Pearl millets flakes developed using different hydrothermal treatments were evaluated for various physical, functional, textural and nutritional properties. Performance of flaking machine was evaluated with the variable roller clearance (0.1 to 0.5 mm) between flaking rolls. The length, width and thickness of flakes was measured by using vernier calliper and also bulk density was evaluated for the flakes obtained at different roller clearance.
Fig 1 presents the physical parameters of pearl millet flakes as the function of roller clearances. As the roller clearance increased from 0.1 to 0.5 mm, length and width of flakes decreased from 5.2 to 3.09 mm and 4.44 to 2.76 mm, respectively whereas the thickness of flakes increased from 0.50 to 1.50 mm. Bulk density of flakes (349.5-590.7 kg/m3) was found proportional with the increasing roller clearance at the mean moisture content of 8.11±0.07 per cent. It was pertinent to note that at the lowest roller clearance of 0.1mm 50% grains were broken. Hence, a roller clearance of 0.2 mm was found optimum.
Colour
Color analysis of hydrothermally treated pearl millet flakes revealed that lower temperatures resulted in higher L* values (lighter color), while increased treatment temperatures reduced lightness due to thermal degradation, pigment leaching and Maillard browning. Initially, lightness increased, but prolonged steaming led to further darkening. Soaking temperature, time and steaming time significantly influenced L* values (P<0.01). Higher soaking temperatures negatively impacted lightness, while extended soaking and steaming initially enhanced it. Color changes were linked to phenolic polymerization and pigment reactions. The ‘a’ value reflected shifts in red-green tones, influenced by soaking and steaming conditions, while the ‘b’ value, affected mainly by steaming, indicated yellow-blue variations. The overall color difference (ΔE) increased with soaking temperature and time, highlighting greater perceptible changes due to combined chemical and physical effects.
Bulk density
Hydrothermally treated pearl millet flour was found to have lower bulk densities. Lower bulk density is desirable criteria for producing good quality flakes. It has been observed that, with the increase in the temperature of soaking and steaming time, the bulk density decreased while it increased in soaking and pressure-cooking treatments Bulk density of flakes ranged from 355-430 kg/m
3 with different pre-treatments. Soaking and steaming produced flakes with lower bulk density compared to pressure cooking. Lowest bulk density (355 kg/m
3) was observed for sample T4 (30 min soaking at 65°C + 20 min steaming) while highest bulk density (430 kg/m
3) in flakes produced after pressure cooking in sample T6 (20 min soaking 75°C + pressure cooking for 2 min at 15 psi) of pearl millet grains.
The decrease in bulk density following prolonged steam treatment can be attributed to the process of starch gelatinization. When starch granules are exposed to heat and moisture, as in steam treatment, they absorb water and swell, disrupting their crystalline structure. This swelling increases the volume of the starch, while its mass remains unchanged, effectively lowering the bulk density of the material. Additionally, prolonged exposure to steam may cause further breakdown of the starch granules, leading to a more porous structure that contributes to the overall reduction in bulk density
(Yadav et al., 2012).
Functional characteristics
Functional characteristics of pearl millet instant flakes are influenced by factors like water absorption index (WAI), water solubility index (WSI) and fat absorption capacity. WAI, ranging from 2.20 to 4.10, indicates starch gelatinization and was negatively correlated with bulk density (r2 = -0.85) but positively correlated with water solubility (r2 = 0.59). The highest WAI (3.44±0.93) and fat absorption capacity (287.84) were observed in sample T4. Water uptake by the endosperm during steam flaking controls gelatinization, with higher WAI in steamed flakes due to limited moisture availability. Hydrothermal treatment (soaking and steaming) resulted in higher WSI than steaming and pressure cooking. Pearl millet flakes exhibited greater water absorption capacity (287.84±3.17) than conventional rice and oat flakes, attributed to their high insoluble dietary fiber. The least cooking time (0.20 min) was found in steamed flakes. High water and fat absorption capacities enhance flavor and mouthfeel, making millet flakes suitable for food applications requiring optimal moisture or oil retention. Variability in functional attributes, as noted by
Narang et al., (2018), is influenced by the carbohydrate/protein matrix and plasticizing effects of water, similar to interactions observed in corn and quinoa flakes.
Textural characteristics
Textural properties were measured in terms of crispness and hardness of flakes. It was observed that hydrothermal treatment, specifically (soaking and pressure cooking), resulted in flakes that require more force to break, indicating they are harder compared to flakes prepared through (soaking and steaming) treatment. This could be due to differences in the structural changes caused by the different hydrothermal treatments as depicted in Table 1. The reason behind was controlled gelatinization and starch retrogradation prior to flaking in hydrothermal treatment (soaking and pressure cooking) and also to higher dietary fiber content. Sample T
3 and T
4 reflected desired crispness. Hardness of T
4 was lowest while significantly higher values were obtained in case of T
6 and T
7. Results indicated that flakes prepared through hydrothermal treatment, which includes soaking and steaming of the grain, particularly in samples T
3 and T
4, exhibited superior textural attributes which suggests that the specific combination of soaking and steaming conditions applied to these samples positively influenced their texture. Hydrothermal treatment often enhances the textural quality of grains by modifying the starch structure, making the flakes softer, more cohesive and improving their overall mouthfeel. The improvement in texture in samples T
3 and T
4 could be due to optimized processing parameters, such as soaking duration and steaming intensity, which led to desirable changes in the grain’s physical properties
(Pawase et al., 2019). Hydrothermal treatment involving soaking and pressure cooking appears to negatively impact the crispness of the flakes, as evidenced by a significantly higher hardness value. Pressure cooking, in particular, can lead to over-gelatinization of starch and increased moisture retention, resulting in a denser, less crispy texture. Consequently, while hydrothermal treatment can improve other qualities, it may render the product unsuitable for applications where crispness is a key requirement.
Nutritional composition
Nutrient composition of pearl millet flakes is shown in Table 2. Moisture content of pearl millet flakes ranged from 3.09-4.39%, with the lowest in T
3 and T
4 (soaking and steaming). Ash content varied between 0.1-0.5 g, with the highest in T
4. Protein content (9.41-12.89%) was higher in soaking and pressure cooking, possibly due to improved bioavailability. Energy content ranged from 410.14-421.11 Kcal, higher than other millet flakes like barnyard (277 Kcal) and oats (343 Kcal)
Takhellambam et al., (2016). Fat content (5.33-5.62%) remained unaffected by hydrothermal treatment, but free fatty acids were lower (2.69%) due to lipid stabilization. Total dietary fiber decreased from native grain (16.11 g) due to fiber disintegration, with soluble fiber reducing more in soaking and pressure cooking. Crude fiber was highest in T
1. Mineral content ranged from 34.5-46.0 mg (calcium) and 138-305 mg (phosphorus), with iron levels (1.20-1.70 mg) higher in soaking and pressure cooking due to reduced antinutrients. Hydrothermal treatment influenced phenolic content, with steaming better for retention while pressure cooking reduced it
Raffaella et al., (2017). Steaming prevented polyphenoloxidase enzyme degradation, aiding phenol retention. Overall, steaming preserved more nutrients, making it a favorable method for processing pearl millet flakes.