The five-second rule
The five-second rule goes like this: if you drop some food on the floor, it is OK to pick it up again and eat it so long as it was on the floor for less than five seconds. Occasionally you see a stricter version of this: the three-second rule. But is there anything even remotely scientific in either of these nuggets of folk wisdom?
Clearly you don’t want to eat any bits of adhering grit or strands of hair, but let us assume that you brush or blow away any visible detritus picked up from the floor. Is the food safe to eat? There is a very simple answer to this question: no, the food is unsafe for consumption once it touches the floor. But when examining this answer, we first need to think about what we mean by safe. Because if this is about safety then it is really about risk, and if it is about risk then it’s time for some risk assessment.
If you put on your health-and-safety hat, then the first thing you do when risk assessing is to identify the potential hazard or, in other words, what it is that could go wrong. In this case, assuming the food is free from inedible grit, then the danger is that you may be about to ingest harmful bugs that cause an upset stomach, probably diarrhoea, stomach cramps and possibly a fever. The most likely cause of this is bacteria called Campylobacter jejuni, which most people haven’t heard of because the food poisoning it causes is normally pretty minor. However, about three-quarters of all upset stomachs are caused by these bacteria. The ones that make up the other quarter of cases are the bacteria you have to watch out for. Food poisoning caused by salmonella and Escherichia coli (or E. coli for short) can be really nasty. In the UK, about 2,500 people end up hospitalized each year with salmonella. But the one you really don’t want to pick up is E. coli. Normally this is a harmless bacterium that we have living happily, and harmlessly, in our guts (see here). But there are a few strains of this bug that have developed a particularly nasty ability to produce Shiga toxin. Named after an early twentieth-century Japanese microbiologist who first described it, Shiga toxin has some extremely unpleasant effects on our bodies. I won’t go into all the gruesome details, but the end result is often hospitalization and it can lead to kidney failure and even death in extreme cases. The most well-known strain of bacteria that can cause this is known as E. coli O157:H7 and it’s been responsible for some high-profile outbreaks all over the world. The somewhat scary thing is that you don’t need that many of these bacteria to give you a dose of food poisoning.
When you drop food on the floor, the hazard is that you then ingest something like E. coli O157:H7 and die. The second part of any risk assessment is to look at how likely it is that the hazard is actually going to happen, and this is where the science comes in. There have been a number of studies on the topic of the five-second rule, going back to 2003 when the first experiments were done by Jillian Clarke, a sixteen-year-old summer intern at the University of Illinois in the US. She took small square tiles, inoculated them with a harmless variety of E. coli and then dropped either a gummy bear or a cookie onto the tile. After five seconds the food was removed and in every case was found now to be contaminated with the test bacteria. For her efforts, she won the 2004 Nobel Prize for Public Health. Well, to be honest it was an Ig Nobel Prize, awarded for making people laugh and then think, and she shared the prize stage with studies on the physics of hula hoops, a man who patented the comb-over hairstyle and the group that discovered that herring fish communicate by farting. What was important, though, was that she took what many would assume to be either a trivial or silly subject and applied rigorous science to it.
After her initial study, other researchers took up the baton and have expanded on the subject, testing different foods dropped on to a range of surfaces. This is also not fringe science; the latest work was accepted for publication at the end of 2016 by the highest-rated scientific journal in the field of microbiology. In this study, the scientists, Robyn Miranda and Donald Schaffner at Rutgers, the State University of New Jersey, used a range of foods including bread, buttered bread, slices of watermelon and, once again, gummy bears. It may seem like a strange mix of food to drop, but it covers a range of foods that are wet, oily, dry and, well the gummy bears are a bit of mystery to me, to be honest. They dropped the food on to carefully inoculated sections of steel, ceramic tile, wood and carpet and then looked at how many bacteria transferred after a second, five seconds, thirty seconds and five minutes. What they found backed up Jillian Clarke’s work: the food was contaminated the moment it made contact, or at least within a second. Which means that the five-second rule is rubbish, but they also found that the longer you left something the more bacteria transferred on to the food. What comes as a surprise is just how few bacteria there are on a surface that is dry and has been dry for a considerable time. Bacteria can survive in dry conditions, but not for long. If a surface has been left dry for a few hours or preferably days, the number of bacteria it may harbour is really small. However, if your surface has a film of water across it, chances are it is teeming with bugs. Similarly, wet food is much better at picking up bacteria. The water is sticky and flows into all the nooks and crannies of the surface on to which the food is dropped.
