The Salt Crystal Liquefaction Test - A Simple Method for Testing the Water Activity of Foods

A simple method for testing the water activity (a_w) of foods has been developed. The test can be used to check the a_w of products which must comply with required a_w standards. The test is based on the property of salt crystals to attract water vapor and to liquefy when they are placed in a jar containing a product with an a_w above the specific a_w of the salt. By using different salts with appropriate specific a_w values, the a_w of products at various a_w levels can be examined. The specific a_w values of salts which can be used for the test are: 0.68, 0.76, 0.79, 0.81, 0.86, 0.87, 0.91, 0.94 and 0.98. The sensitivity of the test is less than 0.02 a_w with a reading time of 3-24 h, depending on the type of salt, type of product and temperature. The test using salt crystals with a high specific a_w is sensitive to a change in temperature and must be performed in a thermostat-controlled cabinet. The test using CuCl₂•2H₂O crystals (0.68 a_w) was evaluated under extreme environmental conditions and it appeared to be most promising for use in field conditions.