Effects of particle size on physicochemical and functional properties of superfine black kidney bean (Phaseolus vulgaris L.) powder

Introduction

As an essential crop, kidney beans (Phaseolus vulgaris L.) are particularly popular in Africa, Latin American, and Asia (Beninger & Hosfield, 2003), and consumed as a human food source throughout the world representing 50% of the grain legumes (Camara, Urrea & Schlegel, 2013). Potential benefits to human health have been explored during the kidney beans consumption, including lowering postprandial glucose and insulin responses, preventing obesity, reducing the risk of cardiovascular diseases and preventing cancers because of the high contents in protein, fiber, vitamin B, and chemically diverse micronutrient compositions (Ai et al., 2016; Anton, Fulcher & Arntfield, 2009; Mitchell et al., 2009). Furthermore, the phenols are rich in kidney beans, which will lead to greater anti-oxidative activity and will be beneficial for preventing oxidative damages (Camara, Urrea & Schlegel, 2013; Lee & Yoon, 2015).

Recently, due to the nutritional and economical values, kidney bean powders have been employed as a food ingredient in the manufacture of value-added products (Anton, Fulcher & Arntfield, 2009; Malav et al., 2016); however, the properties of black kidney bean powder (BKBP) have not been well documented to date. As a necessary process, controlling the particle size in the grinding process is of importance, as it will influence powder behaviors during storage, handling, and processing (Lee & Yoon, 2015). Powder with a larger particle size might be unwieldy for extraction and leaching, and would lengthen heat treatment for blanching and/or cooking (Barbosa-Canovas et al., 2005). In addition, various changes in powder color, texture, and bioactive compounds as well as taste acceptability would be also dependent on variations of particle size (Lee & Yoon, 2013; Liu et al., 2000; Zhu et al., 2010). Thus, searching for appropriate particle size for BKBP would be necessary to improve the application in nutraceuticals and functional food products, as well as a potential and novel biomaterial.

As a useful and novel process technology, superfine grinding has been widely applied in the ceramic, electric material, and chemical powder product developments due to the well contributions to surface effect, mini-size effect, mechanical property, and chemical and/or catalytic properties (Zhao et al., 2009b). Currently, because of the increasing processing requirements, superfine grinding methods, such as ball milling, jet milling, and high-pressure expansion, have begun to be applied in the food industry. Tan et al. (2015) reported that waxy and high-amylose corn starches had a lower viscosity and better pasting stability via planetary ball-milling. Phat et al. (2015) confirmed that Hericium erinaceum powder prepared via jet milling had good water solubility and swelling capacity, which would be suitable to manufacture instant and convenient foods. In the previous study, superfine okra powders exhibited a higher cholesterol adsorption capacity compared with the coarse (Chen et al., 2015b), indicating that the superfine powder might be a potential bio-absorbent for low-cholesterol food developments. It has also been reported that Astragalus membranaceus, wheat (Triticum aestivm L.) bran, and okra (Abelmoschus esculentus) superfine powders have an increased flowability (Chen et al., 2015b; He et al., 2018; Zhao et al., 2010). Several studies found that superfine grinding treatment could enhance the antioxidant activities of some food powders, such as red rice (Oryza sativa L.), Qingke (hull-less barley), and winter wheat (Triticum aestivm L.) bran superfine powders (Chen et al., 2015a; He et al., 2018; Zhu, Du & Xu, 2015) through altering the molecular weight and solution behavior of bioactive components. However, Anguita et al. (2006) reported that a reduction in the particle size of barley after grinding was accompanied by a decrease of water retention capacity (WRC). Choi et al. (2018) and Liu et al. (2015) found that increasing degrees of milling significantly reduced phenolics in rice flours. It has also reported that the antioxidant ability of wheat bran powder was coarse > medium > fine size (Brewer et al., 2014). Then, the reverse phenomena indicated the physicochemical properties seemed to be unpredictable, and would be related to particle size reduction, various grinding treatments, and raw materials. Thus, the effects of superfine grinding treatment on the physicochemical and functional properties of BKBP should be explored due to the little information.

