| Cherry tomatoes?Lycopersicon esculentum?,also referred to as"salad tomato",are a tropical,high commercial value fruit in the international world markets,which makes an important part of daily cuisine in China.The production volume of tomatoes,including cherry tomatoes in China,was 56.8 million tons in 2016,which has increased 2.7%year–on-year.Cherry tomatoes are consumed as fresh,dried,or juicy products.It is a good source of vitamins A and C,with a sweet taste and low in calories,which has made it highly demanding all over the world.However,fresh fruit and vegetables have recently been acknowledged and proven as a major source of chemical and biological contaminants that pose a likely threat to the health of human being globally.Fresh fruit and vegetables can be infected with foodborne diseases from the surrounding environment before,during,and post-harvest processing.Attention is now directed on evaluating and determining the toxicological or microbiological risks that may be involved during the application of novel preservation treatment techniques,and their impact on food quality and safety,for the purpose of obtaining"risk-free fresh"products.There is a need for a non-thermal washing technique that is safe,economical,energy-efficient,and environmentally friendly,which does not negatively affect the quality of the treated fruit and vegetables.Amongst the latest non-thermal technologies in food and their products,the ultrasounds washing process has gained considerable significance in the inhibition of microbial activity and considered as nontoxic and eco-friendly applications employed for enhancing the shelf life of fruits and vegetables.Recently,attention is now focused on the application of multi-frequency ultrasound as an alternative to the conventional mono-mode frequency ultrasound towards a better and improved microbial inactivation and enhancement of fruit shelf life.Higher microbial inactivation is usually obtained at prolonged sonication with the use of mono-frequency ultrasound,which eventually affects the quality of the treated fruit and vegetables negatively.In this light,this study is aimed at investigating the effect of assisted and un-assisted multi-mode frequency ultrasound as a washing treatment on the cherry tomato to ensure microbial safety,enhance,and maintain the nutritional quality of the fruit.This study firstly focused on the application of different modes of frequency ultrasound as a washing treatment on a cherry tomato,thereby determining the best processing system that will give a higher microbial inactivation,maintained,and enhanced the nutritional quality of the cherry tomato.Thereafter,the basic principles behind the working system of these different frequency modes of ultrasound were determined kinetically.Assisted-ultrasound processing has been a common practice to improve the efficiency of ultrasound;therefore,this study investigated the application of the best frequency-mode ultrasound washing treatment with thermal?thermosonication?,chemical sanitizers,and aqueous ozone.Finally,the effect of these washing treatments on volatile compounds,aroma,ethylene production,and respiration of the cherry tomato was determined.The main research contents and findings are as follows:1.Simultaneous multi-frequency:A possible alternative to improve the efficacy of ultrasound treatment on cherry tomato during storageThe productiveness of ultrasound is affected by its frequency,power,and time,which has been reported to be very effective during food treatment when combined with other techniques of treatments.The study was aimed to evaluate the effect of mono-frequency?MFU?,simultaneous dual-frequency?SDFU?and simultaneous tri-frequency ultrasound?STFU?at treatment times of5 to 25 min on microbial inactivation and nutritional quality of cherry tomato.Results revealed that within 10 min of sonication,SDFU was most effective compared with MFU towards inhibiting the spoilage microorganisms,delaying ripening as well as maintaining firmness,total phenolic,flavonoid and antioxidant of the fruit during 21 days of storage at 4 oC.Although STFU caused a higher reduction in microbial load after treatment,it,however,showed several adverse effects such as surface injury,loss in firmness,and loss of bioactive compounds on the fruit during storage.Overall,SDFU treatment had great potential in preserving shelf-life and nutritional value of cherry tomato during storage.2.Kinetic modeling of inactivation of natural microbiota and Escherichia coli on cherry tomato treated with multi-frequency sonication.To determine the inactivation principles behind the simultaneous multi-frequency ultrasound washing process,some non-linear kinetic models?Weibull{with or without tail},Log-linear,Log-linear shoulder{with or without tail},Biphasic linear,Logistic,Multi-target and Single-target models?were fitted to the experimental data.The models were fitted to determine the suitable model?s?that can represent the inactivation kinetics of inoculated E.coli,and natural microbiota?i.e.,mesophilic aerobic bacteria,and mold and yeast?on cherry tomato treated with fixed multi-frequency ultrasound.Almost all the studied model fitted well?R2?0.9?for the inactivation kinetics;however,the Weibull,Log-linear shoulder,and Biphasic linear model showed the highest statistical parameters?0.9?adj.R2?0.99 and smallest RMSE and SSE values?.All the three models could be used to compare the kinetic behavior of E.coli and the first two models for the kinetic behavior of mesophilic aerobic bacteria and mold and yeast during sonication treatment.Two distinctive inactivation curves were obtained for the mono-frequency and the multi-frequency?dual and tri-frequency?for all the microbial inactivation.The remarkable results obtained for dual and tri-frequency sonication shows to be an effective and promising alternative to the traditional microbial inactivation techniques and the common practice of using ultrasound with other sanitizing methods.3.Combination of thermal and dual-frequency sonication processes for optimum microbiological and antioxidant properties in cherry tomatoAccording to our research in the first