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Original Article

Special Issue: Plant Bioactive from the Palms and other Horticultural Crops: Evaluation of particle size on the physicochemical properties of Moringa oleifera Lam. stem powder

Yue Zhang1, Kehong Liang2, Jing Wang2, Aili Wang3, R. Pandiselvam4, Hong Zhu2*

1College of Engineering, China Agricultural University, Beijing, China;

2Institute of Food and Nutrition Development, Ministry of Agriculture and Rural Affairs, Beijing, China;

3School of Food and Biological Engineering, Key Laboratory of Coarse Cereal Processing (Ministry of Agriculture and Rural Affairs), Chengdu University, Chengdu, China;

4Division of Physiology, Biochemistry and Post-Harvest Technology, ICAR-Central Plantation Crops Research Institute, Kasaragod, India

Abstract

Moringa oleifera Lam. stem (MOS) has been used for beneficial dietary and medicinal purposes. In this work, MOS samples of six different particle sizes were produced using sieve-based mechanical grinding to investigate the impact of varying particle sizes on the physicochemical properties of MOS powder. Scanning electron microscopic images revealed the destroyed fiber structures after grinding. The color turned greener and less yellow with decreasing particle size. The angle of repose significantly decreased from 70.36º to 60.25º, as the particle size declined, demonstrating the increasing fluidity of granules. The applied mechanical treatment did not alter the primary conformational properties of MOS except for destructing the intramolecular hydrogen bonds of cellulose and hemicellulose, thereby decreasing the crystallinity and thermal stability. Surface element analysis demonstrated more carbon-rich extractives on the particle surface as the particle size reduced. This study provided reasons behind improved dissolution and bioavailability of functional ingredients in plant-based granular materials by reducing particle size.

Key words: crystallinity, cellulose, functional groups, hydrogen bonds

*Corresponding Author: Hong Zhu, Institute of Food and Nutrition Development, Ministry of Agriculture and Rural Affairs, No. 12 Zhongguancun South Street, Haidian District, Beijing 100081, China. Email: [email protected]

Received: 17 May 2022; Accepted: 23 June 2022; Published: 11 July 2022

Doi: http://dx.doi.org/10.15586/qas.v14iSP1.1123

© 2022 Codon Publications
This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International (CC BY-NC-SA 4.0). License (http://creativecommons.org/licenses/by-nc-sa/4.0/)

Introduction

Moringa oleifera Lam. belongs to the Moringaceae family with other names, including moringa, horseradish tree, and drumstick tree. This plant is commonly cultivated in many tropical and subtropical countries, such as India, Africa, Mexico, Southeast Asia, and southern Japan (Sugahara et al., 2018). Various parts of the M. Ooleifera, such as leaves, stems, roots, flowers, and seeds are rich in protein, dietary fiber, minerals, vitamins, and some bioactive compounds, and can be used as feed, herbal tea, nutritional supplements, food additives, and medicine for its dietary nutrition, functionality, and health benefits (Gharibzahedi et al., 2013; Yang et al., 2020; Astrini et al., 2020). Modern pharmacological research has revealed that M. oleifera possesses multiple nutraceutical or pharmacological activities, such as anti-inflammatory, antioxidant, anticancer, analgesic, hypoglycemic, and blood lipid-lowering functions due to the presence of multiple functional components, especially phenolic compounds, polysaccharides, and saponins (Sharma et al., 2022; Zhou et al., 2018).

A crucial step before using Moringa oleifera Lam. stem (MOS) is micronization using mechanical grinding. Many studies have shown that the reduced particle size was beneficial to increasing the particle-specific surface area and cell wall breakage ratio, thereby accelerating the dissolution of bioactive ingredients and improving bioavailability and bioactivity in vivo or in vitro (Xiao et al., 2017; Zhang & Zhai, 2020; Barretto et al., 2021; Balakrishnan et al., 2021; Qadri et al., 2022). Huang et al. (2020) found that the decrease in particle size significantly reduced the water and oil holding capacities while increasing the lightness and yellowness of M. oleifera leaf powders. Sun et al. (2019) confirmed significantly improved flow properties, hydration properties, thermal stability, antioxidant activity, and cholesterol adsorption efficiency by reducing the particle size of black kidney bean powders. Sieve-based mechanical grinding, a traditional method to reduce the particle size of various materials, has been widely reported for various materials, including tobacco (Zhang et al., 2022), green tea (Zaiter et al., 2016), and black kidney bean (Sun et al., 2019). Particle size significantly affects the physicochemical properties of granular materials. However, limited information is available on the particle size dependence of the physicochemical properties of MOS.

