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  • Guest contributor
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  • Guest contributor
17 Jan 2017

Caroline Gilby MW is well known in wine circles for her unrivalled knowledge of wine production in eastern Europe. She is a notable contributor to the Oxford Companion to Wine

Unnatural science 

There's no such thing as a 'natural' wine. 

Growing grapes hasn't been natural since some pioneering early farmer decided that trying to tame grapevines into orderly rows in a field was a good idea, rather than scrambling through trees to seek out elusive bunches in the canopy. Over millennia since, vines have been pruned, tidied and twisted into forms that are a long way from their original, rambling, free-form nature. They have also been selected and interbred into the vast array of Vitis vinifera varieties known today, bearing little resemblance to the original wild Vitis sylvestris.

Human intervention also plays a huge part in winemaking. If you pick grapes and leave them in a vat, they will ferment. There are so many microbes in the environment which see sweet grape juice as a potential feast, that a fermentation of some sort is inevitable. Luckily for wine drinkers, if yeast get involved then alcohol is a key by-product. However, left to its own devices, this early brew will carry on fermenting all the way to vinegar, which is not what winemakers want. Wine, as we know and love it, is only an intermediate step on the road to becoming salad dressing. The tricks with winemaking are first of all: to get the right microbes to dominate the fermentation, ideally yeast that will produce alcohol and pleasant flavours along the way. The second trick is to stop that fermentation while the wine is still wine. The third trick is to keep it there for the days, months or years until the consumer pulls the cork or untwists the screw cap.

Over history, various ingredients such as tree resins, herbs and sulphur dioxide have been pressed into service, and it's this last one that has become a pretty much universal additive to wine. Sulphur dioxide (SO2 for short) is found in nature and appears in wine anyway as a by-product of yeast fermentation, though at very low levels. No one knows exactly when SO2 was first used in winemaking, but it seems the Ancient Greeks were well aware of its use as a preservative and fumigant, so it's a reasonable assumption that it wasn't long before someone used it in wine. SO2 is extremely handy in winemaking because it does several things: it mops up oxygen and helps prevent damage to aroma and flavour through oxidation, and it kills or inhibits many microbes (including the Acetobacter that play such a role in the progress towards vinegar). This feature also helps select the 'right' microbes, as the wine yeast Saccharomyces cerevisiae can tolerate more SO2 than most yeast species. It can also be used to kill off all the wild microbes hanging around, so a specific commercial strain of yeast can be added. As an extra benefit, SO2 can inhibit certain enzymes from rotten grapes that spoil colour and flavour.

It is definitely possible to make wine without the help of additional SO2, but very tricky and very risky as wine is intrinsically unstable. Certain styles of wine are more robust – those with characteristics such as low pH, high tannin levels and no residual sugar. And, in the winery, there must be scrupulous attention to hygiene and temperature control as cooler conditions slow down reactions, though actually oxygen is more soluble in cold liquids adding to the risk of oxidative damage. Even organic standards recognise the usefulness of SO2 (or 'sulphites' as appears on wine labels because that's the form that is most common in wine), though at lower levels than normal. (The photo below shows a heat exchanger, used in many modern wineries, to cool or heat must.)


Many wine drinkers might be shocked at quite how many other additives and processing aids are also allowed in wine. A weird quirk of EU law is that alcoholic drinks are exempt from ingredient labelling apart from a requirement to list added sulphites (if over 10 mg/l) and certain allergens (egg and milk products used for clarifying if residues exceed 0.25mg/l). The full list of permitted additives is too long to mention here, but includes things like tartaric and citric acids, gum arabic, potassium sorbate, vitamin C, tannins, sugar (to boost alcohol), carboxymethyl cellulose (helps prevent crystals forming), dimethyl dicarbonate (used to kill microbes) and more. Processing aids (which shouldn't remain in the finished wine), include isinglass (from fish swim bladders), albumin, casein, gelatine (from eggs, milk, animal carcasses); PVPP (Polyvinylpolypyrrolidone), and activated charcoal (all used to clarify and adjust colour); potassium ferrocyanide (used to correct excess copper or iron in white and rosé); copper sulphate (to prevent reduction problems); lysozyme and pectolytic enzymes; bentonite clay, plus – of course - yeasts and yeast products. Then there's all the technology used in modern wine making: micro-oxygenationreverse osmosiscryoextraction, electrodialysis, and more. In an ideal world, of course winemakers would not need most of this, but wine is a direct agricultural product and grapes are often less than perfect after a season of being battered by rain and hail, fungal disease and pests like insects and birds. Modern technology and that long list of 'little helpers' has made winemaking much more consistent and able to deliver something drinkable even in difficult years.

