An open-top wine fermenter is an important piece of winemaking equipment - either stainless steel, oak, concrete or plastic vessel of variable sizes without a fixed lid, for the fermentation of red grapes. When making red wines, they are fermented by using the entire grape, skins and all parts of the berry - (whereas white wines are fermented using only the juice). So when making red wines, measures need to be taken into account for an ideal result. Winemaking 101 - during fermentation, yeast creates both alcohol and carbon dioxide, CO2 gas. As the CO2 bubbles up through the wine, a portion gets trapped inside the skins and causes them to float on the surface in a layer called ‘the cap’. The cap causes the volume of ‘must’ to expand inside the tank - which can protect the wine from outside elements.

 

So when you are filling red wine fermenters, one must keep in mind that you will not be able to fill the tank right to the top with ‘must’, you will need to leave enough space for the cap to expand due to the build-up of CO2 gas. Therefore, if you are wanting to be able to ferment a set volume of red wine, you will need to choose a fermenter slightly larger in size than the desired finished volume of wine.
The grape skins in the ‘cap’ tend to dry out quite quickly, plus they contain colour and so much of the wines character, flavour and aroma compounds. The skins need to be pumped-over with the wine several times a day during fermentation. For large volumes this is done with a large pump in a process called ‘pumping-over’ (because you are pumping wine over the top and through the cap). For smaller volumes this wetting of the cap is usually done manually using a specific punching down tool-paddle designed for the job. Using the plunger, you simply break-up the cap and gently stirring, submerging it back into the wine. This process is called ‘punching the cap’. In order to use the punching down tool, however, you will need to have access to the surface of the wine. Therefore an open-top wine fermenter (not sealed or closed) is ideal for fermenting small to medium parcels of red wine.
Winemaking with an open-top fermenter is not for an inexperienced winemaker. As there are a few more risks involved, and if you don’t have a good feel or confidence, you will not know how to best respond to your parcel of wine at the right times during the process for the best results.
Fermenting in an open-top container can be beneficial during the early stages of fermentation - as you get oxygen exposure which helps the yeast build up a strong population. Also there is nothing keeping the heat generated during fermentation from escaping. As lids can help trap heat resulting in a fermentation temperature far too hot, making for a tired wine.
As I mentioned this is not for an inexperienced winemaker - as you are also at risk of picking up too much oxygen or micro-organisms that could spoil your wine, when not protected by a lid and airlock. Open fermentations work because of 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 is enough CO2 being produced you can ferment without a lid. Experienced winemaker will be able to identify when the production of C02 is diminishing and to cover, seal the wine up before it is susceptible to spoiling.
You can use a sterile fabric-cloth, which gives a soft barrier that can help prevent moving air from blowing all the carbon dioxide away. A fermenter should only be open during the ‘primary’ fermentation stage - which is when approximately 70% of the grape sugars are fermented.
Also by using open-top fermenters you can have better control of the temperature and speed of fermentation. During fermentation of red grapes, parcels at lower-temperature can add aromatics and freshness, higher-temperature can add depth to the wine. Also by fermenting in different size tanks, different volumes with varied thickness of the cap and different temperatures, you have different flavours profiles and wine textures. The result is a broader range of blending options at the end for the winemaker.
Several manufactures can also custom fabricate open top tanks of variable capacities with floating lids or removable truncated tops to be used in conjunction with open-top tanks. A custom fabricated open top fermenter including separate heating and cooling jackets and 100% stainless tanks give a winemaker options.
There are some manufactures who make cube and rectangular shaped open-top tanks, designed to maximize the fermentation process. The square surface as opposed to a round tank of the same width, offers 21% more contact surface area between the cap and the wine.
More specifically, these open top fermenters which provide a larger contact surface area also result in having a thinner cap of must. Therefore much easier for a winemaker to punch down and break the cap and submerge it into the wine. All this helps the winemakers to be as gentle as possible with the more fragile grapes, those needing extra care, thinner skinned grapes such as Pinot Noir.