| In recent years,with the increase of consumers' health concern,the demand for healthy food with natural and no added characteristics is increasing.Therefore,in order to meet the need of consumers for natural,fresh and high-quality healthy food,the minimum processing volume has become the strategic goal of modern food processing technology,and has promoted the rapid development of non-thermal processing technology.Pulsed electric fields(PEF),as a new technology of non-thermal processing,not only has the good effect on sterilization,but also has short treatment time,less energy consumption,less change of nutrition and flavor,which has high applicability in heat sensitive food.At present,research on the feasibility of application of PEF technology in citrus juice processing field is relatively less,and the microoganisms' germicidal efficacy and the nutrients' degradation in ponkan juice are rarely reported.Based on the potential advantages of PEF technology to enchance the quality of citrus juice,the germicidal efficacy of different microoganisms in ponkin juice by PEF was estimated,the microbial dynamics model was established.By analysising the mechanism of sterilization by PEF,the influence of PEF treatment on the quality of ponkan juice was evaluated,the degradation kinetic model with effective nutrients to realize the predition of shelf life and the assessment of juice quality during storage period was builed.The main results and conclusions are as follows:1.The inactivation effects of PEF treatment on the main microorganisms including bacteria,staphylococcus aureus,coliform,moulds,yeasts from ponkan juice were evaluated,and the process conditions of PEF sterilization by response surface method were optimized,which provide reliable technical parameters for the application of PEF technology in the fruit juice processing field.The conclusions are as follows: the bactericidal effect by PEF treatment is significant,and the various kinds of microbial communities due to the difference of their structure and shape showed different resistance to PEF treatment.The electric field intensity,flow rate and temperature have significant influence on sterilization.Based on the results of single factor test,the electric field strength,velocity and temperature as independent variables and the total death lgS as dependent variable were selected to analyze the design of the response surface methodology(RSM),the optimum conditions for PEF sterilization were determined as: electric field intensity of 28 kV/cm,flow rateof 15 mL/min,and the temperature of 40 ?,the average death lgS reach to 2.7,that is in good agreement with the model prediction.The total number of colonies obtained by RSM conformed to the requirements of GB 7101-2015.Therefore,the results showed the feasibility and application of PEF technology in the field of ponkan juice processing.2.The effects of different sterilization technology on the physicochemical sproperties and nutritional components of ponkan juice were estimated and its mechanism was analyzed.The results indicated that the total phenolic content in ponkin juice was about 27.62~27.86 mg/100 mL,ascorbic acid was 20.85~21.05 mg/100 mL,and free amino acids was 322.30~331.00 mg/100 mL.Compared to untreated ponkan juice,the content of pH,soluble solids,titratable acid and total sugar(fructose,glucose and sucrose)in ponkin juice by three treatment method such as pasteurization,ultrasonic sterilization and PEF sterilization,had no significant change,which also indicated the content of sugar and acid in ponkan juice is stable under the three sterilization conditons.Compared to pasteurization treatment and ultrasonic treatment,PEF achieve the desired sterilization.At the same time,there is a certain improving effect on nutritional quality of citrus juice.3.The solid phase microextraction of ponkan juice was enriched and gas chromatography-mass spectrometry was tested.With fresh ponkan juice as reference,compared with pasteurization and ultrasonic sterilization,comprehensive analysis of effect of PEF technology on the volatile components of terpenes,alcohols,esters,aldehydes and ketones compounds in ponkan juice.The results show that significant effects of different sterilization methods on volatile components of citrus juice,terpenes,alcohols,esters,aldehydes and ketone compounds changed significantly and PEF treatment of citrus ponkan juice had 43 kinds of volatiles,23 kinds of terpens,compared with the untreated group was increased by 5%,6 kinds of alcohol,the relative content increased by 45%,1 kinds of esters,the relative content increased by 931%,4 kinds of aldehydes,relative content increased 2%,4 kinds of alkanes,the relative content increased by 266%.The volatile components of PEF group to maintain the best,higher degree of retention of the flavor components of the original features,and can inhibit D-carvone and other adverse components.4.Through the real system and model system,the influence of the storage temperature(0?4?25 ?)and sterilization methods on ascorbic acid,free amino acids(Asn,Ser,Glu,Arg,Pro)were evaluated.In addition,the degradation kinetic model was builded to achieve the storage period of ponkan juice nutrition the quality and shelf life prediction.There were extremely significant effects of storage temperature on the changes of the content of ascorbic acid in real system and model system.Low temperature storage is in favor of inhibiting degradation of ascorbic acid,prolonging its half-life and maintaining the nutritional quality of ponkan juice.The way of sterilization has significant effect on the degradation rate of ascorbic acid in real system and model system.Compared with pasteurization and ultrasonic sterilization,the ascorbic acid retention rate in PEF treated system was significantly increased,and the half-life was significantly prolonged.Storage temperature has significant effect on the change of the content of free amino acids in real system and model system.The content of amino acids in real system was affected by many factors during storage period.In the model system,the degradation rate of free amino acids was increased with the increase of storage temperature.At the same storage condition,PEF could effectively inhibit the degradation of amino acids compared with pasteurization and ultrasonic sterilization. |
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