The Sweetest Feeling

Understanding how taste works is a multidisciplinary endeavour; drawing upon scientific subjects like biology and chemistry to understand our bodily receptors and how they experience and process different chemical properties of food, as well as philosophy to plumb the depths of how we experience sensation, and even sociology and anthropology to explore how flavours are socially and culturally constructed. We are generally understood to have five basic tastes or sensory qualities:  sweet, salty, sour, bitter and umami.  When we put food into our mouths, its chemical substance encounters nerve cells on our tongue, these sensory cells are activated and pass on messages to further nerve cells which eventually trigger a particular perception of flavour in the  brain. The little bumps on the tongue, called taste papillae, are where the action happens. The raised nature of the papillae means the surface area of the tongue is magnified in order to taste food more efficiently. On a cellular level the Papillae, together with other cells form a bud-like shape, with a funnel-like hollow where the food substance flows in. Within the hollow it is analysed by sensory cell extensions called taste hairs, before it passes over the tongue & down the gullet. Proteins on the surface of the hollow bind chemicals to the cell to activate the tasting. 

Adults have between 2,000 and 4,000 taste buds in total and the sensory cells recognise the five basic tastes; some cells react to all tastes and others are specialised to only respond to one particular taste. After this initial examination of food on the tongue, the sensory information is transferred to the nervous system by cranial nerves. The information is carried to the brain’s lower section, the medulla oblongata. Information about taste, temperature and touch combine in the brain with smell information to reach our consciousness. The brain combines the information received from all of the sensory cells on the tongue and gives us an impression of what and how intense the flavours of a particular food are. Sweetness is one of our most basic and earliest taste preferences; human milk has a sweet quality (see Ellie Doney’s introductory article for further information). It is a taste that we seem to have an inbuilt predisposition towards, although Sidney Mintz questioned the progress from this predisposition to the primacy of sugar in the global diet.1 Sweet-tasting soluble carbohydrate sugars technically include the naturally occurring fructose (in fruit), maltose (in some cereals and fruits), lactose (in milk). Granulated table sugar is made from sucrose, a disaccharide of glucose and fructose. The sucrose of this kind of refined sugar is perhaps the prototypical processed provider of sweetness in the human diet.

I got the lowdown on sugar, sweetness and taste from Stephen Barrett, who works at the interface between biology and chemistry at Maynooth University and has a past life as a cocktail barman with a keen interest in taste and flavours.  I asked Stephen about the science of how we taste sweetness. He responded, “most cells in the body have what are referred to as receptors. These receptors are made up of various proteins. You can think of these receptors as like a specific size and shape (like a hole in a sheet of paper).  This means that ONLY something of this size and shape can fit into the receptor. The better the fit the more sweetness you taste. If you can imagine table sugar i.e. sucrose being a very good fit to this receptor, then something like lactose would be less of a fit, it fits but not as well. With lactose, like in milks and creams, you can still taste some sweetness but nothing close to the amount of table sugar. When the binding occurs a cascade of chain reactions are set off and neurons (like dopamine) fire on the central nervous system. Your brain says, ‘mmmmm’ this tastes good! The sugars bind into these receptors via what is known as hydrogen bonding in chemistry, it’s a medium to weak strength type of bonding so after a while the sugar dislodges from the receptor and you no longer taste it.”

Taste, therefore, is a multi-step process that involves a sensory perception on the tongue followed by the logging and processing of this sensory data by the brain. Sugars by definition all have the property of sweetness which Stephen explains “is down to the presence of the 5 or 6 membered ring (pentose  or hexose) which is a carbohydrate containing some OH Groups. The OH group is known as a hydroxy moiety in chemistry and is mostly but not fully responsible for the sweet taste in the sugar. The structure is also responsible in order to fit the receptor correctly”.2 OH molecules operate in quite a predictable manner, and Stephen tells me it is possible to chemically block a person’s ability to taste sweetness by interfering with the molecular process. He points out, “You need to find a molecule that blocks the ability of the sugar to bind to the sugar receptor. Gymnemic acid is a great example of one of these molecules. Gymnemic acid belongs to a class of chemical compounds isolated from the leaves of a plant called Gymnema sylvestre. The exact mechanism of how the molecule interacts with the receptor is not fully understood but there’s a lot of evidence to suggest its simply blocks the sugar binding into the receptor and therefore the human cannot taste the sugar anymore.3 The effect is completely reversible if you rinse your mouth with γ-cyclodextrin but the blocking effect also wears off if you wait ten to fifteen minutes”. If you want to try this yourself, you can buy gymnemic acid. I tried a drop on my tongue and experienced a very weird quarter of an hour where sweet foods like honey and beer had a very curious absence of taste; testing something of my psychological expectations of taste

