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.

A commercial wine filter 

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.