It is a widely held misconception that what makes a wine kosher is the blessing of a Rabbi. What actually makes a wine kosher has to do with the ingredients and equipment used to make the wine and who handles the wine.
There are three requirements for a wine to be considered kosher. First, orthodox Sabbath-observant Jews must handle the entire winemaking process under the supervision of a Rabbi, starting with the growing of the grapes all the way through bottling. Second, any ingredients used in the winemaking process must be kosher. Third, the equipment used to make and store the wine must be exclusively for kosher wine, and strict guidelines pertaining to the cleaning and sterilization of the equipment must be followed.
There are two types of kosher wine: non-mevushal kosher wine and mevushal kosher wine. Non-mevushal kosher wine, which is made using the typical winemaking process, loses its “kosherness” if it is opened or poured by someone who is not Jewish. By contrast, mevushal kosher wine remains kosher regardless of who opens, pours or consumes it. To make mevushal kosher wine requires an additional step during the winemaking process. The juice resulting from pressing the grapes needs to be brought to the boiling point and then cooled before fermentation begins. Nowadays, flash pasteurization is used to bring the liquid to the boiling point and cool it down within seconds, thus avoiding the negative impact on flavor that would result from boiling. Most kosher wine made and sold in the US is mevushal kosher wine.
Historically, kosher wine in the US has been associated with sweet wine produced from the concord grape (vitis labrusca), but over the past fifteen years there has been a dramatic increase in the quality and variety of kosher wine available. Today, wine-producing regions around the world, including California, France, Spain, Italy, Chile, Australia, New Zealand, South Africa and Israel are producing kosher wine in a broad variety of styles including Chardonnay, Sauvignon Blanc, Riesling, Pinot Noir, Merlot, Cabernet Sauvignon, Malbec, Zinfandel and Bordeaux.