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](https://laffort.com/wp-content/uploads/FP/FP_EN_Zymaflore_FX10.pdf)**, a yeast suitable for red wine making from the grape varietals Cabernet Sauvignon and Merlot.
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](https://modernwineclub.com/blogs/news/fining-other-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](https://laffort.com/wp-content/uploads/FP/FP_EN_Zymaflore_VL3.pdf?)**.
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](https://wineserver.ucdavis.edu/industry-info/enology/fermentation-management-guides/wine-fermentation/problem-fermentations)**, shows a normal fermentation together with four different problem fermentations
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