Detection and differential analysis of fruit organic acids among different local wrinkled papaya varieties (Chaenomeles speciosa) by GC-MS

Abstract: 【Objective】Organic acid plays a key role in affecting fruit flavor by changing acid-sugar ratio. And GC-MS is also an important detecting platform to inspect fruit organic acids due to its stable,sensitive and accurate features. With methyl ester derivatization, the objective organic acids could be effectively detected for their reduction of ingredient polarity. To provide the basic data for Chaenomeles speciosa(Sweet) Nakai fruit quality improvement, the the organic acid inspection of the fruit sampleswas carried out from 10 main producing areas in China and their compositions and contentswere disclosed by extracting with methal, derivating with methyl esterification and detecting by means of GC-MS.【Methods】In this study, 10 local varieties were used as materials that were collected from 10 main producing areas in China. After methanol extraction and methyl-ester derivatization, the compositions and contents of organic acids with each sample were comprehensively determined by GC-MS.The derivatives were analyzed by a Shimadzu GC-MS 2010 Qplus with a Rxt-5 MS weak polar capillary MS column(30 m × 0.25 mm, 0.25 μm, Shimadzu Technology). Helium was used as the carrier gas at 0.87 mL · min-1 with a split ratio of 53:1 for the testing solution. The GC-MS detecting time was 35 min. Qualitative retrieval was conducted with similarity searching in NIST08 and NIST08 S coupled with Kovats Reservation Index(RI value) matching and quantitative analysis was performed by an external standard method and the ingredient peak responding value was adjusted according to the n-alkanes mixed standards that came from USA O2 Si calibration standards company. The difference in objective organic acids of all the 10 local variety samples was done through the Excel 2007 software and the Hierarchical Cluster Analysis(HCA) of organic acid composition among different samples was completed with SPSS20.0.【Results】Six standards for organic acids including malic acid, citric acid, oxalic acid,succinic acid, fumaric acid and oleic acid as well as their gradient regression equations showed that there was a high correlation between the standard concentration and their component peak area(R2≥92.9%). All baselines of the TICs were stable and all the component peaks were evenly distributed during the detecting period and their resolution was high. So the method for extracting by methanol, derivating by methyl ester and detecting by GC-MS was stable and reliable. A total of 43 organic acids including 9 short-chain carboxylic acids, 22 long-chain fatty acids, 4 aromatic dicarboxylic acids, 4 monobasic phenol acids and 2 amino acids were identified from the 10 fruit samples of different producing areas in China. The top 10 organic acids with the highest contents were dl-malic acid, citric acid, hexadecanoic acid, 9,12-octadecadienoic acid, 9-octadecenoic acid,(+/-)-10-hydroxy-octadecanoic acid, levulinic acid, stearic acid, 9,10-dihydroxy-octadecanoic acid and benzoic acid, respectively. There were 33 common ingredient peaks among the total 10 local varieties and their total contents of organic acids were between 85.02-170.76 mg · g-1. From high to low content, it showed like this: Linyi of Shandong >Jinghong of Yunnan > Zheng'an of Guizhou > Chun'an of Zhejiang > Qijiang of Chongqing> Changyang of Hubei > Xuancheng of Anhui > Yilong of Sichuan > Nanning of Guangxi > Baihe of Shaanxi.The total organic acid content had an extremely significant positive correlation with the long-chain fatty acids, mainly including hexadecanoic acid, 9,12-octadecadienoic acid and 9-octadecenoic acid; a significant positive correlation with the short-chain carboxylic acids, mainly consisting of dl-malic acid and citric acid; a negative correlation with the aromatic carboxylic acids, mainly containing benzoic acid.Analysis of the cluster according to 33 common components showed that all the fruit samples were classified into 4 categories when their clustering distance was 5: Yilong of Sichuan, Xuancheng of An'hui,Baihe of Shaanxi, Nanning of Guangxi, and Changyang of Hubei were clustered into the first group; Qijiang of Chongqing and Chun'an of Zhejiang were clustered into the second branch. Zheng'an of Guizhou and Linyi of Shandong were clustered into the third group, and Jinghong of Yunnan was alone clustered into the fourth group. The Hierarchical Cluster Analysis(HCA) coupled with the total organic acid content showed that Zheng'an of Guizhou and Linyi of Shandong belonged to the high-acid varieties, Qijiang of Chongqing and Chun'an of Zhejiang were the middle-acid varieties, and Yilong of Sichuan, Baihe of Shaanxi and Nanning of Guangxi belonged to the low-acid varieties.【Conclusion】Compared with the acid-base titration method and HPLC, GC-MS method not onlywas more stable, sensitive and accurate, but also could realize a qualitative identification of more chemical components. So it was much better to be used in the determination of total organic acid contents in fruits and their derived products. The results showed that there was a small difference among the different local varieties and an obvious difference existed in organic acid compositions and contents among the samples from different producing areas in China. And dl-malic acid and citric acid were the main components in most of C. speciosa fruit samples from 10 producing areas in China. Therefore, it belonged to the fruit type of malic acid accumulating mode.