Ultrasound-Assisted Washing Of Selected Microorganisms And Pesticide Residues On The Surface Of Fresh-cut Chinese Cabbage (Brassica Rapa Var.Chinensis L.)

It is well known that,the efficient cleaning of harmful microorganisms and pesticide residues on the surface of vegetables is of great significance for improving the food safety of vegetables.Due to the unsatisfactory effect of traditional washing methods,in recent years,many researchers have explored the use of new technologies such as ultrasound,plasma,and ozone to enhance the cleaning effect.Among them,ultrasound is a potential means.Based on this,this research deeply studied the effects of multi-mode ultrasonic assisted cleaning on the harmful microbial content and pesticide residues on fresh-cut Chinese cabbage,and discussed the effect of ultrasonic waves on the related quality indicators of Chinese cabbage.The study was divided into three parts:(1)reduction of natural microbial population;(2)reduction of artificially inoculated Gram-positive and Gram-negative bacteria;and(3)reduction of organophosphorus pesticide residues.The main findings and contents are as follows:1.A comparative study on cleaning methods of the naturally occurring microorganisms on the surface of Chinese cabbage.The application of sweeping frequency ultrasound(SFUS),sodium hypochlorite(NaOCl)and their combinations(SFUS + NaOCl)resulted in significant synergistic reduction of the natural microbiota of Chinese cabbage as well as exhibiting inhibitory effects during post-treatment storage at 4 °C and 25 °C.Hence,40 kHz SFUS + 100 mg/L NaOCl was noted to have the highest reduction of mesophilic counts(4.5 log CFU/g),and yeasts and molds(4.3 log CFU/g).Minimum chlorophyll depletion(0.95 mg/100g),weight loss(1.02 %)and electrolyte leakage(21 %)were associated with the combined treatment whereas total colour and textural characteristics deteriorated.At the end of storage,colour change was visibly different(1.5 < TCD < 3)and samples presented a 55 % loss in firmness after the combined treatment.The combined treatment inhibited the activities of polyphenol oxidase and peroxidase,reducing the activities from 8.3 to 3.6 U/g and 33.6 to 28.5 U/g respectively and demonstrated its preservative effect after FT-NIR analysis.2.An investigation on the sequential multi-frequency ultrasonic cleaning technique of the native microbiota on the surface of Chinese cabbage.The sequential dual frequency ultrasound(DFU)and triple frequency ultrasound(TFU)washing processes exhibited significant effects on the microbial quality,physicochemical parameters,volatile compounds profile and sensory quality of fresh-cut Chinese cabbage.The TFU washing resulted in significant(p < 0.05)reduction of total bacteria,yeast and molds,largely reducing 3.1 and 2.8 log CFU/g respectively.However compared to DFU,TFU washing resulted in low firmness(3.32 N),increased electrolyte leakage(20.6 %)and the formation of micro-channels on leaf surface.Allyl isothiocyanate was the predominant volatile compound after ultrasonication with relative concentration of 25.39 to 102.62 ng/g.Six(6)volatile compounds were identified as aroma active compounds(AACs)contributing substantially to the core aroma profile of Chinese cabbage.Finally,DFU washing according to an affective sensory analysis presented desirable sensory scores compared to TFU washing.3.A study on different modes of ultrasonic cleaning of Escherichia coli on the surface of Chinese cabbage.The assessment of ultrasound washing with sweeping(SF),fixed(FF)and combined fixed(CFF)frequencies revealed a significant inactivation effect on Escherichia coli ATCC 8739 and further improved upon physical and chemical qualities of Chinese cabbage.Largely,reductions in E.coli counts were respectively 2.7,3.4 and 1.8 log CFU/g for SF,FF and CFF ultrasound washing treatments compared to the control(1.17 log CFU/g).Ultrasonic washing of Chinese cabbage significantly(p < 0.05)decreased chlorophyll and carotene contents.However,the FF 40 kHz improved the total phenolic content(51.5 mgGAE/100g),reducing power(115 mgAAE/g)and high 2,2-diphenyl-1-picrylhydrazyl radical scavenging effect(84.2 %)of the treated samples compared to the control.Similarly,scanning electron microscopy revealed the presence of micro-channels while the perceptible visual colour(?E)and the greenish appearance of the leaves improved after instrumental colour analysis.4.A study on the combined decontamination of Listeria innocua on the surface of Chinese cabbage using sweeping-frequency ultrasound and sodium hypochlorite.Combining SFUS and NaOCl for the reduction of inoculated Listeria innocua CICC 10417 a gram-positive bacterium in fresh-cut Chinese cabbage revealed a superior decontamination effect compared to their separate treatments.The combined application of 40 ± 2 kHz SFUS and 100 mg/L NaOCl accomplished a 3.4 log CFU/g reduction of L.innocua,demonstrating an additional 2.1 log CFU/g reduction compared to single treatments.Synergistic reductions of L.innocua were recorded in over 80 % of the combined treatments,though most of the values were less than 1.0 log CFU/g.The combined treatment caused marginal darkening,lowest firmness(3.20 N)and chewiness(9.25 mJ),lowest reflectance and observed photosynthetic cell diminution through perforations and wrinkles.5.The effect of mono-frequency ultrasound washing(MFUW)parameters on the cleaning of L.innocua and E.coli on the surface of Chinese cabbage.Mono-frequency ultrasound washing(MFUW)parameters including ultrasound frequency,ultrasound power,and washing time were established to have significantly influenced the reduction of inoculated bacteria in fresh-cut Chinese cabbage.The optimum decontamination conditions for both responses were reported as ultrasound frequency of 40 kHz,ultrasound power density of 125.45 W/L and a washing time of 15 min.The reduction predictions under these conditions were 5.6 and 4.7 log CFU/g for E.coli and L.innocua respectively with a desirability of 0.966.Experimental reduction values were well correlated to the predicted reduction values with an error margin of about 2 %.Besides,the optimized MFUW prolonged Chinese cabbage storability with acceptable colour,firmness and pigment attributes.6.A study on ultrasonic cleaning of organophosphorus pesticide residues on the surface of Chinese cabbage.The application of sequential dual frequency(DFU)and triple frequency(TFU)ultrasound indicated significant(p < 0.05)improvement in the reduction of dimethoate(20.10–72.42 %)and diazinon(29.60–77.37 %)on Chinese cabbage,as compared to conventional water soaking with 13.24–19.89 % for dimethoate and 17.32–24.16 % for diazinon.Application of TFU and extended washing time(20 min)resulted in high dimethoate(>70 %)and diazinon reduction(>75 %)alongside increase in physical impairment(18.7 %)and reduction in vitamin C content(28.12 %)of samples.The treatment conditions did not significantly affect the colour parameters of the samples despite slight perceptible darkening.In summary,the ultrasonic cleaning of Chinese cabbage and possibly other green leafy vegetables is a very effective way to improve the food safety and quality of fresh agricultural products.This is to effectively reduce or remove harmful microorganisms and pesticide residues while preserving the physical,chemical and sensory properties of vegetables.This non-thermal technique is also beneficial in stimulating the synthesis of polyphenols and improving the antioxidant capacity of vegetables.