Optimization of the Drying Temperature for High Quality Dried Melissa officinalis

Abstract

The drying temperature is one of the main factors affecting the storage of medicinal and aromatic plants (MAPs). The present study aimed to investigate the impact of different drying temperatures (20, 35, 42, and 49 °C) on Mentha officinalis quality attributes (moisture content, color, chlorophyll content) and the composition of its essential oil (EO), as well as the environmental impact, to determine the optimum drying temperature for this herb. According to the current findings, higher temperatures resulted in shorter drying times. However, this was accompanied by increased energy consumption and higher carbon footprint per hour of operation. Both room temperature (20 °C) and high oven temperature (49 °C) led to a darker colored product (i.e., higher browning index). Drying at 20 °C resulted in a higher EO yield compared to drying at higher temperatures (42 and 49 °C). Furthermore, lower temperatures (20 and 35 °C) and the highest temperature (49 °C) significantly decreased the levels of the two major EO compounds (geranial and neral), whereas both compounds were found in higher levels when the plants were dried at 42 °C. On the other hand, plants dried at 42 °C appeared to have the lowest amount of citronellal, significantly lower than those dried at the other tested temperatures. The results suggest that the optimum temperature for drying M. officinalis is at 42 °C, as it maintained the quality attributes of the dried product while also resulting in high quality EO.

1. Introduction

Melissa officinalis (i.e., lemon balm) is a medicinal and aromatic plant (MAP) belonging to the Lamiaceae family. Despite its origins in southern Europe and Asia, it is cultivated worldwide for its culinary and health-promoting properties [1,2]. Lemon balm is most frequently used as an anti-anxiety remedy, but recent research has also explored its effects on the nervous system and neurocognitive processes in cases of depression, insomnia, and stress [3,4,5]. In addition to its use as an infusion/tea, Melissa’s essential oil (EO) is widely utilized in traditional medicine for the treatment of various ailments [6]. The industry employs both fresh and dried MAPs to produce new, natural (less synthetic) and eco-friendly products, including medicines, cosmetics, food and dietary supplements, etc. [5,7,8]. Scientific research focuses on the development of high-quality plant materials [9,10,11] and their derivatives, as well as the preservation of these products [12,13].

Fresh herbs have a relatively high moisture content (up to 75–80%), which must be reduced to below 15% (ideally between 5 and 13% to ensure optimal storage and preservation [14]. The moisture content of dried plant materials should be monitored and kept as low as possible (with a maximum referenced value of 12%) to prevent microbial contamination and deterioration [15]. According to Kunle et al. [16], a low moisture level enhances stability, reducing the risk of product degradation while preserving of important compounds. Additionally, reducing moisture content decreases the weight and volume of the dried product, facilitating packaging and storage while lowering transportation cost [17].

The drying process and conditions are crucial for maintaining the final product’s quality, as they can cause undesirable changes such as alterations in color, texture, flavor, and aroma, as well as reductions in phytochemicals and nutraceuticals [18,19]. The extent of these changes depends on the drying method and parameters used [20,21]. Standard drying techniques, including sun, shade, and oven drying, are commonly applied to MAPs without considering the potential loss of bioactive compounds and the resulting decline in the quality of the final product [22]. Furthermore, the energy consumption associated with the drying process is a critical factor due to its financial implications [23]. Optimizing the balance between dryer design, operational efficiency, energy consumption, and product quality requires significant managerial decision-making [24]. While increasing the drying temperature can accelerate the process, previous studies have reported adverse effects on the dried product and the extracted EO, particularly at temperatures between 50 and 60 °C [22,25]. Research has shown that drying temperatures between 35 and 50 °C can help preserve heat-sensitive compounds [18,26]. For M. officinalis, previous reports show that drying at temperatures higher than 45 °C contributed to color degradation, while plants dried at 30–40 °C resembled the green color of the fresh product [27,28]. Therefore, optimizing drying methods for specific MAPs is essential.

