Alcoholic Fermentation


# **Alcoholic fermentation** Quite a few wrote that you enjoyed the previous blog about **[Fining and Other Wine Adjustments](**. We think that is a fascinating topic as such adjustments, done quietly in the background by skilful winemakers, often have a material impact on how the wine turns out. Yet this is rarely spoken about and the marketing departments of wineries and wine merchants are instead singing the praises of the mystical terroir of the wine estates. To us, that is a bit of a magician's misdirection shifting the focus away from what is really happening. Why is this done? Presumably because rare and poetic properties of terroir sell better than human intervention by winemakers. Today, we'll continue with another winemaking topic, **Alcoholic Fermentation**, a process that is absolutely central to winemaking. Many other steps represent stylistic choices made by the winemaker. In comparison, alcoholic fermentation is a required core process without which there simply won't be any wine made. **Alcoholic Fermentation** Alcoholic fermentation is the process whereby yeast converts grape sugars into ethanol while heat is released. The primary wine yeast is **Saccharomyces cerevisiae**. At the end of alcoholic fermentation, 95% of the carbon atoms in the grape sugars end up as either ethanol or carbon dioxide while heat is released. 1% of the carbon atoms are incorporated into the formation of new yeast cells as yeast multiply during fermentation. And the remaining 4% of carbon atoms end up as various other products including acetaldehyde, glycerol, lactate, acetate and pyruvate.  **BRIX** BRIX is a term used for the percentage sugar by weight in grapes. Among fruit, grapes are unique in having very high sugar content at harvest - often 24-28% sugar. In comparison, ripe apples and tomatoes typically only have 5-8% sugar by weight when they are harvested. This means that the sugar content in grapes at harvest is 4-5 times higher than in apples and tomatoes. And given that grape sugars are the principal food for the wine yeast Saccharomyces cerevisiae, this goes a long way towards explaining why grapes are suitable for wine production given that a high sugar content is a requirement to reach a sufficiently high alcohol level. A conversion factor of 0.60-0.62 for white wines, and 0.57-0.60 for red wines, can be used to estimate the final Alcohol by Volume (ABV) of the wine after fermentation to dryness (i.e. until the yeast has consumed all of the sugars). Thus, if the BRIX at harvest is 25, and you are making red wines, the estimated ethanol content will range from 0.57 x 25 to 0.60 x 25 or 14.25% to 15.0% alcohol in the finished wine. This helps to explain why wines from warmer climates (e.g. Napa Valley and Barossa Valley) often have higher alcohol content compared to wines from cooler wine regions such as France and Germany. The greater sun exposure, and often longer growing season, leads to a greater concentration of sugar in the grapes and this gets converted to ethanol during fermentation. **Saccharomyces Cerevisiae** The hero of alcoholic fermentation is the wine yeast Saccharomyces cerevisiae who does the job of converting the grape sugars to ethanol. What is less commonly spoken about is that alcoholic fermentation is essentially mass murder. The wine yeast initially multiplies rapidly as it feasts on the available grape sugars. However, the saying that none of us are getting out of here alive very much applies also to wine yeast. Towards the end of the alcoholic fermentation, allremaining wine yeast dies from a combination of starvation (as all sugar has been consumed) and ethanol poisoning as ethanol is toxic to wine yeast at higher concentrations. So our friend, Saccharomyces cerevisiae, gets to work, has a short-lived blast while sugars are widely available and the ethanol content is low, and then succumbs in the end leaving dead yeast cells at the bottom of the fermentation tanks or barrels. This deposit of dead yeast cells is known as "lees". Wine may be left to age for a while on the lees, known in French as "sur lies" or "on the lees". In addition, the wine may be stirred to release flavor and aromas compounds from the lees into the wine itself. In French, this is known as "bâttonage". In the end, the wine is "racked" off the lees meaning that it is transferred into a new vessel leaving the lees behind. **Yeast Inoculation** There are three main sources for wine yeast in a winery: 1. Yeast naturally available on the grapes in the vineyard 2. Yeast surviving on winery equipment 3. Commercial yeasts purchased to inoculate the must
If winemakers are relying only on 1 and 2 above, it is called a spontaneous fermentation or sometimes uninoculated fermentation. Some consider this to be more "natural" but it comes at the risk of problem fermentations if yeast is not available in sufficient quantity or if the right strain is not present. When a winemaker purchases a commercial yeast strain and uses it to start fermentation, it is referred to as yeast inoculation. This is considered to lead to more predictable alcoholic fermentations.   **Reading Yeast Instructions** The choice of wine yeast is essential as different yeasts bring different properties to wine, have different tolerance levels to ethanol and temperature and vary in their nitrogen nutrient requirements. Let's use a couple of examples to illustrate this by picking wine yeasts from Laffort. Let's begin with **[ZYMAFLORE® FX10](**, a yeast suitable for red wine making from the grape varietals Cabernet Sauvignon and Merlot.   