What is rested mezcal?

The history and science behind putting mezcal into glass and why it matters 

“Mezcal, the most pure translation of time and space, terroir, and the maestro who translated all of those elements into the spirit, no barrel coming into the ride. There is no other spirit in the world able to achieve that beauty and complexity on its own. There is no dialogue with the barrel but a beautiful monologue.”

Shea Comfort, The Yeast Whisperer

In 2015 I attended my first mezcal trade show in San Francisco, and as I was talking with different reps and trying their mezcals, I suddenly found myself in the position of helping one of them pour mezcal at his table. I quickly learned his mezcal story and helped translate it to those inquiring about it. I remember talking about resting in glass without really knowing the meaning, and the person I was talking to asked skeptically, you can age in barrels but  why  would you use glass, and bury it? Why? I tried to explain but she didn’t seem satisfied. I myself remained curious about this practice and whenever I had the chance, I would choose a rested expression over anything else just to figure out what the special thing was.  Years later, it is great to see these types of mezcales available in the market, but am still curious to understand the science, as well as the origins of this practice in the mezcal world. 

For the people I talked to, it was hard for them to determine a specific time in which this practice started, and because so many aspects of mezcal history are based on oral tradition, I thought maybe every mezcalero would have different memories of how it was kept. In every interview I had, I frequently reminded myself that mezcal consumption was reserved for special occasions and demand did not equal the volume we see nowadays. If resting mezcal was a regular practice,it may have begun as a necessity defined by different factors, many of which were not directly related to the creation of new flavors and textures. 

It wasn’t always glass

During mezcal prohibition, some producers would bury the containers to hide mezcal from the police, and hide them in special rooms. Glass was not always available and the material used to store mezcal varied depending on the region. Commercially speaking, the demand didn’t justify producing large batches. As Miguel Partida from Chacolo recalls, “mezcal was the beverage for the local fiesta, they produced enough for the occasion, let’s say 100 liters and if for some reason, there was mezcal left then they’d keep it for the next celebration; it was not a business.” In Zapotitlán de Vadillo, Jalisco, Partida’s town, mezcal was transported in wooden barrels made by local carpenters. This is how they’d take their production from the mountain into the city, taking a significant loss since each barrel would absorb at least 10 cm of the spirit.

According to Erick Rodríguez, from Pal’ Alma and Almamezcalera, in some regions such as San Luis Potosi, mezcal was stored in large clay pots because it was what they had available. Erick mentioned that glass was used in Puebla long before it was available in Jalisco. To understand this better I looked for dates of when glass began to be used and found a story run by Mexico Desconocido about Puebla’s long blown glass tradition. Antonio de Espinosa, a Spanish artisan, was the first glass maker established in Puebla in 1542 where he made growlers, bell jars and colored glassware. His work started a national glass industry though it depended in most part on imported talent and technology. In fact, most of the accessories used during the 19th century such as demijohns, bottles, containers were brought from Germany. After 1896, with the opening of Corazón de Jesús, Camilo Ávalos Razo would become the most important glass crafter in the country, opening workshops in different states besides Puebla. According to this article, it was around 1935 when the production of glass containers or garrafones for water flourished using the famous green glass, but its production began to decline by the mid 1990s with the introduction of plastic containers.

In Santa Catarina Minas, black clay or barro negro was the only material available to store and transport mezcal. During my conversation with Graciela Ángeles of Real Minero, she reminds me that mezcal has always been a seasonal and complementary activity for the mezcaleros and farmers. She believes that none of the producers in her town stored mezcal for any reason other than having it available to use as a currency to finance the sowing season; this would get them money to buy seeds and supplies to grow their milpa. They needed mezcal year round for wholesale in case they needed money to fund other activities. Most of the people they sold to owned their cántaros and would fill them at every purchase. She remembers her father and grandparents’ collection of cantaros which numbered between 25 to 30 pieces, with capacities of 30-40 liters. Can you imagine a 40 liter cántaro? She tells me they don’t make them in that size anymore and it is getting harder to get the same quality. The traditional gender division of labour is pretty notable in the making of these artesanal products, a cántaro or a large vase with a narrow mouth was made to keep mezcal and was traditionally made by men. A similar vase with a wider mouth, called cántara, was used for water only and made by women. 

