Solving a Challenge in the Tequila Industry: A New Continuous Rectification Process for Reducing Higher Alcohols and Obtaining Products Within the Official Tequila Standard

Abstract

This work is the first to study the effect of residence time on the volatile composition of distilled fractions of ordinario using a horizontal continuous distiller of our own manufacture. The ordinario used in this research had a high amount of higher alcohols, so its adequate distillation is complicated. The fractions rectified by continuous distillation were compared with those obtained by batch distillation. Five distilled fractions were collected, and their combined volume was subjected to gas chromatography (GC) analysis to determine the principal volatile compounds (furfural, aldehyde, higher alcohols, methanol, and ester contents). A variance analysis of each group of volatile compounds was conducted to evaluate the effect of residence time (2 and 4 h) in continuous distillation compared to batch distillation (4 h). Continuous rectification allowed for obtaining a distillate within the permissible limits specified by the Official Mexican Standard (NOM-006-SCFI-2012). For the continuous 2 h, continuous 4 h, and batch 4 h processes, the higher alcohols, esters, and aldehydes showed a decreasing pattern, while methanol and furfural showed an increasing pattern in relation to the fraction number. An analysis of variance showed no statistically significant differences in terms of the regulated volatile composition (higher alcohols, esters, methanol, and furfural) according to process type (continuous 2 h, continuous 4 h, and batch 4 h), except for aldehydes, which presented differences. This new continuous rectification process increases productivity while reducing the processing time by 50%, keeping the composition and volume of the heart fraction.

1. Introduction

Tequila, the most popular Mexican drink, is recognized worldwide for its tradition and aromatic quality [1]. Tequila is obtained by distilling the fermented juice of the Agave tequilana Weber var. Azul [2], which is grown in the Denomination of Origin territory and is the only variety of agave allowed for tequila production [3].

The appellation of origin covers the central–western region of Mexico and includes the state of Jalisco and some municipalities in Tamaulipas, Nayarit, Michoacán, and Guanajuato [4]. The production process includes four main stages: agave jima, fructan hydrolysis, fermentation, and distillation [2]. A current trend in the tequila industry is the standardization and innovation of processes to increase efficiency, such as the use of autoclaves, selected strains, and new distillation equipment designs [5].

The Official Mexican Standard for Tequila [3] states two tequila types based on the total sugars from the A. tequilana Weber var. Azul used: tequila 100% “agave” is an alcoholic beverage made entirely of sugars derived from the plant, while “tequila” is an equivalent alcoholic beverage made using 49% from other sources. The flavor and aroma of tequila are the product of the complex and numerous mixtures of volatile and non-volatile compounds produced by the biochemical reactions that occur at each stage of the process [6]. Some compounds responsible for flavor and aroma are alcohols, fatty acids, aldehyde esters, terpenes, phenols, lactones, and sulfur [7].

The Official Mexican Standard [3] regulates tequila composition and establishes minimum and maximum permissible concentration limits for volatile compounds as methanol 30–300 mg/100 mL A. A. (Anhydrous Alcohol), aldehydes (0–40 mg/100 mL A. A.), esters (2–250 mg/100 mL A. A.), higher alcohols (20–500 mg/100 mL A. A.) and furfural (0–4 mg/100 mL A. A.), depending on the type of tequila. These compounds, along with a wide variety of other volatile compounds present in lower concentrations, are responsible for the unique flavor characteristics of Tequila that consumers associate with its quality [5]. Alcohols are mainly formed during fermentation from the degradation of sugars, the transamination of amino acids, or the reduction in aldehydes by yeast [8]. On the other hand, ester formation could potentially be associated with the esterification of organic acids in the presence of alcohols [8].

Every step of the elaboration process, of which distillation is a crucial part, also impacts its composition. Distillation is a method of separating substances with different volatilities by evaporation and condensation processes [9]. Distillation allows the separation and concentration of ethanol from fermented must [2].

Owing to centuries worth of experience, the spirits industry can undoubtedly produce high-quality beverages. However, the global market demands new products, safer spirits, lower prices, and even higher quality with positive aromas and minimized defects [9].

