The Moisture Endpoint That Is Actually Safe: Water-Activity Targets for Dried Fruit, Fish, and Meat, Computed from Sorption Isotherms

Safety guidance for dried foods is usually stated in water activity, a quantity that a household or small trader drying food in the sun cannot measure. This paper converts three stated water-activity limits into moisture contents that can be weighed, using the Guggenheim-Anderson-de Boer (GAB) sorption model with stated parameters for each food class. At a water activity of 0.60, taken as a level at which mould growth is of no practical concern, the corresponding moisture contents are 14.9 percent dry basis (13.0 percent wet basis) for dried fruit, 11.8 percent dry basis (10.5 percent wet basis) for dried fish, and 13.8 percent dry basis (12.1 percent wet basis) for dried meat. At a water activity of 0.85 the values are 27.8, 24.6 and 27.0 percent dry basis respectively, and at 0.91 they are 34.3, 32.3 and 34.4 percent dry basis. A stated thin-layer drying model then gives the time to reach the 0.60 endpoint in three climates: about 3.1 days for fruit, 3.3 days for fish and 3.0 days for meat in hot-arid conditions, and about 8.2, 8.6 and 8.0 days in temperate conditions. In hot-humid conditions at 75 percent relative humidity, open sun drying cannot reach the 0.60 endpoint for any of the three foods, because the equilibrium moisture at that humidity (20.9, 17.4 and 19.7 percent dry basis) lies above the target moisture in every case. The endpoint table in moisture percent is the usable output: a dryer who owns scales but no water-activity meter can weigh a sample, dry it fully, and stop the main batch when the computed weight is reached. SIMULATED, NOT MEASURED.