This study aimed to examine the effects of different drying temperatures (20, 35, 42, and 49 °C) on the quality attributes (moisture content, color, chlorophyll content) and the EO composition of M. officinalis. Additionally, the environmental impact (energy consumption and carbon footprint) was assessed to determine the optimum drying temperature for this herb.

2. Materials and Methods

2.1. Plant Material and Drying Process

M. officinalis plants were grown on a commercial organic farm in Limassol, Cyprus (34°44′16.53″ N; 32°44′40.86″ E, 427 m above sea level), harvested, and transferred to the laboratory. After sorting and cleaning the plant material, small bundles were prepared. The weight of each bundle was recorded before being placed in an aluminum tray. The samples were then subjected to different drying conditions: in air-ventilated ovens (SANYO convection oven, MOV-212F, SANYO Electric Co. Ltd., Osaka, Japan) at 35 °C, 42 °C, and 49 °C, and at room temperature (20 °C) in a well-aerated laboratory room where the temperature was kept constant. The weight of each bundle was recorded at scheduled intervals (initially every three hours, then every six and twelve hours) until a constant weight was achieved. During every measurement, the plant material was hand-mixed and turned. To ensure equilibrium, the oven was preheated to the designated temperature for at least half an hour before each experiment.

2.2. Moisture Content

For each measuring point, the moisture content was calculated as a percentage (%). The collected data were used to generate graphs with the corresponding curves, while the obtained equations were employed to determine the time required (in hours) to reach a specific moisture content (e.g., 10%). The determination coefficient (R²) for each curve was maximized by fitting the equation and curve to the collected data.

2.3. Energy Consumption, CO₂ Production, and Time Duration

During each experiment, an energy power meter (Energenie ENER007, Bicester, UK) was connected to the drying oven to measure the energy consumption. The amount of energy used and the CO₂ emissions generated during the oven’s operation were computed by recording the energy consumed per day (kWh/day), the power (kW) required, and the total drying time required per day (h/day) as previously reported [29]. Additionally, the carbon footprint and the accumulated energy consumption were calculated [29]; the fact that the CO₂ equivalent emission factor for the electricity in Cyprus is 0.709 kg CO₂/kWh was taken into consideration [30].

2.4. Color and Chlorophyll Content

The color of dried M. officinalis was quantified using a colorimeter (Chroma meter CR400 Konica Minolta, Tokyo, Japan) by measuring the L* (brightness/lightness: 0: black/100: white), a* greenness/redness (−a*: greenness and +a*: redness), and b* (−b*: blueness and +b*: yellowness) values of the CIELAB uniform color space. Based on these values, the hue (h), chroma value (C), color index (CI), and browning index (BI) were calculated [31,32,33]. The determination of the chlorophyll content (i.e., chlorophyll a, chlorophyll b, and total chlorophyll) was performed according to Nagata and Yamashita [34]. Briefly, appropriate amount of dried tissue was homogenized with acetone:hexane (4:6 v/v) and the absorbance of the solution (after sonication for 5 min) was measured at 645 and 663 nm (Multiskan GO, Thermo Fisher Scientific Oy, Vantaa, Finland). The results were expressed as mg of chlorophyll per g of dried weight (mg/g Dw).

2.5. Essential Oil Extraction and Composition

The essential oils were extracted from the dried plants using the hydrodistillation method in a Clevenger apparatus for three hours. The EO yield was calculated as μL of EO per 100 g of dried tissue (v/w on a dry weight basis) and expressed as EO % yield. The extracted EOs were stored at −20 °C in amber glass vials until analysis. The identification of EO components was performed using analytical gas chromatography with a Shimadzu GC2010 gas chromatograph-interfaced Shimadzu GC/MS QP2010 plus mass spectrometer as reported by Chrysargyris et al. [35]. The identification of the EO compounds was carried out by comparing their retention indices relative to n-alkanes (C8–C20), with those found in the literature or with the authentic compounds, available in our laboratory. Additional identification was confirmed by matching the recorded mass spectra of the compounds with those of the NIST08 mass spectra library (GC–MS data system) and other published spectra [36]. Quantitative determination was carried out based on peak area integration, with each compound expressed as a percentage of the total peak area. Components present at concentrations greater than 0.05% are reported in the results tables.