There are also artificial sugars like aspartame and saccharin, often used when people want sweet flavours without the calories. Synthetic sugars work in a similar way to normal sugars but the quantities are different as Stephen explains; “essentially they are a manufactured sugar, or what we refer to as synthetic sugars. The sugar is synthesised in a lab. They work in the exact same way a natural sugar works, only with aspartame its affinity for the sweetness receptor is far greater than sucrose. Therefore the aspartame tastes sweeter but with no calories. The aspartame is usually made from reacting two amino acids i.e. phenylalanine and aspartic acid together.4 Interestingly neither of these molecules taste sweet on their own.  Although there are other methods to make it, like everything in chemistry”. He maintains that any hysteria about artificial sweeteners is not backed up by scientific research. He counters, “aspartame gets a lot of bad press in health magazines and online, there is absolutely no scientific evidence at all to back up any of these wild claims. You’ll often find the people or forums making these claims have actually no background in science at all. The molecule is fully approved by the both the European Food Safety Authority and FDA and has no health risks. The only thing to be aware of with aspartame would be in relation to people who have genetic condition phenylketonuria. Phenylketonuria (PKU) is an error of metabolism that results in decreased metabolism of the amino acid phenylalanine. Untreated, PKU can lead to intellectual disability, seizures, behavioral problems, and mental disorders. This is an issue as aspartame is broken down into phenylalanine and other side products”.

It's not just artificial sweeteners that are demonised. Sugar itself has replaced fat as the villifed food of the day. Many nutritionists and dietary advisors speak of the addictive properties of sugar and its nutritional deficiencies which create a dependency without much bodily sustenance. Stephen comments “it is like any addiction really. Nobody starts out needing sugar. I guess you go from wanting it to needing it. All drugs that can be abused, from nicotine to heroin, cause a particularly powerful surge of dopamine in the nucleus accumbens. This is the part of your brain where you feel motivation and reward. The likelihood that the use of a drug or participation in a rewarding activity will lead to addiction is directly linked to the speed with which it promotes dopamine release, the intensity of that release, and the reliability of that release. Even taking the same drug through different methods of administration can influence how likely it is to lead to addiction. Smoking a drug or injecting it intravenously, as opposed to swallowing it as a pill, for example, generally produces a faster, stronger dopamine signal and is more likely to lead to drug misuse. Sugar is similar, except it doesn’t lead to dopamine flooding on the same levels as narcotics”. So while you may get a sugar high, it won’t be as neurologically captivating as a line of cocaine, so you may need to find a better excuse for raiding the  biscuit tin.

On Sidney Mintz’s question of whether the sweet tooth exits, Stephen is equivocal; “there are suggestions that it is down to genetics.5 Some people have more sugar receptors than others.6 There are also large studies to indicate it is down to diet, so people who eat more sugar as a child will more than likely have a “sweet tooth” as they get older. Both are logical and I’d imagine a mixture of both is plausible”. Yet sugar clearly gives pleasure, which can be enticing and drive people to rely on sweet foods to satisfy that desire. Our consideration of what tastes ‘good’ is driven by that basic individual appreciation of flavour but taste is also a culturally and socially constructed phenomenon. In Stephen’s life as a barman he devised cocktails to meet both individual and cultural aesthetics of taste, believing more refined palates have a more balanced and measured approach to sweetness. He tells me that “when it comes to level of sweetness it is very subjective. Some people enjoy incredibly sweet tastes. In relation to cocktails you absolutely must have a balance. Bad bartenders will just poison everything with sugar because most customers will find it sweet and delicious. The art of a real drink is being able to taste every component of it, including the liquor. Most bartenders these days lean towards natural sugars instead of table sugar”. When it comes to the question of whether something is sweet enough, it seems like individual taste is everything.