Semi-Sweet wines are not made by adding sugar in dry wines, they are wines that contain natural sugar within them because the winemaker intended to do so. This is achieved by controlling the amount of sugar fermented into wine.
Alcoholic fermentation is carried out with the help of yeast cells. The yeast likes ‘eating’ sugar, in grapes this comes in the form of fructose and glucose. Yeast once introduced in the grape juice will start consuming sugars and transforming them into two main byproducts, alcohol and Carbon dioxide (CO2). If the fermentation is not disturbed and continues doing its job, all the sugars will be consumed and thus our finished wine will be dry.
A dry wine usually contains 0-4 gr of sugar per liter, a semi-dry wine 4-11 gr of sugar /L and a semi-sweet wine 12-45 gr of sugar /L, anything more than this is considered a sweet wine.
Winemakers in order to make semi-sweet wines need to stop the fermentation process at the point where the sugar level is ideal and prevent any future restart of the fermentation.
So how do we stop the fermentation?
The first step is to transfer the wine in another tank. Transferring the wine carefully to another tank without the sediments means we are removing the majority of the dead and active yeast cells.
Once we have transferred our wine we now need to cool it down to approximately 10 degrees in order to slow down and stop the fermentation. Yeast cells find it difficult to ferment in low temperatures and thus by cooling our wine we can stop the fermentation fast and easy.
The next step is to add sulfites. Sulfite is used in almost all wines because of its preservative antimicrobial properties but also because it inhibits yeast cells to restart fermentation. Sulfite is very effective in high concentrations but over a certain level it creates unpleasant off odors. The solution is to add Potassium sorbate which is a salt of sorbic acid. Potassium sorbate disturbs the reproduction cycle of yeast. It is widely used in many food products to control yeast and mold spoilage. The combination of both sulfite and potassium sorbate assure a semi sweet wine will remain semi-sweet even in the most unfavorable conditions. Another way to remove yeast is by simply filtering your wine with specific filters that guarantee removal of all yeast (Absolute membrane filters)
So next time your hear someone wondering how semi- sweet wines are made, step in and amaze them.