These near invisible little living wonders are responsible for all life above water. They allow us to thrive on this planet; they are the engines of life that feed the mechanisms which permit us all to live. Microbes have always been undervalued in our society, and in the realm of consumer level wine it is much the same. We never mention them when we discuss terroir or see them detailed on the back of a label, even though they can often make more of a difference to a wine than place or climate.
In trying to explain why two wines from the same variety but from adjacent vineyard plots can taste so different, wine experts nearly always seize upon geology. This has been the case in well-established wine regions all over the world, yet we rarely turn our thoughts to the different microbial communities that populate one square metre of soil compared to the next. Research on forests displays clear evidence of microbial microclimates. The findings do indicate a link to geology, in that certain types of microbes prefer some geological formations over others – the smoothness of the material is often a deciding factor. In general, microbes aren’t fond of smooth surfaces as they struggle to stick. This forest-related research can be extrapolated to vineyards and wineries, but to understand its implications we’ll need to better understand the effect of microbes in wine production. Where better to begin than with the star of the show, Saccharomyces cerevisiae, also known as brewer’s or baker’s yeast.
Saccharomyces cerevisiae
We’ve marvelled at the properties of yeast since the early eighteenth century. It made an appearance in Samuel Johnson’s 1755 Dictionary, where it was described as ‘the ferment put into drink to make it work, and into bread, to lighten and swell it’. Back then we perceived yeast to act more like a chemical catalyser than a living biological entity. It was Louis Pasteur who triumphed in the discovery of fermentation as a biological process rather than a chemical or a mechanical one. Pasteur’s work was later developed by German chemist Eduard Buchner, who received the 1907 Nobel Prize in Chemistry for working out that it was the bits inside of yeast that did all of the fermenting, and not necessarily the living yeast that was needed for fermentation to occur. He discovered this by simply removing the outer membranes of Saccharomyces cerevisiae and using just their innards; surprisingly the yeast insides continued to cause fermentation.
It’s crazy to think that most of the smells and tastes we get from wine are produced by Saccharomyces
The name Saccharomyces derives from the Latinised Greek terms for sugar and fungus, saccharon and myces, while cerevisiae is a Latin-derived species epithet meaning ‘of beer’. As well as beer, this aptly named little organism is responsible for the production of bread and wine. The yeast has a crafty mechanism for defending itself and its sugar resources from nearby competitors: it secretes a toxic and psychoactive alcohol. It’s an evolutionary trait which it’s had to develop a resistance to in order not to poison itself in the process.
In a sugar-rich environment, Saccharomyces will continue to consume this source of energy and create ethanol alcohol as a by-product to keep competitors at bay. When its environmental sugar levels eventually deplete these defences are no longer needed. In environments where there is access to oxygen, Saccharomyces will start to metabolise the ethanol it created earlier, receiving additional energy whilst metabolising its defence system. These are highly adaptive little yeasts, and adaptation has made them very successful.
Many other microbes have built up these sorts of microbial defences. A well-known one is the Penicillium mould which secretes an antibiotic that breaks down the cell walls of nearby bacteria. We have harnessed its antibiotic properties for our own purposes in the form of penicillin. Acetic acid bacteria also have a clever mechanism: these acid-tolerant bacteria secrete acetic acid (vinegar to you and me) to create a harshly acidic surrounding environment for their neighbours. The microbial world is a competitive one, and use of chemical warfare assists in gaining the upper hand.
Our main player, Saccharomyces cerevisiae, functions differently when working aerobically (with free oxygen present) than it does when working anaerobically (without free oxygen). Traditional wine fermentation is considered to be a near anaerobic process since the yeast will sink below the liquid when wetted with the grape must, and therefore won’t have much free oxygen available.
We can see that the link between terroir and wine characteristics grows weaker as our technology advances
The nutrient content of the grapes we grow influences the nutrient content of the must, which in turn influences the performance of the yeast. It’s crazy to think that most of the smells and tastes we get from wine are produced by Saccharomyces. Yet its ability to metabolise and create the flavours and aromas we prize so much is heavily swayed by its nutritional state during fermentation. Nitrogen is key to maintaining healthy yeast nutrition; we need it in a form that the yeast can utilise, called yeast assimilable nitrogen (YAN). Vitamins like thiamine and minerals such as magnesium and zinc are also essential for yeast activity. If soils prove to have poor fertility and are nitrogen-poor, this can cause issues with fermentations getting stuck or not even starting, or the yeasts creating bad aromas and tastes. YAN is consumed by Saccharomyces as it enzymatically breaks down sugars, which means that the more available sugar there is in the must, the more YAN we need. Broadly speaking, to ferment 1 gram per litre of sugar, Saccharomyces will need 1 milligram per litre of YAN. Think of YAN as the oil which keeps the enzymatic engines within yeasts turning smoothly.
Very often in conventional farming, nature’s in-built nitrogen-supplying sources are destroyed by the addition of fungicides, so nitrogen has to be applied to the soil in the form of synthetic fertilizers. In addition, wineries will tend to add YAN during fermentation to help the yeast along. We can see that the link between terroir and wine characteristics grows weaker as our technology advances. Before we had the ability to add nitrogen to soils or YAN to fermentation, soils would have had to generate enough nitrogen on their own or we’d help them along a little with organic manure of some sort. In the isolated case of nitrogen, there is a clear and measurable link between soil, wine quality and final wine taste due to the working conditions of our yeasts. However, with the addition of nitrogen at both the growing and the winemaking stage, we sever this connection with the land but ensure that vines and yeasts get what they need to perform at their best.
