The start of any great wine is in the vineyard, the specific terroir coupled with the specific viticultural practices employed. However, more than exceptional grape quality is needed to guarantee a great wine since winemaking is an interventionist process, and every step of the process requires the skill and experience of a winemaker. If a winemaker makes the right choices given the characteristics of the grapes received, only then is the extraordinary achieved. One of these choices is whether to cold soak (cold macerate) grapes before fermentation.
What is cold soaking and why do winemakers do it?
Cold soaking is believed to have originated as a technique to improve the colour of Pinot noir wines. According to a relatively recent academic review article by Dr Jose Luis Aleixandre-Tudo and Prof Wessel du Toit (2018), the main reasoning behind cold soaking is that during the pre-fermentative period, phenolic compounds are transferred from the solid parts of the berry into the must under low temperatures (to prevent the start of fermentation) and in an alcohol-free environment (aqueous media). This can potentially enhance the phenolic content of the final wines.
Cold soaking is not restricted to Pinot noir anymore. Winemakers worldwide employ this technique for various reasons on many cultivars, including Merlot.
What the science says
The two researchers mentioned above systematically reviewed the scientific literature on cold soaking reported during 2006 – 2017. They found that the cultivar, the vintage, the climate, the vineyard, the grape’s ripening status, initial phenolic composition, grape wall composition, cold soaking temperature, length of cold soaking, cold soaking with or without enzymes, dry ice and sulphur dioxide (SO2), play significant roles in the efficacy of this technique. Many factors, therefore, influence the outcome of cold soaking, and a positive result is not always the case. Some studies reported an initial increase in phenolic concentrations after cold soaking, but this increase disappeared with ageing. Since cold soaking entails various logistical and economic considerations, one must ask oneself, is it worth the effort?
What the Top 10 Merlot winemakers have to say
During 2022 the first Top 10 Merlot competition was organised by the South African Merlot Forum, also known as Hallo Merlot. Nine of the Top 10 Merlot winemakers were interviewed about their specific choices while producing their award-winning wines. They were questioned whether they employed cold soaking and, if they did, how it was performed and what they believed they got out of it. Four of the nine winemakers used cold soaking when they produced the specific Merlots that won the 2022 competition.
Reasons given by the other winemakers not doing cold soaking, include not having the capacity to keep the must very cold for an extended period, not being convinced that it adds something to the wine and the danger of non-Saccharomyces yeasts proliferating during cold soaking and thereby removing much-needed nutrients, for the subsequent fermenting Saccharomyces cerevisiae, from the must.
Morné Vrey, the winemaker of the Delaire Graff Estate Banghoek Reserve Merlot 2019, has been using the same production method for this wine over the past few vintages. He does cold soaking for approximately five days at 8 – 10°C. The grapes are destemmed, and only 30% are crushed. The rest remain whole berries. Sulphur dioxide (30 ppm) and a macerating enzyme (Lafase HE Grand Cru) are added to the different batches of grapes as they fill the tank. Once the tank is filled, the content is mixed with a pump-over, and a biocontrol yeast (Zymaflore Egide) is sprinkled into the tank on the grapes and juice mixture. According to Morné, the yeast, together with the SO2, allow for the suppression of the growth of other indigenous flora long enough for the objectives of the cold soaking to be achieved. When he starts smelling acetaldehyde, he warms the tank up to 15°C and inoculates it with a commercial yeast starter culture (BDX). He also adds a commercial sacrificial tannin (Tanin VR Supra).
For Morné, the cold soaking allows for a controlled start to fermentation, leading to a slow and elegant fermentation where he also controls the temperatures at 23 – 24°C, thus allowing for the production of a fruit-forward wine.
Matthew Copeland, the winemaker of the Vondeling Barrel Selection Merlot 2019, does cold soaking for two days at 10°C. Before cold soaking, the destemmed whole berries, with only limited grape juice volume, receive approximately 40 ppm SO2 to suppress wild yeast growth, a sacrificial tannin (Tanin VR Supra) addition and an acid adjustment. After that, fermentation starts and completes spontaneously (most likely with some help of SO2-tolerant strong fermenting commercial yeasts used on other cultivars in the cellar).
For Matthew, the biggest advantage of cold soaking is the control it offers to the winemaking process. He is not convinced that it offers much else to the wine and argues that it is a controversial topic. He nonetheless thinks it might contribute to more fruitiness in the wine. He does get some potassium drop out via tartrate crystal formation, which is a positive for him.
Matthew also employs two other strategies to control the speed of the winemaking process: starting with whole berries and as little juice as possible and not inoculating with a high dosage of commercial yeast.
“I want my windows of opportunity as big as possible. I want enough time to make a decision. Instead of just hours, I want at least a day.”
Johnnie Calitz, the winemaker of the Glen Carlou Collection Merlot 2018, does five days of cold soaking of his Merlot in 400 L barrels under dry ice. After destemming and sorting, the must containing whole berries is cooled down by a mash cooler, and new French oak barrels without lids are filled to about ¾ of their capacity. Sulphur dioxide (20 – 25 ppm) and acid adjustments are made per barrel. Afterwards, a divider board with dry ice is placed on top to protect the must from oxidation and cool the non-temperature-controlled barrels. The barrels are situated in the barrel room (not on the production side of the cellar), which is temperature controlled at 20°C. Fermentation starts spontaneously after about five days. Johnnie prefers spontaneous fermentations in these barrels without temperature control as it provides better control of the speed and temperature of fermentation.
Johnnie feels he gets better colour extraction on the Merlot if he does pre-fermentation cold soaking. It also allows him to do fewer punch downs if he has a proper cold soaking.
Stander Maass, the winemaker of the Anura Reserve Merlot 2020, does 24 – 48 hours of cold soaking of his Merlot at approximately 14°C in five-ton tanks. After sorting, the grapes are destemmed, but not crushed. The grapes only move through a crusher where SO2 (40 – 60 ppm), enzyme (Rapidase Extra Color) and a sacrificial tannin (Tanin VR Supra) are alternatively added. After the crusher, the grape must, containing about 10 – 20% intact berries at this stage, move through a mash cooler to the tanks, which are covered with floating lids during cold soaking. After cold soaking, the must is inoculated with commercial yeast (BDX) for fermentation.
Stander believes cold soaking gives him better colour and potentially more flavour extraction. He reasons that this is the best time to extract colour and flavour since the grape skins have not yet separated from the must. Once fermentation starts, a mixing action is required to enhance extraction.
In conclusion
These results show that whilst some winemakers experience cold soaking as an integral part of their Merlot production, others produce award-winning wines without pre-fermentative cold soaking. There is also great diversity in how cold soaking is employed; a “one size fits all” recipe is not the order of the day.
Acknowledgements
The author would like to thank Morné Vrey, Matthew Copeland, Boela Gerber, Dr Marinda Kruger, Fanie Geyser, Stander Maass, Johnnie Calitz, Luca Bein and Philip van Staden for agreeing to be interviewed and sharing their winemaking experiences.
Reference
Jose Luis Aleixandre Tudo & Wessel du Toit, 2018. Cold maceration application in red wine production and its effects on phenolic compounds: A review. LWT-Food Science and Technology 95, 200 – 208.
– For more information, contact Karien O’Kennedy at karien@winetech.co.za.
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