fx10 We learn from the above that ZYMAFLORE® FX10 can handle high fermentation temperatures of 20-35 °C, that the yeast can tolerate alcohol levels of up to 16% and that the nitrogen requirements are low. Moreover, the yeast facilitates release of polysaccharides which, as discussed in the blog post about **[Fining and Wine Adjustments](** contribute to mouthfeel and body for wine. The resulting wine expresses the terroir as little aromas are produced during fermentation and the tannins combine with polysaccharides to yield silky, smooth tannins. It is also a yeast that promotes color intensity. Overall, this is a yeast strain for age-worthy, premium wines such as Cabernet Sauvignon and Merlot wines from Bordeaux and Napa Valley. Now, let's contrast this with a yeast for making white wine such as **[ZYMAFLORE® VL3](**. vl3 Here we find almost the opposite characteristics in that ZYMAFLORE® VL3 requires a relatively cool fermentation temperature of 15-21 °C, the yeast requires a lot of nitrogen nutrients and can only tolerate alcohol up to max 14.5%. This is a specialist yeast intended primarily for fermentation of Sauvignon Blanc and other varietals high in thiols such as Colombard and Petit Maseng. Thiols contain sulfur and generate the bittersweet aromas found in grapefruit, passionfruit and cat's pee. And at really high level, gives wine a garlic smell and is then considered to be a wine fault. **Fermentation Vessel** Many different fermentation vessels can be used including stainless steel, cement, plastic and oak barrels. When oak barrels are used, it is known as "barrel fermentation" and research has shown that wine typically gets more of the positive oak characteristics, and a softer tannin structure, from barrel fermentation compared to if the wine is fermented in another vessel and then transferred over to oak barrels for maturation. It is more often favored for high-end wines given the additional complexity of monitoring several separate small-scale fermentation vessels. As you saw above, high temperature is an issue as it may inhibit or even kill the yeast. This is more of an issue for large fermentation vessels, than for small vessels, so large stainless steel tanks often have cooling jackets around them to allow the winemaker to control the fermentation temperature.  **Fermentation Curve** It is common practice to plot the reduction in BRIX (i.e. available sugar) over time, with time measured as either hours or days. The below, taken from **[UC Davis' Department of Viticulture and Enology](**, shows a normal fermentation together with four different problem fermentations  us davis fermentation    The key phases in a normal fermentation are the following: 1. Lag 2. Rapid Fermentation 3. Transition 4. Post-Transition to Dryness
In the chart above, the red line represents an ideal-type, normal fermentation. A key feature of such a fermentation is that it has a relatively short lag before it starts in earnest (e.g. up to 4 days or 50 hours). During the lag phase, the yeast multiplies rapidly and builds up biomass; not much sugar is converted to alcohol at this stage.  The fermentation then accelerates with a period of maximum sugar conversion and the BRIX level drops significantly over a period of perhaps a week or 150 hours. There is then a transition point where the fermentation curve changes slope and the remaining sugar is consumed more slowly on the way to dryness usually defined as less than 0.2% of residual sugar left. As the concentration of ethanol rises towards the end of the fermentation, the density of the aqueous solution drops below that of water and this results in a slightly negative BRIX reading as can be seen in the case of the red fermentation curve above. At this point, the main nutrient for yeast (e.g. sugar) has been depleted and the ethanol level may also have become toxic to the yeast depending on the ethanol tolerance of the yeast strain. The party is over and the remaining yeast dies forming the lees at the bottom of the fermentation vessel. **Problem Fermentations** There are three main types of problem fermentations: _Long Lag_: The first type of problem fermentation is when there is a long lag before fermentation picks up speed and sugar is metabolized by the yeast. This is fairly common when the juice has not been inoculated with a commercial yeast strain; it may then take longer time for the yeast to build up a critical population. Another reason could be the presence of a toxin that inhibits the growth of the yeast. An example of that would be a very high dosage of sulfur dioxide. _Sluggish and Stuck Fermentations_: For the second type of problem fermentations, the yeast fails to consume the sugar as intended. Either the fermentation progresses slowly to dryness or near dryness or the fermentation even stops completely. One of the key reasons is that the temperature gets either too cold or too warm and that this inhibits the yeast. Another reason is nutrient deficiency, especially for nitrogen. A sluggish fermentation can also be caused by ion imbalance with too few potassium ions in relation to hydrogen ions. And as discussed above, ethanol is toxic to yeast at higher concentrations with certain yeast strains being less tolerant to ethanol than others. Then there are other substances that are toxic to yeast including acetic acid, acetaldehyde and sulfur dioxide at higher concentrations. Finally, wine pH drops as a result of the metabolism of wine yeast and if the pH goes below 3.0 it will inhibit the yeast. This is a more common problem for white wines in cooler climates than for red wines in warmer climates given the higher pH of the latter. _Off-character Production_: The third type of problem fermentations is when off-odors are produced. Principal among these are acetaldehyde, acetic acid and volatile sulfur compounds. The cause of these are many and complex including the type of yeast strain used, application of elemental sulphur in the vineyard and nitrogen deficiency during fermentation. Diagnosing problem fermentations, and taking mitigating action, are topics studied at length during university programs for aspiring winemakers. We hope that you enjoyed this overview of alcoholic fermentation and in a later blog, we'll cover secondary malolactic fermentation. **Cheers!** Andreas Birnik