The natural wine movement is a reaction against the industrialisation of wine and a desire to return to simpler times. There's no legal definition, something that might be hard to achieve with a bunch of mavericks who don't like conforming. One of the strongest forces in the natural wine world is the RAW wine fair so it's useful to look at their charter. This requires organic or biodynamic growing, use of natural yeast, no gadgetry and only low levels of sulphites (with no other additives), specifying a maximum of 70mg/l compared to EU limits of 150mg/l to 400mg/l depending on wine style (see Cutting down on sulphur).

In the end, it is what's in the glass that counts and here lies my problem. The first duty of wine is to bring pleasure and far too often self-declared 'natural' wine just isn't pleasant. Fizzy, cidery, funky, varnishy and oxidised characters are common experiences, justified because it's 'natural' but simply not something I want to put in my mouth. And there's frequently a lack of consistency from one bottle to the next. Undoubtedly there are good natural wines, often made by producers who used to make good wine before they went natural, because they already had good vineyards and were passionate about detail.

I do think producers should be encouraged to be more sustainable and do the minimum necessary, rather than the maximum 'just in case'. After all wine is a luxury that we don't actually need in our lives. If being biodynamic, or organic, or natural, is a philosophy that motivates winemakers to put in the extra effort required to make great wine, then it may be no bad thing. I would just like the final product to be worth drinking and to stay that way until it is safely in the consumer's glass (and usually for me that means a judicious bit of SO2).

Taste – uncovering the mysteries

Hands up then – who still believes the story that there are only being four basic tastes and that they are mapped to specific bits of the tongue? This old myth is still repeated widely though it has long been disproved.

It's remarkable how poorly understood taste is; even though it is one the five basic sensations and enjoying the taste food and drink is one of the great pleasures of human life.

As recently as August this year, researchers at Oregon State University announced that humans can taste starch - or at least long chains of polysaccharides. This perception is independent of the taste of simple sugars, as a starchy character can still be detected even when sweetness receptors in the mouth are blocked. As yet, the specific receptors on the tongue haven't been identified, but the real-world physical response is well known ('carb craving', anyone?) and the evolutionary benefits of being able to detect sustaining, long-chain carbohydrates seem fairly obvious. Other weird discoveries about taste receptors include their presence deep in the gut, in the pancreas, and even in the testes – probably involved in controlling hormone production, appetite and reproductive status.

An ability to taste carbs may not have much to do with wine-tasting, but understanding other parameters of how taste works certainly does. There are four types of papillae on the tongue which give it its rough texture. (The photo below is a close-up of lingual papillae stained with fluorescent dies to highlight different molecules - originally published in PLOS Genetics, Nov 2016, under creative commons licence.) The filiform papillae just detect texture (think mouthfeel, astringency and or silkiness - all tactile aspects of wine) while the fungiform, foliate and circumvallate papillae all contain onion-shaped taste buds. Each of these is packed with taste cells and these carry the receptors for the five established sensations. Salt, sweet, acid and bitter are familiar but the fifth - umami - is less well known. This is the savoury receptor which responds to glutamate found in savoury foods. It was first proposed in 1908 by a Japanese scientist called Kikunae Ikeda, but as the original paper was published in Japanese, it was overlooked by western scientists until the 1980s. It took even longer for actual taste receptors for these main senses to be identified. In 2000, the first taste receptors for bitter compounds were identified, then sweet, umami, sour and finally salt as recently as 2010. But the search continues for more with possible candidates being the taste of blood, fatty acids and various amino acids.


