Cold soak or pre-ferment 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.
Background
The principle behind the use of cold soak is that while grape skins are in contact with juice before any alcohol is formed, anthocyanin extraction is enhanced, as is the extraction of aroma and flavour compounds, improving the overall quality of the wine produced.
Cold soaking temperatures typically range from 5 to 10 degrees Celsius. The time for the maceration can be quite variable – from as little as 5 to 10 hours to as much as 10 days, as reported by Cassasa et al. (2015) and references within.
Recent work by Fennessy (2015) showed that cold soaking different red varieties (Cabernet Sauvignon, Shiraz and Merlot) can achieve different results. The work also showed that climate plays a role, with cold-soaked warm-climate fruit showing larger colour differences than cold-soaked cool-climate fruit, with the greatest colour differences occurring with the variety Cabernet Sauvignon.
Practical and logistical considerations
After crushing, the red grape must is pumped to a tank or storage vessel and chilled for a set period of time. Sulfur dioxide should be added at crushing. Dry ice coverage should also be maintained during the cold soak period, to minimise oxidation and avoid aeration. The storage vessel should be covered to prevent unwanted microflora from entering.
Options for cooling must include:
- harvesting at coldest point of the day to maximise cooling potential
- using cool rooms to chill fruit before crushing (only if hand-picked)
- placing picking crates in the picking bins prior to picking (0.5 – 2.5 tonne picking bins). This creates pockets for cool air to circulate when placed in cool room prior to crushing
- crushing into picking bins and storing in refrigerated rooms
- crushing via heat exchanger slowly to maximise cooling potential
- using dry ice in the must, allowing it to sublime and cool
- using tanks with cooling jackets and or internal cooling plates placed into the must
During the 2018 Cabernet trial the cold soak treatment was carried out using the following protocol: Fruit was crushed into a fermenter, crush additions including acid and SO2 were made, then the fermenter was placed into 0oC cool room for five days where the must was held at approximately 3oC. After five days, the fermenter was moved to 20oC and the must was inoculated once it reached 15oC (this took 24 hours).
Potential risks
There are risks that spontaneous yeast and or bacterial growth could occur during cold soaking; however, this risk is considered low due to the low temperature. Covering the fruit during cold soaking is recommended to minimise this risk and using CO2 is advisable to reduce the chance of oxidation. Aeration should also be avoided at this point to prevent oxidation.
Resources
Aleixandre-Tudo, J.L., du Toit, W. 2018. Cold maceration application in red wine production and its effects on phenolic compounds: A review. Lwt-Food Sci Technol 95: 200-208.
Cassasa, L.F., Bolcato, E.A ,Sari, S.E. 2015. Chemical, chromatic, and sensory attributes of 6 red wines produced with prefermentative cold soak. Food. Chem. 174: 110–118.
Fennessy, R. 2016. Influence of climate and variety on pre-fermentative cold maceration. Poster presented at the 16th AWITC.
Godden, P. 2020. Pre-fermentation skin contact: how white wine composition and sensory characters are changed by leaving juice in contact with grape skins and seeds prior to fermentation. AWRI Tech. Rev. (246): 13-18.
WA Department of Primary Industries and Regional Development media release.