Therefore, this study’s objective was to investigate the effect of particle size on the physicochemical properties of MOS, including color parameters, flow properties, hydration properties, thermal properties, and structural properties.

Materials and Methods

Sample preparation

Dried MOS was provided by Dehong Senbao Technology Development Co., Ltd. (Yunnan, China). After being cut into 2 cm segments, the samples were coarsely ground by a high-speed crusher (As One Co., Ltd., Shanghai, China) and sieved sequentially through a series of standard sieves with a pore size of 0.380, 0.250, 0.180, 0.120, 0.096, and 0.075 mm, corresponding to 40, 60, 80, 120, 160, and 200 mesh, respectively, using an electric vibrating screen machine (8411, Shangyu Daoxu xingfeng instrument factory, Zhejiang, China). The MOS powders between 40 and 60 mesh, 60 and 80 mesh, 80 and 120 mesh, 120 and 160 mesh, and 160 and 200 mesh were dominated as M40, M60, M80, M120, and M160, respectively. The MOS powders passed through a 200-mesh sieve were named M200.

Particle size

The particle size of powders was measured using a Mastersizer 3000 laser diffraction particle analyzer (Malvern instrument Ltd., Worcestershire, UK) using a dry test unit. Particle parameters including D10, D50, D90, specific surface area (Asf), and span [(D90-D10)/D50] were obtained. D10, D50, and D90 represented the 10%, 50%, and 90% cumulative percentiles of total volume, respectively. Span characterizes the particle size distribution width, and a larger value indicates a more inhomogeneous distribution.

Scanning electron microscopy (SEM)

A scanning electron microscope (SU8000, Hitachi Co., Tokyo, Japan) was applied to obtain the surface microstructure of MOS powders. Samples were spread onto the conductive adhesive tapes in a single layer, and gold particles were coated under a vacuum. An acceleration voltage of 10 kV and magnifications of ×50 and ×300 were used to obtain the SEM images.

Bulk density and tap density

Powders (5.0 g, M1) were poured into a graduated cylinder with a total volume of 50 mL, and the volume occupied by the sample (V1, cm3) was read. Bulk density (ρbulk, g/cm3) can be calculated using Eq. (1). Tap density (ρtap, g/cm3) was determined using the modified Chinese National Standard methods (GB/T 21354-2008). The graduated cylinder with MOS powders was shaken on a thick sponge until an unchanged volume (V2, cm3). ρtap was calculated using Eq. (2).

ρbulk=M1V1 1
ρtap=M1V2 2

Angle of repose

The angle of repose (º) was tested following the modified method of Meng et al. (2017). A funnel was fixed vertically at 3 cm above the plane. The MOS powders were continuously poured into the funnel until the formed cone touched the funnel outlet. The angle of repose can be calculated as follows.

Angle of repose=arctanHR 3

where R (cm) and H (cm) were the radius and height of the formed cone, respectively.

Water and oil holding capacities

Water and oil holding capacities were determined using the modified method of Zhao et al. (2010). The MOS powders (0.5 g, m0) were poured into a centrifuge tube (m1, g), and 35 g of distilled water at 20ºC was added. After storing in a refrigerator (4ºC) for 24 h, the tubes were centrifuged at 11,180 × g for 10 min. The supernatant was discarded, and the total weight of the remaining sample and tube (m2, g) was recorded. Water holding capacity (WHC) can be calculated by Eq. (4).

