Functional properties of white maize meal stored under tropical conditions

Maize meal, particularly white maize meal, the flour from the ground maize kernel, is the dominant staple food in many parts of Africa. A wide variety of maize meal types, particularly in Eastern and Southern Africa are produced by dry milling-degerming system. Maize meal is stored at various stages of commerce right from production, distribution and as it is being utilized and consumed. The unfavorable climatic conditions of Africa do pose a challenge to the storage and distribution of this important staple food. Studies on other cereal and cereal products have shown that functional properties can be affected during storage. However, data on the functional properties of white maize meal during storage is scarce, particularly for white maize meals in Africa. Therefore, the main objective of this study was to determine the influence of storage conditions on functional properties of white maize meal with a particular emphasis to tropical storage conditions. The experimental work was divided into three parts: (i) moisture sorption properties, (ii) chemical and sensory properties and (iii) pasting and rheological properties. The moisture adsorption isotherms before and after defatting white maize meal were characterized at 25 30 and 45oC. The moisture adsorption isotherms were modeled and best fitted by the GAB model. Thermodynamic properties for moisture adsorption showed that the differential enthalpy and differential entropy increased with decreasing moisture content. The isokinetic theory showed that the moisture adsorption process in white maize meal was enthalpy driven. The white maize meal browned during storage, rapidly at high storage temperatures and gradually at low storage temperature-high humidity conditions. This was further confirmed by sensory evaluation of the color. The polypropylene interwoven sacks had poor barrier protection against the effects of the storage conditions on color change, while the polyethylene plastics depended upon the storage conditions. The browning was attributed to the Maillard reactions. On the other hand, lipolysis as measured by free fatty acid contents increased at all storage conditions, with storage temperature having a higher influence than the storage humidity. At constant storage temperature, free fatty acid contents at high humidity were higher than at low humidity. Packaging material type did not seem to significantly influence lipolysis. Gelatinization temperatures were determined for maize meal and its isolated starches during storage of the white maize meal. The pasting properties in the cooking phase during storage of white maize meal were dependant upon humidity of storage. These observations were explained by hydrothermal effects due to prolonged exposure of the starch granules in the white maize meal to both low or high humidity and temperature. On the other hand, the pasting properties in the cooling phase of the pastes were more influenced by the storage temperature than humidity. The flow behaviour of the white maize meal pastes were best described by Herschel-Bulkley and Mizrahi-Berk models. Herschel-Bulkley and Mizrahi-Berk models could be used to predict the flow behaviour for low temperature storage conditions during the entire study storage period, while only for a limited storage period at high storage temperatures. The maximum stress overshoot values were highly influenced by storage temperature. The flow behaviour indices and the yield stress decreased while the consistency indices increased at all conditions during storage. The pasting properties in the cooling phase and flow behaviour properties were explained by the free fatty acid-amylose complexation. However, at high storage humidity conditions, some properties were highly confounded by mould growths after 28 days of storage of the white maize meal.