The most common systems used in the tequila industry are pot stills and rectification columns. The pot still is considered the earliest form of distilling equipment. It is of the simplest design, consisting of a kettle to hold the fermented wort, a steam coil, and a condenser or a plate heat exchanger. Pot stills are often made of copper, which fixes malodorous volatile sulfur-containing compounds produced during fermentation [2].

A double distillation is usually necessary to obtain tequila. The first is called Destrozamiento, which takes roughly two hours. The product obtained is appointed “Ordinario” and has an ethanol content between 20% and 30% alcohol volume. After 3 or 4 h, a second distillation, known as Rectificación, is required to produce a beverage with an alcohol content of about 55%; this is sometimes referred to as “Tequila Blanco” (Silver Tequila) [10]. The use of batch distillation columns allows for productivity and efficiency obtaining up to 95 percent ethanol recovery. Nevertheless, the tequila produced has less complexity in the content of volatile compounds, which are responsible for flavor [11].

The head volume is usually about 0.5 to 2% of the total volume of the batch fed to the alambic tank; it mainly contains methanol, water, acetaldehyde, isobutanol, isoamyl alcohol, and ethyl acetate [12]. The heart fraction, which is 55–60 percent alcohol volume (% Alc. Vol.), mainly contains ethanol, esters, terpenes, isobutanol, and isoamyl alcohol, organic acids, and other compounds, all of which give tequila its special and unique organoleptic characteristics. The heart fraction size depends on the ethanol content in the ordinario, containing 80–90% of the ethanol from the original ordinario. The tail fraction contains water, methanol, phenylethanol, furfural, acetic acid, and other medium-chain organic acids such as hexanoic and octanoic [4]. In 2016, the Tequila Regulatory Council outlined a sustainability strategy for the agave–tequila production chain with several goals, including minimizing water use and removing steam and energy losses [13]. In addition, in the tequila industry, there is a desire to reduce capital and operational costs while maintaining quality. One of the optimization units may be the process of distillation of enriched fractions, which is characterized by high energy costs and voluminous equipment. These goals can be reached using the proposed new distillation technology. Because of this, new technologies have also been developed to reduce energy consumption in field distillation. A patented technology, referred to as continuous distillation [14], has been shown to be more efficient and economical than the traditional process. However, our continuous horizontal distillation combines distillation columns and the batch distillation process. Based on a study obtaining citrus essential oils, this system made it possible to reduce energy consumption by 50% [15].

The prototype equipment for continuous distillation consists of a horizontal multistage system (number of stages > 2). The pilot prototype is made from stainless steel 304 and has an operational volume of 60 L; it has five stages, which can operate in batch and continuous modes (Figure 1).

The above-mentioned data have motivated the use of this distillation method for potential use in the tequila sector, mainly in the rectification process, with the interest of simultaneously achieving a highly desired aromatic tequila and aiming for increased production and energy savings [15].

One problem in the tequila industry is that, during the traditional batch distillation process, regular manual cuts of different fractions are required—heads, hearts, and tails—from the tequila master or the process engineer. If, for some reason, the separation criterion is not as indicated, the heart fraction may be left with volatile concentrations outside the ranges established by the Official Mexican Standard. Continuous distillation has the advantage that fraction separation is carried out continuously throughout the process; in this way, head fraction separation can be carried out by selecting the necessary heart fractions that lead to a product that meets the Official Mexican Standard [3]. In previous studies, the authors reported that it is possible to obtain distilled fractions within the Official Mexican Standard [3] at a single residence time [15]; however, the effect of residence time on the volatile composition of distilled fractions in horizontal continuous distillation must be studied. Likewise, the shorter the residence time, the smaller the size of the equipment required, with the consequent improvements in its energy consumption.

The goal of this research was as follows: Tequila can be obtained from Ordinario even though it has a high content of higher alcohols utilizing a horizontal continuous distiller by adjusting the residence time (2 h and 4 h) evaluating, thus, the effect of this variable on the volatile composition of distilled fractions. Results were compared with the rectification of ordinario in batch distillation (process time of 4 h).

