Once grapes have arrived at the winery, they are processed in preparation for fermentation. White grape varieties are typically fermented without skins, while red varieties are typically fermented with skins. Either way, a winemaker needs to ensure that the yeast have a happy environment for a successful fermentation and there are several components to keep in mind.
Often considered the most important pre-fermentation characteristic in wine production. Alcohol is produced by the conversion of sugar by yeast during fermentation (1° brix equates to approximately 0.55% v/v alcohol), so typical brix levels prior to fermentation vary greatly depending on stylistic choices made by the winemaker. Of course, harvest conditions can make these choices difficult and lead to some crazy brix levels: way too low to way too high. Every country has its own laws regarding whether water can be added (to decrease sugar levels) or sugar can be added (to increase sugar levels) prior to fermentation, which gives winemakers a little bit of room to play depending on where the wine is being made and where the wine is being sold. Fermenting wine to dryness (less than 2 g/L residual sugar, which is particularly desired for still wine production) is often difficult when there is too much sugar; many yeasts are subject to sugar toxicity levels and alcohol toxicity levels, which is why many commercial yeast producers offer high-brix yeast (they can start ferment with lots of sugar and finish ferment with lots of alcohol).
pH and Acidity
The balance between pH and acidity level is an important indicator of ripeness prior to harvest, used in conjunction with brix levels and sensory evaluation, to determine the best picking dates. Once in the winery, pH and acidity play a major role in winemaking.
Titratable acidity, also referred to as total acidity, is the combined measurement of all acids in a juice/must/wine presented as grams per liter (g/L). There are several different acids present in grapes and wine, but tartaric and malic acids are found in the highest concentrations. Different grape varieties are predisposed to have higher concentrations of one or the other, though environmental factors play a major role. Usually, titratable acidity levels fall between 6-10 g/L prior to fermentation, largely dependent on things such as grape variety and wine style. A winemaker will usually desire higher acid levels for wines destined for malolactic fermentation and even more for those earmarked for sparkling wines.
Most winemakers would likely argue pH carries more importance than titratable acidity. Why? pH has a major effect on microorganisms present. Yeasts require juice/must within a certain pH range to perform non-stressful fermentation. This range is dependent on the specific strain, but usually is somewhere between 3.1 and 3.8 pH. A major issue for winemakers are undesirable microorganisms, many of which can also thrive in this pH range.
There are different regulations for the types and amount of additives a winemaker can use to adjust pH and acidity. Tartaric acid is the overwhelmingly favorite for increasing acidity and therefore decreasing pH. Potassium carbonate is commonly used for decreasing acid, though it has little effect on pH levels.
Managing oxygen levels in wine is a major topic in the modern wine industry. Research continues to show that slight variations in oxygen throughout the winemaking process can have significant effects on wine quality. Nevertheless, oxygen is often left unmentioned when discussing fermentation, one of the stages its levels are most important.
Yeasts are facultative microorganisms, capable of conducting aerobic and anaerobic respiration depending on environmental conditions. Supplying wine yeast with adequate levels of oxygen during the stationary and growth phases is essential for a successful fermentation. Research shows that yeast propagated aerobically contain a higher proportion of unsaturated fatty acids and significantly more steroids than those anaerobically propagated, translating to higher yeast viability.
nce fermentation is underway, consumption of oxygen and subsequent production of carbon dioxide quickly removes oxygen present in the juice/must. In most circumstances, this is desired (not during yeast propagation activities). Oxygen may be added during fermentation of some red varieties. It is sometimes induced as a method of removing carbon dioxide, which becomes toxic to yeast above 0.2 atm concentration.
Yeast cells require several different micro-nutrients during the growth phase, including nitrogen-containing compounds, vitamins, sterols, and minerals. The extent of nutrient requirement is largely based on the amount of sugar the yeast will need to consume and convert to alcohol. The higher the juice/must brix prior to fermentation, the more nutrients the yeast will require. Yeast-assimilable nitrogen (YAN) is a measurement showing the quantity of ammonium salts (NH4+) and free alpha-amino acids (FAN) that are available in the juice/must for yeast consumption.
Proprietary nutrient products are available from several different wine additive companies. Some are specifically designed for use during yeast rehydration, while others are designed for use during fermentation. Diammonium phosphate (DAP) is additive for increasing nitrogen, though varying opinions on its effectiveness exist. Many winemakers use routine rates for nutrient additions, but this can also lead to 'too much of a good thing' situation. Read Yeast Rehydration Nutrients and Fermentation Nutrients for more info.
Most people probably don't realize that sulfur dioxide is a naturally occurring compound in grapes, but it is a very important additive for most winemakers. Sulfur dioxide is added to help protect juice/must/wine against oxidation and spoilage.
Balancing adequate levels of sulfur dioxide to inhibit undesirable microorganisms while allowing desirable yeast to remain active is essential. Prior to fermentation, a good baseline recommendation for free sulfur dioxide is less than 10 ppm and less than 50 ppm for bound sulfur dioxide. These levels can vary widely depending on factors including yeast strain, juice/must pH, presence of infection in the juice/must, and processing considerations. The winemaker's stylistic choices also play a part; for instance, sulfur dioxide helps retain fresh fruit characteristics in white wines.
Just like with pH, yeast have an ideal temperature range in which they are most likely to successfully ferment within. Too low or too high of temperature will stress yeast. Since fermentation produces heat, it's usually a good idea to begin fermentation at a slightly lower temperature than desired.
Winemakers have varied opinions on ideal temperatures dependent on the grape variety, grape quality, and wine style. White grape varieties are typically fermented between 54-63° F (12-17° C), while red varieties will commonly be fermented at higher temperatures upwards of 80° F (26° C).