Realising the benefits of aeration during fermentation

Punchdowns, pumpovers and rack and returns sometimes don’t introduce enough aeration to red wine fermentations to achieve a winemaker’s desired wine style. For those winemakers, they’ve gone the extra step of adding air or oxygen to wine, and now there is solid scientific data on what such a practice achieves, writes Sonya Logan.


As every winemaker knows, a little oxygen is beneficial to wine, particularly red wine, and especially during fermentation. Punchdowns and pumpovers introduce oxygen to musts to help yeasts do their job, thus improving the fermentation process while reducing the chances of unwanted hydrogen sulfide and volatile acidity forming. Rack and returns provide further opportunity to oxygenate fermenting must.

Some winemakers take the additional step of introducing air or oxygen into their red ferments, believing that punchdowns, pumpovers and rack and returns on their own aren’t as effective as they’d like in oxygenating their ferments to suppress sulfidic off-odours and unmask the bright fruit characters they desire. And research carried out over the last decade has backed this up. 

Until recently, the amount of information on the benefits of adding oxygen to ferments has been limited, with scientific data into its effects on wine style lacking. Among the organisations to fill this void in recent times has been the Australian Wine Research Institute (AWRI), which embarked on a project in 2012 to establish the effect of using oxygen at both crushing and during fermentation. Chardonnay and Shiraz were the two varieties selected for the project, which showed that the addition of oxygen during the fermentation of white wines had a positive effect on the kinetics of fermentation, enabling it to finish sooner, but had minimal positive or negative effects on the sensory qualities of the resulting wine. 

When it came to Shiraz, however, oxygen addition during fermentation produced wines with more ‘aged’ characteristics comparable to those achieved in wines made in the absence of oxygen and bottle aged for a year. Specifically, those wines had better hue, fewer anthocyanins, lower tannin concentrations and smaller tannins. 

Subsequent work by the AWRI and published earlier this year has confirmed that aeration during the fermentation of Shiraz does indeed bring out the red fruit characters of wine by stimulating the compounds that impart those characters and reducing reductive characters that might suppress them.

The trial underlined the importance of getting enough air into a ferment for the benefits to be realised: the greater the aeration, the greater the intensity of red fruit characters and decrease in sulfur compounds.

I really do believe that the work that we’ve done with oxygen in our ferments has contributed to the wine show success of our Shirazes.

Anthony D’onise


“In a typical pumpover or plunging operation, you don’t actually get much air in,” explains Simon Schmidt, research manager at the AWRI. “For example, if you’re manually plunging, there’s a blanket of CO₂ over that ferment and plunging it around with a standard small-scale ferment plunger doesn’t really disrupt much of that blanket of CO₂. You’re mixing in the cap, but you’re not really introducing much air.

“There’s an opportunity to potentially introduce a bit more oxygen when you’re doing a pumpover operation, depending on how people do it. People might have irrigators set up in larger tanks; in smaller operations they might have a cellar hand standing at the top, spraying the cap down. But, again, those types of operations tend to introduce a relatively small, if any, amount of oxygen into that ferment. 

“There are other types of cap
management strategies, for example pulsed air operations where large bubbles can be introduced at the bottom of a tank to force changes to the cap structure. Those types of operations can also introduce small amounts of oxygen during the operation itself.” 

Schmidt says even the use of a splash tub in a pumpover circuit introduces limited aeration, even though it might reduce reductive aromas and potentially modify tannin structures.

He explains that the AWRI’s latest research had explored the sensory effect of substantially larger amounts of air being pumped into ferments.

Two methods to introduce air

In the AWRI’s pilot-scale work, two methods were used to introduce air to ferments: spargers that injected air into pumpover circuits and sinters at the bottom of fermenters. Both methods were set-up to exclude cap management.

“We wanted to disentangle the effects of aeration from the effects of cap management, partly because cap management is typically used as a way to manage extraction from grapes in reds in particular,” Schmidt explains. “This also allowed us to do aeration operations that were quite a lot longer in duration than you might get if you were coinciding your aeration operations with pumpover style management of the cap.” 

