Behind the Glass: The Chemical and Sensorial Terroir of Wine Tasting

Behind the Glass: The Chemical and Sensorial Terroir of Wine Tasting

I wanted to write a book that explains the science behind wine tasting in an approachable way.

Wine tasting is often explained through anecdotal evidence and personal experiences, which can be helpful. However, these explanations can be confusing because of differences in genetics, culture, and exposure to food. For example, a wine made from the Gewürztraminer grape can have distinct lychee-like aromas. This aroma descriptor is not relevant to many people in the Western world, who may never have been exposed to lychee fruit, while Chinese consumers can recognize the lychee-like smell in Gewürztraminer wines right away.

While there are some well-written culinary and wine books that explain wine chemistry, such as how rotundone is the aroma compound responsible for the black- or white-pepper smell, not many books discuss variations in people’s sensitivity to rotundone.

That is why my book presents not only the chemical aspects of wine tasting but also sensory-related content. I hope readers discover two strong messages within it. First, wine chemistry is actually very fun, and second, understanding your own sensory perceptions is even more fun.

This is not a textbook; there is no need to have foundational knowledge of chemistry and sensory-related fields to appreciate this book. I summarize and explain the results of scientific research as we explore the subject, so there is no need to understand the methodologies and data analyses behind the studies I refer to. This book can be a reference for anyone during wine tasting, regardless of their level of scientific knowledge.

Finally, this is not a book that teaches specific ways of wine tasting. Rather, it aims to clarify the questions you might have during wine tasting. What is it in wine that causes a certain sensory perception when tasting it, for example, and why might you perceive this wine differently than another person?

The following excerpt unpacks the chemistry and physics that impact the color of red and rosé wines and explores the sensory aspects of how our eyes perceive colors. Other topics in the book include the color of white wines; the science behind bubbles, tears, and solids in wine; how preconceptions can alter our perceptions; the five (or six) basic tastes; why so much of what we taste is because of what we smell; the chemistry behind key wine aroma compounds; and how individuals may react differently to the same aroma compound.

- Gus Zhu

Why Is Red Wine Red?

The Physics and Chemistry of Color

Color is the result of light being reflected and perceived by the human eye. Different wavelengths of light are perceived as different colors by our vision. When an object appears white, it reflects all the visible wavelengths of light into our eyes. On the other hand, when an object appears black, it absorbs all the visible wavelengths of light. All wines absorb light and reflect specific wavelengths of light depending on their chemical composition, resulting in their distinct colors. The simple explanation for the color of red wines is due to the reflection of light in the wavelength range of red. This also means that red wines absorb other wavelengths, such as those that appear as green color.

Among the large population of chemical compounds in nature, certain compounds possess specific configurations of electrons that absorb and reflect photons, which are the elementary particles that we perceive as light. As a deeper look at chemistry and physics is beyond the scope of this book, what we really need to understand is: what are the chemical compounds in red wines that absorb the “not-so-red” wavelengths and reflect the reddish wavelengths of light? The answer is that they are a group of compounds called anthocyanins, which belong to a large category called phenolics.

Phenolics Lesson 1: Anthocyanins

Phenolics, or phenols, are ubiquitous in the plant kingdom and have thousands of different structures that correspond to myriad biological functions. Other than acting as building blocks for color compounds in plants, phenolics play vital roles in forming structural integrity, assisting wound healing, and repelling harmful insects and microorganisms. The human diet also includes plant phenolics, which are essential for better health, since we do not have the ability to synthesize phenolics in the body. Apart from color, there are other sensory attributes based on phenolic compounds, such as the bitter or astringent taste on the palate. Without phenolics, wines would be chemically much simpler and, sensorially, much less interesting.

In nature, many living things have certain colors for a reason. To put it more precisely, organic life possesses compounds that give off certain wavelengths while absorbing others and those wavelengths are tools for attracting or hiding from others. For instance, male peacocks use their colorful tails to charm female peahens. It is also hypothesized that the black and white stripes of zebras provide camouflage from predators by creating visual illusion and confusion in grassland.

