Study On Drying,Quality And Storage Properties Of Walnut Drying With Drum Catalytic Infrared-Hot Air

Walnut is one of the world's famous nuts.The walnut is rich in unsaturated fatty acids,proteins and polyphenol antioxidants,which are beneficial to cardiovascular diseases and diabetes.The current drying method of walnut is mainly hot air drying,which has the disadvantages of low drying rate,long time and high energy consumption,which severely restricts the quality of walnut and the development of the industry.Catalytic infrared drying technology,as a new type of drying technology,uses natural gas as the main energy source.It has the advantages of high drying efficiency,energy saving and environmental protection.Combined with traditional hot air drying(HAD),it can increase the drying rate while taking into account product quality.Low-field nuclear magnetic resonance(LF-NMR)technology can be used in the study of moisture content changes and migration mechanisms during the drying process due to its rapidity and non-damage to materials.Therefore,this paper combines drum catalytic infrared drying with traditional hot air drying,adopts constant temperature and variable temperature drying modes,optimizes the drying process and calculates energy consumption,and examines its influence on the drying characteristics,drying kinetics,and the quality of walnut oil,protein,and polyphenols.LF-NMR technology is used to investigate the moisture migration mechanism in the combined drying process and establish a moisture content prediction model.Finally,the storage characteristics of walnuts after drying are investigated,and storage period prediction models are established.The main conclusions are as follows:(1)A method of constant temperature drum catalytic infrared-hot air combined drying(CTDCIR-HAD)was optimized.The walnut was dried under the optimal infrared temperature of 450?,radiation distance of 30 cm,drum rotation speed of 25Hz for 9.5 min and then transferred to hot air temperature of 43?and wind speed of3 m/s for 16 h,and the total drying time was 16.16 h.Compared with HAD(20 h),the drying rate was significantly increased,10.71%of the moisture was removed in the infrared drying stage,the drying time was shortened by 19.20%,and the energy consumption was saved by 11.60%by CTDCIR-HAD treatment.The dried walnut shell had no brown spots and break and showed bright red and yellow(L of 47.51,a of 7.97,b of 16.37).CTDCIR-HAD was superior to HAD and it is a new,high-efficiency and energy-saving processing method suitable for drying hard shell nut products.(2)A method of variable temperature drum catalytic infrared-hot air combined drying(VTDCIR-HAD)was optimized.The walnut was dried under the optimal infrared temperature combination of 300+400?,radiation distance of 30 cm,drum speed combination of 25+35+35 Hz for 22.50 min,and then transferred to hot air temperature of 43?and wind speed of 3 m/s for 14 h,and the total drying time was14.38 h.The dried walnut shell had no brown spots and break,and showed bright red and yellow(L of 54.24,a of 10.61,and b of 19.89).The quality of dried walnut was good.Compared with HAD(20 h),the drying time was shortened by 28.10%and the energy consumption was saved by 25.04%by VTDCIR-HAD treatment.Compared with CTDCIR-HAD,the drying time was shortened by 11.01%and the energy consumption was saved by 15.21%by VTDCIR-HAD treatment.VTDCIR-HAD was better than HAD and CTDCIR-HAD,which was more suitable for the dry processing of nut products,and was more efficient and energy-saving.In addition,a Henderson-Pabis model for predicting the drying kinetics of VTDCIR-HAD was established.R²was 0.99,RMSE was 0.03,and therefore the fitting effect was good.It can well predict the changes of moisture content and drying rate of walnuts during the drying process.(3)According to the water prediction model based on LF-NMR,the water migration mechanism was revealed in the drying process of VTDCIR-HAD and CTDCIR-HAD.It was found that with the extension of the drying time,the free water peak area continued to decrease until almost disappeared,the weakly bound water peak area also continued to decrease,and the strong bound water and oil peak areas were no significant changes,indicating that the main moisture lost during the drying process was free water and weakly bound water,and the contents of strong bound water and oil do not change much.Under similar moisture ratios,weakly bound water and free water relaxation times of walnut after the treatments of CTDCIR-HAD and VTDCIR-HAD were both higher than that of HAD group,indicating that infrared radiation drying can enhance the mobility of water molecules inside the walnut and accelerate its diffusion from the inside to the surface,thereby improve the drying efficiency.In addition,LF-NMR based moisture content change prediction models for CTDCIR-HAD and VTDCIR-HAD during the drying process were established.R² values were both greater than 0.92,and the fitting effect was good,indicating that LF-NMR technology can better monitor the water migration of walnut during the drying process.(4)After inspecting the quality of walnuts treated with three drying methods,it was found that compared with HAD,CTDCIR-HAD and VTDCIR-HAD reduced the lipid acid value by 17.31%and 28.85%,respectively,and the peroxide value by 4%and20%,respectively,but the iodine value,oleic acid,linoleic acid and linolenic acid contents had no significant changes.The quality of walnut oil treated by VTDCIR-HAD was the best.Compared with HAD,VTDCIR-HAD and CTDCIR-HAD treatments changed the structure of walnut protein,which showed that the infrared absorption intensity increased,the content of?-helical structure decreased by 50%and 18.75%,the fluorescence intensity increased,and the surface hydrophobicity increased by 28.27%and 12.51%.The ratio of free hydrophobic amino acids decreased by 19.24%and9.49%,while the ratio of hydrophilic amino acids increased by 9.84%and 4.85%.There was no significant change in the apparent structure of the protein.Compared with HAD,CTDCIR-HAD and VTDCIR-HAD increased the content of polyphenols by 5.23%and10.92%,respectively,while IC_50(DPPH)and IC_50(ABTS)decreased by 24.36%,11.58%and14.72%,10.60%,respectively.VTDCIR-HAD treatment had the highest content of polyphenols in walnut.In conclusion,VTDCIR-HAD treatment was the best,which can slow down the rate of oil oxidation and rancidity,improve the protein structure,increase the content of polyphenols and antioxidant activity,and was more beneficial to protect the quality of dried walnut.(5)The acid and peroxide values of walnut oils were used as evaluation indicators to investigate the storage characteristics of walnuts treated with three drying methods.It was found that with the extension of storage time,both the acid value and peroxide value showed an increasing trend.Compared with HAD,CTDCIR-HAD and VTDCIR-HAD treated walnut oils had lower increase of acid and peroxide values.Among them,the increase of acid value and peroxide value in VTDCIR-HAD treatment was the least and had the best storage performance.In addition,a storage period prediction modelof walnut treated with VTDCIR-HAD was established.R² values were both greater than0.96 and the relative errors were less than 8.93%,indicating that the prediction model can better predict the shelf life of walnut treated with this drying method at 5-35?.