Jamie Goode: Do winemaking vessels really make that much of a difference?

Go into any wine cellar these days and as well as stainless steel and small oak barrels, there’s a chance you will find a range of fermenting vessels, including different sorts of concrete (tanks and eggs), some large-format oak (foudres or botti or fuders) and perhaps even some clay (amphorae, tinajas, talhas, qvevri), or even glass or ceramics (wine bulb, clayver).

How much difference do all these vessels make? Quite a bit, if we are talking about maturation over many months or even years. But for fermentation, which might last a few days or a couple of weeks? That’s a rather different question.

One of the significant properties of a vessel that’s used for ageing wines is its oxygen transmission properties. Another is whether or not the vessel is flavor neutral. Stainless steel allows no oxygen exchange with the wine, and imparts no flavor: it is inert. However, if there is some headspace then the oxygen present in this can react with the wine. Some wineries have variable capacity tanks, where a floating lid sits on top of the wine, and is sealed in place by an inflatable plastic tube. But if the tube deflates, then some oxygen can get to the wine so the seal needs to be checked frequently. Added to this, oxygen can diffuse through plastic, so this isn’t a completely airtight seal in that it allows a very small amount of oxygen transmission. Oak allows a little oxygen transmission, and this comes through the joins in the stave, but mostly from the bung hole, especially if the barrel isn’t fully topped up. New oak allows a bit more oxygen transmission than older oak, and the larger the barrel or foudre the lower the transmission rate per litre of wine. Topping up itself can be oxidative, and the presence of lees can make the bottom of the barrel reductive. Barrels also impart some flavour, although less as the barrel ages. Concrete and clay both allow a little oxygen transmission. The rate depends on whether or not there is any lining: usually linings slow down oxygen transmission, and epoxy lining will eliminate it altogether. Whether or not flavour is imparted by these materials depends on their construction. For clay, increasing the firing temperature reduces the oxygen permeability. Sometimes the lining used for clay amphorae has a flavour to it. Ceramics and glass behave like stainless steel in that they are inert.

This is for ageing, though. What about fermentation? The wine is in the vessel for such a short time that the oxygen transmission properties don’t make any difference: ferments produce lots of carbon dioxide which will sparge other gases out, and whether or not oxygen gets to the wine (this is a good thing in fermentation because it helps the yeast perform better, and also helps fix colour in red wines) depends more on the shape of the vessel, whether or not it has a lid, and the management technique than it does on the material the vessel is made from.

The shape of the vessel is very important for fermentation. A tall, narrow tank allows less skin contact for red wines, and less oxygen for white wine ferments. A shallow tank allows more skin contact, and also more oxygen availability for the ferment. It’s common to ferment high-end red wines in open top fermenters that aren’t too deep, which allows easier cap management, better contact of juice with the cap, and also allows a little alcohol to blow off.

The material the fermenting vessel is made of does make a difference. Stainless steel conducts heat well, whereas concrete, clay and wood have a bigger thermal inertia, which means that they keep the ferment going at a steady temperature that rises and falls slower than it would with stainless steel. When it comes to the concrete eggs, one of their appeals is that the shape of the vessel keeps the fermenting yeasts, and later the lees, in suspension for longer.

What about wood? Does this impart flavor during fermentation? Not really: the toasty notes from barrel-fermented whites presumably develop during ageing in contact with wood. But whites fermented in barrel acquire less oaky flavours than whites fermented in steel and then transferred to barrel, because the yeasts metabolize some of the oak flavours during fermentation. And for reds fermented in new oak vats, there is the possibility that some of the tannins from the wood will help fix colour by forming complexes with the anthocyanins present in the baby wine. But the cap management for reds will likely have more impact than this: whether it is punched down, or pumped over and how often, and how much.

Often there’s an association with a wine style and the fermentation vessel. Fancy Bordeaux reds are frequently made in tronconic vats, which look a bit like the lunar module. These are associated with pump overs or rack and returns, and then ageing in small oak, and of course fermenting with destemmed fruits. Hipster wineries are associated with using small open-top fermenters, often made of plastic. The one ton or less that can be fermented in plastic bins will affect fermentation temperature (they are small, and the volume generates less heat than a bigger ferment), and often whole bunches will be used, with manual punch down. One issue is whether the bin is cuboidal or cylindrical: the latter is preferred because it doesn’t have cool corners during fermentation. When amphorae are used this is often associated with skin ferment for whites. But the oxygen transmission properties of the vessel, which are so important during ageing, are less important during fermentation.

For whites of course, fermentation and ageing are commonly in the same container, and here it becomes harder to separate the importance of the container during fermentation from its role during ageing. Fermentation sometimes takes a long time for whites, particularly when they have lower pH and the cellar becomes cool soon after vintage. The nature and size of the vessel can affect fermentation temperature. Barrels are frequently without any temperature control, unless they are moved into a cold environment, or they have an elaborate cooling system that enters through the bung hole.

In conclusion, the role of a vessel can affect fermentation, through thermal inertia, or perhaps by imparting some flavour. But its oxygen transmission properties, so important during ageing, are less important during the relatively brief process of fermentation. The vessel size and shape can be significant, particularly for red wines, and if it then affects how the fermentation is managed. And some vessels are easier to cool than others, affecting fermentation temperature.

• Jamie Goode is a London-based wine writer, lecturer, wine judge and book author. With a PhD in plant biology, he worked as a science editor, before starting wineanorak.com, one of the world’s most popular wine websites.