A whole grain kernel consists of the endosperm, germ, and bran. The outer coating of bran is rich in fiber, and lignans, minerals, vitamins and phytochemicals (phenolic acids and phytosterol compounds) are abundant in the inner germ. Pulses are part of a balanced and healthy diet with an important role in the prevention of illnesses, diabetes, cancer and heart-related diseases. Pulses are rich in fiber, are a low-fat source of protein and have a low glycemic index. The grinding procedure is a unit process for reducing the size of the material. Grinding plays a major role in the food industry. Size reduction plays a major role in many food processes and is accomplished by applying diverse forces to produce particles with definite sizes and shapes. All the raw materials were dehulled by a versatile dhal mill to separate the germ, cotyledon, and husk. The raw materials were powdered and passed through a 60 mesh BSS sieve.
The proximity of a sample plays an imperative role in understanding the nutrients in the sample. In addition to its nutritional value, protein content also provides important technological indications. The protein content provides an indicator of the quantity of water absorbed for a certain level of dough consistency and aids in predicting the total time required for development, stability and softening. A constant percentage of acacia gum (1.0%) was used in the formulations due to its function as a thickening agent and stabilizer to complete the characterization of gluten-free flour (Gambuś et al., 2007).
The moisture content of food materials before grinding is a significant parameter for ensuring good flowability. For various food materials, the initial moisture content plays a major role in determining the size distribution of particles and the grinding time. Controlling the moisture content via pretreatment steps such as drying or adding moisture is imperative before grinding.
The approximate compositions of the individual flours are shown in Table 2. The moisture contents of the rice flour, ragi flour, black-gramed flour, and cowpea flour were 5.40%, 11.72%, 6.43% and 6.23%, respectively. The protein content was greater in black-gram-old and cowpea flour.
Table 2
Proximate composition of individual flours
|
Rice
|
Ragi
|
Black gram
|
Cowpea
|
Moisture
|
5.40 ± 0.49
|
11.72 ± 0.69
|
6.43 ± 0.60
|
6.23 ± 0.20
|
Ash
|
0.53 ± 0.10
|
2.54 ± 0.04
|
3.33 ± 0.06
|
3.61 ± 0.03
|
Fat
|
0.72 ± 0.06
|
1.34 ± 0.07
|
1.28 ± 0.02
|
1.32 ± 0.09
|
Protein
|
8.41 ± 0.19
|
6.05 ± 0.65
|
23.13 ± 0.30
|
22.22 ± 0.18
|
Carbohydrates
|
84.94 ± 0.53
|
78.13 ± 1.08
|
65.82 ± 0.42
|
66.56 ± 0.31
|
Calories
|
379.89
|
348.78
|
367.32
|
367.00
|
Together with moisture, protein, fiber, and sodium content, fat content is one of the five key parameters used in assessing food quality. Cowpea and black-gram vegetables have higher protein and ash contents than rice and the ragi. Cowpea is rich in protein, fat and ash, with 22.22%, 1.38% and 3.61%, respectively. The ash content measures the amount of minerals present within a food.
A mixture of different flours from legumes, cereal plants or millet plants was used as the composite flour. It is produced to fulfill specific nutrients and functional characteristics. The final product, with respect to its functional and physicochemical properties and health benefits, mainly depends on the beneficial effects of the use of composite flour.
The moisture contents of the composite flours with and without acacia gum are reported in Table 3.
