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Red Wine Fermenting Procedures

by Staff Writer - B. Shaughnessy | February 08, 2012

Wine fermentation is a natural process where yeasts introduced to the must convert the sugars into alcohol. Vintners have a vast array of yeasts as well as various technologies and methodologies available for the controlling of the fermenting process. In addition to the controls available for directing the fermentation process, various procedures are used for the purposes of extracting additional properties for the resulting wine. In the case of red wine production, procedures ranging from mechanical action of the fermenting vessel to the pre-heating of grape bunches exist for the purposes of extracting color and flavor properties from the grape skins.

In red wine production, juice color elements as well as certain flavor elements are contained within the skins of the grape. Proper extraction of these color and flavor elements is absolutely critical when making a quality wine. The process of crushing the grapes for must production combined with the natural processes of fermentation lead to a natural process of color and flavor element extraction. While this natural process does work for color and flavor extraction, the process can wreak havoc on the resulting wine. This is due to the fermenting yeasts producing carbon dioxide. During the fermenting stage, carbon dioxide is produced and forms gaseous bubbles within the must. These bubbles become trapped in solid matter and force grape skins to float to the surface of the must. As the grape skins have an abundance of acetic bacteria, a potential for vinegar production exists if the grape skins are left floating on the surface of the must. The end result of this process is the conversion of the must into vinegar. As vinegar would destroy the wine, it is critical that control processes be implemented to prevent the production of vinegar, allow for the extraction of color and flavor and promote the synthesis of alcohol.

It is important to remember that a well-balanced wine requires well-balanced winemaking principles. Due to this fact, the extraction of color and flavor elements from the grape skins must be accurately controlled. By extracting too much color and flavor from the grape skins, a wine that is bitter and overpowering with tannins will be produced. Conversely, if not enough color or flavor is extracted from the grape skins, a wine lacking in finesse and character is the final result.

Traditionally, vintners would employ a procedure known as punching down to control the rising of grape skins to the surface of the must. This method was a manual process where a worker would use a special tool to force the floating cap of skins down into the must. The tool used was a pole with a large disk on one end. The worker would balance on the edge of the vat and perform the punch down procedure several times a day. Today this method is still used albeit only at very small wineries. Some winery operations have blended technology with tradition and have created automated punch down machines. The primary benefit is the removal of the human element. It was not uncommon for a worker to fall into the fermenting must and have serious injuries. With the production of wines in larger operations, faster and more cost effective measures of controlling the cap of skins--or chapeau--have been developed.

One process that is available for the prevention of a cap of skins is the use of a submerged cap process. In this process, the skins are held under the surface of the must by the use of a submersed screen. While this may seem like a logical approach to the problem, there are various problems associated with this technique. The main problem with this technique involves the production of carbon dioxide. As more and more carbon dioxide is produced, grape skins are condensed against the screen and tank pressure increases. The increased pressure against the grape skins results in a reduced extraction of color and flavor elements and in some cases can lead to tank failure (ruptures or explosions.) To combat this problem, riser pipes have been introduced within tanks to allow the escape of carbon dioxide gas.

Finally, a process does exist where the liquid of the must is pumped over the top of the forming cap of skins. This process is simple enough, the juices from the bottom of the must are deposited on the top of the must by way of a spray head. These operations are performed in an oxygen free environment and with special zone separated tanks. The zone separated tanks allow for the crushed grapes to be submerged but not to enter the lower part of the tank.

With the inefficiencies found in the punch down methods as well as the submerged cap process and pumping over methods, vintners have turned to completely different technologies for the fermentation of the must. The main purpose of these technologies is not directly related to fermentation. The main purpose is controlling of the release of color and flavor elements as well as preventing a cap from forming.

The delestage process as well as the rotary fermenter are two technological advancements that have resulted in the successful control of cap forming as well as color and flavor release. In the delestage process, techniques are used which are similar to the pumping over method describer earlier. The main difference with the delestage process is that over the course of a day, all of the liquid in the fermenting vat is transferred to a secondary vat. In this process the grape skins are left in the fermenting vat and eventually the liquid from the secondary vat is returned to the fermenting vat by spraying over the grape skins. This process has advantages of seed removal, heat re-distribution, aerated fermenting and the production of soft tannins. Wine produced from this fermentation style obtains smoother characteristics within a shorter time period.

The rotary fermenter is a device where the fermenting vat is designed to follow a similar action of a cement mixer. Blades located within the vat rotate to prevent the formation of a cap and allow for the increased extraction of color and flavor elements. This is especially useful for grape varieties that show difficulty in color and flavor extraction. The problem with rotary fermenters is that too much color and flavor elements can easily be extracted.

Thermo-vinification is another popular method of color and flavor extraction that does not fall into traditional fermenting procedures. Within thermo-vinification, crushed grapes are heated for approximately 30 minutes. After this time the grapes are reduced to standard fermenting temperatures. The heating of the crushed grapes causes a general weakening of the cells found in grape skins. As such, color and flavor elements are more easily extracted from the grape skins. Due to the heat required, there is the potential for a cooked flavor to be present within the wine. This presents a major drawback for the thermo-vinification method. Adaptations to the thermo-vinification method have been developed to prevent a cooked flavor from being introduced. Processes involving rapidly heating uncrushed grapes are successful in weakening the cells of the grape skin while not increasing the temperature of the rest of the grape.

Out of all the various fermentation methods, Carbonic Maceration is interesting in that drinkable wine can be produced at a much faster rate. With the carbonic maceration process, wine is ready for consumption within a few weeks rather than a few months or longer. The carbonic maceration process relies on the use of whole bunches of undamaged grapes and a two stage fermentation process. In the first stage, the grapes are placed in a closed vat with carbon dioxide. An elevated temperature is maintained and alcohol is produced via intracellular fermentation rather than yeasts. In the second fermentation stage the grapes are juiced, cooled and transferred to a vat where a normal fermentation run is executed. This process results in the best possible conditions for the extraction of color and flavor elements as well as lower temperature conditions for standard fermentation.

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