Little Fermented World

How the wine in your glass became more than just grape juice.

Photo by  Thomas Martinsen  on  Unsplash .

We’ve been hearing more and more in recent years about the health benefits of fermented foods and drinks. Fermenting increases nutrient bioavailability, reduces glycaemic load, improves digestibility, increases shelf-life and stability, produces beneficial bacteria, and leads to changes in organoleptic properties that many of us enjoy. But how many of us really understand the process of fermentation and how it produces the wine that it’s in our glass? 

Despite having a science degree, working as a sommelier, and, of course, owning a copy of The Art of Fermentation, it wasn’t until I started to make wine that I truly got my head around one of the most fundamental and fascinating processes – and the micro-organisms behind it – that creates the wines that we know and love. Here’s a quick snapshot of what you need to know as you consume your living beverage. 

Anaerobic respiration

Before we can truly understand fermentation, we need to first understand what it does and why it’s needed. I’m sure that everyone has heard of respiration. It’s one of the most fundamental and basic functions that our cells perform. It’s what keeps us alive, allowing us to move and grow. When we stop respiring, we die. 

Respiration takes place in our mitochondria, tiny organelles found in each of our cells. It’s primary function is to create energy for all of our other cellular functions, using food and oxygen as inputs. It can be represented with the following equation;


This very simple and fundamental process is the reason why we need to eat and breathe. The anaerobic (oxygen-free) version of respiration is known as fermentation. It can be represented by the formula below;


Not all organisms can perform fermentation in the absence of oxygen, but many can. Some micro-organisms only respire anaerobically, while others, such as humans, can do both. Fermentation doesn’t always produce alcohol though – for example, fermentation is undertaken by the same yeast to produce both wine and leavened bread. In bread, the yeast produces organic acids instead of alcohol, hence the sour flavour that many of us enjoy in sourdough. Similarly with foods like sauerkraut, fermented meats, some cheeses, and non-alcoholic beverages. In humans, the by-product of anaerobic respiration is lactic acid, and it’s the reason why your muscles hurt after a particularly intense workout.

Respiration is the preferred method of energy production in many organisms because it’s more efficient. The 38 in front of ATP in the respiration equation signifies that 38 molecules of ATP are produced from one glucose and six oxygen molecules. Fermentation is a far less efficient method of producing energy. In the equation above,  only 2 molecules of ATP are produced from the same amount of glucose. Given that glucose has historically been scarce, while oxygen has been relatively abundant, organisms that have the means to preferentially use respiration to get energy; fermentation is treated as a back-up plan.

Fermentation in wine

Wine fermentation is undertaken by a specific species of yeast called Saccharomyces cerevisiae. There are many strains within the species, specific to various environments and conditions, which are found on grapes as they approach ripeness in the vineyard. This is why wines can ferment spontaneously. In order for fermentation to take place though, S. cerevisiaeneeds an anaerobic environment, otherwise it won’t be as successful as some other kind of organism, such as mould, that thrives in an aerobic environment. Wines, especially those with the skins and stalks left in, need constant management until the sugars are completely fermented for this reason, often with limited oxygen exposure. 

Vin Jaune ageing in a display barrel. Courtesy of Arnaud 25 via Wikimedia. 

Vin Jaune ageing in a display barrel. Courtesy of Arnaud 25 via Wikimedia. 

The rate of fermentation can be controlled by temperature. Warmer temperatures allow for faster chemical reactions; cooler temperatures slow things down (kind of like us). The heat of the fermentation also controls what’s bioavailable to the yeast and thus what kind of flavours they produce. Cooler ferments tend to produce wines with more aromatics and less phenolic compounds. Warmer ferments tend to extract more phenolics but lose some aromatics. In white wines, I can often pick a hot ferment from banana-like esters as well. There’s a range within which these principles work as well – less than 5 or 6 degrees Celsius and things will stop, higher than around 35 degrees Celsius and the yeast will suffer heat stress. 

There are other types of micro-organisms in wine as well. For example, some wines undergo what we call secondary or malolactic fermentation. This is the conversion of malic acid into lactic acid, done by a species of bacteria called Oenococcus oeniHanseniaspora is the yeast found inthe highest concentrations on grapes when they are picked and can play a role in the early stages of fermentation. And the different strains of S. cerevisiaecan perform different functions. Aside from just converting sugar to energy, alcohol and carbon dioxide, there are strains of yeast that form a protective white coating known as a flor, veil or flower, on wines in barrel that are not completely topped up and hold some oxygen. The flor yeast protects the wine from oxygen spoilage whilst at the same time metabolising the ethanol into acetaldehyde, the aroma and flavour that characterises sherry and white wines produced in the Jura, most notably Vin Jaune (my favourite!). There are other types of microbe, including Lactobacillus, responsible for conditions such as ‘mousiness’ and volatile acidity (VA), Brettanomyces, which some people love and others hate (I can tolerate it in small amounts), and Botrytis cinerea, otherwise known as noble rot, which dehydrates the grapes and concentrates the sugars, creating some of the world’s most famous sweet wines. The presence or absence of all of these can be controlled by manipulating cleanliness, air exposure, temperatures, sulphur additions and other processing techniques such as filtering and fining. 

Wine is generally also stored in containers that limit oxygen exposure to protect it from further fermentation from alcohol into vinegar. However, if you did want to make vinegar, then it would be quite simple – expose your wine to oxygen for long enough and it may become contaminated by Acetobacter, which converts ethanol into acetic acid (vinegar) through respiration. 

Bubbles in wine

Have you ever wondered how bubbles get into sparkling wines? There are two common methods that rely on fermentation, which generally fly under the flags of ancestrale and traditional. It’s pretty simple really – in the ancestrale method, the wine is bottled before it has completely finished the primary part of its fermentation i.e. the conversion of sugar to alcohol. It continues to ferment in bottle but, where the CO2 would normally escape from the fermentation vessel into the surrounding air, it’s instead trapped in the bottle and isn’t released until we open it. The traditional method works on the same principle, except that the wine has finished all of its primary fermentation when it’s bottled. However, an addition of yeast and sugar (known as the dosage)at bottling will kick-start fermentation again and produce the same result. In both cases, there is a bit of a balancing act involved to make sure that you get the bubbles but not too many, which basically means bottling at the right time or controlling the dosage. If you bottle with too much sugar remaining or have too high a proportion of sugar in the dosage, the wine will have too much pressure when opened and will explode out of the bottle when opened. Not enough and the bubbles will barely be detectable.

Some wines get their bubbles from a third method, an artificial carbonation known as the tank method, but I tend to steer clear of these wines. Aside from the fact that the bubble is usually coarser and less delicate, it’s also predominantly used to produce cheap, bulk sparkling wine such as Prosecco. I've never found a natural sparkling wine produced in this way.


There is, of course, much more to fermentation but hopefully you understand the basic principles now, especially as they pertain to wine. It’s both incredible and fascinating to think of all the micro-organisms that have played some role in creating what’s in your glass – a little fermented world. 

Lou ChalmerComment