What method is used to determine water activity?

The water activity is the inherent property of the sample, and the relative humidity of the environmental balance is the atmospheric property of the sample, which is only equal in value At the same time, it takes quite a long time for a small number of samples (less than 1G) to reach the balance with the environment, while it is almost impossible for a large number of samples to reach the balance with the environment when the temperature is lower than 50 ° C The relationship between water activity and water content of samples is very important, so it is often necessary to determine the aw of food under certain conditions, which can be done by measuring the bear pressure of food or the relative humidity of environmental equilibrium under such conditions The specific methods are as follows: 1) determination of freezing point: first, determine the freezing point reduction and moisture content of the sample, and then calculate aw according to the equation P / P = Erh / 100 = n = n / (n + n) The error caused by measuring the reaching point at low temperature and calculating the aw value at high temperature is very small (< 0.001aw / ° C) (2) Relative humidity sensor measurement method: at a constant temperature, put the sample with known moisture content in a small airtight chamber to reach the balance, and then use any electronic technology or humidity technology to measure the Erh Of the sample and the atmospheric balance of the environment to obtain aw (3) Constant relative humidity balance chamber method (diffusion method): under the condition of sealing and constant temperature of Conway micro diffusion dish, the diffusion balance of the sample is carried out in the standard saturated solution with higher and lower aw values respectively The aw value of the sample is calculated according to the increase of sample mass (in the standard solution with higher aw values) and the decrease of sample mass (in the standard solution with lower aw values) The diffusion method is very complex in measuring water activity, and the steps are tedious and time-consuming It can not directly get the measurement results (weighing, drawing and solving are needed), and it is not commonly used now (4) Water activity only measured aw of the sample The reported advanced instrument temperature control has reached 0.2 ° C Zui height has reached 0.0001aw Zui short measurement time is only 5min The water activity index is much better than the water content index in judging food spoilage, but it is still not, because some other factors, such as oxygen concentration, pH value, water mobility and solute types, also have a strong impact on the rate of food spoilage in some cases.