130 MISC. PUBLICATION 540, U. S. DEPT. OF AGRICULTURE 



proaches or meets the standards set by tin-plate containers. These 

 standards are (1) hermetic sealing, (2) durability, and (3) prevention 

 of transmission of light. Glass containers can be sealed hermetically 

 but fail to meet other requirements. Flexible materials, such as heavy 

 treated papers, moistureproof cellophane, simple laminations of paper 

 and cellophane, and compound laminations of paper, cellophane, and 

 metallic foils, have certain advantages and disadvantages as packaging 

 materials and, if handled appropriately, can be used satisfactorily. 



Functions of the Package 



Since all unprotected dehydrated foods absorb moisture from the 

 atmosphere, it is essential that the package be moistureproof. The 

 speed of absorption of dried vegetables varies considerably, because 

 of differences in composition of products and also differences in the 

 extent to which the food has been dehydrated. For example, cabbage 

 dehydrated to 5 percent moisture content is more hygroscopic than 

 potatoes containing the same amount. Also cabbage that contains 

 5 percent of moisture will absorb moisture more rapidly than cabbage 

 that contains 10 percent. Excessive absorption of moisture by dehy- 

 drated foods causes chemical changes that quickly impair appearance, 

 palatability, and nutritive values. A suitable high standard for long 

 storage, one that is commonly used, is this : Sufficient moisture-vapor 

 resistance to prevent a maximum absorption of 2 percent of moisture 

 during a storage period of 1 year. 



In a consideration of substitutes for the metal or glass hermetically 

 sealed container, a number of factors that affect moisture-vapor per- 

 meability must be evaluated. The principal factors are as follows : 



The nature and type of the protective materials. — Some materials show a 

 greater moisture-vapor resistance than others. Laminated or compound-lami- 

 nated sheets are more moisture-resistant than single sheets. 



The ratio between the area of the package and the weight of the food contained 

 in it. — The greater the package surface exposed per given weight of hygroscopic 

 food material, the greater the absorption ; that is, well-filled packages of hygro- 

 scopic food materials permit the absorption of less moisture per given weight of 

 contents than partially filled packages. 



Length of time the package will be exposed to humid atmospheres. — The amount 

 of moisture absorbed through a material is in direct proportion to the time of 

 exposure. 



Leakage of moisture through package wall due to defects. — Thin spots or holes 

 in coatings on materials permit the passage of moisture vapor. 



Leakage of moisture vapor through breaks in the protective films caused by 

 rough treatment. — Some moistureproof films are inelastic and their efficiency is 

 impaired by abuse, which causes breaks in the continuity of the film. 



Leakage due to poor sealing properties or careless sealing. — Some thermo- 

 plastic coatings do not make a firm bond after heat sealing, and as a result the 

 seams open. Improper sealing temperatures or technique may result in faulty 

 seals. 



Leakage due to destruction of the coating during heat sealing. — Heat sealing 

 at excessively high temperatures may impair the adhesive properties of thermo- 

 plastic coatings or cause sufficient decomposition to affect moisture-vapor 

 resistance. 



The moisture-vapor differential between the inside of the package and the 

 storage atmosphere. — Since dehydrated foods are hygroscopic, the relative 

 humidity within the package is usually low. Packages are stored in atmospheres 

 of high relative humidity. The attraction for moisture within the package is 

 quite marked and the amount of moisture absorbed is directly proportional to 

 the difference between the internal and external relative humidities. 



Rate of circulation of air in the storage space. — The circulation of humid air 

 across the surfaces of packages increases the rate of moisture absorption through 

 the packaging material. 



