| Tiger nut is an economical oil crop,which is expected to be a substitute for soybean in the future,and reduce the dependence of oil on foreign countries in China.Due to its low planting cost and high yield,the planting area of tiger nut in China is increasing year by year,and the sharp increase in yield also brings many problems.Fresh tiger nut has high moisture content,dense epidermal structure,difficult water loss,and because it is rich in oil and prone to mildew so it needs to be dried and dehydrated in time during transportation and storage.At present,most tiger nut producing areas in China still use the natural drying method of field drying,which is susceptible to the influence of the weather and is faced with serious shortage of resources such as sunning grounds and human resources.So it is far from meeting the development and demand of the existing tiger nut industry.Therefore,the study of efficient mechanized drying and other modern drying technology is an effective way to solve the problem of tiger nut drying.In this study,hot air drying equipment was used for thin layer drying of fresh tiger nut.The effects of hot-blast temperature and wind speed on the drying characteristics of tiger nut were studied.The dynamics model was fitted,and the distribution and migration rules of different moisture states in the process of hot air drying of tiger nut were studied by using low-field NMR technology.In order to solve the problems of difficult peeling and large loss of peeling before oil extraction,the method of hot air drying after Wet Peeling was adopted to explore the effect of hot air drying on the drying characteristics and quality of desquamated tiger nut,so as to provide scientific theoretical guidance for mechanical drying of tiger nut.The main research contents are as follows:Using hot air drying equipment,the change rule of drying characteristics of tiger nut was studied under different hot air temperature and wind speed.The results indicated that the higher the hot-blast temperature,the shorter the drying time of the tiger nut to the same moisture content.The total drying time at 70?was 53.33%shorter than that at 50?.The drying process of tiger nut was mainly slow drying,and the drying rate gradually decreased with the drying process.In the later drying period,the strengthening effect of hot air temperature on the tiger nut's drying rate is clearly weakened.The higher the wind speed was,the higher the drying rate was,However,the effect of hot air temperature on the drying process was more marked t than that of wind speed.The range of effective moisture diffusion coefficient was 2.2856×10-10?7.8112×10-10,and the average activation energy was 35.31kJ/mol.The nonlinear fitting analysis of experimental data of eight commonly used hot air drying models was carried out.The two-term model could well reflect the hot air drying process of tiger nut.The R2 value of the model was the largest,the mean root mean square error(RMSE)and the sum of squares of errors(SSE)were the smallest,and the experimental values were in the best agreement with the predicted values,so the fitting effect is the best.Low field nuclear magnetic resonance(LF-NMR)technology was used to study the water distribution and migration in the hot air drying process of the tiger nut,and the influence of different hot air temperatures on various moisture changes of the tiger nut was compared and analyzed.Meanwhile,scanning electron microscope was used to survey the microstructure of dried tiger nut.The results showed that:there were three kinds of water states in the tiger nut,which were free water,immobilized water and bound water.During the drying process,the free water with the most content was removed first,and the immobilized water presented a trend of increasing first but then decreasing,while the bound water had no significant change during the drying process.With the drying process and the decrease of free water,the tiger nut contracted and deformed,the epidermal accumulation became thicker,and the internal pore became narrower by the extrusion of macromolecules.The relaxation diagram migrated to the left as the fluidity of free water decreased,and the higher the temperature of hot air was,the more free water was converted into water that was not easy to flow.In the late drying period,it is mainly bound water and immobilized water,which leads to the decrease of drying rate.The total peak area of each water measured by LF-NMR was regression analyzed with the dry base water content,and the linear equation was obtained as Y=17.474X+134.05.Scanning electron microscope(SEM)showed the structure changes during drying,the higher the drying temperature,the more serious the deformation of pore structure,the more tightly the starch particles stacked.In order to study the effect of peeling pretreatment on hot air drying characteristics of tiger nut,the effect of drying process on the quality of tiger nut was studied based on color,hardness,rehydration,oil acid value and peroxide value.The results indicated that the peeling pretreatment could accelerate the dehydration rate in the early stage of hot air drying,And half of the total moisture of fresh tiger nut can be removed 2 hours before drying.Because the moisture diffusion is the main factor in the later drying stage,the drying rate significantly decreased,With the increase of hot air temperature,the drying rate will be greatly reduced and the total drying time will be prolonged Therefore,the actual processing application of the hot air temperature is set at 60?is more appropriate.The brightness and redness of peeled tiger nut decreased after drying,and the hardness of peeled tiger nut increased after hot air drying,but it was much lower than natural drying.In the rehydration test,hot air drying at 50?was the best,followed by natural drying,and hot air drying at 70?was the worst,indicating that hot air temperature was the main factor affecting the rehydration.However,uneven drying temperature and long drying time in natural drying would also damage the internal structure of the sample and reduce the rehydration.The acid value and peroxide value of crude oil were much lower than the national standard when the peeled tiger nut was extracted by leaching after drying,which indicated that the drying process of desquamated tiger nut would not have a great impact on the quality of oil.The drying characteristics of hot air drying desquamated tiger nut showed that the drying rate was high in the early drying period,and significantly decreased in the middle and late drying period,and gradually lower than the direct drying rate.In order to further understand the change of drying characteristics of peeled tiger nut,low-field nuclear magnetic resonance(LF-NMR)technology and nuclear magnetic resonance imaging(MRI)technology were used to explore the causes of the above results from the internal water distribution and migration theory of peeled tiger nut.The results proved that peeled tiger nut had three kind of water,which were bound water,immobilized water and free water.At the preliminary stage of drying,the internal moisture of the tiger nut is mainly free water,and there is no epidermis to hinder the rapid drying loss,which is corresponding to the content of higher drying rate in the early stage of drying in Chapter 3.During the process of drying,the immobilized water firstly increased but then decreased,and the increase of drying temperature will cause more free water to be converted into immobilized water.During the whole drying process,the water relaxation peak significantly migrated to the left,and the relaxation time of T23 free water peak at different temperatures shifted from 33.96 ms,31.23 ms and 34.63 ms to about 4 ms,the immobilized water T22,which is not easy to flow,migrated to 0.01?1.0 ms two hours before drying and merged with the bound water T21.The results of MRI images showed that with the continuous decrease of moisture content inside the tiger nut,the brightness and the water-containing area in the images were darkened and decreased.The imbalance between the outward migration rate of water and the removal rate of surface water led to an uneven process of the drying of the tiger nut.To sum up,free water and immobilized water move significantly to the left,as well as more free water turns into immobilized water.These two changes lead to a significant decrease in the drying rate of peeled tiger nut in the middle and late stage of drying,which explains that the drying rate of peeled tiger nut in the late stage of drying is lower than that of direct drying in Chapter 3.MRI images showed that the non-uniform drying process also led to the drying and hardening of the tiger nut epidermis,so the key point to accelerate the drying rate in the later stage was to promote the outward migration of water inside the tiger nut. |
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