The newly-designed eccentric vibratory mill is remarkable nowadays, exhibiting a decisive intensification of the impact force among the grinding rollers for improved effectivity (Gock & Kurrer, 1999). In addition, the power consumption of eccentric vibratory milling is significantly decreased (up to 50% compared to conventional vibratory tube mills), due to the decrease of the ratio between kerb mass and payload, as well as the rational bearing load (Beenken, Gock & Kurrer, 1996), then it is increasingly used for fine grinding and pulverization of raw materials on an industrial scale (Baláž & Dutková, 2009; Godočíková et al., 2006). Thus, the BKBP was developed via eccentric vibratory milling in this study, and the effects of particle size on physicochemical, microstructural, cholesterol adsorption, and antioxidant properties of the resulting powders were investigated. The results are favorable for the development of value-added products using the BKBP.

Materials and Methods

Results

Particle properties and microphotographs of BKBP

The particle size distribution and specific surface area of BKBP were presented in Table 1. With particle size decreasing, all cumulative undersize centiles (D₁₀, D₅₀, and D₉₀) of BKBP significantly (P < 0.05) decreased. D_[4,3] values of the powder decreased from 226.658 to 24.835 μm for a particle size ranging from 250–180 to <38 μm (Table 1). Furthermore, the specific surface area increased with the decrease of particle size, and the BKBP with the smallest particle size (<38 μm) showed the highest specific surface area of 0.520 m²/g, suggesting that the surface parameter of BKBP was negatively related to the projected size of the corresponding particle.

Table 1:

Particle size distributions and specific surface areas of the BKBP obtained from the laser diffraction method.

Powder particles (μm)	Equivalent diameter particles accounted for by measuring the proportion (μm)				Specific surface area (m2/g)
	D10	D50	D90	D(4,3)
250–180	54.366 ± 2.382a	257.167 ± 5.252a	500.742 ± 5.667a	226.658 ± 3.875a	0.125 ± 0.010d
180–125	24.460 ± 2.029b	217.081 ± 3.820b	362.479 ± 6.836b	214.801 ± 7.033b	0.133 ± 0.009d
125–75	25.671 ± 0.337b	140.998 ± 3.799c	255.877 ± 2.529c	146.407 ± 5.660c	0.176 ± 0.017c
75–38	9.694 ± 1.424c	36.594 ± 1.328d	86.295 ± 5.650d	45.962 ± 3.114d	0.297 ± 0.015b
<38	4.810 ± 0.533d	20.706 ± 2.025e	51.331 ± 4.988e	24.835 ± 5.494e	0.520 ± 0.026a

DOI: 10.7717/peerj.6369/table-1

Note:

The results were expressed as mean ± standard deviation. Data in the same column with different letters were significantly different (P < 0.05).

The shape and surface morphology of BKBP were observed using SEM (Fig. 1). As the particles size decreased, it was possible to see the transition of typical blocky shape (coarse powder) into short ones (Fig. 1C, 125–75 μm), until very small parts and fragments were achieved (Figs. 1D and 1E, <38 μm). From Figs. 1B–1E, with the improvement of mechanical force, the transformation of BKBP from an ordered structure to a disordered structure was clearly presented via the breakage of intermolecular bonds as well as the reduction of particle size. It was notable that Figs. 1D and 1E exhibited an increased aggregation of BKBP, due to the various shapes of black kidney bean particles resulted from the extensive milling combination of flattening, aggregation and fracture. Under the higher magnification (Figs. 1F–1J), it could be clearly seen that the particles surface tends to be flat and smooth with the size decreasing.

Hydration property

The hydration property of BKBP was determined by WHC and WRC assays. With the reduction of particle sizes from 250–180 to <38 μm, the WHC values of BKBP increased, ranging from 5.98 to 6.26 g/g, 6.03 to 6.87 g/g, 6.18 to 7.41 g/g, 6.28 to 7.86 g/g, 6.31 to 7.81 g/g, and 6.44 to 8.03 g/g for 10, 20, 30, 40, 50, and 60 min of soaking (Fig. 2A), respectively. A similar tendency was also found in the WRC assay for the BKBP under the same soaking conditions (Fig. 2B). Thus, the hydration property of the BKBP with a particle size of <38 μm was higher. It was also worth mentioning that the WHC values of different sized BKBP increased slowly during the initial 10 min soaking.

Thermal property

The thermal property of BKBP with different sizes was further analyzed via DSC curves (Fig. 3). Compared to the endothermic peaks (T_m) observed in the curve of coarse powder, the peak around 97.07 °C disappeared in the analyses of superfine powder with sizes of 180–125, 125–75, 75–38, and <38 μm. Notably, an intense endothermic peak was found from 128.56 to 178.10 °C in all curves, and the peak temperatures exhibited a significant increasing tendency with the decreasing of particle size.

