New World Winemaker Blog

It’s the time of year after the southern hemisphere harvest when the unfortunate with stuck fermentations seek advice. There are various causes of stuck fermentations with “natural” fermentations being one of the more common ones. Working for a commercial wine yeast producer I am often viewed as not being supportive of this practice. This is not entirely true. I have tasted some exquisite wines produced via spontaneous fermentations. The times that I am critical of the practice it is merely because I am familiar with the risks involved with it. At the end of each harvest I have to help various winemakers to re-inoculate their stuck “naturally fermented” wines with commercial yeast. So over time I have come up with a few guidelines as to how to somewhat make your natural ferments more “secure.” Unfortunately success can never be guaranteed.

Karien’s advice on more secure natural ferments:

1. Do not attempt natural fermentation when the initial grape sugar is above 24°Brix. This works for some people – they are the exception to the rule. Most naturally occurring wine yeasts are not very alcohol tolerant.

2. Add some complex yeast nutrients containing inorganic nitrogen (DAP) at the start of fermentation as to increase biomass formation. One of the main differences between inoculating with commercial yeast and letting nature takes it course is the size of the yeast population starting the fermentation. You need a critical mass to finish a fermentation.

3. Do not stress the yeast by fermenting at extreme fermentation temperatures, i.e. below 17°C or above 25°C. Your specific mix might not be cold tolerant or very alcohol tolerant – the higher the fermentation temperature, the higher the alcohol toxicity.

4. Add yeast cell walls to adsorb medium chain fatty acids produced by stressing yeasts, thereby making the environment less toxic.

5. If you ferment only some tanks natural and others using commercial yeasts, make sure that the commercial yeasts you use are very strong/alcohol tolerant fermenters. That way you can add that lees (once it has completed fermentation) to the natural tanks in case you develop sluggish fermentations.  

This advice is not based on my experience of conducting natural ferments, seeing that I have never done any. It is based on my technical knowledge of what yeasts can and cannot do. Use it, don’t use it. Oh, that reminds me, we once did a yeast trial in our lab (at the yeast factory) and the control – with no added yeast – fermented as fast as the experiments. Yeah….so much for that theory…

This is a favourite claim of the so-called “natural” or “non-interventionist” winemaking movement. One of the reasons why they claim it is better to do natural fermentation instead of using commercial yeasts is that commercial yeasts with their “predictable aromatic profiles” can make, say Sauvignon blancs from Europe, taste like New Zealand Sauvignon blancs. Or worse even, it can make Chenin blanc and Ugni blanc taste like Sauvignon blanc. I have personally been in a tasting with a group of oenologists representing most of the wine countries in the world where the Germans presented a Muller-Thurgau. Everyone, including the French (much to their despair) thought it was a Sauvignon blanc. I have also once given a South African Colombard to a group of French winemakers to taste and they also believed it to be a Sauvignon blanc.

So how does this happen and is it “wrong?” The “naturalists” feel it is wrong. Winery sales figures show it is “right.” Consumers like these aromas. Wines around the world can taste similar because we make wine mostly from one specie – Vitis vinifera. Then we also use yeast which originates from mainly one specie – Saccharomyces cerevisiae. Each grape variety is not equipped with a unique set of flavour active compounds. Only the combination is unique. This combination will differ between grape varieties as well as within the same grape variety in different vineyard blocks and vintages. The main aromatic compounds found in grapes are monoterpenes, C13 – norisoprenoid derivatives, pyrazines, thiols and certain amino acids that can be converted to aromatic higher alcohols and esters. The latter two groups are non-aromatic in grapes and converted by the fermenting yeast to a flavour active form. In some cases pyrazines found in Sauvignon blanc are seen as a positive. When present in reds such as Cabernet and Merlot it is seen as a negative.

Thiols smell like guava, passion fruit, grapefruit, black currant and gooseberry. Sauvignon blanc happens to have the highest thiol precursor concentration in the grapes. Wine yeasts convert these precursors to their flavour active forms and differ in their efficacy to do so. Many other white grape varieties contain these thiols but in lower concentrations. So unless you use yeasts that are very good in expressing these aromas and combine it with certain winemaking practices, these aromas will go unnoticed – as they did for many years. However, winemakers around the world are upping their game – competition is tough. They are using modern technologies and as a result they are tapping into these flavour profiles of their grapes that they did not know exist. As a result they can sell their wine in a bottle and not a box. Is this wrong?

