I've finally worked out why the Booker prize judges left Salman Rushdie's delicious historical fantasy The Enchantress of Florence off their shortlist. Ok, so Rushdie writes prose that carries you along as if on a canopied royal boat on a shimmering lake of gold. And it's true he weaves together the worlds of the Mughal court and the Italian Renaissance with a certain beguiling bravado. But there is a terrible culinary howler in his novel.

Rushdie imagines young Niccolò Machiavelli and his friends improvising a song about polenta, the well-known Italian food. In his acknowledgements he reveals this is really a song he made up with Ian McEwan. Well, that's all very cosy, and doubtless they concocted their neo-con plots while they were singing in some well-equipped north London kitchen, but the fact is that Rushie's got it wrong.

Polenta is made from ground maize, and the scene in the book is precisely dated to the early 1480s. But maize is an endemic American crop that wasn't imported to Europe until after Columbus reached the Americas in 1492. It subsequently became a useful food for the Italian poor who for a long time suffered appalling health problems from eating inadequately prepared maize. Polenta developed as a way of getting the stuff down relatively safely and until very recently it was a low status food - in Bernardo Bertolucci's film 1900 striking workers are miserable because they've had nothing to eat for months but polenta.

Some would claim that polenta originates in ancient Roman meal mush and that before maize was brought to Europe it was made with chestnut flour. But Rushdie's obviously talking about the modern stuff.

Evidently one of the judges is a sybaritic food nut who thinks like this. Anyway I can't think of any other reason to leave this excellent novel off the shortlist except perhaps a spiteful delight in cutting a literary titan down to size, or even anxiety about entangling the Booker in Rushdie's difficult relationship with a major world religion. No, it must be the polenta.