The science and technology of wine making

Introduction

There is a wide range of alcoholic beverages obtained by the fermentation of sweet liquids (vegetable juices, honey, milk) but the most important are wine, beer and cider. Wine is an alcoholic beverage produced by the fermentation of the juice of fruits, usually grapes, although other fruits such as plum, banana, elderberry or blackcurrant may also be fermented and used to obtain products named "wine". In this short article the word "wine" refers to the product obtained from grapes. This product is probably the most ancient fermented beverage and was mentioned in the Bible and in other documents from Asiatic peoples. Exactly where wine was first made is still unclear. It could have been anywhere in the vast region, stretching from Portugal to Central Asia, where wild grapes grow. However, the first large-scale production of wine must have been where grapes were first domesticated, Southern Caucasus and the Near East. In Egypt, wine played an important role in ancient ceremonies and winemaking scenes are represented on tomb walls. Outside Egypt much of the ancient Middle Eastern peoples preferred beer as a daily drink rather than wine. However, wine was well-know especially near the Mediterranean coast and was used in the rituals of the Jewish people. The Greeks introduced wine to Europe and spread the art of grape-growing and winemaking across the Mediterranean hence modern wine culture probably derives from the ancient Greeks. Wine was known to both the Minoan and Mycenaean cultures and referred to as "Juice of the Gods". Dionysus was the Greek god of wine and wine was frequently mentioned in Homer's and Aesop's operas. Many of the grapes grown in Greece are grown nowhere else and are similar or identical to varieties grown in ancient times. Greek wine was widely known and exported throughout the Mediterranean basin, and amphorae for Greek wines have been found extensively in this area.

The Roman Empire was very important in the development of viticulture and oenology. Wine was widely drunk by the Romans and its commerce was a very important business. Many grape varieties and cultivation techniques were known and then exported throughout the Roman Empire. Wine making technology improved considerably during this period and wines were stored in barrels or bottles for shipping in Rome. With the fall of Roman Empire due to the barbaric invasions all the economic activities collapsed. The only stable structure was the Catholic Church. The church preserved viticulture and wine making during this period. In medieval Europe wine was consumed only by the church and the noble classes but since it was necessary for the celebration of the Catholic Mass its production was essential. The Benedictine monks were the largest producer of wine and owned vineyards in Champagne (Dom Perignon was a Benedictine monk), Burgundy and Bordeaux in France and in the Rheingau and Franconia regions of Germany.

Grape vines were transported to Latin America by the first Spanish conquistadores to provide the wine required for the celebration of the mass in the Catholic Church. They were initially planted in the Spanish missions. Later immigrants imported French, Italian and German grapes as replacements for the native grapes that produced a wine with a foxy aroma. In the Americas wine is closely associated with the United States (California), Argentina and Chile where a wide range of varieties of wines are produced.

Grapes are largely cultivated between Sweden to New Zealand and about 250 million hectolitres of wine are produced. The largest producers of wine are France and Italy (see table 1).

Table 1 - Wine production (million of hectoliters; 2005) 

                                                            Source FAO

There are several species of European grape. Varieties of Vitis vinifera such as Pinot Noir, Chardonnay or Nebbiolo are usually used for wine production. The other species (Vitis labrusca, Vitis aestivalis, Vitis muscadinia, Vitis rupestris, Vitis rotundifolia and Vitis riparia) are generally used only for eating in fruit form since their use for winemaking is usually prohibited by law. The North American species are used as rootstocks for the European vinifera grafting since they are resistant to phylloxera, an insect that attack the roots of grape vines. 

Wine can be defined as "varietal" when produced with only one grape variety (minimum 75-85% w/w defined generally by law) as Barolo, Barbaresco or Asti for Italy and "blended" when produced with a mix of grape varieties of the same vintage such as Chianti, Brunello di Montalcino for Italy or Bordeaux wines for France. The quality of blended wines is not inferior to that of varietal wines. On the contrary, winemaking using blended wines is easier as grape characteristics can be complementary.

