What does "cold soaking" do for grapes?

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Dear Dr. Vinny,

I have recently read about a vinification technique used in making Merlot (and possibly other reds). Prior to fermentation, whole berries are "cold soaked" for about 5 days at 50 degrees F in open top stainless steel fermentation tanks. What attributes or benefits are imparted to wine as a result of "cold soaking"?

—Andrew, Mountain Home, Idaho

Dear Andrew,

Pre-fermentation "cold soaking" or "cold maceration" is a way to extract color and flavor from grape skins. Extraction also occurs during fermentation, of course, but many winemakers feel cold soaking brings out different, and beneficial, aspects of the grapes.

It's necessary to keep the grapes chilled to stall fermentation. The length of time varies according to the winemaker's goals and the grape variety. There are other variables, too, such as what proportions of stems and whole berries and clusters they want to be soaked (all adding different characteristics). Some winemakers add enzymes to help the extraction along, sulfur dioxide to inhibit unwanted microbial activity, and inert gas to prevent oxidation.

Winemakers cold-soak all kinds of reds (and some grapes and wine styles are more extractable than others), but since you asked specifically about Merlot, I checked in with Vine Cliff winemaker Rex Smith, who cold-soaks Merlot, to get the skinny on his perspective. This is what Rex says:

"I feel the main advantage to cold-soaking the grapes is to extract color and flavor, in an aqueous environment, without extracting tannin. By pumping over a few minutes a day, without having the solvent affect of alcohol, we are not extracting harsh tannins, bitterness or astringency, just color and flavor. We extract the "good stuff" first, and then we can dial in the tannins [during fermentation], and drain and press when the tannin balance is just right. Also, the nutrients in the grape skins will go into solution and be available for the yeast to use. This period of cold soaking will also allow indigenous yeast to grow and start the fermentation without a lot of temperature that encourages spoilage organisms to thrive."

—Dr. Vinny

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