| The yield of mandarin in China is the largest in the world.The development of fruit juice processing industry is an important way to relieve the pressure of fresh fruits,and to promote the healthy and sustainable development of citrus industry.In recent years,with the increase of consumer's health concern,the demand for healthy foods,such as natural,non-additive,non-thermal processing,is also increasing.As a kind of non-thermal processing technology,ultrasound has the advantages of environmental protection,energy saving and high efficiency.At present,the domestic and foreign research on the feasibility of low frequency ultrasonic technology in mandarin(Citrus unshiu)processing field is relatively fewer,and there is little research about the influence effects and mechanisms of low frequency ultrasonic technology on ascorbic acid,free amino acids and other nutrients in mandarin juice.There are potential advantages of low frequency ultrasonic technology to enhance the quality of orange juice.The article selected mandarin as raw materials,through the research methods of combining real system and model system,researching inactivation effects of low frequency ultrasonic technology.Further,it explored the physical and chemical effect of low frequency ultrasonic key technical parameters.Evaluate the influence of low frequency ultrasonic technology on mandarin processing quality and storage characteristics comprehensively,and construct the effective nutrients degradation kinetic model to realize the prediction of shelf life,as well as the assessment on juice quality in storage period.The main contents and conclusions are as follows:1.Selected mandarin(Citrus unshiu)juice as raw materials,researching inactivation effects of low frequency ultrasound treatment on the main microorganisms(bacteria,fungi,yeast,coliform)of mandarin juice.Further,it explored the physical and chemical effect of low frequency ultrasound key technical parameters(ultrasonic temperature,ultrasonic time,ultrasound power on ultrasonic medium.And to optimize the process conditions of low frequency ultrasound sterilization by response surface method,which provides reliable technical parameters for the application of low frequency ultrasonic technology in the field of fruit juice processing.Through the analysis,it draws the following conclusions: low frequency ultrasound technology has strong bactericidal effect,and different microbial floras have different sensitivity to low frequency ultrasound.Fungi and yeasts showed similar change trend and tolerance in the ultrasonic treatment processing,and coliform is most sensitive to ultra sound.Effect of temperature on ultrasonic sterilization is strong(P<0.05).Use low frequency ultrasound technology with low temperature(4 ?)treating 30 min,the total sterilization rate only reach to 42.5%,which shows the sterilizing effect is not ideal.Use ultrasound combined with medium temperature(40 ?)treatment can significantly improve the sterilization rate(P<0.05).Compared with the low temperature(4 ?)condition,low frequency ultrasound combined with medium temperature can effectively alleviate the lag effect of ultrasound,which significantly shorten critical sterilization time of mold,yeast,and coliform group by 5.8,3 and 9.9 min.Through the response surface testing,it gets optimum conditions for low frequency ultrasound sterilization: ultrasonic time 36 min,ultrasonic temperature 50 ?,ultrasonic power 720 W,the total average sterilization rate reached to 99.89%,the total number of colonies according with the requirements of GB7101-2015.Therefore,the experimental results confirm that the low frequency ultrasound technology has sterilization feasibility on mandarin juice,and combined with medium temperature will have more obvious advantages.2.Selected mandarin as experimental materials,inspecting physical and chemical quality of its fresh juice(the sugar and acid content,optical properties,antioxidant)and nutrients(ascorbic acid,free amino acid)comprehensively.Meanwhile,put mandarin juice as the prototype,establishing mandarin juice standard model system to investigate the influence of different sterilization processes(low frequency ultrasound processing,pasteurized)on physical,chemical properties and nutrient composition change rules of mandarin juice and its model system.The experimental results showes: the content of Vc in mandarin juice range from 25.74 to 27.03 mg/100 mL,total phenolic content range from 20.96 to 23.64 mg/100 m L,citric acid range from 0.66 to 0.68 g/100 mL,total sugar(fructose,glucose and sucrose)range from 5.67 to 6.33 g/100 m L,contains 24 kinds of amino acids,the total content range from 301.72 to 325.52 mg/100 m L.The results show that the sterilization method has a significant impact on the quality of mandarin juice(P<0.05).Compared with the traditional heated sterilization,the sterilization effect of low frequency ultrasound technology plays a certain role in promoting the comprehensive quality stability of Mandarin juice: suspension stability up to 3 times,Vc retention levels and total phenol content increased by 5.2% and 3.3% respectively,the content of total amino acid content increased by 10.64%,antioxidant capacity increased by 9.7%,brightness and the yellow color value significantly improved.After treated by low frequency ultrasound,Vc and AA retention levels of Mandarin juice standard model system were higher than pasteurized model system,and L*,b* increased significantly(P<0.05).It shows a high consistency degree with Mandarin juice system experimental results,confirmed that the model system is reliable and representative.3.Combining the real system and model system,assess the influence of the storage temperature(2,8,20,28 ?),Sterilization method(low frequency ultrasound sterilization pasteurization treatment)on the nutrition content,and identify degradation rule of ascorbic acid during storage of mandarin juice.The experimental result shows that the storage temperature had significant effects on mandarin juice degradation rate of Vc of real system and model system(P<0.01).Low temperature could prolong the half-life of Vc and maintain the quality of mandarin juice.Sterilization methods also has a significant influence on Vc degradation rate(P<0.05).Based on the effect of the removal of dissolved oxygen,low frequency ultrasound technology can effectively delay the degradation of Vc(P<0.05),and indirectly inhibit its anaerobic degradation.first kinetic model was common used(0.84<R2<0.99)in depicting Vc degrading system process of mandarin juice(except 2?-low frequency ultrasound-standard model system experimental group).Vc degradation rate of standard model system is generally higher than that of mandarin juice.At the same time,under the experimental parameters affection(storage temperature,sterilization),degradation rate change trend of Vc in different system is consistent.4.Combining the real system and model system,assess the influence of the storage temperature(2,8,20,28 ?),Sterilization method(low frequency ultrasound sterilization pasteurization treatment)on the total free amino acid and Asp?Glu?Ser?Lys?Arg?Pro,to clear activity of different free amino acid in mandarin juice.Storage temperature has a significant effect on the total content change of free amino acid in real system and model system(P<0.05).In real system,the changes of free amino acid content is caused by microbial activity,enzyme / non-enzymatic browning and some other factors.At low temperature(2 ?)storage conditions,the total content of free amino acid tends to be stable in real system.The degradation rate of amino acid increased with the increase of storage temperature in model system.Low frequency ultrasound treatment can delay the degradation of amino acids.Storage 30 days under 2,8,20,28? respectively,the total free amino acid degradation rate in low frequency ultrasound model system is reduced 3.56,3.15,2.09,and 3.92% than that in pasteurized model system.Different amino acid based on their structural differences are different in reactivity,especially Arg,Pro,Ser loss more,and Asp activity is low.The degradation of Asp,Glu,Ser,Lys,Arg,Pro in the standard model system is in accord with the first zero / first order kinetic model(0.74<R2<0.99).The experimental parameters(storage temperature,sterilization)show the consistency to content changing trends and influence results of amino acids in mandarin juice and standard model system. |
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