2.6. Statistical Analysis

Data (completely randomized design-CRD) were analyzed by IBM SPSS v.22, for the analysis of variance (one-way ANOVA), and results are shown as mean ± standard error (SE). Duncan’s multiple range tests were conducted for comparing the treatment means at the significance level of p = 0.05. Pairwise metabolites effect correlations were calculated by Pearson’s correlation test using the R program.

3. Results

3.1. Moisture Content

The following equations describe the moisture decrease in Melissa: y = −0.3198x + 83.362 (R² = 0.9726) at 20 °C, y = 164.13 e^−0.067x (R² = 0.9688) at 35 °C, y = 189.19 e^−0.077x (R² = 0.9558) at 42 °C, and y = 101.34 e^−0.082x (R² = 0.9258) at 49 °C (Figure 1). Taking in consideration that the initial moisture content of M. officinalis was 80% and using the obtained equations, the time needs for moisture reduction from 80% to 10% is T_20°C = 229.4 h, T_35°C = 41.8 h, T_42°C = 38.2 h, and T_49°C = 28.2 h (Figure 1).

3.2. Energy Consumption, CO₂ Production, and Time Duration

Table 1. Effects of drying temperature on the energy consumption and CO₂ production per h of oven operation and per kg of dried product during the drying of M. officinalis.

Temperature	kWh/h	kg CO2/h	kWh/kg	kg CO2/kg
20 °C	0.000	0.000	0.00 ± 0.00 d	0.00 ± 00.0 d
35 °C	0.0385	0.030	23.61 ± 0.17 c	18.40 ± 0.13 c
42 °C	0.0905	0.085	52.36 ± 0.40 a	49.18 ± 0.37 a
49 °C	0.1170	0.110	48.60 ± 0.16 b	45.69 ± 0.15 b

Values represent the mean ± standard error (n = 5). Different Latin letters in each column indicate significant differences (p < 0.05).

and Figure 2 present the energy consumption, CO₂ production, and time duration of the drying procedure of M. officinalis under different drying temperatures. Drying of M. officinalis at 42 °C (followed by 49 °C) resulted in higher energy consumption (52.36 and 49.18 kWh/kg, respectively) and CO₂ production (49.18 and 45.69 kg CO₂/kg, respectively) compared to lower drying temperatures (Table 1, Figure 2). Plants dried at 20 °C, without the use of an oven, were considered as the treatment with no energy consumption or carbon footprint related to the examined parameters.

Comparing the different examined temperatures, drying at 42 °C versus 35 °C, consumed 21.7% more energy and produced up to 67.3% more CO₂, but reduced drying time by up to 3.6 h. Drying at 49 °C versus 35 °C, consumed more energy (by up to 5.8%) and produced more CO₂ (by up to 48.3%) but decreased drying time by up to 13.5 h. Interestingly, drying at 49 °C versus 42 °C consumed less energy (by up to −84.1%) and produced less CO₂ (by up to −81.0%), while also reducing drying time by up to 9.9 h (Figure 2).

3.3. Color and Chlorophyll Content

The effects of the drying temperature on M. officinalis color are presented in

Table 2. Effects of drying temperature on M. officinalis color.

Temperature	Hue Angle (°)	Chroma Value	Color Index	Browning Index
20 °C	178.56 ± 0.01	15.00 ± 2.50	−4.40 ± 0.91 a	56.14 ± 1.93 a
35 °C	178.81 ± 0.05	16.30 ± 1.62	−11.01 ± 0.18 b	38.12 ± 5.84 b
42 °C	178.60 ± 0.07	13.98 ± 2.30	−4.76 ± 1.13 a	43.59 ± 4.36 ab
49 °C	178.53 ± 0.01	13.33 ± 1.94	−3.40 ± 0.06 a	55.36 ± 2.38 a

Values represent the mean ± standard error (n = 5). Different Latin letters in each column indicate significant differences (p < 0.05).