The concept of "terroir" is very important for wine. This refers to the variety or varieties of grape(s) used the vineyard characteristics (slope, elevation, type and chemistry of soil), the climate and seasonal conditions under which grapes are grown and the activities of the producer during the cultivation of the vine and the winemaking. The wine characteristics are due to the interaction of these factors and this explains the chemical and sensory differences among wines of different vintages.

A lot of texts and articles have been written about the grape, wine and their production. The aim of this short article is to examine only the principles involved in the production of wine and above all to examine the principles of winemaking. The reader is referred to specialized texts for further study of this matter.

Grape as raw material for wine

As reported earlier the grape used for wine production is generally Vitis vinifera L.; over forty species are known but only some are used for winemaking.

Wine grapes grow almost exclusively between thirty and fifty degrees north or south of the equator. The world's most southerly vineyards are in the South Island of New Zealand near the 45^th parallel and the most northerly is in Sweden, just above the 59^th parallel. As a rule, grape vines prefer a relatively long growing season of 100 days or more with warm daytime temperatures (not above 95°F/35 °C) and cool nights (a difference of 40°F/23 °C or more).

It is the grape quality more than any other factor that determines the quality of the wine. This quality is defined by different parameters that can be described as 1) permanent (variety, soil, climate, exposure), 2) variable (the weather during the growing season), modifiable (the time of harvest, pruning) and 3) accidental (hail storm, diseases). The combination of these effects is often referred to as terroir.

The grape berry in its early weeks after flowering is a tiny, green-coloured, very acidic pellet. During ripening the berry increases in volume so that after a few weeks the tiny berry becomes a plump, sweet, coloured fruit. When the berry changes its colour the veraison phase is reached. From an oenological perspective, veraison can be divided into different sub-stages based upon berry metabolism and the continued transport of substances to the vine. During veraison, water, sugars, and nitrogen compounds are transported to the berry via the phloem. Sucrose is hydrolyzed to glucose and fructose and flavour and aroma compounds are synthesized within the berry. The fruit does not increase in volume indefinitely. When the fruit has reached normal maturity there is a cessation of cell enlargement and a decrease in volume can occur under warm conditions. During these physical changes other chemical changes also occur.

When ripening starts there is a continuous increase in sugars in the fruits while the titratable acidity decreases as shown in figure 1.

Figure 1. Changes in sugar concentration and titrateable acidy during ripening of grapes.t- Sugar, n- Acidity.

Sugar concentration increases until a maximum value defined by climate and variety is reached. This value stays stable for a small time and then the sugar concentration increases again due to the over-ripening of grapes and the shrinkage of the berry. The determination of grape maturity is very important because high quality wines can only be produced when the grapes are harvested at their optimum maturity. Generally for white wines harvesting is performed early to preserve acidity that is very important for sensory characteristics (freshness) of wine while for red wines harvesting takes place later when the sugar concentration is higher. It is not possible to fully define this stage because it is correlated not only to the chemical composition of grapes but also to their health and the characteristics required in the wine. Optimum maturity is then defined for each vineyard and can be different each year depending on climate, grape maturity and above all the characteristics of the wine that will be produced. This evaluation is performed generally with analytical methods (determination of sugar, polyphenol and acid concentration) and harvest is conducted when the grapes have reached the correct chemical composition.

Harvesting is generally done by cutting the cluster from the vines by hand with picking knives or scissors. The grapes are moved rapidly to the vinery for winemaking since delay can result in product deterioration. Mechanical harvesting is used when possible but the quality of the wine obtained is generally lower due to some crushing of the grapes. 

Science and technology of wine making

Winemaking, or vinification, is the process of wine production, from the selection of grapes to the bottling of finished wine. The grapes are usually harvested from the vineyard in the fall or autumn.  Harvesting takes place in the northern hemisphere from the middle of October until the beginning of November and from the middle of February until the beginning of March in the southern hemisphere. Grapes used for wine production must be healthy, without defects due to bacterial and/or fungal infections and winemaking must be conducted as soon as possible to prevent wine defects with, if possible, whole grapes. If grapes are broken during harvest and/or transport to the winery defects or problems can arise due to the activity of acetic and/or lactic bacteria and their production of acetic and lactic acid.

