We can be certain that Van Eyck was not cavalier in deciding what fruit to hand Eve. He was obsessed with the spiritual language of plants and flowers. From the triple-leaf clovers that carpet the foreground of the lower register (echoing the trinity) to the purity of lily petals trumpeting from Mary’s crown, he was alive to the emblematic potential of every stem and blade that he has planted in his painting. But the deeper significance of the citron is surprisingly elusive. The fact that, since antiquity, it has been prized for its sweet scent, seems unremarkable. That Romans may have added its essence to perfume and attached healing power to its flesh, could be said of many other botanical entities as well. Given the swell of Eve’s stomach, it is tempting to think that an obscure folk preventative for difficult childbirth, requiring pregnant women eat the tip of the fruit, might edge us closer to something significant. But that custom, it seems, likely belongs to the lore of a later era and was probably not what Van Eyck would have had in mind.
Whatever its concealed cultural connotations, the citron is endowed with a glistening inner architecture that Van Eyck would doubtless have found appealing. The lucency of its solar structure when sliced, and the juice geometry of its fragrant flesh, belie the crude coarseness of its exterior. It alone among the nominees for forbidden fruit can generate a substance equivalent to light. Though the citron that Eve holds in her hand may be eternally intact, Van Eyck has ingeniously inserted another at the very centre of his painting, one that we cut into and violate each and every time that we open the panels of his altarpiece. Once recognised for its resemblance to the cross-section of a citron, with its characteristically thick white pith (or, more correctly, albedo), that strange citric sun at the heart of Van Eyck’s painting zests itself into pictorial pertinence. By doing so, the artist has ensured that we participate in the transgression that triggers the fortunate fall necessary for the narrative of his work to work. Remove the severed citron from the heart of his masterpiece and the altarpiece’s entire theological logic breaks down. The grandeur it celebrates is diminished to a non sequitur. The light goes out.
If you would like to comment on this story or anything else you have seen on BBC Culture, head over to our Facebook page or message us on Twitter.
And if you liked this story, sign up for the weekly bbc.com features newsletter, called The Essential List. A handpicked selection of stories from BBC Future, Culture, Worklife and Travel, delivered to your inbox every Friday.