. A lower color index was observed with drying at 35 °C compared to all the other temperatures. On the other hand, drying at 20 and 49 °C presented as plant material with a higher browning index compared to the other investigated temperatures.

The effects of the drying temperature on M. officinalis leaf chlorophyll content are shown in

Table 3. Effects of drying temperature on M. officinalis chlorophyll content (chlorophyll a-Chl a, chlorophyll b-Chl b, and total chlorophyll-Total Chl).

Temperature	Chl a (mg/g Dw)	Chl b (mg/g Dw)	Tot Chl (mg/g Dw)
20 °C	0.44 ± 0.06	2.34 ± 0.23 b	2.77 ± 0.17 bc
35 °C	0.57 ± 0.00	3.48 ± 0.16 a	4.05 ± 0.16 ab
42 °C	0.51 ± 0.08	3.66 ± 0.37 a	4.16 ± 0.46 a
49 °C	0.36 ± 0.08	1.54 ± 0.34 b	1.90 ± 0.43 c

Values represent the mean ± standard error (n = 5). Different Latin letters in each column indicate significant differences (p < 0.05).

. Drying at 35 and 42 °C resulted in higher Chl b content compared to 20 and 49 °C. In addition, a higher Total Chl content was observed with drying at 42 °C as to 20 and 49 °C.

3.4. Essential Oil Yield and Composition

The effect of drying temperature on essential oil yield and composition of M. officinalis plants is shown in

Table 4. Effect of the drying temperature on the chemical composition (%) of essential oils of M. officinalis plants (>0.05%).