The most important phases of winemaking are:

        Production of must

The grape cluster consists of stem (2.5-8% w/w), skin (6-10% w/w), pulp, grape-stones (2-15% w/w) and liquid or must. It is possible to obtain 60-80 liters of must from 100 kg of grapes.

The stem is a lignified vegetable structure containing a high concentration of polyphenols. During winemaking the stem is generally separated from the grapes and discarded. Polyphenols (anthocyanins and catechins) and the aroma compounds (terpenes) are present in the skin of aromatic grapes such as Muscat. Acids (tartaric, malic and citric; about 0.5-1.5% w/v), sugars (only fructose and glucose as sucrose is absent; about 15-30% w/v) and small quantities of aromatic compounds and polyphenols are present in the pulp.

The first activity in winemaking is grape crushing and the production of must. During this activity the stem is separated, the berry is crushed and the must is produced.

Grapes are crushed with a mechanical crusher/destemmer classified according to which action takes place first. There are destemmer/crushers in which the grapes are broken apart as the grape berries are separated from the stems. Berries are then crushed by rollers and transferred to the fermentation tank. The advantage of this sequence is that since the stems are not in contact with the must then extraction of undesirable stem components does not occur.

Crusher/destemmers break the berries while they are attached to the stems and the destemming action is conducted for the berry skin rather than the whole berries. Generally the destemmer/crusher has a higher capacity but the quality of the must is lower due to the possibility of extraction of stem components.

For white wine production grape crushing must be done very carefully because the compounds present in the skin and stem must not pass into the must. Then crushing is generally obtained with a simple grape pressing.

        Alcoholic fermentation

When the must has been prepared selected yeasts, sulfur dioxide and nutrient substances are added. Yeasts are generally already present on the grapes and in the winery environment but they can give unpredictable results, hence selected commercial strains of yeast are often added to the must. These are supplied in freeze-dried form and many different strains are commercially available. Sulfur dioxide is generally used in winemaking (generally 50-100 mg/L) and has two functions. The first is inhibit or kill the natural microflora (bacteria and yeasts) in the juice and thus facilitate the activity of the selected yeasts added to the must. The second activity is to inhibit oxidative enzymes that would cause the juice to change colour and go brown.  Ammonium salts and vitamins (biotin and thiamin) mixed in special preparations are often added as nutrients for the yeast. 

If the sugar concentration is low, sugar may be added but this addition is strictly subject to local regulations. In Italy it is only possible to add concentrated and rectified must while in some other countries sucrose can be used.

After 8-10 hours from inoculation with yeast the primary, alcoholic fermentation starts. This fermentation generally lasts for 8-10 days and during this period the yeast cells utilise the sugars in the must and multiply, producing carbon dioxide gas and alcohol. The temperature during the fermentation affects both the taste of the end product, as well as the speed of the fermentation.  The temperature used for red wines is typically 25° to 28 °C while for white wines it is 20° to 25 °C.  For every 100 grams of sugars fermented about 60 ml of ethanol, 24 L of carbon dioxide and 24 Kcal are produced. This should result in an alcoholic fermentation yield of 60% v/w but lower value (55-58% v/w) can be obtained if fermentation is conducted erroneously. During fermentation it is important to control the temperature and the oxygen concentration of the must. During alcoholic fermentation other substances (minor products): glycerol, acetic acid, higher alcohols, and acetaldehyde are also produced. Wine quality is also defined by the quantity of these compounds and particularly by low concentrations of acetic acid, higher alcohols and acetaldehyde.

