|Wine is made from the fermented juice of grapes. The sugar in the grapes is converted by a neutral yeast into alcohol and Carbon Dioxide. This will contiinue until all the sugars have been converted. In Britain, many grapes have too low a sugar level to achieve satisfactory fermentation, so sugar is added prior to the fermentation. This is called CHAPTALISATION (See notes below) and was invented by the French Minister of Agriculture (Chaptal) in order to use up France's mountain of sugar beet. Britain has a limit to the amount of sugar that can be added, set for cold climates by the European Union.Currently, the EU is trying to restrict the amount of sugar that can be added in order to reduce the amount of over-strength wine|
|Measurements of sweetness of grapes|
After picking, the grapes taken to the winery, de-stemmed and pressed in a variety of presses.(Picture -left) The juice is then clarified by settling or by centrifuge
before being chaptalised . The yeast is then added and the wine left to ferment in tanks (See picture below- right) . It is important that the Carbon Dioxide
is allowed to escape, otherwise the whole lot will explode ! Red wine is given extended contact with the skins so that their colour
may be incorporated in the wine.
When fermentation is finished (or is stopped), the wine can be racked off or left on the 'lees' for a period to soften. It is then racked off into a clean tank to stabilise. It can then be filtered and bottled and left to mature until the wine-maker deems it ready for release.
The process has been especially valuable in Northern latitudes and in areas where the natural sugar content of grapes is low.In the European Union, grape 'must' can be enriched by between 3.5% and 4.5% in alcoholic stength. About 4lbs of sugar is required to raise the alcoholic strengh of 1 hectolitre of wine by 1%.
Grape concentrates -often known as Sussreserve - are also used for enriching or sweetening basic wine. This is made by concentrating grape juice in a vacuum evaporator. It is often added after fermentation - with the added necessity to kill off the yeast first in order to prevent secondary fermentation.
In the early 1990's , the European Union proposed that the enrichment figures be reduced to a range 1.5% to 2.5%, thus increasing the required natural sugar content of the grapes. This would have had the effect of killing off a large proportion of the English (and the German!) wine industry - or forcing producers to sell their product under such a euphemism as 'fermented grape juice' (FGJ?) Fortunately, the proposal was dropped and the English industry continues to chaptalise to a maximum 4.5%.
Must aeration before fermentation is acceptable, but letting air in after fermentation causes problems. Pre-fermentation
air in red wines produces red hues as opposed to a blue-red colour. The early oxidation of anthocyanic pigments causes
colour development and initiates the polymerisation reactions which stabilise colour in red wines.
Don't overdo it though. It is important to submerge the 'cap' of skins to stop it drying out. The surface also gets easily infected by wild yeasts which can spoil the final flavour. With white wine a pumpover of air is usually beneficial when starting fermentation with low pH grapes (i.e. not over-ripe ones.) The process is called hyperoxidation . This is achieved by bubbling air through the must. The initial phase of exponential growth of the yeast requires this oxygen. At the end of the fermentation, sulphite is added to bind the carbonyl groups created during oxidation and to block the reverse reaction which occurs during the reductive phase of fermentation. This process may make the wine smell oxydised.