits to pressure cookers: they use much less water and hence energy than an equivalent long, slow stewing time and they also have temperatures hot enough to produce delicious flavour molecules through an essential bit of kitchen chemistry called the Maillard reaction (see here).
Pressure cookers have, like Papin, been overlooked in the last few decades. They are undeniably quick and efficient, but suffer from being very heavy and awkward to store and perhaps ever so slightly terrifying when the steam pressure needs to be released. On top of that, the meteoric rise of the microwave means that, at the moment, the pressure cooker remains a fringe interest at best.
Adding air
There is one essential bit of kitchen gadgetry that has not only a long and noble culinary history but, unlike the pressure cooker, is also found in every kitchen. The utensil I am talking of is the humble egg whisk. It’s one of those items that seems so fundamental you can’t imagine a period when a cook would not have one to hand.
Such a time did exist and we can deduce its existence from documentary sources and the rise of the use of whisked egg in culinary instructions. The earliest recipe-book reference to whisked egg comes from the 1602 tome by Sir Hugh Platt called Delightes for Ladies, or to give it its full title, Delightes for Ladies: to adorn their persons, tables, closets, and distillatories with beauties, banquets, perfumes and waters. The book is full of hints and tips for use within the house and includes instructions on how to ‘Break Whites of Eggs Speedily’, which is Elizabethan code for beating eggs. However, the proposed method is to use either a knobbly stick or to repeatedly wring the eggs from a sponge. Neither of which sound very efficient. You may be thinking that a fork would be a better way to do this, but at the time this was an unknown item in northern European households. The fork first debuted in Great Britain in 1611 in an account of travels to Italy and the implement was still seen as an effete southern European affectation well into the late 1700s.
The results of squeezing egg from a sponge or whacking it with a stick barely constitute whisked eggs. At best, you get a light froth. Then in 1651 we start to see recipes that are made with what must be whisked eggs. Sweetened, whisked-up egg was referred to as snow and plopped on top of a variety of different desserts. The assumption by culinary historians is that people must have moved on from knobbly sticks and sponges to whisks of some sort, probably made from twigs. There is even a recipe that suggests using cut and bruised apple twigs to whisk egg whites to impart an apple flavour. I’m not sure if the flavour of apple would come across but this is definitely the first use of a whisk. The thing is, you can’t effectively whisk up egg whites using a single stick: you need a whole bunch made into a whisk, and this is where the science comes in.
The reason you can whisk egg whites is because the proteins in the albumin are denatured by the whisking and can more effectively hang on to bubbles of air. Whisks are very good at creating air bubbles. As each strand of the whisk, or twig if you are old school, moves through the liquid, it pulls air down and creates bubbles. If you do this in water the bubbles quickly rise and burst, but use a more viscous liquid and the bubbles hang around for a while. Something else is going on, though, as the bubbles in whisked egg can last for hours. As the whisk whips through the egg white, the physical action of the individual, bludgeoning whisk-stands breaks open the coiled-up egg proteins (see here for how heat can do the same). This exposes the delicate inner workings of the protein strands – inner workings that are water-hating or hydrophobic. The now exposed, water-hating bits rush to the first place that contains no water, notably the air bubbles in the mixture. Every tiny bubble becomes surrounded with a sheath of broken down or denatured egg protein. These proteins quickly begin to bond to each other, forming a stable web of protein around every bubble.
At this point, a chef would describe your eggy mixture as soft peak. If you lift the whisk out of your mixing bowl any strands of foam will flop over, incapable of holding their own weight. But if you continue to whisk, the egg foam stiffens further to the stiff-peak stage. This is the point where you can infamously hold the bowl of egg over your head and it should stay there. As you continue to beat the egg and put more and more energy into the proteins, the bubbles get smaller and more numerous. This has a couple of effects: it makes the whisked egg look whiter and it reduces the amount of liquid between each bubble. The mess of denatured proteins around each bubble begin to tangle as they bump up against each other. As they do, they start to stick together and it is this that makes the whisked egg stiffen up. Since the proteins also start sticking to your bowl, it’s also why you can do the bowl-over-your-head trick. The stiff-peak stage is optimal for things like meringue that need to hold their shape in the oven.