2. Materials and Methods

2.1. Raw Material

An amount of 1200 L of ordinario with 17.3% Alc. Vol. was acquired by a tequila company in the state of Jalisco. This ordinario had a high amount of higher alcohols, so distillation was a challenge (Table S1, Supplementary Material). The same lot of ordinario was used for the experimental batch and continuous runs.

2.2. Distillation of Ordinario at Pilot Level

The distillation of ordinario at a pilot level for both processes (batch and continuous) was developed using an experimental design unifactorial by triplicate: continuous at a residence time of four hours (continuous 4 h), continuous at a residence time of two hours (continuous 2 h) and batch for four hours (batch 4 h). Each experimental run was conducted in triplicate, giving a total of 9 experimental runs (6 continuous and 3 batch). The response variables evaluated were the volatile compositions of each distillation fraction (fractions 1, 2, 3, and 4). The tails (fraction 5) were not processed because of their high water content. The head cut-off criterion was 0.5% volume or flow of ordinario feed for the batch and continuous processes, respectively.

2.2.1. Horizontal Continuous Distillation Process

The ordinario was continuously fed (F) into the horizontal continuous distiller via a peristaltic pump, as shown in Figure 2; this consisted of five connected distillation stages.

The liquid to be distilled, in this case, ordinario (product of the first distillation), was fed (F) in the first stage (Figure 2) and went through each of the five stages of the equipment until obtaining the waste stream (W). Each one of the stages was separated by screens and contained a heat interchange for heat and evaporating a fraction of the liquid that flows in each stage. In each stage, volatile components were separated from the distillate during the residence time, which was determined by the flow of the feed pump.

The vapors rose in each stage without mixing and were condensed as distillates (D) in each condenser corresponding to each stage. The effluent (W) of the last stage passed through a heat exchanger to preheat the distillate fed (F) into the equipment. The volatile profiles of the five fractions obtained were characterized using gas chromatography [15].

Distillation was carried out manually under the following conditions:

• Residence Time: 4 h (Feed = 250 mL/min) and 2 h (Feed = 500 mL/min).
• Distilled/Feed (D/F) = 0.2.
• During the continuous process, fractions (f1, f2, f3, f4, and f5) were collected once the processes reached a stationary state.

2.2.2. Batch Distillation

In this 4 h process, the ordinario (100 L) was loaded into the batch distillation equipment (alembic) with built-in stainless steel 304 at a volume of 100 L and heated to the boiling point with a heat exchange [12]. At this point, the evaporation started and continued until five fractions were separated: one by the heads, three by the heart, and one by the tails. These conditions allowed for obtaining five fractions with the same alcoholic percentage while the continuous distillation was in operation.

The cut criterion for the head was 0.5% fed volume; for the heart, it was until a distillate accumulated at 55% Alc. Vol. was obtained; and finally, for tails, when 8% Alc. Vol. was obtained in the distillate stream. Batch distillations were conducted in triplicate.

2.3. Alcoholic Content

A portable density meter, DMA-35 (Anton Paar, Graz, Austria), was used to determine alcoholic content. The results are expressed as percent alcohol volume (% Alc. Vol.) at 20 °C, according to the NMX-V-013-NORMEX-2019 [16], described in the Official Mexican Standard [3].

