The blue agave

Botanists recognise roughly 200 species of agave, and about 136 of them grow in Mexico. Only one may be used to make tequila: Agave tequilana Weber var. azul, the blue agave, first described by the German botanist Frédéric Weber in 1902. Despite its spiky looks it is not a cactus. It is a succulent, distantly related to lilies and amaryllis.

What makes the blue agave valuable is the sugar it stores in its core, mostly as inulin, a chain of fructose molecules. Cooking breaks that chain down into fermentable fructose, which is why every bottle of tequila starts in an oven.

Where it grows

Blue agave grows best at altitude, roughly 1,200 to 2,450 metres (4,000–8,000 ft), in the highlands of western Mexico. Most of it is grown in Jalisco, around the towns of Tequila and Arandas. Since 1974, tequila has been a protected appellation of origin, and today that territory covers 181 municipalities in five states: Jalisco, Guanajuato, Michoacán, Nayarit and Tamaulipas. Only agave grown inside it can become tequila.

The heart of that territory is a World Heritage Site. In 2006 UNESCO inscribed the Agave Landscape and Ancient Industrial Facilities of Tequila: some 35,000 hectares of blue agave fields, the towns of Tequila, Amatitán, El Arenal and Magdalena, their old distilleries, and the remains of the Teuchitlán culture that farmed the valley between AD 200 and 900.

Where the agave grows changes the spirit. Lowland agave from the valley around Tequila tends to be smaller and earthier; highland agave from the red soils of Los Altos grows larger and is often described as sweeter and more floral.

The plant

A mature blue agave stands around 1.2 to 1.8 metres tall and can spread 2 to 3.5 metres across, with stiff, blue-grey leaves edged in spines. Like most agaves it flowers only once. Near the end of its life it sends up a flowering stalk, the quiote, that can reach 5 metres, and the plant dies after it sets seed. In the wild its yellow flowers are pollinated by the greater long-nosed bat, as well as insects and hummingbirds.

That flowering would drain the sugars from the heart, so growers cut the quiote off as soon as it appears. The plant then keeps its energy, and its sugar, in the heart (piña), which is what the distillery wants. The catch is that cultivated agave almost never flowers, so it is almost never pollinated, and the crop has to be propagated another way.