Comprehensive evaluation of citrus fruit quality based on principal component and cluster analysis

Abstract:【Objective】Fruit quality of citrus, including color, size and shape, easiness of peeling, seedlessness, flavor and nutritional value, is important to determine the market competitiveness of citrus. However, fruit quality shows great diversity among different citrus varieties. Therefore, objective, accurate and scientific analysis and evaluation of citrus fruit quality play an important role in facilitating breeding work. At present, most studies on the evaluation of citrus fruit quality depends upon conventional methods. To investigate the quality traits of 33 citrus varieties (strains) in the germplasm resource nursery, contents of sugars, acids, total flavonoids, and total phenols were quantified, and the antioxidant capacity of these citrus varieties (strains) were also examined. Then, these data were analyzed through multiple statistical methods to comprehensively evaluate the fruit quality, which included both principal component (PCA) analysis and heat map cluster analysis.【Methods】In this study, samples of all varieties (strains) were randomly collected from all around the periphery of each tree crown, when fruits of all varieties (strains) reached a commercial maturity. To determine its physical and chemical quality, all the fruits were washed clearly, wiped with gauze, peeled and seeded to extract juice from the pulp after harvest. At the same time, the peel was dried in an oven at 50 ℃, crushed through a 60 mesh sieve, put into a plastic bottle, and stored at -20 ℃. The composition and contents of sugars and acids were determined by ion chromatography and high performance liquid chromatography, respectively. In addition to the physical and chemical quality indexes, total flavonoids and total phenol contents were also determined. Then, correlation analysis, principle component analysis (PCA) and heat map cluster analysis were used to comprehensively evaluate the fruit quality of different citrus varieties.【Results】 The external appearance and internal quality of 33 citrus varieties (strains) were nonuniform. The average of the vertical and horizontal diameters was 66.56 mm and 74.71 mm, respectively. However, the diameters of mandarin were generally lower than the average values. The single fruit weight ranged from 90.57 g to 422.50 g, and the average value was 184.99 g. The single fruit weight of mandarin was lower than the average, while that of grapefruit was higher than the average. The peel thickness affects the fruit commercial value, and the consumers always prefer the fruit with thinner peel. Among all the different varieties (strains), Hu You had the thickest peel of 8.02 mm, while the peels of Sokitsu, Himekoharu, Hongmeiren, Orah and Asumi were relatively thinner. The total sugar contents ranged from 43.95 to 145.71 mg·g^-1 . Moreover, the sucrose content accounted for 11.86%-69.12% of the total soluble sugars, and the content of fructose was generally higher than glucose. Besides, the sugar components in different varieties (strains) showed some differences. For example, sucrose was the major soluble sugar in mandarin, while sweet orange, 7 hybrid citrus and flamegrape fruit accumulated both hexose and sucrose, as well as lime mainly accumulated hexose. The citrus fruits contained citric acid, malic acid, tartaric acid, oxalic acid and shikimic acid. The total acid contents ranged from 6.36 to 37.93 mg · g^-1 and the citric acids accounted for 63.21 to 96.70% of the total acid. The Haruka mainly accumulated malic acid that accounted for 52.63%, and the other varieties (strains) accumulated citric acid. The average value of Vc content was 34.30 mg·100 g-1 . The Vc content of sweet oranges was higher than that of citrus hybrid, while the contents of total flavonoids and total phenols in mandarin were generally higher than those in sweet oranges and lime. Among the different varieties (strains), Green Ougan had the highest total flavonoids content of 65.30 mg·g-1 , and that of Chachi Gan was the second highest one containing 56.30 mg · g^-1 . The contents of total phenols ranged from 15.43 to 41.07 mg · g^-1 , and the top 3 were Seihou (41.07 mg·g^-1 ), Green Ougan (36.30 mg·g^-1 ) and Chachi Gan (35.93 mg·g^-1 ). The antioxidant activity of citrus peel was positively correlated with the contents of total phenols and total flavonoids. DPPH free radical scavenging capacity, ABTS + free radical scavenging capacity and FRAP iron reducing capacity of mandarin peel were generally higher than others. The top 3 of antioxidant capacities of all the different varieties (strains) were Seihou, Green Ougan and Himekoharu. In addition, there were some differences in the antioxidant capacities of the same variety (strains) from the three different testing methods. Five principal components were extracted by PCA analysis, where the cumulative contribution rate was 85.120%. The contents of fructose, Vc and sucrose, the single fruit weight, and fruit shape index were the key indicators to evaluate the citrus fruit quality. The evaluation model was established as below: F=0.347 F1+0.265 F2+0.175 F3+0.118 F4+0.095 F5.【Conclusion】33 citrus varieties (strains) were divided into 6 categories by cluster analysis. The fourth category, including Okitsu No. 60, Himekoharu and Asumi, had the best fruit quality. The combination of the heat map cluster and PCA analysis could evaluate the citrus quality systematically and scientifically, and present the results intuitively. Our results will provide a theoretical basis for the structural adjustment of citrus varieties, the breeding of new variety, and the development and utilization of functional components in citrus.【Objective】Fruit quality of citrus, including color, size and shape, easiness of peeling, seedlessness, flavor and nutritional value, is important to determine the market competitiveness of citrus. However, fruit quality shows great diversity among different citrus varieties. Therefore, objective, accurate and scientific analysis and evaluation of citrus fruit quality play an important role in facilitating breeding work. At present, most studies on the evaluation of citrus fruit quality depends upon conventional methods. To investigate the quality traits of 33 citrus varieties (strains) in the germplasm resource nursery, contents of sugars, acids, total flavonoids, and total phenols were quantified, and the antioxidant capacity of these citrus varieties (strains) were also examined. Then, these data were analyzed through multiple statistical methods to comprehensively evaluate the fruit quality, which included both principal component (PCA) analysis and heat map cluster analysis.【Methods】In this study, samples of all varieties (strains) were randomly collected from all around the periphery of each tree crown, when fruits of all varieties (strains) reached a commercial maturity. To determine its physical and chemical quality, all the fruits were washed clearly, wiped with gauze, peeled and seeded to extract juice from the pulp after harvest. At the same time, the peel was dried in an oven at 50 ℃, crushed through a 60 mesh sieve, put into a plastic bottle, and stored at -20 ℃. The composition and contents of sugars and acids