Wine Making

Wine is the product obtained by normal alcoholic fermentation of grapes/grape juice by yeasts and a subsequent ageing process. Wines can also be prepared by fermentation of the juices of fruits (apples, berries, peaches, plums, apricots) and even herbs).

Wine is a non-distilled alcoholic beverage obtained primarily, by fermentation of grape juice resulting from yeast with proper processing. The processes by which raw grapes are converted in to wine include crushing, extraction, fermentation, fining, racking, fortification etc.

 Vinification

Winemaking or Vinification is the process of wine production starting from the selection of grapes to the bottling of the finished wine. Vitis vinifera, the main genus and species of grapes used for wine, produces the best grapes. Grapes must be picked at their optimum maturity stage and  in good condition. The must (fresh pressed juice of grapes) contains sugar, organic acids, tannin, flavoring substances, proteins, mineral, salts, pectin and mucilaginous substances from the grapes. It also contains a large variety of yeasts, bacteria, and fungi, some of which are favorable and some which are not.

The production of wine falls naturally in to two broad divisions, red and white wines, respectively.

 Manufacture of red wine

When the grapes are at the proper stage of ripeness, bunches are plucked and brought to the winery.

1. Stemming: Stemming or removal of stems is done by hand or by use of screens which will pass the grapes but not the stems. It can be operated by hand or by mechanical means. Stemming may also be done after crushing.

2. Crushing: Crushing is done to liberate the juice and inoculate it with yeast. The crushed grapes are taken to the fermenting vat. A treatment with sulfur dioxide or potassium metabisulphite is done to inhibit the growth of other undesirable microorganisms.

3. Fermentation: Crushed grapes are started to ferment as desired either naturally or by means of starter under controlled temperature. Temperature control is desired as yeast works best between 24-27^oC. Sugar is converted in to alcohol and carbon dioxide, and other changes like drop in acidity, formation of volatile compounds (esters and alcohol), extraction of tannin and colouring matter from skin and seeds etc also occur during fermentation.

            A natural “inoculum” (yeast present on the grapes) or a 2-5% special wine yeast, a strain of Saccharomyces cerevisiae var. ellipsoideus, is added to the crushed grapes or must. Contents are mixed daily initially, to aerate and encourage the growth of the yeast and to aid in the extraction of colour from the skins for red wines. Later the mixing is discontinued since anaerobic conditions are favourable for alcoholic fermentation. Temperature control is important, since too high or too low temperatures permit the growth of undesirable microorganisms and lead to wine defects.

4. Completion of First Fermentation: At the best, only 4% of sugar can be converted per day. For complete fermentation, the time required may vary from two to three weeks. The fermented juice after primary fermentation can be collected after filtration and placed in a new vessel for secondary fermentation. The remaining sugar is fermented during secondary fermentation.

 5. Aging and racking: The new wine obtained is low in alcohol and contains some unfermented sugar, excess tartaric acid and tannin. It is also cloudy due to the suspended yeast cell, pectinous material etc. After secondary fermentation, wine is allowed to settle. The addition of fining or clarifying agents like bentonite, egg white, gelatin, isinglass, etc. helps to settle down the colloidal particles.

Racking is the carefully pouring or syphoning off clear wine from the sediment into new clean casks. Racking improves the colour and flavour and also further precipitates the remaining albuminoids by the action of oxygen (aeration). Artificial aeration is often employed to accelerate the desired process.

The wine may be flash-pasteurized before aging to precipitate the proteins. It also helps to arrest further microbial activities without compromising in the flavour.

It is then cooled, held for a few days, filtered and transferred into appropriate containers for aging. Aging for months and years results in desirable changes in the body and flavor of the wine, giving it characteristic aroma or bouquet. Esters and alcohols contribute to aroma or bouquet

After aging, wine is filtered/clarified, barreled or bottled and stored. The final alcohol content varies widely but is usually 8-13% by volume.

Classification of wine

There are endless varieties of wine which differ in so many attributes that they are difficult to classify. Some are 

1. Dry or sweet

2. Fortified or unfortified

3. Sparkling or still

4. Red or white

 These types are defined as:

a) Dry wines are those in which practically all of the sugar has been converted into alcohol by fermentation. They contain very little or no sugar that can be detected by testing. They are of comparatively low alcoholic content of around 8-12%. This includes Hock, Burgundy, and Moselle etc.

 b) Sweet wines are those wines in which sugar content is high enough which can be detected by taste. They contain some unfermented sugar and have alcohol content in the range of 13-15% by weight. Examples are Sauterne and Tokay.