Table 3
Composite flour composition
|
F1 + AG
|
F2 + AG
|
F3 + AG
|
F1-AG
|
F2-AG
|
F3 -AG
|
Moisture
|
8.87 ± 0.30
|
7.94 ± 0.93
|
6.38 ± 0.12
|
10.36 ± 0.18
|
9.85 ± 0.12
|
9.53 ± 0.10
|
Ash
|
2.49 ± 0.15
|
2.53 ± 0.20
|
2.54 ± 0.10
|
2.45 ± 0.06
|
2.48 ± 0.19
|
2.69 ± 0.25
|
Fat
|
1.49 ± 0.12
|
1.41 ± 0.11
|
1.43 ± 0.10
|
1.37 ± 0.01
|
1.14 ± 0.03
|
1.07 ± 0.01
|
Protein
|
13.12 ± 0.93
|
12.53 ± 0.16
|
14.65 ± 0.06
|
11.53 ± 0.84
|
13.22 ± 1.06
|
15.47 ± 0.76
|
Carbohydrates
|
73.93 ± 0.67
|
75.63 ± 1.01
|
74.31 ± 1.00
|
74.29 ± 0.62
|
73.30 ± 0.96
|
71.24 ± 1.02
|
Calories
|
361.65
|
365.31
|
368.72
|
355.61
|
356.37
|
356.52
|
The moisture content varies depending upon the blending ratio. It is clearly shown that there is a decrease in the percentage of moisture content in the composite flours as the amount of ragi flour decreased from 49 − 24% (Table 3). The percentage of moisture content in the composite flour was also strongly affected by the combination of the two materials with acacia gum. The highest moisture content was observed for F1-AG (10.36%), and the lowest was observed for F3 + AG (6.38%). This difference may be due to the absorption of moisture by Acacia gum and cowpea.
Ash content was lower in flour samples with acacia gum than in those without acacia gum. The ash content is an indicator of the nonorganic compound content in food. The ash content varied from 2.45–2.69% in the different flour formulations. The composite flour F3-AG had the highest ash content (2.69%), implying a higher mineral content.
The protein content increased (12.53–14.65%) with increasing percentage of pulsed flour in the formulations with acacia gum. The protein content was greater in the composite flour without acacia gum (15.47%) than in that with acacia gum (14.65%). The ash and protein contents were greater in the F3-AG group. This could essentially be due to the higher content of protein in cowpea and black-gram cowpea. (Abioye et al., 2011) also reported that the higher protein content in composite flour was because of the high protein content in soybeans.
The highest fat content was observed for the F1 + AG flour (1.49%), and the lowest was observed for the F3-AG flour (1.07%). The fat content of the composite flour with acacia gum was greater than that of the flour without acacia gum. This difference may be due to the absorption of moisture by acacia gum. The study showed that the moisture content of the composite flours decreased with decreasing proportion of ragi flour from 49–24%. Similar trends were reported by (Kaushal et al., 2012). The authors used blends of taro, rice, and pigeon pea flour, which resulted in a reduction in the moisture content of the composite flours. The protein content of F3-AG was 15.47%, followed by that of F3 + AG (14.65%), which was > 13%, meeting the desired protein levels (Chillo et al., 2008).
3.1. Physical properties/technological parameters
The flour particle size degrades the water holding capacity of flour by affecting the specific surface area of the flour and the degree of starch damage. A greater amount of damaged starch granules improves flour water absorption, which increases the water holding capacity (Kweon et al., 2014). The characteristics of flour, such as water absorption, conversion of starch by enzymes, damaged starch content, and baking quality, are improved by flour particle size (Alsberg and Griffing, 1925). The particle size distribution data of the individual flours are reported in Table 4.
Table 4
Percentage particle sizes of the individual flours
µm
|
Rice
|
Ragi
|
Black gram
|
Cowpea
|
40
|
91.95
|
84.50
|
60.09
|
70.48
|
60
|
85.23
|
71.15
|
56.77
|
63.77
|
100
|
74.50
|
53.47
|
52.82
|
56.55
|
150
|
65.42
|
43.71
|
48.34
|
49.42
|
The majority of the flour particles had a sieve size less than 150 µm. Table 4 shows that 70.48%, 60.09%, 84.50% and 91.95% of the flour in the 40 µm fraction was produced from cowpea, black gram, ragi, and rice, respectively. The lowest percentage was reported for flour (49.42–65.42%) in the 150 µm fraction.
An alteration in the particle size distribution of raw materials has an effect on the hydration properties. A larger particle size results in heterogeneous hydration of the particles and the formation of large dough lumps (Martens et al., 2010), which leads to uneven drying and white specks.
The incorporation of food gums into flour mixtures has the potential to improve textural characteristics. Gums improve the pasting behavior and granular structure of starch during the cooking and baking of food (Christianson et al., 1974). They form gels and exhibit colloidal appearances in aqueous systems. Gums hydrates to yield viscosity or dispersion in cold or hot water (Scanlon et al., 1988). The gum attributes to the pasting viscosity changes in starch molecules when the starch dispersion is heated in the presence of gums.
