Q. Wine decanters have become a popular gift for a Shabbos or Yom Tov table. The reason to use this specialized bottles is to decant an aerate the wine. (Many young wines can be tight or closed on the nose or palate. As the wine is slowly poured from the bottle to the decanter it takes in oxygen, which helps open up the aromas and flavors). These bottles, for aeration purposes usually don’t have stoppers or covers, as that would defeat the purpose. What is the amount of time that one can leave the wine in these bottles uncovered and it will not become prohibited as megule or uncovered?
If one covers the opening of the bottle with a fiber or plastic mesh or net, how big can the spaces on the material be so it will not be considered megule?
A. On question 4 regarding a soda can left open in a refrigerator overnight, we wrote; “The prohibition of “Gilui” or uncovered is a rabbinic injunction mentioned in Mishnayot Terumoth 8:4, Talmud Chulin 9b, Avodah Zarah 30, and other places, and was instituted as a protection for fear that a snake drank from the liquid while it was unguarded and cast its venom into it. This prohibition applies to wine, water and milk that were left uncovered without supervision long enough for a snake to come out of a hole nearby, drink from it and return to its hole unobserved. The sages issued this prohibition based on the Biblical command of “Greatly beware for you soul’ (Devarim 4:9). The Gemara in Chulin (ibid.) adds that a prohibition instituted because of fear of danger has to be treated more strictly than an ordinary prohibition. Tosafot on Avodah Zarah 35a comments that in our days when snakes are not prevalent in settled areas, this prohibition does not apply anymore. Based on the above, Shulchan Aruch, Yoreh Deah 116:1, states that: In our days when snakes are uncommon it is permitted”.
However, Pischei Teshuvah (ibid.) quotes the opinion of the Shelah that a careful person should distance himself from them as they (the Gilui prohibitions) are all mentioned in the Tur. He mentions too that the Gaon of Vilnah was extremely cautious on this prohibition.
P’as Hashulchan2:32, adds that they are other hidden reasons to Gilui besides the one quoted in the Talmud. The Be’er Moishe (Rav Moishe Stern) p. 230, mentions that nowadays only “Yechudei Segulah” are stringent on this prohibitions, yet his own mother was very careful not to drink water that was left uncovered all night, and that in Eretz Yisroel in certain places (where snakes are common) the prohibition still applies. In sefer Shaarei Torath Habais p. 313, after mentioning that the Gaon of Vilnah and the Chazon Ish were stringent, and it is indeed a quality of “Chasidut” to do so, however it is not for everyone, and if someone is unaware of the details of this Halacha, he should not be stringent and discard good usable food or drink, because of the inherent prohibition of  “B’al Tashchis” (destroying usable items) which is more severe.
It should be mentioned that in regards to kiddush on Shabbat, Mishne Berurah 272:3 mentions that wine that was left uncovered for a long period should not be used for Kidush. Not because of the Gilui prohibition as above, but because it is not anymore of the prime and highest quality of wine that should be used preferably for kiddush.” (The difference to what we mentioned above, may be due to the dryness of our wines as opposed to the sweetened sacramental wines common then, in addition to the sulfites, other preservatives and being pasteurized),
Adding to the above, Horav Shlomo Miller Shlit’a suggested that the fiber mesh covering the bottle’s mouth may have spaces of less than about a millimeter wide. Probably, that would further protect the wine from small flying insects, that are Biblically prohibited
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a