Strength of a package is related to the strength of the packaging material and also the design of the package.  In many situations a weaker material will produce acceptable strength in a package if the design incorporates more material in crucial weight bearing areas of the package.  As you mention, humidity, product weight and time in the refrigerated handling chain are also factors affecting the strength of corrugated fiberboard.  Because of all these interrelated factors it is not possible to specify a specific minimum material strength for produce packages.  Packaging manufacturers often have experience with the performance of their packages in the cold chain and can develop an acceptable design for most situations.  The best way to determine if a package design is adequate is to test it in a packaging laboratory. There are a number of accepted standard protocols for this type of testing.  --Jim Thompson