Refer to the scene when Giuseppe Baldini was explaining about the most amazing perfume ever smelled in the Egyptian tombs.
"Now pay attention to what I tell you. Just like a musical chord, a perfume chord contains four essences, or notes, carefully selected for their harmonic affinity. Each perfume contains three chords: the head, the heart and the base, necessitating 12 notes in all. The head chord contains the first impression, lasting a few minutes before giving way to the heart chord, the theme of the perfume, lasting several hours. Finally, the base chord, the trail of the perfume lasting several days. Mind you, the Egyptians believed that one can only create a truly original perfume by adding an extra note, one final essence that will ring out and dominate the others. Legend had it that an amphora was once found in a pharaoh's tomb, and when it was opened, a perfume was released, after all those thousands of years, a perfume of such subtle beauty, and yet such power, that for one single moment every person on earth believed they were in paradise. 12 essences could be identified, but the 13th, the vital one, could never be determined."
He said every accord was identified except for one. The one that wasnt known was the scent of ancient death. Also, why would Jean-Babtiste put the dead girls in the distillation tanks? He was trying to capture the scent of the first girl he killed on accident. That was his 13th note. The smell isnt the decay of a human body, its the scent of death. Its explained pretty clearly if you pay attention to the clues given to you by the story teller.
The actual perfume he created was probably Aventus with his signature 13th note added.