Ask Wine Wizard

Wine Press Mold Growth


Philip Feiner — San Carlos, California asks,

Last year I brought out our press and found the slats covered in a white mold, so we took it upon ourselves to use 180 °F (82 °C) water to clean the press slats, and figured that would do it. After the 2016 harvest, and press, we sanitized it with SO2 sanitizer solution. The press again has mold on it. Help!


Ah, yes, the joys of wood. We use wood in the winery for barrels and barrel-alternatives of course but, especially for the small-scale winemaker, wooden presses are still often part of our crush equipment.

It’s interesting, but not strange, that this mold pops up periodically. I would scrutinize your ambient humidity and temperature; it’s very possible this mold bloom was caused by a change in the weather or a change in your cellar environment. Once, while working out of a winery in Santa Cruz, California, we noticed that we tended to grow mold on the outside of our wooden upright tanks when we experienced a period of particularly wet, damp, and muggy weather. When the sun came out and it warmed up for good, we cleaned and dried the tanks well (installing a couple of big fans blowing outside air into the tank room) and the white mold didn’t come back until the following fall. At another winery on California’s Central Coast, I used to have a small wooden press and we occasionally experienced the same problem you describe above: White mold growth on the press during storage. If cleaned off periodically I always felt its danger to my wines was limited, but it certainly was unsightly. The key to mold prevention is to maintain cleanliness in a moisture-balanced environment. To follow is some of my best advice to help manage your mold:

Clean, sanitize and dry well.
When I saw my mold develop, I would drag the press out onto the crush pad (a shaded, well-ventilated area is ideal), take it apart and attack it with a pressure washer. A steam cleaner can work great too, and really opens up the pores of the wood. Then I would give it a thorough going-over with a scrub brush or plastic scrubby pad using a solution of sodium percarbonate, known commercially as “Peroxycarb.” It’s good to really get in the nooks and crannies with an old toothbrush to make sure you’re lifting out all of the mold that you possibly can.

Give a good clean water rinse, followed by a rinse-off with a citric acid/sulfur dioxide solution. Dissolve 3 grams potassium metabisulfite and 12 grams of citric acid in 1 gallon (3.8 L) of water. This will produce a strong sulfur-dioxide solution that is very anti-microbial. Be sure to wear a respirator mask when working with sulfur dioxide solutions or gas as it’s not friendly to the lungs. Let the sulfur dioxide stand on the wood for about 15 minutes and then rinse off with water.

Allow the parts of the press to dry. It’s ideal to do this operation early on in a warm day so the wood will have time to dry completely before night falls and dew forms on the wood. Use a fan if necessary to dry the press completely. Store the press assembled or un-assembled, as you choose. If you put it back together again, you’re just going to have to unpack it and re-sanitize before harvest anyway. Depending on how big and awkward it is for your storage space, it seems easier to me to store it unassembled.

Store away from UV light
Over time, UV light can degrade wood so it’s a good idea to store your press (and especially barrels) inside. If you don’t have a good place indoors, a last resort is a well-shaded spot. When I’ve been forced to store a wooden press outside, I’ve covered it up with a loosely wrapped tarp to further protect it from the elements. If you have to store a tarp outside, just make sure it’s somewhere that rainwater won’t fall on or blow into it. Nothing like a plastic tarp “mummy” to trap water and encourage even more mold growth!

Store in a cool place with some (but not too much) humidity
I reached out to a few of my barrel suppliers while answering this question because, after all, they are the wood care experts. With a good French oak barrel averaging around $1,000, they’d better know how to protect their stock. Mario Carofanello, of Radoux Cooperage, says, “We store the barrels between 47-53% humidity at 67–72 °F (19–22 °C) at the cooperage.” Too dry and over time the wooden press slats can dry out and crack. Too wet and you’ll just encourage more mold growth.

Use mildewcides advisedly
Of all the of wineries I’ve worked out of in the last 15 years, only two ever used mildewcide (often sold under the brand name O’Sullivan X650) on the outside of their barrels to help repel mold growth and wood borers. “Mildewcide” as it’s sometimes called, is an acrylic paint impregnated with USDA approved fungicides. It comes in a paint can and is painted on with a brush, roller or sprayer. When I’ve used it in the past it’s sort of waxy, sort of like a clear-ish paint and dries to a varnish-like finish. It does help to repel wine stains and will help keep your barrels looking nicer over the years, but I’ve always wondered if it inhibits barrel breathability. For barrels, it’s meant to be used on the outside only and never contact the wine. It should not be used on a wine press because pretty much every surface on a press will contact the wine at some point, especially on a messy day.

Response by Alison Crowe.