The key compounds that are responsible for the red to purple color in grapes (and thus in wines) are anthocyanins. Recent genetic research indicates that all grapes were “black” varieties in ancient times, meaning their skins (or even flesh in some varieties) were always red to purple in color and rich in anthocyanins. The purpose seems to be obvious: such color is a sign of ripeness in the eyes of animals who will eat the fruit and help the grapevine to distribute its seeds. But almost all plants have evolved other ways of reproduction, such as being propagated via cuttings. Through evolution, some grape varieties may have emerged with “silenced” genes responsible for synthesizing anthocyanins, yet still were able to reproduce perfectly well. Eventually, they became white-skinned, pink-skinned, or brown-skinned varieties. In red and rosé winemaking, the anthocyanins in the skins of black grapes are released into the juice prior to or during fermentation. Essentially, winemakers taint the juice and wine by incorporating the anthocyanins from the grape skins into the juice and resultant wine. So the key factors that contribute to a red wine’s color are the natural anthocyanin composition in the grapes and the level of extraction during winemaking.

The Many Faces of Anthocyanins

Anthocyanin refers to a group of compounds sharing the same type of chemical structure. Rather than delving into the complexities of this structure, in order to appreciate their effect on a wine it is enough to understand that anthocyanins have the chemical property of absorbing green light and reflecting the red, blue, and purple wavelengths of light, hence the red-to-purple color perceived in our eyes. But how can we explain the variations in the color of red wines? For instance, some wines are close to what we might call red, while some look a bit orange, and others seem to be more purple in color. In the wine industry, color-related descriptions such as ruby, tawny, and purple note the variations of color in red wines. It is not about a pure, absolute color called red. Chemically, one of the reasons for the variability lies in the different forms that anthocyanin molecules can take.

There are three forms of anthocyanin based on the chemical environment in wine: the flavylium form, the quinoidal base form, and the carbinol pseudo-base form. Each form absorbs and reflects light differently:

  • The flavylium form gives the red hue.
  • The quinoidal base form absorbs and reflects slightly different wavelengths of light resulting in a blue to purple hue.
  • The carbinol pseudo-base form lost its ability to absorb light and is therefore colorless.

Note that around 90 percent of the anthocyanins in wine are in the carbinol pseudo-base form (colorless), but the rest are sufficient for contributing to the range of colors we call red.

The three forms are pH-dependent, meaning the proportion of each form of anthocyanin present depends on the acidity or alkalinity of a substance. Flowers and fruits can alter their color by changing the pH environment within them. For example, a Brazilian native plant, commonly known as the yesterday-today-and-tomorrow flower, can appear as purple yesterday, turning less purple and more lavender-colored today before becoming white after a few days. The mechanism of such rapid color changes within the flowers is a change of pH that influences the color of anthocyanins.

In wine, the general pH range can vary between 3.0 and 4.0. This determines the percentage of each form of anthocyanin present. Lower pH (more acidic wines) results in more of the reddish flavylium form of anthocyanins, which partially explains why low pH wines, such as quite a few northern and central Italian red wines, often look ruby in color. Higher pH (less acidic wines) increases the percentage of anthocyanins in the blue-purple quinoidal form, which suggests why higher pH wines, such as those made from Syrah and Malbec, tend to display a purple hue.

To understand the influence of pH on the forms of anthocyanin, there is also a little experiment we can do at home. Take a red wine or squeeze the red juice from red fruits, such as strawberry, raspberry, or cranberry, then dilute the wine or juice with water. The pH becomes higher because such dilution results in a decrease in acidity in the solution due to the higher pH of water (tap water has a pH of around 7.5). As the water is added, the juice will turn from red to blue-purple in color, reflecting the switch from the flavylium form to the quinoidal form of anthocyanins. If water is added further, we may see a significant loss of color, not only because of dilution, but also due to the loss of the flavylium or quinoidal form of anthocyanins that contribute to the color.

As we can see, anthocyanins need to be in a particular chemical environment in order to show color. In other words, it is quite easy to lose the red color if conditions are not right for anthocyanins. Besides being pH-dependent, anthocyanins can also react with certain chemical compounds and get bleached. During winemaking or bottling, if the producers add too much sulfur dioxide (SO2), the color of the wine can be lost. This is called bisulfite bleaching and comes from the interaction between the sulfur dioxide added and the anthocyanins. Bisulfite as the major form of sulfur dioxide in wine reacts with anthocyanins and turns them into colorless molecules. Paying attention to the sulfur dioxide levels in wine is crucial if a winemaker is concerned about the loss of color in red and rosé wines.

