| Beta-acids,an important component of hops soft resin,are considered to be nothing serious for the brewing industry.Although?-acids have good antioxidant and antibacterial activity,they are often discarded as by-product in the hop processing because of their instability.However,hexahydro-?-acids(HBA)are hydrogenated derivative of?-acids and have strong antibacterial activity.Its stability is better than?-acids.Therefore,HBA have a great application prospect in the field of food preservation.However,HBA are hydrophobic and can not fully contact pathogens in food with high moisture content,which considerably limits their application in the food industry.Cyclodextrins(CD)encapsulation technology is a useful approach for improving aqueous solubility and stability of low-solubility compounds.And CD is a safe encapsulating agent without toxicity and is well tolerated after oral,intravenous,and parenteral administration.Based on the previous work of our group,we selected2-O-methyl-?-cyclodextrin(M-?-CD)to prepare HBA/M-?-CD inclusion complex to improve its aqueous solubility.Different methods were used to characterze the structural of the HBA/M-?-CD inclusion complex.The inhibitory effect of the inclusion complex against a variety of food-related pathogenic bacteria was investigated,and the inhibitory mechanism against Listeria monocytogenes was also discussed.Then the preservation effect of HBA/M-?-CD inclusion complex in typical aqueous food system(tomato juice and milk)was studied,which provided a theoretical basis for the application of inclusion complex in aqueous food system.Finally,in order to broaden the application field of inclusion complex,it was added to chitosan(CS)as an active substance to prepare the antimicrobial film,which was used for packaging and preservation of chilled mutton.The main contents and results of this paper are as follows:(1)An inclusion complex of HBA and M-?-CD was prepared using the grinding method to improve the water solubility and bioavailability of HBA.Complex formation was assessed using ultraviolet and visible light(UV-Vis)spectroscopy,Fourier transform infrared spectroscopy(FT-IR),X-ray diffractometry(XRD),and nuclear magnetic resonance(~1HNMR).Results showed that the HBA molecule resided within the M-?-CD cavity and there were no new chemical bonds were created between HBA and M-?-CD.Therefore,they may interact via hydrogen bonds and van der Waals force.Scanning electron microscopy(SEM)results showed that the HBA/M-?-CD inclusion complex appeared irregular and block-like,which are different from the original morphology of HBA and M-?-CD.The optimal conformation of the complex was obtained by molecular docking.Phase solubility diagram indicated that HBA and M-?-CD formed an inclusion complex with a host-guest molar ratio of 1:1,which was a spontaneous process.The water solubility test indicated that the effect of the M-?-CD encapsulation extended beyond increasing the solubility of HBA,and the solubility was0.83 mg/m L.(2)The antibacterial activity of the HBA/M-?-CD inclusion complex against a variety of food-related microorganisms,including Escherichia coli,Staphylococcus aureus,Listeria monocytogenes,Candida albicans and so on,was determined.The inclusion complex exhibited strong antibacterial activity against all the tested microor-ganisms with dose–effect relationship.Specifically,the antibacterial activity of the complex to Listeria monocytogenes was higher than that of other pathogens,the minimal inhibitory concentration(MIC)is 12.5?g/m L and the minimum bactericidal concentration(MBC)is 100?g/m L.The bacteriostatic mechanism of the inclusion complex against Listeria monocytogenes was investigated according to the growth curve,cell morphology and cell membrane permeability.The results showed that the complex mainly inhibited the growth and reproduction of Listeria monocytogenes in logarithmic phase,destroyed the integrity and permeability of the bacteria,and caused the bacteria to die due to the leakage of electrolytes,nucleic acids and proteins.(3)The HBA/M-?-CD inclusion complex was applied to the preservation of tomato juice with potassium sorbate as positive control.The results showed that the content of vitamin C in tomato juice added with inclusion complex was about 3%higher than that in tomato juice added with potassium sorbate during the storage of 12days,and the antioxidant activity and acidity were also significantly better than that of tomato juice treated with potassium sorbate.Moreover,the tomato juice with inclusion complex showed obvious fresh-keeping effect at a small concentration.Potassium sorbate,as the most widely used food preservative in the world,has better effect on the maintaining the content of reducing sugar and the stability of lycopene than HBA/M-?-CD inclusion complex.(4)The effects of HBA/M-?-CD inclusion complex on physicochemical properties of pasteurized milk after storage at 4?for 27 days were investigated.During cold storage,the sensory,p H,lactose content,volatile base content,bacterial count,alcohol test,boiling test and acidity of pasteurized milk treated with HBA/M-?-CD inclusion compound were analyzed regularly.In contrast,the volatile basic nitrogen content,bacterial count and acidity of pasteurized milk treated with inclusion compound were lower than those of the control group during storage.The sensory score,p H and lactose were much higher than those of the control group.Among them,HBA/M-?-CD inclusion compound with concentration of 0.15 g/kg and 0.20 g/kg has the better preservative effect on milk,which can effectively inhibit the growth and reproduction of bacteria in pasteurized milk and enhance its sensory and physicochemical properties.Finally,accelerated shelf-life testing(ASLT)method was used to predict the shelf life of pasteurized milk with 0.15 g/kg HBA/M-?-CD inclusion complex under refrigeration(4?)for more than 21 days,which was consistent with the results of cold storage experiment.(5)CS-based films incorporated with various concentration of the HBA/M-?-CD inclusion complex were prepared and evaluated for structural,physicochemical,antioxidant and antimicrobial properties.The morphological and FT-IR demonstrated that there was a good compatibility between CS and HBA/M-?-CD complex and the intermolecular hydrogen bonds were probably formed.After addition of HBA/M-?-CD,the films showed decreased moisture content and increased water solubility,swelling ratio and water vapor permeability.Optical test showed that the addition of inclusion complex could not only maintain good transmittance,but also improved the UV light barrier property.The mechanical properties of the film were greatly increased with the incorporation of 0.10%HBA/M-?-CD,the value of tensile strength and elongation at break were 17.8 MPa and 97.7%,respectivily.The 2,2-diphenyl-1-picrylhydrazyl(DPPH)free radical scavenging activity of HBA/M-?-CD-CS films was enhanced more than ten times in comparison the control CS film.Furthermore,the incorporation of HBA/M-?-CD conferred the films with good antimicrobial activity against various foodborne pathogens.(6)The HBA/M-?-CD-CS composite film was used to package the chilled mutton,and the changes of various indexes of the meat samples during refrigeration were determined.The film could effectively inhibit the growth of bacteria in meat during the storage process,and delay the increase of TVB-N content,TBARS and p H value,while maintaining the sensory characteristics of fresh meat.Compared with PE,the film had a significant effect on the preservation of chilled mutton,which could extend the shelf life of 50%lean meat samples to more than 12 days,and 80%lean meat samples to more than 8 days. |
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