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The Process of Wine Making

The grapes are picked when fully ripened. This is when the sugar level (baume) reaches the correct level, and the fruit reaches its full flavour. Unlike table grapes, the fruit needs to be lacking in moisture to intensify the flavours and sugars. The fruit is harvested by hand picking bunches into buckets and then transferred to larger containers and trucked to the winery or by mechanical harvesters which shake the berries from the clusters.

In modern mechanized wineries the grapes are normally crushed and destemmed by a crusher destemmer which consists of a perforated cylinder containing paddles revolving at 600 to 1200 revolutions per minute. The grape berries fall through the cylinder perforations into a must pump while the stems pass out the other end of the cylinder. In earlier times crushing was carried out by using the feet. Sometimes fruit is not crushed or destemmed at all. This occurs chiefly with Chardonnay and is referred to as whole bunch pressing. Reds are introduced whole into the tanks, which are then closed. The resulting respiration in the fruit, consuming oxygen and producing carbon dioxide, kills the skin cells, which loosens their superpermeability, allowing easy colour extraction. There is also some intracellular respiration of malic acid.

When the juice of white grapes is processed or a white wine is desired, the juice is usually separated from the skins and seeds immediately after crushing. The mass of crushed grapes is referred to as a “must”. When the grapes come out of the crusher they fall into a must pump which then pumps the juice into the wine press. This is a cylinder with an inflatable tube. The grapes are introduced into the cylinder and the tube or membrane of the press is inflated, pressing the grapes against the rotating cylinder sides and forcing the juice out through perforations. The process is designed to be gentle, so the press may take up to 3 hours to complete this process.

White musts are often cloudy and settling is desirable to allow separation of suspended materials. Addition of sulphur and lowering of temperature during settling helps prevent fermentation and allows solids to settle. Pectin splitting enzymes are often added to help settling. Bentonite clay may be added to reduce nitrogen content and facilitate clarification.

The process of alcoholic fermentation requires careful control for the production of quality wines. Requirements include suppression of the growth of undesirable micro-organisms, presence of adequate numbers of desirable yeasts, temperature control, prevention of oxidation and the proper management of the cap of skins floating in the red musts. The desired yeast strain is to be added to a quantity of grape juice and allowed to multiply before adding to the tank of must. This strain would have been chosen for its flavour producing properties. Temperature control during the fermentation process is necessary to facilitate yeast growth, extract flavours and colours from the skins in red must, permit accumulation of desirable by-products and to prevent undue temperature change which could kill off the yeast. Contact with air must be restricted during fermentation. The cap of skins floating on top of the juice in red wine fermentation inhibits flavour and colour extraction and may rise to undesirable high temperatures which may acidify if allowed to become dry. This is avoided by submerging the cap by mixing or pumping 2 or 3 times a day.

Alcoholic fermentation generally ceases when the amount of fermentable sugar available becomes low. About 0.1%. Ferments will not reach this stage if must of high sugar content are fermented, alcohol intolerant yeast strains are used or if fermentation temperature was too high or too low. Fermentation can take from 10 to 30 days. In most cases the major portion of the yeast cells will be in the sediment or lees. Separation of the wine from this sediment is called ‘racking’. The container vessels must be kept full from this time on.

Young wines frequently have a secondary evolution of carbon dioxide, occurring sometime after the completion of alcoholic fermentation. This results from the malolactic fermentation, in which malic acid is broken down into lactic acid and carbon dioxide. Malolactic fermentation is desirable when certain flavour characteristics are required. This is usually in red wines but is also used for barrel fermented Chardonnay. This process can be left to occur naturally or the bacteria can be introduced via inoculation to insure the process happens in a shorter time frame.

Fining: Is an ancient practice in which material is added to aid clarification. The main processes involved are absorption and chemical reaction. Protein and yeast cells are absorbed on fining agents such as bentonite clay or gelatin. Other agents used are egg white, milk powder, casein and pvpp.

Refrigeration aids clarification in several ways. It prevents yeast growth and the evolution of carbon dioxide, which tends to keep the yeast cells suspended. Carbon dioxide is more soluble at low temperatures. A major cause of cloudiness is the slow precipitation of potassium acid tartrate (cream of tarter) as the wine ages. Rapid precipitation is induced by lowering the temperature for 1 or 2 weeks. If the resulting wine is filtered off the tartrate deposit, tartrate precipitation will not usually cause clouding later.


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