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Extraction: making red wines

Machine harvested Merlot (left) and hand-picked Pinot Noir (right)

One of the key issues in red wine making is getting the flavour and colour out of the skins. It’s called extraction, and how you do it is a vitally important factor in the quality of your wines. Here’s a brief illustrated explanation of this important subject.

The pulp of almost all wine grapes is colourless (the exception here being the rare teinturier grapes, such as Alicante Bouschet and Sousão, which have coloured insides). The skins, however, are richly pigmented, containing a range of compounds such as anthocyanins and tannins that are important contributors to the colour and structure of red wines.

There are many different ways of making red wine, but a common theme to all is the goal of extracting these colour and flavour components from the skins without extracting too much: a common analogy used here is making a brew, where the goal is to take out just enough flavour from the tea leaves, but not letting it stew too long. This extraction occurs via a process of maceration, which really is a bit like brewing a cup of tea.  

Pinot Noir grapes entering and leaving a crusher-destemmer

Most red wine making begins with the crushing and destemming of the grapes, which results in a liquid mass, on which floats a raft of skins and pips. One variant on this theme is to include the stems in the fermentation, which can add a lovely spicy, structural element—it’s most commonly used with some Burgundies. But if this is done, the stems need to be ‘ripe’, or else they will make the wine taste green and bitter. Another variant is to leave the bunches uncrushed, and let fermentation begin from inside the grapes: this is called carbonic maceration. But most of the time red wine fermentation will begin with this semi-liquid mass of seeds, skins, pulp and juice.

Left alone, the skins would form a solid cap on the top of the juice. Bacteria would begin to ferment at the cap–air interface, and the result would be a volatile acidity problem—the wine would reek of vinegar, and be spoiled. Winemakers avoid this by keeping the cap moist, either by plunging it regularly, or keeping it submerged by a mechanical device, or by pumping juice over it.

Plunging, also known as pigeage, is the most traditional method, and can be done by machine, or by a special pole, or even by feet. This is typically done with shallow open-top fermenters. Pumping over is potentially more disruptive, because of the forces involved, but is more appropriate for bigger, closed fermentation vessels such as stainless steel tanks. Plunging or pumping over achieve the dual goal of both keeping the cap wet (preventing it going volatile) and extracting colour and tannin.  

Three types of fermenter. On the left we have a lagar (this is a swanky one with a heat exchanger), ideal for shallow fermentation with good contact between the juice and skins. Traditionally, these are foot-trodden, and this one hails from Portugal's Douro. Below we have a row of open-top fermenters, which are quite small and ideal for processing high-quality red wines (again, this is from a winery in Portugal, this time the Alentejo). Then below left there is a rotary fermenter of the type popular in Australia.

A slightly controversial technique is to use rotary fermenters, which have agitators in them that mix up the cap and juice when the whole tank is rotated mechanically. These have been accused of producing wines that have a slight bitterness to them; advocates suggest that this is because not enough oxygen has been provided to the fermenting juice and the bitterness is a problem of ‘reduction’.

Extraction of the good stuff from the skins of red grapes can occur before fermentation, during it, or after it. If the must and skins are kept cool enough, fermentation will be delayed, and maceration will occur in an aqueous medium. Once fermentation begins, alcohol levels gradually rise, and this alcohol aids extraction. After fermentation has stopped, the maceration that occurs is carried out in an alcoholic medium, and may be more severe, taking lots of stuff out of the skins, than that which occurs before fermentation. A key winemaking decision is when to separate wine and skins after fermentation has finished.  

Red grapes removed from fermenters, ready for pressing

This is the point where pressing occurs. After the wine has been run off from the skins, the winemaker is left with a pulp that is a mix of juice, skins and pips. This gets put into a press, which squeezes out the remaining juice from the skins. How this is done – the force that is used and the type of press – determines the quality of the wine that is thus extracted, known as the pressings. These may be finished off separately from the rest of the wine, or blended back into it. Press too hard and you end up extracting bitter compounds from the skins and seeds that can have an adverse effect on quality.

A traditional basket press of the type favoured for red wines

There's a further issue here, which is that of colour. It's a bit complicated, but it is important. The main colour pigments in the skins of red wines are called anthocyanins: they are responsible for the very vivid colour of just-fermented red wines. However, they aren't very stable. To form stable pigments a variety of chemical reactions need to occur resulting in the formation of pigmented polymers. This series of reactions is only just being worked out, but the story emerging is that the presence of oxygen and the use of oak could be important here. 

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