So, it seems that the odds change if the food you drop is dry and not sticky – maybe a bit of toast – and if the surface you drop it on is also dry and has been dry for many hours, perhaps the floor in your kitchen. In this scenario, the chances are that there are very few bacteria to pick up and not many of them will transfer on to your toast. In which case, you may decide that you can eat it. However, although you have probably minimized the chances of anything untoward happening, you can never reduce them to nothing. And don’t forget it only takes a few E. coli O157:H7 and you can become seriously ill.
I should also point out that the work by Miranda and Schaffner discovered that the best thing to drop your food on to, for minimal contamination, was carpet. Presumably the food makes very little contact with the actual surface as it sits on the upthrust carpet fibres. If the toast falls on carpet, the odds are in your favour. However, you may end up with fuzzy toast and, if the toast lands butter-side down, then it’s just a horrible mess.
The microbiota in us and on us
All of this talk of deadly bacteria that you should never take a risk on may give you the idea that all bacteria are bad for us, but that is not the case. Bacteria are an incredibly important part of the way we digest our food and, recently, it has been suggested that they may even be involved in controlling our appetite for food. It has been common scientific knowledge for many years that there are millions and millions of bacteria living on our bodies and inside our bodies. Initially it was thought that these bacteria were essentially hitching a ride with us and, while they caused us no harm, they were equally of no great benefit. You will see various estimates as to just how many bacteria there are, but the media and popular kids’ science books often revel in the statistic that there are ten times as many bacterial cells in a human being as there are human cells. Which implies a great conclusion that we are numerically more bacterial than we are human. The numbers are also really big, which always helps attract attention. The bacterial number usually quoted is 100 trillion cells, that’s a one with fourteen zeros after it. The most recent thinking is somewhat less dramatic and the latest estimates done in 2016 by a trio of scientists at the Weizmann Institute of Science in Israel found that the ratio is much less than ten to one and more like one to one. On average, for a standard 1.7-metre tall, 70-kilogram man (that’s 5 feet 7 inches and 154 lbs) there were slightly more bacterial cells than human cells, the ratio being 1.3 to 1, which does not make nearly as good a headline.
But what is interesting is that the number of bacteria vary wildly between people; some will have twice as many bacteria as the average and some half the number. The massive downgrading of the numbers is mostly due to better data but also down to an understanding of where bacteria are found. Comprehensive surveys have been carried out and the bacterial populations mapped on a wide range of individuals. Different parts of your body host different bacteria. The bacteria on the skin of your scalp live in a radically different environment to the ones between your toes; consequently the types of bacteria found are also completely different. Belly-button bacteria are worthy of a quick mention. Apparently, the environment in the average belly button, or umbilicus to use its proper name, is such that only a single family of bacteria can survive. It is clearly an unusual environment. So much so that on one of the people tested, researchers found members of an entirely different domain of life, the archaea. To clarify, there are three domains of life: bacteria, eukaryota (which includes plants, fungi and animals) and the archaea. These are similar to bacteria but have unique biochemistry and are normally only found in the most extreme environments. That said, the person with the archaea living in their belly button did claim that they had not washed for several weeks, which may explain the extreme environment.
Taken together, all of the bacteria living on and in our bodies are known as microbiota. So, what are they all doing? The idea that these are just hitchhikers on a human body is no longer believed. Your intestines house the bulk of these bacteria, mostly concentrated in the large intestine as the small intestine and stomach are generally too harsh an environment for many bacteria to thrive. It turns out that the bacteria in your gut play a number of essential roles. Firstly, you have bacteria in your large intestine that are capable of digesting some of the plant fibre and complex carbohydrates that would otherwise be completely indigestible. These bacteria break down the fibre to produce what are known as short-chain fatty acids, or SCFAs to their friends. These in turn can now be absorbed by your body and provide energy and help you gather essential nutrients such as calcium, magnesium and iron. You will have encountered the effect of what happens when this bacterial digestion goes wrong. If you take a course of antibiotics, a common side effect is antibiotic-associated diarrhoea. The drug not only kills off the pathological, bad bacteria, but also the good ones, so that your gut bacteria are essentially wiped out, fibre cannot be turned into SCFAs and water is trapped in the large intestine.