Table 5
Percentage particle sizes of the individual flours
Particle size (µm)
|
F1 + AG
|
F2 + AG
|
F3 + AG
|
F1-AG
|
F2-AG
|
F3 -AG
|
40
|
77.33
|
77.16
|
74.89
|
74.52
|
76.68
|
74.50
|
60
|
68.55
|
68.83
|
67.40
|
64.03
|
67.16
|
66.05
|
100
|
56.52
|
57.46
|
57.32
|
51.48
|
56.11
|
56.40
|
150
|
47.20
|
48.30
|
48.45
|
42.07
|
47.50
|
48.55
|
Table 5 reports the particle size distributions of the flour samples produced from the different formulations. A greater percentage of flour (74.89–77.33%) was in the 40 µm fraction in the composite flour with acacia gum. The lowest percentage was reported for flour (47.20-48.45%) in the 150 µm fraction. The particle size distribution was lower for the formulations without acacia gum than for the formulations with acacia gum.
Physical properties play a major role in the behavioral analysis of products during processing. The particle size is inversely proportional to the bulk density (Omimawo and Akubor, 2012). The variation in bulk density is mainly due to the variance in the particle size of the flours. In the food industry, the porosity (bulk density) of a product impacts the nature of the packaging material needed, the package design, and the application of the product in wet processing (Kinsella, 1981).
The bulk density (g/cm3) is the measurement of the density of flour without the interference of any compression. The individual flour bulk density ranged from 0.600 g/cm3 to 0.800 g/cm3. The maximum bulk density of rice flour (0.800 g/cc) was reported compared to that of ragi flour (0.714 g/cc) and cowpea flour (0.696 g/cc), and the lowest bulk density was reported for black-gram flour (0.600 g/cm3) (Fig. 1).
The present study showed that bulk density is influenced by the original moisture content and particle size of the flours. The incorporation of acacia gum into flour increased the bulk density of the composite flour. The highest bulk density of flour suggested its suitability for use in food preparations. In contrast, low bulk density would be beneficial in the formulation of complementary foods (Akpata and Akubor, 1999). Therefore, the high bulk density of the composite flour in the present study suggested that this material is suitable for use as a thickener in food products; subsequently, it helps to decrease paste thickness, which is a significant feature in convalescent and child feeding. The bulk density considerably improved with an increase in the assimilation of black-gram and cowpea flour in formulations without acacia gum.
An increase in the bulk density after mechanically tapping the container with the sample results in an increase in the tap density. This signifies random dense packing of the sample. Figure 1 shows the tap density of the individual and composite flour. The tap density of individual flours ranged from 0.59–0.78. A greater tap density of rice flour (0.78 g/cc) was found. This is due to the density, size, and surface properties of the flour sample (Iwe et al., 2016). The lower tapped density of black g (0.59 g/cc) indicates the noncohesive properties of the material. A higher tapped density is appropriate for packaging, and a larger amount of material can be packed inside a constant unit volume (Van Toan and Anh, 2018). Similar studies on yellow-fleshed cassava flour have been reported (Falade et al., 2019).
The tap density of the formulations without acacia gum ranged from 0.70–0.77 g/cm3. The tap density was found to increase with increasing pulse flour percentage. The maximum tap density was detected for F1 + AG (0.78 g/cm3), which was similar to that of F3-AG (0.77 g/cm3), followed by F3 + AG (0.74 g/cm3).
The capacity of starch molecules to hold water within their structure through hydrogen bonding is the swelling capacity (SC) (Ahmad et al., 2016). SC plays a vital role in the manufacturing and retention of the structure of bakery products. The SC is the maximum volume a flour sample attains due to the absorption of water. This water absorption continued until the formation of a colloidal suspension. The increase in volume stops when intermolecular forces present among swollen molecules prevent water absorption (Adetuyi et al., 2009). The particle size, varietal differences, and processing methods also affect the swelling capacity of flour (Samsher, 2013). Figure 2 shows the comparison of the SC of individual flour with that of the composite flour. The swelling power of the different flour samples ranged from 15.00 to 20.0 ml. The maximum reading was recorded for cowpea (20.0 ml), followed by black (19.50 ml) and ragi flour (16.50 ml). The lowest value was recorded for rice flour (15.0 ml).
