As pizza's popularity spreads around the world, a group of top-secret agents are travelling the globe on espionage missions to determine what "real" pizza is.

On a sweltering day bleached by the fearsome southern Italian sun, a group of international travellers have gathered a stone's throw from Naples' San Gennaro catacombs, named for the city's patron saint.

But these visitors aren't here to venerate the ancient martyr; they've come in service of something equally important to the city's identity. Hailing from Belgium, France, Japan, South Korea, Canada and Brazil, these men and women are all aspiring pizzaioli (traditional Italian pizza makers), and they are about to take the biggest pizza test of their lives.

The trainees are at the headquarters of the Associazione Verace Pizza Napoletana (the "True Neapolitan Pizza Association", or AVPN for short). Founded in 1984, this organisation exists to "promote and protect" an exacting vision of the city's most famous culinary marvel, and was instrumental in inscribing "the art" of Neapolitan pizza-making as a Unesco Intangible Culture Heritage of Humanity several years ago.

From its humble origins as a Neapolitan street food in the late 1800s pizza has become one of the world's most beloved, ubiquitous dishes. Though there are two traditional types of Neapolitan pizza (the Margherita, topped with tomato sauce, mozzarella and fresh basil; and the marinara, which uses oregano and garlic instead of basil and doesn't contain cheese) myriad contemporary varieties have popped up worldwide in recent decades – from slices dressed with blue cheese and honey to the creamy, lemon peel-topped Crisommola del Vesuvio by chef Franco Pepe.

But just as there are strict criteria for determining "authentic" Champagne or Parmigiano cheese, this group of culinary custodians has set out to ensure that the delectable dish stays true to its Neapolitan roots – at least if you're going to call it "real" pizza.

"There is a big connection between this kind of food and the soul of Naples," says Massimo Di Porzio, vice president at the AVPN, who is flecked with flour in his corporate profile photo.

With its training school, competitions, trade fairs and a large bronze pizza statue shining just outside its headquarters the AVPN has become a veritable empire of pizza authenticity. Its lengthy guidelines dictate that all certified pies must consist of a "roundish seasoned disc" with a high-border, puffy crust (cornicione) no taller than 1-2cm. There should be no "big bubbles" or "burned spots". Pizzas must be "soft", "elastic" and foldable. Pizza-makers can't use a rolling pin or baking tray. Cooking a pizza for longer than 90 seconds is sacrilegious. And the final product must be consumed within 10 minutes after emerging from the oven.

On the blistering-hot final day of the AVPN's rigorous monthly training course, the international students will put their newfound pizza knowledge to the test. Attendees have studied dough-leavening techniques and hydration, the ins and outs of yeast, the nuances of picking fresh toppings and ideal salt-to-water ratios. They've practiced the intricacies of placing pizza into ovens – a simple-seeming but deceptively tricky step – all with the goal of baking a consistently perfect pie.

"I was quite nervous, especially as people started coming back from........

© BBC