However, continue to beat the egg and it all goes pear-shaped. You get what is known in the trade as dry peak. The foamy mass becomes almost crumbly and liquid begins to form at the bottom of the bowl. The problem you have here is that all those protein molecules are now pulling so tightly on each other that they start to squeeze the water in the mixture out from between the bubbles. The air bubbles in the foam can’t now move around and the foam becomes partially set.
There are a few methods that have been developed over time to make the whisking process easier, with some proving more successful than others. While the rotary mechanical hand whisk will make the job of whisking less tiring, it never seems to deliver when it comes to beating eggs. The physical effort needed compared to the volume of foam generated does not seem to be worthwhile. You invariably end up resorting to a traditional balloon whisk and wondering why you bother to keep the mechanical version. The issue with the hand-cranked whisk is really that the beating action is all wrong. The recommended chef technique to beat egg whites is not to use a rotating action, but a vertical oval movement that lifts the egg and introduces lots of air. If you have an electric whisk, that will do a great job by virtue of the sheer speed of rotation, far faster than anything you can do by hand. But most chefs reject this as well as it’s too easy to overbeat your eggs. The beaters are going so fast that it can be a matter of just moments between stiff-peak perfection and dry-peak disaster.
Your choice of bowl is also crucial, not least because the egg will increase in volume by eight times and you need big, extravagant motions of your whisk. But size is not the only thing that matter in your choice of bowl. If you can afford it, a copper bowl is universally regarded in the chef profession as best to use when beating your eggs. Tradition had it that you could not overbeat egg whites in a copper bowl. My immediate response when I saw this was that it sounded like a case of unverified tradition, but it turns out to have a significant effect that cooks had noticed way back in the eighteenth century. It wasn’t until 1994 that we worked out why. As you beat your eggs in a copper bowl, a very tiny amount of copper dissolves into the mixture. Before you worry, it’s well below the daily recommended dose for the metal. This copper then binds to chemically reactive sulphur groups on the denatured proteins. This prevents the formation of a particularly strong type of cross-link between proteins called sulphur bonds. The addition of copper effectively reduces the stickiness of the denatured proteins enough that they won’t get too clingy and develop a dry-peak foam, but they are sticky enough to whip up to stiff peaks. It may slow down the whipping process but it definitely makes it easier. You get the same effect with bowls made of silver or gold, although I’ll admit, those are harder to find.
Lacking the resources for a copper bowl, science can come to the rescue. Rather than adding copper to your egg whites, all you need do is add a little acid. A squeeze of lemon juice will do the job or, if you don’t want to change the flavour, add a pinch of a dry powdered acid like cream of tartar (a common baking ingredient with a horrible proper name: potassium 2,3,4-trihydroxy-4-oxobutanoate). This acid will have the same effect as the copper by blocking the formation of sulphur bonds and it will make your egg whisking easier.
The addition of sugar is great for stabilizing your foam although it clearly has a major impact on the taste. The reason it helps is down to the increase in gloopiness or viscosity of the egg mixture. A more viscous liquid will hold on to bubbles more easily and allow the protein networks more time to form, so you need less elbow grease with the whisking. On the other hand, fat is the enemy of the egg whisk. If you have any fat in your egg-white mixture, it will massively reduce and even prevent the egg from being whipped up. The fat molecules effectively do the same thing as the protein molecules because they too are water-hating or hydrophobic. When a bubble forms, the fat will accumulate on the bubble’s surface, competing with the protein for space. Consequently, the network of proteins never forms and your foam flops. So, how much fat will ruin your eggs? Folk wisdom proclaims that the slightest drop of egg yolk, which is packed with fat, will be your downfall. This misconception is easily tested and egg whites will whisk up fine even with a few drops of yolk in them. The other taboo you often see mentioned in cookbooks is that plastic bowls should be avoided. While it is true that oils and fats will cling to the plastic of the bowl and thus could pose a problem for the egg whisking, this is really just a sign that the bowl has not been cleaned properly with hot soapy water, which will strip the fat away from the plastic.