2.4. Volatile Composition Determination

Volatile congeners in distilled fractions and products (accumulated fractions) were determined using an Agilent Technologies 7890B gas chromatograph (Palo Alto, CA, USA) equipped with a flame ionization detector (FID). Separation was performed on an HP-FFAP capillary column (50 m × 200 μm i.d. × 0.33 μm film thickness) using 1.2 mL/min of nitrogen as carrier gas. The temperature oven was initially programmed at 45 °C for 7 min, then increased at a rate of 10 °C/min to 165 °C and held for 2 min. Finally, the temperature was raised to 220 °C at a rate of 20 °C/min and held for 3.5 min. A volume of 1 μL of samples and standard solutions was injected in split mode (1:50). The injection port and detector temperatures were 230 and 250 °C, respectively. Chromatographic data were acquired using Mass Hunter GC/MS software (B.07.02.1938). Quantification of higher alcohols, methanol, esters, aldehydes, and furfural was performed using calibration curves based on internal standardization of solutions containing known concentrations of the analytes of interest, as described in Mexican Standards NMX-V-004-NORMEX-2018 and NMX-V-005-NORMEX-2018 [17,18], which are referenced in [3]. The analyses were performed in duplicate.

2.5. Statistical Analysis

An analysis of variance by factors and by treatments was carried out with a 95% confidence level, according to Tukey’s method, to determine the statistically significant differences between the volatile compounds in the fractions according to process type (continuous 4 h, continuous 2 h, and batch 4 h) using the statistical software, STATGRAPHICS CENTURION 19.

3. Results and Discussion

3.1. Stationary State

The continuous distillation process was monitored in terms of its distillate flows (Figure 3A), vapor temperatures (Figure 3B), and alcohol content (Figure 3C) at each stage of the process to verify that a steady state had been reached.

In these graphs, it can be observed that, from 120 min, the distillate flows, vapor temperatures, and ethanol contents remain constant; thus, a steady state was obtained both for the 2 h residence time and for the 4 h residence time (Figure S1A–C).

3.2. Alcoholic Concentration of Distilled Fractions

The distilled fractions showed decreased alcohol contents according to fraction number for the two residence times. Fraction 1 demonstrated an average alcohol content of 77.03 ± 1.63 for 2 h and 81.55 ± 1.32% Alc. Vol. for 4 h of continuous horizontal distillation (Figure 4A). This is in accordance with [8], who found a maximal ethanol concentration peak in the head fraction, which then decreased as it was extracted from the alembic during distillation. Statistically significant differences (p < 0.05) were observed for fractions 1, 2, 3, 4, and 5 due to both the effect of residence time (Figure 4A) and the effect of fraction number (Figure 4B) on alcohol content.

The alcohol recovery efficiency was 95% for the batch distillation, 94% for the 4 h continuous process, and 84% for the 2 h continuous process. This value was low because the alcohol content of the tails was high, around 17%. This process can be optimized in order to increase recovery efficiency similar to the batch process; moreover, it is necessary to increase the distillate flow rate of fractions to recover more alcohol.

Regarding energy expenditure, the 4 h continuous process consumed 1.01 kg steam/kg of ordinario, while the 2 h continuous process consumed 0.73 kg steam/kg of ordinario, which is equivalent to 84.16% in the 4 h continuous process compared to the traditional process and to a consumption of 60% in the 2 h continuous process compared to the traditional process, thus having savings of 16% and 40%, respectively.

3.3. Volatile Composition of Fractions

A statistical analysis with 95% confidence showed statistically significant differences in aldehyde content according to process type (Figure S2a). For the 2 h continuous process, the content was higher than for the 4 h batch process. However, no statistically significant differences existed in the higher alcohols, methanol, esters, and furfural contents according to process type (batch 4 h, continuous 4 h, and continuous 2 h) (Figure S2b–e). The statistical analysis with 95% confidence also showed statistically significant differences in the higher alcohols, methanol, aldehydes, esters, and furfural contents according to fraction number (Figure S3a–e). For the continuous 2 h, continuous 4 h, and batch 4 h processes, the higher alcohols, aldehydes, and esters showed a decreasing pattern (Figure S3a,c,d). In contrast, methanol and furfural showed an increasing pattern concerning the fraction number (Figure S3b,e). Statistical analysis showed significance values of 0.0000 for the fraction factor for all compounds considered in the Official Standard (Table S2). Likewise, the significance values for the process type factor were greater than 0.05 for all compounds evaluated, except for aldehydes, whose significance value was 0.0105 (Table S2).

