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Development and characterization of acetylated and acetylated surface-modified tapioca starches as a carrier material for linalool

  • Syuzeliana Shaari , Hayati Samsudin , Koh Wee Yin and Uthumporn Utra EMAIL logo
Published/Copyright: December 18, 2024
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e-Polymers
From the journal e-Polymers Volume 24 Issue 1

Abstract

Linalool compound is easily degraded when exposed to light and oxygen during processing or storage. Therefore, the objective of this work was to develop surface-modified tapioca starch as a carrier for linalool. Surface-modified starch was prepared by various treatments, i.e., enzyme treatment (using STARGEN 002 enzyme), alcohol–enzyme treatment, and acetylation; acetylation was carried out with 20% (w/w, starch dry basis) acetic anhydride. Acetyl percentage (Ac%) and degree of substitution (DS) of the acetylated samples significantly increased with different treatments due to the introduction of acetyl groups in the starch molecules. After acetylation, the percentage crystallinity as well as WR1047/1022 values decreased. The acetylated surface-modified starches showed a reduction in terms of thermal properties and gelatinization enthalpy values. Compared with native starch samples, the granules of acetylated surface-modified starch sample exhibited a rough surface and clumped structures. The encapsulation efficiency of linalool significantly increased from 3.63% to 66.76%. The encapsulated linalool with enzyme-acetylated starch showed significantly low values of linalool reduction (11.00%) over 28 days of storage due to its surface erosion and addition of acetyl groups on granular starch, as evidenced by scanning electron microscopy and Ac% as well as DS results. Among them, the enzyme-acetylated starch had the lowest values of linalool reduction, which could effectively encapsulate linalool and control the linalool reduction value.

Graphical abstract

Keywords: acetylation; acetyl groups; degree of substitution; encapsulation; surface-modified tapioca starches

Abbreviations

Ac%

Acetyl percentage

CH3COO

Acetyl

DS

Degree of substitution

EE

Encapsulation efficiency

FTIR

Fourier transform infrared

H

Hydrogen

HCl

Hydrochloric acid

NaOH

Sodium hydroxide

O–H

Hydroxyl

SEM

Scanning electron microscopy

SLC

Surface linalool content

TLC

Total linalool content

WR1047/1022

Wavenumber ratio at 1,047 cm−1/1,022 cm−1

XRD

X-ray diffraction

ΔH

Gelatinization enthalpy

1 Introduction

Starch is one of the natural, low-cost, renewable biopolymers that enable direct use in food and non-food applications. Due to its biodegradability and wide abundance in nature, it displays great potential as a carrier material for the encapsulation, protection, and controlled release of volatile compounds (1,2). Currently, several studies have reported the use of starch as a natural carrier material to protect volatile compounds such as flavor, antimicrobial, and antioxidant components and vitamins which are easily degraded during the processing or storage process (3,4,5,6). For instance, when liquid tomato flavor is encapsulated in porous starch, it significantly retained the flavor content up to 6 months (7). In addition, Jiang et al. (8) reported that by encapsulating procyanidins in porous starch molecules acting as the carrier, their effect was increased up to 96%. Similar to a previous study by Lei et al. (9), the oxidative stability of olive oil was improved using purple sweet potato porous starch granules, where the peroxide value was found to be lower than 10 mmol·kg−1.

To develop starch-based carrier materials, various treatments have been used to improve the functional properties of native starch. Starch can be modified by physical, chemical, enzymatic, or a combination of these treatments. Enzymatic treatment is one of the preferred treatments to modify the granule surface due to its high efficiency and environmental friendliness, which commonly used enzymes such as α-amylase and/or amyloglucosidase (6,10). Apart from these, enzymatic treatment can alter the morphological properties of native starch for the encapsulation proposed, which could cause changes such as pin holes, sponge-like erosion, numerous medium-sized holes, and surface erosion of the native starch granules depending on their sources (10,11).

