Chapter 6

Wine Fermentation

Wine Monitoring Fermentation

Prudent fermentation management cannot exist if the progression of the fermentation is not followed. A decline in sugar levels, more rapid at the start of fermentation but slowing consistently, should be observed. The sugar concentration can either be measured by using a digital refractometer, a hydrometer that measures solution density, a density meter, or by chemical analysis. It is alsoimportant to monitor fermentation volatiles - volatile sulfur compounds, volatile acidity, etc. If a microscope is available the presence and nature of the microbes present can be monitored during fermentation.

Digital Refractometer

Digital refractometer is an optical instrument that measures the density of water-soluble materials, i.e., the proportion dissolved in the water as a ratio. It measures the refractive index, which is the speed at which light passes through a liquid: The denser the liquid, the slower the light will travel through it, and the higher the reading will be on the refractometer.

Hydrometer

Many winemakers use a hydrometer to estimate the residual sugar content and evaluate the progress of fermentation. It provides a quick-and-easy method of assessing specific gravity, an indicator of total soluble solids and sugar content. In the United States, these units are measured in °Brix, whereas in Europe, the units are usually measured as Balling, Baum, or Oechsle. Degrees Brix is a measurement of the apparent concentration of sugar. It is commonly used for grape juice and must and is expressed as a percentage by weight (% w/w).

Density Meter

Although very affordable, the hydrometer is difficult to read and is subject to the interpretation of each sampling technician. The hydrometer also requires a large volume for the sample and can be impractical for some wineries. A mobile density meter provides winemakers with the option of performing density measurements in the cellar or in the lab. A few milliliters of sample are required for making the density measurement.

Chemical Analysis

While °Brix can provide a quick estimate of sugar content, it is not a true measure of the fermentable sugars - namely glucose and fructose - often referred to as reducing sugars, but rather a measure of all dissolved matter in the must. Using °Brix is an inaccurate measure of fermentable sugars and may add an additional layer of uncertainty to alcohol predictions. On the other hand, chemical analysis provides accurate information on the levels of fermentable sugar in the must, which are expressed as grams per liter (g/L).

Rebelein Titration

Reducing sugars in wine can be measured analytically by copper reduction methods or by enzymatic assay. The Rebelein titration is the most commonly used copperreduction method. It consists of causing reducing sugars in a wine sample to react with copper (Cu2+) by using copper sulfate under alkaline conditions.

Enzymatic Analysis

Typically, enzymatic analysis of fructose and glucose is done to determine residual sugar of a wine. Enzymatic analysis is capable of measuring small residual levels of sugar by observing a color change or by ultraviolet (UV) spectrometric measurements.

High Performance Liquid Chromatography

High performance liquid chromatography is a technique that offers several advantages for the analysis of compounds including sugar in wine, including potential for automation, high precision and it is relatively fast.

Plotting the Fermentation

The progress of fermentation can be monitored simply by measuring the degrees Brix of the fermenting must, as in the graph shown below (Figure 6.13). Unfermented must is denser than water, and alcohol is much less dense, so the transformation from must to wine is accompanied by a considerable drop in the density. By measuring the density regularly, it is possible to construct a graph of the progress of the fermentation.

Predicting Potential Alcohol Levels

Knowing the level of fermentable sugar in juice and its conversion rate into alcohol allows winemakers to predict the potential alcohol level in their wines. There are two approaches to estimating potential alcohol. As previously mentioned, one method involves measuring the mass per unit volume (i.e., specific gravity) with a hydrometer while other methods involve chemical analysis in measuring fermentable sugars.

Brix

Brix is commonly used as a measure of sugar in grape juice and must; however, grape juice does not naturally contain sucrose, but rather glucose and fructose and a variety of organic acids and other dissolved solids.

Chemical Analysis

Using chemical analysis (glucose + fructose) to predict potential alcohol provides a more accurate measurement of the levels of fermentable sugar compared to using specific gravity. The official conversion rate formula used in Europe is:

Sampling

Samples should be obtained from mid-barrel or as close to mid-tank (includes bins and open fermentors) as possible. Avoid the bottom of the barrel or tank and the cap area, where insoluble solids are at high levels. It is best to avoid taking a sample during or directly after a tank pump-over or after filling a barrel. The agitation mixes the insoluble solids back into the juice or must and will result in abnormally high Brix readings.

Fermentation Times

Fermentation of normal musts is usually completed in 10 to 30 days. Fermentation times for red wines are often anywhere from one week to a month depending on the winemaker's goal with the wine. White wines usually undergo a much more abbreviated fermentation period, usually in a matter of days and can last up to a week or more for aromatic white wines like Gewurztraminer.

Red Wines

The duration of red wine fermentation should be chosen by the winemaker according to grape quality; it varies from one vineyard to another, one year to another, and even one fermentor to another, since grape quality is never homogeneous. This quality depends on the maturity level of the grapes.

White Wines

The duration of dry white wine fermentation depends on several parameters: juice extraction conditions; sugar and assimilable nitrogen concentrations; turbidity; yeast strain; aeration; and fermentation temperature.

Stopping the Fermentation

As mentioned, fermentation comes naturally to a halt when most of the sugars have been exhausted. Although many fermentations can come to a natural conclusion, commonly known as fermentation to dryness, this takes a long time, longer than commercial wineries can afford. For example, many white wines are not fermented out to complete dryness (i.e., they contain residual sugar), and this is best achieved by halting the fermentation, by either rapid chilling or yeast removal.

Chilling

Toward the end of the fermentation, the alcohol and sugar analysis should be made and, at the desired sugar concentration, the temperature should be drastically lowered under 4 degrees C (39°F).

Yeast Removal

A more complicated method is to remove the yeast from the wine, which typically also involves some racking and fining.

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