Two prohibited animal products are dam – blood – and ḥelev – forbidden fats. Not only are those parts of the animal forbidden to eat, but someone who eats them is liable to receive karet – a heavenly death sentence (see Sefer Vayikra 7:23-27).
The Gemara on today’s daf teaches that the prohibition on both blood and forbidden fats remains in force even if the form is changed. Thus, even if blood was processed so that it became solid, or even if the forbidden fat was melted so that it became liquid, nevertheless the same rules apply.
Regarding solidified blood, the Gemara recognizes that the prohibition should remain in force, since now the blood would be considered a food, and the prohibition to eat blood (see, for example, Vayikra 3:17) applies. Forbidden fats in liquid form, however, appear to the Gemara to present a problem, for the Torah specifically forbids eating these fats, and there is no clear source prohibiting them from being drunk. Ultimately, Reish Lakish suggests a source that would include drinking in the prohibition, as well.
Tosafot and others point out that the Gemara’s question appears difficult. The Gemara in Shevuot (23a) quotes a passage regarding the Second Tithe (see 14:23) that commands the farmer to eat “the tithe of your corn, of your wine, and of your oil, and the firstlings of your herd and of your flock,” noting that wine is “eaten.” From this the Gemara concludes that drinking is considered part of the broad activity of “eating.” Why in this case must we search for a source to forbid drinking forbidden fats?
In answer, Tosafot point out that there is a difference between things that are normally drunk, like wine, and things that are normally eaten, but that have been processed so that they are being drunk in a specific case. In the latter situation, perhaps we would have assumed that such drinking is unnatural and cannot be considered “eating.”
The Maharatz Chajes suggests that the question of the Gemara may revolve around a different issue. Generally speaking, the minimum size necessary to consider someone to have eaten is a ka-zayit – the size of an olive – while the minimum size necessary to consider someone to have drunk is a revi’it – one quarter of a log. Once we establish that drinking ḥelev is considered eating it, the amount required would follow the dry measurement, rather than the liquid measurement.