Li J, Chinachoti P, Wang D, Hallberg L M, Sun X S
Department of Grain Science and Industry, Kansas State University, Manhattan, KS 66506, USA.
J Food Sci. 2008 Nov;73(9):E425-30. doi: 10.1111/j.1750-3841.2008.00938.x.
Beef roast with vegetables is an example of a meal, ready-to-eat (MRE) ration entrée. It is a mixture of meat, potato, mushroom, and carrot with a gravy sauce. The thermal properties of each component were characterized in terms of freezing point, latent heat, freezable and unfreezable water contents, and enthalpy during freezing using differential scanning calorimetry. Freezing and thawing curves and the effect of freezing and thawing cycles on thermal properties were also evaluated. The freezing points of beef, potato, mushroom, and sauce were all in the range of -5.1 to -5.6 degrees C, but moisture content, water activity, latent heat, freezable and unfreezable water contents, and enthalpy varied among these components. Freezing temperature greatly affected the unfrozen water fraction. The unfreezable water content (unfrozen water fraction at -50 degrees C) of ration components was in the range of 8.2% to 9.7%. The freezing and thawing curves of vegetables with sauce differed from those of beef but took similar time to freeze or thaw. Freezing and thawing cycles did not greatly affect the thermal properties of each component. Freezing point and latent heat were reduced by decreasing moisture content and water activity of each component. Water activity was proportionally linear to freezing point at a(w) > 0.88, and moisture content was proportionally linear to freezable water content in all ration components. Water was not available for freezing when moisture content was reduced to 28.8% or less. This study indicates that moisture content and water activity are critical factors affecting thermal behavior of ration components during freezing.
蔬菜烤牛肉是即食餐（MRE）口粮主菜的一个例子。它是肉、土豆、蘑菇和胡萝卜与肉汁酱的混合物。使用差示扫描量热法，根据冰点、潜热、可冻结和不可冻结水含量以及冷冻过程中的焓来表征每种成分的热性能。还评估了冷冻和解冻曲线以及冷冻和解冻循环对热性能的影响。牛肉、土豆、蘑菇和酱汁的冰点均在-5.1至-5.6摄氏度范围内，但这些成分在水分含量、水分活度、潜热、可冻结和不可冻结水含量以及焓方面存在差异。冷冻温度对未冻结水分数有很大影响。口粮成分的不可冻结水含量（-50摄氏度时的未冻结水分数）在8.2%至9.7%范围内。带酱汁蔬菜的冷冻和解冻曲线与牛肉的不同，但冷冻或解冻所需时间相似。冷冻和解冻循环对每种成分的热性能影响不大。通过降低每种成分的水分含量和水分活度，冰点和潜热会降低。在水分活度>0.88时，水分活度与冰点呈线性比例关系，在所有口粮成分中，水分含量与可冻结水含量呈线性比例关系。当水分含量降至28.8%或更低时，水无法用于冷冻。本研究表明，水分含量和水分活度是影响口粮成分冷冻过程中热行为的关键因素。
