Physicochemical properties of heterogeneously acetylated glucomannan of A. oncophyllus and its performance for iron encapsulation

Propriedades físico-químicas do glucomanano heterogeneamente acetilado de A. oncophyllus e seu desempenho para encapsulamento de ferro

Abstract

Well-known as a food additive, glucomannan has excellent biocompatibility and biodegradability properties. However, glucomannan is easily gelled, which limited its use in high concentration. To reduce the gel formation ability of glucomannan, acetylation was conducted. This work aims to study the effect of acetylation on physicochemical properties of glucomannan. Acetylation was performed in heterogeneous system which glucomannan was immersed in ethanol (96%) with various concentrations of glucomannan (5-25%) and acetic acid (5-99%). This modified glucomannan was subsequently used as an encapsulation matrix for producing iron beads. The results showed that higher concentration of acetic acid in acetylation impacted on higher solubility and viscosity of glucomannan. The transmittance intensity of Infrared (IR) spectra and morphology of glucomannan were changed due to the acetylation and encapsulation process. The highest viscosity of the matrix (484.33 cP) led to the highest Encapsulation Yield (EY) (53.3%). Gompertz’s model fitted to describe the release profile of iron in all samples (R²>0.92) that showed the burst phenomena in the initial release. This work found that acetylated glucomannan had higher solubility and has a potency to protect the iron taste during oral consumption as it releases slower in neutral pH solution.

Keywords:
Glucomannan modification; Glucomannan acetylation; Gelation encapsulation; Iron release; Encapsulation yield

Resumo

Conhecido como aditivo alimentar, o glucomanano possui excelentes propriedades de biocompatibilidade e biodegradabilidade. No entanto, o glucomanano é facilmente gelificado, o que limita seu uso em altas concentrações. Para reduzir a capacidade de formação de gel do glucomanano, procedeu-se à sua modificação, usando acetilação. Este trabalho tem como objetivo estudar o efeito da acetilação nas propriedades físico-químicas do glucomanano. A acetilação foi realizada em reação heterogênea, que foi conduzida em etanol (96%), com várias concentrações de glucomanano (5-25%) e ácido acético (5-99%). Este glucomanano modificado posteriormente foi usado como matriz de encapsulamento para a produção de grânulos de ferro. Os resultados mostram que uma maior concentração de ácido acético na acetilação impactou na maior solubilidade e viscosidade do glucomanano. A intensidade dos espectros de IR e a morfologia do glucomanano foram alteradas devido ao processo de acetilação e encapsulamento. A maior viscosidade da matriz (484,33 cP) levou ao maior rendimento de encapsulamento (53,3%). O modelo de Gompertz ajustou-se para descrever o perfil de liberação do ferro em todas as amostras (R²>0,92) que apresentaram os fenômenos de explosão na liberação inicial. Este trabalho verificou que o glucomanano acetilado apresentou maior solubilidade e poder de proteção do sabor do ferro durante o consumo oral, pois é liberado mais lentamente em solução de pH neutro.

Palavras-chave:
Modificação de glucomanano; Acetilação; Encapsulamento por gelificação; Liberação de ferro; Rendimento de encapsulamento

HIGHLIGHTS

Acetylation on glucomannan had successfully conducted using acetic acid in a heterogenous reaction

Acetylation facilitated iron encapsulation ability of glucomannan using gelation method

Gompertz’s model fitted to describe the release profile of iron in all samples that showed a burst phenomenon in initial release

1 Introduction

Iron is naturally presented in the red blood cell in enzymes, hemoglobin, and myoglobin, mainly stored in the liver and bone marrow. Insufficient iron intake causes iron deficiency anemia in humans, which has the highest number of micronutrient deficiency cases globally. This deficiency affects human immunity, cell development, and human productivity of more than 2 billion people worldwide (Scott et al., 2014). The iron fortification helps in fulfilling the iron intake as recommended (Naktinienė et al., 2021). However, direct addition of iron in the food fortification method comes with an unacceptable flavor and taste. Moreover, iron deficiency is also affected by the reduction of iron bioavailability and iron loss during food processing (Shubham et al., 2020). The presence of absorption inhibitors of iron in meals, such as phytic acid and polyphenols, can cause iron degradation which decreases its bioavailability (He et al., 2018; Hurrell, 2021). The iron encapsulation prevents direct contact of iron to its unsupported environment (Naktinienė et al., 2021).

