The most important step in making wine at home is the fermentation process. Vintner's yeast added to the grapes converts the natural fruit sugars into carbon dioxide. During fermentation carbon dioxide is released into the air and the alcohol remains.The first stage of fermentation lasts from four days to a week. More than two thirds of the fermentation takes place during this time. The secondary stage can last between two to three weeks. Fermentation diminishes each day. Pay close attention to the wine during this process to make sure the fermentation process is complete. Letting the wine sit too long after fermentation can damage your wine.
Extract a sample of your wine using a wine thief during the beginning stage of the primary fermentation process, which takes between four and seven days.
Place your sample wine into a hydrometer jar. Float the hydrometer in the hydrometer jar. The hydrometer will give you a gravity reading. The beginning reading should be approximately 1.090.
Take daily readings with your hydrometer to measure the specific gravity of the liquid. As fermentation progresses, and sugars are converted to alcohol, the density will decrease. Your hydrometer lets you know if the wine has any sugars left to be fermented or if they are gone. When your hydrometer reads .998 or less, you will know that fermentation is complete.