I was a little confused throughout the gemara if liquids could be an av hatuma. I had always assumed that food and liquids could only be a rishon l'tumah or lower but I wasn't really sure what that was based on. The thing that bothered me with that is what happens if the food or liquid comes in direct contact with a dead body - is it still only a rishon? Is that possible to just skip a level of tuma? On the other hand, if it really is an av hatuma then why do we always say that food and drink can't make humans and utensils tamei? (I'm not talking about R' Yehuda's opinion in the braisa - I assumed that he must hold that the liquid is an av hatuma and it makes the keilim into a rishon.)
Tosafos (d"h revii) on 17a quotes an imaginary (see Gilyon Hashas who says that he may be referring to the last daf in Eruvin) gemara in nida 28b that says that anything that doesn't have tahara in the mikva can't be an av hatuma. He suggests though that doesn't mean literally that it can't be an av hatuma but just that it can't transmit tuma to humans and utensils. So even though technically it's an av hatuma in that if it touches other food or drinks it makes them into a rishon, it's not a full-fledged av hatuma to make humans and utensils tamei. This pshat is necessary to understand Rashi's explanation of Ravina that the meis made the bread into an av hatuma so the case in the gemara is only talking about a shlishi. Tosafos doesn't like Rashi's pshat though for other reasons so he may understand in the end that food and drinks can never be av hatuma.