Moisture sorption isotherm and shelf life prediction of complementary food based on amaranth-sorghum grains

The quantity and quality of complementary foods is one of the contributing factors to childhood under nutrition. Complementary food based on amaranth and sorghum grains was formulated. It was deemed to be superior nutritionally to the commonly used complementary foods in Kenya. Knowledge on storage and packaging is important for a long shelf life of the product to be achieved. Water activity and moisture content are key indicators of the stability of dried foods. To establish the proper storage conditions of the product, moisture sorption isotherms at 25 °C, 30 °C and 35 °C were determined over water activity range of 0.11-0.97 and data fitted with six sorption models. Water vapour permeability of common packaging materials was also determined at 25 °C and 35 °C and the data was used to predict the product's shelf life. The product exhibited type two sorption isotherm according to Brunner classification an indication of monolayer-multilayer sorption behavior. The critical storage moisture content of the complementary food corresponding to 0.6 water activity a point of microbiological stability was found to be 7.5%. Based on Heiss-Eichner model, a shelf life of 1423 days at 25 °C can be achieved with product initial moisture content of 5 g/100 g and packaged in 110 µm thick aluminum pouch. The shelf life of the product was shortened two to three fold at 35 °C in comparison to storage at 25 °C.