干切牛肉冷冻干燥中高速率升华条件的动态研究

    Dynamic study on operation conditions for the higher sublimation rate during freezing drying of cooked beef slice

    • 摘要: 该文研究了干切牛肉冷冻干燥中使升华干燥速率最大化所需要的操作条件。通过建立升华过程中冻结物料温度在Ti时冰晶以最大速率升华所允许的升华层厚度δis、升华时间tis及升华所需物料表面温度Tis的计算模型,计算了预冻终温–30、–29、–28、–26、–24、–22℃的冻结物料以最大速率升华所允许的物料厚度分别为18、15、12、8、6 mm时,升华过程中物料含水率、升华所需物料表面温度Tis;进而预测了干切牛肉冷冻干燥升华过程中制品含水率、物料中心温度随时间发生的动态变化及升华所需物料表面温度的动态值。设定干燥室压强为10 Pa,以物料表面温度预测值控制加热搁板温度开展验证实验,结果表明:物料厚度分别为15、12、8、6 mm的干切牛肉在升华干燥过程中预测含水率与实测含水率相对误差±10%,物料中心温度计算值与实测值的绝对误差±5℃,说明所建立的物料表面温度预测模型可用于6、8、12、15 mm干切牛肉冷冻升华干燥中搁板加热温度的优化控制,比较不同厚度干切牛肉冷冻升华干燥实验的平均升华速率、脱除水分耗能,6 mm厚物料在升华干燥中升华速率最大、能耗最低。

       

      Abstract: The operation conditions for the higher sublimation rate during freeze drying of cooked beef slice were studied. Mathematical models were established for sublimation layer thickness(δis), sublimation time(tis)and sample surface temperature (Tis) suited for the higher sublimation rate when the temperature of frozen sample is Ti during the sublimation. When the terminal freeing temperatures were –30, –29, –28, –26 and –24℃, respectively, The suitable sample thickness were 18, 15, 12, 8 and 6mm, respectively under the higher sublimation rate and the moisture rate and surface temperature of the sample were calculated. Moreover, the moisture ratio, center and surface temperature changing with time were forecast. The validation experiments were conducted, in which chamber pressure was 10 Pa, and shelf temperatures were controlled according to predictive temperature of sample surface. The results show that the comparative errors between forecast and measured moisture ratio are lower than 10%, and absolute errors between forecast and measured centre temperature of drying sample are lower than 5℃. It can be concluded that the predictive model of surface temperature can be applied in shelf temperature control during sublimation drying of frozen cooked beef slice with the thickness of 6~15 mm. Comparing average sublimation rate and energy consumption for dehydrating water from different samples with the thickness are 6, 8,12 and 15 mm, the highest sublimation rate and the lowest energy consumption are obtained when sample thickness is 6 mm.

       

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