Profiles of soluble sugars and organic acids in grape juice and their application for authentication

Abstract: 【Objective】Soluble sugars and organic acids are the main nutrients and taste components in fruit juices, which show a lower susceptibility to changes as compared with other components such as pigments, antioxidants, and flavour compounds. Therefore, characterization of the composition of sugars and/or organic acids in fruit juices is considered as a desirable approach for fruit juice authenticity as well as quality control. The composition and content of sugars and organic acids of grape juice may be influenced by variety, harvest date and cultivation conditions. This work aims to find the common pattern of grape juices and their implication for authentication by comparing the sugar and organic acid profiles of grape juices prepared from different varieties and harvest dates.【Methods】Grape berries of17 varieties, including‘Ruby seedless' ‘Red globe'‘Gold finger'‘Jingxiu'‘Kyoho'‘Jumeigui'‘Moldova'‘Niagara'‘Centennial seedless' ‘Victoria'‘Summer black'‘Xiazhihong'‘Xiuyu'‘Shine-muscat'‘Flame'‘Zexiang'‘Zuijinxiang'were harvested at mature stage for three times and washed, and then pureed using a lab-scale food processor. After pasteurization in boiling water for 5 min, the puree was cooled and treated with pectinase at 50 ℃ for 40 min, and then centrifuged at 4 000 r · min-1 for 10 min. The resulted supernatant was recovered and stored at-80 ℃ before high performance liquid chromatography(HPLC) analysis for sugars and organic acids. For assessment of the possibility of application of sugar and organic acid profiles for grape juice authentication and quality control, adulterate samples were prepared by adding 20%, 40%, 60%, 80% and 100% of apple juice, pear juice, apricot juice or peach juice and the composition of sugars and organic acids was determined by HPLC. Principal component analysis(PCA) and linear discriminant analysis(LDA) were used to discriminate grape juices from their adulterate samples with apple, pear, apricot or peach juice.【Results】The soluble sugars in grape juices mainly consisted of glucose, fructose and sucrose, while tartaric acid,citric acid and malic acid were the main organic acids in grape juices. There were great differences in contents of individual soluble sugars and organic acids among grape juices of different varieties and harvest dates, with a coefficient of variation of 135.86%, 9.89%, 11.46%, 16.93%, 41.13%, 34.89%,76.65% and 64.82% for sucrose, glucose, fructose, tartaric acid, malic acid, citric acid, shikimic acid,and fumaric acid, respectively. Glucose and fructose contributed 97.88% to 99.86% of the total content of identified soluble sugars, with a ratio in a range of 0.81-1.05. Tartaric acid was the predominant organic acid in grape juices of all varieties and harvest dates, with a proportion of 57.95% to 92.37% in total content of identified organic acids. Malic acid and citric acid accounted for 5.26% to 36.11% and2.08% to 6.67%, respectively. By determining the compositions of soluble sugars and/or organic acids coupled with principal component analysis(PCA) and linear discriminant analysis(LDA), grape juices could be discriminated from their adulterates with apple juice, pear juice, peach juice, and apricot juice.The discriminant effects depended on the ratios of adulterations and dataset used for PCA and LDA.When the soluble sugar composition was used as analytical data for PCA, the adulterate samples could be distinguished from grape juices except juice sample adulterated with 20% of apple juice. The juice sample adulterated with 20% of apricot juice was also not discriminated from grape juices by using analytical data of organic acid content for PCA. However, when combining the soluble sugar and organic acid composition as analytical dataset for PCA, all the adulterate samples could be distinguished from grape juices. As for LDA, the combination of soluble sugar and organic acid composition also showed a better discriminant effect than soluble sugar or organic acid composition alone, with a correct classification rate of 100% for the original and 95.8% for the cross validation method, while they were 95.8%and 93.0%, respectively, when only soluble sugar composition was used as analytical dataset for LDA.As for organic acid composition, the correct classification rates were only 90.1% for the original and83.1% for the cross validation method, suggesting an even worse discriminant effect than soluble sugar.【Conclusion】The profiles of soluble sugars and organic acids in grape juice may be used for the authentication and quality control of grape juice. By using soluble sugar and organic acid composition coupled with principal component analysis and linear discriminant analysis, the samples adulterated with apple juice, pear juice, peach juice and apricot juice could be discriminated from grape juices. The combination of sugar and organic acid composition for principal component analysis and linear discriminant analysis was superior to sugar or organic acid composition alone.