In the must used for red wine production, skins and seeds are present and during the alcoholic fermentation the colour and tannin must be extracted. Seeds fall to the bottom of the tank while skins are pushed to the top of the tank by carbon dioxide. These skins form a surface layer know as "cap of pomace". Grape skins are richer in flavour and colour (anthocyanin pigments) compounds and contact of the skin with the must allows flavour and colour extraction. To extract the colour and tannin in the skins, this cap must be broken and the fermenting must is thoroughly mixed several times a day. In small plants this can be done manually using a metal mixer and in large wineries by drawing the must from the bottom and pumping it over the cap.

When the sugar concentration of the must has reached about 10 g/L or less, usually in 7-10 days, the must is drawn off the pomace.  In some cases the wine may be allowed to remain with the skins and the seeds for seven or more days after the fermentation is complete in order to obtain wines with a high tannin concentration.

Pomace and wine are generally separated using either vertical or horizontal presses. Continuous presses are also used but the wine obtained contains much pulp and sediment and its quality is lower.  The wine is then placed in a storage tank where the alcoholic fermentation runs to completion. This process requires about a week.

        From wine to bottle

When the sugar is fully utilised, the malolactic fermentation can take place. This process is used mainly for red wines but also for some white wines. In this process specific strains of the bacterium, Oenococcus oeni, convert malic acid to lactic acid. The fermentation reduces the titratable acidity and raises the pH which is advantageous in some wines. This fermentation is often initiated by inoculation with desired bacteria but can only be performed if the pH is higher than 3.2 and the temperature is higher than 20 °C. Malolactic fermentation is performed for 2-4 week then the wine is transferred to other tanks for final wine processing activities (fining, filtration, and aging).

The purpose of fining is to remove excessive levels of certain wine components, to achieve clarity and to make that clarity stable especially from a physicochemical viewpoint. Examples of such fining reactions are: the removal of tannic and/or brown polymeric phenols by protein-fining agents such as casein, albumin or gelatin; the adsorption of wine proteins by clays such as the bentonites and the elimination of unpleasant odors by copper sulfate.

Filtration in winemaking is a general operation which encompasses a wide range of conditions from the partial removal of large suspended solids to the complete retention of microbes by perpendicular flow polymeric membranes. Generally all wines are subjected to fining and/or filtration processes but only some wines containing high concentrations of ethanol and tannins are selected for aging.

In general aging is used for red wines but some white wines (e.g. Chardonnay) are also subjected to aging. Aging is a very complex process which has many effects on the wine. It can be performed in two stages, bulk and/or bottle. During bulk storage wine may be exposed to oxidation and other treatments while during bottle storage only reductive reactions occur. Bulk storage can be performed with inert containers like stainless steel or white oak barrels (like French "barriques"). In the latter case, however, the container reacts with wine; these reactions are more intense when small barrels are used. The most important reactions that occur during aging in an oak barrel are: wine oxidation,

evaporation of volatile components and reaction between wine and oak components. The aging period can range from few months to many years and is dependent on the wine type. At the end of this period wine is generally only filtered and bottled. Aging in the bottle ranges from some days to many years and is dependent on wine type. Generally for white wines there is no bulk aging and the period of bottle aging lasts for some months (never more than 1 year). For young red wines, bulk and bottle aging can take place over a very short time period(maximum 2 years) while for particular red wines such as Barolo, Barbaresco, Brunello di Montalcino or some Bordeaux aging can be very long; it is possible to drink wines after 30-40 and more years of aging.

White wine technology

For white wine production the wine-making technology described previously must be changed slightly. White wines can be produced with red or white grapes but there should be no contact between must and skins. For white wine the grapes are directly pressed with or without crushing and the pomace is immediately separated. Destemming is never used for this wine as stems are retained to facilitate the draining of must from the pomace. For production of white wines from red grapes like Champagne, grapes are directly pressed to reduce contact between the must and skins and to avoid the extraction of  anthocyanin and phenols. 

