Introduction: Millet is one of the cereals resistant to drought, pest and disease and has a short growing season. As the world's population grows and water resources decline, this agricultural product will increasingly become- important for human use in the future. Millet is gluten-free grain and can be used to produce special food for celiac patients. Due to the low quality of baking products made from gluten-free cereals, additives and physical treatment of flour can be used to improve the baking properties. Therefore, in the present study, heat-moisture treatment of millet grains was used as a method to modify the flour properties.
Materials and Method: Proso millet (Panicum miliaceum L.) was dehulled using a stone mill and then moistened to 20, 25 or 30% of moisture by spraying sufficient distilled water, mixing and keeping in polyethylene bags at 4°C overnight. The moistened grains were poured into glass containers and heated in an oven at various temperatures (100 or 120 ° C) for 3 h. After heating, the samples were removed from the containers and dried at about 40°C until about 9% moisture content. The samples were then milled and passed from the screen with standard mesh 80. In this study, cakes were prepared from untreated and treated millet grain. The specific gravity of batter was measured by dividing the weight of a specific volume of batter to the weight of the similar volume of water. Batter viscosity was determined by Brookfield viscometer. Cake volume was measured by the rapeseed displacement method. Color of cake crumb and crust were determined by scanner and image J software. The oven method was used for measuring cake moisture. The hardness of cakes was evaluated by texture profile analyzer. For study of the microstructure of cakes, scanning electron microscope (Hitachi) was applied. In this study, the effects of two factors including moisture of heat-moisture treatment (at 3 different levels) and temperature of heating (at 2 different levels) on the batter and cakes properties were studied by a completely randomized design. Statistical analyses were done via analysis of variance (ANOVA) and Duncan’s multiple range tests for significance at p<0.05 using SAS 9.1 software.
Results and discussion: The low specific gravity shows the better aeration of cake batter which can result in greater cake volume. Heat-moisture treatment of millet grains reduced the specific gravity of the cake batter so that the specific gravity of the untreated millet flour batter was the highest. The low batter viscosity reduces the cake volume because the batter cannot keep trapped air bubbles inside it and when the batter is placed in the oven, these bubbles move up to the surface of batter and they get out of the cake. However, a very viscous batter also cannot produce high volume cakes due to the limited expansion of the batter. Therefore, the optimum and appropriate viscosity of the batter is necessary for the production of large volume cakes. Heat-moisture treatment of millet grains significantly increased the viscosity of the batter (p<0.05) so that the lowest viscosity was in the control sample and the highest viscosity was in the treated sample at 30% moisture and 120°C. The samples treated at lower moisture and temperature had higher cake volume compared to the control sample, while the treated samples at higher moisture and temperature showed no improvement in volume. Millet flour treated at 30% moisture and 120°C significantly reduced cake volume (p<0.05) and this effect may be due to excessive viscosity of the batter which prevents the batter expansion in the oven. Increasing the treatment temperature and moisture significantly increased the darkness and redness of the cake color. However, at the baking day and during storage, there was no improvement in the texture and moisture content of the cake crumb after heat-moisture treatment of millet grains. The scanning electron micrographs of raw and treated millet cake showed that heat-moisture treatment of grains increased the uniform and fine cavities in the cake texture. Furthermore, in the cake prepared from treated millet, the gelatinized starch granules are found in the greater numbers.
The development of gluten-free bakery products has a growing market worldwide. Therefore, food industry specialists are looking for the production and improvement of the quality of these products. Due to the increasing consumer demand for non-additive foods, there is a growing interest in the physical treatment of cereals as it changes the starch performance without the addition of external ingredients. In this regard, the results of the present study revealed the ability of heat-moisture treatment to modify the performance of millet flour for application in gluten-free product formulations by improving their qualitative properties. Application of heat-moisture treated millet increased cake batter viscosity and air penetration and reduced its specific gravity. Heat-moisture treatment of millet grain at low moisture and temperature improved cake volume. However, the treatment of millet grains did not improve texture and moisture content of the cake during storage. Study of microstructure of the cakes showed that the heat-moisture treatment of millet grain increased the uniform and fine air cells in the cake structure and also the starch granules were more gelatinized. According to the results of this study, heat-moisture treatment of millet grains at 25% moisture and 100 ° C improved quality parameters of batter and cake compared with control.