Study On The Synergistic Effects And Mechanism Of Carbon Dots Combined With Radio Frequency On Quality And Microbial Safety Of Typical Huaiyang Flavor Prepared Dishes

Prepared dishes generally refer to the food products using agricultural,livestock,poultry and aquatic products as raw materials.After processing and packaging,prepared dishes are usually stored,circulated and sold under the conditions of freezing,refrigeration or normal temperature,and can be eaten directly or after simple processing.To prolong shelf life and ensure consume safety,prepared dishes need to be sterilized.Although the traditional high temperature and high pressure sterilization has good sterilization effect,it has an adverse effect on the sensory and nutritional quality of prepared dishes.To solve the problems above,this paper developed a new mild pasteurization technology,that is the combination of radio frequency(RF)and carbon dots(CDs)or CDs active packaging.Five kinds of commonly consumed materials(pork,crab meal,pakchoi,carrot and dried beancurd)were used to analyze the pasteurization applicability of the new technology.On this basis,the synergistic effects and corresponding mechanisms of the new technology on the microbial safety and product quality of prepared dishes(boiled gansi,boiled crab meatball,Huaiyang flavor meatballs)were further studied.CDs were synthesized by hydrothermal method and characterized.The antibacterial and antioxidant properties,special optical properties and biosafety of CDs were studied.The feasibility of CDs as nano antibacterial agents/antioxidants for food preservation was also discussed.The results showed that the synthesized CDs was monodispersed and spherical,the particle size ranged from 0.54～1.5nm,and there were a large number of hydroxyl and carboxyl functional groups on the surface.The CDs can scavenge DPPH radical,hydroxyl radical and superoxide anion radical,inhibit the growth of Escherichia coli(E.coli),Staphylococcus aureus(S.aureus)and Bacillus subtilis(B.subtilis).It had no obvious biological toxicity to Hep G2 cells within 40～1280μg/m L.Addition of CDs into frying oil can delay oil’s oxidative rancidity,delay the increase of TBARS,carbonyl and TVB-N values of fried meatballs during storage.CDs active film can inhibit the formation of black spots on banana peels and prolong its shelf life.Escherichia coli,Bacillus subtilis,Candida albicans were selected to study the bactericidal effects of CDs,RF,and combination of CDs and RF(CDRF).The effects of CDs concentration,plate spacing and treatment time on the cell membrane/wall permeability,Na~+/K~+-ATPase activity and the leakage of UV absorbing substances were also studied.The results showed that the most suitable plate spacing was 20 mm.The ideal bactericidal effect can be achieved when the CDRF treatment time was 20 min or 30 min at CDs concentration of 0.2 mg/g.CDs and RF treatment changed the permeability of cell membrane,destroyed bacterial cell wall and cell membrane,resulting in the leakage of cellular contents and the secretion of increased Na~+/K~+-ATPase from cell membrane to resist external stimulation.Moreover,the effect of CDRF treatment on bacterial cell membrane/wall and leakage of cellular contents were significantly higher than that of CDs or RF alone.Pork,crab meal,dried beancurd,carrot and pakchoi were selected to compare the effects of CDs,RF and CDRF in terms of microbial level and food quality,as well as the selection of optimum pasteurization parameters.The results showed that under the conditions of 20 mm plate spacing and 0.02 mg/g CDs,the CDRF 20 min and 30 min had ideal bactericidal effect.The synergistic effect of CDRF was not only reflected by the synergistic effect of pasteurization,but also in reducing the lipid oxidation degree of samples.However,CDRF 30min had adverse effects on samples,in particular,that is the color/hardness/TBARS content of pork samples,the color/hardness/conductivity/carotenoid content/water distribution of carrot samples,and the color/flavor substances/TBARS content of crab meal samples.Therefore,it is unsuitable for pasteurization of such samples.CDRF 20 min also had adverse effects on pakchoi’s color/conductivity/chlorophyll content and dried beancurd’s color and texture.Therefore,the pasteurization parameters of such samples need to be further optimized.Furtherly,the pasteurization parameters were optimized to study the bactericidal effect of CDRF treatment on boiled gansi dishes with dried beancurd as raw material.The effects on product quality and corresponding mechanism were studied.The results showed that when the CDs concentration was increased up to 0.24mg/g,CDRF 12 min and CDRF 16 min had better bactericidal effect.Similar to high pressure steam sterilization(HPS),CDRF 16 min changed the secondary structure of protein,facilitated the transformation of water from the gel network to free water.Moreover,HPS increased the odor of spices and promoted the formation of aldehyde,alcohol and other flavor substances.CDRF 12 min was more conducive to maintaining sample quality.After refrigerated at 4℃for 8 months,the total bacterial colonies(TBC)did not exceed the national standard limit,and the sensory score was higher than that of HPS samples.The applicability of CDRF pasteurization on boiled crab meatball dish was tested.The effects of CDRF pasteurization on sensory quality,such as color,texture and flavor,and corresponding mechanism were investigated.The results showed that when the CDs concentration was 0.20 mg/g,CDRF 20 min and CDRF 30 min had better bactericidal effect.Similar to HPS,CDRF 30 min changed the secondary structure of protein,reduced the water holding capacity.In addition,HPS significantly increased the types and quantity of volatile flavor substances,promoted the formation of trimethylamine,and significantly changed the flavor of the sample.CDRF 20 min was more conducive to maintain sample quality.After refrigerated at 4℃for 8 months,the TBC of CDRF 20 min samples did not exceed the national standard limit,and the values of TBARS and TMA were significantly lower than those of HPS samples.Carbon dots/polyvinyl alcohol(CDs/PVA)composite films with different concentration of CDs were prepared.Film’s structure,UV barrier,water resistance,antibacterial and antioxidant properties were tested.Moreover,0.50%CDs/PVA film combined with RF was furtherly selected to investigate their effects on the quality and safety of Huaiyang flavor meatballs during storage.The results showed that the CDs/PVA films have photoluminescence and UV barrier properties.CDs enhanced the thermal stability,mechanical properties and water resistance properties of the films.The active packaging can scavenge ABTS and DPPH free radicals and inhibit the growth of B.subtilis,E.coli and S.aureus.During refrigerated at 4℃,CDs/PVA+RF 20 min and CDs/PVA+RF 30 min extended the shelf life of meatballs from 14 d to 42 d and 49 d respectively,inhibited lipid and protein oxidation and improved sensory quality.