Current developments show that enzymatic treatments, especially the granules of cereal starches were reported with various sized porous granules (10,12,13) due to the presence of natural opening granules which make them more susceptible to penetration by enzymatic molecules. However, in starches without pores such as tapioca, enzymatic erosion is found on the surface of starch granules (13,14). Furthermore, there are few studies that have considered the potential of tapioca starch (Manihot esculenta Crantz) as a carrier material (5,15). Although the native molecular structure of tapioca starch has limitations, such as low adsorption capacity, due to its smooth granule surface, preventing it from acting as an efficient carrier material, it can be pre-treated by alcohol prior to enzymatic treatment. This pre-treatment can enhance enzymatic hydrolysis (16), resulting in high pore volume. In addition, acetylation treatment can improve the binding sites on the surface of starch granules. Treating starch granules with acetic anhydride involves the substitution mechanism where the free hydroxyl (O–H) groups in starch molecules are replaced by acetyl (CH3COO) groups. These substitution groups are allowed to act as binding sites on the granule surface and made it easier for volatile compounds to attach during the encapsulation process (3). Moreover, it has been reported that acetylated starch shows higher encapsulation efficiency (EE) than native starch (3,17,18,19). Therefore, we hypothesized that the sequential modification of alcohol pre-treatment → enzyme treatment → acetylation treatment might increase the pore volume with the addition of binding sites on the starch granules as an alternative carrier for volatile compounds.

In this study, acetylated surface-modified tapioca starches from different treatments were investigated as carrier materials to encapsulate linalool as the guest molecule model. The effects of acetylation treatment on surface-modified tapioca starches in terms of percentage of acetyl groups (Ac%), degree of substitution (DS), granule morphology, molecular structure, and thermal properties were investigated. Besides, the effects of acetylated surface-modified starches on the EE and retention rate of linalool were also studied when applied as a volatile carrier.

2 Materials and methods

2.1 Materials

Tapioca starch (Mannihot esculenta Crantz) was obtained from Thye Huat Chan Sdn. Bhd. (Penang, Malaysia), which contained 11.50% moisture, 1.00% fat, 0.12% ash, and 0.08% protein, as determined by proximate analysis. STARGEN 002 enzyme (DuPont Genencor Science) was purchased from CK Chemical Sdn. Bhd. (Selangor, Malaysia). The enzyme activity of STARGEN is 570 GAU·g−1 (GAU refers to one glucoamylase unit), the optimum pH is 4.0–4.5, and contains α-amylase from Aspergillus kawachi and amy-loglucosidase from Trichoderma reesei. Linalool was used as a volatile carrier with 97% purity (food grade), and the analytical standard was kindly supplied by Sigma–Aldrich (St. Louis, MO, USA).

2.2 Preparation of surface-modified starch

2.2.1 Alcohol pre-treatment

Alcohol pre-treatment was carried out according to the method described by Li et al. (20), with some modifications to remove the surface fat and protein on the native starch granules. The sample was treated with 75% aqueous solution of 2-propanol for 1 h using Soxhlet extraction. Then, the solvent was removed through a vacuum pump equipped with Whatman No. 3 filter paper. The alcohol-treated starch was dried overnight at 40°C in an oven (FD115, Binder GmbH, Germany) and then ground into powder. The resulting starches were sieved through 75 μm sieves and stored in air-tight glass containers at room temperature before enzymatic treatment.

2.2.2 Enzyme hydrolysis

The native and alcohol pre-treated starches were hydrolyzed according to the method of Uthumporn et al. (21), with some modifications. Approximately 25 g dried starch samples were suspended in sodium acetate buffer pH 4.3 at a concentration of 25% (w/v). Then, 5% (w/v) STARGEN 002 was added into the starch suspension and incubated for 24 h in a shaking incubator (SI-600 R, Jelo Tech, South Korea) with the operating conditions of 150 rpm and 45°C. The hydrolysis was stopped by adding 2 M HCl into the slurries until the pH of the slurries reached 1.5–1.6. The slurries were centrifuged (Model 4000 Centrifuge, Kubota Corporation, Japan) for 5 min at 3,000 rpm and washed with distilled water. These series of steps were repeated several times to clean off the top brown layer (excess enzyme). After that, the suspensions were neutralized with distilled water and filtered using a vacuum pump equipped with Whatman No. 3 filter paper. The starch residues were oven-dried for 2 days at 40°C. Then, the resulting starches were ground and sieved through a 75 μm sieve.

