3.4.3. Physicochemical Properties

Changes in color, hardness, weight loss, pH, TSS content, and Vc content of cherry tomatoes packaged with AGO/SPEEK films during storage are shown in Figure 7. Color is the main external quality criteria affecting the marketability [4]. Figure 7a demonstrates that the ΔE of cherry tomatoes in different packages shows an increasing trend, and color changes of AGO/SPEEK films packaged are relatively minimal in different periods,

probably owing to the reduction of ethylene synthesis influenced by the high content of CO2. This is in good agreement with the gas permeability results of AGO/SPEEK films discussed earlier. Specifically, high loading of AGO leads to enhanced CO2/O2 selective permeability of the film and it is easier to form a high CO2 and low O2 gas atmosphere, which thus slows down the accumulation of pigment and the browning process of tissues caused by the aging of cherry tomatoes [4,21].

**Figure 7.** Physicochemical properties of AGO/SPEEK films in the storage of cherry tomatoes. (**a**) Total color difference, (**b**) hardness, (**c**) weight loss, (**d**) pH, (**e**) total soluble solid content, (**f**) vitamin C.

Furthermore, hardness of cherry tomatoes is an important factor for the estimation of maturity [37]. Figure 7b illustrates that the hardness of cherry tomatoes in all groups exhibited a decreasing trend with the extension of storage time, and the control group was more significant, which depends on the progress of the conversion of insoluble pectin to soluble pectin or even pectin acid [38]. The hardness of cherry tomatoes packaged with 0.9% (*w/w*) AGO/SPEEK film decreased by 47.4%, which is similar to the result reported by Zhang et al. [37]. Thus, the AGO/SPEEK film can efficiently slow down the softening of the cherry tomatoes.

Transpiration and respiration of the fruits during storage lead to loss of water and weight, which can cause a wilting phenomenon and shorten the postharvest life of fresh fruit. According to the reported, cherry tomatoes will cause severe wilting when the weight loss is about 11%, while at about 6% weight loss they will not wilt [39,40]. Figure 7c shows that the weight loss of cherry tomatoes was the least when the packaging was done with 0.9% (*w/w*) AGO/SPEEK film and exhibited an increasing trend in the following order: 0.5% (*w/w*), 0.1% (*w/w*), and the original film. Undoubtedly, the samples in the control group were significantly softened and cracked and exhibited the highest loss of weight of up to 8% after 30 days of storage, while that of others increased to 3–4%, which benefits from the water resistance of films inhibiting the evaporation of water. Notably, that is coherence with the results of WVP shown in Figure 4c.

Moreover, pH is an important reference for evaluating the quality of cherry tomatoes during storage because it uses organic acids as a substrate for respiration [19]. Figure 7d shows a slight increasing trend of pH from 4.2 to 4.8, which coincided with the results of hardness caused by increasing pectin acid. The pH of control samples increased more significantly mainly resulting from no packaging, which is consistent with the report by D'Aquino et al. [41].

TSS refers mainly to soluble sugars and is a very important indicator for determining consumer acceptability [42]. Figure 7e exhibits that the TSS content increased at the beginning of storage (5 days), benefiting from the accumulation of nutrients brought by maturity. However, it decreased to varying degrees in the later stage, caused by the hydrolysis of sucrose for respiration and maintaining the physiological activity of plant raw materials [42]. Noteworthily, cherry tomatoes packaged in a 0.9% (*w/w*) AGO/SPEEK film showed a slower decrease in TSS compared with others, indicating a delay in the ripening of cherry tomatoes under high concentration of CO2 [43].

Figure 7f shows that the Vc content in cherry tomatoes increased initially and then decreased during the storage period, similar to the behavior described by Zhou et al. [30]. The increased VC content of cherry tomatoes was attributed to the accumulation of postripening effect, while the decrease resulted from the high rates of respiration, oxidative deterioration, and accumulation of CO2 [30,41]. Noteworthily, the VC content of cherry tomatoes packaged in 0.5% (*w/w*) or 0.9% (*w/w*) AGO/SPEEK films was basically maintained at 30 mg/100 g at the end of storage, much higher than others, benefited from favorable oxygen barrier properties of AGO/SPEEK films that effectively prevented the oxidation of VC.

#### **4. Conclusions**

AGO/SPEEK films were prepared by physical blending for modified atmosphere packaging. A series of data revealed that the loading of AGO improves the mechanical properties, gas permeability, and selective permeability of CO2/O2 compared to the original film, while reducing the water uptake ratio, the swelling ability, and the water vapor permeability. Preservation experiments revealed that 0.9% (*w/w*) AGO/SPEEK films delayed the maturation process and doubled the shelf life of cherry tomatoes. Overall, these findings demonstrated that AGO/SPEEK films have great application potential in the preservation of fruits with high moisture content and low respiration rate. Undeniably, many more systematic explorations are still demanded to confirm the safety and service life of the film when storing various food products, which will be pursued in the future.

**Author Contributions:** Conceptualization, K.X., Y.C., and H.D.; methodology, Y.C. and H.D.; formal analysis, K.X., Y.C., and H.D.; data curation, Y.C. and H.D.; writing—original draft preparation, Y.C., H.D., and Y.W. Both Y.C. and H.D. are first authors and contributed equally to the manuscript and are listed in alphabetical order. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was financed by the Research and Development Plan in Key Areas of Guangdong Province (2019B020222001), Guangzhou Rural Science and Technology Commissioner Project (GZKTP201932), and Guangdong Basic and Applied Basic Research Foundation (2019A1515111213).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The study did not report any data.

**Conflicts of Interest:** The authors declare no conflict of interest.
