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Keeping Fermentations Cool

The heat of summer can lead to problems when it comes to winemaking. When the temperature of your fermenting wine becomes excessive, your resulting wine could develop off flavors, off aromas or fusel alcohols. Fusel alcohols not only smell offensive, but they can cause headaches, severe hangovers and other ill effects.

So, rather than giving up your winemaking during the hot summer months, here are some tips to keep your fermentations cool.

The cool towel

The wet towel trick is perhaps the most common method for taking the edge off a hot fermentation. It’s simple and works under the scientific principle that as water evaporates from a surface, the temperature of the water left behind drops. This is why body temperature decreases when we get wet.

To cool a carboy using evaporative cooling, all you need is a towel, a basin and a bungee cord to hold the towel in place. Find a basin (bathtub, large picnic cooler, garbage can), fill it with a few inches of water and set the carboy inside. Wet the towel, wrap it around the carboy and fasten it tight with your bungee cord (or rope). (You can also use a sweatshirt or a T-shirt.) Make sure the bottom of the towel touches the water in the basin so that it works as a wick to continually soak up the water as the water evaporates. Believe it or not, this little trick can cool your carboy by 5–10 °F (~3–5 °C), depending on humidity and airflow over the evaporative cooling surface (the towel).

You will need to change the towel every few days to prevent it from mildewing. Likewise, if you don’t have pets (or kids) that might drink the water, you may want to add a splash of bleach to the water in the basin to prevent mold. The towel will get bleached, of course, so use something old.

A “temperature strip” stuck to the outside of your carboy will tell you the temperature of the wine. However, you will need to protect the strip from the soggy towel by covering it with clear packing tape.

Ice bath

If your basin is deep enough, you can create an ice bath containing enough water to surround most or all of your fermenter. All you need to do is add ice or ice blocks periodically to keep the temperatures down. This is a simple method, but it does require some testing to determine how often you’ll need to replenish your ice to maintain your target temperature.

When using the ice bath method, avoid subjecting your must to large temperature swings. It is better to ferment steadily a few degrees above your desired temperature than to put your fermentation on a thermal roller coaster (a sure way to end up with off flavors).

Ice box

The ice box is an age old device dating back before electricity. You don’t need to go out and find an antique ice box, but you can steal the concept and create your own. There are basically two components to this contraption: ice and a box. OK, it is a little more complicated than that, but not much. The box needs to be insulated to a degree where it will be able to hold the cool air generated from the ice blocks (milk jugs or 2 or 3-L soda bottles filled with water and frozen work well). You can construct the box out of cardboard and line it with a few inches of Styrofoam or build it out of plywood and line it with builder’s insulation. It all depends on how much time, money and energy you want to invest. Once you’ve got your box, gather your ice blocks and put them inside the box with your carboy and monitor your temperature every few hours to see how the temperature holds. This will give you an idea of how often you should rotate your blocks.

You will need to have a pan to catch water dripping from the frozen blocks and should check your box periodically for the presence of mold or mildew.

The fan and AC

If you have central air or window air conditioning units, you can effectively lower the temperature of your fermentation 10 degrees (ºF) or more. Make it your goal to route the cold air from the AC directly to your carboy. You can accomplish this by building makeshift barriers out of cardboard to direct the cool air. You can even expedite the airflow from the AC with a fan, to drop the temperature more efficiently.

Now that you have a few easy tips, you should look forward to keeping your cool this summer and making some wine. Stay cool!