This, though, is not what has got microbiologists excited at the moment. Just recently, a number of experiments have been done that show that the bacteria in your gut may have an ability to change your body in pretty radical ways. It looks like the gut microbiota in mammals can control not only weight but also mood and possibly even behaviour. Now, these experiments have all been conducted using mice, which means that there is a chance that the results do not directly apply to humans. Mice are a pretty good substitute for humans in this kind of science, though, and all of the work revolves around the use of special germ-free mice. These are bred in completely sterile environments, and over generations have come to the point where there are no bacteria anywhere on the mice or even inside them. The most immediate impact on the mice is that they have to eat a lot more food to maintain a healthy weight, which is a direct result of having no bacteria in their gut to digest fibre and produce nutritious SCFAs. Researchers from St Louis in the US then colonized germ-free mice with the gut microbiota of other ordinary mice. Another term for colonized is faecal transplant and, yes, that does indeed mean a poo transfer. What is interesting is what happens if you transfer gut microbiota from mice that don’t have a normal weight. Put bacteria from an obese mouse into a germ-free mouse and it too becomes obese. Similarly, bacteria from an underweight mouse will make a germ-free mouse become underweight. There is clearly something that the gut bacteria are doing to the germ-free mice that changes their metabolism. Exactly what is going on is just beginning to be worked out.
A group based in Yale University in the US showed that it may be those nutritious SCFAs that are to blame. They fiddled with the gut bacteria of mice so that they produced more SCFAs and noted that this somehow turned on a whole slew of signalling systems in the brain that resulted in the release of the hunger hormone called ghrelin. Ghrelin is normally released into your bloodstream when your stomach is empty and increases your sensation of being hungry. So, when the mousey gut bacteria make too many SCFAs, you get hungry mice that then become fat. Perhaps even more fascinating are two sets of experiments done in 2016 by scientists in Cork, Ireland, and Houston in the US.
The Irish scientists showed that if you took gut bacteria from people suffering from severe depression and transplanted these into the germ-free mice, the mice became depressed too. Now, it may seem bizarre to call a mouse depressed, but there are ways to gauge the mental state of mice in a laboratory setting. Note that this was depressed human gut bacteria that changed the moods negatively in mice. On top of that, the Houston group made young mice antisocial by giving them the gut microbiota of obese mice, and then made the juvenile mice social again by feeding them bacterial supplements. It looks like, in mice at least, not only is their weight partially regulated by their gut microbiota but also their moods and behaviour.
So, what does this all mean for humans? Well, while we can’t yet be certain that the bacteria inside us are doing the same things, there is clearly a lot more going on than we thought. It has been known for a long time that stress in humans is linked to various digestive and bowel complaints. Irritable bowel syndrome, for example, often goes hand in hand with clinical depression. The assumption is usually that the depression is causing the irritable bowels, but it may be possible that it is the other way around and that the root of both problems lies with the bacteria in the large intestines.
However, there is one simple way you can change your gut microbiota. A change of diet can have a profound effect on the variety and numbers of bacteria in your large intestine. What is more, the effect can be really rapid. In a 2013 study from Harvard University in the US, it took just twenty-four hours from switching to a radically different diet for volunteers to show an equally radical change in their microbiota. Which brings us back to the things we eat and how it can have a profound effect on us in more ways than we previously knew. While it is probably not true that we are more bacteria than human cells, it looks like those bacteria play an essential role in our relationship to food.
The subtle science of killing bacteria
Given that the world is so permeated with bacteria, viruses and other bugs, on every work surface in your kitchen, on you, in your belly button and inside your gut, is it possible to protect food from these microscopic organisms? Any food grown outside of a sterile laboratory is, just like you, going to be covered in micro-organisms of one sort or another. Furthermore, we know that bacteria in particular will in time cause food to spoil, turning palatable nutrition into inedible rubbish. So, can you get the bacteria out of food? Yes, but there is a cost. If you want to completely remove bacteria and other micro-organisms from food there are several ways you can do this, but in so doing you will change either the taste or texture of the food. Consequently, most food-preservation techniques don’t remove all the bacteria, they just reduce the numbers. It’s usually a compromise situation where we balance the longevity of the food with alterations to taste and texture.
