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Major Volatile Compounds Analysis Produced from Mezcal Fermentation Using Gas Chromatography Equipped Headspace (GC-HS) 75 2.3 Milling After cooking, the agave piñas are crushed with wood or steel mallets, or in a rudimentary mill using a large stone wheel 1.3 m in diameter and 0.5 m thick. The stone wheel is pulled by animals around a circular pit, crushing the cooked agave. The agave juice and the bagasse are collected and placed in fermentation tanks. Some factories have recently begun to use mills similar to those used in the sugar industry but smaller. In this case, water is added and the bagasse is separated, and only the agave juice is placed in the fermentation tank. 2.4 Fermentation Mezcal factories start fermentation after the agave juice and the bagasse have been put into the fermentation tank; either by adding specific yeast, or by allowing the microorganisms present in the wort to ferment the juice spontaneously. Water is added as the temperature increases or an ethanol smell is detected (the quantity of water in the majority of factories is not precise). When yeast is added the fermentation last between 2 to 4 days, and spontaneous fermentation requires 2 or 3 weeks. The fermentation tanks are open and are fabricated from wood, stone, animal skins, or steel. 2.5 Distilling After fermentation, the wort (agave juice and bagasse) is collected and put into the distillation equipment. Distillation separates ethanol and volatile compounds. The distillation is done in a pot still and rectifying column consisting of a kettle to hold the fermented wort and a condenser or a plate heat exchanger. Some factories use a steam coil to heat the wort, but most heat the kettle in a stove. Mezcal is distilled twice. In the first distillation, the alcohol concentration is between 20 to 40% by volume, after removing the first (heads) and last extraction (tails). The second distillation brings the concentration to 45 to 60 % by volume. 2.6 Aging The maturation step is not mandatory and the white mezcal (silver) may be bottle at 35 to 50% alcohol volume. But, some factories put the mezcal in oak barrels for at least 2 months then the mezcal is called "mezcal reposado", and if the time is more than a year is called "mezcal añejo". The alcoholic degree is adjust to the level wanted. 3. Volatile compounds produced in mezcal The aroma and taste of alcoholic beverages are critical to their acceptance by consumers, and the primary determinants of aroma and taste are volatile compounds. For mezcal, some of the volatile compounds are specified by Mexican laws (SCFI 1997), including methanol, higher alcohols (fusel alcohols), and the compounds that influence volatile acids like acetic acid. These specifications are shown in table 1. Volatile compounds are produced throughout all mezcal production stages. Some derived directly from the raw material, but most of them are produced during fermentation, and, to

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