The cántaros can only be closed with a piece of olote, or corn cob, but Angeles’s grandparents used a carved piece of quiote instead, a true piece of work. Cantaros are custom made and if used for mezcal, the artisan needs to follow a specific process requiring it to be cooked for a longer period in the oven, with high heat so it is as cooked as possible to avoid leaks. Once it’s made and sold to the mezcalero, he or she cures it with either paraffin or wax spread out all over the exterior and then filled with water and buried in the embers resulting from distillation and left overnight. If it holds the heat, then it can be used to store mezcal. Most of the cantaros don’t survive this process. 

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The question of terminology

What is the best terminology when talking about resting spirit? Is aging only accurate when using barrels, either in wine or whisky? Or is aging the same as maturing and resting? In researching the origin of the concept, I learned about elevage, a French term that describes the process of bringing a wine from its raw state to its final point where it gets bottled. As Maggie Campbell, Head Distiller at Privateer Rum described it on Artisan Spirit, it is the raising, the rearing of something as though it is a child. Either with wine or spirits, elevage is witnessing how a creation of your own grows in a given environment, changing and maturing throughout time. This definition helped me understand how aging or maturing involves a process of ripening that helps harmonize the final product (alcohol) as well as answering part of my question about terminology.

But there is also the question of what that word resting means and how it relates to mezcal. As I look for more examples, I think of food and how some meals need to rest, or sit, in order to develop textures and flavor. This takes me to the times I’ve made my favorite morita salsa and disobeyed my grandmas’ instructions by not waiting for it to come together. As the dish cools and sits over time, the different flavor and aroma compounds mingle together and develop more seasoned notes. The individual flavors are still there, but much less pronounced and the dish is therefore more mellow or rounded in flavor. At the end of the day, the three words in question, resting, aging and maturing, relate because they are all about specific chemical reactions. 

The chemical reaction

A natural way to process a phenomenon we don’t fully understand is by comparing it with something more familiar. That might be the reason why the closest reference to explain mezcal resting is by comparing it with the aging of wine. For both, oxygen and time play a key role in similar but slightly different ways. I was able to talk to Shea Comfort, a fermentation and winemaking consultant who painted a clearer picture for me. Basically, he explained that post-fermentation and in the cellaring phase; for white wine you want to avoid oxygen exposure; and for red wines you want to limit oxygen exposure to very small amounts (mL oxygen/L wine/month!). The wood where wine is put to age is porous and allows the natural transfer of just the right amounts of micro-dosing of oxygen, “the liquid permeates into the barrel, about 6 to 7 ml, then as it travels through the thickness of the wood, it changes into a vapour gas phase. What happens is that there are a lot of beneficial reactions that happen in the middle of the wood stave with the controlled amount of oxygen and the compounds found in the pores of the wood. These reactions create a more textured, better-harmonised wine, that is more complex and interesting to enjoy. The results are quite different to anything rested in inert stainless steel or glass, because you don’t have the beneficial micro-oxidative reactions. As a result, these wines are more narrow, linear, and more simplistic. The same thing actually happens with spirits, the only difference is that when ageing spirits you actually want oxygen exposure in larger amounts”.

Since temperature also has a massive effect on the rate of these reactions, how wine or spirits is stored matters significantly. He explained, “If you have a lot of humidity in the cellar, preferentially more alcohol goes out of the barrel and the proof goes down, if you have a dry cellar then more water goes out of the barrel and the proof goes up in the barrel. In addition, every time a wine barrel is opened up, it has to be filled back up all the way to the top to eliminate the oxygen in the headspace, or you will oxidise and ruin the wine. With spirits, you don’t need to do this and most barrels are allowed to remain at whatever level they have evaporated down to during the ageing process.”

Science with a touch of alchemy

If glass is not a porous surface, then how do these interactions happen? Karina Abad, Director of Production at Los Danzantes explains that once the product is distilled, its different compounds are still changing since the molecules that control the organoleptic profile are dispersed. When putting the distillate to rest, these compounds calm down and start coming together, the fermentation creates flavors and aromas that become softer with maturation. When the spirit is poured into the glass container, a certain headspace is left. Karina explains it allows for an expansion of volatile compounds. Comfort considers this headspace fundamental in the creation of a more edgy profile only if fermentation happened under the right circumstances. 