Figure 2 clearly shows that the maximum swelling power was detected for F3-AG (25.00 ml); however, the lowest swelling power was detected for F1-AG (16.00 ml). An increasing trend was noted in the case of the formulation without acacia gum. In the case of the formulation with acacia gum, there was a decreasing trend from F1 + AG (20.00 ml) to F3 + AG (18.00 ml). Furthermore, an increase in temperature causes leakage of amylose and acacia, leading to the formation of films around the granules, which inhibits swelling.
The swelling capacity of the composite flour was strongly affected by the proportion of cowpea flour due to pregelatinization resulting in a high starch content. Acacia gum may inhibit water absorption by limiting the water availability available to starch, thus reducing the swelling-promoting effect of the formulations.
The color values of the individual and composite flours were estimated by CIELAB color values, where L* represents lightness, a∗ indicates redness, and b∗ indicates yellowness. The color parameters of the individual flours and blends of composite flours are shown in Table 6 in terms of L*, a*, and b*. The L* values of the individual flours ranged between 70.76 and 89.57. The L* value of the composite flour blend (F3-AG to F1-AG) decreased from 81.70-77.29. Similarly, the L* value of the composite flour blend with acacia gum (F3+AG to F1+AG) decreased from 81.93–77.31. There was no significant difference between the L* values of the composite flour blends with and without acacia gum.
Table 6
Color characteristics of individual and composite flour
|
Color values
|
Flours
|
L* white
|
a* green
|
b* blue
|
Rice
|
89.57 ± 0.11
|
-0.45 ± 0.02
|
11.34 ± 0.06
|
Ragi
|
70.76 ± 0.12
|
5.18 ± 0.02
|
29.24 ± 0.11
|
Black gram
|
89.10 ± 0.66
|
-0.26 ± 0.02
|
14.09 ± 0.49
|
Cowpea
|
88.38 ± 0.52
|
0.08 ± 0.06
|
15.20 ± 0.33
|
F1 + AG
|
77.31 ± 0.33
|
3.02 ± 0.18
|
22.63 ± 0.53
|
F2 + AG
|
78.85 ± 1.15
|
2.57 ± 0.10
|
21.09 ± 1.11
|
F3 + AG
|
81.93 ± 0.25
|
1.62 ± 0.04
|
18.12 ± 0.10
|
F1-AG
|
77.29 ± 0.64
|
2.94 ± 0.12
|
22.43 ± 0.75
|
F2-AG
|
78.63 ± 0.35
|
2.37 ± 0.02
|
21.02 ± 0.38
|
F3-AG
|
81.70 ± 0.33
|
1.67 ± 0.02
|
18.53 ± 0.31
|
Similarly, a significant difference was not observed in the redness and yellowness color values of the two flours, with and without gum. The values are presented in Table 6. The a* values of the flour were positive, ranging from 1.67 to 3.02. However, the b* values of the flour were found to be positive, ranging from 18.53 to 22.63. The alteration in the color value is due to the polyphenolic pigments in the pericarp, aleuronic layer and endosperm region.
3.2. Functional properties
The solubility of proteins is the percentage of nitrogen in a protein product that is in the soluble state under specific conditions. To improve the efficacy of the use of raw and composite flours in various food products, the protein solubility of the flours was evaluated in water and 0.5 M NaCl extract at pH 7.0.
