| According to the report on nutrition and chronic diseases of Chinese residents,the average daily intake of cooking oil of Chinese residents is 43.2 g,which is higher than the recommended value(25～30 g/day)in the Dietary Guidelines for Chinese residents.Therefore,the outline of "Healthy China 2030" focuses on encouraging the whole society to reduce salt,oil and sugar,so as to improve the health level of residents.On the other hand,since the establishment of a string of Western-style fast-food restaurants in China,a large group of potato chips and French fries consumers has been cultivated and the development of the domestic potato processing sector has boomed.However,blanching and freezing during the preparation of prepared potato strips significantly increased the oil content of the French fries.Physical field processing techniques are characterized by high heating efficiency,strong sterilization capabilities,and it is unknown whether they can replace blanching and freezing treatments and reduce oil content of fries.Therefore,this thesis takes potato chips and French fries as the research object to study the influence of physical field pretreatment technology on its quality,focusing on the processing technology of physical field technology,oil reduction effect and mechanism.The main findings and results of this thesis are as follows:Firstly,the oil reducing effect and mechanism of pulsed electric field(PEF)treatment on potato chips were studied.The results showed that the electric field strength was the main factor in the oil content of the chips,with an electric field strength of 5 k V/cm able to reduce the oil content of the chips from 50.8% to 40.3%.The texture results showed that the PEF treatment was able to increase the hardness and springiness of the chips,increasing the crushing force from 233.93 g to 484.79 g.The cell viability results indicated that the PEF could cause cell membrane damage,the formation of irreversible transient pores,resulting in the overflow of cell contents and eventually apoptosis.The results of scanning electron microscopy and pore characteristics indicated that high intensity(10～20 k V/cm)electric field treatment could also destroy and merge smaller micropores(10～50 nm)in chips,increasing the number of larger micropores(50～100 nm),and increasing the diameter and volume of micropores.This mechanism will help to increase the evaporation rate of water in the potato chips during frying,while reducing the oil adsorption,thereby reducing the oil content of chips.The above results indicate that the pretreatment of the PEF before frying has some oil reduction effect.Then,the effects and mechanisms of blanching and PEF treatment on the quality of the French fries were investigated.The color and texture results showed that the blanching gives the fries a golden color and a softened texture.The results of the mass transfer dynamics showed that the blanching treatment increased the oil absorption rate constant by a factor of 1.33 and the oil content of the fries increased from 29.6% to 41.3%.On the other hand,PEF treatment was able to slightly reduce the oil content of the fries from 29.6% to 27.0%,but was unable to give the fries a golden,bright color.The results of the combined PEF and blanching treatment showed that blanching had a dominant effect on the microstructure of potato tissue,cells,and the inner core of fries.The shortcomings of blanching,such as increased oil content in fries and high water consumption during processing had not been resolved.Therefore,it is necessary to seek alternative blanching processes that guarantee both the color and texture of the French fries.Then,the feasibility of electromagnetic wave heating technology of radio frequency(RF)and microwave(MW)replacing the blanching process of French fries was studied.The results of the processing characteristics showed that the RF treatment could reduce the cutting force and facilitate the processing of potatoes.In addition,this treatment inhibited the activity of potato polyphenol oxidase by more than 60%,and the enzymatic browning reaction could be hindered during processing.The color results showed that the fries obtained after 15 to 20 minutes of RF treatment had a similar golden color compared to the blanching process.The results of scanning electron microscopy showed that the inner core structure of fries became smoother after RF treatment,which helped to reduce oil adsorption.Compared to the blanching treatment,the oil content of the fries was reduced from 41.3% to 18.4% after 20 minutes of RF treatment.The texture results showed that RF treatment caused the texture of potato chips to deteriorate,resulting in a hard core.On the one hand,30 s of MW treatment was able to inactivate more than 90% of the activity of polyphenol oxidase,and the passivation effect could be further enhanced by increasing the MW treatment time.The quality results of French fries showed that the fries produced by moderate MW treatment(60 s)had a bright and golden color and similar texture characteristics to that of the blanching process.It is worth noting that MW treatment for 60 s significantly reduced the oil content of potato chips from 41.3% to 18.1%.The microstructures of the fries’ crust and inner core suggested that the MW treated fries form a thicker "protective shell," which plays a key role in preventing the absorption of fat during frying.MW heating,rather than blanching,therefore helps reduce water consumption during processing and significantly reduces the oil content of the fries.In addition,the effect of the combined MW and hot air drying processes on the quality of French fries and the synergistic mechanism of oil reduction were discussed.The texture results of fries showed that compared with the traditional low fat French fries prepared by blanching combined with hot air drying,the new low fat French fries prepared by MW(30 s)combined with hot air drying(30 min)had stronger hardness,springiness,and chewiness.The results for moisture and oil content showed that reducing the moisture content of the potato strips before frying helped reduce the adsorption of oil during frying.The new low-fat fries were significantly lower in oil than traditional low-fat fries,reducing the oil content from 31.7% to16.3%.The results of microstructure of the crust and inner core of French fries showed that the combination of MW and hot air drying makes the fries form a thicker and more ductile "protective shell",which plays a key role in preventing the adsorption of oil during frying.As a result,the fries prepared by MW combined with hot air drying treatment had a lower oil content.Finally,the effect of cold plasma treatment on the quality of fries was investigated and the potential of refrigerated storage and transportation of prepared potato strips was evaluated.The effect of freezing treatments on the quality of French fries had been assessed.Freezing somewhat reduced the brightness and hardness of fries and significantly increases the oil content of traditional low-fat fries prepared by blanching combined with hot air drying from31.7% to 49.5%.However,the new type of low-fat fries prepared by MW combined with hot air drying could resist this negative impact,and their oil content(16.3%)had no significant change,and was significantly lower than the oil content of commercially available frozen potato strips after frying(31.5%).However,the problem of high energy consumption in frozen storage and transport remained.The results of the bacterial counts showed that the cold plasma technology was able to reduce the initial bacterial counts of the prepared potato strips by approximately 1.72 logarithmic cycles,which was beneficial for the cold storage and transportation of the prepared potato strips.Prepared potato strips treated with cold plasma still retained a golden,bright color after 12 to 14 days of refrigeration,significantly longer than the4 to 6 day shelf life of the control group.The results of sensory evaluation showed that the fries prepared by multi-physics field combined process had similar appearance,flavor,crispness,taste and lower greasy feeling compared with commercial French fries.In summary,PEF,RF and MW technologies can all play a role in reducing the oil content of potato chips and French fries.Preparing the potato strips for frying by microwaving instead of blanching reduces water consumption,effectively reducing the oil content of the fries and improving their quality.Finally,a process route for preparing low-fat prepared potato strips by MW combined with hot air drying and using cold plasma sterilization technology to extend the refrigerated shelf life of prepared potato strips was formed,achieving energy conservation and consumption reduction,and reducing the negative effects of freezing treatment on the quality of French fries. |
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