3.1. Proximate Composition and Physicochemical Properties
Varying the particle size affected some physicochemical and proximate properties of the fava bean flour samples (
Table 1). The moisture content among the three particle sizes was significantly different (
p < 0.05), with the value increasing as the particle size increased. This trend could be due to the increased porosity in coarse particles compared to the finer ones. This increased porosity can provide more space for moisture to accumulate within the particles, resulting in higher overall moisture content. The results also showed that fava beans had a very high amount of protein (greater than 30%) and a low amount of fat (less than 2%) for all the particle sizes. These values are similar to the 30.4% and ~1% values reported for fava bean flour [
1]. Similar trends were observed for the protein (%), starch damage (%, db.), and fat (%) contents of the flour. The smallest particle size had a higher value for these properties than the coarse size. For the fat content, varying the flour particle size did not result in appreciable differences (
p > 0.05) among the samples. The flour with the largest particle size had significantly lower starch damage (%, db.) than the smaller ones. Different researchers have reported that pulse flours ground into fine particles exhibited more starch damage than coarse samples [
17,
33]. This could be attributed to a higher mechanical force needed to break the particles to go through the smaller screen [
1,
38]. Variations in the particle size did not produce a significant effect (
p > 0.05) in the amount of total starch in the flour samples.
For the color characteristics, except the
L* value for the 1.00 mm sample that was significantly different (
p < 0.05) from the others, all the other color properties were unaffected by varying the fava bean flour particle size. Generally, varying the fava bean flour particle size had less impact on its physicochemical and proximate properties. Compared to other commonly consumed pulses like peas and lentils, fava beans have a substantially lower amount of starch and a higher amount of protein [
1,
19]. This result further demonstrates that fava beans are a superior and promising source of plant protein that can address the increasing demands for protein-rich ingredients in the industry.
3.2. Pasting Properties
The implication of fava bean flour particle size variation on pasting properties is presented in
Table 2. The pasting property offers insights into the viscosity and the degree of retrogradation of the flour, which are important parameters that affect functionality. From the table, it can be deduced that there was a significant difference (
p < 0.05) in the hot paste viscosity, peak viscosity, and final viscosity (cP) of the flour, with the flour with a 0.5 mm particle size having the highest value and the 1.00 mm flour having the least. The peak paste viscosity shows the capability of the flour to swell quickly before breaking down [
39]. The results showed that the 0.5 mm flour particle size performed the best for these parameters, while the 1.0 mm flour performed the worst. The improved dispersibility of the fine flour particles could cause this. The increased surface area in fine particles allows for faster heat transfer, leading to faster and more even cooking. Gu et al. (2021) also reported that the 0.5 mm screen aperture yielded the best viscosity values for yellow pea flour in comparison to the 0.25 mm and 1.00 mm particle sizes [
38].
For the breakdown, fewer variations were observed based on the particle size. This implies that the flours have comparable stability after hydration in water. However, the lower particle sizes (0.14 and 0.5 mm) would have been expected to have higher stability due to the higher starch damage than the 1.00 mm screen aperture. There were no significant differences (p > 0.05) in the peak time and pasting temperature values for the three particle sizes.
For the setback value (an indicator of temporary retrogradation), the smaller particle sizes (i.e., 0.14 and 0.5 mm) exhibited comparable behavior, which was significantly greater (p ˂ 0.05) than the 1.00 mm particle size. From this result, it can be concluded that the fava bean flour with a 0.5 mm particle size had the best pasting properties. The improved pasting properties of this particle size could be credited to the increased flour surface area for heat transfer and increased dispersibility of the particles. An important area to explore is understanding the effects of the nexus between the starch and protein content and the amylose–amylopectin ratio on the pulse flours’ pasting characteristics. This knowledge would help product developers adjust the flour’s composition to the desired levels to achieve baked products with the desired texture, structure, and crumb characteristics.
3.4. Functional Properties
The capability of flour to retain oil and water is an essential characteristic in different baked goods, such as bread and meat formulations, as it influences the structure, texture, and mouthfeel of the end products [
18,
34].
