Wine is grape juice that has been fermented; it is essentially the result of a metabolic by-product of the one-celled organism, yeast, which converts its favourite food, sugar, into ethyl alcohol, carbon dioxide and heat. Grape juice at 16°-26°C represents a sugar and nutrient-rich medium which is ideal for flourishing reproduction of the many species of yeast. Yeasts have limited generation times and are fiercely competitive. If conditions suit one yeast strain better than another, it will quickly establish itself as the primary fermenting organism. Whether a spontaneous fermentation with indigenous yeast or an inoculated fermentation with cultured yeast, it is nearly always a strain of the alcohol and SO2 tolerant Saccharomyces species (usually cerevisae or bayanus) that successfully completes the fermentation. As grape juice metamorphoses into wine, yeast shapes the aroma, flavour, mouthfeel and alcohol content of wine. Spontaneous fermentation with indigenous yeasts has been the traditional way to make wine for thousands of years. Deliberate inoculation with cultured yeast is a fairly new winemaking technique. Some winemakers argue that authentic expression of terroir and vintage can only be achieved through fermentation with indigenous yeasts. Other winemakers prefer the greater security and controlled variability of specialized strains. There are in fact, advantages and disadvantages for both indigenous and cultured yeasts and these shall be examined in this essay.
There are numerous indigenous yeast species found in the vineyard. In a study of spontaneous at the Geisenheim Research Centre, there were nearly10,000 yeast species identified and isolated in the 2009 vintage – Saccharomyces cerevisiae made up less than 1% of the population on the grapes in the research vineyard. The surface of grapes, particularly at the place where the grape berries are attached to the stem, is typically dominated by the Hanseniaspora and Kloeckera species. These species initiate the fermentation and die off once low alcohol levels of around 5% by volume are achieved and others will take over. Twenty or more different yeasts may participate in a fermentation. The source of Saccharomyces is predominantly winery machinery and equipment surfaces and it is this yeast genus that will take over a successful fermentation. One of the major advantages of fermentation with indigenous yeast lies in the timing and duration of fermentation. A delay in the onset of vigorous fermentation allows oxygen to react with anthocyanins and other phenols present in the must to enhance colour stability and accelerate phenolic polymerization which enhances texture and mouthfeel. It has been demonstrated that in long slow fermentations fewer aromatics may be burnt off and in addition small amounts of sulphide compounds may be produced that make a wine interesting without full hydrogen sulphide stink. Producers of “natural” wines and/or organic wines are often proponents of non-intervention in their philosophy for expression of terroir. The philosophy of a wine with no additives or processing agents is an attractive one for many consumers and phrases like “spontaneous fermentation” or “wild” or “natural” yeasts have become positive slogans in sales communications.
Yeast populations vary from vineyard to vineyard, vintage to vintage, and cellar to cellar and determining which yeasts are present is difficult because this is in constant flux. The main risk of indigenous yeast lies in delayed onset of fermentation and limited fermentation capacities. Deficient yeast strains can expose the must to the danger of bacterial contamination, oxidation and the formation of off-flavours and/or undesired components like acetaldehyde, biogenic amines, acetic acid and excessive volatile sulphur compounds. Risks connected with inefficient fermentations can be mitigated by ensuring that juice and fermenting must provide an environment that is more advantageous for the type of yeast the winemaker wants to promote. Superb cellar hygiene is not the only means of control; indigenous yeasts are also less tolerant of SO2 and warmer fermentation temperatures. Since converting to certified biodynamic viticulture, Kurt Feiler of the Feiler-Artinger Estate in Burgenland, Austria also now abstains from use of pure cultivated yeast. He says, “I use compost and return grape pomace and stems to the vineyard. I believe this has helped in establishing a flourishing and dependable yeast flora. It also promotes nitrogen and provides grapes with the necessary nourishment for fermentation. Lag times in vinification onset can be minimized by the use of a pied de cuve. You’ve got to be patient and willing to accept the idiosyncrasies of the vintage. My Pinot Blanc fermented only to halbtrocken in 2008.” The Feiler Artinger Estate is small, producing only 160,000 bottles divided between about 25 different wines each year. None of his fermentations occur in huge vessels. If one fermentation is not successful, the potential financial implications will not be disastrous. The financial risk of spontaneous fermentation for large volume vessels is potentially staggering, whether of an entry-level or premium quality wine.
