| As a nutritious and unique flavored food,scallion stems are generally made into pickles or eaten raw.Since they are not heat-treated,they can be contaminated by microorganisms during harvesting,transportation,and marketing,which leads to foodborne diseases.NaClO is used as a disinfectant in the traditional sterilization techniques of scallion stems in the industry.However,there are still many problems such as the large amount of water consumption and residual microorganisms.In addition,it is resisted by consumers due to the controversial issues concluding byproduct residues generated by the high concentration chlorite reagents.Ultrasonic cleaning,as a non-thermal physical sterilization technology,has the advantages of environmental-friendly and easy to carry out continuous operation.When ultrasound is combined with NaClO,it enhances the ability to kill microorganisms of NaClO.That can reduce the concentration of NaClO and the amount of water consumption,while ensuring the effectiveness of sterilization and cleaning.Therefore,the study was carried out on the ultrasonic-assisted NaClO to clean and sterilize scallion stems.The applicability of ultrasonic-assisted NaClO cleaning technology for scallion stems was explored,which provided theoretical basis and data support for industrial popularization by comparing the cleaning and sterilization effects of the NaClO cleaning and the ultrasonic-assisted NaClO cleaning,comparing the physicochemical quality and quality during the storage of the samples after different treatments,revealing the bactericidal mechanism and kinetics of ultrasonic-assisted NaClO treatment,and verifying the feasibility of the combined treatment in pilot scale and calculating the cost.The main research contents and conclusions were as follows:(1)The ultrasonic-assisted NaClO cleaning technology was developed by using tri-frequency sweep container-shaped diverging ultrasonic equipment.The optimal parameters of ultrasonic-assisted NaClO cleaning were selected based on the cleanliness and microbial biomass.The effects of different cleaning methods on the quality of scallion stems were evaluated by comparing the changes of color,firmness,residual chlorine content,allicin retention,vitamin C retention and microstructure of scallion stems.The results showed that under the optimal parameters of(20±2)+(28±2)k Hz frequency sweep ultrasonic-assisted 100 ppm NaClO cleaning with bubbling and ratio of sample to water of 1:4,the cleanliness was 0.374%,the total number of bacteria reduction was 1.48 lg CFU/g and the number of Escherichia coli was 1.49 lg CFU/g,compared with 250 ppm NaClO cleaning,the cleanliness increased by 20.4%,the total number of bacteria decreased by 20.3% and the number of Escherichia coli decreased by 57.6%.Moreover,it could save 19.99% water and 68% NaClO.In addition,the color of scallion stems reduced 3.76%(ΔE=16.61),and firmness increased(887.19 g)1.12%.The allicin retention has improved 52.5%.Compared with the NaClO cleaning,the VC retention the structural tissue was better.The residual chlorine content on the surface of scallion stem was reduced to 0.27 mg/kg,which was 42.5% lower than that of 250 ppm NaClO cleaning.(2)The mechanism of lethality of Escherichia coli by NaClO and ultrasonicassisted NaClO was investigated and the bactericidal kinetic model was developed.First,before and after ultrasound,the value of pH and ORP,the content of hypochlorite in the NaClO solution were determined.Second,the cause of death was explored at the cellular level by measuring the electrical conductivity and the increase rate of nucleic acid protein which was related to cell membrane permeability.Observe the cell morphology of Escherichia coli by scanning electron microscopy.Finally,Wei-bull model,Wei-bull+Tail model and Log-Linear+Tail model were used to study the bactericidal kinetics of NaClO and ultrasonic-assisted NaClO on microorganisms in scallion stems.The results showed that after ultrasonic treatment,the pH value of the NaClO solution decreased,the content of hypochlorite and the ORP value increased,which indicated that ultrasonic treatment could improve the oxidizing property of the solution,and thus enhance the germicidal efficacy of the NaClO solution.After treated with 100 ppm NaClO and ultrasound-assisted 100 ppm NaClO,the electrical conductivity,extracellular nucleic acid and extracellular protein of E.coli increased by 36.23% and 77.03%,44.81% and 108.51%,44.53% and 107.46%,respectively,compared with 100 ppm NaClO treatment,the electrical conductivity,extracellular nucleic acid and extracellular protein increased by 112.61%,142.16%respectively.Combined with scanning electron microscopy,the surface of E.coli was depressed by NaClO alone,and the intracellular chlorine content increased to 0.2%.Under the synergistic effect of ultrasound,E.coli even burst,and the intracellular chlorine content rose to 0.28%,which was 40% higher than that of NaClO treatment.Wei-bull+Tail model had a better fit and could better describe the inactivation characteristics of microorganisms in scallion stems under the sterilization mode of single NaClO and ultrasonic-assisted NaClO.(3)Study on the effect of ultrasound-assited NaClO under scanning frequency on the quality of scallion during storage.Under the optimal parameter of ultrasonic cleaning above,it had good germicidal effect and structural retention on scallion stems.Furthermore,the enzyme activity relative to cell senescence and browning of scallion stems was inhibited effectively.Consequently,compared with the NaClO cleaning technology,the ultrasonic-assisted NaClO cleaning technology can extend the shelf life by 4 days,and the respiration of the cells was inhibited,which resulted in the decrease of the metabolic activity and the activity of enzymes related to browning.Scallion stems obtained by ultrasound-assisted 100 ppm NaClO cleaning under scanning frequency had a weight loss rate of 7.44%,electrolyte leakage rate of73.33% and hardness value of 482.56 g at the end of storage period.Compared with250 ppm NaClO cleaning,the weight loss rate and electrolyte leakage rate were reduced by 19.2% and 11.9%,respectively.These results suggest that ultrasoundassisted NaClO cleaning can effectively alleviate the dehydrating and fibrosing problem of scallion,and at the same time protect the quality of the product during storage.(4)Batch and continuous industrial cleaning experiments were carried out using frame and belt ultrasonic cleaning equipment in conjunction with NaClO solution treatment.The cleanliness and microbial biomass were used as the evaluation indexes of cleaning efficiency,and the color value was used as the evaluation index of the product quality.A comparison was made between the 250 ppm NaClO cleaning technology and the ultrasound-assisted 100 ppm NaClO cleaning technology.The results showed that applying the best parameters screened above to the pilot-scale ultrasonic-assisted NaClO cleaning process of scallion stems,ultrasonic cleaning using both the frame ultrasonic equipment and the belt ultrasonic equipment.can achieve the same germicidal effect of 250 ppm NaClO cleaning technology and reduce the concentration of NaClO to 100 ppm.At the same time,it can guarantee the quality of product.The total bacterial count of the products after ultrasonic-assisted NaClO cleaning using frame and belt ultrasonic equipment decreased 1.51 lg CFU/g and 1.24 lg CFU/g respectively,and all were within 5 lg CFU/g,which can meet the requirements of industry standards.The values of ΔΕ cleaning using frame and belt ultrasonic equipment were 22.31 and 19.9 respectively,and all within the acceptable range for consumers.In addition,in the cost calculation of the cleaning experiment used industrial equipment,ultrasonic-assisted 100 ppm NaClO cleaning can reduce the NaClO concentration by 40% and save the cost by 4.88%.Therefore,ultrasoundassisted NaClO cleaning technology has a broad application prospect. |
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