Encapsulation is a method to cover and protect iron using an encapsulant matrix that limits the iron contact with other components. The undesirable sensorial changes can be prevented as the iron is protected (Bryszewska et al., 2019). Encapsulation using polysaccharides as the matrix improves the bioavailability of active substances as it increases the solubility of the components encapsulated in particles. Suitable polysaccharides also protect the active substances in the digestive tract conditions and increase the residence time of the active substances (Fathi et al., 2014). In drug-delivery use, the properties of matching matrix allow to support the drug-targeted delivery, which maintains and releases the active substances in specified organs (Wang et al., 2019). Chean et al. (2021) used chitosan-alginate matrix to protect the Lactiplantibacillus plantarum against simulated gastrointestinal conditions.

Glucomannan, a natural polysaccharide extracted from Amorphophallus oncophyllus Prain ex Hook. f., shows excellent biocompatibility and biodegradability properties which suitable for manufacturing drugs and medical dressing (Liu et al., 2012). However, glucomannan has low solubility as hydrogen bond formation during purification and drying treatment (Alonso-Sande et al., 2009). In addition, glucomannan's high molecular weight and viscosity limit its application. Glucomannan with specific molecular weight could have specified biological functions, such as anti-tumor, immunoregulation, and cytothesis (Liu et al., 2012). Thus, glucomannan should be modified to conform its use and broaden its application (Wardhani et al., 2019; Zhu, 2018).

For drug excipient use, lower solubility in neutral pH is preferred to cover the drug taste. Glucomannan solubility is depended on its acetyl group, as it prevents the glucomannan self-aggregation and promotes the water penetration to amorphous phase (Wani et al., 2012; Wardhani et al., 2022). Acetylation is a proven method to increase glucomannan solubility. Liu et al. (2011) produced more soluble glucomannan with a degree of acetylation of 0.8-0.9. Wang et al. (2020) used acetic acid to produce acetylated glucomannan which has solubility improvement in organic solvents. In previous research, Wardhani et al. (2016) conducted glucomannan acetylation homogenously using acetic acid in various concentrations. However, in that work only 3% maximum of glucomannan can be reacted with acetic acid as glucomannan has high gelation properties in water. Heterogeneous acetylation can be an alternative that allow to conduct the reaction in the presence of non-solvent. Quintana et al. (2018) performed heterogenous acetylation of chitosan.

Hence, in this work heterogenous acetylation of glucomannan was conducted in ethanol using acetic acid. The acetylated glucomannan was subsequently used for iron encapsulation using a gelation method. Physicochemical properties of the acetylated glucomannan and performance on the iron encapsulation were studied.

2 Materials and methods

2.1 Materials

Crude flour of A. oncophyllus tube was bought from a local farmer at Nganjuk, East Java, in Indonesia. The glucomannan was extracted based on previous method using isopropyl alcohol (IPA) (Wardhani et al., 2020). The extraction found that the flour contained 76.3% glucomannan. Ethanol (96%) was purchased from PT. Indoacidatama Tbk, (Surakarta, Central Java, Indonesia). Ferrous sulfate heptahydrate (FeSO₄·7H₂O), HCl, 1,10-phenanthroline monohydrate (C₁₂H₈N₂.H₂O), acetic acid glacial and other reagents were pro analysis grade from Merck Chemical Co. (Darmstadt, Germany).

2.2 Modification of glucomannan

Glucomannan was dispersed in acetic acid-ethanol solution (100 mL) and stirred for 1 h in ambient conditions. The concentration of acetic acid and glucomannan were varied for 5, 10, 25, 75, 99% and 5, 10, 15, 20, 25%, respectively. The acetylated glucomannan was obtained by filtering using filter paper and subsequently oven-drying at 80 °C for 30 min.

2.3 Iron encapsulation

Glucomannan matrix solution was prepared by dissolving acetylated glucomannan powder (3 g) in distilled water (100 mL) under constant stirring for 10 min. After addition of ferrous sulfate (0.3 g), the solution was continuously stirred for 15 min. Subsequently, the solution was dropped to ethanol (96%, 100 mL) to form iron beads. The mixing and beading process were conducted at ambient temperature (~25 °C). The beads were immersed in the ethanol solution for 3 h before being filtered and oven-dried at 105 °C for 120 min.

2.4 Functional group and morphological analysis

Functional groups of native glucomannan, acetylated glucomannan, and encapsulated iron were obtained by Infrared (IR) spectra using a Fourier-transform infrared (FTIR) spectroscopy (Perkin Elmer Spotlight 200, Perkin Elmer Inc., Waltham, MA, USA) at a range of wave number between 4000-400 cm^-1. The surface morphology of the samples was observed using a Scanning Electron Microscope (SEM) instrument (JEOL JSM-6510 LA, JEOL Ltd., Akishima, Tokyo, Japan).