Compound	RI	20 °C	35 °C	42 °C	49 °C
1-Octen-3-ol	975	0.51 ± 0.00 a	0.00 ± 0.00 b	0.01 ± 0.01 b	0.00 ± 0.00 b
5-Hepten-2-one,6-methyl	983	0.22 ± 0.01 b	0.25 ± 0.01 ab	0.28 ± 0.01 a	0.00 ± 0.00 c
β Myrcene	989	0.06 ± 0.00 a	0.05 ± 0.00 a	0.04 ± 0.00 b	0.00 ± 0.00 c
trans β Ocimene	1046	0.11 ± 0.00 a	0.07 ± 0.01 b	0.07 ± 0.00 b	0.00 ± 0.00 c
Bergamal	1050	0.06 ± 0.00 b	0.06 ± 0.01 b	0.07 ± 0.01 a	0.00 ± 0.00 c
Linalool	1100	0.04 ± 0.00 a	0.01 ± 0.01 b	0.06 ± 0.00 a	0.00 ± 0.00 b
cis Rose oxide	1109	0.02 ± 0.00 b	0.06 ± 0.01 a	0.02 ± 0.02 b	0.00 ± 0.00 c
exo Isocitral	1142	0.04 ± 0.00 a	0.03 ± 0.00 b	0.03 ± 0.00 b	0.00 ± 0.00 c
neo Isopulegone	1148	0.17 ± 0.00 a	0.08 ± 0.01 c	0.13 ± 0.00 b	0.00 ± 0.00 d
Citronellal	1153	33.32 ± 0.08 a	27.51 ± 0.10 c	17.86 ± 0.09 d	31.33 ± 0.08 b
Z Isocitral	1162	0.45 ± 0.00 a	0.46 ± 0.02 a	0.42 ± 0.08 a	0.06 ± 0.01 b
Rosefuran epoxide	1173	0.03 ± 0.00 c	0.07 ± 0.01 b	0.10 ± 0.01 a	0.00 ± 0.00 d
E Isocitral	1180	0.76 ± 0.00 b	0.90 ± 0.02 a	0.87 ± 0.01 a	0.44 ± 0.01 c
Neral	1242	23.26 ± 0.03 c	25.63 ± 0.04 b	29.66 ± 0.01 a	23.07 ± 0.10 c
Carvone	1244	0.61 ± 0.01 a	0.11 ± 0.03 c	0.67 ± 0.01 a	0.37 ± 0.05 b
Methyl citronellate	1259	0.98 ± 0.02 a	1.01 ± 0.03 a	0.44 ± 0.01 b	0.35 ± 0.06 b
Geranial	1271	36.30 ± 0.00 d	39.18 ± 0.08 b	45.23 ± 0.05 a	38.47 ± 0.13 c
Thymol	1292	0.01 ± 0.00 d	1.44 ± 0.05 b	0.94 ± 0.01 c	2.02 ± 0.15 a
Methyl geranate	1321	0.29 ± 0.01 a	0.24 ± 0.02 b	0.22 ± 0.01 ab	0.26 ± 0.02 ab
iso Dihydrocarveol acetate	1326	0.10 ± 0.00 b	0.00 ± 0.00 c	0.00 ± 0.00 c	0.28 ± 0.05 a
Geranyl acetate	1381	0.23 ± 0.01 c	0.42 ± 0.01 b	0.49 ± 0.01 a	0.11 ± 0.01 d
β Caryophyllene	1425	1.95 ± 0.02 a	1.72 ± 0.02 b	1.35 ± 0.01 c	1.00 ± 0.02 d
Caryophyllene oxide	1587	0.75 ± 0.02 b	0.53 ± 0.03 c	0.84 ± 0.02 a	0.33 ± 0.01 d
Total identified		99.37 ± 0.03	99.45 ± 0.02	99.38 ± 0.01	98.25 ± 0.52
Monoterpenes hydrocarbons		0.05 ± 0.00 a	0.05 ± 0.00 a	0.04 ± 0.00 b	0.00 ± 0.00 c
Oxygenated monoterpenes		93.07 ± 0.03 b	93.52 ± 0.11 ab	94.04 ± 0.02 a	93.79 ± 0.26 a
Sesquiterpenes hydrocarbons		1.96 ± 0.01 a	1.72 ± 0.03 b	1.35 ± 0.00 c	1.07 ± 0.09 d
Oxygenated sesquiterpenes		0.75 ± 0.02 b	0.53 ± 0.03 c	0.84 ± 0.02 a	0.33 ± 0.01 d
Others		4.02 ± 0.02 a	2.06 ± 0.05 c	3.01 ± 0.01 b	1.12 ± 0.21 d

Values are expressed as means ± standard (n = 3). Values in rows for each harvest followed by the same letter are not significantly different, p ≤ 0.05.

and Figure 3, along with the retention indices of each compound. Essential oil analysis identified twenty-three individual compounds, each contributing more than 0.05% to the total oil profile. The majority of these compounds (93.07–94.04%) were oxygenated monoterpenes, with the dominant constituents being geranial (36.30–45.23%), neral (23.07–29.66%), and citronellal (17.86–33.32%), followed by β-caryophyllene (1.00–1.95%). The rest of the compounds were present in amounts below 1% of the total oil profile. Drying at both low (20 and 35 °C) and high (49 °C) temperatures significantly reduced the content of the two major compounds of the EO, geranial and neral, which reached their highest concentrations (45.23% and 29.66%, respectively) when plants were dried at 42 °C. At the same time, plants dried at 42 °C appear to have the lowest citronellal content (17.86%), significantly lower than those dried at the other tested temperatures. While 20 °C maximized EO yield, 42 °C optimized the composition of key compounds like geranial and neral. The formation of these compounds is derived from geraniol, which follows two different metabolic pathways, producing either geranial and neral or citronellal. The remaining identified compounds were also influenced by drying temperatures; however, since they were present in low amounts (<1%), the effect was likely less pronounced. As for the EO yield, it was relatively low compared to other MAP species, ranging from 0.03 to 0.10%. The highest EO yield was observed in Melissa plants dried at 20 °C (Figure 3).