For David Suro, owner of Siembra Valles and distributor of Don Mateo and Mezonte, the porosity of the olotes (corn cob) or corks used to close the glass containers, allows for a continuous transfer of oxygen. Luis Loya, from Lamata and Nación de las Verdes Matas,  prefers to hermetically close his demijohns with wax or a synthetic cap to avoid oxidation. Karina, on the other hand, explains that these volatile compounds are let go every three months when the garrafones are opened up. This is the personal touch that makes each brand unique, or as comfort puts it, “Mezcaleros worked years to learn so every single decision has a reason and a consequence.”

Both Partida and Angeles believe that resting mezcal is a way of preservation. While Partida thinks glass is the best way to keep the original characteristics of mezcal such as aroma, color and flavor, Angeles is convinced that black clay adds more flavor. “During the first three months of resting, there is a very strong and intense flavor, I guess due to the porosity of clay similar to wood barrels which allows for oxygen transfer.” She adds, “But after a year, the flavors become softer.”  Because of that, last year, Angeles and her family created their own cava of mezcal rested in black clay. Some of the expressions just finished their first year of maturation, and they are adding new ones hoping to have their first batch released in 2022. 

At many of the places that Rodriguez has visited, resting in glass was a rule for these makers so he decided to respect that tradition before putting the different expressions out in the market. The resting time depends on each expression as well as the ingredients he uses for his well known pechugas. “One time we made a mango distillate and we had to leave it resting for about five years before I was satisfied with the flavor profile. The presence of mango made it too aggressive to the palette but time helped creating an exceptional product.” Being fully aware of how the full process of mezcal making happens allows those personal touches to be present and recognizable. “Every now and then, I would release some batches of maguey from Oaxaca that haven’t been rested for long because I personally like them, their flavors and aromas that result from a single distillation.” 

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If time is money, let’s do it faster

Back in 2018, Max Garrone reported on an interesting case of resting mezcal which must have raised some eyebrows about a very specific practice. Ansley Cole from Craft Distillers wanted to send a unique rested expression to the US under the brand Mezcalero but he was aware of the great investment in time and resources this venture represented. Luckily, there was Don Valente, the maestro mezcalero behind Alipús San Andrés, who suggested that in order to accelerate maturation, they could bury the equivalent liters of 120 bottles of mezcal and cover them with a mixture of bagazo and  lamb manure for about six months.

Could this be compared to the “hot and cold cycle” used in whisky? Would burying the mezcal perform the way a cellar does? Would the combination of compost and manure increase temperature and speed up the rate of reactions?  comfort Comfort responded to my inquiries once more by explaining “a lot of whisky producers do the hot-cold cycle when aging a product for 2 or 3 years, for a couple months they will warm up the cellar and others months they will cool it down. Warm is pushing more spirit into the wood and cold is pulling it back to bring the extracts in so you are getting a deeper cycling of reactions and compounds coming in that you would normally have in a traditional kind of way in about 5 or 10 years.”

The mezcal world is still a work in progress which allows those involved in the making process to work in continuous trial and error mode. Partida says they have mezcales that have been resting for up to fifteen years and according to his experience, it is not until the tenth year that the aromas start changing and the intensity of the alcohol is reduced. 

Comfort offers the example of fruit brandies like pear, plum and quince. He says that in the beginning of the ageing process of the spirit you can warm it a little bit to increase the ester reactions for some of the fruit brandies like pear, plum and quince required. “When you first distill them it takes time for the full fruit aromas and flavours to actually develop, and by storing it in a warmer part of the cellar (considering the standard cellar temperature to be 55 to 65 F), a lot of perfume will eventually magically come out of nowhere. This works if you are trying to develop esters, but if storage under warm circumstances goes too long, you may end up ruining the spirit so you definitely need to smell and taste as you go along.”

A traditional almost unaffordable but necessary

The NOM-070 recognizes “mezcal madurado” as a category as long as it has been rested for at least twelve months, whereas destilados de agave are able to put out expressions with different lengths of maturation starting at 3 months and 6 months as minimum. Currently one barrier of entry for brands into this category is the high cost of glass containers. In 2008 when Angeles and her family decided to start using glass, they were able to get 500 garrafones for $50 pesos a piece, in 2021 the price has gone up to $500 pesos if you know where to find them. A quick Google search showed prices as high as $1,800 pesos per garrafon and up to $2,499 per demijohn, both of a 20 liter capacity. Storage availability is fundamental since demijohns take space and need to be handled with care, Luis Loya has containers in all the sizes he can get, from 10 to 15 liters. He recently had a 20 liter glass garrafon with a two year rested mezcal break, which is why he describes storage as a heroic act. 