2.5 Viscosity

The viscosity of glucomannan powder was determined using Cannon Fenske Viscometer size 100 (Schott AG, Mainz, Rhineland-Palatinate, Germany) (Wardhani et al., 2018). Glucomannan solution was prepared by dissolving and stirring the glucomannan (1 g) in distilled water (40 mL) at 45 °C for 90 min. After centrifuging at 2300xg (EBA 21 centrifuge, Hettich®, Kirchlengern, North Rhine-Westphalia, Germany) for 15 min, the supernatant was transferred to the viscometer and compared with the data of water.

2.6 Solubility

Solubility determination was conducted following the method of Wang et al. (2014) with slight modification. Glucomannan solution (1%) was prepared and heated in a water bath at 60 °C for 30 min. The mixture was centrifuged at 2300xg for 20 min to obtain the supernatant. The supernatant was weighed and oven-dried at 105 °C until constant-weighted. The weight of supernatant and its oven-dried precipitate was determined for solubility calculation (Equation 1).

$Solubility\mathrm{}\left(\mathrm{\%}\right)\mathrm{}=\frac{Dried\mathrm{}supernatant\mathrm{}weight}{initial\mathrm{}supernatant\mathrm{}weight}\mathrm{x}\mathrm{}100\mathrm{\%}$ (1)

2.7 Encapsulation yield

Iron bead (0.4 g) was dispersed in HCl solution (6 M, 10 mL) and stirred for 1 h in ambient conditions. The mixture was subsequently filtered, and the filtrate was diluted to 10 mL using distilled water. The solution (0.1 mL) was added by sodium thiosulphate solution (1.1 mL, 100 ppm), 1,10-phenanthroline (1.5 mL, 1000 ppm), acetate buffer (pH 4.5, 1.5 mL) and acetone (5 mL). After diluting to 10 mL, the solution was stirred for 120 min. The absorbance of the solution was obtained by Ultraviolet-visible (UV-Vis) spectrophotometer (Shimadzu UV Mini 1240, Shimadzu Asia Pacific Pte Ltd, Singapore, Central Region, Singapore) at 520 nm. The iron content was obtained by plotting the absorbance data to the iron standard curve. The encapsulation yield was calculated using Equation 2.

$Encapsulation\mathrm{}yield\mathrm{}\left(EY,\mathrm{}\mathrm{\%}\right)=\frac{concentration\mathrm{}of\mathrm{}iron\mathrm{}in\mathrm{}bead\mathrm{}}{concentration\mathrm{}of\mathrm{}iron\mathrm{}added\mathrm{}}x100\mathrm{\%}$ (2)

2.8 Iron release and its kinetics

The iron release was determined by an in-vitro test in 2 pH solutions. Dried iron bead (0.5 g) was added to either 150 mL of HCl solution (0.1 M, pH 1.2) or phosphate buffer solution (0.1 M, pH 7.4) under constant stirring. Sample solutions were taken every 30 min for the iron concentration determination as described in the previous section.

The iron release at a certain time $t$ ($C_t$) per total iron content ($C_{total}$) was mathematically modeled using four equations, i.e., Weibull, Hopfenberg, and Gompertz models (Equation 3, 4, and 5, respectively) using linear regression (Ramteke et al., 2014; Shah & Rajput, 2019).

$C_t=\mathrm{}{C}_{total}\left[1-exp\left(\frac{{-(t-Ti)}^b}{a}\right)\right]$ (3)

$\frac{C_t}{C_{total}}=1-{\left[1-\mathrm{}{k}_{1}t\left(t-l\right)\right]}^n$ (4)

$\frac{C_t}{C_{total}}=\exp \left[-\alpha \mathrm{}{e}^{\beta \log t}\right]$ (5)

where:

T_i or l = lag time for dissolution process. There was no lag time occurred in this dissolution process (T_i = l= 0)

a = time-dependent parameter

b = the shape of dissolution curve progression

k₁₌ erosion rate constant

$\alpha$ = the undissolved iron at time =1

$\beta$ = dissolution rate per unit of time.

3 Results and discussion

This research focused on acetylation of glucomannan to modify its properties and subsequently, applied it for encapsulation iron using gelation method. Impacts of acid-glucomannan ratio and concentration of glucomannan on the viscosity and solubility of acetylated glucomannan were studied. The modification was conducted heterogeneously in ethanol to prevent the gel formation. Performance of the encapsulated iron using acetylated glucomannan, including its release studies in acid and neutral conditions, was observed.

