## Shelf Life Moisture

### Quick Start

This is a specialist app about how the moisture content in food changes with time. It does some great industry-standard calculations if you need them. Otherwise it's an interesting lesson in how these sorts of things are done.

The shelf life of a food can depend on how long it takes to reach a critical moisture content mc starting from an initial moisture content mi and with the capability of reaching an equilibrium moisture content me. The analysis here follows the methodolgy and terminology of Prof Cooksey.

The moisture absorption follows an isotherm (dependency of %moisture on the vapour pressure of water) with a slope b. The calculation requires the saturated vapour pressure of water at the given temperature - this is calculated in the app from your input T via a standard formula.

The food is stored at a temperature where the vapour pressure of pure water is p0 which can readily be found from the literature, but is calculated for you by entering storage temperature T°C.

Finally we have the weight of the food, W, the area and thickness of the packaging, A and L, and the Permeability, P, of the packaging material. From all this we can calculate the time, t, to reach the critical moisture content via this formula. The formula is a mix of ratios - of how far the moisture is from initial and critical concentrations, Permeability/Thickness, Area/Weight, vapour pressure/slope.

Instead of entering Permeability and Thickness we enter the WVTR (Water Vapour Transmission Rate) for this sample of packaging (i.e. P/L), in units of g/(day.m².mmHg). To suit different needs, the time is calculated in terms of hours, days and (30-day) months.

 me mc mi WVTR Area m² Dry Weight g b gw/gs T°C p0 mmHg t hr t day t month