This month's question is in regards to suitable methods for packaging--in particular, using plastic.
One of the simplest ways to estimate the shelf life of your product is to calculate the vapor pressure of water under both your test conditions and the ambient conditions where it will be stored. For example, you find that your product is stable after one week at 85 degrees C and 100 percent relative humidity. The vapor pressure of water under these conditions is 433.6 mm (from tables in any good materials handbook). You can calculate the vapor pressure of water under ambient conditions by first checking weather tables for any particular location. For example, if average conditions at a location are 20 degrees C and 50 percent relative humidity (RH), the average vapor pressure of water under these conditions is 17.535 mm (from tables) x 0.5 (50 percent RH) = 8.76. If you divide the vapor pressure under your test conditions by the value at the location, then this should allow you to estimate the shelf life. In our example here: Shelf life = 433.6/8.76 = 49.5 weeks.