# Optimization of the Drying Temperature for High Quality Dried Melissa officinalis
## 4 Discussion
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 \({}^{\circ}\)C) and high oven temperature (49 \({}^{\circ}\)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 \({}^{\circ}\)C). The concentrations of the two primary EO components, geranial and neral, were significantly reduced at both lower temperatures (20 and 35 \({}^{\circ}\)C) and higher temperatures (49 \({}^{\circ}\)C), whereas higher levels of these compounds were found in EOs extracted from plants dried at 42 \({}^{\circ}\)C. In contrast, citronellal levels were lowest in plants dried at 42 \({}^{\circ}\)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., geparmint, lemon balm, and thymne) 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 aquatic_[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 \({}^{\circ}\)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 \({}^{\circ}\)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 \({}^{\circ}\)C (as opposed to 20 \({}^{\circ}\)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].