## 1 Introduction  
Collectively, citrus trees comprise the most abundant crop of fruit trees in the whole world, providing fruits which have a high commercial value on both the fresh market and the food processing industry [1]. At present, the citrus varieties produced in Taiwan are the Ponkan (_Citrus reticulata_ Blanco), Tankan (_C. tankan_ Hayata) and Murcott (_C. reticulate \(\times\) C. sinensis_) varieties, with 108,058 tons of Ponkan, 52,023 tons of Tankan, and 24,751 tons of Murcott having been produced in one recent year [2]. The harvest months for Ponkan, Tankan, and Murcott are October-December, February-March, and January-March, respectively. The peels and pulps of processed citrus product waste contain high levels of active biological components, including carotenoids, vitamin E, flavonoids, limonoids, phenols, polysaccharides, lignin, cellulose, pectin, and essential oils [3; 4; 5]. Essential oils and oleoresin are found in citrus peels, the bulk of which are mainly obtained from the byproducts of the juice manufacturing industry, and are extracted for use in beverages and cosmetics with approximately 56,200 tons being used annually [6; 7]. Extracted in different manners with different solvents, essential oils are the volatile aromatic plants prepared by steam distillation; oleoresins are extracts of spices obtained by using organic solvents, after the solvent is removed from the extract. Oleoresin itself is composed of non-volatile resins and volatile essential oil. According to several studies, a variety of the non-volatile oleoresin compounds, such as piperine from black pepper oleoresin, gingerol from ginger oleoresin, and lycopene from tomato oleoresin, can strengthen sensory stimulation [8; 9; 10]. Supercritical fluid extraction (SFE) is a relatively new technology that has been widely used in recent years in producing perfumes and spices, as well as in the extraction of active ingredients, particularly plant extracts. SFE is regarded as a green technology due to the fact that it is low cost, high purity, nontoxic, and solvent-free [11; 12; 13; 14; 15]. The use of SFE to extract oleoresin can produce oleoresin with an enhanced flavor and more intense aroma, and its use also yields oleoresin with improved storage stability because it prevents the degradation of thermally unstable components during the extraction process [13; 14; 15]. The specific properties of a given sample of citrus essential oil and oleoresin will affect its quality in terms of its utility for industrial applications, and these properties depend in turn, at least in part, on the origin of the raw material used (including the different fruit varieties of which it consists) and the different extraction processes used. Volatile essential oils of plants can be divided into three categories: monoterenes, sesquiterpen, and oxygenated terpen. Essential oils extracted via the cold-pressing method contain at least 200 compounds and some natural pigments, such as chlorophyll and carotenoids [16; 17; 18; 19; 20]. The flavor characteristics of cold-pressed citrus essential oils are associated with the contents of the oil, consisting of terpene compounds and oxygen-containing derivatives. Among those contents, volatile components account for about 96%-98% of the total volume, whereas non-volatile compounds, such as long-chain hydrocarbons, fatty acids, sterols, carotenoids, and oxygenated heterocyclic compounds, account for about 1%. The oxygenated heterocyclic compounds, including coumarins, furanocoumarins, and flavonoids, are associated with essential oil safety and efficacy, as well as tests to determine essential oil authenticity [17; 18; 19]. The non-volatile compounds in citrus oils account for an extremely low portion of their total content, but these non-volatile compounds are nonetheless highly important, with key compounds strongly determining the quality of a given oil sample in terms of sensory evaluations [20].