## 3 Results and Discussion
### Changes of VOCs in the Flesh of Eight Pear Varieties at Different Shelf Life  
A total of 18 shared VOCs were simultaneously detected in the flesh of the eight pear varieties, as seen in Figure 3. These included two alcohols: 1-Hexanol and 2-Ethylhexanol; one ester: 2,2,4-Trimethyl-1,3-pentanediol disobutyrate; seven aldehydes: Hexanal, (E)-2-Hexanal, Octanal, (E)-2 Heprenal, Nonanal, (E)-2-Octanal, (E)-2-Nonenal; three ketones: 1-Hepten-3-one, 6-Methyl-5-heptene-2-one, (E)-1-(2,6,6-Trimethyl-1,3-cyclohexation-1-yl)-2-buten-1-one; three acids: Hexanoic acid, Octanoic acid and Nonanoic acid; and two other compounds: trans-\(\alpha\),\(\alpha\),5-Trimethyl-5-ethenyltetrahydro-2-furanmethanol and 3,5-bis(1,1-Dimethylethyl)phenol.  
Aroma is a complex mixture of multiple VOCs [10], and the flavor of peers is closely related to the content and relative proportion of VOCs [34]. The types and contents of VOCs in different pear varieties changed in the course of shelf life (Figure 3). Throughout shelf life, the VOCs in Korla peer were mainly alcohols and aldehydes, and the high concentration of aldehydes and alcohols and the low concentration of esters contributed to a unique \"fresh green\" flavor [17]. The esters, alcohols and aldehydes were dominant in Yali and Hanhongli. This was similar to the results obtained in Laiyang pear (_P. bretschneideri_) [35]. With the prolongation of shelf life, the content of alcohols in Yali was always higher than that of Korla pear and Hanhongli, while the content of alcohols in Korla pear was lower than that of Hanhongli at shelf life of 0-7 d and higher than that of Hanhongli at shelf life of 14-21 d, as seen in Table 2. The aldehyde content of Korla pear flesh during shelf life was always higher than Yali and Hanhongli, while aldehyde content in Hanhongli was only higher than that of Yali during the shelf life of 14 d, and was lower than that of Yali in the rest of shelf life. Ester content in Hanhongli was lower than that of Yali only in the shelf life of 0 d and 21 d, and higher than that of Yali in the rest of shelf life. The content of the rest of the compounds in the three crisp-flesh pears was at a low level throughout shelf life.  
Figure 3: Composition and content of VOCs of the eight pear varieties at different shelf life.  
Among the soft-flesh pears, the VOCs in Nanguoli, Jingbaili and Ruanerli were mainly esters, alcohols and aldehydes during the whole shelf life, as seen in Figure 3. Hydrocarbons were also the main VOCs of Nanguoli and Jingbaili during the shelf life of 14 and 21 d. Other researchers observed similar results that esters, alcohols, aldehydes and alkenes were the main aroma constituents in seven _P. ussuriensis_ cultivars, including Anli, Jingbaili, Nanguoli and so on [34]. Esters and aldehydes were the key VOCs shared by 33 Chinese _P. ussuriensis_ cultivars [22]. The VOCs in Cure and Louis were mainly alcohols, aldehydes and ketones throughout shelf life. Moreover, esters were also the major VOCs in Louis flesh at 21 d of shelf life. In a previous study, it was noted that the main VOCs in European pear varieties were esters, alcohols and alkanes [9]. With the extension of shelf life, among the five soft-flesh pears, the alcohol content in Ruanerli was always at a higher level during the shelf life of 0-14 d, and was lower than that of Nanguoli and Jingbaili during the shelf life of 21 d in Table 2. During the whole shelf life, the alcohol content in Ruanerli, Nanguoli and Jingbaili was consistently higher than that of Louis and Cure. The aldehyde content in Jingbaili and Ruanerli was at a high level throughout the shelf life. The ester content in Ruanerli was always at a higher level during the shelf life of 0-7 d, but lower than Nanguoli in the shelf life of 14-21 d. The ketone content in Cure was higher than that of Louis during the shelf life of 0-7 d, but lower than Louis during the shelf life of 14-21 d.