Avodah Zarah 68 - Mouse That Fell Into Beer

There was a case when a mouse fell into beer. It stayed there soaking for more than 24 hours, and was therefore considered as if cooked, imparting flavor to the beer. Rav ruled it forbidden.

The student related this to Rav Sheshet and told him that Rav must consider forbidden flavor that ruins the taste as forbidden. "Nor necessarily," - told them Rav Sheshet. In general, Rav may consider such food permitted. Only here, since people are generally disgusted by mice, the Torah, by prohibiting mice, told us to avoid even its flavor. The students told him that they don't see this logic in such cases as semen, and furthermore, Rav Shimi said that in some places they go on the tables of kings.

Therefore, Rava concluded that when a forbidden flavor ruins the taste, the dish remains kosher. Here, however, he was not sure why Rav prohibited the beer. It could be that Rav held that bad flavor made it non-kosher, and then the law would not agree with Rav. It could also be that Rav considered the flavor of mouse improving the beer, and in this case the law about prohibited flavor would agree with him.

Art: Willem Van Aelst - Still-Life with Mouse and Candle