Table 7
Protein solubility of native and composite flours
Flours
|
Protein soluability (%)
|
|
Water, pH 7.0
|
0.5 M NaCl, pH 7.0
|
Rice
|
18.83 ± 3.23
|
7.80 ± 0.16
|
Ragi
|
239.42 ± 8.41
|
11.52 ± 0.17
|
Black gram
|
45.86 ± 2.73
|
45.35 ± 5.00
|
Cowpea
|
32.68 ± 2.98
|
83.26 ± 1.00
|
F1 + AG
|
118.63 ± 4.09
|
7.17 ± 0.09
|
F2 + AG
|
91.18 ± 8.64
|
11.68 ± 0.16
|
F3 + AG
|
143.72 ± 2.05
|
11.59 ± 0.28
|
F1-AG
|
147.88 ± 1.08
|
8.64 ± 0.91
|
F2-AG
|
151.13 ± 1.08
|
10.96 ± 0.14
|
F3-AG
|
138.31 ± 10.52
|
13.56 ± 0.21
|
The influence of water and NaCl at pH 7.0 on the protein solubility of the raw and composite flour is presented in Table 7. The highest protein solubility (239.42 ± 8.41%) was recorded for the ragi, followed by the black gram (45.86 ± 2.73) in water. However, the protein solubility of cowpea flour decreased in water (32.68 ± 2.98) followed by rice flour (18.83 ± 3.23). Moreover, there was no difference in the protein solubility of black blood in water or NaCl solution. The highest protein solubility (83.26 ± 1.00%) was recorded for cowpea flour, followed by black g (45.86 ± 2.73 in NaCl). A minimum solubility of 7.80 ± 0.16% was observed for the rice flour in the 0.5 mM NaCl solution.
The results showed that there was a decrease in protein solubility in the presence of NaCl compared to that in the presence of water extract in formulations with and without acacia gum. However, the protein solubility of these three formulations was greater than that of raw flour, except for that of ragi flour. Compared with formulations with gum, formulations without gum have been reported to have the highest protein solubility. F2-AG showed the highest protein solubility (151.13 ± 1.08) in water, followed by F1-AG and F3-AG, with solubilities of 147.88 ± 1.08 and 138.31 ± 10.52, respectively. F3 + AG showed the maximum protein solubility (143.72 ± 2.05).
The water absorption capacity reflects the amount of water absorbed and retained by the flour. The type of protein, amino acid composition, and protein polarity and hydrophobicity affect the water and oil absorption capacity (Chandra and Samsher, 2013). Additionally, a deviation in the amylose/amylopectin ratio also contributes to alterations in the water and oil absorption capacity of flour (Chandra et al., 2015). High carbohydrate content increases the WAC of flour due to its hydrophilic constituents, which enable it to bind additional water (Mbaeyi, 2005).
Table 8 shows the water absorption capacities of the raw flour and its composite flours, which are influenced by the flour constituents and their relationships.
Table 8
Water and oil absorption capacities of individual flours
|
Flours
|
|
Rice
|
Ragi
|
Black gram
|
Cowpea
|
Water absorption capacity (%)
|
112.05 ± 0.85
|
130.77 ± 1.19
|
320.57 ± 0.84
|
83.97 ± 0.65
|
Water solubility index (g/100 g)
|
1.19 ± 0.01
|
3.88 ± 0.11
|
13.42 ± 0.79
|
28.94 ± 0.64
|
Oil absorption capacity (%)
|
65.04 ± 1.35
|
68.52 ± 3.35
|
63.21 ± 0.69
|
65.64 ± 0.14
|
The water absorption capacity was highest for black g of flour (320.57 ± 0.84%) and lowest for cowpea flour (83.97 ± 0.65%). The maximum water absorption values were attributed to the higher content of starch and fiber (Klunklin and Savage, 2018). A high protein content tends to improve water absorption (Patil and Arya, 2017). In the present study, a good association was established between water absorption and protein content in black-gram flour. A higher protein content in black-gram flour leads to increased water absorption capacity. The water solubility index (g/100 g) was greatest for cowpea (28.94 ± 0.64), followed by blackberry (13.42 ± 0.79).
The major chemical component affecting OAC is protein, which is composed of both hydrophilic and hydrophobic parts. Nonpolar amino acid side chains can form hydrophobic interactions with the hydrocarbon chains of lipids. The oil absorption capacity of flour is determined by physical binding of proteins to fat through capillary attraction. The maximum OAC reflects the enhanced hydrophobicity of the proteins in the flours, which results in more nonpolar amino acids being transferred to the fat and enhanced hydrophobicity via the absorption of oil. OAC enhances the shelf life of sausages (Akinyede and Amoo, 2009).