For most wineries, the easiest way to introduce air to ferments is via a sparger in the hose line of a pumpover circuit which is connected to a compressed air supply. Another option is to connect a venturi device to the hose used for pumpover operations which doesn’t require compressed air but simply sucks air into wine as it flows past the attachment.

Winemaker Anthony D’Onise, from organic wine producer Pig in the House, based in the Central Ranges of New South Wales, has gone one step further and introduced oxygen to his ferments.

“We’ve always done open pumpovers,” says D’Onise. “In 2015, we started to seriously question how we best get oxygen into ferments. It’s well documented that the ingress of oxygen during open tub pumpovers is quite limited. What we decided to do was use oxygen cylinders with regulators and flow meters. And then we use weighted sinters. So, we’ll do an open tub pumpover with an oxygen sinter in it. Over time, we’ve simply trialed which rate of oxygen works best.”

We certainly wait until the ferment is up and running. We want to see quite an active ferment before we start introducing any air.

Matt Godfrey

D’Onise says he was first exposed to the addition of oxygen to ferments at a winery in France where oxygen was dosed into ferments during the first quarter of fermentation. It has been practiced at Pig in the House for seven vintages now.

Initially, oxygen was applied to daily or twice daily pumpovers at rates of between 2L and 10L a minute. Fine-tuning has since seen the rate settle at around 5L a minute.

“In order to look at the exact impact of the oxygen aromatically we did initially trial it in a rack and return so we could have a look at what the racked tank was like before we returned it,” D’Onise explains. “Sometimes you don’t see the difference in the tank because you’re washing the oxygenated wine back over the skins through a pumpover, whereas by using that rack and return, we were isolating that oxygenated wine in another tank, assessing it there and then pumping it back. That way we thought we had a clearer picture of what the effect was with the oxygen; you can actually see an immediate ‘before and after’.”

The technique is applied to all the reds made at Pig in the House, including Merlot, Pinot Noir, Cabernet Sauvignon and Shiraz, the latter making up around 50% of the winery’s output.

Asked why he went down the path of using oxygen as opposed to compressed air or a venturi device, D’Onise says he felt the former delivered more control.

“It was easier for me to calculate the oxygen use with a regulator and an accurate flow meter, as opposed to just an air compressor. I can guarantee what we’re putting in, at what rate; everything’s regulated. With an air compressor, what you receive may be dependent on the pressure that the compressor’s set at,” says D’Onise, adding that the amount of oxygen introduced via venturi valves is also hard to regulate.

“You don’t actually know the amount of oxygen that you’re getting in, you just know that you’re putting some in. From tank to tank, depending on what you do with your valves, some might get too much, others might not get enough. It’s just about knowing exactly what we’re doing. 

“It’s not that expensive to set up either,” D’Onise continues. “For the sake of a $80 sinter, $80 for a hose and a couple of 100 bucks for a regulator — the oxygen cylinder is negligible — you can get set up doing it this way quite cost effectively. We do 300,000 litres of red a year. In the initial year, let’s say we spent $1000 getting set up. Since then, we’ve only had to hire a couple of cylinders of oxygen. It’s quite cheap really.”

Improving wine quality

D’Onise has no doubt that the use of oxygen at Pig in the House has improved the quality of the winery’s reds.

“The main outcome I was expecting was improved aromatics. Being a winery that makes organic wine we’re limited to how much nutrition we can add in the form of DAP and we can’t use copper sulfate. I initially went down this path to look at how I get around that,” he says. 

“We’ve also got a clone of Shiraz, in particular, that is higher in tannin. I wanted to make sure that we softened the wine as well, making them ready for release sooner. I’ve looked at oxygen, but also tannin management during ferment, oak alternatives and the enzymes that we use as well. So I can’t say that the result that we’ve achieved is solely dependent on the oxygen in ferment because it hasn’t been a controlled trial. But we have achieved that one way or another. And I believe that the oxygen has assisted us in that. 

“Our wines are generally far more attractive and far more fruit driven and aromatically superior to what we used to produce and I attribute that to using the oxygen in ferments,” D’Onise continues. “I really do believe that the work that we’ve done with oxygen in our ferments has contributed to the wine show success of our Shirazes. In a line up they look very attractive in their aromatics and they are bright and vibrant. I put that down to the fact that we’ve addressed that reductive issue that Shiraz can have. 