White wines can also contain a low level of anthocyanins if they are made from black or pink/brown-skinned varieties. For example, the pink- to brown-skinned Pinot Gris/Grigio grapes can be made into white or rosé wines depending on the length of skin contact during winemaking. Even if a white wine has a yellow, golden color at the beginning, some pink color can be picked up as the wine is exposed to oxygen. The reason for this is that while the sulfur dioxide added during production bleaches the anthocyanins, oxygen can consume the sulfur dioxide to reverse the bisulfite bleaching effect, resulting in the return of anthocyanins to the colored forms. Known as the pinking phenomenon, this is an unexpected and undesired change for makers of white wines.

Color Properties

So far, our discussion has focused on the chemical origins of color. Hence, we simplified the sensory perceptions of color to absolutes such as red and purple. But our life experience tells us that there are many different variations of the same class of color. If we take two glasses of the same red wine and view them under different lighting conditions, one can appear to be brighter in color, while the other is darker. Color is never one-dimensional, and multiple factors can tweak the appearance of color. To explain the variations of color in wine, three important color properties are highlighted: hue, saturation, and value. We’ll consider each of these in turn.

Hue

Hue is used to describe the basic categories of color, such as red, green, and blue. When we talk about changing hue, we are referring to a change between distinct colors, not a change in other parameters, such as brightness or darkness. As explained previously, we can see a given color when a specific wavelength of light is reflected into our eyes. But it is important to keep in mind that the wavelengths on the color spectrum are continuous, meaning that within the range of red-colored wavelengths, some wavelengths may appear closer to the color we call red, while others will look more orange.

Here, some numbers may help provide more intuitive understanding of the minor change of hues. The range of wavelengths in the visible light spectrum is from 380 to 700 nanometers (nm). As the number slowly increases or decreases, the transition of the corresponding colors is gradual. For example, a specific wavelength between 625 and 750 nm gives the red color, while 590 to 625 nm represents the zone of orange color. This means that the wavelength of 625 nm, which sits at the meeting point of the two ranges, can appear as a red-orange color.

The reality is that there are often multiple different wavelengths of light being reflected by an object. As we have seen in a glass of red wine, there are multiple forms of anthocyanins and anthocyanin-derived compounds. Hence, the resulting color is a mixture of wavelengths, whether the range is narrow or broad. The best example is a red wine that appears both ruby and purple in color. Such a blend of color is likely due to the purple-colored anthocyanins in the high-pH environment of the wine, along with anthocyanin-derived pigments that reflect wavelengths in the more reddish color spectrum. The same theory explains wine with a ruby to garnet color: these wines contain color compounds that reflect wavelengths with higher and lower numbers (they can co-exist in the same pH environment) in the red-to-orange spectrum.

Saturation

Although the hue helps anchor the basic categories of color, this property alone cannot explain other observations of color. By looking at the visible light spectrum, you may find a popular color that is missing: pink. Rosé wines, favored by markets worldwide nowadays, attract consumers through their various shades of pink. Winemakers create such lovely colors by releasing a low level of anthocyanins from grape skins into the wine. It seems obvious that low levels of anthocyanins account for the pink color in wine. But still, the color property of hue alone does not explain the color pink, which is non-existent in the visible light spectrum.

In order to understand this, another property of color needs to be introduced: color saturation. Saturation is how pure or brilliant a color is. The alternative term for saturation is intensity, which is often gauged when observing a glass of wine. When saturation is reduced, the color is more diluted, with a white tone. Although there is no wavelength of light with the hue of pink, when white color is mixed with red, our brains interpret this as the color pink (more discussion on this later). When a wine has a low concentration of anthocyanins, the red color becomes weak. Therefore, other perceived wavelengths from the environment can have a significant impact on what we see. In this case, the light with all or most wavelengths reflected from a white background goes through the wine glass and cannot be overpowered by the small amount of red wavelengths in the wine. The result is that our eyes perceive a mixture of red wavelengths and the white ambient light to produce what we see as pink. In technical terms, rosé wines are pink because of the white background added to them, causing the saturation of the red hue to be decreased.

