Subtle or robust? Wine is an interplay of the type of grapes, the soil, the climate, the time of picking and the craftmanship of winemaking. During the winemaking process, the many components of the grapes are transferred into the wine, and yeasts take care of the conversion of sugars from the grapes into ethanol. As a natural product, red wine is a mixture of hundreds of components. About 85% of these are water and 12 to 14% are ethanol. It is precisely the large variety of “small” components that determine the wide range of red wines.
The line “A bottle of white, a bottle of red …” is the start of Billy Joel’s masterpiece ‘Scenes from an Italian restaurant‘. The typical red colour of red wine is due to the fact that the skins of blue grapes containing the colouring agents are involved in the fermentation (winemaking) process. In white wine, the skins of the same (blue) grapes are removed before fermentation begins.
Thousands of different types of grapes produce an equally large variety of wines. For example, the well-known blue Cabernet Sauvignon grape is smaller, slightly darker blue, has a thicker skin and is more acidic than the equally popular Merlot grape.
Tannins – bitter flavours, polyphenols that make the mouth contract slightly when consumed – and colourings are present in the skin, while sugars and acids are in the flesh. The sugars are mainly glucose and sucrose – together about 20% – while the acids are mainly tartaric and malic acid – together about 1% of the grape. Water forms the main constituent of grapes at almost 80%. Many trace elements including vitamins, minerals and flavourings already present in the grape complete the composition.
During the approx. 100 days of ripening, sugars are formed by photosynthesis and the acid content decreases. This process is influenced by the climate: sun and heat result in a lot of sugars and relatively little acidity, while clouds and cold lead to little sugar and relatively much acidity.
The composition of the soil (e.g. clay, sand or limestone) determines which minerals the grape can absorb. A thin top layer must allow sufficient permeation of rainwater, while the bottom layer must retain the water absorbed by the grapevine via the roots.
The winegrower also plays a role by choosing the right time to pick – not too early and not too late – for an optimal balance between sugars and acids.
The harvested grapes are stripped of their stems and carefully crushed, so that the flesh mixes with the skin and the juice comes free. To prevent the released compounds from being oxidised by oxygen from the air, a small amount of sulphur dioxide (SO2, ‘sulphite’) is added. Next, fermentation takes place. Grapes are fermented by adding selected wine yeast cultures, usually of the Saccharomyces cerevisiae type, in a controlled manner. In this biochemical process, the sugars present in the grapes are converted anaerobically into ethanol, popularly known as ‘alcohol’, with mainly carbon dioxide and glycerol as by-products. This process stops when all the sugars have been converted, when the winegrower removes the yeast from the wine or when so much alcohol has been formed that the yeast cells succumb. When all the sugars have been converted into alcohol, we speak of a ‘dry wine’ – which has nothing to do with a lack of water. Here ‘dry’ is in contrast to ‘sweet’, where (residual) sugars are left in the wine. Whereas grapes consist of about 20% sugars, wine contains about 12 to 14% ethanol.
Apart from the conversion into alcohol, during this fermentation process numerous aromatic compounds and flavours are formed – or passed on from the grape – which give each wine its own character. It is striking that in addition to the main components water, ethanol and glycerol, less than 1% of colourings, flavourings, volatile aromas and (remaining) sugars and acids determine the distinction. Fruity notes, for example, can be introduced into the wine by esters produced by the yeast cells. Phenolic compounds with an umbrella name of flavonoids are largely responsible for the taste, colour and character of wine. These compounds are already produced in the grape – mainly in the skin – and are simply transferred to the wine during the winemaking process. Anthocyanins, for example, with or without the help of flavonols, give wine its red colour. Catechins and tannins contribute to the “bitterness”. Tannins, which are natural polymers of catechins, are antioxidants that literally counteract the oxidation of wine in a natural way.
For red wines, alcoholic fermentation is followed by malic lactic fermentation in which lactic acid bacteria convert the ‘hard’ malic acid into the ‘softer’ lactic acid.
After fermentation, the fermented mass is filtered to separate ‘the solid’ from ‘the liquid’. After this, the wine matures for some time – traditionally in wooden barrels, but nowadays increasingly in airtight stainless steel tanks. The wine is still turbid due to particles of grape skin or flesh floating in it, or due to (dead) yeast cells and other substances such as proteins and pectins. To improve the aroma and taste but especially the appearance of the wine, these have to be removed. There are various possibilities for this. With some wines, the ‘floating’ material settles quickly, so that pouring off the liquid above it is enough to create a clear wine. The settling of floating particles is accelerated by having this process take place in a centrifuge. Often small quantities of fining agents are added to clarify, which can adhere to the suspended particles or neutralise their electrical charge so that they agglomerate and settle. For example, proteins and yeast cells can adsorb to bentonite or gelatine. Another option is filtration, such as coarse filtration with diatomaceous earth or the use of membranes with sufficiently small pores to remove yeast cells and bacteria. Cold treatment can also be helpful; potassium bitartrate and calcium tartrate precipitate quickly at low temperatures. It is precisely the slow precipitation of these tartrates at room temperature that causes turbidity in wine.
Red wine and materials
The inner surface of the fermentation vessel or maturation tank must be smooth so that no pathogenic micro-organisms and scale formation can attach to it. Moreover, this surface should be corrosion resistant and easy to clean. Stainless steel and (fibre-reinforced) plastics are well suited for this. Hoses and pipes between the vessels are also made of these materials. Tools for crushing grapes and machines for bottling are also largely made of the ‘workhorse’ stainless steel.
Wood, especially oak, plays its own role in the ageing of some wines. Wines that have spent several years ‘resting’ in wine barrels absorb components from the wood, such as tannins and other aromas and flavourings, which can be observed in high concentrations in the wine. A vanilla flavour, for example, is typically a result of the wine absorbing vanillin from the wood. Wood also allows small amounts of oxygen to pass through in a controlled manner, where the wine oxidises slightly – softening the tannins – and develops in its own way.
Wine bottles for packaging and storing red wine are usually made of green glass – and sometimes of brown glass. Apart from tradition, aesthetic and marketing reasons for a certain type of bottle, the green colour blocks ultraviolet light from the sunlight sufficiently to prevent the wine from oxidising. Brown glass blocks UV light even better. In addition, the glass also seals off the wine from oxygen in the air. After all, under the influence of oxygen, ethanol can further oxidise to acetic acid (and water) and thus spoil the wine. By the way: here too the tannins come to the rescue, because red wine suffers less from oxidation than white wine due to the presence of these ‘natural miracle compounds’. An equally airtight ‘real’ cork, a plastic cork or a metal screw cap closes the bottle. Sometimes red wine comes in waterproof, opaque cardboard packaging with a plastic tap or screw cap.
Glass has another role to play in wine, namely that of the wine glass. Preferably it should be as colourless and translucent as possible in order to be able to see the (colour of the) wine clearly. The shape of the wine glass – like a tulip on a stem – also plays a role: a large and wide wine glass has a large interface between wine and air, allowing the aromatic compounds to penetrate the air. The tapered top ensures that this scent (the “bouquet”) remains somewhat captured in the glass, so that your nose can enjoy it properly – even while drinking. Cheers!