A comprehensive evaluation of mineral elements and active substances in citrus fruits

Abstract:【Objective】Mineral elements play an important role in the growth, development, and yield formation of fruit trees and are also essential nutrients for human health. Citrus fruits contain abundant mineral elements required by the human body, which can be used as one of the important indicators for evaluating nutritional quality. In addition, citrus fruits are rich in phenolic substances, which have antiinflammatory, antioxidant, and anti-tumor effects. However, there are currently few reports on the accumulation characteristics and comprehensive evaluation of mineral elements in different parts of newly cultivated and introduced citrus varieties. Meanwhile, as consumers increasingly pay attention to health foods, a large amount of data related to nutrition and antioxidant activity is needed to screen for excellent citrus germplasm. Therefore, this study aimed to analyze mineral elements, polyphenols, antioxidant properties, and their relationship in different citrus varieties and to screen citrus resources with high nutritional value.【Methods】Mature fruit samples of all varieties were collected from Zhejiang Citrus Research Institute under consistent cultivation and management conditions. All citrus fruits were thoroughly washed, wiped with gauze, and seeded. The peel and pulp of samples were dried in an oven at 40 ℃, crushed through a 60-mesh sieve, and stored at −80 ℃. Atomic absorption analysis was usedto analyze mineral nutrients (K, Ca, Mg, Na, Fe, Mn, and Zn), while the P content was determined by spectrometry. Total flavonoid content, total phenolic content, and antioxidant capacity were also determined. Subsequently, correlation analysis, principal component analysis (PCA), and heat map cluster analysis were employed to comprehensively evaluate the fruit quality of different citrus varieties. The data were organized using Excel 2010, and principal component analysis and comprehensive analysis were conducted using SPSS 22. Correlation graphing and heat map cluster analysis were performed using Origin 2024.【Results】The total mineral element content in citrus pulp ranged from 10 918.22 to 17 042.22 mg·kg^-1 and from 12 612.91 to 19 777.92 mg·kg^-1 in the peel. The coefficient of variation of mineral elements ranged from 11.10% to 41.26% in pulp and from 15.62% to 46.23% in peel. Among the 13 citrus varieties, the total mineral element contents in pulp was in the order of Sour orange＞Ougan＞Green Ougan＞Bendizao＞ Himekoharu＞Yuhuanyou＞Tarocco blood orange＞Ponkan＞Cocktail grapefruit＞Gaocheng＞Hongmeiren＞Chazhigan＞Miyagawa wase, and in the peel the order was Sour orange＞Yuhuanyou＞Himekoharu＞Miyagawa wase＞Hongmeiren＞Gaocheng＞Chazhigan＞ Cocktail grapefruit＞Ougan＞Green Ougan＞Bendizao＞Tarocco blood orange＞Ponkan. The average mineral element contents among the 13 citrus varieties in pulp was in the order of K＞Ca＞P＞Mg＞ Na＞Fe＞Mn＞Zn, and the order was K＞Ca＞P＞Mg＞Na＞Mn＞Fe＞Zn in the peel. K and Na accounted for 73.79% and 2.09% of the total analyzed mineral elements, respectively. The characteristics of citrus fruits were high in K and low in Na. From the perspective of mineral element distribution, the contents of Ca and Mn in the peel were higher than in the pulp, while P was the opposite. Except for Yuhuanyou, the Fe content in the peel was higher than in the pulp, while the K content was higher in the pulp except for Miyagawa wase and Chazhigan. The content of phenolic substances varied among varieties. Sour orange had the highest contents of total flavonoids and total phenolics, and those of Chazhigan were the second highest. The antioxidant activity was positively correlated with the contents of total phenolics and total flavonoids in citrus pulp. The top three antioxidant capacities among all the different varieties were Sour orange, Gaocheng, and Tarocco blood orange, and the lowest was Hongmeiren. The correlation analysis showed that P, K, Ca, Mg, and Fe were significantly positively correlated. Na showed a highly significant negative correlation with Ca and a significant negative correlation with K. Ca promoted the accumulation of total flavonoids, while P and Na had antagonistic effects on the accumulation of flavonoids and phenolics. Three principal components were obtained using PCA analysis with a cumulative variance contribution of 77.939%. The contents of K, Ca, Mg total flavonoids, and total phenolics were the key indicators for comprehensive nutritional evaluation of citrus fruit. The comprehensive evaluation model was established: F=0.508F1 + 0.357F2 + 0.135F3. The comprehensive scores of mineral elements and polyphenols in the 13 citrus varieties was in an order of Sour orange ＞ Green Ougan＞Himekoharu＞Tarocco blood orange＞Ougan＞Chazhigan＞Bendizao＞Yuhuanyou＞ Gaocheng＞Ponkan＞Cocktail grapefruit＞Hongmeiren＞Miyagawa wase. The 13 citrus varieties were divided into 4 categories by cluster analysis. Sour orange was in the first group; Yuhuanyou was in the second group; Miyagawa wase was in the fourth group, and the other varieties were clustered into the third group.【Conclusion】There was spatial specificity in the distribution of mineral elements in citrus, and there were significant differences in the content of phenolic substances among different citrus varieties. Sour orange, Green Ougan, Himekoharu, and Tarocco blood orange were rich in mineral elements and active substances.