Diffusion Coefficient of Liquid Foods in the Drying Process

在分析液态食品喷雾干燥过程中，需要了解水扩散系数D和水活度<a_w>。本研究的目的是建立一种基于D和<a_w>数据预测液态食品干燥行为的方法。在许多液态食品中，水D的扩散系数随着水浓度的降低而急剧下降，下降幅度达几个数量级。首先利用D的各种类型的浓度依赖性得到收缩坐标下扩散方程的数值解，然后建立了积分平均扩散系数与干燥速率以及干燥试样的平均浓度与中心浓度之比之间的关系，由此可以由测量的干燥（解吸）速率得到D作为水浓度的函数。我们首先将该方法应用于不同分子量糖（葡萄糖、蔗糖、低聚糖和麦芽糊精）的等温干燥实验。还测量了这些糖的<a_w>数据。无限稀释<D_0>时的扩散系数可以用Wilke扩散系数与分子量相关联。糖的浓度依赖性D/ <D_0>，可以用一个方程表示，与分子量无关。同样，D对这些糖的活化能的浓度依赖性也没有明显变化，可以用一个方程表示。采用单液滴干燥实验，研究了蛋白质（牛血清白蛋白）、液态食品（脱脂乳）和模型食品（乳糖、酪蛋白、白蛋白和氯化钠组成）的干燥行为。结果表明，液滴的不均匀收缩加速了样品的干燥过程，用蔗糖的D和<a_w>数据计算的干燥行为可以很好地预测样品的干燥过程。

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.