experimental chapter,dual-frequency ultrasound?SDFU?was found to be the best treatment for microbial safety,preserving shelf life and nutritional values of cherry tomato;therefore,SDFU was applied under thermal treatment as an assisted-ultrasound treatment.Cherry tomato fruits were treated with three dual-frequency ultrasound treatment at different temperatures and times.The three treatment variables,i.e.,dual-frequency ultrasound,temperature,and time,were optimized with the aid of response surface methodology?RSM?.20/40 kHz,52°C,and 6 min were the optimum condition obtained for total bacteria count?TBC,1.08 log CFU/g?,firmness?15.90 N?,and total phenolic content?TPC,34.34 mg GAE/100g?.All the linear relations of the treatment variables showed a significantly negative and positive effect on the TBC and TPC,respectively,while significant positive effects were obtained for some of the relations of the firmness.Hence,dual-frequency sonication is effective for microbial safety and maintaining the quality of the cherry tomato.In addition,RSM was found to be a useful tool in setting and optimizing the variables responsible for sonication treatment.4.Efficacy of dual-frequency ultrasound and sanitizers washing treatments on quality retention of Cherry tomatoEven though a better result was obtained within a short duration of sonication when the multi-frequency ultrasound was assisted with thermal treatment,there is still a need for further exploration to minimize cost and energy that will be incurred as a result of the high temperature obtained.Furthermore,as ultrasound has been applied to enhance the sanitizing efficiency of chemical sanitizers,there will be a need to explore this with the multi-frequency ultrasound.In this light,experiments were performed to analyze the effect of 10 min post-harvest treatment with individual and/or combined treatment of dual-frequency ultrasound?SDFU??20/40 kHz?and some chemical sanitizers?sodium dichloroisocyanurate?SD?,peracetic acid?PAA?,peracetic acid with5%H2O2?PAAH?,hydrogen peroxide?HP??on the microbial and nutritional quality of cherry tomatoes during 21 days of refrigerated storage.The results showed that the treatments significantly reduced the spoilage organisms with the highest inactivation observed for the individual and combined treatment of PAAH and SDFU.Also,firmness,total phenolic,and flavonoid contents were significantly affected after treatment,especially for the SDFU and combined treatment.The other quality parameters such as pH,total soluble solids,titratable acidity,and maturity index were not influenced,which signifies that the ripening process had a similar pace,especially in both control and chemically treated fruit.The result obtained at the end of the storage indicated that the combined treatments enhance the quality of the fruit and increase its shelf life.5.Sonozonation:Enhancing the antimicrobial efficiency of aqueous ozone washing techniques on cherry tomato.Further studies on the assisted-ultrasound washing process were also carried out with aqueous ozone,as this?aqueous ozone?has gained the interest of many researchers and food-related industries.Ultrasound requires high power and longer treatment times to inactivate microorganisms when compared to ultrasound combined with other technologies.Also,the antimicrobial efficiency of aqueous ozone increases with an increase in its concentration and exposure time,but with a detrimental effect on the quality of the treated food.In this study,the effect of aqueous ozone at low concentration,multi-mode frequency sonication,and their combination on microbial safety and nutritional quality of cherry tomato was investigated.Individual washing with aqueous ozone and mono-mode frequency sonication resulted in<1 log CFU/g reduction in the spoilage microorganisms,while dual-mode frequency ultrasound?SDFU?resulted in higher microbial reduction?1.3-2.61 log CFU/g?.The combined system?20/40 kHz+aqueous ozone?,on the other hand,resulted in>3 log CFU/g microbial reduction.The application of SDFU enhanced the antimicrobial efficiency of aqueous ozone without any detrimental effect on the physicochemical properties?except the firmness?,bioactive compounds,and antioxidants of the cherry tomato during 21 days refrigerated storage.The result obtained indicates the promising substitute to the single washing technique for microbial safety as well as preserving the nutritional quality and enhancing the shelf life of cherry tomato.6.Effect of assisted and unassisted ultrasound washing on respiration rate,ethylene production,enzymes activity,volatile composition and odor of cherry tomatoHowever,all the treatments under study have proven to be best in guaranteeing the microbial safety,maintaining,and enhancing the quality of the cherry tomato.Nevertheless,there is still a need to further evaluate the effect of these postharvest treatments on the metabolic activity,volatile profile,and sensorial quality of the fruit.The aim of this current study was to evaluate the enzyme activity,respiration rate ethylene production,volatile profile,and sensorial quality of cherry tomato treated with optimized assisted and un-assisted simultaneous multi-frequency ultrasound.The treatments,though not very significant,enhanced the enzymes activities and inhibit ethylene production and respiration rate.The treatment had no significant effect on the volatile compound of the cherry tomato when compared with the control samples.However,the results indicated that the treatment of cherry tomato with 20/40 kHz and aqueous ozone?0.85±0.2 mg/L?for 10 min enhanced and maintained the volatile compounds.Out of the forty-four?44?identified and quantified volatile compounds,one that had odor activity value>1 and thereby considered as active aroma compound contributing significantly to the essential aroma profile of the cherry tomato.The result obtained in this study further verified that none of the washing treatments showed a very significant negative effect on the cherry tomato after treatment,and the treatment with ultrasound and aqueous ozone proved to be the best treatment. |
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