A study on sugar and organic acid components in different apple cultivars

Abstract:【Objective】Soluble sugars and organic acids are important components of fruit flavor and have a strong impact on the overall organoleptic quality of apple fruit. Current researches are mostly limited to individual main cultivars, and the assessment of sweet and sour flavor is not comprehensive. Therefore, understanding the fruit quality traits of different cultivars can provide breeders with valuable information and assessment tools. The composition and contents of soluble sugars and organic acids in 86 different apple cultivars were analyzed in the study with an aim to screen appropriate indexes to eval- uate and classify apple taste.【Methods】The soluble sugar and organic acid components and contents of 86 different apple cultivars were determined and analyzed with high-performance liquid chromatogra- phy (HPLC); correlation analysis and principal component analysis were used to determine the quality index of taste; and cluster analysis was performed using K-means method to analyze and evaluate the fruit quality of different cultivars.【Results】The results indicated that the soluble sugars in apple fruit included fructose, sucrose, glucose and sorbitol, of which fructose content was the highest, with an aver- age value of 51.93 mg·g^-1, accounting for 50.39% of the total sugar content, and the two were extremely significantly related (r=0.867). The highest fructose content was found in Golden Delicious (68.96 mg·g-1). The average sucrose content was 31.33 mg·g-1, which accounted for 30.14% of the total sugar content. The sucrose content of Envy was 52.25 mg · g^-1. The content of the four soluble sugar compo- nents (fructose, sucrose, glucose and sorbitol) in most apple fruits showed a pattern of fructose>su- crose>glucose>sorbitol, and the only three cultivars with glucose content higher than sucrose content were Fuji, Cameo and Qiufu No.1. The glucose content in Dailv and RuiXianghong was close to su-crose content. The dominant organic acid in apple fruit was malic acid, with an average value of 4.97 mg·g-1, accounting for 89.440% of the total acid content, followed by quinic acid (7.81%) and citric acid (1.1%), and the contents of oxalic acid and shikimic acid were low. Among them, the cultivar with the highest malic acid content was Granny Smith. Malic acid was extremely significantly related with total acid, with a correlation coefficient of 0.904, and significantly related with fructose. The sweetness/ acid ratio in different cultivars ranged from 11.81 to 53.96. There were 9 cultivars with sweetness/acid ratio greater than 40, such as Dailv and Qiujing; and 63 cultivars between 20 and 40; and 14 cultivars less than 20. The soluble sugars and organic acids in different apple cultivars were significantly different, with coefficients of variation of 8.16% to 46.95%, respectively. The content of quinic acid had the largest variation and the content of soluble solids had the smallest variation. The correlations between soluble sugar and organic acid components were analyzed. It could be concluded that malic acid was significantly correlated with fructose (r=0.226), and citric acid and fructose were extremely significantly correlated. The sweetness/acid ratio had a very significant negative correlation with total acid. The principal component analysis method was used to analyze the 9 main indicators of apple taste evalua- tion. The first 2 factors were the main factors contributing to 85.124% of the total variation. Combined with correlation analysis, three indexes including total sugar, malic acid and sweetness/acid ratio were selected as indexes for sugar and acid evaluation. Cluster analysis of the 86 apple cultivars showed that they could be divided into 4 categories: high sugar and low acid, high sugar and high acid, low sugar and high acid, and low sugar and low acid. Middle-late and late-ripening cultivars accounted for 92%, 95%, 80% and 53% in the four categories of high sugar and high acid, high sugar and low acid, low sug- ar and high acid, and low sugar and low acid, respectively.【Conclusion】The sucrose content in most apple cultivars were higher than glucose. The major organic acid was malic acid, followed by quinic acid and citric acid, and the content of the other organic acids is lower. The total sugar, malic acid, and sweetness/acid ratio in apple fruit are important indexes to evaluate the sweet and sour flavor of the fruit. Middlelate and late-ripening cultivars had a higher proportion of high-sugar and low-acid varieties, and a lower proportion of low-sugar and lowacid cultivars.