“In a lot of reductively made wines, even if they’re not obviously displaying H₂S, the nose might be a little muted and a bit dull. 


We’ve moved past that. They’re really expressive and they win awards and for a Shiraz — a medium bodied, savoury Shiraz that you get in the Central Ranges of New South Wales — to present in wine shows against other regions that have greater pedigree, I think is a great result. Oxygen has been a part of us getting to that point.”

Across the other side of the country in Margaret River, Devil’s Lair senior winemaker Matt Godfrey can also be heard singing the praises of adding air to red wine. The practice has been used increasingly since he joined the wine company in 2014, particularly on its commercial products, but progressively more on its more premium offerings.

“The addition of air is in its infancy with our premium program,” Godfrey clarifies. “We’re probably focusing more on that commercial space but, from all we see and all we read and everything that we see from other people and what we want to try and create, it’s got a huge place in the future.”

All reds — including Cabernet Sauvignon, Shiraz, Merlot and Malbec — are aerated at Devil’s Lair either via rack and return or injection of compressed air into pumpover lines.

“We have a lot of static fermenters, and with our premium fruit, we would rack and return those anything from two to four or five times in their life. We also

have the ability to inject air through a sinter into our pumpover lines. That happens whenever the pump is in operation,” Godfrey explains.

It’s around day two or three of fermentation when the added air is introduced.

“We certainly wait until the ferment is up and running. We want to see quite an active ferment before we start introducing any air. They get pumped over every four hours and the duration will depend on the ferment and the extraction we’re trying to achieve. But there’s quite a few days there where they’re basically getting aerated every few hours.”

Godfrey says the introduction of oxygen has a two-fold benefit.

“You’re introducing oxygen, which is going to benefit your yeast and produce a healthy yeast culture. And then you’re getting less sulfide production which is producing bright ferments and more of those compounds we really want. Sulfides add to your astringency and the dulling of fruit; everything that we perceive as negative in the style of wine.

“When you add oxygen it changes the redox potential which alters the ability of tannins to polymerise We’re looking for more subtle tannins so we look to push that ability for tannins to polymerise and create those longer chain tannins which are going to make a softer wine that will age just as well,” says Godfrey.

Of course, introducing air to active ferments isn’t for everyone and is very much dependent on style considerations. For Dylan Rhymer, winemaker at Ballandean Estate in Queensland, the amount of oxygen introduced while splashing wine into tubs during pumpover operations has so far been sufficient in achieving the winery’s stylistic goals.

“I haven’t found it necessary. If we were doing young reds, as in going to market in the year of production, then maybe I’d look at micro-oxygenation for softening tannins, but our reds see 12-14 months in oak,” Rhymer remarks.

“We’ve been doing splashing and pumping over since I’ve been here. That was in 2001. We used to do heading down of the cap too when I got here, but I found that just made the ferments more reductive and the extraction of colour, flavor and tannin wasn’t as good as pumping over.

“We aerate all our reds but with Shiraz we’ve explored different levels to try and alleviate H₂S production,” Rhymer continues. “We’ve found with new nutrient products and splashing that H₂S isn’t so much of a problem now. We splash the must into tubs and then pump it back over the top. All our ferments are pumped over three to four times a day. We have one rotary fermenter which is a static fermenter so it turns on the inside. With that we leave the top cover open with mesh to stop bugs getting in.”

Rhymer says they’ve also become accustomed to tolerating a small amount of H₂S.

“We’ve learned not to worry too much about a bit of H₂S. We now know through experience how much H₂S is ok to go to barrel with as the barrel ageing and O₂ transfer that happens with ageing sorts out small amounts of H₂S anyway. If it’s more than we like then a small dose of copper after malo cleans them up and the copper is taken out by the lees resettling before going to barrel.”

The AWRI is in the process of finalising a practice change project on introducing oxygen to ferments which will incorporate a range of extension activities that will draw together the latest research on the practice as well as offer guidance on its application. The project is expected to be released in the lead up to vintage 2022.


This post was originally featured in the December 2021 issue of the Grapegrower & Winemaker.

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