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Wine making

Just like bread, wine relies on the action of yeast to create alcohol and carbon dioxide in a process called fermentation.

Just like bread, wine relies on the action of yeast to create alcohol and carbon dioxide in a process called fermentation. Quite simply, fermentation is the action of yeast on sugar to converting the sugar to alcohol and carbon dioxide. In bread making, the carbon dioxide is the desired by-product to make the dough rise, whereas in wine it is the alcohol that is sought.

Jim Harre, Chair of Judges, New World Wine Awards

Grapes are remarkable in that they are a completely self-contained unit. They have the highest sugar source of any fruit. Ripe wine grapes will have around a quarter of a kilo of dissolved sugar in every litre of grape juice. This is enough to make the finished wine ‘stable’ with about 12 percent alcohol by volume.

Red or white?

Wine grapes have clear juice. The tannins and colour of the grape is in the skin, so it’s possible to make both red and white wine from red grapes by separating the grape skins from the juice before fermentation. In the case of some grapes like the popular Californian planted Zinfandel, wineries create white Zinfandel, blush Zinfandel, rosé Zinfandel and deep red Zinfandel all from the same grape variety.

On the outside of the grape are naturally occurring yeasts that will, in the right conditions, convert the grape juice to wine. Hence grapes are a truly self-contained unit!


After the grapes are harvested, the berries are normally separated from the grape bunch stems. When making white wine, the berries are pressed and the clear juice separated from the skins and transferred into a fermentation container. In New Zealand this is usually a large stainless steel vessel. These will range in size from 5,000 to 100,000 litre volumes and are often refrigerated to control temperature during fermentation.

Yeast is then added or in some cases the naturally occurring wild yeasts are encouraged to commence fermentation. Once the fermentation has finished (the yeast has consumed the available sugar) the wine settles and the clear wine is siphoned off (racked) from the sediment of dead yeast. The wine is then clarified and bottled.

With oak influenced white wine, the fermentation of the grape juice will often take place on oak barrels. During the fermentation the sediment of dead yeast (lees) in the barrel is stirred to impart a textural character in the wine. When the fermentation is finished and all of the sugar has been converted into alcohol and carbon dioxide, the wine is siphoned off, clarified and then bottled.

Red wine is fermented with the grape skins as the colour is contained in the skins. It’s the action of the alcohol that helps extract the colour, flavour and tannins from the skins. During fermentation the carbon dioxide released helps lift the skins to the surface creating a cake of skins. These need to be pushed (plunged) back into the liquid to gain the maximum amount of contact with the wine and extraction of colour and flavour.

After fermentation has finished (all of the available sugar has been converted into alcohol and carbon dioxide) the wine is drained off and the skins are pressed to extract all of the wine. The wine will then be stored in oak barrels. This is usually a mixture of new and used barrels to allow the oak flavour, sometimes referred to as a cedar or pencil wood flavour, to develop. After 12 to 18 months the wine is removed from the barrels, blended together, clarified and then bottled.