The handling of grapes by non-Jews in relation to kosher laws is primarily concerned with the concept of "Yayin Nesech," which refers to wine that is produced by non-Jews and is prohibited for consumption by Jews unless specific conditions are met. One of these conditions is that if the grapes have been pasteurized, they can be handled by non-Jews, as pasteurization removes their status for concern regarding ritual impurity and the potential for their involvement in idol worship.
Before harvesting, grapes are not considered wine, and thus there is no issue with handling them. Once they are made into wine or used in such a context, however, the rules come into play, which makes the pasteurization process key. Additionally, the Sabbath rules govern many activities, including food preparation and handling, which adds complexity to the situation. Therefore, the ability for non-Jews to handle grapes hinges significantly on whether those grapes have undergone pasteurization, allowing them to sidestep the restrictions associated with wine.