Diffusion Coefficient of Liquid Foods in the Drying Process

点击翻译按钮获取中文摘要

In the analysis of the spray drying process of liquid foods, the knowledge of the water diffusion coefficient D and the water activity <a_w> is required. The purpose of this study is to establish a method for predicting the drying behavior of liquid foods on the basis of the D and <a_w> data.The diffusion coefficient of water D in many liquid foods decreases sharply with decreasing water concentration by a factor of several orders of magnitude. We first obtained the numerical solution of the diffusion equation in the shrinking coordinate by using various types of concentration dependence of D. Then, the relation among the integral averaged diffusion coefficient, the drying rate and the ratio of average concentration to the center concentration of the drying specimen was established, by which D can be obtained from the measured drying(desorption) rate, as the function of water concentration.We first applied this method to the isothermal drying experiment with sugars of various molecular weights(glucose, sucrose, oligosaccharose and maltodextrins). The <a_w> data were also measured for these sugars. The diffusion coefficient at infinite dilution <D_0> can be correlated to the molecular weight using the Wilke's diffusion factor. The concentration dependence of D of sugars, D/ <D_0> , can be presented by a single equation regardless of the molecular weight. Similarly, the concentration dependence of the activation energy of D for these sugars was not markedly varied and can be expressed by a single equation.The drying behavior of proteins(bovine serum albumin), liquid foods(skimmilk) and model foods (composed of lactose, casein, albumin and sodium chloride) were examined by using the drying experiment of single droplets. It was found that the drying of these samples was accelerated by the non-uniform shrinkage of the drop and can be predicted well by the numerically calculated drying behavior with the D and <a_w> data of sucrose.