Tequila is the Mexican distilled beverage par excellence; from a regional consumption and confidential production (i.e. low volume production); since 35 years ago, it has become a renowned beverage exported worldwide. And, despite its ancestral origin, some of the stages of the production process have remained almost unchanged; others have received the contribution of new technologies aimed at optimizing processes.
Agaves -and in particular, Agave tequilana Weber var. Azul-, characterized by a high concentration of sugars in the form of polysaccharides (mainly fructose), are known as fructans or agavines, which constitute the main substrate for the microorganisms involved in fermentation. Currently, the industry uses two processes to recover different sugars: an ancestral, classic process that consists of cooking the agave and then extracting juice with the sugars already hydrolyzed. The second is a process that appeared no more than 15 years ago, which consists of extracting the polysaccharides by leaching the crushed raw agave.
The hydrolysis of the fructans is carried out at a later stage and it may happen that the compounds obtained differ from those acquired by cooking the agave cones, especially in the case of the minority compounds that may give certain distinctive notes to the product. The great advantage of this new process is the reduction in extraction costs, as well as an almost 100% recovery of sugars.
Fermentation, as in all alcoholic beverages, is a key stage in the production of tequila. During this process, the agave's sugars (mainly fructose) are transformed into ethanol (the main alcohol in tequila), carbon dioxide, and other compounds important for the drink's smell and taste. Fermentation is carried out thanks to the action of different microorganisms, particularly yeasts. Like any process that involves the participation of living beings (in this case, microorganisms), the conditions of fermentation influence the characteristics of the final product.
In the industry, this stage has been and, in many cases, still is a kind of black box, to which, the industrialist pays little attention, in spite of its importance.
Development of fermentation in the tequila industry
The tequila industry is divided into two groups, in terms of how this process is carried out: first, the majority of small distilleries, which continue to use spontaneous fermentation (no previously cultivated yeast is added). The second group consists mainly of large industrial companies that use specific pre-selected yeasts, which ferment in a liquid medium, without the presence of fibers. Generally, the fermentations are carried out in open stainless steel tanks of variable volume (between 5,000 and 120,000 liters) - although some companies use wooden tanks, whose volume fluctuates between 1,000 and 5,000 liters - and the development time of the fermentation can vary from 20 hours, in industrial processes, to four or five days in fermentations with fibers. The process is subject, among other variables, to temperature changes, since, in general, this parameter is not controlled throughout the fermentation.
The fermented musts are of two types: only integrated by agave juice - for the elaboration of "100% agave tequila" - or with the addition of other sugars (currently, most companies use fructose corn syrup) for the commonly named mixed tequila. In both cases, it is allowed to add some compounds that favor fermentation; at this point, the most common is to resort to a source of nitrogen. The initial sugar concentration normally ranges from 80 g/l to 140 g/l, which is a relatively low concentration of sugars compared to other fermentations, such as wine. In general, fermentation is considered to be complete when the sugars have been consumed, which is reflected in low and stable Brix degrees (between zero and two Brix degrees). Some companies, after this, still let the fermented musts rest to encourage the formation of other volatile compounds, before passing the musts to distillation.
This is the stage in the process that serves to obtain a rectified distillate, an indispensable requirement, according to the official standard in force. In the traditional distillation process, which uses stills, this rectification is obtained after two "differential" type distillations. In the first - or destruction - the fermented must is distilled in order to obtain a so-called ordinary distillate, whose alcoholic content varies between 20 and 30% by volume. On the other hand, undesirable materials are removed, such as residual sugars, agave fibers, yeast residues, mineral salts, etc.
In the second distillation - or rectification - three fractions are obtained: the first fraction of the distillate is called heads, which is generally recycled by adding it to the fermented must. The second fraction is the rectified tequila itself, with an alcohol content of at least 55% alcohol by volume, and the third fraction is the colas, which can be recycled but are usually discarded. Rectified tequila is then subjected to finishing processes, which are specific to each processing plant. These processes can be: maturing in barrels, diluting with treated water to reduce its alcohol content, mixing with other batches, etc.
The use of stills or continuous distillation columns depends on the physicochemical and sensory characteristics to be obtained, as well as the size of the tequila production line. That is, a still, even though it is an equipment with less energy efficiency and lower productivity (expressed in liters/day, for example), allows to obtain distillates with a high aromatic richness, which translates into more attractive tequilas to the smell and taste of the consumers, which may have more attractive prices in the market for the tequila producer. On the other hand, a continuous distillation column is appropriate for processes that require high productivity and continuity in production.
A third alternative for distillation is the use of stills with separation plates on top of the still. This equipment combines some advantages such as obtaining an alcoholic strength of more than 55% alcohol by volume in a single distillation step and a very attractive aromatic complexity. This distillation technique, called fractional batch distillation, is not well known in the production sector and its advantages have not been appreciated to date.
Another factor involved in defining the aromatic profile of tequila is the construction material used to build the distiller used. It has been determined experimentally that equipment built entirely of stainless steel produces distillates with undesirable sensory notes. In contrast, the use of copper, as a construction material, inhibits the formation of chemical compounds with an outstanding but unpleasant odoriferous effect, which is detected in the distilled fractions immediately during the process.
Tequila quality and composition
Official Tequila Standard NOM-006-SCFI-2012 establishes two categories, according to the proportion of agave sugars used in the must formula to produce: 100% agave tequila and tequila. Due to the characteristics acquired in processes subsequent to distillation, tequila has been classified into five classes or categories: White or Silver, Young or Gold, Reposado, Añejo, and Extra Añejo.
Each stage of the production process has a direct influence on the composition of the product and, consequently, on its quality. In this sense, tequila is constituted by diverse non-volatile and volatile components present in small concentrations that provide the color, texture in mouth, taste, and characteristic aroma that contribute, to a great extent, to the identity and quality of tequila over other distilled beverages.
One of the most studied aspects of tequila has been its volatile composition. Among the most important chemical families that constitute tequila, different alcohols, esters, aldehydes, acids, ketones, furans, lactones, and hydrocarbons, among others, have been described. The different components and their concentration in the drink make up the footprint or volatile profile that characterizes the tequila produced by the different producing companies.
The sensory character of white tequila is defined mainly in the distillation process, since during this process, besides separating and concentrating the ethanol, the relative concentrations of the minor volatile components that also form part of the fermented must are modified. Thus, by separating the heads and tails, during the first and second distillations, the volatile compounds that will essentially dictate the character of the final product remain.
Like other distillates, tequila is also aged in barrels. During its rest in wood, gradual changes are made to the tequila, due to the direct extraction or decomposition of some components of the wood by the ethanol. All the compounds added at this stage have a sensory impact on the spicy and woody character of the product, as well as on its color.