Cholesterol adsorption of BKBP

As shown in Fig. 4A, the adsorption capacity for cholesterol significantly increased with the reduction of particle size. The BKBP with a size of <38 μm showed the strongest adsorption capacity (27.27 mg/g); thus, it was chosen for further evaluation. The cholesterol adsorption capacity decreased dramatically from 26.95 to 15.51 mg/g with adsorbent dosage increasing (Fig. 4B). With the increase in initial concentration of cholesterol, the adsorption capacity increased (Fig. 4C). Furthermore, the cholesterol adsorption capacity for different adsorption time (min) and temperature (°C) was shown in Figs. 4D and 4E, respectively. The adsorption increased quickly with increasing time from 10 to 60 min, reaching a plateau in the following 60–150 min (Fig. 4D); nevertheless, the cholesterol adsorption capacity decreased when temperature increased (Fig. 4E).

Antioxidant activity analysis

The antioxidant activity of the BKBP with different particle sizes was evaluated by different in vitro assays (Fig. 5). Regarding radical scavenging activity using DPPH assay, the finer powders with particle sizes of 75–38 and <38 μm exhibited higher DPPH scavenging activities of 87.30% ± 1.77% and 89.40% ± 0.81%, respectively (Fig. 5A). As shown in Fig. 5B, the powder with particle size of 75–38 μm exhibited the strongest hydroxyl radical scavenging activity (88.92% ± 1.38%) among all tested samples, while the BKBP with the particle size of 250–180 μm, obtained via coarse grinding, showed the lowest activity (69.72% ± 2.49%) (Fig. 5B). Furthermore, an increase in ferrous ion-chelating effects was observed when the particle size decreased from 250–180 to 75–38 μm, and the BKBP with a size of 75–38 μm exhibited the strongest chelating activity of 81.16% ± 1.72% (Fig. 5C).

Discussion

Taking into account the nutritional and economical aspects of black kidney beans, fortifying varied bean powders appears to be promising for the production of health food (Lee, Hung & Chou, 2008). Conventional milling methods have generally been used in the pulverization and research of kidney bean (Anton, Fulcher & Arntfield, 2009; Malav et al., 2016). However, until now, systematic studies on superfine kidney bean powder are still limited.

In the present study, BKBP with sizes between 180 and <38 μm was prepared via eccentric vibratory mill. Elliptical, circular and linear vibrations could be generated via eccentric vibratory mill instead of homogeneous circular vibrations, which would increase the amplitude of the individual grinding media and increase the rotational speed of the grinding media filling (Gock & Kurrer, 1999). Consequently, as shown in the particle size and SEM analyses (Table 1; Fig. 1), BKBPs were efficiently broken into smaller fractions, and the shape and original structure of particles were changed to be smoother by the inhomogenous impact force. Therefore, physical–chemical properties of BKBP would be altered with the sieving of special size parameters (250–180, 180–125, 125–75, 75–38, and <38 μm), confirmed the importance of micronization equipment on the fluidity, dissolution, and surface activity of powders (Muttakin, Kim & Lee, 2015).

The color parameters of BKBP (Table 2) depicted their relations to particle size and morphology. The increase of L* values was as expected with the reduction of particle size, due to the increase in surface area, and that would allow more reflection of light (Ahmed et al., 2016). Meanwhile, the loss of pigment and the exposure of internal materials during superfine grinding also could contribute to the improvement of brightness. Thus, the BKBP with a size of <38 μm was brighter, which might be favorable for the applications as food ingredients. The decrease of bulk density and increase of tapped density of fine powders (Table 2) exhibited the enhancement of inter-particulate interactions, which indicated the improvement of flowability of the BKBP. Moreover, according to Table 2, the decreasing angle of repose and slide of superfine BKBP with smaller size also might indicate the increase of flowability (Zhao et al., 2010). But the result was not in agreement with the investigation of Lee & Yoon (2015), who found that the soybean powders with the smallest particle size (250–150 μm) showed a poor flowability because of the cohesion. However, Fu et al. (2012) reported that powder shape significantly affected the flow characteristics of the powder, and stated that more circular and smooth shaped particles had the higher flowability, which was consistent with our results for the morphology analysis (Fig. 1), and confirmed the efficiency of eccentric vibratory mill. Thus, the BKBP with a size of <38 μm had a larger number of particles per unit weight and achieved the higher flowability, which would be beneficial to fill tablets or capsule products to achieve homogeneity state when mixed with other additives.

