Kitzur Chapter 49:7 - 50:2

RE: Why is wine made mevushal There is the concern of an idolater having the thought of using the wine for worship which would render the wine forbidden for any form of use or benefit for a Jew. Such wine is called yayin nesech. Being that idolaters would not use wine which was mevushal, cooked, as it isn't a premium wine, there is no concern of mevushal wine ever being rendered forbidden. This is why many kosher wines are mevushal. Reply