Eating Food Cooked with Meat or Wine Vinegar During the Nine Days
Beginning Rosh Chodesh Av until after Tisha B’Av (the Nine Days) the custom of Ashkenazim is not to eat meat or drink wine, in remembrance of the destruction of the Beis Hamikdash. What about a food that was cooked with meat, or a pareve soup that was cooked in a fleishig pot? Can they be consumed? Also, can one eat foods that contain red wine vinegar?
The Mishna Berura (551:63) writes that the common custom is not to eat foods that were cooked together with meat. For example, one should not eat a potato from a fleishig cholent, even though it does not contain actual meat. Since it absorbed from the meat, we refrain from eating it. He adds that one may cook a pareve food in a fleishig pot, even if the pot had been used to cook meat immediately beforehand.
Rama (OC 551:9) writes that using wine vinegar is acceptable during the Nine Days. He explains that wine vinegar does not promote simcha (joy), and was not included in the wine restriction. Wine “vinegar” refers to wine that has fermented to the point where one would not drink it (MB 551:57).