3.1 Acetylation

Homogenous acetylation had successfully modified the physicochemical characteristics of 3% glucomannan solution (Wardhani et al., 2016). However, high gelation capability of glucomannan limited diffusivity of reactants that reduced the effectiveness of the homogeneous reaction. Only low glucomannan concentration involved in the reaction. In this research, glucomannan acetylation was performed heterogeneously in ethanol to increase its yield. The heterogeneous reaction might hinder glucomannan gelation, allowing more glucomannan to contact with the acid. In this works, up to 25% glucomannan was used in the heterogeneous acetylation, higher concentration than that of the homogenous one.

The viscosity and solubility of glucomannan were affected by its acetyl group (Mehboob et al., 2020). Hydroxyl group of glucomannan forms hydrogen bonds with water, leading to gel formation (Impaprasert et al., 2017). In acetylation, the hydroxyl groups of glucomannan were replaced with the acetyl ones. This latest group inhibited the inter-chain hydrogen bonds between adjacent glucomannan chains as intramolecular and intermolecular hydrogen bonds weakened (Adeyanju et al., 2016; Colussi et al., 2015; Wardhani et al., 2016). Moreover, the presence of acetyl group reduced the molecular bond strength and eased the water diffusion. This condition led to increase the swellness (Hoover & Sosulski, 1985). The less association of glucomannan chains eased the water penetration and increased the glucomannan solubility. Hence, the viscosity and solubility of glucomannan tended to increase after modification when there is more acetyl group involved in the reaction (Figure 1b). A similar result of increasing viscosity and solubility after acetylation was also reported by Shubeena et al. (2015) on chestnut starch and Siroha et al. (2019) on pearl millet starch, respectively.

Figure 1
Effect of (a) glucomannan concentration and (b) acetic acid concentration on viscosity, solubility, and encapsulation yield of acetylated glucomannan.

3.2 Iron encapsulation

The gelation method was used for iron encapsulation by dropping glucomannan-iron solution into ethanol which acted as an anti-solvent for glucomannan and formed iron beads (Wardhani et al., 2019). The matrix ability to entrap the active agent was represented by the value of Encapsulation Yield (EY). Figure 2 shows that higher viscosity of glucomannan performed higher Encapsulation Efficiency (EE). Matrix solution with high viscosity entrapped more iron and prevented leaching process during encapsulation process (El-Say, 2016). Moreover, the viscous solution also prevented harsh solidification. Xin et al. (2008) suggested that the EE of the dropping method was related to the molecular structure of matrix compound, i.e., the length of the chain and molecular weight. Better encapsulation performance of higher viscosity materials was also obtained by Silva Carvalho et al. (2019) on anthocyanin bead formation.

Figure 2
Correlation between viscosity and encapsulation yield.

The study of iron release of all samples was determined at acidic (pH 1.2) and neutral conditions (pH 7.4) for 180 min. These solutions represented the condition of non-enzymatic human gastric and saliva, respectively. Figure 3 shows the iron release fraction from iron beads encapsulated by acetylated glucomannan with different degrees of acetylation (high DA=AA99; low DA=GM25). Acetylation accelerated the iron release from the beads as the matrix was more soluble. Initial burst releases were observed in all iron bead due to the surface iron dissolution. Gelation method of encapsulation produced glucomannan bead with equally distributed iron around the bead, including on bead surface. This iron was instantly dissolved as the bead immersed in water. Moreover, the release was lower in neutral pH immersion. The gelled glucomannan bead maintained its form at neutral pH as described by Wardhani et al. (2022) and Cui et al. (2013). This release profile was preferable to avoid the unpleasant flavor of iron during chewing process. However, the release pattern of the iron in both pH solutions was insignificantly different.

Figure 3
Profile of iron release and the Gompertz model fitting: (a) AA99 pH 1.2; (b) AA99 pH 7.4; (c) GM25 pH 1.2; and (d) GM25 pH 7.4.

Four mathematical models were fitted into the release profiles from iron-glucomannan beads, as shown in Figure 3, and the constants for their regression analysis were summarized in Table 1. Judging from the coefficient of determination (R²), Gompertz model was the most suitable model to describe the iron release. This model started with a steep increase and converged slowly to the asymptotic maximal dissolution. Gompertz’s model described the good solubility with intermediate release rate of iron from acetylated glucomannan beads (Dash et al., 2010).

