# Optimization of the Drying Temperature for High Quality Dried Melissa officinalis
## 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,  
Figure 4: Heat map matrices of the correlation between plant physiological traits, essential oil yield, and main components. Each square indicates r (Pearson’s correlation coefficient of a pair of metabolites). Figure 5: Metabolite changes in _Melissa officinalis_. Heat map representing relative expression of plant physiological traits, essential oil yield, and main components elicited in plant tissue following subjection of different drying temperatures as compared to control (air-dried plants at 20 \({}^{\circ}\)C). 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 \({}^{\circ}\)C) might present lower energy consumption and CO\({}_{2}\) production due to shorter operation time. It is also worth mentioning the importance of determining the CO\({}_{2}\) equivalent emission factors in each country when investigating the environmental footprint of such processes. For the case of Cyprus, the CO\({}_{2}\) equivalent emission factor for electricity is higher than the one for fuel oil (0.709 and 0.616 kg CO\({}_{2}\)/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.