The ragi flours had the highest oil absorption capacity (68.52 ± 3.35%) because they retain the flavor and enhance the mouthfeel in foods. With other flours, the oil absorption capacity ranged between 63.21 ± 0.69 and 65.64 ± 0.14%. (Di Cairano et al., 2020) reported no significant difference in the oil absorption capacity of gluten-free flour. Similar observations were made for rice, cowpea and black-gram flour.
However, for the composite flour, F2-AG had the highest WAC (%) (135.74 ± 1.80), followed by F2 + AG (126.73 ± 1.08) (Fig. 3). These findings suggested that water absorption was affected by the addition of rice flour. This difference might be due to the molecular structure of the rice starch, which initiates water absorption, as reflected by the increase in the WAC and decrease in the proportion of black-gram flours. The observed variation in the different flours may be due to differences in protein concentration, degree of interaction with water and conformational changes (Butt and Rizwana, 2010). The WSI increased from F1 + AG to F3 + AG with acacia gum. This difference may be due to the increase in the concentration of cowpea flour from F1 to F3. Similar trends were observed both with and without acacia gum.
The increase in the WAC of formulations with acacia gum might be due to the ability of gum to absorb water in its interrelated network and interaction with starch granules. These results were attributed to structural modifications resulting from the assimilation of gum to allow additional absorption of water through hydrogen bonding (Ognean et al., 2006).
The formulations with acacia gum exhibited the highest oil absorption capacity compared to the formulations without gum. With flour formulations, F2 + AG had the highest oil absorption capacity (66.71 ± 0.82%), followed by F1 + AG (65.90 + 1.20%). The lowest percentage was reported for F1-AG (53.34 ± 0.39%). The results indicate that the OAC capacity of acacia gum tended to increase compared to that of the flours without acacia gum.
The amount of interfacial region that can be formed by a protein reflects the foam capacity of the protein (Fennama, 1996). Foam formation occurs when colloidal gas bubbles surround a liquid or solid. The foaming and emulsion activities are shown in Table 9.
Table 9
Foaming capacity and emulsion activity of individual flours
|
Rice
|
Ragi
|
Black gram
|
Cowpea
|
Foaming capacity (%)
|
0.80 ± 0.00
|
4.00 ± 0.00
|
20.00 ± 0.00
|
32.00 ± 0.00
|
Foaming stability (%)
|
0.00 ± 0.00
|
60.00 ± 0.00
|
70.00 ± 0.00
|
52.50 ± 0.00
|
Emulsion activity (%)
|
4.41 ± 0.00
|
4.62 ± 0.00
|
44.78 ± 0.00
|
42.03 ± 0.00
|
Emulsion stability (%)
|
33.33 ± 0.00
|
33.33 ± 0.00
|
40.00 ± 0.00
|
34.48 ± 0.00
|
The foaming capacity (FC) and foaming stability (FS) of the different flours ranged from 0.80 to 23.00% and from 0.00 to 70.00%, respectively. The highest foam capacity was observed for cowpea flour (32.00%), black-gram flour (20.00%), and the lowest for rice (0.80%). Foam stability (FS) is the ability of proteins to stabilize against mechanical stresses and gravitational forces (Fennama, 1996). The highest FS was detected for black-gram flour (70.00%), followed by ragi flour (60.00%) and cowpea flour (52.50%), and the lowest was detected for rice flour (0.00%).
Proteins can alleviate emulsions by creating electrostatic repulsions on the surface of oil droplets (Kaushal et al., 2012). The EA and ES of the individual flours are tabulated in Table 9. The emulsion activity (EA) of the different flours ranged from 4.41 to 44.78%. The maximum EA was observed for black-gram flour (44.78%). The emulsion stability (ES) of the different flours ranged from 33.33 to 40.00%. The maximum ES was shown for black-gram flour (40.00%), followed by cowpea flour (34.48%), and the lowest was shown for rice and ragi flour (33.33%).
The FC and FS of the composite flours improved with increasing combination ratio of the different flours. There was an inverse relationship between the foam capacity and foam stability. Samples with maximum foaming ability might form large air bubbles enclosed by a thin flexible protein film. These air bubbles can easily collapse and subsequently decrease the foam stability (Jitngarmkusol et al., 2008).