Table 4 shows the fava bean flour’s water absorption capacity (WAC) as affected by varying particle size. The WAC of the 0.14 mm particle flour was significantly lower (
p ˂ 0.05) in comparison to the 1.0 mm particle size. The lower WAC value of the smaller particle sizes could result from the further degradation of the starch granules due to the extended milling time. This decrease in the WAC can also be partly due to the reduced dietary fibers in the finer particle size flours [
18]. This trend in the WAC of the flour agrees with the results reported by Bourré et al. (2019), who found a decrease in the WAC of yellow pea, navy pea, and red lentil flour milled using a Ferkar mill with decreasing particle size [
17]. There was also a corresponding decrease in the WAC of pea, lentil, barley, and oat flours as the particle sizes decreased [
18]. A more recent study showed that pea (yellow and green) flours milled using smaller screen size apertures had a lower WAC than the bigger particle size flours [
41]. The result is, however, contrary to the general anticipation that the flour samples with finer particle sizes, having a larger surface area and more starch damage, would absorb more water than flours with coarser particle sizes. This further confirms that the WAC of flour depends on multiple factors that include the total starch, starch damage, protein and fiber content, and particle size. Thus, varying the particle size alone may not influence the WAC of flours [
17,
20,
33].
The oil absorption capacity (OAC) (
Table 4) spanned from 3.66 to 3.72 g/g of flour. The result of the OAC showed that the flour samples were not significantly different (
p > 0.05) based on the particle size variations. Similar results have been reported that varying the particle sizes of navy bean, yellow pea, and red lentil flours milled with a Ferkar mill did not significantly affect its OAC [
17]. In addition, varying the particle size of pea, lentil, barley, and oat flours did not affect their OAC [
18]. Also, varying the particle size of pea (yellow and green) flours did not affect its OAC property [
41]. However, some studies reported contrary results. Of note is a study that showed that Indian and Turkish lentil flours that have the finest particle size possess the lowest OAC value [
20]. Another study has also reported that green lentils and yellow pea flours with larger particle sizes milled with a Ferkar mill had a higher OAC than those milled with a roller mill [
42]. They attributed the high OAC value in the flours to the elevated protein levels in the flour, as the hydrophobic proteins possess a better cohesive strength to lipids [
19,
20,
43].
The least gelling capacity (LGC) of the flour is the lowest concentration needed for the flour to form a successful non-sliding gel as a result of gravity when turned upside-down in a tube. Hence, flour with the lowest LGC has the highest gelling ability [
34]. From
Table 4, the gelling capacity of the finer particle sizes (i.e., 0.14 and 0.5 mm) was 14%, while that of the coarse particle size was 20%. The result implies that increasing the fava bean flour particle size reduces its gel-forming capacity. The improved gel-forming capacity of the finer flours could be associated with a higher surface area, resulting in better hydration rates. It has also been found that increasing pea, lentil, and fava bean flour particle sizes decreased their gel-forming capacity [
1].
There was a gradual increase in the bulk density of the fava bean flour with a corresponding increase in the particle size. Also, the largest particle size possessed a significantly higher (
p < 0.05) bulk density than the finer particle sizes. Factors influencing the bulk density include the particle size, moisture content, and other chemical constituents, as well as the method of determination. Also, the coarse particle size has a higher porosity value, contributing to the higher bulk density [
44].
There was an insignificant difference (
p > 0.05) in the swelling power of the flour based on the variation in the particle size. For the WSI, the finer particle size flour (0.14 and 0.50 mm) had similar WSIs, and the coarse particle had a significantly lower (
p < 0.05) value. This difference in the WSI can be associated with the greater starch damage in the finer particle sizes (1.3%, db.) compared to the coarse particle size (1.0%, dry basis). The WSI provides knowledge about the strengths of the bonds in the starch granules and is a crucial way to predict the behavior of the flour during cooking [
20,
35].
