What actually happens during the fermentation process and how can this affect the flavours in our favourite beverage? Wine writer Caroline Gilby MW explains. Caroline Gilby is a Master of Wine and a scientist by training. She is a wine writer with a passion for the wines of Central and Eastern Europe and contributes to several wine books, magazines and websites.
Life - or at least much of the food and drink that gives us pleasure - would be so much duller without microbes. We are surrounded by these microscopic life forms, some of which play a key role in transforming simple raw ingredients into something with complex flavour. Bread, beer, cheese, cider, coffee, tea, chocolate, yoghurt, soy sauce, olives, vinegar and, of course, wine are just some of the fermented foods people enjoy so much.
In the wine world we talk a lot about the winemakers, the terroir and the grape variety, but without the array of microbes that transform simple grape juice into the delicious liquid we know as wine, none of the rest of the wine story would be possible.
The magic of microbes
I would like to introduce you to, taking centre stage, Saccharomyces cerevisiae, literally translated as 'Sugar Fungus'. It was Louis Pasteur who first connected yeast to fermentation back in 1876 and today we know that the most important species in winemaking is S. cerevisiae. It metabolises (or feeds on) simple sugars like glucose and fructose found in grape juice (and sucrose added to enrich alcohol levels) and produces ethanol and carbon dioxide (CO2 for short) along with a whole host of other by-products that add flavour and aroma to wine. For still wines, it is the alcohol that is important (and actually getting rid of the CO2 safely to avoid its suffocating effects is a challenge in the winery), while for makers of sparkling wine, trapping the CO2 bubbles inside the bottle is a vital part of the process.
'So what is special about S. cerevisiae?'
Alcohol is toxic to all living cells but this yeast can actually survive at much higher alcohol levels than other species of yeast, particular the variants that ferment wine. This is essential where wines like Amarone (for instance Allegrini Corte Giara Amarone) or weighty Australian numbers (such as Rockford Basket Press Shiraz) can reach 15% alcohol and more quite easily. It also survives a good dose of the universal wine 'disinfectant': sulphur dioxide (SO2). This is widely used by winemakers early on to kill off or inhibit more sensitive rogue microbes that could cause spoilage and help leave the way clear for a good strong fermentation by the right yeast.
Often people believe that yeasts come into the winery on the skin of ripe grapes - hidden in the waxy 'bloom' that coats the fruit surface. However, research suggests that S. cerevisiae is pretty rare in the vineyards, if it can be found at all (for instance it is even rarer in wet years), but there are a whole bunch of other yeasts (with nice easy-to-spell names like Pichia, Kluyveromyces, Candida, Kloeckera, Hanseniaspora, Rhodotorula) that do appear on grape skins and almost certainly start the fermentation (unless they have been killed off).
These other yeasts tend to die off at around 3-4% alcohol at which point S. cerevisiae takes over and if all goes well, it finishes the fermentation off fully to end up with a dry wine. But there's still the question of how this yeast appears and it seems it is almost certainly from winery equipment, which is impossible to sterilise, and the ubiquitous 'fruit flies' found in wineries around harvest are great at carrying microbes around the winery.
That's how wine might ferment naturally, but needless to say there is a huge commercial industry based on selecting strains of S. cerevisiae that have useful winemaking characteristics, originally selected from wineries. These are for sale as bags of active dried yeast, which can be added to a vat early on to ensure a good strong fermentation gets under way quickly.
Most winemakers use selected yeast nowadays as they like to sleep at night without worrying about 'stuck' ferments where yeast struggles to finish its job. Winemakers can select strains that are good at releasing flavour from sauvignon blanc for instance, or can tolerate particularly high alcohol levels. There are strains that can help reduce the level of sharp malic acid, or contain 'killer' factors to help outcompete other microorganisms that happen to get into the vat. Sparkling winemakers can choose yeast that flocculate (clump together) well to help them remove the sludge of dead yeast from the bottle more easily, and so on.
The call of the wild
However, there are increasing numbers of winemakers who are returning to tradition and insisting on 'natural', 'wild', or 'spontaneous' fermentations using so-called indigenous yeast. This often (but not always) goes hand-in-hand with organic or biodynamic winemaking. Domaine Guillemot-Michel in Burgundy is one example, 'We limit our interventions in the cellar to those that are strictly necessary. Our musts (grape juice) are left to settle in the cold overnight to separate the clear juice. The alcoholic fermentation then happens naturally thanks to our indigenous yeast.'
Paul Draper at Ridge in California is another who believes in natural fermentations. He has been quoted (in an article for Enology) as saying, 'Originally, we did it because philosophically, it allows us to step out of the way and guide the development of the wine instead of thinking of ourselves as creators. You are allowing the wine in a sense, to make itself.' He also admits though, 'There is no guarantee that you're going to make a better wine - it may be lousy. On the other hand, you may get a really lovely wine from someone who has used an inoculated fermentation, has watched his wines and maybe even sterile-filtered the wine.'
Kevin Judd of Greywacke in New Zealand is a keen proponent of wild yeast fermentation. His Greywacke Wild Sauvignon, chardonnay and pinot noir are made exclusively using indigenous yeasts while all other aromatic whites are made using 50% wild yeasts. While he loves the extra element that he believes wild yeasts bring to a wine, due to their unpredictable nature he says that sometimes the outcome can be a bit disappointing if the wine is not 'funky' enough!
This is recognition that so called wild ferments can be very risky - especially for wines from warmer regions with high alcohol and higher pH (or low acid levels). Typically, the number of yeast cells at the beginning is low, so the fermentation takes longer to get going. This can be good for complexity but is risky too as there is plenty of opportunity for rogue microbes to get going, with production of acetic acid (vinegar), ethyl acetate (unwanted nail polish smell) and other dirty flavours. In addition, a winemaker doesn't know what yeast strain is going to ferment the wine and whether it is actually any good for winemaking. Some yeasts get 'stuck' and die off too early leaving wine with unfermented sugar while others can give bad flavours like hydrogen sulphide (bad egg gas).
Even believers in 'wild ferments' will resort to bags of yeast on occasion. A recent innovation is to try and mimic the complexity of such wild fermentations by using a commercial non-Saccharomyces yeast to start things off (Torulaspora delbruecki is one showing promise) then following up with S. cerevisiae.
Whichever route winemakers choose, we have a lot to thank those microscopic workers for in making our favourite drink. And next time we can take a look at the other group of important microbes in winemaking - the lactic acid bacteria. The microbial mysteries continue…