Table 1
Linear regressions of the release modelling data.

Variables	Weibull			Hopfenberg			Gompertz
	a	b	R2	k1	n	R2	α	β	R2
AA99 1.2	1.03 ± 0.005	0.025 ± 0.002	0.567 ± 0.005	0.003 ± 0.001	-0.14 ± 0.021	0.93 ± 0.009	0.973 ± 5.27	-0.45 ± 0.94	0.99 ± 0.057
AA99 7.4	1.04 ± 0.001	-0.03 ± 0.003	0.537 ± 0.002	0.004 ± 0.001	-0.11 ± 0.043	0.93 ± 0.012	0.979 ± 5.31	-0.28 ± 0.94	0.98 ± 0.062
GM25 1.2	1.12 ± 0.001	-0.03 ± 0.002	0.127 ± 0.001	0.004 ± 0.001	-0.10 ± 0.032	0.96 ± 0.015	1.043 ± 1.79	-0.27 ± 0.94	0.92 ± 0.032
GM25 7.4	1.07 ± 0.001	-0.01 ± 0.003	0.069 ± 0.002	0.004 ± 0.001	-0.11 ± 0.027	0.95 ± 0.010	1.006 ± 5.45	-0.31 ± 0.94	0.99 ± 0.053

• The constants and their definitions are followed: a=time-dependent parameter; b=the shape of dissolution curve progression; k₁=erosion rate constant; α=the undissolved iron at time=1; β=dissolution rate per unit of time.

3.3 Functional groups and morphology

The IR spectra of native glucomannan, acetylated glucomannan, and iron bead are shown in Figure 4, which described that these samples had similar peaks but in different transmittance intensities. The hydroxyl and acetyl groups of glucomannan were found at ~3430 and ~1620 cm^-1, respectively (Asghari-Varzaneh et al., 2017). A broad peak around ~3430 cm^-1 of native glucomannan and encapsulated iron spectra was due to O-H stretching vibration. High intensity of hydroxyl group was detected in the dried beads of iron encapsulation that could be contributed by water owing to the crystallization of the iron. The C=O bond became a parameter to analyze acetyl groups in glucomannan spectra at ~1620 cm^-1. The iron (FeSO₄.7H₂O) addition in iron beads caused the broader peak of O-H bond identified around ~3430 cm^-1, which was also found by Asghari-Varzaneh et al. (2017).

Figure 4
IR spectra of native glucomannan, acetylated glucomannan using 99% acetic acid, and dried encapsulated iron.

The change of surface morphology due to the acetylation and encapsulation is shown in Figure 5. Acetic acid eroded the glucomannan and produced a rougher surface of the acetylated glucomannan. Shah & Rajput (2019) showed that the acid attack on polysaccharide surfaces during acetylation. Cuenca et al. (2020) found that internal damage after acetylation led to starch aggregation. Furthermore, the iron bead showed a wrinkled surface due to water diffusion during bead drying. This water loss also led to an agglomeration of the beads. Wang et al. (2022) also found the rough and cracked surface on their acetylated starch granules.

Figure 5
Surface appearances of (a) native glucomannan; (b) acetylated glucomannan; and (c) dried encapsulated iron in a magnification of 5000x.

4 Conclusions

Heterogeneous acetylation in ethanol changed the physicochemical properties of glucomannan. Higher concentration of acetic acid as the acetylation agent promoted the acetylation process of glucomannan, that led to improve its solubility and viscosity. The iron encapsulation produced more yield when higher viscosity of acetylated glucomannan was used. The acetylation and gelation process influenced the glucomannan surface. Gompertz’s model fitted to describe the release profile of iron in all samples that showed the burst release phenomena.

• Cite as: Ulya, H. N., Devara, H. R., Wardhani, D. H., Chusnullita, A., Purwati, D., & Aryanti, N. (2022). Physicochemical properties of heterogeneously acetylated glucomannan of A. oncophyllus and its performance for iron encapsulation. Brazilian Journal of Food Technology, 25, e2022036. https://doi.org/10.1590/1981-6723.03622
• Funding: Directorate of Research and Community Service, Directorate General of Higher Education, Ministry of Research, Technology and Higher Education of the Republic of Indonesia through PTUPT Scheme (101-152/UN7.P4.3/PP/2018).

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Edited by

Section Editor: Mateus Petrarca.

Publication Dates

• Publication in this collection
  07 Nov 2022
• Date of issue
  2022

History

• Received
  12 Apr 2022
• Accepted
  20 Sept 2022