The first person to really show that it was possible to make something completely free of bacteria was the eighteenth-century Italian scientist Lazzaro Spallanzani. Ironically, he was not investigating food preservation but something more fundamental than that. By the start of the 1700s the development of the microscope had allowed us to see bacteria and other micro-organisms for the first time. They were everywhere, it became apparent, and the question was where did they come from. The prevailing wisdom was that they spontaneously arose when non-living matter fused with a mystical spark of life. In 1768, Spallanzani set out to prove otherwise. It had already been shown that if you boil up meat broth you kill all the micro-organisms, but they always seemed to repopulate and grow anew. Spallanzani took broth and placed it in a sealed container, then dropped the whole lot into boiling water for an hour. The heat transferred through the container and into the broth, killing all the bacteria, and since the container was already hermetically sealed, there was no way for recolonization by micro-organisms. Thus proving that spontaneous generation of life was not possible. It was an experiment that others had tried before, but where they had failed was in creating a vessel that could be heated without the contents expanding and bursting out. However, Spallanzani was not interested, or it did not occur to him, that his proof that life did not spontaneously arise had an application in the kitchen.
It wasn’t until 1810 that food preservation by heat treatment was cracked by a Frenchman, Nicolas Appert. Fifteen years earlier, in the closing years of the French Revolution, the military leaders of the recently minted republic (including the twenty-five-year-old General Napoleon Bonaparte) offered a substantial 12,000-franc prize for a new method of preserving food. Appert was a confectioner in Paris when the prize was first offered but set to work using technology he was familiar with from his home town, notably champagne bottles, and Spallanzani’s technique. After initial successes, he moved to wide-necked bottles with corks sealed with a paste of lime and cheese. An odd mixture, but apparently one that survived the boiling treatment. His results were evidently impressive, his bottled peas being described in his 1810 book as having ‘all the freshness and flavour of recently gathered vegetables’, and he was duly awarded the prize money by the now Emperor Napoleon. But it was a different Frenchman, Philippe du Girard, who made the leap to working with metal containers. Since he couldn’t get a patent in France, as Appert had that market sewn up, he essentially stole the idea and filed a patent in the UK through a London-based agent called Peter Durand, whose name is on the patent. Which is why Durand gets all the credit for inventing the tin can even though he did none of the work. By 1813, the first canning plant opened in Bermondsey, south London, but the canned food was expensive and only really used by the military and as a frivolous expense by those that could afford it. A particular impediment to the adoption of canned food was that the can opener was not invented until 1845. The instructions placed on early cans was that they should be opened with the aid of a hammer and chisel, a distinctly hazardous operation.
While those early cans were clearly revolutionary, as you could store food for years in them, the contents were hardly fresh. Despite the accolades in Appert’s book, canned food is, by the very nature of the process, thoroughly cooked. That is not to say that the food is less than nutritious and tasty; in fact, for some items like grapefruit the canned version is more nutritious than the uncooked fresh alternative. The process of canning not only stops ripening in grapefruit, which uses up some of the fruit’s vitamin content, but it also breaks down some of the fibre, making more nutrition available when you eat it. However, given that in Appert’s case the food was cooked for an hour, I think it is safe to assume that his canned veg was probably overcooked.
The key to this is clearly reaching a compromise situation. Do you need to kill every single one of the bacteria? Can you get away with a less harsh process of heating? The answer is yes, you can, so long as you are willing to sacrifice some of the extended shelf life, and that’s where pasteurization comes in. It’s a process named after Louis Pasteur, who performed the definitive experiments in 1864. However, as is often the case in the history of technology, he was not the first to discover it. That honour should probably go to Japanese sake-brewing monks some 400 years before Pasteur. Either way, Louis ended up with his name attached to the process that is used to treat large amounts of the food we buy from our supermarkets. Take milk, for example, or any milk-based product for that matter. Raw milk, straight from the dairy, is pumped through a system of metal pipes that run in a hot water bath. By adjusting the heat of the bath and the flow rate of the milk you can ensure that the milk reaches a precise temperature for a precise length of time. For the UK, and standards do vary a little around the world, the milk needs to hit 72 ºC (162 ºF) for fifteen seconds.
It’s a specific requirement; the heat and temperature must be spot on. If you heat for too long or at too high a temperature you start to alter the taste of the milk. Whereas if the temperature is too low or held for too short a time, not enough bacteria are killed. So, how many bacteria are enough bacteria? And what is an acceptable number of bacteria in a carton of milk?