Yes yes stone me, I work for a wine ingredients company and I have a commercial interested in winemakers using yeast. However, my clients who’s Chenins, Colombards, Ugni blancs, Muller-Thurgaus and Verdelhos that have “Sauvignon-like” aromas are certainly not complaining about their sales. If you have very good quality grapes then you have many other flavour active compounds that can “sell” your wine for you and you don’t necessarily have to make such an effort to express the thiols. When you have less than top quality grapes, then optimising what you have is a good idea and if that means optimising thiol expression then so be it.

Just for the record – contrary to popular belief I do support natural / un-inoculated fermentations when conditions are right for it. See my earlier blogpost: Natural vs. inoculated fermentations.

I am not a non-interventionist winemaker, simply because I do not trust fate at the wheel of my winemaking career.

We are often warned of the danger of processed food to our health, but what constitutes processed food? The common definition of processed food is any food that is handled through a process. Basically slaughtering and animal to get meat, or picking or handling food constitutes a process that leads to processed food. William S. Burrough’s name of his novel “Naked Lunch” refers to the fact that food is never as naked, as it is at the end of a fork. But the purest would argue that often food is not “raw” anymore, but “dead” by the time it is naked at the end of your fork. Is wine natural by the time it is in your glass? My current winemaking philosophy is that I do not want add anything to wine that will take anything away from it.

Basically I do not want to add fining agents, as far as possible, while at the same time I would consider tannin additions, acid and similar additions that will add something to the wine and make it better, more favourably. Dominique Delteil always warned of the effect yeast has on a wine’s colour, because the proteins in the cell wall would bind tannins that are bound to colour, and thereby reduce he colour of the wine. The real world effect of yeast reducing colour is difficult to fathom, but the possibility and effect exists.

One should therefore in theory either use wild yeasts (very small amount of yeast) or yeast that you know will do the job of fermentation well, so you would not need to re-inoculate and thereby fine some colour out with the new yeast cells. One of my pet peeves is the economical use of the truth by chemical and additive suppliers. Their graphs show the speed and efficiency clearly visible on their graph at presentations. They never ever show the correlation of their product with quality.

Does speed of malolactic fermentation correlate with quality? Was their microbial experiment that they used to get the data from, representative of the whole industry? When I buy malolactic bacteria, imagine my reaction, when together with my expensive packet of bugs, I am given a bag of yeast hulls, to help along the kinetics of the malolactic bacteria, and to help fine the colour out of my wine.

What??

Wine is a complex soup of chemicals, with interactions and kinetics quite impossible for any human brain to fathom exactly. Just because there is potassium in potassium metabisulphate, will it affect the pH of my wine significantly? Will my wine treatment improve the quality of the wine, or will throwing salt over my right shoulder have the same effect? I have said it many times, may we never stop learning, and wondering what is around the next corner.

Louis Nel is the owner and winemaker of Louis wines in South Africa.

Some time ago I posted a comment on a site of a chap who is very pro natural fermentation. My comment was very neutral, in my opinion, but some people perceived it as pro using commercial wine yeasts. The fact that I work for a wine yeast manufacturer was like a red rag to a bull for the “naturalists” since “I have a commercial interest in people not doing natural fermentation.” True, I would like to retire comfortably, but in this case I felt I gave a very good recollection of the facts.

The pro’s and cons of both types of fermentations. You get to choose. You could always try both and see what works the best for you – that is if you still have your job the next vintage after you discovered natural is not such a good option. Natural fermentation means you utilise the yeast naturally occurring on grape skins and on your winery equipment to conduct the fermentation for you. Some people do true natural ferments, i.e. they have never introduced commercial yeasts in their cellars. Others do un-inoculated fermentations, meaning they have used (or still use for some wines) commercial yeasts.

What are the advantages of natural fermentations in all their deviations?

There are approximately 16 yeast species associated with winemaking. The yeast most commonly associated with winemaking and also called the wine yeast is Saccharomyces cerevisiae. The so-called “wild yeasts” or I believe they are called “feral yeasts” in Australia, comprise such yeasts as Kloeckera, Hanseniaspora, Pichia, Candida and Torulaspora, to name a few. When the grape skin is broken during crushing the juice is exposed to the yeasts on the skin and the yeasts will all start growing depending on the size of their populations. The strongest fermenter will eventually dominate due to the selective pressure in winemaking conditions.

