Study On Pretreatments Of Okra By Flat Sweep Frequency Pulsed Ultrasound And Freeze-Thaw Based On Drying Efficiency And Quality Control

Okra is a kind of vegetables that contains high moisture and rich nutrition.During the storage period after picking,the internal moisture gradually loses,and wood aging occurs.Moreover,it deteriorates quickly during transportation because of mechanical loss.To prolong the shelf life of okra,drying process is necessary.An appropriate drying method plays a key role in the quality and subsequent processing of dried products.At present,the most popular method for production of fruits and vegetables is hot air drying.However,such method follows with poor product quality,which is not suitable for high nutritional value of fruits and vegetables.Instead,vacuum freeze drying and vacuum pulse drying methods are better in maintaining the quality of okra.However,the dense villi and wood fiber structure of the outer epidermis of okra,as well as the rich water-soluble pectin in the pulp,make the moisture diffusion slow in the drying process,resulting in long drying time and high cost.A method of pretreatment to address this problem,therefore,needs to be explored.In this paper,the physical processing pretreatments(flat sweep frequency,pulsed ultrasound and freeze-thaw)were combined with vacuum pulse drying and vacuum freeze-drying to improve the water diffusion ability in the drying process and reduce the drying time.Meanwhile,the impacts of drying technologies on dried products,i.e.,physical properties and nutritional qualities,were also investigated.The main contents and results are as follows:(1)The flat sweep frequency and pulsed ultrasound(FSFP)and freeze-thaw(FT)pretreatment methods were applied to the vacuum pulse drying of okra.The microstructure of okra pretreated by FSFP and FT,the signal of ultrasound field,the drying kinetics of okra vacuum pulse drying,drying time,and the physical properties(color,hardness and brittleness),chemical components(flavonoids,phenolic acids,chlorophyll),flavor and antioxidant capacity of dried products were discussed.The results showed that the drying time of vacuum pulse drying can be significantly shortened by the FSFP and FT pretreatments(20.44%,52.91%);the optimal parameters of FSFP were center frequency 40 k Hz,power density 25.0 W/L,scanning cycle180 vms,amplitude ±1 k Hz;fluorescence microscopy results showed that the FSFP pretreatment had little effect on the cell viability of okra,and the cell viability decreased after FT pretreatment;scanning electron microscopy and optical microscopy showed that FSFP made the surface of okra more fluffy.The results also showed that the noise signal collected by hydrophone spectrum was the strongest when the parameters of FSFP were the best;the browning index of dried products was decreased after the pretreatment of FSFP.After FT pretreatment,the quality of vacuum pulse drying products was deteriorated seriously,with higher browning index,collapse of tissue structure,loss of flavor,lower content of chemical components and lower antioxidant capacity.It was concluded that the drying time of vacuum pulse drying can be significantly reduced by the pretreatments of the FSFP and FT.The quality of the dried products after FSFP was higher than control.Therefore,the FSFP was suitable for pretreatment before vacuum pulse drying,and FT was not suitable for vacuum pulse drying,but FT pretreatment can significantly shorten the drying time was still worthy of attention.(2)The flat sweep frequency and pulsed ultrasound(FSFP)and freeze-thaw(FT)pretreatment methods were applied to the vacuum freeze-drying of okra.The microstructure of okra after pretreatment with FSFP and FT(freezing water thawing(WT),freezing ultrasonic thawing(UST),freezing air thawing(AT))were studied.The effects of drying kinetics during vacuum freeze drying were discussed.The physical properties(color,hardness and fragility),chemical components(flavonoids,phenolic acid,chlorophyll),flavor and antioxidant capacity of dried products were discussed.The results showed that the drying time was reduced by 25.0%-62.50% through pretreatments,and the total energy consumption was reduced by 24.28%-62.35%.The cell structure of okra was less damaged by FSFP,which promoted the water mass transfer and significantly shortened the thawing time.In addition,most of the quality characteristics(flavor,color,hardness and brittleness,flavonoids,and phenolic acids)and low chlorophyll degradation were retained;In the FT pretreatments,AT had the longest shortening time(62.5%),lower chlorophyll degradation rate and higher contents of total phenols,flavonoids and pectin,UST method had shorter thawing time and stronger antioxidant capacity.It was concluded that the drying time of vacuum freeze-drying can be significantly reduced by the pretreatment of FSFP and FT,and the quality of products obtained by FSFP,UST and AT are better.Therefore,FSFP and FT pretreatment methods were suitable for vacuum freeze drying.(3)The effects of flat sweep frequency and pulsed ultrasound(FSFP),freeze-thaw(FT)pretreatment,vacuum pulse drying(VPD)and vacuum freeze-drying(VFD)on the structural,physicochemical,and functional properties of pectin in okra were studied.The results showed that: 1)pretreatments and drying methods had no effect on the molecular structure of pectin and did not affect the amorphous polymer state of pectin;the molecular chain of pectin obtained by VFD kept the original state and crosslinked after FSFP and FT;the molecular chain of pectin obtained by VPD was crosslinked,and the structure was compact,and the structure became loose after FSFP.2)Drying method could affect the main chain of pectin molecular chain,VPD pectin had better retention of Hg main chain,FSFP reduced the decomposition of HG main chain of VPD and VFD,but FT had the opposite effect;the molecular weight of VFD pectin was unchanged,VPD molecular weight decreased,FSFP and FT had little effect on molecular weight;the galacturonic acid contents of VFD and VPD pectin were the same,FSFP increased the galacturonic acid contents of VFD and VPD,but FT had the opposite effect;3)the viscosity of pectin obtained by VPD increased,and FSFP reduced it;the viscosity of VFD remained unchanged,while FSFP and FT increased it;the emulsifying stability of VFD was better,and the emulsifying activity of VPD was better;the thermal stability of VFD was the worst,and the thermal stability of VPD was better.The antioxidant capacity of pectin obtained by VPD was better than that of VFD,and FSFP increased its antioxidant capacity.The gel strength,hardness,adhesiveness,gumminess,and chewiness of VFD samples were higher than those of VPD,and FSFP enhanced VFD and VPD characteristics.In conclusion,the structure and functional properties of pectin were greatly affected by the drying methods.The pectin obtained by vacuum freeze-drying can maintain the natural molecular state,while the conformation of pectin molecular chain obtained by vacuum pulse drying changes.The flat sweep frequency and pulsed ultrasound and freeze-thaw pretreatment also had an impact,and the effect was greatly related to the drying methods.In summary,the FSFP pretreatment improved the drying efficiency of vacuum pulse drying and vacuum freeze drying.It had a favorable effect on the quality of dried products obtained by the two drying methods.The FT pretreatment improved the drying efficiency of vacuum pulse drying and vacuum freeze-drying to a greater extent.It had little effect on the quality of dried products obtained by vacuum freeze-drying method.However,it caused the quality degradation of dried products obtained by vacuum pulse drying method.