3.5. Correlation Between Measured Parameters

The correlation matrix of plant physiological traits, essential oil yield, and main components is presented in Figure 4. Among parameters that were activated, a positive correlation was found between chlorophylls content and neral and geranial content, while the above parameters were negatively correlated with browning index, color index, and citronellal content.

Figure 5 illustrates the heat map based on the relative expression of plant physiological traits, essential oil yield, and main EO components of M. officinalis subjected to different drying temperatures. The increase in drying temperature from 20 °C up to 42 °C increased the chlorophylls content. Relevant changes in the EO profile were more profound at 42 °C, with stimulation of neral and geranial and suppression of citronellal. Thymol was also stimulated at the highest tested temperature of 49 °C, whereas β-caryophyllene was decreased at that temperature.

4. Discussion

Drying is an ancient physical process essential for the preservation of MAPs, their direct application in the industry, and the preparation of plant extracts for future use. It is well established in industry that the drying process and its variables (e.g., temperature, relative humidity, and air velocity) significantly influence the final product’s quality, affecting the appearance, color, texture, and flavor of the dried plant material [18,37]. Furthermore, improper drying can lead to loss of vital phytochemicals, resulting in low-nutritional-value products [26]. Each plant material exhibits a unique behavior, so drying procedures must be carefully assessed to establish optimal and customized drying conditions [37]. The drying time for MAPs and other plant materials depends on temperature and relative humidity throughout the drying process, as well as product volume, initial moisture content, and other physicochemical factors [18,25]. Improper drying can also lead to the degradation or loss of the nutraceutical components, resulting in a final product with reduced biological and nutritional value [26,37,38,39,40].

The primary goals of the drying process are moisture removal and the elimination of enzymatic and degradative activities. Plant tissues contain two different types of moisture: bound and unbounded [37]. While unbound water evaporates from the plant surface during drying, bound water must also be removed to ensure a stable, dried product. Fresh MAPs typically have high moisture content (75–80%), whereas certain Mediterranean semi-arid species range between 55 and 60% [41]. To ensure proper preservation and storage, the moisture content of dried MAPs must be reduced to below 15%, typically between 5 and 13% [41]. The present study confirmed that higher drying temperatures result in shorter drying times. These findings align with previous research, which supports the inverse relationship between drying time and temperature [25,42]. However, this was accompanied by increased energy consumption and higher carbon footprint per h of oven operation. On the other hand, when one looks at the overall energy consumption of the whole process, higher temperatures (i.e., 49 °C) might present lower energy consumption and CO₂ production due to shorter operation time. It is also worth mentioning the importance of determining the CO₂ equivalent emission factors in each country when investigating the environmental footprint of such processes. For the case of Cyprus, the CO₂ equivalent emission factor for electricity is higher than the one for fuel oil (0.709 and 0.616 kg CO₂/kWh, respectively) [30]. While high drying temperatures accelerate water removal, they are also linked to the degradation or even loss of vital phytochemicals and leaf pigments [18,22]. Additionally, an increase in drying temperature raises energy demands, as was observed in this study. Notably, prolonged drying times have been associated with both increased energy consumption and quality degradation. Since drying temperature affects both final product quality and process’s environmental impact, selecting the optimal drying temperature is crucial from both an economic and sustainability perspective [29,43].

The drying process should preserve the organoleptic qualities of the fresh plant material, particularly aroma and color, which influence consumer acceptance and marketability. Leaf pigment degradation is the primary cause of color loss in dried plant materials [19]. Studies suggest that applying high temperatures for a short period through convection drying may help maintain the plant’s natural color [22]. In the current study, drying M. officinalis at both room temperature (20 °C) and high oven temperature (49 °C) resulted in darker-colored products (i.e., higher browning index). A moderate drying temperature with a steady drying could help preserve the color, chlorophyll content, and nutraceutical compounds of the final product [22].