WHC=m2m1m0m0 4

The MOS powders (W0, 0.5 g) and corn oil (10 mL) were mixed in a plastic centrifuge tube (W1, g). The tubes were kept at room temperature for 1 h and then centrifuged at 11,180 × g for 15 min. The tubes with MOS powders (W2, g) were reweighted after gently discarding the supernatant. Oil holding capacity (OHC) was obtained using Eq. (5).

OHC=W2W1W0W0 5

Color difference

Color parameters of MOS powders were quantitatively measured using a spectropolarimeter (Labscan XE, Hunterlab, USA). The CIE color parameter of L* represents lightness, while a* and b* are greenness/redness and blueness/yellowness, respectively. The color difference (ΔE) can be calculated following Eq. (6).

ΔE=L*L02+a*a02+b*b02 6

where L0, a0, and b0 were the color parameters of the M40 sample, and L*, a*, and b* were the color parameters of other MOS powders.

Fourier transform infrared (FTIR) spectroscopy

Samples were prepared using the pellet method by mixing potassium bromide (100 mg) and MOS powders (1 mg). An FTIR spectrometer (Nicolet iS5, Thermo Scientific, USA) was used for determination, and the spectrum was obtained over a wavenumber range of 4000-400 cm−1 with 32 scans at a resolution of 4 cm−1.

X-ray diffraction (XRD)

The XRD patterns of MOS powders with different particle sizes were measured by a wide-angle X-ray diffractometer (SmartLab, Rigaku, Japan) with Cu-Kα radiation (40 kV and 200 mA). Samples were loaded in a standard quartz sample holder and scanned from 5º to 45º (2 θ) at 0.02º step size and 2º/min speed. The MDI Jade 6 software (Materials Data Inc, Livermore, CA) was applied to calculate the crystallinity index (CrI) by the peak area method (Toba et al., 2013).

Thermogravimetric analysis (TGA)

TGA was carried out using a thermogravimetric analyzer (STA 449 F3 Jupiter, NETZSCH Instruments, Bavaria, Germany). Thermograms were recorded between 30 and 1000ºC at a heating rate of 10ºC/min and a 50 mL/min nitrogen flow rate. First derivatives of the thermograms (DTG), the weight loss percentage of each sample, and the related temperature parameters were analyzed using Universal Analysis Software (New Castle, Delaware, USA).

X-ray photoelectron spectroscopy (XPS)

The surface analysis of MOS powders was performed by an XPS system (ESCALAB Xi+, Thermo Fisher Scientific, UK) with a monochromatic Al Kα source (1486.6 eV, 14.4 kV, 13.6 mA) and a basic chamber pressure of 8 × 10−10 Pa. Each MOS sample was first analyzed with a survey scan using 100 eV pass energy at an energy step size of 1 eV. The pass energy of 30 eV’s energy step size of 0.1 eV was used for data acquisition. The spot size was 500 μm, and the surface analysis depth was ~10 nm. The chemical-bond analysis of the carbon was performed by deconvoluting and fitting the C1s peak area region using Avantage software (Thermo Fisher Scientific Inc.). The oxygen to carbon (O/C) atomic ratio was estimated by the following Eq. (7) (Hua et al., 1993).

O/C=I0/2.85Ic 7

where Io and Ic were the normalized integrated area of the O1s peak and C1s peak, respectively.

Statistical analysis

Data were presented as means of determinations ± standard deviation (n=3) and analyzed by SPSS 25.0 software (SPSS Inc., Chicago, IL, USA) to obtain the significance of the difference. Analysis of variance (ANOVA) with Duncan’s multiple range test was applied, and a 0.95 confidence level (P < 0.05) was considered significant.