3.3.1. Content of Higher Alcohols and Methanol

Regarding higher alcohols, the statistical analysis showed with 95% confidence (p < 0.05) no statistically significant differences in the content of these compounds according to process type, Figure 5 and Figure S2b. However, statistically significant differences by fraction number were observed (Figure S3a). Furthermore, two homogeneous groups were obtained for the fraction factor, the first formed by fractions 1 and 2 and the second by fractions 3 and 4. The value of fractions 1 and 2 was close to 800 mg/100 mL A. A. (an average content for f1 of 764.35 ± 268.45 mL A. A. and f2 of 797 ± 11.10 mg/100 mL A.A.) at residence time of 4 h; as such, the heads was the sum of the volume of fraction 1 and a percentage of that of fraction 2.

This would present a problem in the batch process because the product obtained would be outside the limits of the Official Standard. However, with the continuous distillation process, it is possible to select fractions or percentages of fractions to generate a product compliant with the Official Standard. The Official Mexican Standard for Tequila establishes the maximum limit permissible for higher alcohols at 500 mg/100 mL A. A. (

Table 1. Volatile profiles for final distillates obtained by continuous distillation.

Compound	Final Distillate, P1 a	Final Distillate, P2 b	NOM-006-SCFI-2012 [3] c Specifications
	mg/100 mL of Anhydrous Alcohol		Minimum	Maximum
Aldehydes	16.18	14.65	0	40
Methanol	264.91	268.30	30	300
Esters	96.22	88.46	2	200
Higher alcohols	461.76	444.82	20	500
Furfural	1.76	1.80	0	4

^a Mean concentration of continuous distillation (∑ fractions: 80% F2, 100% F3, 100% F4). ^b Mean concentration of continuous distillation (∑ fractions: 60% F2, 100% F3, 100% F4). ^c Specifications for major volatile components in Silver Tequila according to the Official Tequila Standard.

). In the case of ordinario processed, the initial content of higher alcohols was greater than 429.21 mg/100 mL A. A. (Table S1). Hence, when processing is by rectification, it was expected that the first fractions would be high, too.

For tequila, the higher alcohols content includes 2-butanol, 1-propanol, 1-butanol, isoamyl alcohol, and 1-pentanol [3]. Of these compounds, the highest concentration corresponds to the isoamyl alcohol in the heads, in this case, f1 and f2 (

Table 2. Chromatographic characterization of tequila fractions obtained via continuous distillation at a residence time of 2 h.

Compound	Fraction 1	Fraction 2	Fraction 3	Fraction 4
	Concentration in mg/100 mL of Anhydrous Alcohol
Acetaldehyde	156.57 ± 2.58 c	36.12 ± 0.78 b	12.50 ± 0.33 ab	3.91 ± 1.26 a
Ethyl acetate	785.79 ± 28.25 b	183.98 ± 5.42 a	36.84 ± 1.22 a	12.24 ± 0.52 a
Methanol	201.01 ± 2.49 a	220.74 ± 1.22 a	254.17 ± 5.11 b	310.98 ± 15.44 c
2-Butanol	39.06 ± 0.45 a	26.64 ± 0.54 a	15.65 ± 0.07 a	8.40 ± 0.19 a
1-Propanol	36.32 ± 45.52 a	198.66 ± 0.97 a	168.72 ± 0.66 a	136.68 ± 3.30 a
1-Butanol	1.14 ± 0.01 b	1.26 ± 0.09 b	1.03 ± 0.01 ab	0.48 ± 0.014 a
Isoamyl alcohol	499.66 ± 14.06 b	455.05 ± 1.98 b	273.40 ± 0.44 a	141.29 ± 6.92 a
1-Pentanol	0.66 ± 0.04 ab	0.34 ± 0.26 a	0.68 ± 0.01 b	1.06 ± 0.11 c
Ethyl lactate	3.59 ± 0.08 a	13.08 ± 0.01 a	22.40 ± 0.74 b	40.28 ± 4.02 c
Furfural	0.74 ± 0.08 a	1.20 ± 0.01 a	1.55 ± 0.03 b	2.41 ± 0.05 c
∑Higher alcohols	576.74 ± 60.08 b	681.94 ± 3.84 b	459.47 ± 1.18 a	287.90 ± 10.54 a
∑Esters	789.34 ± 28.33 b	197.06 ± 5.44 a	59.24 ± 1.95 a	52.53 ± 4.53 a

Values are expressed as the mean ± standard deviation. Different letters in the same line indicate statistically significant differences between fractions at p < 0.05.

