Drying And Quality Characteristics Of Ultrasound Combined Far-infrared Radiation Drying On Pear Slices

Pear is a plant of genus Rosaceae.It is appreciated for its high value of nutritional attributes,and it could produce saliva,embellish dryness,reduce heat and dissolve sputum.In addition to fresh use,pears could be dried to get pear slices,which not only ensures the edible value of pears,but also broadens the consumer market of pears.Contact ultrasound drying technology has the characteristics of promoting mass transfer of materials.Far-infrared radiation drying has the advantage of improving material heat transfer.Combining contact ultrasound technology with far-infrared radiation technology could theoretically improve the heat transfer and mass transfer of materials simultaneously,thereby improving drying rate and product quality.The objective of this study was to investigate the drying characteristics and quality characteristics of contact ultrasound combining far-infrared radiation dried pear slices.The research results are intended to provide reference for the theoretical research and practical production application of ultrasonic combined with far-infrared radiation drying technology.The main contents of this study are as follows:1.The strengthening effects of ultrasound on hot air drying of pear slices were studied.The results showed that the application of ultrasound during hot air drying of pear slices is beneficial to accelerating drying rate,and higher ultrasound power could lead to higher dehydration rate and shorter drying time,When the ultrasound power of48 W was applied,the drying time can be shortened by more than 50%.The scanning electron microscopy was applied to observe the microstructure of pear slices under different drying conditions,and the results showed that the higher the ultrasound power,the more favorable the formation of a looser structure on the surface of the pear slices.Higher FIR temperature could produce more cavities and larger channels in dried samples.Higher ultrasound power could produce more pores and larger channels in dried samples,which could be a consequence of the better moisture migration inside pear slices.The peak area of free water with the highest mobility inside the pear slices showed a downward trend during the drying process,and it was completely removed first.Increasing ultrasound power could reduce the time when free water is completely removed.The peak area of bound water and immobilized water generally showed a trend of rising first and then decreasing.The higher the ultrasound power,the more significant the effect on the removal of immobilized water and bound water.When the temperature was 35℃and the ultrasound power was increased from 0 W to 48 W,the total phenolic content of pear slices increased by32.97%,the total flavonoid content increased by 21.06%,the rehydration rate increased by 21.61%.However,when the temperature was 55℃and the ultrasound power was increased from 0 W to 48 W,the total phenolic content was reduced by30.25%,the total flavonoid content was reduced by 8.66%,and the rehydration rate was increased by 16.34%.It shows that application of ultrasound at low temperature was beneficial to improving the quality of pear slices,and the total phenolic content,total flavonoids content and rehydration rate of pear slices increased with the increase of ultrasound power,while the excessive temperature was not conducive to the quality of the product.2.The drying characteristics and moisture transfer rule of ultrasound strengthened with far-infrared radiation drying of pear were investigated.The results showed that:both ultrasound power and far-infrared radiation temperature had significant effects on drying characteristics（P<0.05）,higher ultrasound power and far-infrared radiation power could significantly shorten drying time and improve drying rate.The average drying rate increased from 25.81%to 62.24%.The BP neural network was used to train and predict the change of water content during drying process,and the established mathematical model could achieve higher prediction accuracy,The coefficient of determination R² is 0.9992.The higher the ultrasound power and the far-infrared radiation temperature,the looser the longitudinal structure of the pear slices,and the larger the pore diameter.The LF-NMR results showed that with the increase of ultrasound power and far-infrared radiation temperature,the signal of the T₂ inversion spectrum reduced faster,and the time for free water to be completely removed was significantly shortened.The immobilized water and the bound water showed an upward trend in the early stage of drying,and increased with the increase of ultrasound power and far-infrared radiation temperature.The immobilized water and bound water show a downward trend after free water is completely removed,as increasing ultrasound power and far-infrared radiation temperature led to faster removel.The H⁺proton density image can visually and intuitively characterize the change of water distribution of the pear slices during drying process.With the increase in ultrasound power and far-infrared radiation temperature,the redness value and water-containing area decreased faster with the extension of drying time.3.The quality characteristics and parameters optimization of ultrasound strengthened far-infrared radiation drying of pear were investigated.The results showed that:increasing ultrasound power was conducive to improve the quality characteristics of pear slices at low far-infrared radiation temperature.Excessive high far-infrared radiation temperature and excessive ultrasound power combined with drying could impair the quality of pear slices.At the same ultrasound power,with the increasing far-infrared radiation temperature,the total phenolic content,total flavonoid content,total sugar content and rehydration rate of the pear slices increased first and then decreased,the color difference first drops and then rises,Vc and hardness values decreased.The optimum process parameters of ultrasound strengthened far-infrared radiation drying of pear slices were determined as radiation temperature of 220℃and ultrasound power of 24 W,by adopting AHP and comprehensively weighted grading method.And the corresponding contents of total phenolic,total flavonoids,Vc,and total sugar were 459.06 mg/100 g,231.36 mg/100g,34.86 mg/100 g,553.55 mg/100 g,the rehydration rate was 3.34,the color difference and hardness are 11.62,653.44 g,respectively.