The bacterial content of milk varies enormously from farm to farm and from cow to cow. The bacteria can come from the cows themselves, either from dirty udders or in extreme cases from a teat infection called mastitis. Then there is the issue of the cleanliness of the milking equipment, the pipes, the storage vessels and the tankers used to transport the raw milk. All of which means that milk companies expect, on a good day, from healthy cows and well-run farms, about 10,000 bacteria per millilitre of raw milk (about 300,000 per fluid ounce). Which may seem like a lot, but remember that in an average human turd there are about 10 trillion bacteria weighing ten grams in total. So, it’s all relative. Pasteurization of milk at just 72 ºC (162 ºF) for fifteen seconds will kill 99.99999 per cent of these bacteria. Which sounds comprehensively lethal, but let’s break down the maths. There could be 100 million bacteria in a litre of raw milk (about 2.1 pints) and after pasteurization you would still have 10 bacteria left, give or take. So, what does that mean? Is that safe to drink? How long will a litre of milk last if it has 100 bacteria in it? To answer these questions, you need to consider what would happen to milk if it was not pasteurized.
Firstly, there is a chance that some of the bacteria in raw milk are pathogenic, or capable of causing a disease. These can range from the unpleasant, like food-poisoning bacteria, through to downright dangerous beasties like diphtheria, typhoid and even tuberculosis bacteria. These are all diseases of the past thanks to pasteurization, so we have forgotten how nasty they are, but they are really horrible and will kill about one in ten people infected. Clearly, swallowing down millions of potentially disease-causing bacteria is a very risky thing. Your body’s own defence system will probably cope. However, if you are already a little under the weather, your immune system may just have too much on its plate and you could succumb to the bacteria in the milk.
There is another problem with having millions of bacteria in your milk: what all those bacteria eat. At their most benign, they will start to digest the lactose sugar in the milk, converting it to lactic acid. Initially this makes the milk taste sour but given enough time this acid will curdle the milk protein, turning it into a smelly, wobbly block of yoghurty stuff. On top of that, there are many other odorous molecules produced and I’ve not even mentioned the possible fungal spores and how mould will start to grow.
Clearly, we don’t want any of that to have happened to the milk we consume. But if you can reduce the bacterial levels low enough you can significantly increase the potential safety and longevity of the milk. If you only have a few hundred or thousand bacteria, even if you are unlucky, only a small number of these will be pathogenic and your body’s natural defences will cope with them. Similarly, those few bacteria will start producing lactic acid but only tiny amounts. On top of this, if you keep the milk cold in a refrigerator, you massively slow down the bacteria that are there. Unpasteurized or raw milk will last for maybe a day at room temperature. Refrigerate the same milk and it will last a week. Pasteurize it, though, and it will easily last two weeks and probably three. So, when it comes to killing off bacteria in our food, there is a balancing act we need to achieve between shelf life and taste.
Around the globe, food manufacturers put a variety of helpful dates on the products we buy in our supermarkets. The exact wording varies a bit from country to country and what those dates actually mean can be significantly different. In the US, for example, most food will carry one of three types of shelf-life date: sell-by, use-by and best-before. The UK and the European Union have a similar set of date stamps, although the placing of sell-by dates is no longer recommended to food producers. The problem is that all these different date stamps cause an understandable confusion: can you eat food that is past its shelf-life date? Well, it depends which shelf-life date is being used and which set of regulations you are using. In the UK, the use-by date is the one you have to look out for. This means literally what it says, that you should have used the food, presumably by eating it, by midnight on the date shown. Once that date has passed the food is no longer safe for you to eat and it is recommended that you throw it away. The best-before date gives you more leeway; what this is telling you is that the manufacturers recommend that you eat the food by this date to appreciate it at its optimum condition. If you want to eat it after the date, go ahead but it won’t be as palatable. The sell-by date is the one that used to cause the most confusion as this was just put there to help retailers keep track of how long their stock had been sat on the shelf. Sell-by was specifically not there to give the consumer any advice on when to eat the product. Which is why you don’t see it any more in the UK. In the US, the equivalent labels have roughly the same meaning, but crucially none of them are officially safety labels and carry no legal weight behind them. The exact wording is also not fixed so the Food Product Dating code is more what you would call a guideline than an actual rule.