The ability of the flour to produce a continuous phase with vegetable oil is known as the emulsion capacity, while emulsion stability describes how stable the formed emulsions were after 30 min. In this study, the emulsion capacity and stability of the flour were examined at different pH levels. Studies have shown that the functionality of proteins is a function of both intrinsic (structure and molecular size) and extrinsic factors (i.e., its interactions with the processing conditions, including pH, thermal treatment, and ionic strength). Therefore, understanding the behavior of the flour under different pH conditions is essential for product manufacturers, especially those producing low-acid foods like salad dressings and mayonnaise [
45].
Figure 2a,b show the emulsion capacity and stability of the flour at 20 mg/mL and different pH values.
Figure 2a shows that the variations in the pH level affected the average oil droplet size (
d3,2) of the flour, as the flour behaved differently under different pH conditions. At pH 3, the 1.00 mm particle size flour had the largest oil droplet size, while the smaller particle size flours had similar but lower oil droplet sizes. At pH 5, there was a significant increase (
p < 0.05) in the oil droplet size with the corresponding increase in the flour particle size. At pH 7 and 9, the flour had a similar oil droplet size, with only the 1.00 mm particle size flour having a significantly higher (
p < 0.05) oil droplet size. Overall, under all the pH conditions examined, the 1.00 mm particle size flour had a significantly higher (
p < 0.05) oil droplet size, while the medium and fine flours had smaller and similar oil droplet sizes. The smaller oil droplet sizes with the smaller particle sizes imply they have a better emulsion-forming capacity. This could be due to their greater interaction with oil droplets as a result of their higher surface hydrophobicity [
34]. However, it is possible that the improved surface area of the flours with smaller particle sizes was also a contributing factor by providing a larger binding area for the oil droplets in contrast to the coarse flour with a smaller surface area. This study confirms that modifying the particle size of fava bean flour has an effect on its emulsion capacity and its application in different food formulations.
The stability of the emulsion after allowing the emulsion to rest for 30 min undisturbed is shown in
Figure 2b. At pH 3, the 0.14 mm and 0.5 mm flour exhibited a significantly reduced (
p < 0.05) emulsion stability compared to the 1.00 mm particle size. At a pH 5, the medium particle size flour possessed a significantly higher (
p < 0.05) stability than the fine and coarse particle sizes. However, at pH 7, the medium and fine particle size flours were more stable (
p < 0.05) than the coarse particle size. At pH 9, the flours were stable in a similar pattern. The results suggest that varying the fava bean flour particle size affects emulsion stability. Overall, the fine and medium particle size flours tend to produce more stable emulsions, irrespective of the pH, which may be due to the smaller oil droplet sizes. Also, the smaller particle size flours had a more stable emulsion because of the increased surface area for protein adsorption, and the increased protein content in these smaller particle sizes ensures the stability of the emulsion interface [
46,
47,
48].
The foaming capacity describes the volume increase of a sample after air is incorporated, while the foaming stability assesses the ability of the flour to retain air and prevent bubble collapse after 30 min. These properties are important in food products where aeration and overflow are necessary, such as cakes and ice cream, to enhance the texture and visual appeal of the products [
34].
Figure 3 shows the foaming capacity of the flour samples at different concentrations and pH. At pH 3, the foaming capacity increased for each particle size as the flour concentration increased; a similar trend was observed at pH 5. However, at pH 7, a decline in the foaming capacity was observed with increasing flour concentration. At pH 9, lower foaming capacities were observed for the smaller particle size flour than the coarse particle size. Thus, the flour produces more foam in an acidic-to-neutral medium than in a basic medium. This could be caused by increased surface tension in the basic medium, making it more challenging for air bubbles to form and stabilize. Overall, increasing the flour concentration tends to significantly affect the foaming capacity of the flour than the variation in the particle size. These could be caused by increased viscosity, particle aggregation, and protein content of the solution as the flour concentration increases [
49]. The foaming capacity of flour depends largely on the solubility of its proteins [
34,
50]. As shown in
Figure 4, all the emulsions were mostly stable, irrespective of the variations in the particle size and flour concentration. The foam’s stability over an extended period is a desirable characteristic, especially when used as a whipping agent in food [
44,
51].
