Studies On Optimization Of Drying Technology, Mathematical Modeling And Quality Changes Of Traditional Cooked Sweet Potatoes During Hot-air Drying

There are rich resources of sweet potatoes in China. After dehydration, it can be processed to dried sweet potato which is not only nutritious but edible convenient. During the traditional production process, because drying temperature and air velocity are hard to control, the traditional dried sweet potato was hard in the surface structure and dusky brown in colour, which, therefore, seriously affect its quality and popularity. In this paper, we examined the hot air drying characteristics of sweet potato slices, established the best drying mathematical model and select the best process conditions by the response surface optimization, which can provide a theoretical basis for the industrial production of dried sweet potato. Meanwhile, it was beneficial to reduce energy consumption and improve product quality. Firstly, the hot air drying characteristics of sweet potato slices were examined under different temperatures (60,70and80℃), air velocities (0.8,1.2and1.6m/s) and slice thickness (0.6,0.9and1.2cm), respectively. The drying data were fitted to12semi-theoretical models, such as Page, Newton, modified Page, Henderson and Pabis, two-term, Logarithmic, Wang and Singh, Thomson, diffusion approach, modified Henderson and Pabis, Werma, et al. and two term exponential models. As a result, the Wang and Singh model gave better predictions than other models with the decision coefficient (R2) of0.998, chi-square (χ2) of0.000168and root mean square error(RMSE) of0.0117. The effective moisture diffusivity and activation energy were also computed, and drying activation energy of this experiment was40.103kJ/mol.The hot air drying characteristics and effective moisture diffusivities Deff of sweet potato slices were examined after infused in different concentration of sugar solution. The experimental data were fitted to9mathematical models, respectively. And the Page model had the best perfonnance. Sugar content c had a linear relationship with parameter k and had nothing to do with the parameter n, and the value of R2and PMSE was0.982and0.043. The pre-exponential factor D0and activation energy Ea were also calculated by the Arrhenius relationship, and both A (6.66×108m2/s) and Ea (12.423kJ/mol) value of sugar infused sweet potato slices were larger than D0(9.69×104m2/s) and/:a(40.103kJ·mol-1) value of slices which didn't have the sugar infused pretreatment.Based on the drying kinetic model research of cooked sweet potato, single factor experiments were conducted using drying rate, hardness, springiness and total color difference as evaluation indexes. Through the BBD response surface optimization, the best processing conditions of dried sweet potato were determined:drying temperature67℃,air velocity1.15m/s and slice thickness1.26cm.The moisture change of sweet potato slices during hot air drying was researched. As a result, with the drying time prolonged, both moisture content of drying basis and water activity declined, and water reabsorbing capacity showed a trend of rise first then unchanged. During drying, free water inside the sweet potato slices was removed firstly, and then tectnotic water. The bound water did not change significantly.The basic nutritional composition of dried and fresh sweet potato was determined and compared. The results showed that the starch and vitamin C content of dried sweet potato were relatively lower, and the reducing sugar content was higher than those of fresh samples. The content of crude fiber and crude protein did not change significantly due to its highly stable chemical properties.The trend of sweet potato texture during hot air drying were researched. As a result, with the time passed by, the hardness, adhesiveness and chewiness of the sample increased, and cohesiveness showed a trend of rise first then decline. The relationship between hardness and sugar content of sweet potato was also studied. The results indicated that the increased sugar content may be one of the most important reasons for the increasing of hardness during drying.In order to control and improve the color quality of sweet potato slices during hot air drying, the effects of drying temperature and air velocity on dried sweet potato were studied. The experimental data were fitted to two kinetic models (zero and first) and two polynomials models (quadratic and cubic). And, the best fits were obtained with the cubic model which could predict the color change of cooked sweet potato during hot air drying.