Results and Discussion

Particle size analysis

The particle size parameters are presented in Table 1. The particle size range of M40, M60, M80, M120, M160, and M200 samples lay in 8.0-1100.0 μm, and their median particle size (D50) was 458.3, 329.5, 230.0, 139.7. 53.7, and 36.3 μm, respectively, with significant differences between each other (P < 0.05). Generally, M40, M60, M80, and M120 samples belonged to tissue scale powders (500-100 μm), while M160 and M200 were cellular scales (50-30 μm) (Barakat et al., 2015). These findings demonstrated that the plant scale could be reduced from organ to tissue and cell using sieve-based grinding. Fine powder (M200) owned a significantly (P < 0.05) higher span value than coarse powders (M40 and M60 samples). Similar observations were reported by He et al. (2019), who confirmed that more agglomerates appeared on the surface of ultrafine water dropwort powders. After grinding, the exposed polar groups may increase the electrostatic interaction of molecules, resulting in inhomogeneous powders and increasing span value (Xiao et al., 2017). Moreover, with the decrease in particle size, the Asf value significantly increased from 59.6 to 526.8 m2/kg. Liu et al. (2019) and Zhang et al. (2021) also reported a similar correlation between particle size and Asf. Decreasing particle size was accompanied by more particles per unit weight, owning a higher potential to achieve rapid dissolution and homogeneous mixing with the active pharmaceutical and food ingredients (Zhao et al., 2010; Li et al., 2020).

Table 1. Particle size parameters of Moringa oleifera stem powders.

Samples D10(μm) D50(μm) D90 (μm) Asf(m2/kg) Span
M40 214.7 ± 0.9e 458.3 ± 1.2f 1100.0 ± 10.0e 59.6 ± 0.2a 1.93 ± 0.02ab
M60 152.5 ± 0.5d 329.5 ± 0.5e 750.0 ± 2.0d 73.1 ± 0.1b 1.81 ± 0.01a
M80 77.8 ± 1.1c 230.0 ± 1.0d 600.5 ± 6.5c 92.6 ± 1.1c 2.27 ± 0.04bc
M120 24.8 ± 1.3b 139.7 ± 2.4c 337.3 ± 28.3b 130.0 ± 1.6d 2.23 ± 0.06bc
M160 10.6 ± 0.3a 53.7 ± 1.5b 126.0 ± 2.0a 346.6 ± 3.9e 2.16 ± 0.10abc
M200 8.0 ± 0.1a 36.3 ± 0.9a 105.7 ± 10.8a 526.8 ± 2.2f 2.46 ± 0.07c

Results were represented as mean values ± standard deviation of triplicate tests. Different letters superscripted on the results were significantly different at P< 0.05.

SEM micrographs

The SEM micrographs of MOS powders with different particle sizes are presented in Figure 1. Particle size was markedly decreased from M40 to M200. Strip-shaped structures were present in M40, M60, and M80 samples, and vascular bundles could be observed. The fiber structure was gradually destroyed by further reducing particle size and irregular shapes, especially strip and sheet structures. Granules were the smallest in the M200 sample, and original fiber structures were broken and destroyed entirely. It is worth noting that the porosity of the M200 sample was markedly reduced, and its particles had almost no internal pores. However, dimensional inhomogeneity was found in samples with smaller particle sizes due to the agglomeration of small granules, which correlated well with the span values.

Figure 1. Scanning electron microscopy micrographs of Moringa oleifera stem powders with different particle sizes (M40, ×50; M60, ×50; M80, ×50; M120, ×300; M160, ×300; M200, ×300).

Bulk density and tap density

Density, a fundamental physical parameter of powdered materials, is crucial to the industrial processing of food powders. Density parameters can help manufacturers design packaging container volumes for powder products (Gao et al., 2019). As shown in Table 2, the bulk and tap density decreased from 0.263 to 0.177 g/cm3 and 0.303 to 0.206 g/cm3, respectively, as the particle size decreased from M40 to M160. The low density of tiny particles might be that the inter-particle forces were comparable with the particle mass, avoiding the powders forming dense structures (Liu et al., 2017). However, the M200 sample owned higher bulk and tap density than M160, while still significantly lower than coarse MOS powders such as M40, M60, and M80. Decreasing intragranular closed pores and inter-particle voids in the M200 sample were responsible for this phenomenon (Meng et al., 2017). Overall, cracks, hollows, closed pores in the particles, and the thin air film between the particles affected the density of the plant granules (Gao et al., 2020).

