## 3 Results and Discussion
### Changes in VOCs during Storage
As shown in Table 2, total ester content increased from 32.93 to 36.29 mg/kg FW after 14 days storage at 4 \({}^{\circ}\)C, and then rapidly reached 60.18 and 63.43 mg/kg FW after a further 7 days storage at 4 \({}^{\circ}\)C and 20 \({}^{\circ}\)C, respectively. In general, esters with various carbons exhibited distinct patterns (Table 2). Butyl acetate (C6) and hexyl acetate (C8) were the crucial esters in pear at each stage, and their content experienced a significant increase. This finding was similar to that in a previous study, in which acetyl esters yielded a strong production in Bartlett pears during storage [30]. The contents of C9-C10 esters exhibited no significant difference between fruits at various storage stages (Table 2). The content of hexyl hexanoate (C12) decreased from 0.30 to 0.11 mg/kg FW during 21 days storage at 4 \({}^{\circ}\)C. However, C9-C12 esters had a 2-4-fold increase after 7 days at 20 \({}^{\circ}\)C. Thus, C9-C12 esters might be the specific aroma-active VOCs in Dr. Guyot pear after storage at room temperature [29]. During the storage period at 4 \({}^{\circ}\)C, the content of C17-C19 esters notably decreased from 0.14-2.12 mg/kg FW to undetectable. On the contrary, the content of ethyl oleate (C20) gradually increased during 21 days storage at 4 \({}^{\circ}\)C (from 0.14 to 0.40 mg/kg FW). Specially, its content decreased to 0.09 mg/kg FW after transfer to 20 \({}^{\circ}\)C. It is well known that the greatest amounts of flavor compounds are derived from esters during the growth of the fruit [31]. Therefore, C17-C20 esters might be the precursors of flavor compounds in pears after harvest. As shown in Table 2, 12 alcohol compounds with content of at least 0.10 mg/kg FW were detected in Dr. Guyot pear at the beginning of the storage period. After 21 days storage at 4 \({}^{\circ}\)C, five of these compounds became undetectable. Specially, the total content of alcohols increased from 6.45 to 16.47 mg/kg FW after 14 days storage at 4 \({}^{\circ}\)C, and exhibited a slight decrease at the remaining stages. However, only five of the alcohols identified had strong production, especially 1,3-octanediol, the content of which increased from undetectable to 11.06 mg/kg FW. Storage at 20 \({}^{\circ}\)C contributed to an obvious degradation of 1,3-octanediol (from 11.06 to 5.28 mg/kg FW), which was the main reason for the decrease in total alcohol content (from 16.47 to 12.31 mg/kg FW) after 14 days. As the primary contributors to fruit aroma, aldehydes are the predominant VOCs in immature fruit [27]. Nine of the aldehydes with content within the range of 0.10-0.88 mg/kg FW were provisionally identified in Dr. Guyot pear. During 14 days storage at 4 \({}^{\circ}\)C, the concentration of aldehydes showed a significant decrease, with four of them becoming absent (Table 2). According to previous studies, degradation of aldehydes in fruits during storage was caused by catalyzation of alcohol dehydrogenase (ADH) [32; 33]. The only exception was hexanal, the content of which was relatively stable at all stages. There were 3 ketones detected in pear with a concentration of 0.32-1.05 mg/kg FW, including 1-hepten-3-one, sulactone, and damascenone. As shown in Table 2, storage led to a decrease in 1-hepten-3-one content, while, sulactone and damascenone had a relatively stable content at all the stages. Total content of alkenes gradually increased from 8.28 to 25.04 mg/kg FW during storage at 4 \({}^{\circ}\)C. Only the content of \(\beta\)-farnesene exhibited a stable or decreasing trend. Production of (Z,E)-\(\alpha\)-farnesene, (E,E)-\(\alpha\)-farnesene, and \(\alpha\)-terpinene experienced a rapid rise during storage, increasing from 0.18-7.69 to 1.83-17.64 mg/kg FW. Storage at 20 \({}^{\circ}\)C result in a strong production of all alkene compounds, especially (E,E)-\(\alpha\)-farnesene, which increased from 13.10 to 30.63 mg/kg FW.