Exploring differing taste thresholds in tasters is important to help us understand why our preferences for wine vary so much. It's fairly widely known that tasting sensitivity for bitterness is divided roughly into super-tasters, normal tasters and non-tasters. This is tested using a compound called 6-n-propylthiouracil (PROP), which tastes intensely bitter to super-tasters, mildly bitter to normal tasters and flavourless to the rest. About 25% of us are believed to be super-tasters and the trait is more common in women, while a further 25% are non-tasters – including some Masters of Wine [see The PROP test and reactions to it - JR]. It was also believed that this sensitivity to bitterness was linked to the number of taste papillae on your tongue, but recent research in Denver, Colorado tested 3,000 people and found no link at all between number of papillae and perception of bitterness with PROP. Another long-held belief destroyed. With wine tasting – especially with levels of phenolic bitterness (tannins, polyphenols and many more complex compounds) common in wine, it is really useful to know where you fit on the scale, but there's actually no reason why even non-tasters can't be successful in wine.

Our enjoyment of the taste of fizzy drinks has been another fruitful area for research. It seems there's much more to it than just the theatre of the pop of the champagne cork and the prickle of bubbles bursting on the tongue. If you get people to taste fizzy drinks in a pressure chamber where bubbles are prevented from bursting, the sensation is just the same. As recently as 2009, US scientists discovered a protein receptor in mice (which have very similar taste buds to us) for the taste of carbonation. They discovered a specific enzyme called carbonic anhydrase-4 that is attached to the sour-sensing taste cells. This enzyme binds CO2 to form carbonic acid, helping to give a mildly sharp taste perception. It also seems that CO2 triggers both touch and pain receptors so may give a little endorphin boost to add to our enjoyment of fizz.

Another tasting myth that has been busted is that different parts of the tongue taste different components of wine. It's now known that taste sensing cells are spread across the tongue but it's not the same in the brain itself. Functional MRI studies have found that by dropping bitter and sweet flavours on the tongue, specific and quite separate groups of neurons light up, which can be as much as 2.5mm apart (a long distance in brain terms). In evolutionary terms, this probably makes sense as bitterness tends to be associated with aversion to protect us from poisoning, while sweetness would have been a cue for attraction. Specific hotspots of nerve response have been found for umami and salt too, though not, as yet, in response to acid.

This leads neatly onto odour sensing, which is well known to be a key part of the tasting experience as it is the combination of odour and taste that makes up flavour. Humans have around 400 different types of odour sensing cells and the old story was that we could detect around 10,00 different aromas. Recent research at Rockefeller University in USA has put paid to this myth too – differential testing (where two identical and one different odour are tested to pick out differences) suggests that we can detect trillions of odour differences. Unlike taste though, odour sensing in the brain is not linked to specific nerve cells, or hot spots but is a result of small differences in nerve firing patterns across the brain.

It's also becoming increasingly clear that everyone experiences their own odour and flavour world. Smell is critical in wine tasting – just think how dull wine is when you have a cold. Often people worry when they can't detect all things mentioned in the florid descriptions of exotic fruit or flowers that some wine critics use so liberally. Sometimes, it may well be that wine critics are more about creating an impression than accuracy, but it is also probably the case that not everyone can smell or taste the same things. Many of the population are anosmic, or physically unable to smell certain chemicals because they simply don't have the right functioning receptors in their olfactory bulb. This includes up to 3% who can't detect vanilla (a very common flavouring but also a component of the flavour from new oak barrels), while a massive 30% can't smell androstenone, a key component in the scent of truffles. Researchers in New Zealand have been looking at detection of the smell of violets, due to a compound called beta-ionone, which often appears in Pinot Noir and sometimes aged red wines. Perception for this turns out to be controlled by a single gene called OR5A1 and research guinea pigs varied from insensitive to highly sensitive depending on gene expression. And this is just one of the hundreds of components that make up the bouquet and flavour of wine.

Periodically stories crop up in the press of electronic noses replacing wine tasters, but given the complexities we already know about taste and odour, I can't see machines taking that job any time soon [phew - JR]. It's also clear that there cannot be any single right or wrong answer in wine tasting, as everyone will have their own individual responses to what's in the glass – one of the joys of the complex and varied world of wine.