Table 2. Density, angle of repose, water and oil holding capacities, and color parameters of Moringa oleifera stem powders.

Samples WHC
(g/g)
OHC
(g/g)
Bulk density
(g/cm3)
Tap density
(g/cm3)
Angle of repose (°) L* a* b* ΔE
M40 4.28 ± 0.02a 3.02 ± 0.02ab 0.263 ± 0.004e 0.303 ± 0.005d 70.36 ± 0.02c 71.02 ± 0.17b 1.58 ± 0.06f 27.67 ± 0.10d 0
M60 4.66 ± 0.12ab 3.02 ± 0.04ab 0.248 ± 0.002d 0.297 ± 0.002d 68.46 ± 0.71bc 70.43 ± 0.16a 1.32 ± 0.08e 26.98 ± 0.08c 0.96 ± 0.15a
M80 4.88 ± 0.12ab 3.44 ± 0.24b 0.229 ± 0.003c 0.285 ± 0.001c 67.77 ± 0.43b 70.31 ± 0.10a 0.73 ± 0.02d 27.50 ± 0.04cd 1.12 ± 0.08a
M120 5.69 ± 0.67b 3.98 ± 0.01c 0.182 ± 0.003a 0.225 ± 0.003b 66.29 ± 0.14b 71.34 ± 0.18b 0.26 ± 0.08c 27.21 ± 0.20bc 1.47 ± 0.16b
M160 5.29 ± 0.13ab 3.45 ± 0.17b 0.177 ± 0.002a 0.206 ± 0.002a 62.12 ± 1.31a 70.53 ± 0.04a -0.60 ± 0.01b 26.02 ± 0.01b 2.77 ± 0.01c
M200 4.25 ± 0.47a 2.71 ± 0.01a 0.204 ± 0.004b 0.225 ± 0.003b 60.25 ± 0.36a 70.45 ± 0.07a -0.99 ± 0.03a 25.65 ± 0.09a 3.31 ± 0.05d

Results were represented as mean values ± standard deviation of triplicate tests. Different letters superscripted on the results were significantly different at P< 0.05.

Angle of repose

Angle of repose was used to characterize the fluidity of granular materials, and a smaller angle indicated the better flowability of granules. Table 2 presents the angle of repose of MOS powders. It was significantly (P < 0.05) reduced from 70.36º to 60.25º with decreasing particle size, demonstrating that the MOS particles became flowable. Similar results were also reported for Quercus salicina (Blume) leaf powders (Hong et al., 2020) and hard white winter wheat (Triticum aestivm L.) bran powders (He et al., 2018). Therefore, the fluidity of the MOS powders could be significantly improved by sieve-based mechanical grinding. However, Huang et al. (2020) found a significantly increased angle of repose for M. oleifera leaf powders as the particle size was reduced. This may be due to the different chemical compositions of M. oleifera leaf and stem. Finer M. oleifera leaf powders tended to agglomerate and arranged in a cone due to the significantly higher protein and fat contents than MOS (Zhao et al., 2015b; Shih et al., 2011).

Water and oil holding capacities

As shown in Table 2, the WHC values increased first with the decrease in particle size from M40 to M120 and then decreased from M120 to M200. As the particle size decreased, the increasing capillary attraction and material porosity and exposed hydrophilic groups in hemicellulose and cellulose improved the hydration capacity of MOS (Meng et al., 2017). However, polysaccharide chains, which could hold water by forming hydrogen bonds, may be destroyed in finer MOS powders, negatively affecting the hydration properties (Gao et al., 2020). The OHC displayed a similar trend to WHC. The reduced particle size from M40 to M120 may also improve the capillarity absorption of the oil (He et al., 2018). In contrast, fine particles, especially in the M200 sample, owned high bulk density, namely, smaller inter particulate spaces, reducing the capacity to hold interstitial water and oil (Zhao et al., 2017). These results indicated that the decreasing particle size possessed the potential to integrate with water and absorb the dietary fat in the intestinal tract (Chen et al., 2015; Zhong et al., 2016), while the M200 sample negatively affected the functional properties of MOS.

