Analysis of quality differences of the fruits harvested in different times in Huangguo pear at different harvest stages

【Objective】The study aimed to clarify the best harvest stage and provide a theoretical basis for timely harvest of Huangguo pear (Pyrus bretschneideri Rehd.) through investigating the changes of vitamin C (Vc), soluble solids, titratable acid, total phenols, total flavonoids, sugars, organic acids, and volatile compounds in Huangguo pears harvested in different stages.【Methods】Huangguo pears from Tongren City, Qinghai Province were harvested in different stages of 149 d (S1), 159 d (S2), and 168 d (S3) after flowering. The Vc in Huangguo pear was determined by Indophenol method, soluble solids were determined by Refractometer, titratable acid was determined by Titration of intrinsic acids, total phenols were determined by Folin-ciocalteu method, and total flavonoids was determined by Aluminum chloride colorimetric assay. High-phase liquid chromatography (HPLC) was used to determine the organic acids, high-performance liquid chromatography-mass spectrometry (HPLC-MS/MS) was used to determine the soluble sugars. Additionally, the volatile compounds were determined by headspace solidphase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) and gas chromatography- ion mobility spectrometry (HS- GC- IMS), respectively. The differences of volatile compoundswere analyzed by odor activity value (OAV) and relative odor activity value (ROAV) combined with variable importance in the projection (VIP).【Results】With the extension of the harvest stages, the contents of Vc and soluble solids in Huangguo pear increased continually, among which the contents of S3 samples were higher than those of S2 and S1 samples significantly. The titratable acid was decreased significantly in S2 samples. The solid-acid ratio showed a significant upward trend with the extension of harvest stages. The total phenol content in the peel was significantly higher than that in the pulp, and decreased with the extension of the harvest stages. It was pretty much the same for the change of total flavonoid content. A total of 8 organic acids were detected in Huangguo pear with great differences in different harvest stages. The content of malic acid accounted for 65.94%-73.97% and 88.89%-92.06% of the total organic acids in the peel and pulp, respectively. The malic acid in peel was increased significantly in S3 samples. The content of quinic acid was lower than malic acid and increased significantly in the pulp. The tartaric acid was increased and the citric acid was decreased with the extension of harvest stages. The oxalic acid, shikimic acid, succinic acid, and fumaric acid were also detected with lower contents in Huangguo pear. A total of 9 soluble sugars were detected in Huangguo pear fruit, among them glucose, fructose, sucrose, and sorbitol accounted for more than 98% of the total sugar content. The contents of fructose, glucose, and sucrose in the peel and pulp were accumulated significantly with the extension of the harvest stages (p＜0.05). The arabinose, maltose, maltitol, erythritol, and xylitol were also detected with lower contents in Huangguo pear. Based on HS-SPME-GC-MS and HS-GCIMS, 112 and 70 volatile compounds were identified in Huangguo pear, respectively. The volatile compounds were mainly esters, which contributed to the aroma of Huangguo pear mostly. To evaluate the contribution of each volatile compounds to the aroma of Huangguo pear, the OAV and ROAV were applied for analysis. 32 and 6 characteristic volatile compounds were identified, respectively, in which ethyl 2-methylbutyrate contributed to the aroma of Huangguo pear prominently. The ethyl 2-methylbutyrate with sweet odor, had the highest aroma activity value in the peel and pulp, and gave the greatest contribution to the aroma of S3 samples. To analyze the contribution of volatile compounds to the aroma Huangguo pear in different harvest stages, a total of 21 differential volatile compounds were screened and confirmed using OAV＞1 or ROAV＞1, with p＜0.05 and VIP＞1 as screening conditions. Among them, 12 and 15 differential volatile compounds were in the peel and pulp, respectively, with 6 differential volatile compounds, including ethyl caprate, ethyl caproate, ethyl 2,4 decadienoate, ethyl 2-methylbutyrate, ethyl isobutyrate, and octanal in both of peel and pulp. Most of differential volatile compounds with fruit aroma, pear aroma, and sweet aroma had the highest OAV values in S3 samples. 【Conclusion】The best harvest stage of Huangguo pear was S3, when the contents of Vc, soluble solids, solid-acid ratio, malic acid, fructose, and glucose were highest. The differential volatile compound of ethyl 2- methylbutyrate was accumulated significantly in S3 samples. The differential volatile compounds screened and confirmed in this study could be used as potential markers for the distinguishment of Huangguo pears harvested in different stages.