In addition, the pressure of the market to sell right away is one of the most important reasons why rested mezcal is limited as it requires a more cautious planning and of course time. Brands must come up with a specific criteria to release batches little by little, some of them do it upon demand some others, upon availability. At Chacolo, for the past eight years they have about producing around two thousand liters per year and divide up each production follows: one half of each production is for distribution (30% destined to exports and 20% for national sales) and the other half is stored, so that 30% is kept rested and 20% is held for local direct sales and special celebrations. Interestingly, some of their wild expressions like Ixtero Amarillo and Cimarrón Silvestre are in high demand within their local community, people like it so much that they don’t want it to be rested. These are released only once a year. 

At Real Minero, the production of mezcal depends on maguey availability. Occasionally there are a few batches that aren’t purchased so they are kept to rest. The idea is to keep an inventory of the production of the prior year, for example this year they are selling batches made in 2020 and 2019. If by the end of the year, they still have mezcal from 2019, they will put them away to mature. This process allows them to build a more diverse and interesting cava, as of now, they have different kinds of espadin, tobala and mezclas or ensambles, smaller batches made by her brother Edgar and pechugas from her dad. 

Another barrier might be that many consumers are yet not familiarized with rested mezcales and agave distillates, however this category could be useful in attracting those whose palates are not accustomed to mezcal. David Suro says that a rested mezcal helps people appreciate agave in an easier way. Time is an inherent part in the production of mezcal, from the years needed to get a ripe plant to the weeks required to cook agave, to the time needed for a proper fermentation. The way in which the mezcal world has evolved in the marketplace, is by bringing new expressions and flavors as often as possible to the market. In times when we keep worrying about scarcity of the plants, perhaps going back to basics like patience is what can save us. As mentioned before, resting in glass comes as an alternative for agave preservation and also, as the natural progression of our taste of mezcal. 

List of mezcales rested in glass released in the US. 

Past editions (no longer available)

• Tosba: Dua Warash, a non classified type of agave from Sierra Negra in Oaxaca, similar to rhodacantha. It rested in glass for  about one year and a half. 
• Mezcales de Leyenda: Released a limited edition called Cementerio in 2016, they used agave americana from Michoacán that had been rested for 9 months. They are now helping the same maestro mezcalero launch his new brand call Desentierro

Currently available:

• Mezcalero and Los Danzantes: 
  • They started resting in glass in 2014 releasing their first batch in 2017.
  • They currently have six batches of 300 to 500 liters each. 
  • They use old blown glass containers of 15 to 18 liters
  • The first batch of mezcal rested for four years will be released in 2022.
  • Mezcalero Special De Cabra, Don Valente, Espadín/Bicuishe, 47.4% Alc. Vol.2018 rested in glass for six months 
  • Mezcalero 22 Don Valente, Bicuishe 47.4% Alc. Vol 2015, rested four years in glass
  • Los Nahuales Ed. Sp. 4, Espadín 100%, 49% Alc. Vol. 2013 rested five years in glass. 
• Don Mateo and Siembra Valles
  • Don Mateo is currently transitioning to mature all of their expressions. 
  • Siembra Valles has small batches of 50 to 90 demijohns of 20 liters each. 
  • Siembra Valles Ancestral rested from six to eleven months in glass 
  • Siembra Valles High Proof is rested for 11 months in glass
• Chacolo
  • All of their expressions are matured in glass for at least six months
• Nación de las verdes matas 
  • They use old blown glass garrafones and old demijohns of 10, 15 and 20 liters 
  • They rest all their 14 expressions for at least six monthd.
  • They just exported a batch of Lechuguilla Of six months, A Maguey Verde from 2019 and s Castilla from Durango, 2019
• Real Minero
  • Soon to be released batches from 2006 to 2014 by  Don Lorenzo Ángeles
• Pal’ Alma
  • All 76 expressions are rested in glass. About 450 containers total includes 150 glass demijohns between 3,5, 8 and 18 liters. The Cuarenteno expression is kept in 50 and 60 liter demijohns.
• Macurichos
  • All of their 20 expressions are matured in glass for at least six months. ​​760 garrafones of 19 liters and 40 garrafones between 4 and 6 liters. All are old blown glass.