Pre-Fermentation Treatments
Cold Soak
Cold soak, also known as cold maceration or prefermentation maceration, is a common winemaking technique that is widely used in the production of red wines. It involves holding crushed red grapes at a low temperature for a period of time prior to the start of fermentation (Figure 3.2). Winemakers typically choose to employ this technique for two major reasons: to improve the color and/or mouthfeel of the wine, and/or to modify the flavor and aroma of the wine (Casassa et al., 2015). The color of a red wine is primarily driven by the concentration of anthocyanins present in the grape skins. Anthocyanins are more soluble in an aqueous solution than in an ethanol solution.
Grape Varieties
This technique has become highly successful among a growing number of winemakers that macerate red grape skins from Bordeaux grapes. Pinot Noir is a very popular variety that commonly undergoes a cold soak as well as Cabernet Sauvignon. In terms of red wines, some varietals just don't seem to play well with a cold soak.
Practical and Logistical Considerations
Following the harvest of the grapes selected for cold soaking, the crusher is typically set to barely bruise/split the fruit. After crushing, the grape must is cooled with a heat exchanger and pumped to stainless steel tanks with temperature-controlled cooling jackets and chilled for a set period of time; the tank having been filled beforehand with a layer of carbon dioxide to avoid oxidation. An effective cold-soak requires that the must temperature be lowered rapidly (under 24 hours). This key point is, unfortunately, frequently ignored by winemakers, who take risks that can result in serious spoilage of their wine.
Microbiological Risks
Extending the skin contact phase as during cold maceration can be quite risky for the winemaker, since it raises the chances of spoilage to occur. Yeasts and bacteria are present in quantities, with the various populations in differing proportions, according to the condition of the grapes. Even if precautions are taken to avoid the microbiological risks, the grapes must be healthy and at optimal ripeness. If the grapes were exposed to powdery mildew and Botrytis the grapes will have a much higher load of non-Saccharomyces yeast species and bacteria than healthy undamaged fruit. There are many species of yeast that are found on the grape berry naturally including many non-Saccharomyces.
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