One significant advantage of cultured yeast is its ability to quickly establish dominance in the yeast population in early stages of fermentation. Inoculation with one of the many Saccharomyces species ensures an efficient and dependable fermentation. By limiting the danger of a stuck fermentation, microbial stability is encouraged and the development of sulphur compounds (H2S) is hindered. There are numerous species of Saccharomyces and they can be chosen to enhance specific wine styles. The current trend of longer hang times to achieve higher physiological ripeness in tannic red wines is resulting in problematic musts with high potential alcohol and high pH. High pH diminishes the effectiveness of SO2 and even some Saccharomyces species will die off at high alcohol levels before the wine is rendered dry. Commercial yeast strains such as Lalvin Clos have been cultivated specifically for efficient and complete fermentation in a high alcohol, high pH, and low-nutrient milieu and are adaptable to different fermentation temperatures and help promote MLF. At the Shaw and Smith winery in the Adelaide Hills, Michael Hall Smith MW has been able to demonstrate how different cultured yeast strains consistently promoted unique sensory profiles in Chardonnay independent of vintage and has chosen different yeast strains for different vineyard plots that enhance vineyard character and minerality. In an effort to replicate the aroma complexity derived from the diversity of indigenous yeasts in a spontaneous fermentation, the AWRI has developed two pre-mixed blends of compatible non-Saccharomyces und S. cerevisae strains that display strong performance and aroma enhancement. Production of traditional bottle-fermented sparkling wine requires the addition of cultured yeast and they are chosen for their ability to ferment dry at high alcohol levels and for their flocculation tendencies which aid degorgement. Makers of botrytized sweet wines will also often chose cultured yeasts that are resistant to the natural antibiotic properties of noble rot. There are currently two authorized gene manipulated wine yeasts permitted for use in North America. The ML01 initiates MLF simultaneously with alcoholic fermentation thus excluding the production of bioamines by lactic bacteria – potentially a welcome development for wine lovers with allergies and asthma.
With the higher dependability and predictability of cultured yeast, their disadvantage is largely philosophical and consumer perception. Unlike indigenous yeast, cultured yeast must be purchased, but their cost when weighed against the potential financial impact of a fermentation gone wary is minimal. Many different yeast strains and combinations of yeast strains are available on the market and they are not all of equal quality and they must be stored and re-hydrated and properly dosed for their successful use. Unlike opponents of all wine additives and processing agents, a winemaker using cultivated yeasts will not likely have a philosophical argument against adjustment of pH or additions of a yeast nutrient like Fermaid® or commercial yeast protectors. A listing of these additives and certainly the mention of the use of a gene manipulated yeast on the label will not be viewed as positively by most consumers as “use of native, natural yeast only”. Laws to protect consumers and their rights to know the contents of a food product and processing agents used to produce it will require more specific labelling in the future.
The choice between the use of indigenous and cultured yeast is largely a philosophical discussion. On one extreme, there is a movement towards more “natural” wines and wines that express an authentic and unadulterated expression of vintage and terroir – wines sourced from organic fruit with no additives or processing agents and even minimal or no sulphur dioxide. On the other side we have cultured yeast technologies that are constantly being perfected to fit specific musts and wine styles, including even gene manipulated yeasts with potential human health benefits. Beyond the philosophical discussion, the choice of indigenous yeast or cultured yeast will also depend on the style of wine that is to be made and the volume in which it is produced. Stricter labelling regulations in the future will shed more positive light on wines with less additives and processing agents unless the consumer can be convinced that these additions are good for him. Whichever option a vintner chooses, whether a fermentation is successful in achieving the desired wine style and quality goal will be dependent on the winemaker ensuring that juice and fermenting must provide an environment that is more advantageous for the type of yeast the winemaker wants to promote.