The egg whisk is a remarkable utensil that brings about a complicated change in an everyday food, turning it into something quite out of the ordinary. Armed with the science behind the transformation it creates, you should be better placed to whisk eggs like a pro.
Keeping cool
For all the wonders of the pressure cooker, the egg whisk and the sous vide gadgetry, none of these are as transformative as our ability to preserve food with cold and our dependence on electrically produced cooling. In 2015, the combined value of the chilled and frozen food sold in UK supermarkets was £18 billion, which is such a big number it is hard to get a grip on what that means. Put it this way: you could say that in 2015 every woman, man and child in the UK bought and presumably consumed £275 of frozen or chilled food. Of course, we throw away huge amounts of food and it is unlikely that a newborn infant would get through their allotted £275 worth, but you get the idea. It’s a huge chunk of the food that is sold in the UK and it all relies on refrigeration.
There is one reason we refrigerate so much of our food chain and it all comes down to the Arrhenius equation, which you will remember from the pressure cooker explanation (see here). The simplest version of this is that the rate of a chemical reaction doubles for every increase of 10 ºC (18 ºF). By the same token, but in reverse, a drop of 10 ºC halves the reaction rate. All of the things that can spoil your food rely on chemical reactions to change the molecules within the food. Reduce the temperature enough, the reactions slow down and the food stays in top condition for longer. Bacteria are the most common reason food spoils and these, like all living things, are essentially bags of chemical reactions. These conform to the Arrhenius equation just like all chemistry must, although due to the complex interactions within a living organism, the chemistry slows down even more quickly as the temperature drops. Which is why, if you freeze a lump of meat, it can last for much longer than the equation predicts. The safe storage time for meat that has a temperature of 21 ºC (70 ºF), or room temperature, is just two hours. After that the bacteria levels could be too high for safe consumption. Freeze the meat to -18 ºC (0 ºF), a drop of nearly 40 ºC (70 ºF) and the chemical reactions should slow to one-sixteenth of the rate (½ × ½ × ½ × ½). According to the maths, it should be safe to store the item for thirty-two hours at this temperature (2 hours × 16). Clearly biology has little respect for maths and you can store meat for much, much longer in a freezer since bacterial growth comes to a complete standstill at -18 ºC (0 ºF). Although, it’s important to note it won’t kill the bacteria.
People have been using artificial cooling for well over 3,000 years. Ice and snow would be harvested and stored underground or in special, thickly insulated ice houses and then used once the weather began to warm. However, most antique cultures just used their icy refrigeration to cool drinks. It is harder to find evidence for using the cold to preserve food. The closest we get in antiquity is the Persian dome-shaped yakhchals from about 400 BC. These were huge, 10-metre-tall (33 feet) cones made from a special mortar that used evaporative cooling to create ice in the winter. The ice was then stored through to the summer.
We know the Persians used the ice to cool drinks and also to make faloodeh, a frozen dessert, but if they used the yakhchal to preserve food with cold, there doesn’t appear to be any evidence. Many authors will claim they did, but this appears to be purely speculative. We take it for granted that if you have a chilled place, you are going to keep perishable food there so it keeps longer, but that is because we also take it for granted that we understand the science of food preservation and what causes food to perish (see here).
A more likely example of the first use of cold for food preservation is the native Inuit of what is now northern Canada. When Clarence Birdseye visited Newfoundland in around 1912, he observed Inuit flash-freezing the fish they had caught and then thawing it out later to eat. Mr Birdseye is often credited as the inventor of frozen food, but it’s clear that the Inuit were using the frozen environment as a walk-in freezer long before.
