Effect of the GTP/KMG composite coating treatment on the quality of passion fruit
Shrinkage index
As presented in Fig. 1A, passion fruit shrinkage index increased gradually in both the control and treatment groups, while only modestly in the treatment group. Values for the various treatments showed significant differences (p < 0.05) up to day 6 in the fruit peel, while those of the T2 group showed significant differences until the 9th day. Moreover, the T2 also had a lower final shrinkage index than the other groups. Therefore, the T2 had the best effect on inhibiting the shrinkage index of passion fruit in comparison with other treatments, and the result was similar to the report of Bai et al. (2022). Different GTP/KGM composite coating treatments can effectively alleviate the shrinkage of passion fruit, and this membrane may affect the anaerobic respiration and metabolism of passion fruit.
Weight loss rate
The rate of weight loss in each group increased rapidly before day 3, and there was no significant difference between the treatment groups and control group (p > 0.05) (Fig. 1B). However, after day 3, the rate of weight loss of fruit slowed down, and there was a significant difference between the treatment groups and control group (p < 0.05). The result showed that the rate of weight loss of passion fruit in the treatment group was much less than the control group during storage, which revealed that the GTP/KGM treatment had remarkable effects on reducing the rate of weight loss compared with the control treatment.
Respiratory intensity
As seen in Fig. 1C, the respiratory intensity of fruit in each group tended to increase first and then decrease. It can be seen that passion fruit's respiratory peak reached the maximum in the T1 group, T3 group, and control group. Besides, the respiratory peak appeared on day 9 in the T2 group and was the best among all the treatment groups. The result showed that the GTP/KGM composite coating treatment could control the respiratory intensity and delay the time to reach the respiratory peak.
Total Soluble solids
As shown in Fig. 2A, the TSS of each group first increased and then subsequently decreased. The TSS reached its peak on day 6 and then decreased rapidly. The ripening process of the fruit may cause this phenomenon. A significant increase in TSS was observed in the treatment group over the control group, and the TSS in the T2 group was the highest in the period of post-harvest. On day 15, there was no significant difference between the treatment groups and the control group (p > 0.05). The result showed that the GTP/KGM composite coating treatment was beneficial in maintaining the TSS content of passion fruit.
Titratable acid
The TA content in passion fruit tended to increase first and then decrease during the preservation, and such a change was consistent with the change in the TSS content (Fig. 2B). On day 9, the peak values of TA in the control group, T1 and T3 group were 11.01%, 11.71%, and 12.2%, respectively, with no significant difference (p > 0.05). In contrast, the peak value of TA in the T2 group was 12.3%, which was significantly different from the control group (p < 0.05). Thus, the GTP/KGM composite coating treatment can inhibit the loss of TA and thus allow for better preservation of passion fruit.
Total sugar content
The total sugar content of passion fruit was observed in the control group over the treatment group within the first three days (Fig. 2C). Apart from the T1 reaching its peak on day 6, the total sugar level of the T2 and T3 were larger than the control group and reached the peak on day 9. The total sugar content of the treatment group was more than that of the control group after day 9, and there was a significant difference on day 15 (p < 0.05). The result demonstrated that the GTP/KGM composite coating treatment was beneficial in maintaining a high total sugar content in post-harvest passion fruit.
Vitamin C
The VC content fluctuated downward, which could be attributed to the breakdown of the peel cell membrane during the middle and late stages of preservation. (Zheng et al. 2019). Additionally, the VC content in the treatment group was larger than that in the control group (Fig. 2D). For day 18, the VC content of the control group was 18.13 mg/100 g, while it was 67.95 mg/100 g (T1), 47.99 mg/100 g (T2), and 19.97 mg/100 g (T3) in the treatment groups, respectively. The result showed that the GTP/KGM composite coating treatment could retard the degradation of VC and extend the shelf-life.
MDA content
MDA is one of the decomposition products of membrane lipid peroxidation, and it is a direct index of cell membrane injury and cell oxidation. The higher the MDA content, the higher the degree of membrane lipid peroxidation and damage (Khalifa et al. 2016). The MDA content of passion fruit fluctuated much less in the treatment groups than in the control group, as seen in Fig. 3A. Subsequently, the MDA content of passion fruit in the treatment group fluctuated slightly from 9 to 15 days, and the fluctuations were significantly different compared to the control group (p < 0.05). At the end of storage, its content in the control group reached the maximum value (3.583 nmol/g); T1, T2, and T3 were increased by 23.89%, 29.49%, and 24.97%, respectively. The result showed that the GTP/KGM composite coating treatment (T2) could significantly inhibit the accumulation of MDA, reduce the damage of tissue structure, and maintain the cell membrane integrity of passion fruit.
Flavonoid content
The flavonoid content of passion fruit in the T3 group reached its maximum value (61.77 mg/g) on day 9, which was significantly more than that in the other groups (p < 0.05). Besides, the flavonoid content of the control group, T1, and T2 reached maximum value on day 15 (Fig. 3B). There was a significant difference in flavonoid content between the treatment groups and the control group at the end of storage. However, no significant difference among tea polyphenol concentrations was observed (p > 0.05). The result showed that the GTP/KGM composite coating treatment could prevent the degradation of flavonoids and maintain the excellent quality of passion fruit.
Total phenolics content
The total phenolics content of passion fruit increased rapidly within the first six days of storage, and a significant difference was recorded between the treatment groups and the control group (p < 0.05). As presented in Fig. 3C, on day 9 and day 15, the total phenol content of treated passion fruit was significantly different (p < 0.05), which was more than that of the control group. The result showed that the GTP/KGM composite coating treatment could substantially retard the decline of total phenols in passion fruit.
Effect of the GTP/KMG composite coating treatment on the antioxidant enzyme activity of passion fruit
PPO activity
The maximum PPO activity of passion fruit in the treatment group was significantly much less than that of the control group on day 6, and there was a significant difference in PPO activity between the treatment groups and the control group on day 18. However, as shown in Fig. 4A, no significant difference among the coating treatment groups was observed (p > 0.05). The PPO activity in all treatment groups was much less than that in the control group at the end of the storage, and the T2 showed the best performance. Overall, the result showed that the GTP/KGM composite coating treatment could effectively inhibit PPO activity and delay the senescence and browning of passion fruit.
POD activity
As seen in Fig. 4B, the treatment of composite coating GTP/KGM significantly inhibited the POD activity of passion fruit compared to the control group, and the POD activity of passion fruit reached its maximum on day 6. The result showed that the GTP/KGM composite coating treatment could effectively inhibit the increase of POD activity in passion fruit, and the treatment effect of the T2 group was the best among all groups.
CAT activity
CAT can catalyze the decomposition of H2O2 into water and oxygen, thus removing free radicals and enhancing the antioxidant activity of plants (Gao et al. 2018). As presented in Fig. 4C, the CAT activity of passion fruit in each treated group tended to increase first and then decrease during storage. Meanwhile, the CAT activity of fruit in each group gradually increased and was recorded as highest in the treated group on day 12. Besides, the CAT activity of the T2 was recorded up to the maximum value in the T2, indicating that treatment could improve its activity. As the storage time increased, CAT activity gradually decreased in the control group and was higher in the treated group (p < 0.05). Therefore, the GTP/KGM composite coating treatment could improve the CAT activity and enhance passion fruit's defense and anti-aging ability.