A study on the components and characteristics of sugars and acids in 8 hybrid citrus cultivars

Abstract: 【Objective】Fruit quality is an important basis for evaluating fruit varieties and determines their competitiveness in the market. Sugar and acid components are important indicators of fruit flavor and quality. Hybrid citrus from natural or artificial hybridization between citrus species have the charac-teristics of nutrition, flavor and peeling easiness of citrus, tangerine, orange, or pomelo. Therefore, hy-brid citrus play an important role in the adjustment of the citrus variety structure. At present, research on hybrid citrus mainly focuses on the aspects of botanical characters, biological characteristics and cul-tivation techniques, and very few studies have made on fruit quality, especially sugar and acid composi-tion. This paper comprehensively evaluated the sugar and acid components and sweet-sour flavor char-acteristics of 8 introduced hybrid citrus cultivars in Zhejiang.【Methods】Ion chromatography and high-performance liquid chromatography were used to determine the compositions and contents of sugars and acids in the fruits, and the sweetness value, sweet-sour ratio, and sugar-acid ratio were used as the indexes for evaluation. An ion chromatography electrochemical detector and a standard four-potential acquisition method with a PA10 anion exchange column (4 mm × 250 mm) and the same type of guard column were used to determine sugar components. Mobile phase A was water while B was 200 mmoL·L^-1 NaOH with gradient elution and equilibration for 5 min. High-performance liquid chromatography with a C18 column (4.6 mm×300 mm, 5 μm) and the same type of guard column was used to determine acid components. The column temperature was 40 ℃, the flow rate was 0.5 mL·min ^-1, the detection wave-length was 210 nm, and the mobile phase was KH2PO4 buffer with a pH value of 2.65 (containing 0.5% methanol). The collection time was 35 min and quantification was made using the external standard  method. The soluble solids (TSS) were measured by a Brix meter (PAL-1), and the titratable acid (TA) was measured with titration. Data processing and statistical analysis were performed using Microsoft Excel 2007 and SPSS 19.0 software, respectively.【Results】The total sugar content in fruits of the 8 cit-rus cultivars was in the range of 83.78-127.66 mg · g^-1, in an order of‘Hongmeiren’>‘Huangmeiren’>‘Kanpei’>‘Asumi’>‘Orah 091’>‘Haruka’>‘Himekoharu’>‘Harumi’, and the main sugar components were glucose, fructose and sucrose. Sucrose accounted for 41.54%-64.60% of the total soluble sugars, and the contents of glucose and fructose were similar. The total acid content ranged between 6.28 and 10.70 mg · g -1, and the content of citric acid, malic acid and shikimic acid accounted for 45.58%-88.06%, 7.84%-53.22% and 1.14%-9.59% of the total acid content, respectively. The total acid content in the eight hybrid citrus cultivars was in the order of ‘Kanpei’>‘Asumi’>‘Huangmeiren’>‘Hongmeiren’>‘Himekoharu’>‘Harumi’>‘Orah 091’>‘Haruka’. The content of malic acid in‘Haruka’and‘Orah 091’was higher than in the other cultivars, and the contents of fructose,glucose and shikimic acid in‘Huangmeiren’were the highest.‘Hongmeiren’had the highest total sweetness, sweet- sour ratio and sugar-acid ratio. The taste of the 8 hybrid citrus cultivars fell in 5 grades including sour, sweet, sweet-sour, palatable sour-sweet and sour-sweet, and the rate of sweet-sour was in the order of‘Hongmeiren’>‘Huangmeiren’>‘Orah 091’>‘Haruka’>‘Asumi’>‘Himekoharu’>‘Harumi’>‘Kanpei’. The total sweetness of‘Kanpei’was not low, because of the influence of organic acids, its sweet-sour ratio and sugar-acid ratio were the smallest among the 8 hy- brid citrus.【Conclusion】The sugars of‘Huangmeiren’,‘Asumi’and‘Kanpei’among the 8 hybrid citrus accumulated both hexose and sucrose, while the other 5 varieties accumulated mainly sucrose. The sum of citric acid and malic acid in the 8 hybrid citrus cultivrs accounted for > 90% of the total acids. The ratio of malic acid to citric acid in‘Haruka’and‘091 Wokan’was similar, and the others were dominated by citric acid. Due to the differences in the contribution of sugar and acid components to taste, the sweet-sour ratio, sugar-acid ratio and solid-acid ratio were used in this study to rank the varieties. The results of sugar-acid ratio and sweet-sour ratio were generally consistent, but the evaluation results based on solid-acid ratio, sweet-sour ratio and sugar-acid ratio had greater deviations. Finally, it was suggested that the sweet-sour ratio or sugar-acid ratio can be used as the quality evaluation index. This study provides data for selection and promotion of citrus varieties as well as reference of citrus fla-vors for consumers.