2.3 Acetylation treatment

The acetylation treatment on the starch samples (native, enzyme hydrolysis, and alcohol–enzyme treatment) was prepared according to the method of García Tejeda et al. (22), with some modifications. Approximately 50 g of the dried sample was dispersed in 112 mL of distilled water with continuous stirring via an overhead stirrer. The pH of the suspension must be maintained between 8.0 and 8.4 using 3% (w/v) aqueous sodium hydroxide (NaOH) solution. Then, about 20% (w/w) acetylation agent (acetic anhydride, starch dry basis) was added dropwise into the starch suspension, while the pH was maintained within the range of 8.0–8.4 using 3% NaOH solution. Then, the mixture was continuously stirred for 15 min after completion of the acetic anhydride addition process. After treatment with the acetylation agent, the pH of the mixture was reduced to 4.5 with 5% (v/v) HCl solution and centrifuged at 2,500 rpm for 5 min. The starch residues were washed several times with distilled water until the pH of the samples was neutralized. The samples were filtered with Whatman No. 3 filter paper through a vacuum pump and dried at 40°C for 2 days in an oven. The dried residues were ground and sieved through a 75 μm sieve.

2.4 Determination of starch properties

2.4.1 AC% and DS

The procedure for the determination of AC% and DS for all the acetylated samples (i.e., acetylated starch, enzyme-acetylated starch, and alcohol–enzyme-acetylated starch) was modified using the titration method from Sahnoun et al. (23). About 1.0 g of acetylated starch samples was weighed in a 250 mL conical flask and mixed with 50 mL of distilled water. Then, the mixtures were titrated with 0.1 M NaOH solution using phenolphthalein as an indicator until a permanent pink color was detected. After that, 25 mL of 0.5 M NaOH solution was added, and the mixtures were continuously shaken in an incubator shaker (120 rpm) at room temperature for 30 min. Finally, the excess alkali in the mixture was titrated with 0.2 M HCl solution until the mixture turned into a white solution. The native tapioca starch was treated in a similar step and used as a blank sample. The Ac% and the DS were calculated according to Eq. 1:

(1) Ac ( % ) = ( V 1 – V 2 ) × 10 3 × M × 43 × 100 w

(2) DS = ( 162 × Ac % ) [ 4,300 − ( 42 × Ac % ) ]

where V 1 is the volume of 0.2 M HCl used for the titration of native tapioca starch; V 2 is the volume of 0.2 M HCl used for the titration of acetylated starch; M is the molarity of HCl solution; W is the weight of the acetylated starch; 43 is the molecular weight of the acetyl group, g·mol−1; and 162 is the molecular weight of an anhydroglucose unit, g·mol−1.

2.4.2 X-ray diffraction (XRD)

The crystallinity pattern of starch granules after treatment was evaluated through X-ray diffractometry (D8 Advance, Bruker, AXS, Germany), and the percentage of crystallinity (%) was estimated using the Bruker Diffrac.Eva (version 4) with Cu Kα radiation (λ = 1.5406 Å) set at 40 kV (tube voltage) and 40 mA (tube current) using a nickel filter. The dried native acetylated starch sample powder was equilibrated in 100% relative humidity at room temperature overnight prior to XRD analysis (24). Then, the samples were scanned over a range of 4°–40° (2θ), with a step size of 0.02° and step times of 0.1 s per step.

