| With the increasing demand of food quality and environmental protection,higher requirements have been put forward to food packaging materials.Degradable,natural and non-toxic edible films have become a research hotspot.Tuna skin is the waste of tuna processing,which is unemployed or even abandoned.It is economical and environmental to extract collagen from tuna skin.At present,edible films still have the disadvantage of poor mechanical properties,which restricts its industrial application seriously.As a green physical processing technology,ultrasound can accelerate grafting reaction of protein and polysaccharide,change the polymer structure and improve the mechanical properties of edible film.Pomegranate peel is the waste of pomegranate processing,which is rich in active polyphenols.Adding it to edible film can improve the antioxidant and bacteriostatic properties of the film.In this paper,tuna skin collagen was fully utilized to prepare collagen-chitosan edible films,sweep frequency and pulsed ultrasound technology was introduced to prepare composite films,and pomegranate polyphenols were added to modify the film to optimize the preparation process.The effects of ultrasound and polyphenol modification on the structure,thermal stability,antioxidant and bacteriostasis properties of the composite films were investigated.Finally,the films were used to preserve pork and investigated its preservation effect.The main conclusions are shown as follows:1.The optimum preparation parameters of ultrasound assisted films were obtained as follows:ultrasonic frequency of 28 kHz,power density of 100 W/L,sweep frequency cycle of 100 ms,sweep frequency amplitude of±0.5 kHz,duty cycle of 77%(10 s/3 s)and ultrasonic time of 10 min.Under these conditions,compared to traditional composite films,ultrasonic treatment significantly increased the tensile strength and elongation at break of the films by 27.14%and 16.54%,respectively,and decreased the water vapor permeability by 12.15%.Moreover,it was concluded that ultrasonic treatment improved the structure of composite films.Fourier transform infrared spectrum(FTIR)results showed that ultrasound enhanced the hydrogen bonding and covalent binding between collagen and chitosan,and made the combination of the two closer.X-ray diffraction(XRD)results showed that ultrasonic treatment reduced the crystallinity of composite films,and improved the toughness of the films.Circular dichroism(CD)results showed that ultrasonic treatment changed the secondary structure of composite films,increased?-helix and?-sheet and reduced?-turn and random coil,and made the structure more compact and stable.Scanning electronic microscopy(SEM)results showed that ultrasonic treatment made the surface structure of composite films more compact.The thermal decomposition temperature(T_d)of the composite films was significantly increased by ultrasonic treatment,but the thermal denaturation temperature(T_g)and antioxidant and bacteriostatic properties were not significantly affected.2.The tensile strength and antioxidant properties of ultrasound prepared composite films were further increased by gallic acid(GA),tannic acid(TA)and ellagic acid(EA)modification,while the elongation at break,water solubility,water vapor permeability,transmittance and chrominance were decreased.Among them,GA modified composite films had the best comprehensive properties.The optimum modification conditions were obtained as follows:GA concentration of 1.0 g/L,and reaction time of 10 minutes.In these conditions,compared to the ultrasound prepared composite films without GA modification,the tensile strength and DPPH radical scavenging rate of ultrasound prepared films increased by 36.85%and 23.70 times,respectively.The water vapor permeability and transmittance decreased by 13.70%and 29.65%,respectively.Moreover,it was concluded that GA modification changed the molecular structure of ultrasound prepared composite films.FTIR results showed that the addition of GA enhanced the hydrogen bonding between chitosan and collagen,but reduced the covalent binding and made the structure loose.CD results showed that GA modification changed the secondary structure of composite films,reduced?-helix and?-sheet,increased?-turn and random coil,and made the structure looser.XRD results showed that GA modification increased the crystallinity of ultrasound prepared composite films.SEM results showed that the surface structure of ultrasound prepared films was loosened and roughed by GA modification.The T_d of the ultrasound prepared films was slightly decreased by GA modification,but it had no significant effect on the T_g.The GA modified ultrasound composite films still had a good thermal stability.GA modification showed a significant improvement on the bacteriostasis of four pathogenic bacteria.The order of bacteriostasis effect was Bacillus subtilis>Escherichia coli>Listeria spp>Staphylococcus aureus.3.The results of pork preservation test showed that the values of pH,total volatile basic nitrogen(TVB-N),and total bacterial count in the control group and packed group by GA modified ultrasound films increased with the increase of storage time.However,the increase rate of packed group was significantly lower than that of the control group.The indexes of hardness,elasticity,chroma and sensory in the control group and packed group by GA modified ultrasound films decreased with the increase of storage time.However,the decline rate of packed group was significantly lower than that of the control group. |
PDF Full Text Request