The functional properties of the blends will vary according to the component of the blend. Figure 4 shows the percentages of FC, FS, EA and ES in the different formulations with and without acacia gum. The foam capacity of the different flour formulations ranged from 11.47 to 15.20%. The FC of the F3 + AG group (15.20%) was greater than that of the other formulations. The highest foam capacity was observed for F2 + AG (13.07%). FS was found to improve when blended with cowpea and black-gram flour. However, FS was more common in F2-AG (88.55%), followed by F2 + AG (80.58%). The least amount of FS was observed for F1-AG (60.63%). The same pattern was found for formulations with and without acacia gum.
The highest EA was observed for F1 + AG (42.05%), followed by F2 + AG (40.58%). The emulsion stability (ES) of the composite flours varied from 12.91 to 24.00%.
Rigid globular protein molecules are highly resistant to mechanical deformation. Cohesive films are formed by the absorption of these globular proteins. This in turn increases the emulsion stability (Graham and Phillips, 1980). All the composite flours exhibited relatively good emulsion activity. The EA and ES of the flours are shown in Fig. 4. The emulsion activity (EA) of the different composite flours ranged between 29.77 and 42.05%. The maximum EA was observed for F1 + AG (42.05%). The emulsion stability (ES) of the different composite flours ranged from 41.89 to 59.18%. The highest ES was observed for flour F2 + AG (59.18%), followed by F3 + AG (50.18%), and the lowest was observed for F1 + AG (41.89%).
The least gelation concentration (LGC) is the lowest concentration of protein at which the gel retains its structure even in the inverted position. The difference in the gelling properties can be attributed to the differences in the constituent ratios of the pulse/legume flours, such as carbohydrates, proteins, and lipids. The interactions among the above constituents play a substantial role in determining their functional properties. The LGC data for the raw and composite flours are given in Table 10. The raw flour (rice and cowpea) samples exhibited 100% gelation at a concentration of 25%. However, black-gram and ragi flour showed 100% gelation at a concentration of 30%.
Table 10
Effect of flour concentration on the least gelation capacity
Conc. (%, w/v)
|
Samples
|
Rice
|
Cowpea
|
Black gram
|
Ragi
|
F1-AG
|
F2-AG
|
F3-AG
|
F1-AG
|
F3 + AG
|
F3 + AG
|
2
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
5
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
10
|
+
|
+
|
+
|
+
|
+
|
+
|
+
|
+
|
+
|
+
|
15
|
++
|
+
|
+
|
+
|
+
|
+
|
+
|
+
|
+
|
+
|
20
|
++
|
+
|
+
|
+
|
+
|
+
|
++
|
++
|
++
|
++
|
25
|
+++
|
+++
|
++
|
++
|
+++
|
+++
|
+++
|
+++
|
+++
|
+++
|
30
|
+++
|
+++
|
+++
|
+++
|
+++
|
+++
|
+++
|
+++
|
+++
|
+++
|
Each value represents the mean of three determinations.
-= no gelation, += 50% gelation, ++ = 75% gelation, +++ = complete gelation.
F1-A = formulation 1 without gum; F2-A = formulation 2 without gum; F3-A = formulation 3 without gum; F1 + A = formulation 1 with gum; F2 + A = formulation 2 with gum; F3 + A = formulation 1 with gum.
The composite flours exhibited 100% gelation at 25% and 30% concentrations of flour. The composite flours formed a gel at a significantly lower concentration (25%) or a higher concentration (30%).
3.3. Bioactive compounds and their antioxidant activity
Whole grains are rich sources of phenolic acids. These phenolic acids have antimicrobial, anticancer, antioxidant and anti-inflammatory potential. Phenolic acids exhibit antioxidant properties due to the presence of an aromatic phenolic ring (Rice-Evans et al., 1996). Polyphenols are involved in defense mechanisms against biotic and abiotic stresses.
Phenolic compounds were extracted from the individual and composite flours using a previously described method (Chethan and Malleshi, 2007). The bioactive components, such as polyphenols, flavonoids, and proanthocyanidins, were extracted from the flours with methanolic HCl, and the antioxidant activity was assessed.