). At the same time, the 1-propanol increases its concentration from 36.32 mg/100 mL A. A. in fraction 1 to 136.68 in fraction 4 for the continuous process residence time of 2 h. This is consistent with what has been reported by Nolasco-Cancino et al. (2022) [19], where the higher alcohols come out at the beginning of the rectification in the head fraction despite their high boiling point (e.g., isoamyl alcohol 131 °C), which is probably due to their low solubility in water and their high affinity to ethanol.

Methanol is mainly generated during maguey cooking by the demethoxylation of the pectins present in the agave plants [19]. Keeping the methanol concentration within the permissible limits by Mexican Standards is the main challenge for artisan producers [19]. The Official Mexican Standard for Tequila establishes that the maximum limit permissible for methanol is 300 mg/100 mL A. A. (Table 1).

As illustrated in Figure 6 and Figure S2c, no statistically significant differences by process type were observed in the methanol content with 95% confidence (p < 0.05). On the contrary, the statistical analysis showed significant differences according to fraction number, Figure S3b. In addition, two homogeneous groups were obtained for the fraction number factor, the first formed by fractions 1, 2, and 3 and the second by fraction 4 (Figure S3b). The concentration of methanol increases with fraction number, similar to batch distillation, where the concentration of this compound increases during rectification when the second half of the process begins and rises significantly at the end [12]. The average methanol concentrations in fractions 1, 2, and 3 ranged from 200 to 250 mg/100 mL A. A., while fraction 4 reached an average value of 350 mg/100 mL A. A. Although fraction 4 exceeds the permissible limits of the Official Standard, the mixture of fractions 2, 3, and 4 (equivalent to the heart in a batch distillation) would generate a distillate complying with the maximum limit allowed for this volatile compound. The concentration of methanol depends on its content in the wort, the ordinario content, and the point or moment at which the tails are cut.

Regarding methanol content, this compound was present in all fractions during our distillation process; however, an increase in its concentration was observed, mainly in the tails. This behavior is consistent with that reported for distillation in traditional stills for tequila [12], mezcal [19], and other distilled alcoholic beverages [20]. Some authors explain this fact due to the formation of hydrogen bonds between methanol and water [19] or due to the unlimited mutual solubility between these compounds [20]; however, both explanations cannot be conclusive considering that both phenomena also apply to the ethanol–water system.

It is important to consider that tequila distillation is a complex system; however, a possible explanation for this phenomenon could be found considering the phase equilibrium of a methanol–ethanol–water system where presumably, during the initial phase of the process, the composition of the mixture is in the distillation area where ethanol concentration is high and methanol content is very low which favors greater separation of ethanol because the volatility ratio of ethanol to methanol is greater than 1 in the initial phase and for most of the distillation time (heads and heart). Nevertheless, in the tails distillation phase, the ethanol concentration decreased while the water fraction increased, and the activity coefficient of methanol also increased, favoring its volatility and output in this last stage of the distillation process [21,22]. More precise studies of phase equilibria in complex systems containing azeotropes and polar substances (congeners) at very low concentrations are necessary in order to supply the lack of thermodynamic information required in this regard.