Color analysis

Color is a crucial sensory parameter related to the consumer preference for food products developed from MOS. Color parameters of MOS powders with various particle sizes are listed in Table 2. Both a* and b* values significantly decreased as the particle size decreased, indicating that the powders turned to be greener and less yellow. The increase in the green spectrum may be related to the enhanced exposure to internal pigment compounds such as chlorophyll (He et al., 2019). Ramachandraiah and Chin (2016) found that decreasing particle size reduced persimmon peel in a* and b* values. Sun et al. (2019) also observed a decrease in b* value as the particle size of black kidney bean powders decreased from 250–180 μm to 125–75 μm. Decreasing particle size increased ΔE dramatically, and a maximum value of 3.31 was found for the M200 sample compared to the M40 sample. Perceivable color difference is divided into three categories: very distinct (ΔE > 3), distinct (1.5 < ΔE < 3), and small difference (ΔE < 1.5) (Adekunte et al., 2010). Based on these obtained results, sieve-based grinding displayed considerable influence on the color of MOS.

Functional group analysis

The FTIR is a useful tool for exploring organic materials’ chemical composition and conformation by characterizing chemical bonds and functional groups (Xu et al., 2018). The FTIR spectra of MOS powders are illustrated in Figure 2. Various absorption peaks related to chemical constitutes were observed. A broad band at 3500–3300 cm−1 represented the stretching vibration of O-H in polysaccharides and phenolic substances (He et al., 2019). The absorption peak at around 2920 cm−1 indicated stretching vibrations of C-H from proteins and polysaccharides in MOS (Zhao et al., 2017). Three bands at approximately 1738, 1650, and 1510 cm−1 were attributed to carbonyl groups’ bending or stretching vibrations (C=O), aromatic CH bonds, and C=C stretching bands, respectively (Zhao et al., 2015b; Ramachandraiah and Chin, 2016). The peaks centered at 1419 cm−1 and 1377 cm−1 indicated CH2 and CH symmetric bending for cellulose and lignin, and the absorption at 1319 cm−1 suggested the C–N stretching vibration (Zhao et al., 2013). The peaks at around 1244 and 1030 cm−1 were ascribed to stretching vibration of C–O groups, corresponding to the pyranose ring from polysaccharides (Zhao et al., 2015b; He et al., 2019). With decreasing particle size, no new functional group was found. However, the accurate position of some chemical bands was altered to a higher wavenumber, suggesting that the sieve-based mechanical grinding did not alter the conformational properties of MOS except for the destruction of the intramolecular hydrogen bonds. Similar observations have been reported for persimmon by-products powders (Ramachandraiah and Chin, 2016) and red grape pomace powders (Zhao et al., 2015b). The hydrogen bond cleavage could enhance the material’s external surface area and correspondingly improve the extraction yield of bioactive compounds (Hong et al., 2020).

Figure 2. Fourier transform infrared spectra of Moringa oleifera stem powders with different particle sizes.

XRD analysis

Figure 3 shows the XRD patterns of MOS powders with various particle sizes. Two crystalline peaks at around 16º and 22º, as well as a relatively weak peak at 35º, were observed, which were typical cellulose regions and corresponded to (101), (10—1‒), (002), and (040) lattice planes of cellulose, respectively (Ji et al., 2016; Park et al., 2010). A broad peak appeared around 16º because the 101 and 10—1‒ planes were close and overlapping (Zhao et al., 2017). As shown in Table 3, the CrI values were significantly (P < 0.05) reduced from 51.66% to 33.12% with decreasing particle size. The ordered structure of cellulose was destroyed, and other studies have also reported a similar tendency (Yang et al., 2014). Combined with FTIR findings, a fact can be generalized that the applied mechanical force disrupted the hydrogen bonds associated with the crystal structure of cellulose and hemicellulose, and new amorphous cellulose and soluble saccharides might be formed. However, some opposed results were reported that the reduced particle size increased the CrI values of corn stalk (Zhao et al., 2013), tobacco (Zhang et al., 2022), ginger (Zhao et al., 2015a), Dendrobium officinale (Meng et al., 2018), and M. oleifera leaf powders (Huang et al., 2020) because amorphous cellulose was more reactive. Some new crystals were reconstructed during processing (Zhao et al., 2013).

