There’s a lot of discussion around open fermentations. Beginners especially wonder why some recipes call for open fermentations and kit instructions call for an airlock to be placed on the fermenter as soon as you’ve pitched the yeast.
The truth is, you can do it either way.
When you’re first getting into winemaking I do not recommend fermenting your wine in an open container. There’s a bit more risk involved if you don’t have a good feel for when to put the lid on your wine.
The Benefits of Open Fermentation
Fermenting out in the open can be beneficial during the early stages of fermentation for a couple reasons. First, you get some oxygen exposure which helps the yeast build a strong population.
Second, there’s nothing keeping the heat generated by fermentation from escaping. Lids can help trap heat and, depending on your ambient temperature outside the fermenter, your wine could get too hot with a lid in place.
Most commercial wineries will ferment red wines in open containers to allow heat to escape and to have better access to the cap. When you ferment on the skins they will float to the top and for the cap. This cap protects the wine from outside elements but it must also be punched back down into the wine from time to time to help extract more flavor and aroma compounds from the skins.
The Risks of Open Wine Fermentation
As you might imagine you’re at a higher risk of picking up too much oxygen or spoilage micro-organisms when your wine is not protected by a lid and airlock.
Open fermentations work because the carbon dioxide produced by the yeast during alcoholic fermentation acts as a blanket over the wine. As long as the air around the fermenter is still and there’s enough carbon dioxide being produced you can happily ferment without a lid.
However, at some point carbon dioxide production will fall to a point where there’s no longer enough to protect your wine. With some experience you’ll be able to identify when this time is approaching and get your wine covered up before it’s too vulnerable. This is why I never recommend to first time winemakers that they ferment in an open container.
Here’s a great introductory article from EC Kruas entitled Wine Fermentation 101
Something else to consider is air movement around the fermenter. Even an aggressive fermentation, with lots of carbon dioxide being produce, can be susceptible to oxygen and spoilage micro-organism exposure if you have an air vent or ceiling fan constantly blowing the carbon dioxide blanket off your wine.
Then there’s also curious cats and dogs but we won’t get into that.
Advice for Beginning Winemakers
If you’re preparing for your first fermentation stick to using a sealed primary fermenter with an airlock. You’ll want to be sure you have a few inches of headroom in case the yeast foams up.
As your wine ferments pay attention to the rate of carbon dioxide production by keeping an eye on your airlock. At first it will bubble quite slowly when the yeast is acclimating to their new environment. Then the pace will pick up as the yeast goes into the growth phase. For some time the rate will remain constant before it starts to slow down.
Record your observations and perhaps after you have a few wines under your belt, then try an open fermentation. You want to have a feel for when fermentation is slowing down. so you know when to put the lid on.
Trying an Open Fermentation
When you’re ready to try an open fermentation I recommend placing some sanitized cheese cloth over the fermenter. This gives you a soft barrier that can help prevent moving air from blowing all the carbon dioxide away.
Keep a close watch of the rate of bubbles coming to the surface and measure your specific gravity. As soon as you notice fermentation starting to slow down rack into a carboy and put the airlock in place.
Your fermenter should only be open during “primary” fermentation which is when the first 70% or so of your sugars are fermented (here’s more on Primary vs Secondary Fermentation). This corresponds to a specific gravity of around 1.030 to 1.020 depending on your original gravity.