Must one cover the wine bottle during Kiddush and may the wine still be used if it was left open?
One may not recite Kiddush over wine that has been left open/revealed. Nevertheless, if the wine remained open/revealed for only a short amount of time, one does not need to be so stringent to avoid using it, so long as the smell and taste of the wine has not gone off. However, one is not to initially leave the wine open for even a short amount of time. Accordingly, it is best for the wine that is used for Kiddush is to always remain covered, and hence one is to cover the wine bottle as soon as he finishes pouring from it and is not to leave it open even momentarily, and so is the worldly custom. [Likewise, one is to recite Kiddush as soon as the wine is poured into the cup, and is not to pour the wine prior to being ready to say Kiddush.] This applies before, during and after Kiddush and is not limited to only while making Kiddush. Many however are particularly careful in this regard during Kiddush based on an oral [unsubstantiated] tradition of the Baal Shem Tov that the wine is to be covered especially during this time.
Bedieved: In the event that the wine bottle was not covered while making Kiddush, then if it was left open for only a short amount of time, it may be used even initially for another Kiddush.
One may not make Kiddush over wine which was left open for a long time. If, however, it was left open for only a short amount of time it remains valid, although initially it should not be uncovered for even a short amount of time. Accordingly, one is to cover the wine bottle as soon as he finishes pouring from it and is not to leave it open even momentarily, and so is the worldly custom. Likewise, one is to recite Kiddush as soon as the wine is poured into the cup, and is not to pour the wine prior to being ready to say Kiddush. the event that the wine bottle was not covered while making Kiddush, then if it was left open for only a short amount of time, it may be used even initially for another Kiddush.
How long is considered “left open for a long time” which invalidates the wine for Kiddush?
Some write that if the wine was left open for even 3-4 hours it is still valid. Others write it is up to 5-6 hours. Others rule that if it was left open for the entire night it is invalid. Other rule that it is all dependent on the wine and as to whether it’s taste or smell has dissipated with the time that it was left open. Practically, it is only invalid if it was left open throughout the night, or long enough for its smell/taste to dissipate.
 Admur 272/1; Michaber 272/1; Bava Basra 97a
The reason: Although today we are no longer careful against drinking revealed liquids, being that snakes are no longer commonly found, nevertheless [it is not to be used]. [Admur ibid; Michaber ibid; Tur in name of Maharitz Geios] This is due to the obligation to “offer G-d the most pleasant items…” and thus the wine must be fit to serve to princes and aristocrats. [Admur ibid; M”A 272/1; Taz 272/2; M”B 272/3; Bach 272; Reb Nechmia in Baba Basra ibid] The reason why revealed wine is not considered pleasant before Hashem or fit for aristocrats is because a) Leaving it open causes its taste and smell to dissipate. [Admur ibid; M”A ibid] and b) Leaving it open can allow dirt and insects to fall inside. [Divrei Malkiel 4/1; Pesech Hadvir 272/2; See Kaf Hachaim 272/11]
Other reasons: The wine is not to be left open being that it corresponds to the “Yayin Hameshumar” which Hashem will give us in the future and hence the wine used for Kiddush is to be guarded. [Minchas Shabbos 77/18 in name of Toras Chaim Bab Basra 97b “Even a small amount of time one is to be particular”; Brought in Tosefes Shabbos 242/5; Daas Torah 272; Piskeiy Teshuvos 272/2 footnote 9]
 Admur ibid; M”A ibid; Elya Raba 272/3; Chayeh Adam 6/7; Chesed Lealafim 272/1; Kaf Hachaim 272/7
The reason: As even the aristocrats and ministers are not particular in this after the fact [and they still drink the wine that was revealed for only a short period] unless the smell and taste of the wine has gone off [and thus it is no longer considered unfit to be brought to the king]. [Admur ibid; M”A ibid]
 Implication of Admur and M”A ibid; Minchas Shabbos 77/18 in name of Toras Chaim Bab Basra 97b “Even a small amount of time one is to be particular”; Tosefes Shabbos 242/5; Daas Torah 272; Kaf Hachaim 272/7; Az Nidbaru 1/7; Piskeiy Teshuvos 272/2; Orchos Rabbeinu that so was custom of Steipler
Implication of Admur: The following implications from Admur lead one to conclude that initially the wine is not to be uncovered even for a short amount of time: a) Today wine is commonly found, and Admur partially based his allowance on the scarcity of wine back then. [See Az Nidbaru ibid] b) Admur here writes one does not need to be so stringent, thus implying that there is room to initially be stringent. [See Az Nidbaru ibid] c) The aristocrats would only drink it after the fact, implying that initially they would be careful to cover the wine d) Why rely on a leniency if it can be avoided, and Admur was only referring to the law of after the fact when it was already left revealed.
 Az Nidbaru 1/7; Hakashrus 18/16 footnote 72 “The custom is to cover the bottle immediately after pouring out the wine and not to leave it open at all.”; Piskeiy Teshuvos 272/2; Harav Eli Landa in an email correspondence
 Pashut, and so I was told by Harav Eli Landa that the custom is to be always careful and not only during Kiddush.
The reason: According to the reason of “revealed wine” brought in Admur 272/1, there is no difference between before, after or during Kiddush in this regard. Those that are accustomed to only cover the wine while making Kiddush, and leave it open until they begin Kiddush, or after Kiddush, during the meal, seemingly have no basis for their custom, as there is no reason for why the wine must be covered during Kiddush other than due to the reason of “Giluiy.” Making Kiddush in the presence of an open bottle does not make the wine Pagum, and does not cause the wine to be any worse than any other wine, other than the fact it was left open, which is relevant to be before and after Kiddush as well.
 See Reshimos Devarim [Chitrik] 1/9
 As rules Admur 272/1 that momentary openness does not disqualify the wine. Likewise, making Kiddush in the presence of an open bottle, which is considered as if he made Kiddush on the bottle as well [See Admur 271/28] does not make the wine Pagum, and does not cause the wine to be any worse than any other wine, other than the fact it was left open, which is relevant to be before and after Kiddush as well. Thus, one may even initially use this wine for Kiddush even though it was left open during Kiddush and had Kiddush said over it as wine only becomes Pagum when one drinks from it and not when one says Kiddush over it. [See Admur 182/4 and 190/5 and 272/19-20]
 Ketzos Hashulchan 46 footnote 2
 The reason: Admur does not specify any amount of time, although, one can learn from the Seder on Pesach that it is at least 3-4 hours being that the second cup is poured at the beginning of Maggid and not drunk until the end. [Ketzos Hashulchan ibid; See Az Nidbaru 1/7]
 Divrei Malkiel 4/1 that 20 minutes is a short time and it takes 5-6 hours long for the wine’s taste to dissipate; See Az Nidbaru 1/7; Piskeiy Teshuvos 272/2
 Beis Yehuda 53; Ben Ish Chaiy Bereishis 2/25; Kaf Hachaim 272/7; Aruch Hashulchan 272/5;
 Implication of Admur ibid and Chayeh Adam 6/7; Minchas Shabbos 77/18; Piskeiy Teshuvos 272/2