3.3.2. Content of Aldehydes, Esters, and Furfural

In general, the ester contents decreased with fraction number, with fraction 1 showing the highest concentrations in both the 2 h and 4 h continuous distillation processes and the 4 h batch distillation process, as shown in Figure 7. The analysis of variance, with 95% confidence (p < 0.05), showed no statistically significant differences by process type, as indicated in Figure 7 and Figure S2d. On the other hand, the statistical analysis confirmed the statistically significant differences in the ester contents according to fraction number (Figure S3d). Concerning the fraction number factor, two homogeneous groups were obtained. The first was formed by fraction 1 and the second by fractions 2, 3, and 4 (Figure S3d). As shown in Figure 7, fraction 1 showed an average ester contents of 716.96 ± 76.26 mg/100 mL A. A. for continuous 4 h and 789.38 ± 28.33 mg/100 mL A. A. for continuous 2 h. The concentrations of fractions 2, 3, and 4 were within 2–200 mg/100 mL A. A., the range established by the Official Tequila Standard for Silver Tequilas.

The concentration of esters is highest in the first fraction, which is rich in ethyl acetate (Table 2). According to Prado-Ramírez, 2015 [12], ethyl acetate is distilled in the heads in the first half of the process in batch distillation, so only traces remain in the tails. On the other hand, due to its physicochemical properties and boiling point of 154 °C, ethyl lactate behaves completely differently since it is a typical compound in the tails, fractions poorer in ethanol. This behavior is consistent with what has been reported for mezcal distillation, where esters content decreased rapidly at the beginning of distillation, which is attributed to the diminishes in ethyl acetate concentration, while ethyl lactate increases during rectification [19]. The ester content considers the sum of ethyl acetate and ethyl lactate, according to the Official Tequila Standard (Table 2). Therefore, a decrease and/or increase in either of these two major esters will impact the total content of this chemical group of compounds.

The acetaldehyde is the main aldehyde monitored in tequila production. At low concentrations, this compound can give a fruity character to alcoholic beverages; however, at higher concentrations, it can have an adverse effect on the sensory characteristics, causing a pungent smell [19]. The Official Mexican Standard for Tequila establishes a maximum permissible concentration of 40 mg/100 mL A. A. for aldehydes (Table 1).

Statistical analysis showed with 95% confidence (p < 0.05) statistically significant differences in aldehydes content both by fraction number and by process type (continuous 4 h, continuous 2 h, and batch 4 h), Figures S2a and S3c. Regarding the process type factor, two homogeneous groups were obtained: one for the 4 h and 2 h continuous process and another for the 4 h continuous process and the 4 h batch process (Figure S2a). On the other hand, four homogeneous groups were obtained regarding the fraction factor, one for each fraction (Figure S3c). As observed in Figure 8, fraction 1 showed an average aldehydes content of 139.63 ± 0.32 mg/100 mL of A.A. and 156.56 ± 2.58 mg/100 mL of A.A. for the continuous distillations of 4 h and 2 h, respectively.

The aldehyde concentration, referred to as the acetaldehyde concentration in the case of tequila, decreases as the fraction number increases. The above agrees with what was informed by Prado-Ramírez (2015) [12], who confirmed that the acetaldehyde concentration is highest in the first minutes of distillation in the heads fraction due to its low boiling point (20.5 °C) and high solubility in ethanol.

The Official Mexican Standard for Tequila establishes a value of 4 mg/100 mL A. A. as the maximum permissible limit for furfural (Table 1). This compound and other furans, such as 5-(hydroxymethyl) furfural and 5-methyl furfural, are formed primarily during heat treatments to hydrolyze agave fructans [19]. Regarding furfural, the analysis of variance showed, with 95% confidence (p < 0.05), no statistically significant differences in its content according to the process type (Figure 9 and Figure S2e). On the other hand, statistically significant differences were observed with respect to the fraction number (Figure S3e). According to this last figure, three homogeneous groups concerning the fraction factor were obtained from the analysis of the comparison of means. The first is formed by fractions 1 and 2, the second by fraction 3, and the third by fraction 4. Furfural increased with fraction number, with fraction 4 presenting the highest concentration value (Figure 9). Being a low-volatile compound (b.pt. = 162 °C), it is usually present at the end of batch distillation [12].

