Composition and variability of organic acid in different fruit juices

Abstract：【Objective】Organic acids are important nutrients and flavor substances in fruits and their processing products. Determination and analysis of the composition and contents of organic acids in fruits and their processing products can not only contribute to identifying their flavor characteristics and nutritional value, but can also be used for quality control and identification of adulteration of fruit processing products. The composition and contents of organic acids in different fruit juices were analyzed and researched, and the organic acid HPLC fingerprint database of fruit juices was established to provide a basis for quality control and scientific evaluation of fruit juices.【Methods】Six fruit species, a total of 53 cultivars of fruit, were taken as test materials, the high performance liquid chromatography method was used to determine the organic acids of fruit juices after all the fruits were made into juice. The HPLC system contained a quaternary pump, an autosampler, and a photodiode array detector with an Ultimate®AQ-C18 (4.6 mm×250 mm, 5 μm) column. Chromatography separation was performed at 30℃, the mobile phase consisted of a 0.02 mol·L^-1 (NH4) 2HPO4buffer solution (p H=2.40) using an isocratic elution procedure with a flow rate of 1.0 m L·min^-1, the injected sample volume was 10 μL, and absorbance was measured at 210 nm. Quantification was calculated by an external standard method. The chemometric methods (principal component analysis and cluster analysis) using SAS 9.3 statistical software was used to evaluate differences among the fruit juices from different fruit species.【Results】Eight organic acids could be successfully separated under the above experimental conditions. All the calibration curves for these organic acids displayed a good linear relationship, the relative standard deviation of precision was 0.24%-1.73%, andthe spiked recoveries were between 88.33% and 103.55%. There were 4-5 organic acids identified in all the fruit juices. Four organic acids of malic acid, shikimic acid, citric acid and fumaric acid were identified in cherry juices; malic acid was the most abundant acid, accounting for more than 90% of the total organic acid content, with levels ranging between 6 666.12 and 14 189.78 mg·L^-1. Five organic acids of quinic acid, malic acid, shikimic acid, citric acid and fumaric acid, were identified in peach, apple, blueberry and pear juices, but the relative contents of the five organic acids were extremely different in the fruit juices from different fruit species. Malic acid and quinic acid were established to be the major organic acids in peach juices, accounting for 28.44%-45.46% and 35.73%-57.92%, respectively. Among the five peach juices, the content of malic acid in peach juices of‘Huangjinmi 3'and‘Zhongtao 6'was highest, and quinic acid was the second dominant acid, while the content of quinic acid was highest in the rest of the peach juices, and malic acid was the second dominant acid. In apple and pear juices, except for the juice of the‘Huangguan'pear, which was with a higher citric acid than malic acid, malic acid was the predominant organic acid, accounting for 51.84%-88.30% and 32.40%-68.30%, respectively. Citric acid, accounting for 73.83%-85.99%, was the most abundant organic acid followed by quinic acid in blueberry juices, regardless of the cultivars. In all the juice samples studied, oxalic acid was only found in pomegranate juices, but the content of oxalic acid in different cultivars of pomegranate juices was quite different and the variation coefficient of oxalic acid was up to 90.26%. The contents of oxalic acid in the four pomegranate juices of‘Yushizi'‘Dabenzi'‘Qiuyan'and‘Tongpi'were significantly higher than in the rest of the pomegranate juices. The composition of organic acids was similar in juices of different fruit species, and the composition of organic acids was the same in juices of the same fruit species, but the contents of organic acids were different. Three principal components were extracted from the data by means of principal component analysis according to the eigenvalues being greater than 1, the first three principal components accounted for 71.98% of the total variance, which could basically reflect the information of the six original organic acids. Except for pomegranate juices and pear juices, the distribution of the juices from the same fruit species in the score plots was relatively concentrated. The result of cluster analysis was consistent with the result of principal component analysis. Principal component analysis and cluster analysis visually reflected the constitutive characteristics of organic acids in different fruit juices. All the fruit juices, except for pomegranate and pear juices, could almost be distinguished and clustered by the principal component analysis and cluster analysis.【Conclusion】Using the composition, contents and ratio of organic acids in fruit juices, differentiation and identification for different fruit juices could be realized by means of principal component analysis and cluster analysis, which could provide a theoretical basis for the identification and quality control of different kinds of fruit juices.