Wine

The Complete Grape Growing System

The Complete Grape Growing System

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Wine is the product of the alcoholic fermentation of naturally occurring sugars in the juices of fruit, most commonly grapes. An amazing variety of microbial transformations occurs during both the fermentation and aging of wines, giving rise to complex flavors and aromas. Enzymes of yeast and other microorganisms catalyze the conversion of sugars, organic acids, amino acids, pigments, and many other components of fruit into numerous different substances including alcohol. Only a few of these reactions are completely understood.

One of the most important variables in the quality of wine is the variety and quality of grapes used. The growing conditions and ripeness as well as other factors affect the grapes' content of sugar, acids, and various organic compounds, which in turn critically influences the final product. Commercially, wine is made by crushing carefully selected grapes in a machine that removes the stems and collects the resulting solids and juices, or must (figure 32.4). For red wine, the entire must, including skins and pulp, of red grapes is put into the fermentation vat. The red color and complex flavors contributed by the tannins of red wines are derived from components of the grape skin and seeds. The solids are removed during fermentation by a wine press that separates them from the partially fermented juice once the desired amount of color and tannins have been extracted. For the production of white wines, the solids are removed immediately and only the clear juice of white, or occasionally red, grapes is fermented. Rose wines obtain their light pink color from the entire crushed red grape fermented for about 1 day, after which the juice is removed and fermented alone.

The fermentation must be carefully controlled to ensure that desired reactions occur. Sulfur dioxide (SO2) is generally added to inhibit the growth of the natural microbial population of the grape, including acetic acid bacteria. These bacteria can convert alcohol to acetic acid (vinegar) and are the most prevalent cause of wine spoilage.

The fermentation process is initiated by the addition of specially selected strains of Saccharomyces cerevisiae. Species of Saccharomyces are more resistant to the antimicrobial action of SO2 and produce a higher alcohol content than naturally occurring yeasts. Fermentation is carried out at a carefully controlled temperature, which varies with the type of wine, for a period ranging from a few days to several weeks. During fermentation most of the sugar is converted to ethanol and CO2, generally resulting in a final alcohol content of less than 14%. Dry wines result from the complete fermentation of the sugar, whereas sweet wines contain residual sugar.

In addition to the alcoholic fermentation of the grape sugars, a distinctly different type of fermentation, called malolac-tic fermentation, may occur during wine production. Lactic acid bacteria, primarily species of Leuconostoc, convert malic acid to the less acidic lactic acid. Red wines made from grapes grown in cool regions tend to have high levels of malic acid, and their flavor is mellowed by this fermentation.

After fermentation, the wine is siphoned several times to remove the clear juice from the sediment of yeast and particulate debris. Most red wines and some white wines are then aged in oak barrels, contributing to the complexity of the flavor. Red wines are generally aged for 1 to 2 years. Wine is clarified by filtration and bottled. The CO2 produced during fermentation is usually released before the wine is bottled, resulting in a "still" (non-carbonated) wine. Other processes are used to prepare carbonated wines such as champagne.

The Japanese wine, sake, depends on several microbial fermentation reactions. First, cooked rice is inoculated with the fungus Aspergillus oryzae. The fungus produces the enzyme amylase, which degrades the rice starch to sugar. Then, a strain of

808 Chapter 32 Food Microbiology

Fruit, most commonly grapes, is crushed to yield must, composed of solids and juices. Stems are removed. Red wines are made from the entire must of grapes. White wines are made from the clear juice of white or red grapes.

Sulfur dioxide is added to inhibit wild yeasts and spoilage bacteria. Specially selected strains of Saccharomyces are added. The fermentation process begins.

Settling vat

The wine is siphoned several times to separate the fermented juice from the particulate debris.

During the aging process, chemical and microbial changes occur that contribute to the complex flavors of wine.

Settling vat

During the aging process, chemical and microbial changes occur that contribute to the complex flavors of wine.

Wine is clarified by filtration and bottled.

Wine is clarified by filtration and bottled.

Figure 32.4 Commercial Production of Wine

Saccharomyces cerevisiae is added to convert the sugar to alcohol and CO2. Lactic acid bacteria add to the flavor by producing lactic acid and other fermentation end products.

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