The winemaking process of cold soaking (or cold maceration) is where temperatures of fermenting grapes are kept low to encourage slow, soft extraction of colour and flavours from red grape skins. Rather than relying on heat and alcohol to act as a solvent and without extracting harsh tannins.
This technique was made popular in Burgundy during the 1970s & 1980s with the production of Pinot Noir. There is still debate among winemakers about the overall benefits to and resulting quality of the final wine. But many winemakers feel cold soaking brings out different and beneficial, aspects of the grapes which you see and taste in the wine. Many winemakers use this technique to improve the richness and flavour of the ‘must’ juice.
So that when fermentation starts the wine has had a head start on colour and flavour extraction. Also during cold soaking the process encourages and allows time for the native or indigenous yeasts in the winery to get established for fermentation. In addition to the advantages of the character of the finished wines, cold soaking also has some practical advantages.
For some the process can take anywhere from 3-10 days (on some occasions longer) - though the length of time of cold soaking will vary according to the winemaker's interpretation of the fruit parcels and the grape varietal. During this time, you can get a more accurate measurement of the natural sugar content and chemistry of the grape must.
Following the harvest of the grapes selected for cold soaking, the crusher is typically set to barely bruise/split the fruit. Then depending upon the temperature of the incoming fruit, dry ice can be added (don’t worry it does not affect the wine and it simply burns off) to the gently crushed grapes while they are being placed into the tank to drop the temperature down to anywhere from 5°C to 16°C. This process can also be done in stainless steel tanks with temperature controlled cooling jackets (as shown below). Then a small amount of sulphites are added to discourage any undesirable microbes.
Since you are have only ever so gently crushed the fruit, there is not enough juice to start full fermentation right away. Plus the cold temperatures inhibit the ability of yeast to start converting the grapes sugars. Without cold soaking it is much easier for fermentation to start prematurely prior to inoculation. Cold soaking gives grapes a gentler crush and introduces less harsh acids, bitterness or astringency to the resulting wine.
During this process a great deal of care and attention is required from and by the winemaking team. As there is a rick of spoilage to the grape must by organisms, so typically a small amount of sulphur is added to the must before the cold soaking starts to prevent spoilage.
During cold soaking pectin enzymes in the fruit are released during the crushing, which start to break down the pectin and thus releasing the juice from the grape cells. This also liberates or creates aroma compounds which enhances the flavour of the finished wine. Also nutrients in the grape skins can go into solution and will be available for the yeasts to use. So the aim is for a more coloured and inviting wine with more natural varietal flavours and less harsh characters in the wine.