Furthermore, the decrease of particle size has a substantial effect on the hydration properties of BKBP. Particles with a size of <38 μm exhibited the highest hydratability during soaking (Fig. 2), which was higher than previous data on soybean flours (4.1 g/g) (Heywood et al., 2002) and superfine wheat (Triticum aestivm L.) bran superfine powder (7.0 g/g) (He et al., 2018). Superfine grinding treatment might result in the surface properties changes of the BKBP, such as the increase of surface energy, greater surface area, and the exposure of hydrophilic groups, which led to an easy integration with water (Zhao et al., 2009a). Additionally, high content of protein (20–30%) within the BKBP could also held water through weak forces, such as hydrogen bonds (Shi et al., 2016). Similar results were also confirmed by Chen et al. (2015a) and Zhao et al. (2009a, 2010). In contrast, Raghavendra et al. (2006) found that the hydration properties of coconut dietary fiber were decreased when its particle size was decreased from 550 to 390 μm. It has been reported that grinding the dry fibrous material to fine powder adversely affected its WHC and swelling capacity, presumably attributed to the collapse of the fiber matrix by milling (Kethireddipalli et al., 2002). Hence, various physicochemical characteristic would be discovered because of the diversity of materials and grinding treatments. Eccentric vibratory milling treatment might result in the damage on BKBP structure, and the particle size would be too smaller to compensate differences on the hydration properties. High hydration capacities of BKBP would increase the affinity between the powder and water, and might keep more water in the inner part (He et al., 2018), which would lead to the enhancement of evaporation energy, and exhibited an improved thermal stability (Fig. 3). Therefore, BKBP with the smaller size (such as <38 μm) could be more suitable for water retention, and might thus be more potentially applied as functional ingredient to prevent syneresis and improve textural properties, as well as be utilized in the higher-temperature processing, such as baking or steaming.

It is well known that the surplus cholesterol in the human body forms an initial pathogenic factor of arteriosclerosis, resulting in apoplectic stroke, angina pectoris, and cardio sclerosis (Soh, Kim & Lee, 2003). Food material as biosorbent for cholesterol reducers/extractors is of growing interest, due to many advantages, such as natural, wide availability, healthy, and nontoxicity. Good cholesterol binding capacities have been found using the cereal brans, such as rice bran, oat bran, wheat bran, and corn bran (Kahlon & Chow, 2000). Adsorption properties of four legume seeds (green lentil, white small bean, yellow pea, and yellow soybean) have been evaluated by Górecka, Korczak & Flaczyk (2003), grinding degree was found to be significantly influenced the adsorption properties. In the present study, superfine BKBP was found to have a high cholesterol adsorption capacity by in vitro assays, which was probably correlated with their high contents of dietary fiber, especially hemicelluloses and lignin (Górecka, Korczak & Flaczyk, 2003), thus it could be recommended in the in lipid disorders prophylactic. Particle size, powder dosage, initial concentration of cholesterol, absorption time, and absorption temperature were all found to significantly affect the cholesterol adsorption of BKBP (Fig. 4). Decreasing particle size could effectively improve cholesterol adsorption capacity of BKBP due to the increase of specific surface area, thus lead to a larger contact area with cholesterol and shorter absorbing path distance (Chen et al., 2015b). It was interesting that the relative lower temperature would be favorable for the cholesterol adsorption, thus cholesterol adsorption process using BKBP should be controlled at below 18 °C for a 60 min reaction.

