Abstract:

Objective The annual waste of mulberry branches is high. In order to recycle and utilize the discarded mulberry branches and achieve high-value utilization of by-products from the sericulture industry, this research aimed to develop functional mulberry fibers from mulberry and silkworm resources and its products for preservation applications following the concept of sustainability.

Method The research used microbial-enzyme-ultrasonic extraction method and a wall-breaking technology to extract mulberry micro-nanofibers and prepare eugenol/mulberry micro-nanofiber membrane. The influences of eugenol content on antibacterial, moisture permeability, and mechanical properties of the membrane were studied and analyzed. Packaged kyoho grapes were used as example for preservation evaluation by using the membrane when the grapes were stored at 25 ℃. Grape appearance, weight loss ratio, decay rate and vitamin C(VC) content were used to evaluate the quality of kyoho grapes.

Results The physical properties of eugenol/mulberry microfiber membranes and nanofiber membrane were measured and analyzed. It was found that eugenol/mulberry microfiber membranes and nanofiber membranes showed good moisture permeability, with moisture permeability rates ranging from 1 000.58 to 1 048.83 g/(m²·d) and 1 918.68 to 1 996.19 g/(m²·d), respectively. Eugenol was proved to improve the antibacterial properties of the mulberry micro-nanofiber membrane, and the antibacterial rate of eugenol/mulberry skin microfiber membrane with eugenol content of 0.4 mg/mL(E/U0.4) and eugenol/mulberry skin nanofiber membrane with 0.3 mg/mL content of syringol(E/N0.3) against Staphylococcus aureus and Escherichia coli were close to 80% and 90% respectively. As the content of eugenol was increased, the breaking strength of the eugenol/mulberry micro-nanofibers membranes was decreased, and the breaking elongation was increased and then the followed by a decrease, reaching its maximum values of (6.14 ± 1.53)% and (7.51 ± 1.29)%, respectively. Subsequently, physical and chemical indicators such as appearance, weight loss rate, decay rate, and VC content of grapes were analyzed with the extension of storage time. The results demonstrated that grapes exhibited some degree of dehydration, softening, and skin wrinkling after storage. However, on the 8th day, the kyoho grapes wrapped in eugenol/mulberry skin microfiber membrane with eugenol content of 0.4 mg/mL (E/U0.4) and eugenol/mulberry skin nanofiber membrane with 0.3 mg/mL content of eugenol (E/N0.3) only show slight shrinkage and darkening of luster, and their overall sensory scores are higher than those of the other groups; The weight loss of grapes increases with the extension of storage time. After 4 days of storage, E/U0.4 and E/N0.3 can significantly inhibit the increase of grape quality loss rate; On the 8th day, the quality loss rate of kyoho grapes wrapped in ordinary plastic wrap is 24.20%, which is about 34% higher than that of E/U0.4 and E/N0.3; Grapes wrapped in E/U0.4 and E/N0.3 begin to rot on the 6th day, which is relatively late; On the 8th day, the grape decay rates of the E/U0.4 and E/N0.3 groups are 29.94% and 60.73% lower than those of the ordinary plastic wrap, respectively; The VC content of grapes shows a decreasing trend with increasing storage time. On the 8th day, the VC content of grapes in the E/U0.4 and E/N0.3 groups are 50.84% and 51.52% higher than that of ordinary plastic wrap, respectively.

Conclusion The results of above characterizations indicate that eugenol/mulberry microfiber membranes and nanofiber membranes have good moisture permeability and mechanical properties. Among them, E/U0.4 have an antibacterial rate of nearly 80% against Staphylococcus aureus and Escherichia coli, while E/N0.3 have an antibacterial rate of about 90%. Both have good moisture permeability and mechanical properties, and can effectively maintain the freshness of grapes at room temperature, slow down water loss, decay, and inhibit the loss of VC. Further research is needed on the preservation effect of mulberry micro-nanofiber membrane on different fruits and under different environmental conditions.

Key words: mulberry fiber, microbial-enzyme degumming, micro-nanofiber membrane, eugenol, fruit preservation