Characteristics And Regulation Of Porous Morphological Of Chinese Yam During Microwave Freeze-drying(MFD)

Chinese yam,also known as Dioscorea opposita Thunb,has a special health effect on the human body.It can prevent the cardiovascular and cerebrovascular diseases of the human body,and it can also protect the stomach of the human body.It is necessary to extend the shelf life of the Chinese yam by using a suitable drying method,because the fresh yam is not resistant to storage.Hot air drying?AD?is economically applicable,but the serious loss of nutrients in the products often makes the products fail to meet the expected requirements.Freeze drying?FD?reduces the nutrient loss of the product and achieves the highest quality products,but is also considered to be the most expensive drying method.In order to reduce the cost,the pre-frozen material can be heated by microwave under vacuum,which is called Microwave vacuum freeze drying?MFD?.Compared with FD,MFD has a fast drying speed and good product quality,and has the potential to replace FD.Therefore,in this paper,the Chinese yam was dried by MFD,and the changes of water migration,pore morphology and dry quality during the drying process were studied,which provided a theoretical basis for the quality control of the Chinese yam microwave freeze-drying process.The main conclusions are as follows:1.The study on the water diffusion characteristics of the Chinese yam microwave vacuum freeze-drying process found that the whole drying process of the Chinese yam had obvious speed increase and speed reduction process,no obvious constant speed process.The water with high degree of freedom was converted to the water with low degree of freedom during the drying process of MFD.The content of combined water generally decreased first and then increased,and the content of combined water at the end of drying was significantly higher than that at the beginning of drying.The effective water diffusion coefficient of the Chinese yam drying process of different microwave power densities ranged from 1.129�10^-95.439�10^-9 m²/s.As the drying progresses,the value of the effective moisture diffusion coefficient was gradually reduced.2.The study on the evolution of the pore structure of the Chinese yam microwave vacuum freeze-drying process found that it was first formed into closed pores in the MFD process,and then some closed pores were transformed into open pores,and the open porosity increased gradually.In general,higher porosity is obtained when the slice thickness is smaller and the microwave power density is higher,but the phenomenon of coking and surface cracking is likely to occur.It is difficult to reach the drying end point when the microwave power density is low and the slice is thick.Therefore,it is necessary to select a suitable microwave power and slice thickness for drying.The pore diameter of the Chinese yam was basically in the range of 10 to 106nm during the whole drying process.The median pore diameter increased and then decreased and then increased.3.The study on the dry quality of Chinese yam based on the change behavior of porosity showed that the brittleness increased with the extension of drying time.The hardness increased with the extension of drying time,and remained basically unchanged after reaching the maximum value.At the end of drying,the peaks of the sensory scoring fuzzy matrices of 0.125,0.225,and 0.370 W/g were 0.3,0.375,and0.333,respectively,which means that proper microwave power density can produce better product quality,and excessive microwave power density may be Will reduce consumer acceptance of the product.The brittleness of the Chinese yam is related to the moisture content,and the hardness mainly depends on the porosity and moisture content.The established brittleness model was y=A₁+?A₂-A₁?/(1+10^??logx0-x?p?),R²>0.99.The hardness regression model had a better fitting degree,which could better describe the texture quality changes of Chinese yam.4.Higher open porosity and shorter drying cycles can be obtained by pretreatment,but at the same time the damage to the cell structure of the material was enhanced.The open porosity of osmosis pretreatment,blanching pretreatment and ultrasonic pretreatment was about 1.23,1.98 and 1.92 times of the untreated sample group,respectively.Ultrasonic pretreatment can obtain the most ideal pore network structure,which means that the pore network structure of the material showed a uniform sponge shape.Ultrasonic pretreatment can obtain the most ideal pore network structure,which means that the pore network structure of the material showed a uniform sponge shape.