VEGETABLE AND FRUIT DEHYDRATION 131 



Single Sheets, Double Laminations, and Compound Laminations 



A single-sheet substitute material to take the place of tin-plate or 

 other hermetically sealable containers has not been discovered. For 

 example, a close-textured paper heavily waxed on both sides satisfies 

 all of the requirements with the exception that rough treatment seri- 

 ously impairs or destroys its efficiency. Laminations of two or more 

 sheets of similar or different characteristics will frequently offset the 

 weaknesses of single sheets. 



Such laminations as glassine paper to cellophane, cellophane to 

 parchment paper, cellophane to cellophane, and other combinations 

 show increased resistance to the passage of moisture vapor as compared 

 with single sheets. With some laminations the increase in resistance 

 to moisture is due to the thermoplastic adhesive used to cement the 

 materials together. If the membranes are subject to "pinholing" or 

 other imperfections during their manufacture, the adhesive seals the 

 imperfections during the process of lamination. 



Far superior to double laminations is the recently developed com- 

 pound lamination that includes a metal foil, which is an extremely 

 moisture-vapor resistant material. Kraft paper is laminated to the 

 foil with asphalt on one side and cellophane with thermoplastic adhe- 

 sive on the other side (fig. 64). This compound structure protects 



ADHESIVE 

 CELLOPHANE 



w 



PAPER 



[asphalt 

 [lead FOIL 

 ^ADHESIVE 



fCELLOPHANE 



■yl-W-fr L ACQUER 



Figure 64. — Construction of laminated packaging materials. 



the lead-foil sheet against pinholing or rupture due to flexing and other 

 stresses. The finished laminated sheet has strength, waterproofness, 

 resistance to moisture vapor, lightproofness, and exceptional heat-seal- 

 ing properties. Its heat-sealing properties are due in part to the 

 lacquer coating on the surface of the cellophane, and the seal strength 

 is further enhanced by the thermoplastic coating applied to the inner 

 cellophane surface. Its unusual moisture- vapor resistance is due to the 

 multiple barrier that it presents. In order to pass through this struc- 

 ture, moisture vapor must penetrate kraft paper, asphalt, lead foil, 

 thermoplastic adhesive, and moisture-vaporproof cellophane. 



In use, this material is first fashioned into flat envelope-type bags. 

 Before these bags are filled they are shaped by the use of a mandrel. 

 After they are lined and filled, a partial vacuum is created in the con- 

 tainer by means of suction and the bags are quickly and securely heat 

 sealed. The sealed package is placed in an outer shipping container. 

 This package has been found to approach closely the standards set 

 by the hermetically sealed tin-plate container and, when the closure 

 has been carefully completed, it complies with virtually all of the re- 

 quirements necessary to protect dehydrated foods against deterioration 

 caused by absorbed atmospheric moisture. 



The use of the less efficient packaging materials is always fraught 

 with danger. However, certain of the less hygroscopic products can 

 be packaged in less efficient materials, especially if they are to be held 

 only a short period. With the less hygroscopic foods, such as certain 

 of the dried fruits, and especially those with moisture contents 10 or 