Furthermore, the maximal adsorption capacity (53.476 mg/g) of BKBP was successfully predicted by Langmuir adsorption isotherms analysis (Table 3), which was higher than the ability of okra superfine powder around 18.75 mg/g (Chen et al., 2015b), and carrot pomace insoluble dietary fiber around 30 mg/g (Yu et al., 2018), but lower than thyme (Thymus vulgaris L.) powder of 84.74 mg/g (Salehi et al., 2018). Besides, the value of 1/n (0.697) obtained from the Freundlich model was less than unity, indicating the favorability of the adsorption. The two fitting models suggested that BKBP would be effective as a potential adsorbent. Therefore, it seemed that the BKBP could be applied in the functional food manufactures, such as biscuits and other healthy products, to reduce calories and cholesterol without loss in physical and structural properties (Prokopov, 2014). It has been confirmed that the plant source of seed powder might have hypolipidemic effect on diabetic patients (Kassaian et al., 2009). Thus, superfine BKBP might act as a novel nutraceutical additive/excipient in tablets, such as simvastatin, to provide synergistic effects for lowering serum cholesterol level (Swami et al., 2010). Besides, it would be also interesting to employ the BKBP as the potential biosorption materials in the developments of low cholesterol milk or milk beverages (Oliveira et al., 2015), and even in the extracorporeal perfusion to immediately reduce the content of the lipids in the blood (Salehi et al., 2018).

In addition, multiple antioxidant assays including DPPH and hydroxyl radical scavenging activity, as well as ferrous ion-chelating effects, were carried out in the experiments, and particle sizes showed significant effects on the activities (Fig. 5). The capability of stable free radical 2,2-diphenyl-1-picrylhydrazyl to react with H-donors, including phenolics in natural materials, could be evaluated by the DPPH• test in the visible region after a fixed incubation time (Roginsky & Lissi, 2005; López-Alarcón & Denicola, 2013). In this study, higher DPPH• scavenging activity was obtained with the decrease of BKBP particle size (Fig. 5A). Meanwhile, the powder with a particle size of 75–38 μm exhibited the strongest hydroxyl radical scavenging activity in Fig. 5B. Hydroxyl radicals (•OH) are the most commonly formed reactive oxygen species and have been linked to many clinical disorders, such as brain ischemia, cardiovascular disease, and carcinogenesis (Hu, Chen & Ni, 2012). Several reports also indicated that the •OH scavenging effects could be related to hypoglycemic activity (Chen et al., 2009; Chen, Zhang & Xie, 2005). As shown in Fig. 5C, the ferrous ion (Fe²⁺)-chelating activity of the BKBP was favorably affected by the reduction of particle size, which would prevent the generation of free radicals, oxyradicals, and lipid peroxidation (Singh & Rajini, 2004).

According to the previous studies, polyphenols and flavonoids compounds were main antioxidant compounds presenting in kidney bean (P. vulgaris L.), containing free and bound forms (Cardador-Martínez, Loarca-Piña & Oomah, 2002; Malav et al., 2016). Meanwhile, as one kind of black coat bean, a high accumulation of anthocyanins relating to antioxidant activity would be found in the epidermis palisade layer, and was up to 13,955 (mg CGE/kg) (Žilić et al., 2013). Therefore, the increase of antioxidant availability in the BKBP with the smaller particle sizes (such as 75–38 and <38 μm) might be attributed to the fact that finer particles would be beneficial for the dissolution of free-form antioxidant compounds. In addition, superfine grinding broke the structure of protein and fiber matrix (as shown in SEM images), and thus increased the availability of bound-form antioxidant compounds linked or embedded in the matrix. However, as shown in Figs. 5B and 5C, compared with the sample size of 75–38 μm, the antioxidant activities of BKBP with a size of <38 μm have a slight decrease (P > 0.05), which might be attributed to the inevitable mechanical impact and heating effect during superfine grinding, leading to altering or disrupting of antioxidant compounds within BKBP. Therefore, controlling grinding degree is of importance, as it will influence powders’ functional properties, and superfine BKBP with a size of 75–38 μm exhibited a potential application as antioxidative products.

Conclusions

Fine BKBP with smooth surface was obtained using the eccentric vibratory milling, and the application potential of BKBP was improved with the decrease of particle size. The BKBP with a particle size of <38 μm exhibited good flowability, hydration properties, and thermal stability. Adsorption isotherm analysis highlighted the promising potential of the superfine BKBP with a particle size of <38 μm as a cholesterol sorbent or an alternative source to absorb harmful lipids. Moreover, compared with the other particle sizes, the superfine BKBP with sizes of <75 μm showed improved antioxidant activities in the free radical scavenging activities. Overall, the BKBP prepared by eccentric vibratory mill with a particle size of <38 μm showed great potentials in the food industry and pharmaceutical field for the health product developments. In the future, in vivo evaluations of the BKBP would be urgently carried out, and the BKBP produced using the eccentric vibratory mill should be further evaluated under various processing conditions to better understand the attributes of the grinding technology.

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