Extracting the skin’s colour and the flavour is a key aspect of red wine production. This is called extraction, and it plays a crucial role in the quality of your wine. This is a short illustrated explanation of the importance of extraction.
Most wine grape pulp is colourless, except rare teinturier grapes like Sousao and Alicante Bouschet, which have coloured interiors. However, the skins are richly pigmented and contain various compounds, such as anthocyanins or tannins, which are crucial contributors to red wine’s colour and structure.
Although there are many ways to make red wine, the common thread is that you want to extract the skins’ colour and flavour elements. A common analogy is making a brew. The goal is to extract the right flavour from the tea leaves while not letting the tea steep too long. This is done by maceration. It’s a similar process to brewing a cup.
The process of making red wine begins with the crushing of grapes. This results in a liquid mass on which is floating skins and pip. The stems can be included in the fermentation. This variation on the theme is most often used with Burgundies. The stems must be ‘ripe’ to avoid wine tasting bitter and green. You can also leave the bunches undamaged and allow fermentation to begin inside the grapes. This is known as carbonic maceration. This semi-liquid mass, consisting of seeds, skins, and pulp, will most often be used for the fermentation of red wine.
The skins will form a solid cover on top of the juice if left alone. The cap-air interface would allow bacteria to grow, resulting in a volatile acidity problem. The wine would smell of vinegar and become spoiled. This can be avoided by winemakers keeping the cap moist by either plunging it or keeping it submerged with a mechanical device or pumping juice.
The most common method of removing a fermented product is plunging. It can be done manually, by special poles, or even by foot. This is usually done with small open-top fermenters. Pumping over can be more disruptive due to the forces involved, but it is better for larger, closed fermentation vessels like stainless steel tanks. Pumping over or plunging achieves the dual purpose of keeping the cap moist (preventing it from becoming volatile) and extracting colour/tan.
Rotating fermenters are a controversial method. They have agitators that mix the juice and cap when the tank is turned mechanically. They have been accused of making wines with a slightly bitter taste. Advocates suggest that this is due to a lack of oxygen to the fermenting juice and that the problem of reduction” is the cause.
The good stuff from red grape skins can occur before, during, or after fermentation. If the must and skins can be kept cool enough, fermentation may be delayed, and maceration in an aqueous media will occur. After fermentation starts, alcohol levels slowly rise and aids in extraction. The maceration occurs after fermentation has ended. It is done in an alcoholic medium and can take more stuff out of the skins than fermentation. The key decision in winemaking is how to separate the wine from skins after fermentation is complete.
This is where pressing takes place. The winemaker will have a mixture of juice, skins, and pips after the wine is removed from its skins. This pulp is then put in a press to extract the juice from the skins. The pressings are the process that determines the wine’s quality. It is determined by the force used and the type of press used. These can be added to the wine or left alone. You can extract bitter compounds from the skins or seeds if you press too hard. This can adversely affect quality.
The issue of colour is another. This is a complicated issue, but it is crucial. Anthocyanins are the main colour pigments found in red wineskins. They are responsible for the vivid colour of red wines that have just been fermented. They are not very stable. A variety of chemical reactions are required to form stable pigments. These reactions can lead to the formation of pigmented copiers. Although this series of reactions is still being figured out, it is becoming clear that oxygen and oak could play a significant role in forming stable pigments.