178 Novel food packaging techniques Molecular sieve type 13X Moisture vapour adsorption at 25C Time(hours) 上10%-20%-30%—4% Fig 9.5 Adsorption isotherms of common types of molecular sieve 9.5 Humectant salts A common and economical method of controlling humidity in moist environments is the use of humectant salts Such salts will adsorb moisture until they go completely into solution. As this occurs, what will be seen is a mixture of solid salt and a salt solution. This solution will be saturated and will have an equilibrium relative humidity characteristic of the particular salt used 9.5.1 Sodium chloride As an example, a saturated solution of common table salt, sodium chloride, will have an ERH of -75%. A corollary of this is that sodium chloride will not adsorb moisture if the ambient humidity is less than 75% 9.5.2 Magnesium chloride Similarly, a saturated MgCl2 solution has an ERH of about 35%. Numerous tables can be found in handbooks, that list the erh of many salts and common ambient temperatures. Wexler defined a relationship between ERH and temperature; constants for Wexler's equation for some commonly used humectant salts have been published. Therefore it is usually possible to select a salt that will control humidity in a range suitable for the system or packaged product being protected9.5 Humectant salts A common and economical method of controlling humidity in moist environments is the use of humectant salts. Such salts will adsorb moisture until they go completely into solution. As this occurs, what will be seen is a mixture of solid salt and a salt solution. This solution will be saturated and will have an equilibrium relative humidity characteristic of the particular salt used. 9.5.1 Sodium chloride As an example, a saturated solution of common table salt, sodium chloride, will have an ERH of ~75%. A corollary of this is that sodium chloride will not adsorb moisture if the ambient humidity is less than 75%. 9.5.2 Magnesium chloride Similarly, a saturated MgCl2 solution has an ERH of about 35%. Numerous tables can be found in handbooks, that list the ERH of many salts and common ambient temperatures. Wexler defined a relationship between ERH and temperature; constants for Wexler’s equation for some commonly used humectant salts have been published. Therefore it is usually possible to select a salt that will control humidity in a range suitable for the system or packaged product being protected. Fig. 9.5 Adsorption isotherms of common types of molecular sieve. 178 Novel food packaging techniques