Comparative Study on the Content and Antioxidant Activity of Flavonoids in Citrus Peel of Different Cultivated Varieties
Citrus has a cultivation history of more than 4000 years in China. In the Song Dynasty, there were dozens of varieties of citrus. The “Orange Record” recorded: “There are eight varieties of citrus, including vermilion and milk citrus…; there are four varieties of citrus, including yellow and vermilion. In recent years, in pursuit of the edible value and economic benefits of citrus fruit pulp, a large number of new varieties of citrus cultivation have emerged, such as Ganping, Qingjian, and Mingmei. It can be seen that citrus fruits have had the characteristic of multiple varieties from ancient times to the present, and there are also records of their skin being used as medicine. The Shennong Bencao Jing recorded for the first time that tangerine peel comes from the peels of various cultivated varieties of the broad skinned citrus genus in the Rutaceae family. According to the 2020 edition of the Chinese Pharmacopoeia, under the category of dried tangerine peel, tangerine peel refers to the dried and mature peel of the Rutaceae plant Citrus reticulata Blanco and its cultivated varieties. Four citrus varieties for medicinal use were specifically mentioned, namely Chazhigan C Retina ‘Chachi’ (dried tangerine peel), Dahongpao C reticulata ‘Dahongpao’、 Wenzhou honey orange C. reticulata ‘Unshui’, Fu orange C reticulata ‘Tangerina’。 It can be seen that although there are multiple varieties of citrus, the medicinal value of their peels varies. In 2002, the Ministry of Health issued the “Notice of the Ministry of Health on Further Standardizing the Management of Health Food Raw Materials (No. 51)” and issued a new “List of Items that are Both Food and Drug”, which stipulated that orange peel is a dual-use substance for food and medicine. Nowadays, as a dual-use substance for medicine and food, citrus peel not only faces the problem of variety selection. With the increase of citrus planting area, the citrus processing industry has emerged, producing by-products such as fruit peels. Some are circulated in the market as “mixed tangerine peel”, while others are buried as waste. The research on the application value of different citrus peels lags behind, leading to a chaotic application status, and the problem of unreasonable and insufficient utilization of their medicinal and edible dual-use value is prominent.
The quality control of Chenpi medicinal materials under the Chenpi section of the 2020 edition of the Chinese Pharmacopoeia only specifies the limit of hesperidin content, which has certain limitations. Flavonoids, as one of the main active ingredients in citrus peel, have pharmacological activities such as improving glucose and lipid metabolism, protecting cardiovascular health, antioxidant, anti-inflammatory, and anti-tumor effects. Research by Luo Meixia et al. has shown that using naringin and hesperidin, which have high and significant differences in content, as multiple indicator components for comprehensive evaluation of tangerine peel quality is more scientific and comprehensive. Furthermore, naringin and hesperidin have the effects of preventing obesity, assisting in lowering blood lipids, and fighting inflammation. Although studies have shown significant differences in flavonoid content among different varieties of citrus peels, no comparison has been made with the medicinal varieties specified under the Chenpi section of the 2020 edition of the Chinese Pharmacopoeia. Therefore, this study took 9 types of citrus peels as research objects, determined total flavonoids, and identified 5 flavonoid components with stable chemical properties, easy extraction and separation, high content, and clear biological activity (Yunxiang pomelo peel glycoside, hesperidin, sweet orange flavonoids, Sichuan tangerine peel extract, and tangerine peel extract) and antioxidant activity. Cluster analysis and TOPSIS analysis were conducted to provide some reference for the evaluation of the medicinal and edible value of citrus peels.

The total flavonoid content and antioxidant capacity of Dahongpao Red Orange (DHP) were not significantly different from those of Wenzhou Honey Orange (CLU), Ponkan (PK), Qingjian (QJ), and Mingjian (MRJ), but the hesperidin content of CLU and QJ was significantly higher than that of DHP. In the food industry, citrus peel is used in various fields due to its strong antioxidant properties. For example, citrus peel extract can be used as a natural preservative for pork and as a functional food with antioxidant properties such as citrus jelly. CLU, PK, QJ, and MRJ that have not been used for medicinal purposes can all be used as natural sources of antioxidant extraction. The content of hesperidin in flavonoids exceeds 50% of the flavonoid content, and it has physiological functions such as anti-inflammatory, antioxidant, antibacterial, and lipid metabolism regulation. Monomers are the most effective antibacterial agents used to combat SARS-CoV2 virus. Hesperidin transdermal nano formulations have also been developed to treat Pseudomonas aeruginosa and hesperidin nano particles to treat environmental organic pollutants. The experimental results indicate that CLU and QJ can provide raw materials for extracting hesperidin.
Cluster heatmap analysis can quickly and comprehensively identify and classify samples by screening, extracting, and dimensionality reducing complex data, and present the results intuitively. The entropy weight TOPSIS method is a multi criteria decision analysis method that has been widely used in the quality evaluation of traditional Chinese medicine. Combining these two methods can provide a more scientific, comprehensive, and objective evaluation of the quality of medicinal herbs. The clustering heatmap analysis results showed that 9 types of orange peels were divided into 3 categories, and the quality of the first category samples (DHP, PK, AY38) was better than that of the second to third category samples. At the same time, this study established an overall quality evaluation model for Chenpi medicinal materials using the entropy weight TOPSIS method. The results showed that the comprehensive scores of DHP and PK of the samples ranked in the top two, indicating that their quality was relatively good. The above analysis methods are combined and complementary to each other, and the analysis results are mutually confirmed, which can comprehensively evaluate the quality of Chenpi medicinal materials. Based on the results of total flavonoid content determination, the total flavonoid content of Dahongpao Red Orange (DHP) was significantly higher than that of Ehime 38 (AY38), Chunjian (CJ), Wogan (WG), and Ganping (GP) (P<0.05), and the PK and DHP flavonoid chemical compositions and antioxidant activities were found to be similar, indicating its potential as a medicinal cultivation variety, which is consistent with literature research results.
In summary, this study compared and analyzed the chemical components and antioxidant activities of 9 citrus peel flavonoids, and found that DHP, CLU, PK, QJ, and MRJ can all be used as natural sources of antioxidant extraction. CLU and QJ provide raw materials for the extraction of hesperidin compounds and serve as references for the development of citrus peel by-products. The flavonoid chemical composition and antioxidant activity of PK and DHP in 9 different cultivated varieties of orange peel are similar, which has the potential to be used as medicinal cultivated varieties and provides reference for the mixed use of different cultivated varieties of orange peel in medicine. However, this study only focused on determining the content of flavonoid indicators in 9 cultivated varieties of orange peel, with relatively few categories of components and sample batches. In subsequent research, sample batches can be increased and multiple categories of components such as alkaloids, polysaccharides, and volatile oils can be analyzed comprehensively to evaluate the medicinal and edible value of orange peel more comprehensively.