# Compositional Analysis and Aroma Evaluation of Feijoa Essential Oils from New Zealand
## 2 Results and Discussion  
### Comparison of Steam-Distillation (SD) and Hydro-Distillation (HD)  
The extraction of feijoa essential oil was successfully conducted by both SD and HD. As shown in Figure 1A, the essential oil yielded by SD gathered rapidly during the first 90 min of extraction and gradually become stable thereafter. With regards to HD, two parameters, material to water ratio and extraction time, were evaluated to optimize the extraction conditions. The amount of essential oil reached a peak at the material to water ratio of 1:4, although it is not significantly higher than the amount at the ratio of 1:5 (Figure 1B-1). Similar to SD, a time-dependent generation was also observed in the process of HD, in which the essential oil yield tended to reach a plateau after 90 min of extraction (Figure 1B-2). Notably, the extractable quantity of essential oil by HD was significantly higher than that by SD, indicating that HD could be more suitable than SD for the commercial extraction of feijoa essential oil. Thus, HD with a material to water ratio of 1:4 and an extraction time of 90 min was selected as the extraction method with the highest yield for feijoa essential oil.  
Both SD and HD are well developed and widely applied traditional methods for the extraction of essential oils from natural plants. The history of the extraction and pharmaceutical application of essential oils can be dated back to the 13 th century [1]. Previous studies have compared the extraction efficiency of SD and HD, and results have varied depending on the plant material. [PERSON] et al. [23] found SD was more suitable than HD for the extraction of rosemy essential oil. [PERSON] et al. [24] reached a similar conclusion that SD was more efficient than HD in the extraction of essential oil from both Kaffir lime peel and leaves, while [PERSON] et al. [25] discovered a higher essential oil yield using HD than SD from _Satureja hotensis_. Similar findings from [PERSON] and [PERSON] [26]were also in agreement with our present study. It has been suggested that, due to the different communication mode occurring in SD and HD, incomplete distillation may occur when materials clump together [26]. In the case of feijoa peel, significant agglutination and clumps formed during SD, which may have led to inefficient extraction.  
Nevertheless, there are very few studies carried out on the extraction and analysis of feijoa essential oil. The earliest report on feijoa essential oil was by [PERSON] and [PERSON] [16], who extracted feijoa volatile oil from the whole feijoa fruit by an initial HD using a rotary evaporator, followed by the re-distillation of the collected essence with trichlorofluomethane. Following this, [PERSON] et al. [27] extracted essential oil from feijoa peel by low temperature vacuum SD and subsequent liquid-liquid extraction using hexane and diethyl ether. Both of the reports included organic solvents to aid in the separation of the oil phase from the aqueous phase and, unfortunately, neither of these publications mentioned the extraction yield. In a more recent study on feijoa peel essential oil, the extraction method of HD was employed and yielded 0.1% oil [28], which is consistent with our results. Despite the aforementioned studies on feijoa essential oil, our research is the first to optimize and compare the extraction efficiency of SD and HD under solvent free conditions. In addition, as evidenced from our results, feijoa peel, as a by-product from the juicing industry, has great potential for utilization in the essential oil industries.