2.4.3 Thermal properties

The thermal properties of starch samples were analyzed using differential scanning calorimetry (DSC Q100, TA Instruments, USA) at 50 mL·min−1 of nitrogen flow rate and equipped with Universal Analysis 2000 software. The procedure for sample preparation was modified based on the Benavent Gil and Rosell (11) method, with a starch–water ratio of 1:3 (g·g−1) before sealing hermetically. Approximately 2.5 mg of each dried sample of starch and distilled water was weighed and placed directly in an aluminum pan. Then, the sample pans were equilibrated at 25°C for 1 h after sealing. The sealed pans were heated from 20°C to 130°C with 10°C·min−1 heating rate. An empty aluminum pan was used as a reference prior to analysis. The onset temperature (T o), peak temperature (T p), and conclusion temperature (T c) were analyzed based on the heating curve, while the gelatinization enthalpy (ΔH) was analyzed based on the area between the connection points of T o and T c in the heating curve and was expressed in J·g−1 (dry basis).

2.5 Linalool encapsulated in surface-modified starches

The procedure for linalool encapsulation was prepared using freeze drying based on the modification method of Samakradhamrongthai et al. (25). The emulsion of starch sample (50% w/v) was prepared in distilled water at 25°C and mixed with a 2:1 (starch/linalool) ratio. Then, the emulsion was mixed with an overhead stirrer for 10 min, frozen at −18°C, and freeze-dried at −50°C (FreeZone 12-L Console Freeze Dryers, Labconco Corporation, USA) for 48 h. The encapsulated samples were collected and stored in universal glass bottles at 23°C until further analyses.

2.5.1 Granule properties

A small amount of each dried encapsulated starch powder was spread on the surface of the aluminum specimen stub, which contained a double-sided adhesive tape. All of the samples were coated with a thin layer of gold and observed using a scanning electron microscope (Quanta 650 FEG, FEI Company, Netherlands) at a magnification of 3,000×, under 10 kV acceleration voltage.

2.5.2 Fourier transform infrared (FTIR) analysis

The molecular structures of all samples were analyzed using an FTIR spectrophotometer (Nicolet iS10, Thermo Fisher Scientific, USA). Each of the samples was blended with spectroscopic grade solid potassium bromide (KBr) powder and pressed into pellets. Then, the FTIR spectrum of the samples was recorded with 32 scans in the wavenumber range of 4,000–400 cm−1. The spectra of samples were recorded and compared with the spectra of the native tapioca starch and linalool compound. The intensity ratio of wavenumber values 1,047 cm−1/1,022 cm−1 (WR1047/1022) of the sample without an encapsulation process was calculated according to Lacerda et al. (10) for evaluating the characteristics of the crystalline and amorphous structures in starch samples.

2.5.3 Determination of EE

The surface oil, total oil, as well as EE of linalool were determined according to Jarunglumlert et al. (26), with some modifications. Dried encapsulated linalool was extracted by using 75% (v/v) ethanol solution. The dried encapsulated sample (1.0 g) was added into 10 mL of ethanol and stirred for 10 min and 2 h at 23°C. The mixture was centrifuged (Model 4000 Centrifuge, Kubota, Corporation, Japan) at 3,000 rpm for 5 min. The absorbance of the supernatant was measured using a UV–visible spectrophotometer (UV-160A, Shimadzu, Japan) at 291 nm. The absorbance of the extracted sample after 10 min was used to measure the amount of surface linalool content (SLC), while the extracted sample after 2 h was used to measure the total linalool content (TLC). The following equations were used to calculate the SLC, TLC, and EE, which were obtained from Jafari et al. (27).

(3) SLC ( % ) = M so M i × 100

(4) TLC ( % ) = M to M i × 100

(5) EE ( % ) = M to − M so M i × 100

where M so is the surface oil (linalool) content on surface-modified starch; M i is the initial amount of loaded linalool, and M to is the total oil content (linalool) in surface-modified starch.

A calibration curve of linalool compound was prepared using a linalool–ethanol solution (ranging from 0 to 6 mg·mL−1) to measure the EE of the encapsulated linalool. A linear equation of the calibration curve (R 2 = 0.9998) was generated, and it was used to measure the concentration of linalool in each encapsulated starch sample.

