Lipid stabilisation and partial pre-cooking of pearl millet by thermal treatments

Abstract

Pearl millet is a cereal crop cultivated by subsistence farmers in semi-arid parts of Africa and Asia. In Namibia, pearl millet porridge is a staple food for over half of the population. Healthful Harvest, a cooperative of subsistence farmers in a rural area in Namibia is developing a flour product with extended shelf life and a short cooking time comprising pearl millet and cowpea. This requires the application of simple technology. The pearl millet grain is small (3-15 mg) but has a proportionally larger germ than all other cereal grains, except perhaps maize. Therefore, it tends to contain a high content of triglycerides, which are rich in unsaturated fatty acids. Pearl millet flour is susceptible to rancidity within a few days due to lipolysis and subsequent oxidation of the de-esterified unsaturated fatty acids. To try to prevent rancidity and to pre-cook, pearl millet grain was subjected to toasting, boiling and toasting then boiling before reduction to flour. The effects of these different thermal treatments on fat acidity, peroxide value (PV) and conjugated diene and triene values of pearl millet flour before and after three months storage at ambient conditions were determined. The degree of cook of starch was determined on fresh flours. The porridges made from the flour of the treated grains were evaluated by a trained panel and by consumers. Analyses of energy demands and practicality of the thermal treatments and extrusion cooking as processing technologies in manufacturing pearl millet flour in rural parts of Namibia were made. Fat acidity for the untreated flour increased significantly from 0.11 to 3.72 g KOH kg-1, whereas no significant increase observed in the flours of wet thermally-treated grains. This indicates that wet thermal treatments inhibited triglyceride hydrolysis. The PVs of the flours of the wet thermally-treated grain increased seven-fold, while the PV of the untreated flour decreased. A similar trend was observed for the conjugated diene values. The conjugated triene values increased significantly for all the samples. These results indicate autoxidation in the thermally-treated samples and that there was accumulation of hydroperoxides. The degree of cook of the wet thermally-treated grain (~40%) was twice that of the untreated and toasted grains, indicating that the wet thermal treatment partially gelatinised the grain. Porridges prepared using untreated flours were associated with rancid flavours, while those of other treatments were not, indicating that the thermal treatments can prevent rancidity. Consumers preferred the porridge prepared using flour of the boiled grain, presumably because it was fully cooked, whereas others were not. Thus, the boiling treatment can be applied to extend the shelf life of and pre-cook pearl millet flour. The energy demands for boiling and extrusion cooking were estimated to be 0.6 and 0.2 kWh kg-1, respectively. The energy demand for the boiling process can be minimised by sun-drying instead of using electricity. The cost of an extruder would be prohibitively costly for Healthful Harvest. Thus, boiling the grain is a suitable appropriate technology that can be applied in the Healthful Harvest situation by ordinary people, with no specialist skills.