| Freeze-dried fruits and vegetables have a porous sponge-like structure,which makes it easy for moisture to enter the interior of freeze-dried fruits and vegetables,resulting in oxidation reactions and collapse of the texture,reducing product quality and shortening shelf life.Therefore,moisture protection has become an important process and research priority in the processing,transportation,storage and consumption of freeze-dried fruits and vegetables to ensure food quality and safety.Traditional damp-proofing techniques are time-consuming,wasteful of film materials,poor damp-proofing and short product shelf-life,so it is particularly important to choose an efficient,high-quality damp-proofing technique.Zein has good film formation,moisture barrier,flavour retention and degradability,making it a more suitable edible film material;electrospray coating technology has the advantages of uniform film formation,strong adhesion,material saving and anti-bacteria,etc.Zein combined with electrospray technology can effectively reduce the moisture absorption of freeze-dried fruits and vegetables.In this project,freeze-dried hawthorn,freeze-dried apple and freeze-dried carrot products were developed by using the pulse spray bed microwave freeze-dried pilot platform,and the hygroscopic characteristics of lyophilized products were studied,and the moisture-proof effect of electrospray spraying of freeze-dried fruits and vegetables was explored by using Zein as the film-forming material.The results of this study provide technical support for freeze-dried fruits and vegetables to prevent moisture.The specific research content is as follows:Firstly,the preparation and hygroscopic characteristics of freeze-dried fruits and vegetables（hawthorn,apple and carrot）were studied.Developed hawthorn,apple and carrot freeze-dried fruit and vegetable products using the pulse spouted bed microwave freeze-drying（915 MHz）（PSBMFD-915）,analyzed the hygroscopic characteristics and product quality of three freeze-dried fruits and vegetables,and combined with conventional freeze-drying（CFD）for comparison.The results showed that compared with CFD,PSBMFD-915hawthorn,apple and carrot had lower density and loose porous structure,showing higher moisture absorption rate,which was 5.99%,6.13%and 1.93%at equilibrium and higher rehydration capacity at equilibrium at equilibrium of 3.29 g/g,4.53 g/g and 2.70 g/g,respectively.In terms of nutrient retention,the polyphenol retention rates of PSBMFD-915hawthorn,apple and carrot were 92.77%,98.48%and 90.21%,the total flavonoid retention rates were 72.29%,76.54%and 64.12%,and the ascorbic acid retention rates were 30.71%,73.96%and 51.45%,respectively.In terms of antioxidant capacity,the retention rates of DPPH were 72.98%,74.79%and 88.12%,and the retention rates of reducing power were87.27%,92.80%and 42.16%,respectively.Secondly,the optimization of film material formulation and electrospray spraying process parameters was studied.The effects of the addition ratio,voltage,flow rate and needle diameter electrospray spraying parameters of zein,citric acid and glycerin on the water vapour permeability rate（WVPR）of the membrane were studied.The results showed that the concentration of Zein,voltage,flow rate and needle diameter significantly affected the moisture resistance of the film,and that the moisture resistance of the film was proportional to the concentration of Zein and voltage,and inversely proportional to the flow rate and needle diameter.Using WVPR as the response value,the optimum film material formulation and electrospray coating process parameters were determined by response surface analysis,i.e.5%Zein concentration,5%citric acid addition（in terms of Zein amount）,20%glycerol addition（in terms of Zein amount）,as well as a voltage of 50 k V,a flow rate of 0.25 m L/min and a needle diameter of 0.25 mm,under which conditions The experiment was repeated three times and the mean WVPR of the film was 7.24 g·mm·m-2·d-1·k Pa-1.Thirdly,the effects of electrospray spraying treatment on the film liquid‘s and film’s structure and function were investigated,the mechanism of electrospray spraying film formation and moisture resistance were analyzed.The results of the study showed that the PDI,zeta potential and conductivity of the film liquid after electrospray spraying were 0.13,-34.71 m V and 45.1μs/cm respectively;the microstructure showed that the surface of the film was smooth and neat without obvious pores and cracks;the FT-IR test showed that the electrospray treatment enhanced the compatibility between Zein,citric acid and glycerol;The results of XRD analysis showed that the electrospray treatment was beneficial to the formation of dense and orderly structure of the film;the results of TGA analysis showed that the electrospray treatment improved the thermal stability of the film;the results of water sensitivity study showed that the contact angle of water droplets in the electrospray film became larger,hydrophobicity was improved and the water barrier performance was excellent compared with the pressure atomization film.Finally,the effect of electrospray coating on freeze-dried hawthorn,freeze-dried apple and freeze-dried carrot products was investigated.The results of the study showed that electrospray spraying formed a uniform film on the surface of freeze-dried fruits and vegetables,which had good water resistance and reduced the moisture absorption,rehydration rate and loss of nutrients and functional components of freeze-dried hawthorn,freeze-dried apple and freeze-dried carrot products.Under the storage conditions of 25℃,75%relative humidity and no packaging,the moisture absorption rate of freeze-dried hawthorn,freeze-dried apple and freeze-dried carrot were 1.01%,1.15%and 0.70%,the rehydration rate was2.38 g/g,3.05 g/g and 1.82 g/g,the retention rate of total phenols was 86.60%,76.69%and91.44%,and the retention rate of total flavonoids was 91.87%and 91.87%,respectively,after7 d.91.87%,89.11%,72.60%,91.62%,83.47%,79.88%,88.35%,86.23%,89.97%and83.74%,85.89%,89.63%for the retention of DPPH,respectively.In addition,the freeze-dried fruit and vegetable products formed by electrospray coating were able to maintain the original colour and flavour matter. |
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