| In recent years,the fruit-vegetable crisps produced by vacuum frying?VF?have been widely concerned due to natural color,superior sensory and nutritional quality.However,there are certain defects in the application of VF technology,such as backward heating method,low efficiency especially with low-temperature frying and relatively high oil uptake in products.Focused on these problems,the effectiveness of microwave and its synergistic treatments,such as dielectric pretreatments,vacuum infrared pre-drying and low-frequency ultrasound,on the frying efficiency and quality of vacuum frying were comprehensively investigated in this research with potato chips?Solanum tuberosum L.Atlantic?as experimental material,and the related mechanisms were studied as well.Firstly,microwave?2450 MHz,600 W1000 W?was applied in the process of vacuum frying as high-efficient heating method and the effects of microwave on the efficiency and quality of VF potato chips were comparatively studied.Results showed that the moisture loss rate of VF potato chips was significantly?p<0.05?improved by microwave,reducing the frying time by 16.7%.The oil content was reduced from 34.89±0.78?g/100 g db?to24.12±0.23?g/100 g db?26.75±0.39?g/100 g db?,the crispness and color of VF potato chips were significantly improved.The microstructure observation showed that the surface cell morphology was well kept and more micron diameter porous structures were obtained by microwave.The factors containing microwave power,frying temperature,vacuum degree,thickness and loading coefficient were discussed,respectively.The liquid medium and the motion of material resulted with a good drying uniformity in MVF.The deterioration of oil during VF was slowed down by microwave at the same frying batch due to higher efficiency.Then,the mechanism of high-efficiency in MVF was studied through the variation of microwave output power,dielectric properties and temperature of both potato slices and oil.Results showed that the synergistic heating effect of microwave and oil made the heat transfer to the same direction with water removal,thus increasing the driving force for moisture loss.The thin layer drying dynamic model of MVF was created and the moisture loss kinetics was analyzed.The Deff of VF was improved from 0.501×10-8 m2/s to 0.667×10-8 m2/s0.739×10-8m2/s by microwave?600 W1000 W?.Simulated first-order model was developed for the variation of oil content in potato chips during MVF.In this study,total oil content in fried samples was divided into surface oil content and structural oil content to reveal the de-oil mechanism of MVF.Results of quantitative analysis and CLSM observation showed that both the surface oil content and structural oil content in MVF samples were less than that in VF.The total oil content was determined by the surface morphology and the size and distribution of the pores in the structure.The formation of porous structure was accelerated and the vapor pressure was increased by microwave,which prevented the adhesion of fat to the surface and porous structure.The surface cell morphology was well kept and more micron sized porous structures were produced,which makes more surface oil removed in the centrifugation and reduced the penetration of oil to the structure,thus revealing the de-oil mechanism of MVF.According to the dielectric heating and de-oiling mechanism of MVF,the dielectric pretreatments?freezing,osmotic dehydration,coating?and vacuum infrared?VIR?pre-drying was combined to reduce the oil uptake of MVF potato chips,respectively.Results showed that the oil content of MVF samples was increased by 33.1%with freezing pretreatment,while it was effectively decreased by 16.6%31.4%using the NaCl osmotic pretreatment with the concentration of 1%5%.In addition,the oil content of MVF potato chips was reduced by about 53.5%,36.8%and 15.2%with the 25%maltodextrin,1.0%CMC-Na and 1.5%chitosan coating pretreatment,respectively.The VIR pre-drying?120?,0.085 MPa,20 min35 min?made a dense and hard surface cell morphology,and accelerated the formation of the porous structure during MVF.The oil content of MVF potato chips was reduced from 22.39±0.35?g/100 g db?to 13.49±0.29?g/100 g db?14.48±0.46?g/100 g db?,which was reduced by about 54.4%57.5%compared to VF samples.The shrinkage and color change of MVF samples was increased by VIR pre-drying,the overall acceptability achieved highest when the connective moisture content reduced to about 75%?wb?.Therefore,osmotic dehydration,coating and VIR pre-drying could be utilized as effective de-oiling pretreatments for MVF.In order to improve the frying efficiency of low-temperature frying,the effectiveness of the vibration effects of low-frequency ultrasound on VF process was studied with the combination of microwave.Results showed that ultrasound?28 kHz,300 W600 W?revealed significantly the vibration effect.The combination of ultrasound and microwave had a synergistic effect on the energy efficiency and quality attributes of vacuum fried potato chips,shortening the frying time by 36.4%54.5%,increasing the Deff by 60.5%144.5%and decreasing the total energy consumption by 34.9%48.3%.The oil content of VF potato chips was reduced from 19.98±0.42?g/100 g db?to 13.53±0.53?g/100 g db?14.51±0.36?g/100 g db?,reducing by 27.4%32.3%with the synergistic effect of ultrasound and microwave,even lower than that in MVF samples?17.96±0.57 g/100 g db?.The crispness,shrinkage,color and storage stability of VF samples were ultimately improved,and more porous structures were induced by combined ultrasound and microwave.The low-frequency ultrasound showed a more remarkable effects in the low-temperature?90??frying condition.Finally,the synergistic mechanism of low-frequency ultrasound combined microwave on the frying efficiency and quality of VF potato chips was investigated with water/oil medium simulation systems.Results showed that the changes of moisture content,water loss,solid gain,dielectric properties and conductivity in samples were enhanced by ultrasonic treatments.The binding force between moisture and structure in the sample was weakened by ultrasound.Intercellular expansion and micro-channels were also produced by ultrasound in the microstructure,which made it easier for the moisture removal and affected the microstructure of the fried product.The Deff of VF potato chips was improved by about 72.0%84.3%and70.4%with the combined microwave and ultrasound treatment in water and oil medium simulation system,respectively.The oil content,breaking force,shrinkage,color change and water activity of VF samples were consequently reduced.Thus,the synergistic mechanism of ultrasound combined microwave during VF was revealed. |
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