Study On Drying Characteristics And Viscoelastic Behaviors Of Maize Kernels

The moisture content of harvested maize is usually very high, so it has to be dried before storage. The traditional drying methods for maize kernels have some shortcomings, for example, high energy consumption. Besides, the qualities of dried maize kernels are often poor. Moreover, due to various mechanical effects, the maize kernels may crack or break during drying. The study on hot air drying combined vacuum drying and viccoelastic behaviors of maize kernels can be of great importance in saving energy, optimizing processing machinery, and reducing the loss of maize.The drying characteristics of maize kernels under hot air and vacuum conditions were studied. The results indicated that the drying temperature and average precipitation rate showed a quadratic function in the two kinds of drying methods. And the two kinds of drying processes both showed two-stage deceleration. Modified Henderson and Pabis (Three Term Exponential) model fitted best with the drying data of hot air drying. And modified Midilli model fitted best with the drying data of vacuum drying. The moisture diffusivity of hot air drying was between 2.7559×10-8m2/h and 21.8854×10-8m2/h. And the moisture diffusivity of vacuum drying was between 4.0528×10-8m2/h and 32.9091×10-8m2/h. Furthermore, the moisture diffusivity of vacuum drying was greater than the moisture diffusivity of hot air drying at the same temperature. Compared to hot air drying, vacuum drying could improve the efficiency of drying significantly.The effects of moisture content on the viscoelastic behaviors of maize kernels were analyzed. The results indicated that the Tg values for the maize kernels decreased with increased moisture content. And the Tg values were 114℃,96℃,86℃,82℃,78℃,65℃ at moisture content values of 11.9%,14.6%, 16.4%,20.7%,23%,25.9%, respectively. The four-element Burgers model was found to adequately represent the creep behavior of the maize kernels (R2>0.97). The 5-element Maxwell model was able to better predict the stress relaxation behavior of maize kernel than the 3-element Maxwell model. The magnitude of the strain of the maize kernels increased with increased moisture content in the creep tests. The magnitude of the stress of the maize kernels decreased with increased moisture content in the stress relaxation tests. The effects of moisture content on the morphology of cut section of maize kernels and X-ray powder diffraction of maize kernels were analyzed. The results indicated that the starch granules of maize kernels expanded with increased moisture content. And the maize kernel structure exhibited A-type crystalline pattern.The effects of hot air drying and hot air drying combined vacuum drying on the time-temperature superposition properties, mechanical properties and morphology of cut section of maize kernels were analyzed. The results indicated that the R2 of all treatments were over 0.93 except the treatment of hot air drying(50℃) combined vacuum drying(45℃) whose R2 was 0.870 in the tests of time-temperature superposition of creep. And the longest time of master curves of creep was 1.81×107s. The R2 of all treatments were over 0.92 in the tests of time-temperature superposition of stress relaxation. And the longest time of master curves of stress relaxation was 1.62×107s. The R2 of all treatments were over 0.95 in the tests of frequency-temperature superposition. And the highest frequency of master curves was 1.51×105Hz. By using time-temperature superposition principle, the mechanical properties of maize kernels during much longer time and much higher frequency could be studied. The results could help studying the long-time storage of maize kernels. The magnitude of the storage modulus and loss modulus increased with increased frequency in the mechanical tests. For all treatments, the magnitude of the storage modulus was higher than the magnitude of the loss modulus. And the dried maize kernels exhibited elastically. Compared to hot air drying, the starch granules of maize kernels dried by vacuum drying were plumper. And the qualities were better.