To complement the ongoing food chemistry posts, this supplementary series is going to be looking at the key chemicals (or families of chemicals) that give alcoholic drinks their characteristics. The first in the series looks at some of the families of chemicals in red wine which contribute towards its colour and flavour, with more detail provided in the post below.
Red wine has been espoused over the years for its supposed health benefits, from claims that it could let us live to 150, to assertions that chemicals within it can help prevent cancer. Whilst these are no doubt a very pleasant means to justify a glass of wine (or several), how correct are they? Similarly, ‘tannins’ are commonly mentioned in wine circles when discussing the flavour and quality of wines, but what are tannins, and what effect do they have on the quality of the wine?
In general, red wine is a complex mix of a large number of chemicals; there’s no exact figure, but estimates range from around 800 different compounds to over 1000. An average red wine will contain 86% water, and 12% ethyl alcohol. Glycerol (also known as glycerin) makes up around 1%, with a variety of acids making up an additional 0.4%. Compounds referred to as tannins and phenolics comprise just 0.1% of your average red wine – but it’s to these we’ll look when examining the contributing compounds to colour and flavour.
First, we should explain what phenolic compounds are. Phenol, shown below, is the simplest example of a phenolic compound. It consists of a hydroxyl group (an oxygen atom bonded to a hydrogen atom) bonded to a benzene ring, which contains six carbons. The phenolic compounds in wine are somewhat more complicated than this, but they’ll all include several phenol units in their structures.
In red wine, we’re primarily interested in looking at flavonoids – a class of phenolic compounds commonly found in a variety of plants. The four sub-classes found in wine are catechins (or flavan-3-ols), flavonols, anthocyanins and tannins. Each sub-class in turn contributes in some way to either the flavour, colour or character of the wine due to differences in the huge range of compounds contained therein.
The first family of compounds we’ll consider, anthocyanins, originate from the skins of the grapes used to make the wine. These compounds, along with their derivatives, contribute the majority of red wine’s colouration. Their colouration is, in fact, dependent on the surrounding acidity; the acids in wine give rise to the red colouration, but in alkaline solutions, the same compounds can give a blue colour, or even green & yellow at higher alkalinities. These are the same compounds that give fruits such as blackberries and raspberries their colour, as well as the wide variety of shades seen in autumn leaves.
As wines age, molecules of anthocyanins can undergo a wide variety of reactions to form larger ‘complexes’, which can also contribute to the red colouration of the wine. As a result, although the concentration of anthocyanins in a bottle of wine will constantly decrease as they are incorporated into these larger complexes, the red colour will still remain.
The second family of compounds, the flavan-3-ols, contribute to the bitterness of wine. They originate primarily from the seeds of the grapes, and their concentration in red wine can reach up to 800mg/L. 20mg/L is the threshold amount required for the bitterness to be registered from the wine, and higher alcohol concentrations have been shown to enhance this bitterness. Catechin and epicatechin are the primary flavan-3-ols found in red wine; these compounds are also found in high concentrations in tea and dark chocolate, and have been associated with health benefits due to antioxidant activity.
The similar sounding flavonols also have a similar looking structure to the flavan-3-ols, with just a couple of minor differences. However, these are significant enough that flavonols don’t contribute to the bitterness of the wine, as the flavan-3-ols do – in fact, they’ve yet to have any sensory impact attributed to them. They, too, have antioxidant properties, but research suggests they’re present in red wine in too low a concentration to be considered a good source, at least in comparison to other natural sources such as yellow onions or tea. They do, however, help contribute to the colour of red wine by forming complexes with the previously mentioned anthocyanins.
The final family of compounds to consider is the tannins. Tannins are polymers – that is, many smaller molecules joined together to make a long chain. More common examples of polymers are man-made plastics, or the cellulose in plants. Condensed tannins are the main class found in red wines, which consist of many different flavan-3-ol molecules joined together – as many as 27 in one polymer molecule when the grapes used to make the wine are first harvested. Some tannins can also come from the barrels in which the wine is aged.
The tannins in red wine contribute to its astringency, or dryness, as well as the bitterness. When you drink wine, the tannins react with the proteins in your saliva. This forms a precipitate, and leads to the sensation of dryness. Obviously, variation of tannin concentration will affect the amount of dryness that is perceived. They can also contribute to the colour by combining with the anthocyanins.
Over time, it was originally thought that the long tannin polymers that form can eventually precipitate out of the wine itself, and that this was one of the causes of the appearance of sediment at the bottom of the bottle. However, this has yet to be conclusively proven scientifically, and more recently it’s been suggested that the tannin polymers may actually get shorter as the wine ages. It’s incredible to think that, in a single bottle of wine, there’s a myriad of chemical reactions constantly occurring – but it also makes the chemicals within very difficult to study!
Finally on the subject of tannins, they may also be the reason that some people experience headaches or migraines after drinking red wine. This has been shown to be a demonstrable effect of red wine in some people, and it was suggested that tannins could cause this by altering serotonin levels. With this too, however, the jury is very much out – a number of other possibilities have been suggested, but we’re currently no closer to being able to pinpoint a specific molecule.
We also mentioned the supposed health benefits of red wine at the start of the article, and we’ve touched on the antioxidant properties of some of the compound families we’ve examined. Another molecule in red wine, however, has been the main focus of health benefits in recent years: resveratrol, shown below. It has been shown in studies that resveratrol, as well as having antioxidant properties, can help prevent high blood pressure (hypertension) in mice, and also has anti-inflammatory effects.
At this point, it’s worth pointing out that, for all the hype about antioxidants, we do actually need some of the free radicals they react with in our bodies – it’s not simply a case of the more antioxidants, the better. The story of resveratrol also shows that, whilst animal testing has countless benefits in the testing of pharmaceuticals for the treatment of diseases, it can also produce results than sometimes aren’t replicable in humans. That’s exactly what’s occurred recently with resveratrol, with research (detailed here) seeming to suggest that the levels of resveratrol present in red wine aren’t enough for any discernible benefit to be observed – at least over the nine year period of the study.
Whilst this doesn’t mean the end of the road for resveratrol research, whether or not drinking red wine for the health benefits is worthwhile is questionable. However, next time you have a glass, you can at least marvel at the myriad number of chemical compounds that go into producing its colour and flavour.
References & Further Reading