Are neutral yeasts really neutral?

Some winemakers choose neutral yeasts to let terroir speak — but are they truly invisible?

If you’ve landed on this article, it’s probably because you’ve been around for a little while and felt like nerding out on this subject for a bit. That said, I know many of you are also just starting to explore the world of wine, so allow me to give you a very simplified summary of today’s topic: when it comes to fermenting a wine, there are essentially two paths — using indigenous or native yeasts (those naturally present in the environment that surrounds us) or using exogenous yeasts (in other words, selected yeasts).

And this is where the whole debate begins, of course. For purists, there is no other truth than relying on native yeasts if the aim is to faithfully capture the sense of Origin in a wine. I do believe, through spontaneous fermentations, it is possible to achieve greater complexity and a more authentic expression of the vineyard — provided the grapes start from a pristine sanitary condition, winery hygiene is rigorous, and the winemaker carries out extremely close monitoring to minimise any deviations that may occur.

Because, in the end, purity is only purity until it becomes tainted by a fault. No matter how many people choose to bottle it, a fault is still a fault.

So, if wine must is capable of fermenting spontaneously, why do we choose using selected yeasts?

The crux of the matter is that spontaneous fermentation comes with its own set of risks; including fermentations that progress too slowly, stuck fermentations, or even fermentations that fail to complete, leaving some residual sugar behind that can potentially encourage the development of undesirable microorganisms.

Furthermore, depending on which family of native yeasts ends up dominating the fermentation process — something you cannot control when working spontaneously — they may produce elevated levels of volatile acidity, hydrogen sulphide (unpleasant aromas reminiscent of rotten eggs, sulphur springs or blocked drainage), ethyl acetate (usually associated with nuances of glue or nail polish remover), or other compounds responsible for what we consider aromatic faults.

Reduction or Volcanism?

That is why many winemakers prefer to avoid unnecessary headaches by having greater control over the fermentation and achieving more predictable results. This often leads to the development of what are known as house styles, creating wines with an identity that is easily recognisable to the final consumer.

Can I understand it? Absolutely. Do I defend it or prefer it as a consumer? Not quite as much.

That said, I do appreciate that there are some outstanding wines made in this way. At the end of the day, when you are a huge winery producing large volumes of wine every year, you need to make sure you can maintain the viability of the business through consistent production year after year. In that context, there is often no alternative but to intervene and control certain processes like this one.

Likewise, I can understand why a small grower producing tiny quantities may not want to take the risk that a minor fermentation issue could ruin their entire crop. That does not change the fact that, in the latter case, I sometimes find it a shame to see people who work their vineyards brilliantly — and whose grape quality is clearly there — end up with wines that feel a little flat because they have not taken the risk, among other things, of fermenting spontaneously.

In the same way, I miss seeing more large wineries willing to dedicate part of their production to making more exciting wines, taking the gamble of pursuing far greater expression in certain cuvées. But that is a topic for another post, otherwise we’ll end up going off on a tangent.

Amidst all this confusion, it seems that many winemakers have found a safe haven in a practice that helps them sidestep the issue, because increasingly at wine fairs, I find myself having the following conversation:

— Do you kick things off spontaneously?

— No, I use selected yeasts. But I use a neutral strain!

And suddenly everyone seems reassured. Of course, if it’s neutral… then the sense of Origin remains untouched. Doesn’t it?

Before I get myself tangled up in a whole world of trouble, I should say that there are some very talented people working with neutral yeasts and making wines where a sense of purity can genuinely be appreciated.

However, two questions keep coming to mind.

The first one is obvious. Bloody hell, if people are making outstanding wines using neutral exogenous strains, what would happen if they simply allowed indigenous yeasts to do their job?

The second question leads me to wonder whether these yeasts are truly neutral, or whether some winemakers simply resort to that explanation because they feel uncomfortable admitting — in certain circles and at specialised wine fairs — that they select specific yeast strains to achieve a more terpene-driven or thiol-driven profile. Because in many cases, what’s in the glass gives them away. At least, when I taste certain wines, I have serious doubts that neutral strains have actually been used.

And if they really have used this type of neutral yeast, then it brings me to today’s question:

Are neutral yeasts really neutral?

Let’s be clear, an exogenous yeast will always modify the sensory profile of a wine to some extent.

Now, as you already know, I like getting to the bottom of things, so I have done a bit of digging to bring some science into the conversation.

Let’s start from the basis that, when discussing oenological yeasts, we need to explore the sources where these products come from — namely, the companies that supply them such as Lallemand, Laffort or Agrovin among many others. After going through their catalogues and reading various academical studies published on yeast and fermentations, I believe the most accurate way to refer to so-called neutral yeasts is as yeasts with low or moderate sensory impact.

And that is actually how they are marketed to you; I’m not making this up. In other words, there is an impact on the wine’s profile. It may be low, but it is still there.

Because of this, these are the yeasts chosen by producers who, for whatever reason, do not feel comfortable relying one hundred per cent on indigenous yeasts, but still want to preserve as much as possible the varietal character or sense of place in a given wine.

Those yeasts are told to provide the assurance of carrying out highly reliable fermentations, even under extremely demanding conditions such as high alcohol levels, low pH, reduced temperatures, high pressure and limited nutrient availability.

However, neutrality does not exist, because there is no group of Saccharomyces Cerevisiae strains that can be considered objectively neutral.