2.5.4 Determination of linalool retention

The procedure for sample storage was modified from Jarunglumlert et al. (26); the samples were placed in desiccators at room temperature for 28 days to determine the effect of surface-modified tapioca starches on the stability of linalool compound. At selected day intervals, about 0.25 g samples were collected for the analysis of TLC in each treatment.

Each of encapsulated samples (at 0, 14, and 28 days) were extracted with 10 mL of distilled water and 5 mL of n-hexane in a 15 mL glass tube. Then, the mixture was vortexed for 2 min before immersing in an ultrasonic bath (TI-H-10 multi-frequency device, Elma, Germany) at 45°C for 15 min (70% amplitude, 20 kHz). This extraction step was repeated three times and vortexed for 30 s after every 15 min extraction. Then, 1.5 mL of the resulting mixture was filtered through 0.45 μm nylon disposable syringe filters. The filtration sample was kept in GC vials and placed in the refrigerator until analysis.

Gas chromatography with flame ionization detection (GC-FID) was performed by a GC-2010 Plus (Shimadzu, Japan) equipped with a BPX5 capillary column (30 m, length × 0.25 mm, 0.25 μm film thickness), with the temperature set from 35°C to 200°C at a speed of 1°C·min−1, the injection temperature set to 250°C, and the detector temperature set to 280°C. The extracted sample (1–2 μL) was injected by an autosampler. The retention rate of linalool in surface-modified starch was calculated by Eq. 6 (28):

(6) Retention rate (%) = M t M 0 × 100 %

where M t is the remaining linalool content in surface-modified starch at time t; M 0 is the initial linalool content encapsulated in surface-modified starch.

The concentration of encapsulated linalool on surface-modified tapioca starch was determined according to the standard calibration curve by using linalool–n-hexane solutions (ranging from 0 to 20 ppm). The linear equation of the calibration curve (R 2 = 0.9943) was generated, and it was used to calculate linalool concentrations in the samples before and after storage.

2.6 Statistical analysis

Data are presented as mean ± standard deviation. Data were analyzed by one-way ANOVA for Ac%, DS, wavenumber ratio at 1,047 cm−1/1,022 cm−1 (WR1047/1022), percentage of crystallinity, thermal properties, and EE analyses. For determining the retention rate of linalool analysis, two-way ANOVA and a post-hoc test were run for statistically significant differences among the treatment and storage time by Tukey’s HSD test at α = 0.05 using SPSS Statistics for Windows (Version 20.0, IBM Corporation, USA).

3 Results and discussion

3.1 Characterization of surface-modified tapioca starches

3.1.1 AC% and DS

Ac% and DS of acetylated surface-modified tapioca starches were significantly higher than those of the acetylated tapioca starch sample (Table 1). From the results, it can be seen that the Ac% and DS values of the starch samples increase in the following order: acetylated < enzyme-acetylated < alcohol–enzyme-acetylated. The Ac% value increased with increasing DS value among the samples. The Ac% of starch samples ranged from 0.58 to 1.54 g/100 g, while DS values ranged from 0.02 to 0.06 at different treatments of the surface starch granules. These findings also show that the Ac% of the samples is lower than 2.5 g/100 g, which is the maximum value recommended by the US Food and Drug Administration (US FDA) for food applications. In addition, DS values of acetylated samples ranged between 0.02 and 0.06, indicating that the samples can be classified as low DS (<0.1) (29).

Table 1

Ac% and DS of acetylated surface modified tapioca starch

Starch samples Ac (%) DS
Acetylated 0.58 ± 0.00a 0.02 ± 0.00a
Enzyme-acetylated 1.16 ± 0.00b 0.04 ± 0.00b
Alcohol-enzyme acetylated 1.54 ± 0.00c 0.06 ± 0.00c

Values are expressed as mean ± SD of triplicate samples; a, b, c represent significant differences among treatment at p < 0.05.

