## 1 Introduction  
Citrus, belonging to the Rutaceae family, are well known as one of the world's major fruit crops that are produced in many countries, such as China, USA, Brazil, etc. [1, 2]. Due to the flavorful and pro-health properties, citrus fruits are one of the most readily consumed fruits in the world. In addition to large scale consumption as fresh fruits, the citrus fruits are mainly processed to produce juice, which generates a huge amount of citrus waste [3]. Citrus peel, the main byproduct of the citrus industry, has rich bioactive compounds, including flavonoids, phenolic acids, volatile oil, etc. [4], which were reported to possess health-promoting benefits including antioxidant, antifungal, antibacterial, hypolipidemic effects [5, 6, 7]. Thus, the fruit peels resulted from juice processing, were fully utilized in food industry, cosmetics, and folk medicine. With the risk of being prone to microbial spoilage, the citrus peel was often dried before being utilized as culinary seasoning, dietary supplement, and food additive [8] for its dietary values in smell, flavor, and curative effects [4], and as the herbal medicines for treating cough, indigestion, inflammatory respiratory diseases, and others [9]. Since the citrus plant and fruit were prone to pest infection, a multitude of pesticides were used for citrus fruits during the cultivation process [10]; thus, the danger of dried citrus peel from pesticide residues should not be ignored. For dried citrus peels, most of the literature reports focused on flavonoid content [11]; however, aroma was also the important quality characteristic affecting consumer acceptance [12; 13]. Phytochemical studies demonstrated that approximately 160 volatile compounds have been identified from dried citrus peel, including monoterpenes, sesquiterpenes, and \(O\)-containing compounds (acids, esters, aldehydes, ketones, and alcohols, etc. ), with monoterpenes of the highest content among different cultivars [14; 15]. In addition, for dried peel of citrus from different cultivars or origins, the volatile composition was often different. In fact, the volatile profile has been widely studied for classification of citrus species and citrus taxonomy [16; 17; 18]. Generally, the dried peel of _Citrus reticulata_ \"Chachi\" from Xinhui County (Jiangmen, Guangdong, China) has always been well-regarded to have superior quality when being compared with dried peel derived from other cultivars with respect to geo-herbalism [19; 20]. Thus, recently, under the economic adulteration that the dried Chachi peel was often superseded by cheaper or inferior variants in the market, the volatile profile of dried citrus peel has also been studied to develop accurate methods differentiating the dried peel of Chachi and other cultivars [14; 21; 22]. The preliminary market survey in China showed that the dried peel of Chachi was mainly prepared by natural sun-drying, while the dried peel of other citrus cultivars was mainly prepared by hot-air-drying due to the climate differences in different production areas. Therefore, for dried peel of different citrus cultivars, which were made from different drying methods, it is necessary to explore the effect of drying methods. Principal component analysis (PCA), a multivariate statistics-based detection method, could be used to transform multiple indicators into a few comprehensive indicators for extracting features and revealing the relationship between variables. Recently, PCA has been widely used in food classification [23], such as identification of significant organic substances in raw materials or food products [24], as well as the demonstration of changes in food or raw material under the influence of technological processes [25; 26]. Thus, in this study, fresh _Citrus reticulata_ \"Chachi\" (Xinhui District, Jiangmen, Guangdong, China) and _Citrus reticulata_ \"Ponkan\" (Jian 'ou County, Nanjing, Fujian, China) were collected to prepare dried peel by using three drying methods, that is, sun-drying, hot-air-drying, and freeze-drying.