But how do manufacturers work out the dates and choose which foods get use-by and which best-before? The answer to the second of these questions is relatively straightforward. You need to consider what will happen when the food goes past a hypothetical shelf-life date. If the food product can become dangerous to eat, maybe because bacteria will have proliferated or poisoned the food, then you slap a use-by date on it. On the other hand, if the food just becomes less appetizing, maybe because it loses its crunch or becomes stale, then you put the best-before date on it. Foods in the use-by category tend to be wet foods like milk, meat or cheese, where bacteria will readily grow. Dry foods, on the other hand, usually get a best-before date instead.
As for how you determine the date, particularly the use-by date, well, that is a bit more involved. The key to this is the idea of what constitutes a minimum infectious dose of bacteria. This is the smallest number of bacteria you need to cause an infection in an ordinary, healthy person. Note that this dose of bacteria won’t cause an infection in everyone, but it can cause an infection if you are unlucky. By which I mean that your immune system is not working optimally, maybe because you have a cold, or are sleep deprived, or particularly stressed about work. The minimum infectious dose varies from bacteria to bacteria, so you need to know what sorts of bacteria are likely to be found in the food you are trying to date stamp. Take, for example, raw chicken. A common contaminant of raw chicken is the salmonella bacteria that can cause particularly nasty food poisoning. The minimum infectious dose of salmonella is usually in the order of a hundred thousand bacteria. So, meat with less than this is deemed safe and the question now becomes, if you have a lump of raw chicken breast kept in the refrigerator, how long will it take for those bacteria to multiply to this potentially dangerous level? Once you have worked that out, it gives you a use-by date. Except the food producers err on the side of caution and use a date a few days before this, just in case the food was not stored at the optimum temperature.
To work out this date you can either take a load of samples of the food, stack them in a refrigerator and then test one each day to check the number of bacteria, or deliberately contaminate the food with the bacteria of choice and see how long it takes to grow to dangerous levels. But that is not how most shelf-life dates are determined as this is a laborious and expensive process. Instead, most manufacturers use a computer model specifically designed for a type of food. They input the various conditions of manufacture, transport and storage into the model and, using previous scientific data, it predicts the safe use-by date.
There is a considerable industry behind all of these calculations and determinations of shelf-life dates – an industry that makes it possible for us to safely buy our food from supermarkets with the certainty that it is fit for consumption. But there are many people who ignore the dates, regularly eat food after a use-by date has expired and are none the worse for it. Are the food producers being too cautious with their dates, presumably fearing causing harm and maybe suffering litigation? Are we needlessly throwing away food because it hits its use-by date, but is still good to eat? The answer to that is definitely yes, people do throw away good food that has hit its shelf-life date. But that is because the system has a built-in level of caution. It all comes down to risk and how you rate it (see also the five-second rule here). Sure, you could eat that ham that is only one day past its use-by date, but are you sure it’s not now contaminated with dangerous levels of E. coli bacteria that cause severe food poisoning? It’s worth mentioning that food poisoning is not a small-scale or trivial illness. In the US, there are in excess of 50 million cases of food poisoning a year, 150,000 of these lead to hospitalization and over 3,000 deaths are caused. If the ham you are considering eating is within its use-by date and has been stored correctly, you can be sure that the chances of it containing dangerous levels of bacteria are so small that you need not worry. The point of shelf-life dates is to use the science of microbiology to take the guesswork out of knowing what is safe to eat. Which means that if we are agreed that throwing food away is a bad thing, then the sensible option is surely to only buy food you know you are going to eat and then make sure you scoff it before the date comes up. Although I appreciate that this is easier said than done.
The good, the bad and the fungi
So far in this section of the book I have sometimes glibly been using the deeply unscientific term ‘bugs’ to refer to the micro-organisms that share our world. I have also been pretty negative about the bugs in general, maybe with the exception of our gut microbiota. So, at this point I really need to set the record straight. Not only are many bugs incredibly helpful in the kitchen and in the preparation of food, but also not all bugs are bacteria. While it may be true that the majority of bugs we encounter in our day-to-day life come from the various families of bacteria, there are also fungi, tiny plants and even animals too small to see, although examples of microscopic animals that are both beneficial and used in food are vanishingly rare. How do the bugs help us make food?
