Alcoholic fermentation is the process of converting sugars into alcohol through the presence of yeast. The yeast (collection of microscopic fungi) seeks out the sugars within the grape juice and consumes with the resulting product being alcohol (hence why high sugar content grapes have a higher alcohol percentage). For this process to occur the juices must be placed within a temperature controlled, carbon-dioxide filled vessel.
Monitored closely, the temperature can have a major impact on the speed of the fermentation process and therefore a major impact on the flavours and style of the resulting wine. More tannins and colour can is extracted from the grapes when fermented at a higher temperature, with maceration also accelerating. Red wine producers tend to lean into warmer fermentation to bring out the red colours and the tannic structure necessary for a bold red wine. Contrastingly, white wines are fermented at a cooler temperature to encourage fresher and fruitier wines. Maceration also is not often occurring at this time as stems, skins and seeds all would have been removed beforehand, reducing the tannins further.
For wines that continue their maceration through the fermentation process there are a few choices made to balance out the amount of tannins and flavours transferred. Cap Punching is a act of pushing down the skins and stalks that float to the top of the vessel so it mixes back into the juices. If left as is, the skins and stalks create a ‘cap’ above the juice and traps the heat of the fermentation beneath and can ‘cook’ the wine and kill the yeasts. The practice of cap punching does increase contact between juice and skin, further adding tannins and flavours throughout the process. Often done manually, it is a laborious task performed with rods and forks to break up the cap one to three times a day.
Alternatively, Pumping Over is performed for the wines that don’t want the extra tannic structure in the final wines. This is the practice of extracting wine from the bottom of the vessel beneath the cap and pouring it over the top to cool it down, helping to homogenise the temperature in the vessel over all. There still remains the option to do high pressure pump-overs where the juices sent towards the cap with strong force and breaking up the cap, resulting in highly structured and tannic wines. Yet most modernised machines do a gentle sprinkle so to not disrupt the cap but simply cool it down and continue the process.
Once decisions have been made regarding temperature and vessel choice, the winemakers cannot just walk away. Monitoring the process is key to ensuring the temperature does not rise above 35°C, this can halt the fermentation process before all the sugars have been converted. For winemakers that decide they want to stop fermentation early, to keep a sweet element in the wine, they kill the yeast with the addition of sulphur-dioxide or grape spirits to the vessel. Additionally, pausing the process is an option when temperatures drop below 5°C.
Ecoulage & Décuvage
Once alcoholic fermentation is complete, the ‘free-run’ wine is drained from the fermentation tank into another tank or barrel. This is the Running Off stage that is reasonable straight forward and separates the solids from the pure wine. The solid remains left behind, the pommace, is then collected manually and put into a press. The pressing process forms another collection of juice, pressed wine. Said to be of lower quality that free-run wine, it can be added to the free-run wine or kept separate.