The Nazir is somebody who takes a special vow to deprive himself of certain enjoyments for the purpose of drawing closer to G-d. One of the commands which the Torah introduces in its discussion of the Nazir is a prohibition against eating “Zag” – the peels of grapes (Bamidbar 6:4).
The question arises as to why the Torah would go this far in imposing restrictions on the Nazir. It is understandable that a Nazir would be forbidden from drinking wine, which is intoxicating and can thus lead to sin. (Incidentally, the Torah (6:3) forbids a Nazir from drinking “Yayin” and “Shechar.” The Rambam understands that “Yayin” refers to plain wine, whereas “Shechar” refers to a mixture of beverages consisting mostly of wine. Targum Onkelos translates “Yayin” as newly-pressed wine, which has yet to become alcoholic, and “Shechar” as wine which has become alcoholic and is thus intoxicating.) We must wonder, however, why the Torah forbids the Nazir from partaking of grapes, which are not intoxicating and thus do not lead to sin. Intuitively, we might explain that the Torah forbade eating grapes as a safeguard against drinking wine. However, it seems somewhat far-fetched to say that the Torah would go so far as to forbid even the peels of grapes as a safeguard against drinking wine.
The Sefer Ha’hinuch answers by positing that in truth, grapes do, in fact, have the effect of intensifying a person’s Yeser Ha’ra (sinful inclination). Even without intoxication, the properties of grapes themselves can strengthen our negative tendencies. Therefore, although the Torah permits eating grapes, in the case of a Nazir, who seeks to exercise particular care to avoid wrongdoing, the Torah forbids the consumption of wine and even grapes, as part of the Nazir’s effort to cling to the service of G-d.
The Rambam summarizes the prohibitions related to a Nazir’s eating and drinking by listing five commands, forbidding wine, grapes, raisins, a grape’s pits, and a grape’s peel. The Gemara in Masechet Nazir comments that these constitute five separate Biblical prohibitions, such that one who consumed all five products, and was warned about all five prohibitions, is liable to five sets of Malkut.
It should be noted that the prohibition against drinking wine includes drinking vinegar, meaning, wine that went sour. The Torah specifies both “Yayin” (“wine”) and “Hometz Yayin” (“wine vinegar”), but these are in truth two aspects of the same prohibition, forbidding wine, as wine does not lose its status as “wine” even once it becomes vinegar.