Wines with 7-9% alcohol are known as ‘light’, those with 9-16% are ‘medium’ and those with 16-21% are known as ‘strong’ wine.

 c) Unfortified wines are those in which all the alcohol content is entirely derived from fermentation. The above two classes (a & b) are unfortified wines. 

d) Fortified wines are those wines in which distilled spirits such as grape brandy is added to increase the alcohol content in addition to those derived from fermentation. They usually contain 18-22% alcohol. Examples are Sherry, Port, and Champagnes etc. 

e) Still wines also known as table or dinner wine are those whose fermentation has been completed before bottling so that they don’t retain carbon dioxide produced in the fermentation. Most of those mentioned above are still wines.

 f) Sparkling wines are those wines which are bottled before the fermentation process is ceased so that they contain carbon dioxide gas in solution. These wines are characterized by effervescence when served. They may be dry or sweet, light or strong. Champagne, sparkling burgundy are sparkling wines.

 g) Red wines are those wines in which the skin, stems etc. of the grapes are present during the fermentation to extract the pigment and thereby colouring the fermented juice. Example includes Port, Burgundy, and Chianti etc.

 h) White wines are fermented grape juice only i.e. without skin, stems etc. The colour of the white wines varies from a pale straw colour to deep gold colour. Riesling, Sauterne and Champagne may be cited as examples. 

The distinction between the red and white wines is particularly based on a difference in manufacturing process and on the basis of flavor and use. For example, red wines are more flavourful, typically drier, and more astringent (bitter) than white wines. In contrast, white wines are generally more acidic, floral in nature, and come in a wide range of sweetness styles. Since most white wines are intended to be consumed with meals, they typically are produced to possess an acidic character. 

Factors affecting wine quality

 A number of factors affect the wine quality, the most important being the type of grape used. Others include location, climate, weather etc. Climate and weather are the most important factors that influence the growth of grapes for high-quality wines. Climate is determined by geographical location, whereas weather is the result of how nature decides to affect that climate on a daily basis. Apart from these factors, chemical composition of the grapes also affects the sensory properties of wine and the fermentation process.

 These are

 

1. Water: Moisture content of the grapes varies from 70 to 85% which directly influence the yield of juice.

2. Sugar: The major sugars of fruits which are normally used for making wine are glucose, fructose and sucrose. The two reducing sugars predominate in grapes and occur approximately in equal proportion and one of the important parameter in deciding the maturity of grapes.  Harvesting at the proper stage gives the optimum wine quality.

3. Organic acids: Amount of organic acids is low in wine, but it is an important constituent for tartness (sour/acid taste). Tartaric and malic acid have a significant effect on wine pH. pH of wine is associated with wine colour, spoilage and stability. Sulfur dioxide is widely used in the wine industry as sanitizing agent and antioxidant and a slight decrease in pH in the range of 3.0-5.0 increases the antiseptic action of sulfur dioxide.

4. Nitrogenous substances: Nitrogen compounds such as amino acids, ammonia and ammonium salts are essential for the growth and development of the yeast during the fermentation process. If these nitrogen compounds are not utilized completely during fermentation, re-fermentation and bacterial spoilage may occur in the finished wines.

5. Phenolic substances: Phenolic substances significantly affect the colour, appearance, taste and body of wines. The colour of red wines is primarily derived from the fruit anthocyanin through heat extraction or fermenting on the skins. Tannins, another group of phenolic substances, influence the body of wine and produce an astringent taste.

6. Inorganic constituents: Inorganic components of grapes significantly influence the production and quality of wines and are important for human nutrition. Generally excessive levels of inorganic constituents produce undesirable effects by altering the colour, appearance or taste of the wine.

7. Enzymes: Certain enzymes present in grapes may be responsible for wine disorders such as clouding, browning, and darkening or an oxidized taste. Enzymes found in grapes include peroxidases, catalases, invertases, tannases, polyphenol oxidases and pectic enzymes.