Fermentation of Wine

Fermentation of Wine

Winemaking is a labor-intensive process by which grapes are stemmed, crushed, fermented and bottled. The basic process is ancient, changed little from the earliest wine-making cultures. Modern technology has resulted in refinements to the process such as better quality control, greater consistency and greater accuracy. While traditional wineries leave some of the character of their wine to chance -- to the elements in the vineyards and to fermentation -- many modern wineries seek to control these elements as well through a variety of techniques. This paper will describe the winemaking process and the equipment involved.

Wine begins with grapes. The science of growing wine grapes is known as viticulture. As this is considered a separate discipline, it will not be included in this discussion. It is assumed that the process of winemaking begins with grapes, although some wines are made from grape juice. In that situation, the juice is purchased from a bulk vendor and then fermented at the winery. Grapes are typically harvested from the vineyards by hand for high end wines, and using mechanical harvesters for lower quality wine.

Wine production has changed in recent decades, beginning with 19th century developments in biology that allowed winemakers to understand the nature of fermentation. Prior to the discovery of yeast, the process by which grape sugars were converted to alcohol was unknown to winemakers. With the discovery of yeast came further understanding of its role in the winemaking process. From there, the rise of large-scale wine production brought about more technological changes, some of which were borrowed from the brewing industry, which had already industrialized by the mid-19th century.

Process, Technology and Equipment

A grape cluster consists of stem, skin, pulp, grape stones and liquid, known as must. In general, around 60-80 liters of must can be derived from 100kg of grapes. The first step in the winemaking process is to remove the stem and crush the grape in order to extract the liquid. This task was traditionally done manually; today equipment is used. This process can be done concurrently or the stemming can be done post-crushing. Stemmer-crushers are relatively low-technology machines. The crusher "consists of a perforated horizontal drum and a shaft with blades set spirally. The drum and shaft turn at low speeds in opposite directions." The must and skins fall through the perforated drum while the stems are collected and subsequently expelled from the equipment. For high volume low quality wine the crusher-stemmer may use centrifugal force instead, but this is a rough process not used in high quality winemaking (Peynaud, 1984). Stainless pumps take the must to the fermenters. A saccrometer is a basic device that weighs the must relative to the weight of water, giving the winemaker an indication of the amount of sugar in the must, in other words the potential alcohol. Such a device is also used to measure progress of the wine through the fermentation process -- some partially fermented wine is drawn off from the a tap in the fermentation vessel to the measured.

Before the understanding of yeast, most wine was wild-fermented using the yeasts found naturally on the grape skins. Today, many winemakers eschew wild yeasts in favor of laboratory strains that are introduced to the must. Winemakers using this technique must first introduce sulphites to the wine in order to kill the wild yeast (LaMar, 2006). Different yeast types have different properties, so winemakers using laboratory yeast must choose their strains carefully in order to achieve the character they wish to achieve (Sanz, 2003).

The next step is fermentation. The basic equipment for fermentation is tanks, traditionally made of either wood or concrete. Because the yeast is a living organism, it survives best within a specific temperature range. Traditionally, this was an advantage for concrete fermentation vessels but today stainless steel is preferred. Stainless equipment is typically equipped with "temperature controls, including jackets through which refrigerant can be pumped, thermostats, internal stirring components and computerized controls that maintain the temperature of the must within one or two degrees" (LaMar, 2006). The yeast also needs a healthy level of oxygen concentration in order to thrive. The process of stemming and crushing generally provides this, but there may be a need to aerate the must through agitation prior to pitching the yeast in order to ensure proper oxygenation. This is typically done with an agitator, a small motor device inside the fermentation vessel that simply consists of a spinning blade. Such devices are typically retractable so that they can easily be removed once the aeration process is finished (Australian Wine Makers.com, 2010). Makers of red wine will also allow their wine to sit on the grape skins for a period of around 2-5 days at temperatures of between 70-80F in order to bring the color of the skins out into the wine, as grape juice is naturally clear, even for red grapes (Ibid.)

Commercial, laboratory yeast strains can come in freeze-dried or liquid form, the latter being preferable for its freshness and ability to begin fermentation quickly. With the wine in the tanks at the appropriate temperature range for the yeast to be comfortable and given the appropriate oxygen concentration in the must, the yeast will begin the fermentation process within 8-10 hours. The process lasts between 8-10 days. The yeast are organisms, and they perform very basic life functions. In essence, the yeast eat the sugars present in the must and they excrete alcohol and carbon dioxide. The yeast also produce by-products. As yeast science has advanced, so has the ability of winemakers to control the production of these other substances -- glycerol, acetic acid, higher alcohols and acetaldehyde (Zeppa, 2009). Temperature and oxygen control are central to achieving a higher level of control over the production of these compounds and as such temperature control is essential to the character of the wine. Greater investment in temperature control equipment is one of the processes by which industrial winemakers achieve a consistent product.

The next step in winemaking is to separate the wine from the pomace that remains. The pomace consists of the skins, pulp and sediment that remain after the primary fermentation. Presses are either hydraulic or air presses. An air press is essentially a membrane bag that is placed into the fermentation tank and then expanded, pushing the liquid out of the tank but not the pomace. A hydraulic basket press is made either from wood or stainless steel and is pushed into the tank, forcing the liquid out. Controls such as drain mats allow for a minimum of solid pomace material to be extracted with the liquid.

The next step is malolactic fermentation, especially in red wines. A specific bacterium is introduced to convert malic acid to lactic acid, raising the pH of the wine. Beyond this step, the wine must be filtered to remove the remaining solid material from the bottle. Rustic wines that are not filtered can have a quarter of the bottle taken up by sediment, so this process is essential in professional winemaking. A centrifugal separator can be used as part of this process but in general a more sophisticated filter will be required for finer matter.

There are many different types of filters, including plate and frame, cartridge, rotary drum vacuum, cross flow and (diatomaceous) earth (Australian Wine Makers.com, 2010). Each of these is a different type of membrane through which the wine flows, removing particulate matter from the liquid at varying degrees of fineness. Once the wine is filtered, it is ready to bottle.

Current Uses and Benefits

Modern winemaking equipment represents an improvement over traditional techniques in a number of ways. The most important thread that runs through all of these innovations is consistency. Finer control of temperature, oxidation, filtration and stemming allows the winemaker to achieve the precise character that is desired in the wine. Some of this is achieved through control of internal elements such as yeast and fermentation byproducts; other improvements control externalities such as stems and pomace. The result is that winemakers today have a tremendous amount of control over the quality and character of their beverage. They no longer rely on wild yeasts and the element of chance to produce great wine -- the process can be controlled, can take place almost anywhere and can yield very good results.