Calling them neutral simply means that their footprint is smaller, not that it disappears.

The fact that we talk about a ‘neutral’ character is the direct result of decades of research carried out by the companies that provide these oenological products. Essentially, they are the result of genetic selection of yeast strains — in most cases Saccharomyces Cerevisiae — that possess very specific metabolic characteristics.

To understand this, let’s start with the basics. In very simple terms, the equation for alcoholic fermentation can be summarised as follows:

Glucose (C₆H₁₂O₆) → Ethanol (2C₂H₅OH) + Carbon Dioxide (2CO₂) + and because it is an exothermic process, Energy.

However, alcoholic fermentation does not follow such a simple equation where the only transformation taking place is the conversion of fermentable sugars from the must into alcohol, carbon dioxide and energy, alongside a broad range of secondary metabolites that will modify the aromatic and structural profile of the wine.

Rather, it is a complex network of metabolic pathways regulated by the physiological state of the yeasts and by the conditions of the must.

Among the secondary metabolites of greatest oenological relevance are higher alcohols, esters, volatile organic acids and certain sulphur compounds. The extent to which each of these processes takes place depends on the interaction between the genetics of the Saccharomyces Cerevisiae strain and the fermentation conditions — nutrient availability, temperature, dissolved oxygen and fermentation kinetics.

It is precisely this metabolic distribution that explains why some strains are considered aromatic while others are regarded as more neutral.

How?

The main characteristic of low-impact strains is their ability to generate moderate quantities of secondary aromatic metabolites, allowing the aromas derived from the grape variety, the vineyard and winemaking practices to remain at the forefront, without being masked by aromas produced during fermentation.

One of the most influential groups of compounds is esters — those molecules responsible for the fruity and floral aromas we find in wines when young. They are formed through the combination of acids and alcohols, with enzymes such as acetyltransferases playing an important role in the synthesis of acetate esters. During ageing and over time, some of these esters undergo hydrolysis, reducing their concentration, which explains why the fruity notes of a wine gradually lose intensity as it evolves. While aromatic yeasts have a high capacity to synthesise these volatile esters, neutral yeasts produce much more moderate quantities, allowing the focus to remain on the varietal aromas.

Yeasts do not only create new aromas during fermentation; they are also capable of revealing aromatic precursors that are initially odourless. Some strains produce enzymes, such as β-glucosidases or certain lyases, capable of breaking these bonds and releasing terpenes, norisoprenoids or thiols, significantly enhancing the varietal expression of a specific grape variety. Neutral yeasts generally display much more limited activity from these enzymes, meaning they preserve the natural aromatic potential of the must without artificially enhancing specific families of compounds.

Likewise, the so-called Ehrlich pathway — one of the main routes through which yeasts metabolise amino acids such as leucine, valine, isoleucine and phenylalanine — plays a role in the formation of what are known as higher alcohols. When these alcohols are present at moderate concentrations, they contribute to the aromatic complexity of wine, providing delicate floral nuances and subtle spicy notes. However, when certain alcohols, such as amyl alcohol or isoamyl alcohol, are produced in excessive amounts, they can unbalance the wine by introducing a certain solvent-like, glue-like or nail polish remover character.

This is where low-impact strains can help, producing metabolites that are far less dominant from a sensory perspective. This is due, among other factors, to the fact that each strain incorporates and metabolises the amino acids present in the must differently. Differences in their transport systems and in the regulation of their metabolic pathways mean that some strains produce higher levels of higher alcohols than others, even when fermenting the same must.

To this, we must add another key aspect: nitrogen metabolism. As we discussed a month ago, in musts with insufficient levels of assimilable nitrogen, yeasts can become stressed, excreting hydrogen sulphide and other sulphur compounds responsible for highly reductive aromas. For this reason, many neutral strains have been selected for their ability to efficiently utilise available nitrogen and minimise reductive characters.

A different approach

Out of curiosity, and just to bring this to a close, I should mention that there are also companies — such as LEV2050 (no, this is not sponsored) — that work closely with wineries over several years, carrying out annual microvinifications in order to isolate and reproduce those strains of yeast that are in their vineyards to find the ones that best align with the ‘terroir expression’ the winery believes reflects its own Origin.

Are indigenous yeasts still being used? Yes.

Have some terroir factors been completely altered? Also yes.

I think there is something genuinely honest about isolating indigenous yeast strains in order to continue preserving your particular sense of place. However, by reproducing a specific strain from a given vintage and vineyard plot, what you ultimately achieve is the industrialisation of that strain, removing variables such as exposition and vintage from the equation.

I see it as a more appealing alternative than simply buying a yeast from a third party, but it still does not fully convince me.

As always, these articles are simply intended to open up a conversation and continue learning together, so any feedback is always very welcome. I have tried to condense a subject that is far more technical than it may first appear, and one that could easily have taken me down an endless rabbit hole. I hope, at the very least, that it has helped to provide a clearer understanding of the topic and sparked some further discussion.

🍷

Miguel Crunia

Galician Sommelier does not publish advertising or accept paid collaborations of any kind. If you like what I write, share it with anyone who enjoys discovering and experiencing new wines. And if you feel like supporting my work through crowdfunding, you can do so here. You’d be helping me bring to life the book on Galician wines that I’m currently working on 🍷