Question:
If someone is making kiddush on a cup of grape juice on shabbos morning but does not want to drink most of it, and would rather be yotzei the kiddush with a kizayis of mezonos, can the grape juice be poured back in the bottle and reused later?
Is the person making kiddush allowed to be yotzei with mezonot?
Answer:
In fact, ideally a person should fulfill his “kiddush bmakom seuda” with mezonos. All one is required to drink from kiddush is “mlo lugmav” – a cheekful which amounts to more than half of a Reviis. Only one who does not have mezonos or can’t eat mezonos can fulfill his obligation by drinking a full Reviis of wine.
Sources:
Shulchan Aruch O:C 271:13, 273 MIshna Brura 25.
