## 3 Results and Discussion
### Comparative Analysis of Egg Yolk Volatiles  
In Figure 2A, it could be seen that the volatile substances in egg yolk were mainly alkanes, terpenes, aldehydes, ketones, alcohols, and amines. The content of alkane compounds decreased with increasing ozone treatment time, and no volatile alkane compounds were detected after 30 min and 40 min of ozone treatment time. The content of terpenids in the egg yolk remained low and did not change crucially with the increase in treatment time. In addition, aldehydes and alcohols could be found as the main contributing components of egg yolk volatiles [22]. Among them, the content of aldehydes produced a certain degree of increase with the prolongation of ozone treatment time, while the alcohol compounds showed a decreasing trend. The content of ketones and amines in the egg yolk also had a certain degree of reduction with increasing treatment time. Ozone acted as an oxidizing agent to induce oxidative degradation of some compounds in the egg yolk that were sus  
Figure 1: Distribution of volatile species and relative content of egg yolk under different ozone treatment times. (**A**) 0 min; (**B**) 10 min; (**C**) 20 min; (**D**) 30 min; (**E**) 40 min.  
ceptible to oxidation [23]. The decrease in alkanes, alcohols, ketones, and amines indirectly reflects the ozone attack on these compounds under the oxidation of ozone, which in turn triggers their conversion to other oxidation products [24].  
Figure 2: Composition of egg yolk volatile fraction species under different ozone treatment times. (**A**) Total volatiles; (**B**) aldehyde volatiles; (**C**) Wayne diagram.  
Since aldehyde volatiles were the main egg yolk volatiles, the mechanism of ozone action on egg yolk volatile substances was investigated by analyzing the classification and relative content of aldehydes. Figure 2B shows the classification and relative contents of egg yolk aldehydes volatiles under different ozone treatment times. Valeraldehyde, hexanal, heptanal, octanal, 2,5-dihydroxybenzaldehyde, decanal, undecanal and nonanal were the main components of egg yolk aldehyde volatiles, among which nonanal and octanal had the highest contents. In addition, it was found that the content of hexanal and octanal in the egg yolk showed a decreasing trend with the increase in ozone treatment time. However, a significant increase in the relative content of glutaraldehyde occurred with increasing treatment time. Aldehyde volatiles were also transformed after ozone treatment, and it is assumed that aldehydes with longer carbon chains in the yolk were attacked by oxidation, resulting in shorter carbon chains [25]. As shown in Figure 2C, a Venn diagram was used for the classification of egg yolk volatiles at different ozone treatment times, and 14 volatiles were identified collectively from the five treatment groups. One volatile substance was identified with the control group by ozone treatment for 10 min, and one volatile substance was identified with the control group by ozone treatment for 20 min. Overall, ozone treatment reduced unfavorable volatiles, such as amines, which tended to have a negative impact on the flavor of egg yolks [26]. In addition, ozone did not destroy components with a relatively large contribution of volatiles and also increased the relative content of aldehyde volatiles. Low molecular weight aldehydes had an unpleasant odor, but as the molecular weight of the aldehyde increased, fruit flavors began to emerge [27]. The content of aldehydes with long carbon chains in the egg yolk increased substantially after ozone treatment (10 min, 20 min, and 30 min), thus contributing to the improvement of the egg yolk's flavor. Egg yolks contain the highest relative proportion and variety of aldehyde volatiles [26]. Therefore, aldehyde volatiles make an important contribution to the flavor of egg yolks. The main reason for the increase in aldehyde volatiles could be the oxidation of fatty acids caused by ozone treatment.