Study On Energy Saving And Quality Loss-reduction Of Pulse-spouting Combined With Microwave Freeze-drying Of Chinese Yam

Fresh fruits and vegetables are rich in nutrients,but they are perishable due to their high water content.Drying processing allows for a significant extension of shelf life,ease of storage and transportation,and breaks seasonal constraints to achieve annual supply.In recent years,people's demand for high-quality dried fruit and vegetable products has gradually increased.Freeze-drying(FD)is a kind of drying method which can keep the original nutrients and color,aroma,taste and shape of fruits and vegetables to the maximum extent.However,long drying time and high energy consumption limit its application.Microwave freeze-drying is a new type of freeze-drying method developed on the basis of FD.It has the characteristics of significantly shortening drying time,reducing drying energy consumption and maintaining product quality comparable to FD.In this study,Chinese yam was used as the test material,and the advantage of improving the drying uniformity of the pulse-spouting system was combined with the microwave freeze drying,the pulse-spouting combined with high/low frequency microwave freeze-drying process with high-efficiency,energy-saving and loss-reduction was studied systematically.The pretreatment methods for improving microwave freeze-drying efficiency were developed by combining ultrasonic,dielectric treatment and vacuum impregnation technology.The mechanism of accelerated drying was revealed.Meanwhile,the uniformity of drying was studied and evaluated.Firstly,the energy-saving and loss-reducing processing technology pulse-spouting microwave freeze-drying(PSMFD-2450)using high frequency microwave(2450 MHz)as heating source were studied.The drying characteristics and product quality of PSFD-2450 with different processing parameters(microwave power,spouting parameters and upper limit temperature)were studied.The results showed that increasing microwave power and upper limit temperature could significantly shorten drying time,but resulted in deterioration of nutritional quality and color of the product.The spouting interval had no significant effect on drying time.However,the longer spouting interval leaded to the deterioration of product quality.Considering comprehensively,microwave power of 3 W/g,spouting interval of 20 min and upper limit temperature of 50? were suitable.Compared with FD,PSMFD-2450 under this process can significantly shorten drying time,reduce energy consumption,and maintain good quality similar to FD.Compared with static microwave freeze-drying(MFD-2450),PSMFD-2450 can significantly improve product quality and drying uniformity.Secondly,the drying characteristics of pulse spouting low-frequency microwave(915 MHz)freeze-drying(PSMFD-915)was studied,and its effect on the energy saving and loss reduction was also evaluated.The results showed that a multi-stage variable power microwave loading scheme desgined according to the dielectric properties evolutions of yam and the discharge characteristic during PSMFD-915 avoided the gas discharge effectively.Under the test spouting interval,the longer the spouting interval was,the shorter the drying time was.However,for the quality of dried products including color,rehydration rate,total phenol and total flavonoid content,the shorter spouting interval could produce products with the same quality as FD and PSMFD-2450,and was conducive to improving the drying uniformity.Compared with FD,PMFFD-915 can save up to 34.4% of total energy consumption.Compared with PSMFD-2450,the specific energy consumption was greatly reduced due to the large sample processing capacity per unit time.In addition,the peak viscosity and final viscosity of flour paste produced by microwave-assisted freeze-drying(PSMFD-2450 and PSMFD-915)was high,showing a good thickening effect.In addition,microwave-assisted freeze-drying showed destructive effect on yam starch granules.Thirdly,the transition points of sublimation drying and desorption drying in PSMFD-2450 and PSMFD-915 were studied.The melting point of Chinese yam was taken as the transition point to study the difference,characteristics and influence of drying process on the moisture content of the transfer point.The results showed that the different moisture content at the transition points showed different drying rates and sample temperature variation characteristics.In order to ensure the quality of the product,especially the shrinkage rate,the moisture content at the transition points of PSMFD-2450 and PSMFD-915 should not be higher than 0.87 g /g d.b.Then,ultrasonic combined with dielectric(ultrasonic assisted salt solution osmotic dehydration,USOD)was used to treat the yams to study its effects on the drying characteristics,product quality and drying uniformity of PSMFD-2450 and PSMFD-915.The results showed that USOD treatment changed the microstructure of fresh yam tissue,made the water in the material organization more active,and increased the material loss factor,thereby accelerating the drying process;USOD treatment with ultrasonic intensity of 151 W/L had the best effect on accelerating PSMPD-2450 drying and maintaining product quality.It was applied to PSMFD-915,which significantly shortened the drying time,and saved energy consumption up to 22.55% compared with control.It produced product quality similar as FD sample.The feasibility of pulsed vacuum osmotic dehydration pretreatment(PVOD)with sucrose solution as osmotic solution to change the unfrozen water content of yam was also discussed,and its effect on improving the drying rate and energy saving of PSMFD-2450 and PSMFD-915 was studied.The results showed that PVOD treatment increased the content of unfrozen water,decreased the eutectic point,changed the dielectric properties of frozen materials and the evolution of dielectric properties during drying process.PVOD shortened the drying time,increased the drying rate of PSMFD-2450,and obtained products with good color and rehydration ability.The best treatment group--40% sucrose solution PVOD treatment group was applied to PSMFD-915 can significantly shorten the drying time of PSMFD-915 and save energy consumption up to 54.65%(compared with FD).Same product quality as FD samples was achieved,and drying uniformity was improved compared with control.Finally,in order to provide a basis for on-line non-destructive monitoring of dielectric properties of yam in the future drying process,microwave vacuum drying was used as carrier to provide several samples with different dielectric properties.The difference of water state between different samples was monitored based on low-field nuclear magnetic resonance technology(LF-NMR)and the dielectric properties prediction model of Chinese yam was established by stoichiometric analysis.The results showed that the dielectric constant and loss factor of yam decreased nonlinearly with the decrease of water content.The corresponding NMR parameters showed a regular variation similar to the dielectric properties.A partial least squares regression model based on different variables was established by chemometrics.The optimized 4-variable partial least squares regression model had a great prediction performance.