Step 3 of 6
Roasting & milling
The agave hearts are cooked to turn their starches into sugar, then milled to extract the juice.
Raw agave is not sweet enough to ferment. Its sugar is locked up as inulin, so the piñas are cooked first to break it down into fructose. There are three ways of doing this: stone ovens (hornos), autoclaves and diffusers.
Roasting
Stone ovens
The traditional method. The piñas are split, stacked in brick or masonry ovens and steamed slowly, for up to 36 hours at around 90 °C (194 °F), then left to cool. Slow cooking is widely believed to develop more of the cooked-agave flavour that defines traditional tequila.
Autoclaves
Autoclaves are, in effect, giant industrial pressure cookers. Steam under pressure cuts the cooking time to around 18 hours or less, and gives a more consistent result from batch to batch. Both hornos and autoclaves are heated with steam from large boilers called calderas, which also supply heat to the stills. The cooked piñas are then left to cool for 8 to 24 hours before milling.
Diffusers
A diffuser skips the oven altogether. Shredded raw agave passes through a long chamber where hot water and steam, often with acid, wash the sugars out. Diffusers extract 10 to 20% more fermentable sugar than traditional methods, which matters at industrial scale. Many enthusiasts argue that the shortcut costs flavour, and diffuser use has become one of the most debated topics in tequila.
Milling
Once cooked, the agave has to be crushed to separate the sweet juice, sometimes called “cooking honey”, from the fibres, called bagasse.
Tahona
The original method uses a tahona, a stone wheel weighing about two tons, rolled around a circular pit to crush the cooked agave. Mules once pulled it; today it is often driven by a tractor or motor. It is slow and labour intensive, and most distilleries have abandoned it, but some keep a tahona for their premium brands because they believe it makes better tequila.
Roller mills
Most distilleries now use mechanical shredders and roller mills, which tear and press the agave several times. Water is sprayed over the fibres between passes to rinse out as much sugar as possible.
Inulinases
Researchers have also tested enzymes called inulinases as a way to release more fermentable sugar from agave. If adopted, enzymes could reduce the use of acids in large-scale production, though how widely they are used commercially is unclear.