One of the implications of the pink story is the importance of the surrounding light while observing an object. In most cases, the ambient light is white in color, so that the saturation of color is always lower when our eyes receive strong natural light. Thus, looking at a rosé wine against a shiny white background makes it appear pinker in color. On the other hand, a rosé wine seems to be darker and more reddish in color if you observe it against a piece of black paper. Given that color is an important factor for the way in which consumers make decisions about a purchase, it is crucial to consider the lighting conditions and the ambient colors when displaying a product, for example if using fluorescent lights as the preferred source of lighting in a shop.

Value

Value is the brightness or darkness of a color. As the value gets lower, the color becomes darker and duller (grayer or with more shadow). The value of color is influenced by the degree of light reflected from an object. The more light an object absorbs, the lower the color value. On the other hand, if light goes straight into our eye without losing any power, the color value will be at its highest. Saturation, discussed above, is about how much other light is mixed in to dilute the purity of the hue, whereas value is about the intensity of each hue.

When we check on the color of red wine in a glass, such as by tilting the glass at a 45-degree angle and looking through the liquid in the glass against a white background, the common observation is that the center of the wine appears to be darker than the rim. This is because the center has more wine and therefore a higher concentration of compounds (more than just anthocyanins) that “devour” the light, resulting in lower value and more darkness of the color when compared to the rim, with less concentrated light-absorbing compounds. If you care about the observation of color in wine tasting, it is important to have sufficient natural light. Tasting in an old, underground wine cellar can be atmospheric, but the dim lighting in those places makes the color of wine irrelevant, as everything looks gray and dark. Once again, the ambient light conditions have a significant influence on the properties of color.

Thanks to some magical software and electronic devices, these days we can understand color properties more easily and intuitively without necessarily understanding the physics involved. Graphic editors on our computers and phones give us access to color manipulation, enabling us to adjust various parameters of color. A glass of dark-red-colored wine in a photo turns pink as you lower the level of saturation. As the value of color is edited to a high level, the color looks very bright. The result is a shiny rosé wine that conceals the true color. When shopping online, be careful when you look at the color of a dress based on the images shown on the screen. If the colors in the photos have been heavily modified (or even due to the difficulty in calibrating a computer screen to match reality), you may receive a dress of a significantly different color.

Color Receptors in the Eyes

So far, our discussion has been focused on the physical properties of light and color, without really examining the physiology of human vision. Since this book also explains the sensorial terroir, we need to look at how exactly our eyes perceive light and how our brain interprets those wavelengths.

Human eyes have color receptor cells called cones. There are three types of cones in the retinas of our eyes: the L-cone (L for long), the M-cone (M for medium), and the S-cone (S for short). As the name suggests, the L-cone is more sensitive to long wavelengths, in the red-orange color zone; the M-cone is more sensitive to medium wavelengths, in the yellow-green color zone; the S-cone is more sensitive to short wavelengths, in the blue-purple color zone. A given wavelength hitting our eye might fully trigger one type of cone, partially trigger another, or not trigger another cone at all. Here is a list of examples to help you get a feel for how the cones work:

  • When a long wavelength of 700 nm hits our eyes, the L-cone gets fully switched on, whereas the other two cones don’t respond to it at all. The result is a pure red hue being perceived.
  • If the wavelength is switched to around 570 nm, both the L-cone and the M-cone are highly activated, but the S-cone is still inactive. At this moment, the lively L- and M-cones are translated into the color yellow by our brains.
  • Now say the wavelength becomes shorter, to around 500 nm. The L-cone becomes less active and the M-cone dominates, while the S-cone is still sleeping. Our brain interprets this as the color green.
  • If the wavelength is a short one, of 450 nm, the red and green cones are less sensitive but still partially responsive, while the S-cone is finally awake, resulting in the perception of the color blue, or even slightly purple.

Common sense tells us that our vision should perceive more than just the different types of hues. As indicated by the color properties of saturation and value, color intensity and background lighting also affect the colors we see. Indeed, our vision does contain another group of receptor cells responsible for sensitivity to light. Those receptors are called rods, and they are crucial in our daily lives, for example, in giving proper vision at night when the light is weak. Rods also have a prime zone of wavelengths, which is around 500 nm, corresponding to the green-to-blue colors. No wonder most objects look dark green and dark blue in the evening.

This excerpt first appeared in Behind the Glass: The Chemical and Sensorial Terroir of Wine Tasting, written by MW Gus Zhu and published by Académie du Vin Library. It has been minimally edited for style and length. Used with permission.

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