The most mysterious aspect of wine is the endless variety of flavours that stem from a complex system of interactions among many hundreds of compounds. In its widest sense, wine flavour refers to the overall impression of both aroma and taste components. Aroma is usually associated with odorous, volatile compounds from the grape varietals. The bouquet of wine refers to the more complex flavour compounds which evolve as a result of fermentation, and ageing. With the exception of terpenes in the aromatic grape varieties and alkoxypyrazines or pyrazines in the herbaceous cultivars, perceived flavour is the result of many of these interactive compounds, rather than being attributable to a single “impact” compound.
This lecture will focus mainly on the contribution of yeast fermentation to the sensorial quality of the final product. Yeast and fermentation conditions are claimed to be the most important factors influencing the flavours in wine. Both spontaneous and inoculated wine fermentations are affected by the diversity of yeasts associated with the vineyard and winery. During the primary alcoholic fermentation of sugar, the wine yeast, Saccharomyces cerevisiae and is some cases Saccharomyces bayanus, together with other indigenous non-Saccharomyces species, produce ethanol, carbon dioxide and a number of by-products. Of these yeast-derived metabolites, the alcohols, acetates and esters are found in the highest concentration in wine. While the volatile metabolites contribute to the fermentation bouquet common to all young wines, the production levels of these by-products are variable and yeast strain specific.
There is a hidden wealth of indigenous yeast species present on grapes. This, combined with the selection of cultured yeast strains can add improved flavour profiles in your wines. In the future, some winemakers may prefer to use mixtures of indigenous yeast species and tailored S. cerevisiae strains as starter cultures to reflect the biodiversity and stylistic distinctiveness of a given region. This may help winemakers to fulfil their demand for individual wines with intact local character and to ensure the survival of wine’s most enthralling aspect – its endless variety. However, without knowing the characteristics of the indigenous wild yeasts there is the potential to end up with a product that may not meet your expectations.
Esters are a group of volatile compounds that impart a mostly pleasant smell in wine. Most esters found in alcoholic beverages are produced by yeasts during fermentation as secondary products of sugar metabolism and constitute one of the largest and most important groups of compounds affecting flavour. However, a particular aroma property can only rarely be associated with a specific ester. The concentration of esters usually found in wine is generally well above their sensory threshold levels and they make up numerically the largest group of aroma compounds in alcoholic beverages.
Therefore it is not surprising that some of the descriptors used in sensory evaluation of wine and alcoholic beverages coincide with the associated aroma of these compounds. The fresh, fruity aroma of young wines results in large part from the presence of the mixture of esters produced during fermentation, which is why it is usually called fermentation aroma/bouquet. Fermentation compounds, especially the acetate esters, are responsible for the desirably fruity, ester-like character of young wines from neutral cultivars such as Chenin Blanc, Palamino, and Colombard.
Yeast and Wine Aroma
|Ethyl acetate||Varnish, nail polish, fruity|
|Isoamyl acetate||Banana, pear|
|2-Phenethyl acetate||Rose, honey, fruity, flowery|
|Ethyl isovalerate||Apple, fruity|
|Ethyl butanoate||Floral, fruity|
|Ethyl 2-methyl-butanoate||Strawberry, pineapple|
|Ethyl hexanoate||Apple, banana, violets|
|Ethyl octanoate||Pineapple, pear|
Re-hydrating freeze dried yeast
Pure culture yeasts that are grown in a lab are often freeze dried and packaged for commercial use. Prior to their addition into must, these yeasts need to be re-hydrated in “starter cultures” that must be carefully monitored (particularly in regards to temperature) to insure that the yeast cells are not killed off by cold shock. Ideally winemakers want to add enough inoculum to have a viable cell population density of 5 million cells per milliliter. The exact amount of freeze-dried culture varies by manufacturer and strain of yeast but it is often around 1 gram per gallon (or 25 grams per 100 liters). Wines that could have potentially problematic fermentation (such as high sugar level late harvest or botryized wines) may have more yeast added.
Similarly, re-hydration procedures will also vary depending on the manufacturer and wine maker. Yeast is often inoculated in a volume of water or grape must that is 5–10 times the weight of the dry yeast. This liquid is often brought to temperature of 40°C (104°F) prior to the introduction of the yeast (though some yeast strains may need temperatures below 38°C (100°F) to allow the cells to disperse easily rather than clump and sink to the bottom of the container. The heat activation also allows the cells to quickly re-establish their membrane barrier before soluble cytoplasmic components escape the cell. Re-hydration at lower temperatures can greatly reduce the viability of the yeast with up to 60% cell death if the yeast is re-hydrated at 15°C (60°F). The culture is then stirred and aerated to incorporate oxygen into the culture which the yeast uses in the synthesis of needed survival factors.
The temperature of the starter culture is then slowly reduced, often by the gradual addition of must to get within 5–10°C (41–50°F) of the must that the culture will be added to. This is done to avoid the sudden cold shock that the yeast cells may experience if the starter culture was added directly to the must itself which can kill up 60% of the culture. Additionally, surviving cells exposed to cold shock tend to see an increase in hydrogen sulfide production.
