| As one of the most commonly used spices in daily life,dried chilies are popular for their unique pungent flavor and attractive color.However,processing technology for dried chilies is deficient in China.During storage,the dried chilies absorb moisture very easily,this led to microorganisms multiply quickly even excessive the standard required level.The currently used sterilization methods are resisted by consumers due to long sterilization time,high energy consumption,lack of environmental protection,and irradiation residues.Therefore,there is an urgent need for a safe,efficient and environmentally friendly technology to sterilize microorganisms in dried chilies to meet the increasing demand for high quality products from both companies and consumers.Pulsed light technology as a non-thermal sterilization technology,has received widespread attention in the food processing industry due to the advantages of environmental protection,efficient sterilization,easy operation.However,pulsed light is light sterilization with weak penetration,which can only sterilize microorganisms on the food surface,so it has certain limitations in dealing with complex materials and needs to be combined with other technologies to make up for the deficiency.Curcumin is a natural photosensitizer that can be stimulated by light irradiation for photodynamic sterilization to achieve a sterilizing effect on the surrounding environment.The applicability of pulsed light combined with curcumin to the sterilization of dried chilies were explored deeply from several aspects,included comparing the bactericidal efficacy of single treatment methods and combined treatment methods and the physicochemical qualities of samples after different bactericidal treatments,revealing the bactericidal mechanism and bactericidal kinetics of combined treatment methods,and verifying the feasibility of combined bactericidal test at pilot scale and storage tests,so as to provide a theoretical basis and practical reference for industrial promotion.This study provided theoretical basis and practical reference for industrial promotion.The main research work and conclusions were as follows:(1)The dominant bacteria and fungi in dried chilies were isolated and identified.High-throughput sequencing was used to determine the bacteria and fungi of dried chilies from seven dominant producing areas in China,in order to study the inactivation effect of sterilization technology on microorganisms in dry chilies.The results showed that bacterial and fungal diversity in dried chilies from the seven main production areas were similar.On the surface bacteria of dried chilies,Proteobacteria accounted for the highest proportion in all samples at the phylum level,Gammaproteobacteria and Enterobacteriaceae were dominant at the class and family levels,respectively.At the genus level,the Escherichia was present and was the dominant bacterial genus in dried chilies.From the analysis of fungal colony diversity in dried chilies,Ascomycota dominated at the phylum level,Eurotiomycetes had the highest percentage among all samples at the class level.Aspergillaceae dominated at the genus level,with Aspergillus niger being the dominant species in dried chilies.These bacteria and fungi are commonly found in soil,water and crops,suggesting that dried chilies are contaminated by the external environment during harvesting and processing.(2)Small-scale experiments were conducted using laboratory pulsed light equipment,the disinfection effect of curcumin-mediated photodynamic,single pulsed light and pulsed light combined with curcumin treatment were studied by measuring inactivation amount infected with Escherichia coli,Aspergillus niger and naturally growing total colonies,The effects of different sterilization methods on the quality of dry chilies were evaluated by comparing the changes of color,capsanthin content,capsaicin content and volatile components of dry chilies before and after sterilization.The results showed that curcumin mediated photodynamic technology has the worst sterilization effect on dry chilies.Under the optimal sterilization conditions of curcumin concentration of 6 mmol/L,dark incubation time of 30 min and LED illumination time of 20 min,the sterilization amount is less than 2 log CFU/g.The sterilization effect of pulsed light was the second most effective,after 60 flashes at a pulse intensity of 0.66J/cm~2/pulse,Escherichia coli,total colonies,and Aspergillus niger in dried chilies were reduced by 3.60 log CFU/g,2.29 log CFU/g,and 2.18 log CFU/g,respectively.Some recessed parts of dried red chilies cannot be irradiated resulting in incomplete sterilization and large amount of residual bacteria.Pulsed light combined with curcumin had the best bactericidal effect.Under the condition of pulse intensity of 0.66 J/cm~2/pulse and 60 flashes,Escherichia coli,total colony and Aspergillus niger decreased by 5.64,3.65 and 3.82 log CFU/g respectively.Comparing the quality of untreated dry chilies with that treated by pulsed light combined with curcumin and single pulsed light,it was found that while ensuring the sterilization effect,the color,capsaicin content and volatile component content of dry chilies were not significantly different from those of untreated samples(P>0.05).In general,the quality of dry chilies treated with pulsed light combined with curcumin is better than that treated with single pulsed light.(3)The mechanism of lethality of Escherichia coli and Aspergillus niger spores by pulsed light and pulsed light combined with curcumin technology was investigated and the bactericidal kinetic model was developed.The cell structures of Escherichia coli and Aspergillus niger spores before and after treatment were observed by scanning electron microscopy,transmission electron microscopy,flow cytometry and fluorescence spectrophotometer,to explore the cause of death at the cellular level.The results showed that the apparent morphology of Escherichia coli and Aspergillus niger spores were depressed,Escherichia coli appeared to be perforated resulting in the cell contents leaked out,the internal structure of Aspergillus niger spores cells were destroyed,the organelles were degraded,the cell membrane became transparent,and the phenomenon of mass wall separation appeared.Escherichia coli and Aspergillus niger spores DNA double-stranded were unwinded,and the level of reactive oxygen species in cells increases,disrupting cell integrity,interfering with the normal physiological activities of the bacterium resulting in inactivation,it is the result of the synergistic effect of photochemical effect,photothermal effect,photosensitization effect.Weibull model and Weibull+Tail model were used to study the bactericidal kinetics of pulsed light and pulsed light combined with curcumin on microorganisms in dried chilies.The results showed that the Weibull+Tail model had a better fit and could better describe the inactivation characteristics of microorganisms in dry pepper under the sterilization mode of single pulse strong light and pulse strong light combined with curcumin.(4)Pilot-scale pulsed equipment was used to verify the feasibility of industrial production of dried chilies sterilization by pulsed light combined with curcumin.The results showed that the pilot-scale pulsed light equipment could reduce the total bacteria in dried chilies by 2.71±0.34 log CFU/g.In the accelerated storage experiment at 35°C,based on the standard that the total number of bacterial colonies of corporate dried chilies should not exceed 4.7 log CFU/g,the dried chilies treated with pulsed light combined with curcumin sterilization could be stored for about 8 d at 35°C with a relative humidity of 30%,which is equivalent to 27 d at 25°C and one year at 4°C.Comparing the quality of dried chilies during storage,it was found that the color of dried chilies untreated and with pulsed light combined with curcumin did not change significantly(p>0.05)as the storage days increased.The water activity of both groups increased gradually with the increase of storage time,and the water activity of the untreated group was higher than that of the sterilized group.In conclusion,compared with the single treatment method,the pulsed light combined with curcumin technology achieved effective sterilization and good preservation of food quality under the synergistic effect of photochemical,photothermal and photosensitization effects,which has potential for industrial production. |
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