Our Gemara has been discussing the case of noten ta’am le-fegam – when a forbidden mixture that is inadvertently added to a food imparts a worsened flavor to it. Although we ordinarily require that a significant amount of permissible food (in many cases 60 times the amount of forbidden food) be mevatel – nullify – the forbidden food that was mixed in, many of the Sages believe that if the additive is noten ta’am le-fegam – if it imparts a worsened flavor to the food – we consider it to be nullified on its own.
The Gemara relates a story in an attempt to clarify this law.
A mouse fell into a barrel of beer and Rav prohibited the beer. Some Rabbis mentioned this in the presence of Rav Sheshet and remarked that Rav evidently was of the opinion that even when it is noten ta’am le-fegam – when it imparts a worsened flavor – nevertheless it is prohibited. Rav Sheshet said to them: Rav certainly maintains elsewhere that when it imparts a worsened flavor it is permitted. Here, however, we have an anomaly since a mouse is something repugnant and people naturally find it impossible to eat. Since the Torah teaches that in such a case it is forbidden, it must come to teach that although it imparts a worsened flavor it is nevertheless prohibited.
Rashi explains that according to Rav Sheshet, there was no need for the Torah to prohibit eating mice, since no one would do it in any case. We therefore infer that the Torah is prohibiting this case even if it is a mixture that is noten ta’am le-fegam. Tosafot argue that there is certainly a need for the Torah to teach this law, since otherwise it would not be clear that eating a creature like a mouse is truly forbidden by the Torah. They explain that there was no need for the Torah to forbid eating a mouse. The fact that the Torah chose to prohibit something that was disgusting is the ḥiddush – the novel concept – which teaches that even in a mixture it will be forbidden.