Introduction: Microwave-assisted extraction (MAE) is one of the newest techniques for extracting essential oils from medicinal plants. Microwave waves penetrate into plant cells and affect the polar molecules, causing heat to build up inside the plant tissue, destroying the cells and releasing the active ingredients. Although in terms of essential oil extraction efficiency, MAE method is slightly higher than Clevenger, but in terms of energy consumption, process time and amount of solvent consumed has an inherent advantage over the Clevenger method. In this study, the aim is to evaluate the quantity and quality of rosemary essential oil in the developed microwave extraction system and process optimization in this system.
 
Materials and Methods: Rosemary plant (Rosmarinus officinalis L.) with 200 g used for each experiment. In this study, a microwave-assisted essential oil extraction system was developed. The system consisted of a home microwave, a distillation condenser, a cold-water source and a pump. In this study, the response surface methodology (RSM) used in the form of a central composite design with 13 treatments. The effect of two independent variables including power in the range of 300 to 900 watts and time in the range of 5 to 35 minutes on the efficiency of essential oil as dependent variables was investigated. To evaluate the quality of the extracted essential oil and compare it with Clevenger treatment, GC-Mass analysis was done using an Agilent technology apparatus (Agilent 7990B, USA).
 
Results and Discussion: A quadratic model was proposed for modelling of extraction efficiency and it was able to model with the values of R², Adj R² and Pred R², which were 0.9521, 0.9180 and 0.574, respectively. The "Pred R²" of 0.547 is not as close to the "Adj R²" of 0.9180 as one might normally expect. The difference is more than 0.2. This may indicate a large block effect or a possible problem with the proposed model and/or data. Things to be considered for model modification are model reduction and response transformation. Results of ANOVA showed that only the linear terms of time, power and quadratic power of time are significant. The normal probability plot showed the normal distribution of errors observed in the experimental design space. The actual measured values R² of the extracting efficiency from the experiments versus the values R² predicted by the model indicated that this model could estimate the extracting efficiency with a correlation coefficient of 0.746. Box-Cox plot demonstrated for obtaining better results, the data of the amount of essential oil efficiency should be transferred to the power of 0.68 by the power function and the new model should be fitted to the data. Therefore, based on the Box-Cox diagram and the transfer of response data to the power space, as well as the use of a step-by-step method to remove meaningless terms from the model, the proposed model is as shown in equation 6. The results of the new model shows that the Pred R² increased from 0.5574 to 0.8644, which indicates an  
improvement in the performance of the new model. Alpha-pinene, camphene, borneol, camphor, cineole, and caryophyllene were the predominant compounds of the essential oil. In 300 and 600-watt treatments with a time of 35 minutes, the amount of dominant compounds of essential oil is slightly higher than Clevenger. However, in the power of 900 watts, due to the increase in temperature, some compounds have been destroyed and the extraction level of some compounds has decreased drastically, and in contrast, the level of caryophyllene was increased to 23.595 %, which indicates a sharp increase in its value at 900 watts. On the other hand, the components of alphapenine, camphor and cineole have the highest medicinal value, which according to Table 8, the highest values for these compounds have been extracted in 300 watt treatment. At the higher microwave power, due to the decomposition of the compounds, the amount of extraction of these compounds was significantly reduced. Also, at 300 watts, the extraction efficiency of these compounds was higher than that of Clevenger, which shows the ability of the MAE method in extracting essential oils from medicinal plants.
The results showed that the amount and quality of essential oil in the MAE with power of 300 and 600 watts was almost the same as Clevenger in most compounds. Essential oil compounds are degraded at 900 watts, so it is recommended not to use maximum power in the experiments. However, if the goal is to extract the highest amount of cariophylene, verbenone and cariophylene oxide in the extracted essential oil, higher power can be used.