By far and away the most common way bugs help make food is through the process of fermentation. You are probably aware that fermentation is the way we make all our alcoholic beverages, but it is the same process that underlies how we produce a huge range of food products. It’s a surprisingly common undertaking that also gives us yoghurt, soy sauce, miso, fish sauce, crème fraîche, sauerkraut, kimchi and kombucha, and even salami is somewhat fermented. In addition, some primary ingredients need a fermentation step before they become usable; both chocolate and vanilla are essentially bland, lacking their distinctive aromas before the fresh beans or pods are carefully fermented. Each fermented food product has its own fermentation process that requires a specific micro-organism and specific conditions. However, at its heart is a very simple principle: a lack of oxygen.
Within all living organisms the energy needed to survive comes from a chemical process called respiration. The form of respiration we are most familiar with, since it is how we extract energy from our own food, goes like this. Step one is to take a glucose sugar molecule and split it into two identical pyruvate molecules, which are just intermediary molecules as far as respiration is concerned. This process alone releases a bunch of energy that our cells can use to survive. Then step two takes the intermediate pyruvate and adds oxygen, which breaks it down further and releases a whole load more energy. The key to this process is that while it is preferable to use both steps for maximum energy yield, you don’t need to. Step one of respiration produces energy and some organisms choose to stop there, ignoring the second, oxygen-dependent step. In some cases, they don’t have a choice. Some bacteria living in waterlogged environments, where there is no oxygen in the first place, have become so used to sticking with just step one that oxygen is positively poisonous for them.
There is, however, a wrinkle to this. To get step one to happen – breaking your glucose molecule into two pyruvate molecules – you need to use another chemical called nicotinamide adenine dinucleotide, or NAD for short. A molecule of NAD has two forms, NADH and, if you take off the hydrogen, NAD+. It is relatively easy to convert between the two and NAD is endlessly cycled between NAD+ and NADH in all the cells in our body. When you do step one of respiration you flip a NAD+ to NADH. So how then do you turn the NADH back to NAD+? You could just use up some of the energy you created in the first place, but if you are stopping at step one and not doing the step that needs oxygen, you can use fermentation.
This is where the scientific definition of fermentation comes in. If you take the result of step one of respiration, notably pyruvate molecules, you can use them to turn NADH back to NAD+. There are two ways to do this and each produces a desirable waste product. The simple option is a direct conversion from pyruvate to lactic acid. Alternatively, you first snip off a carbon and two oxygens, making carbon dioxide gas, and the remaining bit of the pyruvate is then converted to alcohol, specifically ethanol. If an organism is going to use oxygen-free respiration, it is going to produce either lactic acid or ethanol as a waste product. Which process it uses depends on the organism. Bacteria tend to favour the lactic acid option, while microscopic fungi, like yeast, go for the ethanol route. Which begins to explain the two main types of fermented food.
If you ferment with an organism that produces lactic acid you end up with things such as yoghurt, sauerkraut and kimchi. The most common bug responsible for these foods is a bacterium called Lactobacillus. If you put Lactobacillus in an oxygen-free environment, it will switch from full-blown respiration to lactic acid fermentation. As it grows it begins to churn out lactic acid and that acid can be used in a couple of different ways. If you introduce lactic acid to milk it causes the proteins in the milk to change shape: they denature (see here) and turn into long spaghetti molecules. These then tangle up with each other and turn the liquid milk into a solid, set lump most commonly known as yoghurt. Alternatively, if the starting material is a vegetable, the end result is an acid-pickled food like sauerkraut (German pickled cabbage) or kimchi (a Korean dish, usually of pickled cabbage and radish).
On the other hand, if you are looking to produce alcohol or carbon dioxide gas, you need to add a fungus to your food. The most popular is the single-celled Saccharomyces cerevisiae or yeast. If you put yeast in a mixture with lots of food, either sugar or starch, it will take the easy route and only perform step one of respiration, fermenting the results to make carbon dioxide and ethanol. If you start with something like grape juice, the sugar is converted to pyruvate and that in turn becomes the alcohol that makes the juice into wine. What may be less obvious is that the exact same process is how we make bread. The yeast ferments the sugars in the starch (see here) and makes carbon dioxide gas, which makes the bread rise. In each case, the other product of the fermentation is also produced. Wine produces carbon dioxide, but it is allowed to escape. And when making bread some ethanol is made, which usually evaporates in the baking process. Something like champagne uses both waste products of the yeast. The waste alcohol makes the grape juice into wine and the carbon dioxide is trapped to give it the fizz.