Must produced from presses must first be clarified before it can be used in wine making. The solid fragments of skin must be removed from the juice to prevent the extraction of phenols. This juice clarification can be performed using different techniques. The common practice is the use of gravity settling in vertical tanks. The time required will be determined by the rate at which the skin cap rises; clarification agents such as clay are used as adjuncts. To use this technique it is essential to prevent any natural alcoholic fermentation taking place; this is achieved by cooling the must rapidly (0-5 °C) and sulfiting. Generally after 12 hours the clarified juice can be sent to tanks for the alcoholic fermentation. Juice can be clarified with centrifuges also. This technique is very fast but has considerably higher capital costs. Cross-flow filtration and flotation are largely used for must clarification. Generally cross-flow filtration is used in conjunction with other equipment that can provide partially clarified juices such as a centrifuge. Flotation is a new technique that has received renewed interest as a continuous, relatively fast and low-energy process for clarification. In this method fine gas bubbles, usually nitrogen are introduced into a static juice and the suspended pulp floats upwards toward the surface where it can be collected.

When juice is ready selected yeasts, sulfur dioxide and nutrient substances are added and alcoholic fermentation can be started as described for red wine. In this fermentation the control of temperature is very important and for high quality products temperature during the alcoholic fermentation must not exceed 25°-27 °C. When sugars are completely consumed wine is transferred to other tanks for final wine processing activities (fining, filtration). Generally malolactic fermentation and aging are not executed for white wines.

Special wines

Some wines are produced using special winemaking technology and processes. In this paper only the most important wines are reported along with a short explanation of the technology used in manufacture.

    Rosé wines: production of rosé wine is very similar to that of red wine but contact between the must and the grape skins and other parts  during the alcoholic fermentation is reduced to 1-3 days, just long enough to extract sufficient colour. Pomace and wine are then separated with vertical or horizontal presses and the must transferred to containers where the alcoholic fermentation runs to completion. Rosé wine is submitted to treatment similar to red wine with the exception that it is not aged. Also the malolactic fermentation is generally not performed with rosé wines.

    Passiti wines: "Passiti" wines are typical products of the Mediterranean area although Reno wines and ice wines can also be classified as "passiti" wines. For these products grapes are sun dried to obtain a higher sugar concentration (more than 300 g/L), pressed and the must is fermented with special yeasts. Because the sugar concentration is so high the fermentation is only partial and the wine has a high alcoholic (more than 14% v/v) and also sugar (more than 60 g/L) concentration. These wines are sweet, very alcoholic, amber-coloured and have typical odors like hazelnut and honey.

     Reno wines are very particular "Passiti" wines. The grapes for these products are dried by Botrytis cinerea, a mould that generally cause a grape disease and rottenness but in this area with low environmental temperatures this activity is very scarce with only a reduction of water concentration.

    Another type of "passiti" wine is "ice wine" typical of some alpine areas in Germany and Canada. For the production of these particular wines, grapes are left on the plant during the winter to freeze. When grapes are frozen they are immediately pressed without defrosting and the must is fermented similarly to white wines. As a large quantity of water is frozen the sugar concentration in the remaining water is very high and a sweet wine with a high ethanol concentration is produced.     

    Sparkling wines: these contain a visible excess of carbon dioxide (at least 1.5-2 atmosphere pressure). There are three sources of this carbon dioxide: fermentation of residual sugar from the primary fermentation (Asti Spumante DOCG), from fermentation of sugar added after the process of fermentation (Champagne AOC), and from direct addition (carbonate wines).  In the first case the must is partially fermented in special tanks. When the pressure reaches the value required, the wine is clarified, filtered and bottled. The product is sweet with a low quantity of ethanol (about 4-7% v/v). This technique is generally used to produce aromatic wines such as Asti Spumante DOCG or Brachetto DOCG. In the second technique, sugar and yeast are added and the wine is re-fermented. This new fermentation can be carrying out directly in the bottle as for Champagne AOC or in a special pressurised fermentation vessel. In the first case the wine should be left in contact with the yeast for at least a year in order that the second fermentation will be complete and to allow the yeast cells to die and to permit development of the "champagne" bouquet. The yeast sediment is then removed from the bottle and liqueur is added to replace that lost in decanting. In the second case fermentation can be carried out in a tank or pressure vessel and when the pressure reaches the required value the wine is clarified, filtered and bottled. Sparkling wines can be obtained also by the direct addition of carbon dioxide. While this is a very simple production process with low costs it is not used to produce quality products.