Nutritional needs of wine yeast
In order to successfully complete a fermentation with minimum to no negative attributes being added to the wine, yeast needs to have the full assortment of its nutritional needs met. These include not only an available energy source (carbon in the form of sugars such as glucose) and yeast assimilable nitrogen (ammonia and amino acids or YAN) but also minerals (such as magnesium) and vitamins (such as thiamin and riboflavin) that serve as important growth and survival factors. Nutrients that are added to the must to aid fermentation include Go-Ferm, DAP, Fermaid K and Superfood to name a few of the more common nutrient additives.
Wine faults related to yeast
Either directly or indirectly, wine yeast can be a culprit behind a wide variety of wine faults. These can include the presence of “off flavors” and aromas that can be the by-product of some “wild yeast” fermentation such as those by species within the genera of Kloeckera and Candida. Even the common wine yeast Saccharomyces cerevisiae can be behind some wine faults with some strains of the yeast known to produce higher than ideal levels of acetic acid, acetaldehyde and volatile sulfur compounds such as Hydrogen Sulphide. Also any yeast can have a low tolerance to nutritional deficiencies, temperature fluctuation or extremes and excessive or low sugar levels that may lead to a stuck fermentation.
In the presence of oxygen several species of Candida and Pichia can create a film surface on top of the wine in the tank of barrel. Allowed to go unchecked, these yeasts can rapidly deplete the available free sulfur compounds that keeps a wine protected from oxidation and other microbial attack. The presence of these yeasts is often identified by elevated levels of volatile acidity, particularly acetic acid. Some strains of Pichia will metabolize acetic acid (as well as ethyl acetate and isoamyl acetate that may also be produced) with the side-effect of substantially decreasing the titratable acidity and shifting the pH of wine upwards to levels that make the wine prone to attack by other spoilage microbes. Commonly called “film yeast”, these yeasts are distinguished from the flor sherry yeast that are usually welcomed by winemakers in producing the delicate fino-style Sherry wines.
Growth of many unfavorable wild yeasts is generally slowed at lower cellar temperatures, so many winemakers who wish to inhibit the activities of these yeasts before the more favorable Saccharomyces yeast kick in, will often chill their must, such as the practice of “cold soaking” the must during a pre-fermentation maceration at temperatures between 4–15°C (39–50°F).
Though some species of yeast, such as Brettanomyces, will not be inhibited and may even thrive during an extended period of cold soaking. While some wine regions view the influence of Brettanomyces in the wine, in limited amounts, as added complexity, it can also be viewed as a negative influence that needs to be controlled. The wine yeast Brettanomyces (or “Brett”) produces very distinctive aroma compounds, that can have a wine being described as smelling like a “barnyard”, “wet saddle” or “band-aid”. To some winemakers and with some wine styles (such as Pinot noir from Burgundy), a limited amount of these compounds could be considered a positive attribute that adds to the complexity of wine. To other winemakers and with other wine styles (such as Riesling from the Mosel), the presence of any Brett will be considered a fault. Fruit flies are a common vector in the transfer of Brettanomyces.
As a fermentation yeast, Brettanomyces can usually ferment a wine up to 10-11% alcohol levels before they die out. Sometimes Brettanomyces already present in a wine that has been inoculated with Saccharomyces cerevisiae will out compete the Saccharomyces strain for nutrients and even inhibit it due to the high levels of acetic acid, decanoic acid and octanoic acid that many strains of Brettanomyces can produce.
The primary role of yeast is to convert the sugars in the grapes into alcohol. The most common winemaking yeasts are different strains of Saccharomyces cerevisiae, although strains of Saccharomyces bayanus are also used under certain circumstances. There have been several hundred different strains of Saccharomyces cerevisiae identified, however not all of them are suitable for winemaking. Some distinct difference among various strains include, suitable fermentation temperature ranges, fermentation speed, competitive factor with other yeasts, alcohol tolerance, relative nutrient requirements, and added sensory and flavour effects. The selection of specific yeasts strains is a choice of the wine maker and it can significantly influence the structure, aroma and flavour profile of a finished wine.
Most yeasts come with some limitations. These limitations can be temperature, nutrient requirements, alcohol tolerance, sugar tolerance, and competitive factors, to name a few. When selecting a yeast that may be sensitive to a specific fermentation condition that is expected, it is sometimes recommended to finish off the fermentation with a very hardy yeast such as EC1118 to avoid stuck or incomplete fermentations. If desired, the EC1118 should be inoculated before the fermentation is completed at around a specific gravity of 1.010.
This is a practical exercise to illustrate how yeast influences aroma and flavour in wine. These wines were all made from exactly the same grapes but different strains of yeast were used. 2 of the yeasts selected were yeasts recommended for red wine, 1 was a neutral wine yeast, and 2 were recommended for this varietal of white wine.
Taste and evaluate the wines, paying particular attention to the differences in them. Label your glasses (A, B, C, D, and E).
Contributed by by Rick Homer