Regarding the issue of roasting the korban Pesah, the Gemara quotes a baraita which teaches that two sacrifices cannot be roasted together. At first the Gemara suggests that this ruling stems from the fact that the two sacrifices will absorb some of the smell from one another, and someone who had signed up to eat one particular korban (as everyone is obligated to do) will end up eating something of another korban. Based on this understanding, the Gemara proposes that this might be an important statement which could shed light on a general question with regard to cooking – is there significance to the smell that food gives off during cooking or roasting, or not?
This is an argument that appears in the realm of the rules of kashrut.
Rav said: Fatty kosher meat that one roasted in an oven together with lean non-kosher meat is forbidden, even if the two meats never came into contact with one another. What is the reason for this halakha? It is that they are flavored from one another. The fatty meat emits an aroma that is absorbed in the non-kosher meat. The aroma is then transferred back to the kosher meat, causing the kosher meat to absorb some aroma from the non-kosher meat.
Levi argues, claiming that all we are dealing with here is smell, and halakha does not recognize smell as being significant. The Gemara goes so far as to record that Levi applied this and permitted kosher meat that had been broiled in close proximity to pork (referred to by the Gemara as davar aher, “a different thing”).
Our Gemara concludes that the law restricting the roasting of two Pesah sacrifices together stems from a different concern – that perhaps once they are roasted we will not remember which korban belonged to which group. Based on this understanding of the rule, there is no connection between the law as taught in the case of the korban Pesah and the rules of kashrut.
There are areas of halakha where it is clear that smell is considered something of significance. For example, someone who smells the ketoret, the incense made for use in the Temple, would be considered to have derived benefit from kodashim (sanctified items), which is forbidden. The question that our Gemara deals with involves the issue of whether the smell that is absorbed by something permissible is significant enough to forbid that food. The general principle of ta’am ke-ikar, that a change in taste gives food the status of the thing itself, may only apply when the food has actually absorbed the forbidden taste, but with regard to smell, there is less of an actual transfer.