The main constituents of M. officinalis include hydroxycinnamic acid derivatives, such as caffeic acid, chlorogenic acid, metrilic acid, and rosmarinic acid [3,4,5]. It is important to note that M. officinalis with preserved EO content is the best suited for tea and infusion preparation [6,44]. Although higher drying temperatures accelerate the process, they may also lead to the degradation of nutraceutical compounds [20]. Water evaporation and diffusion occur both internally within the plant tissue and externally during the drying process. Water migrating from the inner plant tissue can facilitate the translocation of EO particles and other volatile compounds to the surface, which may lead to reduced aroma, lower EO yield, and diminished biological properties [45,46]. A higher EO yield was observed in lemon balm plants dried at room temperature compared to those dried at higher temperatures (42 and 49 °C). The concentrations of the two primary EO components, geranial and neral, were significantly reduced at both lower temperatures (20 and 35 °C) and higher temperatures (49 °C), whereas higher levels of these compounds were found in EOs extracted from plants dried at 42 °C. In contrast, citronellal levels were lowest in plants dried at 42 °C, differing from those dried at other tested temperatures. The formation of these key constituents originates from geraniol, which follows two distinct metabolic pathways, leading to synthesis of neral and geranial or citronellal. It is important to note that MAPs from the Lamiaceae family (e.g., spearmint, lemon balm, and thyme) store their EOs in glandular trichomes on the leaf surface, making their volatile compounds highly susceptible to evaporation during drying [25]. Previous studies indicate that drying temperature, plant material features, drying method, and total process duration significantly affect EO yield and composition [25,46,47]. Preserving the integrity of glandular trichomes or minimizing their damage during drying can help maintain EO yield and aroma [19].

In the present study, a positive correlation was observed between chlorophyll content and neral–geranial levels, whereas these parameters were negatively correlated with browning index, color index, and citronellal content. Similarly, previous research found a correlation between chlorophyll content and EO yield/composition in Mentha aquatica [48]. Preservation of chlorophyll content may indicate better cell wall integrity and controlled water loss, which, in turn, helps retain volatile compounds and prevents excessive EO loss during the drying process. As it seems from the current study, drying at 42 °C appears to be the optimum temperature for drying M. officinalis, as it contributes to the preservation of the product’s green color and chlorophyll content, as well as the key compounds of its EO, with shorter drying operation time. This could be attributed to the lower thermal stress (compared to 49 °C), which allows for the rapid but steady release of moisture, avoiding the loss of heat-sensitive vapor EO components [49]. In addition, the rapid drying process at 42 °C (as opposed to 20 °C) could also have contributed to the inhibition of enzymatic activities that result in the formation of brown-colored compounds such as o-quinones, leading to a dark-colored dried product [28,50].

5. Conclusions

The storage stability and quality of dried MAPs are primarily influenced by drying temperature, along with other parameters. This study investigated the impact of different drying temperatures (20, 35, 42, and 49 °C) on the quality characteristic and essential oil composition of M. officinalis. Additionally, the environmental impact of the drying process was evaluated to determine the optimal drying temperature for this herb. The results show that higher drying temperatures resulted in shorter drying times accompanied by increased energy consumption and higher carbon footprint per hour of operation. Drying at room temperature (20 °C) and high oven temperature (49 °C) resulted in a darker-colored product (i.e., a higher browning index). M. officinalis dried at room temperature (20 °C) presented higher EO yield, whereas drying at high temperatures (42 and 49 °C) reduced EO yield. Furthermore, geranial and neral levels were significantly reduced at both lower (20 and 35 °C) and higher (49 °C) drying temperatures. In contrast, EO from plants dried at 42 °C contained the highest levels of geranial and neral. Based on these observations, the optimal drying temperature suggested for M. officinalis is 42 °C, as it balances product quality, EO composition, and overall drying efficiency while minimizing negative environmental impacts. However, producers prioritizing EO yield may prefer 20 °C despite longer drying times.