Requirements for being kosher:
Because of wine's special role in many non-Jewish religions, the kashrut laws specify that wine cannot be considered kosher if it might have been used for idolatry. These laws include Yayin Nesekh (יין נסך), wine that has been poured to an idol, and Stam Yainom, wine that has been touched by someone who believes in idolatry or produced by non-Jews. When kosher wine is yayin mevushal (יין מבושל – "cooked" or "boiled"), it becomes unfit for idolatrous use and will keep the status of kosher wine even if subsequently touched by an idolater.
While none of the ingredients that make up wine (alcohol, sugars, acidity and phenols) is considered non-kosher, the kashrut laws involving wine are concerned more with who handles the wine and what they use to make it. For wine to be considered kosher, only Sabbath-observant Jews may handle it, from crushing until the bottles are sealed or the wine is pasteurized, whichever occurs first.
Wine that is described as "kosher for Passover" must have been kept free from contact with chametz. This would include grain, bread and dough.
As mentioned above, when kosher wine is mevushal ("cooked" or "boiled"), it thereby becomes unfit for idolatrous use and will keep the status of kosher wine even if subsequently touched by an idolater. It is not known whence the ancient Jewish authorities derived this claim; there are no records concerning "boiled wine" and its fitness for use in the cults of any of the religions of the peoples surrounding ancient Israel. Indeed, in Orthodox Christianity, it is common to add boiling water to the sacramental wine. Another opinion holds that mevushal wine was not included in the rabbinic edict against drinking wine touched by an idolater simply because such wine was uncommon in those times.
Mevushal wine is frequently used in kosher restaurants and by kosher caterers so as to allow the wine to be handled by non-Jewish or non-observant waiters.
The process of fully boiling a wine kills off most of the fine mold on the grapes, and greatly alters the tannins and flavors of the wine. Therefore, great care is taken to satisfy the legal requirements while exposing the wine to as little heat as necessary. There is significant disagreement between halachic deciders as to the precise temperature a wine much reach to be considered mevushal, ranging from 165°F (74°C) to 194°F (90°C). (At this temperature, the wine is not at a rolling boil, but it is cooking, in the sense that it will evaporate much more quickly than usual.) Cooking at the minimum required temperature reduces some of the damage done to the wine, but still has a substantial effect on quality and aging potential.
Recently, a process called flash pasteurization has come into vogue. This method rapidly heats the wine to the desired temperature and immediately chills it back to room temperature. This process is said to have a minimal effect on flavor, at least to the casual wine drinker.
Irrespective of the method, the pasteurization process must be overseen by mashgichim to ensure the kosher status of the wine. Generally, they will attend the winery to physically tip the fruit into the crush, and operate the pasteurization equipment. Once the wine emerges from the process, it can be handled and aged in the normal fashion.
According to Conservative Judaism:
In the 1960s, the Committee on Jewish Law and Standards approved a responsum ("legal ruling") by Rabbi Israel Silverman on this subject. Silverman noted that some classical Jewish authorities believed that Christians are not considered idolaters, and that their products cannot be considered forbidden in this regard. He also noted that most winemaking in the United States is fully automated. Based on 15th–19th century precedents in the responsa literature, he concluded that wines manufactured by this automated process may not be classified as wine "manufactured bygentiles", and thus are not prohibited by Jewish law. This responsum makes no attempt to change halakhah in any way, but rather argues that most American wine, made in an automated fashion, is already kosher by traditional halakhic standards. Some criticism was later made against this teshuvah, because (a) some wines are not made by automated processes but rather, at least in some steps, by hand, and (b) on rare occasions non-kosher fining ingredients are used in wine preparation. Silverman later retracted his position.
A later responsum on this subject was written by Rabbi Elliot N. Dorff, and also accepted by the CJLS. Dorff noted that not all wines are made by automated processes, and thus the reasoning behind Silverman's responsum was not conclusively reliable in all cases. He explored rabbinic thought on Jewish views of Christians, also finding that most poskim refused to consign Christians to the status of idolater. Dorff then explored the traditional halakhic argument that avoiding such wine would prevent intermarriage. Dorff asserted, however, that those who were strict about the laws of kashrut were not likely to intermarry, and those that did not follow the laws would not care if a wine has a heksher or not. He also noted that a number of non-kosher ingredients may be used in the manufacturing process, including animal blood.
Dorff concluded a number of points including that there is no reason to believe that the production of such wines is conducted as part of pagan (or indeed, any) religious practice. Most wines have absolutely no non-kosher ingredients whatsoever. Some wines use a non-kosher ingredient as part of a fining process, but not as an ingredient in the wine as such. Dorff noted that material from this matter is not intended to infiltrate the wine product. The inclusion of any non-kosher ingredient within the wine occurs by accident, and in such minute quantities that the ingredient is nullified. All wines made in the USA and Canada may be considered kosher, regardless of whether or not their production is subject to rabbinical supervision. Many foods once considered forbidden if produced by non-Jews (such as wheat and oil products) were eventually declared kosher. Based on the above points, Dorff's responsum extends this same ruling to wine and other grape-products.
However, this teshuvah also notes that this is a lenient view. Some Conservative rabbis disagree with it, e.g. Isaac Klein. As such Dorff's teshuvah states that synagogues should hold themselves to a stricter standard so that all in the Jewish community will view the synagogue's kitchen as fully kosher. As such, Conservative synagogues are encouraged to use only wines with a heksher, and preferably wines from Israel