扩散法测定水分活度.doc

扩散法测定样品水分活度1理想公式计算法根据水分活性（以下简称Aw）的定义，它可近似等于食品在密封容器内的水蒸汽压（P）与在相同温度下的纯水蒸汽压（Po）之比：一般说来，实际上测定食品水分活性都采用直接测定法。2扩散法（直接测定法）根据蒸汽压、湿度动力学等原理相应出现了不少直接测定仪器。国外也发展了许多测定水分活性的电子仪器，其测定原理有的是根据二电极中吸湿性物质的电导变化，也有的是直接依靠气体热传导的湿度传感器来检测。这类仪器具有快速、灵敏、精确度高的优点，我国可加强这类仪器的研制。在目前情况下，这种电子仪器的造价高，有些尚需进口，不利于推广。下面介绍一种坐标内插法，它不需要特殊的仪器装置。一般实验室都可采用。2.1仪器及用具康维皿容器，分析天平，恒温箱。2.2试剂表4-1标准饱和盐溶液的Aw值表（25）标准试剂Aw标准试剂AwLiClH2OK2C2H2O2MgCl2 6H2OKCO3Mg(NO3)26H2O0.110.230.330.430.52NaBr2H2ONaClCdCl2KNO3K2C2O30.580.750.820.930.98注：不同温度下标准饱和盐溶液的Aw值见附录2.3测定方法主要测定容器是康维皿容器，它分内外二室，测定时在外室加入标准盐饱和溶液，在内室的铝箔皿中加入1克左右的待测试样。试样应用天平精确称量，记下初读数。固体食品试样最好切细后放入。然后用玻璃盖涂上真空脂密封，放入恒温箱在25条件下保持23小时，然后取出滤波皿再次精确称出试样的重量，算出试样的增减量。如试样重量增加，说明内室的试样水分活性比外室的盐饱和溶液水分活性低，因此在密封容器内试样由于吸附水分而增重；反之，如试样的水分活性比盐饱和溶液水分活性高，则试样重量减少。2.4计算根据试样与二种以上标准饱和盐溶液平衡后试样重量的增减作坐标图（见图4-1），纵坐标为试样重量增减的毫克数，横坐标为水分活性值。如图4-1的A点是试样与标准MgCl26H2O饱和溶液平衡后重量减少20.0毫克，试样与标准Mg(NO3)26H2O平衡后失重5.2毫克，相应作出B点，与NaCl饱和溶液平衡后试样增加11.1毫克作出C点，把三点连成一线与横坐标交于D点，得出试样的水分活性为0.60。图4-1 坐标法测定水分活性2.5注意事项（1） 注意称重试样的精确度，否则会造成测定误差。对试样的Aw值范围预先最好有个估计，以便正确选用标准盐饱和溶液。（2） 若试样中含有酒精一类水溶性挥发物质时难以正确测定Aw值。（3） 如有米饭类、油脂类食品在25下放置23小时测不出Aw值，可继续放置14天，先测定2小时后的试样重量，然后间隔一定时间称重，再作坐标求出。把首次与横坐标的相交点作为测定值。为防止试样腐烂，可以加入0.2%的山梨酸钾作为防腐剂。附录：不同温度下饱和盐溶液相对湿度对照表（标准饱和盐溶液的Aw值表）对照表#1#氟化铯2#溴化锂4#氯化锂7#乙酸钾8#氟化锂9#氯化镁11#碳酸钾12#硝酸镁07.750.8311.230.5433.660.3343.130.6655.521.97.430.7611.260.4733.600.2843.130.5060.350.55104.891.67.140.6911.290.4123.380.5333.470.2443.140.3958.860.43154.331.46.860.6311.300.3523.400.3233.300.2143.150.3357.360.33203.831.16.610.5811.310.3123.110.2533.070.1843.160.3355.870.27253.390.946.370.5211.300.2722.510.3230.851.332.780.1643.160.3954.380.23303.010.776.160.4711.280.2421.610.5227.271.132.440.1443.170.5052.890.22352.690.635.970.4311.250.2224.590.6432.050.1351.400.24402.440.525.800.3911.210.2122.680.8131.600.1349.910.29452.240.445.650.3511.160.2121.460.7031.100.1348.420.37502.110.405.530.3111.100.2220.800.6230.540.1446.930.47552.040.385.420.2811.030.2320.600.5629.930.1645.440.60602.030.405.330.2510.950.2620.770.5329.260.18652.080.445.270.2310.860.2921.180.5328.540.21702.200.525.230.2110.750.3321.740.6127.770.25#13#溴化钠14#碘化钾16#硝酸钠17#氯化钠19#溴化钾21#氯化钾23#硝酸钾24#硝酸镁075.510.3488.610.5396.332.9563.510.7073.300.3478.570.5275.650.2785.090.2687.670.4596.272.11062.150.6072.110.3177.530.4575.670.2283.750.2486.770.3995.961.41560.680.5170.980.2876.460.3975.610.1882.620.2285.920.3395.410.962059.140.4469.900.2675.360.3575.470.1481.670.2185.110.2994.620.662557.570.4068.860.2474.250.3275.290.1380.890.2184.340.2693.580.5597.880. 493056.030.3867.890.2373.140.3175.090.1180.270.2183.620.2592.310.6097.080.413554.550.3866.960.2372.060.3274.870.1279.780.2282.950.2590.790.8396.420.374053.170.3766.090.2371.000.3474.680.1379.430.2482.320.2589.031.295.890.374551.950.3665.260.2469.990.3774.520.1679.180.2581.740.2887.031.895.50.405050.930.3564.490.2669.040.4274.430.1979.020.2881.200.3184.782.595.250.485550.150.2863.780.2868.150.4974.410.2478.950.3280.700.356049.660.2663.110.3167.350.