Keeping Dried Food Dry in a Wet House: Moisture Uptake Through Stated Containers, and the Barrier a Ninety-Day Store Requires

Dried food stored in a humid house takes up water through its container until its water activity reaches that of the room, and mould growth becomes possible once the water activity passes about 0.60. This paper computes how long that protection lasts for a stated food in stated containers. The food is a starchy dried food packed at a water activity of 0.40, with moisture content given by a stated GAB isotherm (M0 = 0.07 g per g, C = 12.0, K = 0.85): its moisture is 0.0913 g per g of dry solids at packing and 0.1323 g per g at the safe limit of 0.60, so a 2.0 kg pack, containing 1832.7 g of dry solids, can absorb 75.1 g of water before it reaches the limit. Its equilibrium moisture is 0.1844 g per g in a room at 75% relative humidity and 0.2904 g per g at 90%, both well past the limit, so every container considered here fails eventually; only the time differs. With ingress proportional to the humidity difference across a wall of 0.24 m², a thin polyethylene bag (transmission rate 18.0 g per m² per day under the stated reference gradient) reaches the limit in 68.5 days at 75% relative humidity and 40.8 days at 90%. A thick polyethylene bag (4.5 g per m² per day) lasts 274.0 days and 163.1 days. An unlined woven sack (400.0 g per m² per day) lasts 3.1 days and 1.8 days. A glass jar, whose wall passes nothing and whose stated seal leakage is 0.15 g per day under the reference gradient, lasts 1972.9 days and 1174.0 days. For a store of 90 days the wall transmission rate must not exceed 13.70 g per m² per day for the 75% room or 8.16 g per m² per day for the 90% room, the lower figure governing. A buyer can test a filled pack against that specification by weighing: a 2.0 kg pack that gains more than 75.1 g in 90 days, an average of 0.834 g per day, has passed the limit whatever its material is claimed to be.