Figure 3. X-ray diffraction patterns of Moringa oleifera stem powders with different particle sizes.

Table 3. Weight loss due to water desorption (WL), temperature values corresponding to the onset of the decomposition process (Tei) and the maximum degradation rate (Tdeg), and crystallinity index (CrI) of Moringa oleifera stem powders.

Samples WL (wt%) Tei(ºC) Tdeg(ºC) Maximum deriv. weight (wt%/ºC) CrI (%)
M40 2.89 ± 0.00ab 278.40 ± 1.41dc 324.62 ± 0.40dc 0.94 ± 0.02dc 51.66 ± 2.48d
M60 2.68 ± 0.21a 276.16 ± 0.69d 323.83 ± 0.10d 0.87 ± 0.04c 47.87 ± 0.30c
M80 2.78 ± 0.00a 277.67 ± 0.55db 323.79 ± 0.41db 0.89 ± 0.01cdb 45.25 ± 1.11bc
M120 2.86 ± 0.03ab 272.68 ± 1.05c 322.62 ± 0.05cb 0.85 ± 0.01c 42.73 ± 1.31b
M160 3.07 ± 0.04bc 265.96 ± 1.29ba 320.55 ± 0.65ba 0.74 ± 0.03ba 35.90 ± 0.49a
M200 3.14 ± 0.10c 262.28 ± 1.31a 319.59 ± 0.00a 0.66 ± 0.00a 33.12 ± 0.90a

Results were represented as mean values ± standard deviation of triplicate tests. Different letters superscripted on the results were significantly different at P< 0.05.

Thermal analysis

The thermal properties of the MOS powders with different particle sizes are displayed in Figure 4. The TGA (Figure 4A) and DTG (Figure 4B) curves were analyzed to study the thermal behaviors of the MOS powders. Weight loss of material occurred at three different temperature regions. In the first stage, around 3% of the initial weight of the MOS powders was lost as the temperature ranged from 30 to 200ºC. This was probably due to the evaporation of adsorbed and structural water and light volatiles (Meng et al., 2018). At a temperature of 200-500ºC (the second stage), the powders lost most of their initial weight because many organic substances, including hemicellulose, cellulose, and lignin, were decomposed (Chen and Kuo, 2010). When the temperature was higher than 500ºC (the third stage), thermal decomposition of other heavy components occurred, while the weight loss was not very distinct. Carbonized residues were finally retained, which accounted for about 17%-25% of the initial mass.

Figure 4. Thermogravimetry (A) and derivative thermogravimetry (B) curves of Moringa oleifera stem powders with different particle sizes.

Percentage weight loss due to water desorption (WL) and temperature values corresponding to the onset of the decomposition process (Tei) and the maximum degradation rate (Tdeg) are summarized in Table 3. It can be found from the first weight-loss stage that the WL increased with the decrease in particle size. This phenomenon was related to crystal structure and the affinity of the water to the powder. Increasing amorphous fraction in finer powders would absorb more water molecules, while crystalline domains were less accessible to water (Avolio et al., 2012). In the second stage, the samples with smaller particle sizes possessed lower Tei value, Tdeg value, and maximum derivative weights. This is owing to the higher content of amorphous cellulose, which could be degraded without overcoming the energy barrier of the crystal structure (Wang et al., 2013). These results demonstrated that sieve-based grinding could destroy the crystal structure of cellulose and form amorphous domains, reducing the thermal stability of MOS powders. These observations correlated well with the FTIR and XRD results.