Differences in Ac% and DS values among the acetylated surface-modified starch samples might be due to different treatment conditions on the starch granules before acetylation treatment. The treatments could have caused the disruption and weakening of hydrogen (H) bonds in the starch granules, which make it easier for acetic anhydride as an acetylation reagent to react with starch molecules (30). The penetration of enzyme molecules on the surface of starch granules could cause the breaking of the H bonds within the starch molecules, resulting in an erosion on the surface and an increased surface area. It is believed that erosion is beneficial to increasing free hydroxyl (O–H) groups in the starch molecules and could lead to increased substitution of acetyl groups during the acetylation process.

The higher Ac% and DS values of alcohol–enzyme-acetylated starch sample were found to be due to the modification of surface granules, allowing acetylating agents to attach during the subsequent process. This could be explained by the disruption of lipid and protein layers surrounding the starch granules by alcohol reagent, contributing to the penetration of enzyme molecules into the surface granules (31) and formation of surface erosion (21). This treatment might provide a synergistic effect of reducing the amorphous region in the starch molecules and enhancing the penetration of enzyme molecules during the hydrolysis, providing deep surface erosion and increased free O–H groups. The presence of free O–H groups in starch molecules increased the contact area between the acetic anhydride and starch molecules, so more acetyl groups would be introduced during the acetylation treatment. This result showed that the surface granules had a greater influence on the Ac% and DS of the samples.

3.1.2 Thermal properties

The thermal properties of native and different surface-modified acetylated starches are summarized in Table 2. The acetylation treatment significantly affected the thermal properties of starch samples, except for the T c. From the result, it can be seen that after the acetylation treatment, the surface-modified starches had significantly lower thermal properties with increasing Ac% and DS values of samples, compared to those for native tapioca starch. Similar findings have been reported for microwave-acetylated wheat starch compared with native starch (30). The reduction in thermal properties is related to the incorporation of acetyl groups into the starch granules, causing the disruption of the molecular structure and deformation of the crystalline structure of the sample with an increment of the amylopectin short chain. This result is expected since the reaction of acetic anhydride successfully substituted the O–H groups in starch molecules with acetyl groups, which affected the molecular structure of the starch granules.

Table 2

Thermal properties of native and acetylated surface modified starches

Treatments DSC properties
T o (°C) T p (°C) T c (°C) ΔH (J·g−1)
Native 62.64 ± 0.65d 67.93 ± 0.24d 73.62 ± 0.57a 6.87 ± 0.67b
Acetylated 57.61 ± 0.16b 63.99 ± 0.38b 72.78 ± 0.16a 1.43 ± 0.09a
Enzyme-acetylated 59.69 ± 0.14c 64.79 ± 0.13c 72.42 ± 0.38a 1.32 ± 0.22a
Alcohol-enzyme acetylated 54.27 ± 0.84a 59.63 ± 0.06a 74.05 ± 2.00a 0.92 ± 0.16a

Values are expressed as mean ± SD of triplicate samples; a, b, c, d represent significant differences among treatment at p < 0.05.

Moreover, the incorporation of acetyl groups into surface-modified starch granules significantly reduced the gelatinization enthalpy (ΔH) value compared to that of native starch. ΔH is mainly related to the loss of the crystalline region in the starch granules as well as the breaking of the double helices after treatment (29). However, the effect of acetylation treatment was not significant for ΔH among the surface-modified starch samples. The ΔH of the samples in the decreasing order was as follows: native > acetylated > enzyme-acetylated > alcohol–enzyme-acetylated. These observations are in accordance with the Ac% and DS values of the samples, and the greatest reduction in ΔH was found from the alcohol–enzyme-acetylated sample. This might be due to the substitution of acetyl groups in the starch molecules, which increased the hydrophobic properties, inhibited the interaction among starch molecules, and disrupted the double helical structures. An increase of the Ac% and DS values in samples led to a decrease in O–H groups, indicating that more acetyl groups were introduced into the starch molecules after treatment. This behavior might have contributed to the changes in double helical structures by the substitution reaction in the starch molecules. El Halal et al. (32) and Colussi et al. (29) also reported that the presence of acetyl groups in the starch molecules caused changes in the double helical structures. This suggested that the substitution of acetyl groups in the starch molecules also indicated a decrease of hydroxyl groups, which disrupted the internal hydrogen bonding and allowed the granules to swell at lower temperatures. Based on the results, it was suggested that the acetylated surface-modified tapioca starches had a lower percentage of organized crystal structure, which required less energy to gelatinize the starch. Consequently, acetylated surface-modified tapioca starch samples can possibly be applied at low temperature to improve their potential use in food applications.