In fractions 1 and 2, furfural was in the range of 0.74 ± 0.08 to 1.2 mg/100 ± 0.01 mL A. A.; while for fraction 3, the range was 1.26 ± 0.01 to 1.67 ± 0.07 mg/100 mL A. A.; and finally, for fraction 4, the range was 1.92 ± 0.16 to 2.44 ± 0.79 mg/100 mL A. A., (Figure 9). The concentrations of fractions 1, 2, 3, and 4 were within 0–4 mg/100 mL A. A., the range specified for furfural [3].

Nolasco-Cancino et al. (2022) [19] reported a similar behavior in mezcal distillation, where furfural levels increase as ethanol levels decrease because furfural is highly soluble in water and has a high boiling point. Just as cutting the heads is important for the concentration of higher alcohols, cutting the tails is also important for regulating the concentration of methanol and furfural. When methanol and/or furfural concentrations are high, the heart fraction is reduced, and a larger tail fraction is obtained [19].

Once the fractions have been obtained, it is possible to design a product by mixing fractions whose volatile compositions are within the permitted limits or, even more, mixing different proportions of the same to obtain a product within the standard required. In this work, the final distillates were obtained by mixing fractions to generate two product scenarios. Product 1 consisted of 80% of fraction 2 and 100% of fractions 3 and 4, while product 2 contained 60% of fraction 2 and 100% of fractions 3 and 4. These were within the maximum permissible limits regarding methanol, higher alcohols, esters, aldehydes, and furfural contents, as established by the Official Mexican Standard for Tequila [3] (Table 1).

Table 2 presents the chromatographic characterization results of each of the volatile compounds present in the distillate fractions obtained in horizontal continuous distillation for a residence time of 2 h. These fractions, whose volumes are practically equal, can be mixed in multiple ways to comply with the limits established by the Official Mexican Standard and generate volatile profiles of interest for the target market. In the continuous process, 94% and 84% of the alcohol was recovered in the residence times of 2 h and 4 h, respectively, across five fractions—including head, heart, and tails. In the batch processes, 95% of the alcohol was recovered. The product obtained was subjected to a sensory test by untrained personnel, and no differences were detected in the distillates.

Vertical distillation columns operate on the principle of vapor mixing as they rise through the column, enriching the vapor with more volatile components. The condensed liquid that falls is enriched with less volatile components. This results in a more neutral product. Sometimes, tequila obtained from columns is mixed with that obtained from batch distillation to balance the amount of organoleptic compounds since the product obtained from columns has less aroma and flavor than that obtained from batch distillation [2]. However, in the case of continuous horizontal distillation, the vapors from each stage never mix with the other stages, meaning that the type and concentration of the complex mixture of volatile components are preserved at each stage, similar to batch distillation, where the vapors condense over time, giving rise to each of the fractions: heads, heart, and tails. This advantage of continuous horizontal distillation in preserving the volatile profile of the compounds regulated by the Official Mexican Standard, similar to batch distillation, in addition to the benefit of reduced energy consumption, provides elements for its future scaling in the distilled beverage industry such as the tequila industry.

4. Conclusions

The horizontal continuous rectification process represents a good alternative for obtaining Tequila from Ordinario even though it has a high content of higher alcohols, allowing fraction production in accordance with Official Mexican Standard, NOM-006-SCFI-2012 for methanol concentration, higher alcohols, esters, aldehydes, and furfural. Statistical analysis with 95% confidence showed statistically significant differences in the contents of higher alcohols, aldehydes, esters, methanol, and furfural according to fraction number. However, there were no statistically significant differences in the contents of higher alcohols, esters, methanol, and furfural according to process type (continuous 2 h, continuous 4 h, and batch 4 h), except for aldehydes, which did present differences.

It is possible to design final products by adjusting fraction cutoffs and strategically blending the obtained fractions to ensure high alcohol yield, regulatory compliance, and sensory characteristics of fractions produced. It is possible to reduce the residence time of up to 50% (2 h) of the continuous prototype without affecting the chemical quality of the product, obtaining an alcohol recovery of up to 94% with a significant improvement in productivity and energy efficiency compared to the traditional batch distillation method (4 h).