XPS analysis

XPS characterized the surface composition of MOS powders, and the O1s peak and C1s peak were found to be present in noticeable amounts (Figure 5). According to the classification of carbon atoms in plant-based materials, the C1s peak can be deconvoluted into four subpeaks at approximately 284.8 eV (C1) and 286.3 eV (C2), 287.8 eV (C3), and 289.0 eV (C4). C1 corresponds to C–C/C–H, mainly from lignin and extractives; C2 corresponds to C–O, mainly from cellulose and hemicellulose; C3 corresponds to C=O/O–C–O; and C4 corresponds to O–C=O (Sinn et al., 2001). The C1s spectra of MOS powders with different particle sizes were deconvoluted into three Gaussian peaks, including C1, C2, and C3 (Figure 6). At the same time, C4 was not found in the XPS spectra, indicating an undetectable concentration of O=C–O groups in MOS samples, as seen elsewhere (Yang et al., 2014). The theoretical O/C ratios of cellulose, hemicellulose, lignin, and extractives were 0.83, 0.8, 0.33, and 0.11, respectively (Ji et al., 2016; Kocaefe et al., 2013). With the decrease in particle size, the O/C atomic ratio and the area percentages of C2 peak and C3 peak decreased, while the area percentage of C1 peak increased (Table 4). The increasing C1 component and decreasing O/C ratio indicated more carbon-rich extractives, including fats, terpenes, and lignin guaiacyl units on the surface of MOS powders (Kocaefe et al., 2013). Therefore, the present study results demonstrated that the reduced particle size improved the exposure, dissolution, extraction, and bioaccessibility of bioactive ingredients in plant-based granular materials.

Figure 5. X-ray photoelectron spectra of the O1s and C1s peaks for Moringa oleifera stem powders with different particle sizes.

Figure 6. Deconvoluted C1s peak area region for Moringa oleifera stem powders with different particle sizes.

Table 4. Area percentage of C1-C3 peaks by deconvoluting C1s peak region and surface O/C atomic ratio of Moringa oleifera stem powders with different particle sizes.

Samples C1 (%) C2 (%) C3 (%) O/C
M40 55.42 ± 1.73a 30.27 ± 1.81a 14.31 ± 0.08c 0.35 ± 0.01b
M60 59.52 ± 1.44a 26.68 ± 0.80a 13.80 ± 0.64bc 0.32 ± 0.01ab
M80 57.56 ± 6.30a 28.13 ± 5.33a 14.31 ± 0.97c 0.32 ± 0.04ab
M120 60.23 ± 0.23a 26.85 ± 0.05a 12.92 ± 0.18ab 0.30 ± 0.01ab
M160 59.01 ± 0.89a 27.70 ± 0.73a 13.29 ± 0.16bc 0.31 ± 0.00ab
M200 62.59 ± 1.03a 25.59 ± 1.03a 11.83 ± 0.01a 0.29 ± 0.01a

Results were represented as mean values ± standard deviation of triplicate tests. Different letters superscripted on the results were significantly different at P < 0.05.

Conclusions

Moringa oleifera Lam. stem powders with median particle sizes ranging from 458.3 to 36.3 μm were produced using sieve-based grinding. The microstructure showed that the fiber was gradually destroyed, and irregular strip- and sheet-shaped structures were observed with decreasing particle size. The decreasing particle size reduced density and angle of repose while increasing water and oil capacities. However, the smallest particle size of 36.3 μm reduced hydration properties and increased powder density due to reduced porosity. Regarding the color, decreasing particle size increased greenness and reduced yellowness. After grinding, no new functional group was found in Fourier transform infrared spectra, but the accurate position was shifted to a higher wavenumber. The decreasing particle size reduced the crystallinity index (from 51.66% to 33.12%) and the thermal stability of powders (e.g., maximum degradation temperature from 324.62 to 319.59ºC). The above structural analysis demonstrated the destroyed hydrogen bonds associated with cellulose and hemicellulose crystal structure. The decreasing surface O/C ratio of powder with decreasing particle size indicated more carbon-rich extractives on the powder surface. Future studies must investigate the dissolution and bioavailability of active compounds in M. oleifera Lam. stem powders.

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