Difference analysis on the berry quality among six juicing grape cultivars

Abstract: 【Objective】In China, the number of juicing grape varieties is few, excellent parents are fewer and varieties suitable for production are scarce, which cannot meet consumer’s demands for highquality grape juice. Evaluation on berry quality of juice grape cultivars is key for development of grape juice industry. The study aims to analyze the berry quality and aroma components of different juicing grape varieties, and screen out the excellent juice-making varieties.【Methods】Six juicing grape varieties, Triumph, Roudingxiang, Wyoming Red, Delaware, Heihuxiang and Campbell Early, in Taigu Grape Nursery of National Germplasm Resource Nursery were used as experimental materials. The berry weight, the contents of soluble sugar, total acid, vitamin C (Vc) and total phenol, and components and content of aroma in the mature berry were determined and analyzed. The berry weight was measured with a 1/10 000 electronic balance, the vertical diameter and transversal diameter were measured with a vernier caliper, the content of soluble solids was determined with a digital refractometer, the content of soluble sugars was determined by the anthrone- H2SO4 method, the total content of acids was titrated with alkali solution, the content of Vc was determined by molybdenum blue colorimetry, and the total content of phenols was determined by Folin-Schoka method. Aroma components were analyzed by theheadspace solid phase microextraction gas chromatography-mass spectrometry. Extraction method: the berry pit was removed, 2 mL Vc (50 mg·L^-1 ) was added, the sample was homogenized with a juicer and centrifuged for 20 min. 6 mL supernatant was taken and 1.5 g NaCl and 5 μL internal standard (2-octanol, 155 mg·L^-1 ) were added into the 20 mL headspace bottle. Extraction conditions: It was balanced at 40 ℃ for 15 min (rotating speed: 250 r · min^{- 1} , shaken for 3 s, intermittently for 4 s), and extracted at 50 ℃ for 40 min. The flow rate was 1 mL·min-1 , with SPME automatic injection. The temperature procedure of the column temperature box was 40 ℃ for 5 min. When the temperature rose to 230 ℃ at the rate of 5 ℃·min^-1 , and then the temperature raised to 250 ℃ at the rate of 20 ℃·min-1 for 5 min. The mass spectrometry interface temperature was 280 ℃, the ion source temperature was 230 ℃, the ionization mode was EI, the ion energy was 70 EV, and the mass scanning range was 20-400 m·z^-1 .【Results】 The berry mass of 6 juice grape varieties was 2.57-4.78 g, and the juice yield was 68%-75%. Except for soluble solids, the soluble sugar, total acid, total phenol and Vc contents were significantly different. A total of 65 aroma compounds were detected from 6 juice grape varieties, with different compositions and contents. According to chemical functional groups, it was divided into C6 compounds, alcohols, esters, aldehydes, acids, terpenes, ketones, C13- hypoisoprenes and aromatic benzene- containing. There were the most kinds of aroma compounds in the Roudingxiang berry, especially for the terpenes. Triumph was followed, it contained 58 kinds of aroma compounds, and the kind of esters was the most. There were only 41 kinds of aroma compounds in the Delaware berries. The data showed that aroma compounds were different according to the juice grapes varieties, and the main compounds were esters and terpenes. OAV (odor activity value) is a measurement of the contribution of aroma compounds to fruit flavor, so the presentation of berry characteristic aroma was related to the composition and content of aroma compounds and OAV. Only when OAV value was larger than or equal to 1, was this compound considered as contributing to aroma. Among aroma components, the number of compounds with OAV≥1 was 19. Principal component analysis was carried out for the aroma compounds with OAV≥1 in the berries of six juicy varieties. And the results showed the contribution rate of PC1 was 35.71, the contribution rate of PC2 was 56.67, and the cumulative contribution rate was 92.38. There were obvious differences among varieties, Triumph, Roudingxiang, Wyoming Red and Campbell Early can be clearly distinguished in location, Roudingxiang and Wyoming Red were on the positive half axis of the first principal component, while Triumph and Heihuxiang were on the positive half axis of the second principal component. Aromatic compounds such as linalool, rose oxide, ethyl phenylacetate, heptaldehyde, nonaldehyde, phenylacetaldehyde, damastone, ethyl valerate and myrcene were more contributed. Hierarchical cluster analysis was carried out for the aroma compounds with OAV≥1 in the berries of six juice grape varieties. And the result showed that the aroma compounds were divided into four categories, with Delaware and Heihuxiang were the most similar, followed by Triumph, Campbell Early, Wyoming Red and Roudingxiang.【Conclusion】According to the contents of berry soluble sugar, Vc and total phenol, and pH of grape juice, the quality of Delaware grape was better; According to the composition and content of aroma compounds, six grape varieties were distinguished clearly. The kinds of aroma compounds with Roudingxiang berries were the most abundant, and the contribution rate of terpenes was higher. In Heihuxiang berries, the contribution rate of C13-pentadiene was the highest. Among the six juicing grape varieties, the aroma of Roudingxiang and Heihuxiang was more special. Grape variety with different aroma feature should be chosen according to different consumers.