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Explainer: what is whole bunch fermentation and why are more winemakers embracing it?

March 15, 2023

Everyone seems to be doing it. From the stalwarts of Hunter Valley Shiraz to Great Southern Pinot Noir boundary pushers, winemakers across the country are embracing whole bunch fermentation in ever-increasing numbers.

Why? Why are we seeing many more wines made using whole bunches (aka whole clusters)? And what is this winemaking process anyway?

To help answer, we’ve enlisted the expert help of Dr Wes Pearson – who is not only a scientist at the Australian Wine Research Institute (AWRI) but a winemaker with his own range of McLaren Vale-based wines under the Juxtaposed & Dodgy Brothers labels.

This does get a bit science/winemaking process focussed, but it’s worth a deep dive. This handy AWRI seminar goes even deeper if you want more information too. In the meantime, let’s get started!

Dr Wes in action. Source: Instagram.

What is whole bunch fermentation?

When it’s time for grapes to be picked, wine producers have a few choices. The cheapest/fastest way is to use a mechanical harvester, which basically beats the berries from the vine (it can be quite violent but fast and effective). Then, the berries are transported to the winery, where they are put through a crusher (which crushes berries. Keeps it simple) to produce a slurry of juice and skins called the must.

If you’re making white wine, the must is then pressed (again, in a press. Winemaking is more straightforward than you think), which ekes out the juice and leaves behind the skins. Then the grape juice goes off fermentation (and sometimes the berries skip the crusher and go straight into the press).

It’s a bit different with red winemaking, as the must is transferred to a barrel/tank for fermentation and then pressed after the ferment is finished.

The alternate choices, however, start at the moment of harvest.

Instead of using a machine, producers can pick by hand and leave the whole bunch of grapes intact. Once in the winery, they can then use a destemmer to separate berries from stems, and then send the berries off to the crusher to follow the same process as above (with destemmers and crushers typically a combined machine. You can watch a version in this video).

Winemakers can also choose to keep the bunches completely intact, and put the whole bunch, stems and all, into the fermenter.

Hello, whole bunch fermentation!

Kangaroo Island Tempranillo destined for a Juxtaposed wine (and still whole bunches). Source: Instagram.

The whole bunch challenge

While whole bunch fermentation might seem novel, it is how wines were produced before the mechanical era, with red grapes picked and thrown into a vat for fermentation (and foot treading). Simple! But using whole bunches is not always simple – it influences the aromas and structure of the finished wine, which can be good or bad, with the possibility of herbal aromas/flavours, increased tannins and a change in the colour intensity of the finished wine.

In a worst-case scenario, using whole bunches can mean that you end up with dull, green, herbaceous and rustic-tasting wines.

Those ‘green capsicum’ characters – associated with a group of highly aromatic compounds including 3-isobutyl-2-methoxypyrazine or IBMP – can be the biggest challenge. Numerous studies (like this one from Adelaide University) have shown how disliked ‘green’ aromas and flavours are by consumers – especially in an era where fruity wines are king, and tannins are often downplayed.

As a result, you’ll also mainly see the use of whole bunches confined to Pinot Noir & Shiraz. That’s because many varieties – including Cabernet Franc, Cabernet Sauvignon, Merlot & Sauvignon Blanc – have natural levels of IBMP in the berry, while Shiraz and Pinot, for example, have almost none.

So why use whole bunches?

Studies (like this one using Pinot Noir) have shown that including whole bunches ‘significantly increase tannin and monomeric phenolics’. This is very useful for thin-skinned grape varieties like Pinot Noir or Grenache, which can lack skin tannins. More tannins can notably help improve the mouthfeel and structure, ultimately making for more impressive, longer-lived wines.

Further, whole bunches can introduce more aromatic complexity, with studies showing that Tempranillo wines made using the process have improved aromatic qualities.

That’s certainly what Pearson sees:

‘To me, the biggest positive from adding whole bunches are structure (and) the aromatic profile’ he said.

‘For Pinot and Grenache especially, the additional structure you can get from the stems is the appeal’.

Indeed, it has been estimated that up to 1 in 3 new world Pinot Noir producers are using some whole bunches in their Pinot Noir fermentation. It’s not just Pinot Noir either, with Shiraz, Grenache, Tempranillo, Gamay and a smorgasbord of red varieties now seeing more whole bunches.

As Pearson notes, it’s a relatively recent thing:

‘I think there was a general increase in their use around 10 years ago across the varieties you would normally associate it with like Shiraz, Grenache, Pinot, and it’s probably pretty consistent since then’, he said.

‘I would also say that we (now) see less of the 100% whole bunch wines and more measured use by winemakers as a tool to complement their wines’.

That last bit is important. By using a measured amount of whole bunches, winemakers can avoid the green notes, problematic methoxypyrazines, firm tannins and changes in colour – all of which are highly correlated with whole bunch concentration, especially with more than 20% whole bunches. What counts as ‘too much’ whole bunches, however, is something Pearson sees as a subjective choice:

‘How long is a piece of string?’ he said.

‘To me, the difference would be the wine looking bunchy vs looking stemmy. To me, bunchy is a stylistic choice, so when done well, it just fits in with the rest of the wine holistically’.

He also sees important parallels with another dominant red wine element – oak.

‘Judicious use of oak is a tool a winemaker uses to compliment the wine. When it dominates, it’s never good. Stemmy is kind of the same as too much oak – it means the stem inclusion is out of balance and doesn’t sit in the framework of the wine properly’ he explained.

‘Your wine can look stemmy with 30% inclusion if you add under-ripe bunches and green rachises (stems). It’s not really about a number – it’s more about the resulting sensory effect of their inclusion. The skill is knowing when to add them and how much to achieve the desired effect’.

As the Pinot Noir study we mentioned above pointed out (here it is again), increased concentrations of whole bunches also have other impacts. These might include a higher pH in finished wines (due to the extraction of potassium from stems) and higher levels of the highly desirable antioxidant resveratrol. Moreover, the desirable tannin levels in some wines may not increase unless whole bunches lift above 50%, just to reinforce how delicate the winemakers balancing act can be.

So, do whole bunches have a place beyond Pinot Noir? What about McLaren Vale Grenache & Shiraz?

Pearson certainly thinks whole bunches have a place:

‘(Using whole bunches) is not something I tend to do with my own wines as they are terroir pieces generally, and I try to minimize the winemaking effect, or at least standardise it across my winemaking’.

‘But for me, it’s just another tool to use for the skilled winemaker, and if making delicious wines is your goal, I think (whole bunch fermentation) is a tool that can definitely help you achieve that goal’.