Wine is made by the action of yeast fermenting natural fruit sugars into alcohol. Vitis vinifera - wine grapes are the perfect fruit for making wine as they usually contain enough natural sugars, when ripe, to produce an alcohol level that is strong enough to be stable and inhibit spoilage. No other fruit contains enough natural sugars to produce a stable wine. Natural wild yeasts are found everywhere, especially on the outside skins of of fruit, so by simply crushing grapes and allowing the wild yeast to mix with the juice will produce wine. This is how it all began. Other fruits and juices can be used to make wine at home, but sugar must be added to produce enough alcohol to make a stable wine. A stable wine should contain around 12% alcohol. At this level the antiseptic property of alcohol will inhibit the growth of spoilage organisms and the wine is stable. Below this level wine is very susceptible to mold and bacteria. Alcohol levels much higher than 14% will not only prevent the growth of unwanted bacteria but will inhibit wine yeast as well.
Unfortunately many other organisms, along with the wild yeasts, occur everywhere and some of these organism can spoil wine. Acetobacter is a very common bacteria that will turn good wine into bad vinegar. Modern winemaking does not rely on wild yeast to produce wine. Purified yeast strains have been developed for specific wine making requirements. Other techniques are used to prevent unwanted organisms from contaminating the wine.
Home wine makers can benefit from using purified wine yeasts and by preventing spoilage organisms from reaching the wine. Keeping all equipment and containers clean and sanitary is the most important rule for making good wine. Using a closed fermentation system is another technique for making good wine at home. Closed fermentation simply means using a sealed container that blocks both air and any unwanted bad things from contaminating the wine. During the fermentation process yeast converts sugars into alcohol and carbon dioxide gas. It is necessary to have some way to vent off this excess CO2 gas otherwise pressure can build up inside a sealed container and it will overflow making a big mess and may even rupture the container. This is easily avoided by the use of a one-way valve or airlock. These are commonly available at any wine-making supply shop or on the Internet. They work by allowing the excess gas to "bubble" out through a one-way liquid trap while blocking air from entering your closed fermentation container.
White grape wines are typically made by fermenting grape juice that is pressed and strained from crushed grapes while red grape wines are made by fermenting crushed red grape pulp for several days before pressing out the juice. This is done to extract the red pigments which are contained in the grape skins. Home winemakers can use many different fruits and juices for making wine. Home grown berry wines are usually made in the same manner as red grape wine and the wine starts with crushed fruit pulp and after several days of fermentation the pulp is strained off and the juice pressed out. Either way the first fermentation period is called primary fermentation and will be very active - foaming and churning away inside the container. In a week to ten days this active period will slow down considerably as most of the sugars are consumed and converted into alcohol.
A large glass jug makes an excellent closed fermentation system to make wine from fruit juice but a bucket is needed when working with crushed fruit pulp. Just remember not to fill your container too full. Four to six inches of air space is needed to be safe - otherwise the excess foam that is produced during the primary fermentation can overflow your container and will make a big mess. You can attach a large plastic tube called a blow-off hose to a jug as illustrated below to create a giant airlock. This is my method when working with juice.