From Mexico City it takes about an hour and a half to get to Jesus Abel Monroy's 13 hectare farm in the state of Hidalgo. The region is mountainous and arid - only mesquite bushes and cacti thrive. Nopal (prickly pear) cacti grow wild or in plantations, with their flat, wide 'ears' and red fruit they make for a delicious vegetable - as well as excellent fences. The organ pipe cacti give the landscape a distinct character, as if one had stepped into José María Velasco painting.

The dry, brittle leaves rustle as Ernesto Rancaño steps into the field ahead of us. A few steps in and he has all but vanished from view among the tall maize plants. Now, at the beginning of December, the fat corn cobs are nearly ready to be harvested. Carefully Rancaño peals the outer layer back to reveal the ripe kernels which are blue, almost black, with a few lighter, purple ones in between. On his one hectare plot he not only grows traditional red, blue and white maize, but also heritage beans, which climb all the way to the top of the maize stalks. The beans inside the well filled pods are huge and have a startling blue colour. Ayocote morado is just one of many Mexican heirloom bean varieties. Normally their colour ranges from lavender to deep shades of purple, ocean blue ones like those in Ernesto Rancaño's field are rare.