      Beaujolais wines: these wine are very special products as the grapes are subjected to a special treatment, carbonic maceration. To produce these wines the whole grapes are put in a tank full of carbon dioxide gas which is renewed  periodically. After ten-fifteen days of immersion in carbon dioxide the anthocyanins passes from the skin to the pulp. At this stage grapes are crushing and destemming and the red must obtained is  fermented as a white product. The wines are very fruity with strawberry and raspberry odors and a red ruby colour. This is a young wine and must be drunk within a few months of its production. 

Wine tasting

Wine quality is given by three production phases (grape production, winemaking and bottle conservation) but the factors that determine quality are numerous and a high quality wine is the optimum result of a large number of these factors. Wine quality evaluation can be performed only with sensory analysis because, generally, chemical data are not sufficient to define this parameter.

To correctly evaluate wines a defined temperature and wine glass must be used.

White wines must be drunk at 9°-12 °C, red wine must be drunk at 20°-22 °C and rosè wines must be drunk at 14°-16 °C. As wines are preserved generally at 8°-12 °C (a domestic refrigerator should not be used) time for temperature adjustment must be allowed. For wines with a very long aging period (more than 5 years) it is necessary to decant the product in new special bottles some hours before the tasting. To taste wine a large number of glasses will be required and in some cases special glasses have been produced  for particular wines (e.g. Reno wines, Champagne). A special glass for wine tasting has been defined by ISO and although it is  largely used to drink wines it was defined only for sensory analysis. As the glass to drink wines it was defined only for sensory analysis. As the glass shape influences sensory evaluation of wine it is very important to use a correct glass or the "better glass" for that wine. During wine tasting it is necessary to evaluate colour, odor, taste, aroma and trigeminal sensations. The optimal combination of these parameters results in a high quality wine.

A white wine will be yellow or pale-yellow with a fresh odor where fruit, flower and herb sensations are predominant along with acidity. A young red wine will be ruby-red with a fresh odor, fruit and flower sensations are predominant, along with an acid and astringent taste. An aged red wine will be orange-red with a special odor such as vanilla, butter, spice, or cooked fruits. In the mouth the taste is sweet and gently bitter with a predominant astringency. Because of the differences in the sensory characteristics of wines it is possible to link food courses and wines. Generally white wines are linked to starter, fish, and salami courses but not to ripened cheeses. Young red wines are linked instead to first courses, meat and ripened cheeses. Aged wines best compliment meat and game dishes. Sparkling wines can be used also as table wine but generally are linked to starter courses while sweet wines are used only for dessert and cake. The "passiti" wines obtained by partially dried grapes are generally drunk with mould ripened cheeses or chocolate.

How to cite this article

Giuseppe Zeppa (2007).   [On-line] UK: Available:  Accessed:

Recommended further reading

Colagrande O. (1999). Preparazione dei vini di qualità. Ed. Chiriotti, Pinerolo, Torino (I).

Ribéreau-Gayon P., Glories Y., Maujean A., Dubourdieu D. (2000). Handbook of Enology. Ed. Wiley & Sons Ltd, GB.

FAOSTAT. FOOD AND AGRICULTURE ORGANIZATION OF THE UNITED NATIONS. On line: http://faostat.fao.org (accessed May 28, 2007).

Flanzy C. (1998). Oenologie. Ed. Lavoirier Tec-Doc, Paris, France.

Wikipedia contributors, "Wine," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Wine&oldid=153895592 (accessed May 27, 2007).

© Copyright September 2007. All rights reserved. W M A Mullan.
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