*3.1. Analysis of Apple Volatile Gas*

The volatile gas production during fungal spoilage of apples was recorded using the sensor and represented as sensor response value. The sensor response value indicated the response value of each sensor to the presence of certain gaseous chemicals over time.

The average response data of the VOC, CO2, O2 and C2H4 gas sensors of simulated warehouse apple samples are shown in Figure 4. The response value of the VOC sensor was 7 > 8 > 4 > 6 > 5 > 1 > 3 > 2, and the VOC content on the seventh and eighth days was much higher than that on the first and second days (Figure 4a). The results showed that the content of VOC gradually increased during the degradation of apples from fresh to severe spoilage. The microbial spoilage of apples led to changes in VOC emissions, which are classified as alcohols, terpenes, ketones, alkenes, benzenoids and sulfides [39].

**Figure 4.** Response of each sensor for different days of spoilage. (**a**) VOC, (**b**) CO2, (**c**) O2 and (**d**) C2H4.

The responses of the CO2 sensor indicated that the content on the eighth day was much higher than that on the first day: 1 < 4 < 3 < 2 < 7 < 5 < 6 < 8 in sequence (Figure 4b). The results showed that during the spoilage of apples, the CO2 release decreased gradually. The responses of the O2 sensor were 8 > 5 > 6 > 7 > 3 > 4 > 2 > 1 in sequence, and the content on the eighth day was much higher than that on the first day (Figure 4c). The results showed that the consumption of O2 gradually decreased during apple spoilage process due to the natural senescence process that causes cell and tissue to breakdown.

The responses of the C2H4 sensor were 7 > 6 > 8 > 5 > 4 > 3 > 2 > 1 in sequence, with the highest content on the seventh day and the lowest content on the first day (Figure 4d). During the decaying process of apples from fresh to severe spoilage, the production of C2H4 increased gradually, except that the content decreased slightly on the eighth day of treatment [24].

Based on the results obtained [24,40], it was found that with the intensification of the degree of spoilage, the release of VOC and C2H4 gradually increased, while the release of CO2 and the consumption of O2 were generally reduced. This was because the metabolic capacity of apples decreases as they spoil, leading to a lower consumption of O2 and a lower release of CO2. With the increase in spoilage time, the apple samples consume O2 and release CO2, VOC and C2H4, but as the degree of spoilage progresses, the metabolic capacity of apples is reduced, leading to the low consumption of O2 and the release of CO2.