3.5. In Vitro Protein and Starch Digestion
Table 5 depicts the impacts of varying the particle size on the starch and protein digestibility properties of fava bean flour. It was found that increasing the particle size had an inverse relationship with the
in vitro protein digestion of the flour. Though the fine particle size flour had the highest
in vitro protein digestion, it was insignificantly different (
p > 0.05) from other particle sizes. Tinus et al. (2012) also reported that the milling methods and particle size affect the digestibility of cowpea flour [
52]. Byars et al. (2021) also reported an increase in navy bean flour’s
in vitro protein digestion with decreased flour particle size [
5].
For starch digestion, it was observed that varying the flour’s particle size produced a significant effect (
p ˂ 0.05) on the starch digestibility property. The smaller particle size flour exhibited higher starch digestion, while the coarse particle size had more resistant starch. The starch digestibility of the fava bean flour is in the range of the values observed for different cultivars of lentil, pea, and chickpea flours. The improved digestibility of the finer particle sizes could be attributed to the increased starch damage [
53]. Overall, the flour particle sizes impact
in vitro protein and starch digestibility. Flours with finer particle sizes have a larger surface area, with the ability to produce an increased enzymatic action and improved digestibility. Coarser particles, in contrast, have reduced surface area, potentially leading to slower digestion. In addition, finer flour particles can absorb water more quickly, promoting starch gelatinization during cooking. The gelatinized starch is more readily broken down by enzymes, leading to speedy glucose release and improved digestibility. On the other hand, coarser flour particles may not fully gelatinize during cooking, causing slower starch digestion and a slower release of glucose. This can be beneficial for controlling blood sugar levels but might reduce overall nutrient absorption.
In summary, finer flour particles generally lead to an increased rate of nutrient digestion bioavailability but may also cause quicker spikes in blood glucose levels. Coarser flour particles can produce slower digestion rates, providing more sustained energy release and potentially benefiting gut health, but may reduce the immediate bioavailability of some nutrients. The particle size of flour influences the rate and extent of enzymatic reactions that break down proteins and starches during digestion [
42,
52,
54,
55,
56].
3.6. In Vitro Protein Digestion of Fava Bean-Fortified Bread
Table 6 shows the effect of the fava bean flour substitution at varying particle sizes and levels of substitution on bread’s
in vitro protein digestibility. Variations in the particle size and levels of substitution had less effects on the protein digestion of the bread. The complex processes involved in baking, including mixing, kneading, and baking (and the chemical processes involved), profoundly influence the bread’s digestibility [
57]. Hence, merely varying the particle size of the pulse flour might not produce a pronounced effect on protein digestion. The values of the protein digestion of the bread were similar to the result (~84%) reported for bread with different formulations [
58]. Overall, baking improves the protein digestibility of the bread compared to raw pulse flour. Different processing techniques, including baking, have been reported to positively affect the protein digestion of food products [
59]. The baking process breaks down complex proteins into more digestible forms through denaturation, gelatinization, and other biochemical reactions. These processes help to improve the bioavailability of proteins by making them more easily digestible in the human digestive system.
In order to enhance the nutritional profiles of baked products using fava bean flour, it is imperative to study the effects of varying its particle sizes on the physicochemical, proximate, and functional properties. A good comprehension of the implication of varying particle size on the end product quality characteristics will allow product developers to make informed decisions on the flour particle size to achieve the desired end product quality [
28]. Different studies have confirmed that varying the particle size of pulse flour affects the end product quality features. For example, it has been reported that varying the particle sizes impacted the textural properties of cookies produced from black soybean-fortified flour. They reported an increase in the hardness of the cookies as the flour particle size decreased. However, the fracturability, chewiness, and gluing properties of the cookies decreased with the decrease in particle size [
60]. These textural features have an enormous impact on the sensory characteristics of the cookies. The crispiness, an important sensory feature, is negatively correlated with its fracturability, while the chewiness affects the taste and stickiness of the cookies [
60,
61]. It has also been reported that varying the particle size affected the color and specific volume of soy flour-fortified sponge cake. Thus, producing pulse flour-fortified baked products with the desired textural, nutritional, sensory, and handling characteristics demands a careful selection of flour particle sizes [
21].