*3.2. Decay Index*

As shown in Figure 2, the cowpeas in the control group started to decay from the 3rd day of refrigerated storage. The decay index increased steadily with time during refrigerated storage. After the 6th day, the decay index increased sharply and reached 53.14% on the 15th day. However, the decay index of the cowpeas in the treatment group increased slowly during the entire storage period, and the decay index was only 21.04% on the 15th day, which is much lower than that of the control group (*p* < 0.05).

**Figure 2.** Effect of pullulan-based coatings on the decay index of cowpeas during refrigerated storage. Bars represent the standard deviation (*n* = 6).

### *3.3. Rust Spot Index*

As shown in Figure 3, the cowpeas in the control group started to show rust spots from the 3rd day of refrigerated storage. The rust spot rate continued to increase with time during refrigerated storage. After the 6th day, the rust spot rate increased sharply, reaching 64.07% on the 15th day. However, the rust spot rate of the cowpeas in the treatment group increased slowly throughout the entire storage period, and the value was only 5.72% on the 15th day, which is much lower than that of the control group (*p* < 0.05).

**Figure 3.** Effect of pullulan-based coatings on the rust spot index of cowpeas during refrigerated storage. Bars represent the standard deviation (*n* = 6).

### *3.4. Respiratory Rate*

The peak respiratory intensity of the control group was observed on the 6th day, while that of the treatment group was delayed to the 9th day and is lower than that of the control group (*p* < 0.05, Figure 4). On the 15th day of storage, the respiration intensity of the cowpeas in the treatment group was 17.09 mg/kg·h, which is 69.09% lower than that of the control group (*p* < 0.05), indicating that treatment with pullulan-based coatings incorporating E-polylysine and glutathione significantly inhibited the respiration intensity of the cowpeas and reduced the nutrient consumption during refrigerated storage (*p* < 0.05).

**Figure 4.** Effect of pullulan-based coatings on the respiratory rate of cowpeas during refrigerated storage. Bars represent the standard deviation (*n* = 6).

### *3.5. Soluble Solids*

Soluble solid contents in the cowpeas in the control group continuously decreased with time during refrigerated storage. After the 6th day, the soluble solid contents decreased sharply, reaching 3.81% on the 15th day. However, the soluble solid content of the cowpeas in the treatment group decreased slowly throughout the entire storage period with a value of 5.14% on the 15th day, which is much higher than that of the control group (Figure 5, *p* < 0.05).

**Figure 5.** Effect of pullulan-based coatings on soluble solids in cowpeas during refrigerated storage. Bars represent the standard deviation (*n* = 6).

### *3.6. MDA*

As shown in Figure 6, the MDA content of the control group increased slowly in the first 9 days of refrigerated storage and then increased sharply after 9 days of refrigerated storage, while the MDA content of the treatment group increased slowly during refrigerated storage and at lower levels than those of the control group (*p* < 0.05).

**Figure 6.** Effect of pullulan-based coatings on the MDA content of cowpeas during refrigerated storage. Bars represent the standard deviation (*n* = 6).

### **4. Discussion**

The edible part of the cowpeas was fresh and had tender pods, and the pods maintained a high moisture content, which is an indication of the preservation technology. After refrigerated storage, the treatment with pullulan-based coatings incorporating E-polylysine and glutathione significantly inhibited the increase in the weight loss rate of the cowpeas and effectively maintained a high moisture content (*p* < 0.05). Pullulan solution treatment could form a dense protective layer on the surface of the cowpeas, thus effectively reducing the respiration, transpiration and water loss of the pod [8].

Glutathione has potent antioxidant activity and can suppress PPO activity and browning and thus decrease the rust spot rate [10,11]. The preservation treatment formed wax-like coatings on the cowpea surface, thus reducing the mechanical loss during refrigerated storage. In addition, the coatings blocked the passage between the outside world and the fruit cells, preventing infection with aerobic microorganisms such as fungi, which effectively prevented spoilage of the cowpeas during refrigerated storage. Moreover, E-polylysine has broad-spectrum antibacterial activity, suppressing the growth of spoilage bacteria and preventing the spoilage of the cowpeas during refrigerated storage [9].

The browning of cowpeas after harvesting is a common phenomenon, and it often occurs in injured areas, especially the stem. After the harvest, the color of the cowpeas changed from dark green to light green or even yellow, resulting in rust spots. Glutathione has potent antioxidant activity and can suppress the PPO activity and browning, thus decreasing the rust spot rate [10,11].

Cowpea is a respiratory climacteric vegetable. As shown in Figure 4, the respiration intensity of the cowpeas during the whole storage period increased first and then decreased. The treatment with pullulan-based coatings incorporating E-polylysine and glutathione affected spontaneous air conditioning and preservation. The coatings formed on the surface of the cowpeas, delaying their physiological metabolism and remarkably

reducing the respiration intensity of the cowpeas, thereby inhibiting the process of other metabolic activities, reducing the consumption of nutrients and delaying the senescence of the cowpeas.

Soluble solids closely contribute to the taste and flavor of cowpeas. After the harvest, some sugar was degraded to carbon dioxide and water respiration, thus decreasing the soluble solid content in the cowpeas. The coatings that formed on the surface of the cowpeas reduced the physiological metabolism during refrigerated storage and inhibited the consumption of organic matter [12]. The treatment with pullulan-based coatings incorporating E-polylysine and glutathione maintained a higher soluble solid content during the refrigerated storage of the cowpeas and maintained a higher nutritional quality of the cowpeas than the control group (*p* < 0.05).

Environmental stress and tissue aging can decrease the ability of the tissue to scavenge active oxygen and induce the production of many free radicals that then produce MDA, and the MDA content can reflect the degree of membrane damage. The treatment with pullulan-based coatings incorporating E-polylysine and glutathione inhibited the respiration of the cowpeas during refrigerated storage, thereby inhibiting other physiological and biochemical activities in the cowpea cells, delaying the senescence of the cowpeas and inhibiting the accumulation of MDA. The coating treatment also reduced the weight loss rate of the cowpeas during refrigerated storage, thereby reducing the increase in cell membrane permeability. This indicated that the coating treatment can inhibit the occurrence of membrane lipid peroxidation to a certain extent, thus preventing the destruction of the cell structure and inhibiting cell senescence and death. Glutathione has potent antioxidant activity and can suppress membrane lipid peroxidation and thus inhibit the accumulation of MDA [10,11].

In conclusion, the treatment with pullulan-based coatings incorporating E-polylysine and glutathione effectively decreased the weight loss, decay and rust spot indices, the respiratory rate, and the MDA content, and increased the soluble solids during refrigerated storage compared with the control group. Therefore, the treatment with pullulan-based coatings incorporating E-polylysine and glutathione might be a promising method for extending the shelf life of cowpeas.

**Author Contributions:** A.H.—Investigation, Software, Data curation and Writing Original draft preparation. Y.M.—Conceptualization, Supervision, Validation, Reviewing and Editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by a project funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** No new data were created or analyzed in this study. Data sharing is not applicable to this article.

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

### **References**


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