Both types of fermentation are important in the production of huge amounts of our food. Most rely on just one type of fermentation but a few mix them together. Sourdough bread relies on a combination of yeast and bacteria to produce a distinctive result. The bacteria begin to digest sugars that the yeast can’t cope with, producing lactic acid. This gives the bread its distinctive tang, but also provides a food source for the yeast, which makes carbon dioxide through fermentation and the bread rises.
Fermentation has been around since the Stone Age, for probably 10,000 years, and is the earliest and most extensively used form of food processing. Without it your supermarket shelves would be decidedly bare.
Fermentation is not the only way we use micro-organisms to make our food. There are a few rare cases where it is the bugs or micro-organisms themselves that we eat. The two common examples here in the UK are both based on microscopic single-celled fungi. The first is pretty straightforward but also uniquely British. Marmite is a thick, black and salty paste that some British folk like to spread on toast. It tends to split the population into those that love it and those that can’t abide it. It would also seem that this is one of those things with such a peculiar flavour that you need to be brought up on it from childhood to have any chance of liking it.
It was invented by a nineteenth-century German professor of chemistry called Justus von Liebig. He found that if you took yeast that had been used to ferment beer, it could be turned into a concentrated black paste that smelled and tasted distinctly meaty, even though it was entirely vegetarian. It was mostly ignored until 1902 when the Marmite Food Extract Company set up a factory in Burton upon Trent in the UK to make the stuff. They chose Burton as their base so that they could use the waste yeast from the Bass brewery just next door. The product was named Marmite after the French-style ceramic pots it was originally sold in. Since Liebig’s time, science had discovered that certain chemicals, dubbed vitamins, were vital for life, and Marmite turned out to be packed with these chemicals. Consequently, it quickly became a popular, cheap and nutritious food within the UK.
The production process for Marmite is ridiculously simple because the yeast cells do a big chunk of the work. The waste brewer’s yeast is mixed with lots of salt and allowed to stand for about twelve hours. The salt has a peculiar effect on the yeast, causing the tiny fungal cells to self-destruct. The salt kicks off a cascade of chemical and biochemical reactions that kill the yeast cells and release enzymes that digest resulting debris. It’s all a bit gruesome, but the result is a creamy brown, thick soup. This is then concentrated by allowing it to flow down a long series of heated pipes. It goes in the consistency of cream and comes out like treacle, thick and black. But unlike treacle, it’s very salty.
They are a few different versions of yeast extract produced around the world. The Australians have Vegemite, which has added celery and onion concentrate but is still basically yeast extract. The New Zealanders have a product also labelled as Marmite, although to the connoisseur it has a sweeter and less salty taste. Differences aside, all are made from yeast, a single-celled organism.
The newest kid on the block in the field of microscopic fungal-based foods is Quorn, a vegetarian, protein-rich, meat-substitute product that can now be bought in supermarkets around the world. Quorn is named after the small British village just north of Leicester where it was first manufactured as a commercial product in 1985. But it was back in the 1960s that work began to identify new sources of protein. It had been predicted that by the 1980s the world would be suffering from a global food crisis. Thankfully this never materialized, but it did spur food scientists to start looking around for alternatives. In 1967 a fungus was found in a sample collected from a local field by scientists working at the Rank Hovis McDougall Research Centre near Marlow, to the west of London. That fungus, called Fusarium venenatum, proved to be ideal. Not only could it be persuaded to grow in huge tanks but, rather than making little round blobs like yeast, it produced short, thin filaments. Those filaments turned out to be about the same dimensions as individual muscle fibres. Consequently, if you press the filaments to form a block, it has an uncannily meaty texture.
It took eighteen years to perfect a way to grow the fungus in bulk and produce a palatable product from the microscopic fibres. These days Quorn and similar products have been made into a whole range of meatless products from lumps that resemble chicken-breast meat to slices of bacon. It is probably worth noting that while the global meat alternative market in 2016 was worth in excess of $4 billion, it is not vegetarians that are eating it. After all, if it looks likes meat, tastes like meat and has the same texture as meat, it’s probably the last thing a vegetarian is going to want to eat. Some 90 per cent of Quorn, for example, is consumed by omnivores who are just trying to cut down on the amount of actual meat they eat. However, whoever the consumers of Quorn are, it remains a remarkable product set apart from the vast majority of our food as, like Marmite, it’s made from a fungus you can’t even see.
