Yeast; The Silent Winemaker

Yeast; The Silent Winemaker

Contributed by Dave March CWM
from wine.co.za

Imagine a midday snack, some cheeses, marmite on fresh bread toast, maybe a croissant and a glass of wine or cold beer. None would be possible without yeast.

“It’s an incredible thing”, says Elda Lerm, International Product Manager of Anchor Yeast in Cape Town. Anchor was the second in the world to produce wine yeasts and the first to create hybrid wine yeasts (more of later).

The world of wine yeasts is a complicated one, but the simple version is that yeasts have transporters in their cell walls that allow for the uptake of sugars, mainly glucose and fructose and release by-products such as CO2and ethanol. This is fermentation and a number of Saccharomyces cerevisiae yeast strains have proved robust enough and have become dominant in winemaking.

Saccharomyces and other yeast genera occur naturally on grapes, probably dozens of strains on the same grape, (and on every other surface nearby) and some wineries rely on this natural population to ferment their wines. The problem is that some yeasts strains might negatively affect fermentation – most notably stopping the ferment prematurely. In addition, many yeast populations contain non-Saccharomyces strains which can also change the outcome. There is little control over how these yeasts behave and little opportunity to ‘tweak’ what you get.

There are very few non-Saccharomyces commercialised strains. Anchor normally does a fermentation (rather than just isolating from the vineyard) and selects a Saccharomyces strain that is able to out compete the other strains in the natural population during fermentation.

Director of Oenology at Anchor, Dr Danie Malherbe, explains that this is not genetic engineering or anything like it, but by providing conditions which force the selected strains to adapt to changing stresses or environments you can encourage ‘adaptive evolution’ and thus create strains which offer certain capabilities.This ‘evolution’ could take years to achieve.

The new strains can be dried, (yeasts that are dried have a better shelf life than liquid forms which have less certainty of viability over time),  packaged and offered to winemakers who may be subject to difficult fermentation conditions or looking for certain characteristics in their wine.For example, take a strain which is unable to cope with higher alcohols; change its environmental pressures, this will change its metabolism which could make it cope with higher alcohol levels. This ‘evolution’ could take years to achieve.

Commercial yeasts offer a range of characteristics, the packaging for Alchemy I reads, ‘Alchemy I mainly enhances fruity and floral esters and to a lesser extent, volatile thiols (passion fruit, grapefruit, gooseberry and guava aromas) in white wines’.

Elda says the process is completely natural – after all, yeasts (and vines) have been adapting to changing environments for centuries – and that only calling a wild yeast fermentation ‘natural’ is incorrect, packaged yeasts are natural as well. She prefers the term ‘commercial yeast’ for her products.

Presently, winemakers are looking for yeasts which cope with climate change, and for lower alcohol levels, for yeasts compatible with organic and biodynamic farming and for pronounced aromatic profiles and mouthfeel. Danie is always looking for yeasts which offer something new to add to the 300-400 commercial yeasts already available, but, “they must bring something unique to the table”.

Every wine farm, every micro-climate, every varietal and every style is considered when selecting a yeast product.  Anchor does not suggest yeasts for certain grape varieties, but yeasts for certain characteristics. “You need to listen to what the winemaker wants, what they are trying to achieve”, says Danie.

Many commercial yeasts are for the most part,in fact, terroir driven (less so for hybrid yeasts) because they originate from natural yeasts drawn from berries within vineyards and climate pockets.  It might be appropriate to select a yeast which enhances the characteristics of the vineyard if seeking to retain terroir effects. Grape composition differs from vineyard to vineyard, formed by climate influences, the soil, stresses such as wind and disease and some winemakers use different yeasts for each vine block – even if they are adjacent and of the same variety.

This is terroir based on the selection of yeasts with characteristics that match, balance or enhance the winemaker’s terroir. Hybrid yeasts are yeasts that have been selectively sporulated and hybridized from either Saccharomyces cerevisiae or non-cerevisiae parent strains (eg Saccharomyces cerevisiae x Saccharomyces cariocanus) to provide a better combined sum of the parts. Says Anchor, ‘the concept behind these unique hybrids is to provide you with all the benefits and complexity of a spontaneous fermentation, without the associated risks’.

Theoretically, one can isolate strains from a winery, grow the biomass, dry the selected yeast population and return them to the winery. This might offer ‘natural’ terroir reflecting yeasts, but with greater reliability and which could be stored for future harvests.

Most commercial yeasts possess the ‘killer factor’ (really a ‘survival factor’) used to nullify any negative yeast strains(and on any killer sensitive strain in the population) occurring naturally on grapes. Some natural isolates also have this characteristic. The introduced yeasts dominate or outlast those indigenous strains.

They can also help restart a stuck fermentation, which in our hot climate is often due to a sugar imbalance. Ideally, yeast uptake glucose:fructose in a 1:1 ratio. However, when there is a glucose:fructose imbalance, usually at the end of fermentation when transport mechanisms start suffering from the high alcohol, the ratio can skew to 1:4 or more and the yeast will then have issues completing the fermentation. Commercial yeasts are more adapt to withstanding the high alcohol and thus their transporters stay active for longer.A sluggish fermentation at the beginning is more easily fixed than later in the fermentation (when you have more challenging conditions: high alcohol, low nitrogen content etc.).

Anchor doesn’t promote a yeast solely suitable for Shiraz, or for Robertson, or for cold fermentation. It is a matter of looking at all the characteristics and content of the berries and juice and matching it to the desired outcome. Some winemakers use a natural ferment for some juice and commercial strains for the rest and blend the two. “Winemakers want a wine that stands out”, says Danie.

Commercial yeasts, such as Anchor’s popular Vin 7 and Vin 13, come from natural isolation  and hybridization respectively, and are used because of the particular by-products (like aroma compounds) they provide and the stresses they can cope with. Two examples, a winemaker with too much acidity in their wine could use a yeast strain which could degrade malic acid levels, or one who is looking for lees enrichment of the wine might select a yeast strain that when autolysed may offer more polysaccharides and yeast extracts like mannoproteins to provide mouthfeel. This could magnify the effect or reduce the time spent on the lees.

Anchors offer yeasts which can ferment at lower temperatures (to preserve aromatics) less foam, more fruity flavours, and even to withhold fermentation during a cold soak (Gaïa™ can delay fermentation for weeks).  Its Alchemy range offers blends of yeasts creating a composite of influences, “why blend wines when you can blend yeasts”, says Danie.

The use of ‘natural’ fermentation is a misnomer, then, as commercial yeasts are natural, just selected for certain qualities.  Danie sums up, “Winemaking starts in the vineyard, you cannot make good wine with bad grapes, but you can have a bad wine by making the wrong choices in the winery. Just like most things in life, there needs to be a balance”.

Whether selecting yeasts for aromatic or flavour profiles rather than correcting imbalances or avoiding problems is not truly reflecting terroir, is a question for the winemaker.
 

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