Table 11
Phenolic indices and biological activity of individual and composite flours
Samples
|
Total phenolic content (TPC, GAE/100 g)
|
Total flavonoid content
(TFC)
|
Proanthocyanidin content (PAC)
|
DPPH (%)
|
Rice
|
4.07 ± 0.23
|
0.76 ± 0.05
|
0.47 ± 0.09
|
81.74 ± 0.50
|
Ragi
|
47.69 ± 1.68
|
23.66 ± 0.60
|
5.49 ± 0.19
|
82.83 ± 0.61
|
Black gram
|
9.14 ± 0.54
|
1.43 ± 0.16
|
1.25 ± 0.10
|
77.96 ± 1.21
|
Cowpea
|
6.52 ± 0.59
|
2.08 ± 0.17
|
2.18 ± 0.19
|
76.48 ± 1.05
|
F1 + AG
|
28.43 ± 0.41
|
13.72 ± 0.47
|
6.70 ± 0.23
|
72.99 ± 1.41
|
F2 + AG
|
24.16 ± 0.29
|
10.70 ± 1.15
|
5.12 ± 0.18
|
73.09 ± 0.34
|
F3 + AG
|
17.77 ± 1.08
|
7.50 ± 0.28
|
6.04 ± 0.21
|
69.31 ± 1.69
|
F1-AG
|
27.61 ± 2.10
|
9.62 ± 0.37
|
3.22 ± 0.37
|
81.77 ± 0.68
|
F2-AG
|
23.61 ± 0.81
|
10.43 ± 0.70
|
2.83 ± 0.24
|
72.09 ± 2.58
|
F3-AG
|
18.07 ± 1.71
|
6.87 ± 0.43
|
2.69 ± 0.21
|
74.41 ± 0.79
|
The TPC, TFC and PAC of the individual and composite flour samples are shown in Table 11. Ragi flour possessed the maximum TPC (47.69 ± 1.68), followed by black-gram (9.14 ± 0.54) and cowpea (6.52 ± 0.59) flours. TFC and PAC were also found to be more abundant in ragi flour. This is because the anthocyanin contents in colored seed coats are very high (Sreerama et al., 2012). It was reported that grains with a dark-colored pigment and pericarp had higher soluble phenolic fractions than those with a light color (Chandrasekara and Shahidi, 2010). In this study, a ragi and husk were used for flour preparation. In contrast, the soluble phenolic extracts of the finger millet in this study had greater TPC and TFC than did those of the other grain flours.
Significant variations were noted in the phenolic indices of flour subjected to composite flour preparation. The formulation with acacia gum had a greater PAC than that without acacia gum. The PAC values for F1 + AG and F3 + AG were greater than that for F2 + AG. In formulations without acacia gum, the PAC decreased (range 2.69 ± 0.21 to 3.22 ± 0.37).
The TPC, TFC and PAC in the composite flours were significantly different from those in the control flours. However, there was no significant difference observed with the addition of acacia gum to the composite flour. However, a decreasing trend is observed. This may be due to a decrease in the content of ragi flour from F1 to F3. TPC and TFC were greater in F1 + AG, at 28.43–17.77 mg/g and 13.72–7.50 mg/g, respectively. Similar observations were made in the absence of acacia gum. PAC was greater in the composite flour (6.40–6.70 mg/g) than in the individual flours (0.47–5.49 mg/g).
The antioxidant activities of the phenolic extracts obtained from the raw and composite flour samples were studied for their radical scavenging capacities. Natural antioxidants help increase the shelf life of food products without affecting nutritional or sensory qualities (Rajalakshmi and Narasimhan, 1996). The DPPH radical scavenging activities of the raw flour and composite flour extracts are presented in Table 11. The DPPH radical scavenging activities of the raw flours (76.48 ± 1.05 to 82.83 ± 0.61%) were slightly greater than those of their composite flours (69.31 ± 1.69 to 81.77 ± 0.68%), but the differences were not significant.
The addition of acacia gum improved the functional properties of the final product. The addition of gums results in increased dietary fiber and decreased caloric value by diluting the moisture content (Rodge et al., 2012).