For the rest of us, the chilled and frozen-food revolution began with the discovery of the vapour-compression cycle. At the heart of this is an observation made in 1755 by a Scottish chap named William Cullen. If you take a low-boiling-point liquid, such as ether, and put it at low pressure it evaporates and cools down, drawing in heat from its surroundings. It was something a lot of eighteenth-century scientists went on to investigate. When they soaked something in ether, the evaporation would cause significant cooling. Benjamin Franklin even had a go and commented in a letter to a friend that ‘from this experiment, one may see the possibility of freezing a man to death on a warm summer’s day’. Finally, in 1805, courtesy of American inventor Oliver Evans, we had a full description of how to create a cycle that pulls in heat during the evaporation step and gives it up with condensation. All that was needed now was to hitch the vapour-compression cycle to a machine that would pump the heat from inside a box, which became cold, to outside the box.
The first demonstrably workable and practical such device was the invention of Scotsman James Harrison, who had emigrated to Australia to work as a journalist. In 1856, he patented a machine that was used initially to make ice for the good burghers of Geelong, a city just 75 km (46 miles) to the west of Melbourne. Those canny Aussies were not slow to make further use of Harrison’s cooler and soon had it installed in breweries and meat-packing companies. It was this second use that was to become Harrison’s undoing. At the time, there was a major trade in beef from the US to the UK. The journey time was less than two weeks, the weather on the way predictably cold and, with a bit of ice on board, the carcasses would easily survive the voyage unspoiled. The boat journey to Australia was considerably longer and, in an effort to open up a competing trade, Harrison decided to send frozen beef to the UK. However, he was persuaded that it was too risky to install a refrigeration unit on a ship and instead, in 1873, built an insulated ice room on board the sailing ship Norfolk. Hundreds of cattle carcasses were duly frozen solid and packed in ice made using his refrigeration system. Sadly, his calculations were wrong, or maybe it was hotter than expected on the voyage, and the ice thawed. History does not record what became of his cargo, but the first venture in frozen-food delivery was not a success.
The birth of the frozen-food industry and the whole attendant revolution in our consumption habits came just a few years later with an even more epic journey. On 15 February 1882, the good ship Dunedin set sail from New Zealand to England fitted with a steam-powered refrigeration unit. The system burnt two tonnes of coal a day but kept the entire hold below freezing throughout the long passage through the tropics. The journey was not without excitement: there were fires, broken crankshafts and Captain Whitson even developed hypothermia while working in the hold. However, on 24 May the ship arrived in London with its perfectly frozen cargo of 4,331 mutton carcasses, 598 lamb carcasses, 22 pig carcasses, 250 kegs of butter, an unrecorded number of hare, pheasant, turkey and chicken, and 2,226 sheep tongues. Quite why you need that many sheep’s tongues I don’t know, but the age of refrigeration had well and truly begun.
Today the chain of logistics that runs from food producers through to our tables is rarely without a refrigerated step at some point. All your fresh vegetables, fruit and bags of salad need refrigeration to prevent degradation and retard ripening too soon. Then you have all of the frozen and chilled ready meals, dairy products, sliced meats, fish, juices and, harking right back to the beginning of refrigeration, cooled drinks. Most of the process that brings all this food to us is hidden; we just see the end result. However, a little while ago, I was fortunate enough to see just one section of how it all functions when I was involved in making a film. England has just three central hubs for frozen-food distribution. These massive facilities, known as ice cubes, are strategically placed around the country and take in frozen produce grown locally. This produce is stored and then sorted back onto refrigerated lorries destined for shops, not just regionally but all over the world. The scale of the operation is quite breathtaking. To sum up, within science, the results of any experiment are ultimately the end goal. However, the technical know-how and expertise to perform scientific experiments is clearly a vital component of understanding science as a whole. Sometimes how a result was achieved is as important as the result itself. In the same way, the science of food is as much about how we prepare the food as it is about the chemistry, biology and physics of the food itself. So, whether it is the blade of a knife, a fancy high-pressure or low-temperature cooking machine, or the humble egg whisk, the technology of our kitchens deserves a second look and there’s more on the future of this technology later in the book (see here).