The most important selective pressure is alcohol and it is usually the more alcohol tolerant S. cerevisiae that completes the fermentation. However, many yeasts can take part in the fermentation before the first 5% alcohol is formed. Each yeast will leave its unique mark in the wine. Yeasts are known to produce flavour active compounds during fermentation. Yeasts can also modulate grape flavour compounds, converting odourless compounds to very flavour intensive compounds in wine. An example of such aromas is passion fruit, grapefruit and boxwood aromas associated with Sauvignon blanc varietal character. So, the more yeasts taking part in the fermentation, the bigger the possibility of a broader range of aromas.

That is, if you have the good guys. And this is exactly where the risk associated with natural ferments comes in. The chances of you having the optimal mix of yeasts on your grapes year after year are very slim. Some people do, and they make exquisite wines. I have to add you can also only make exquisite wines with exquisite grapes. The chances that you have alcohol tolerant enough yeasts on your grapes year after year are even slimmer. Many natural fermentations lead to stuck fermentations nowadays because we just don’t pick grapes at the sugar levels we used to in the previous century. Blame it on global warming, Robert Parker, the Australian success, whatever. The reality, even Bordeaux is now picking at 24°Brix. Commercial yeasts are mostly yeasts isolated from nature that proved to be alcohol tolerant enough to complete fermentations. The risk of leaving things up to nature / fate is therefore that

1. Your fermentation has a very good chance of getting stuck if you pick your grapes above 23°Brix.
2. The specific aromatic profiles of the yeast mix might not be optimal for your specific style of wine and as a result you bottle un-tapped potential.

The “naturalists” feel you should bottle this untapped potential since it is a true expression of your terroir that year. Try selling a bottle of untapped potential in this economic climate, with the competition in the wine world as is. The consumer wants a wine that tastes nice now. Not a wine that has the potential to maybe taste nice in the future. So the bottom line here is that natural fermentation has the potential to be way superior to inoculating with a single commercial yeast. Some wineries are very lucky to have the ability to utilise natural ferments to help them produce exquisite wines. They are, however, in the minority. The advantages of using commercial yeasts – I can sum this up in three phrases: fermentation reliability, fermentation and style predictability, repeatability. With natural ferments you have none of this.

Oh yes and you keep your reputation and job as a winemaker. The winemaker from Chateaux Margaux summed it up very well when asked earlier this year by a group of wine journalists if he does natural ferments. His answer was “Absolutely not. I inoculated each tank with a commercial wine yeast. With the expectation the world has of Chateaux Margaux, I simply cannot risk a natural fermentation.”

The reality is that the majority of the best wines in the world are made with commercial wine yeasts. Commercial yeasts have been selected specifically because they can withstand the extreme conditions of fermentation, specifically rising alcohol levels. They are also selected based on their ability to optimise wine aroma and flavour. In the case of white winemaking your choice of yeast is critical – especially S. blanc – but this is a topic for another day.

Chances that you have the exact mix of yeasts in you S. blanc vineyard to maximise varietal expression are very slim –probably only slightly better than your chances to win the lotto. Then again if you don’t want to maximise varietal expression and bottle less of a quality wine than you could have, then I guess it is your prerogative. Personally, I would like to make the best wine I possibly could, keeping the process as natural as possible.

That brings us to what is natural and what is unnatural when it comes to winemaking? Well, that is also a whole new discussion. Using sulphuric acid to adjust your wine’s pH, is for instance “un-natural” in my opinion. I would stress about that if you feel the need to stress about the naturalness of wine. Adding commercial yeasts that originated from grapes in the first place, is not unnatural. The potential shortcoming of using only one commercial yeast to conduct the fermentation is that the wine can be less complex than a successful natural fermentation where many yeasts made a contribution to wine aroma and flavour. However, using the right commercial yeast by far outperforms “less optimal” natural ferments.

The good news is that researchers are currently tapping into the advantages of natural fermentations. A vast amount of research is being done by various institutes around the world on non-Saccharomyces yeasts. Two companies have just commercialised Torulaspora delbrueckii. By inoculating your grape juice with Torulaspora and a day later with S. cerevisiae you obtain enhanced complexity in both mouthfeel and wine aroma – so they claim. Two companies have commercialised inter-specie hybrids, meaning you stay with the genus Saccharomyces but the species can be hybrids between cerevisiae and paradoxus, bayanus, cariocanus etc. These hybrids also introduce new possibilities into the wine aroma pool that in the past were only obtained with successful natural fermentations.

The commercialisation of wild yeasts / interspecie hybrids provides winemakers with a tool to safely reap the benefits of a natural fermentation without the risks, i.e. offer them reliability, predictability and repeatability.