3.1.3 XRD

The XRD patterns of native and acetylated surface-modified tapioca starches are illustrated in Figure 1. Based on the results, the native and acetylated surface-modified starch samples had a similar A-type crystalline pattern, suggesting that the acetylation treatment on the starch granules did not affect the diffraction pattern. The A-type starch had strong diffraction peaks at 2θ about 15°, 19°, and 23° and also a connected double peak at 17° and 18°. Similar findings were also reported by Li et al. (33) and Zhang et al. (34), who observed the characteristic diffraction patterns of tuber starch.

Figure 1 XRD of native and acetylated surface modified starch samples.
Figure 1

XRD of native and acetylated surface modified starch samples.

However, a reduction in diffraction peak intensities was observed for the acetylated surface-modified starch samples compared to the native one. Acetylation treatment was able to cause a significant decrease in the diffraction intensities at 20° (2θ), which was related to amylose–lipid complexes in starch molecules. This finding suggests that the presence of acetyl groups in starch molecules could destroy the double helical structures of the starch molecules. Moreover, the substitution of O–H groups with acetyl groups led to a reduction in the formation of inter- and intra-molecular H-bonds between the starch molecules (32).

Nevertheless, the percentages of crystallinity decreased significantly with the increase in Ac% and DS values, which indicated the acetylation process, when compared to native tapioca starch (Table 3). The percentages of crystallinity of samples in an increasing order were as follows: alcohol–enzyme-acetylated < enzyme-acetylated < acetylated < native starch. The decreased crystallinity among the samples resulted in an increase in the amorphous region in the starch molecules due to the acetylation process that disrupted the crystalline regions of the granules (18,35). In addition, when more O–H groups were substituted with acetyl groups of starch, less intermolecular H bonds were formed in the starch molecules and resulted in partial destruction of the crystalline structure.

Table 3

The percentage of crystallinity (%) and WR1047/1022

Types of starch Percentage of crystallinity (%) WR1047/1022 (cm−1·cm−1)
Native 45.40 ± 0.00a 1.31 ± 0.05c
Acetylated 41.60 ± 0.00b 1.28 ± 0.02ab
Enzyme-acetylated 40.20 ± 0.00c 1.24 ± 0.00ab
Alcohol-enzyme acetylated 37.90 ± 0.00d 1.21 ± 0.02a

Values are expressed as mean ± SD of triplicate samples; a, b, c, d represent significant differences among treatments at p < 0.05.

3.2 Characterization of encapsulated linalool in surface-modified tapioca starches

3.2.1 Granule properties

The microstructures of encapsulated linalool in native, acetylated, enzyme-acetylated, and alcohol–enzyme-acetylated tapioca starches are displayed in scanning electron micrographs (Figure 2). Overall, morphological differences in the encapsulated linalool depend on the treatment. According to this, the encapsulated linalool on native tapioca starch granules (Figure 2a) remained relatively smooth with one or more truncated areas on the surface and showed no visible pores or cracks, similar to the findings reported by Mbougueng et al. (36) and Alzate et al. (5). In general, the absence of porous structures on the granules suggests that the encapsulated compound was only absorbed onto the granular surface (37). Therefore, in this study, no entrapment of linalool compound occurred in the sample due to the absence of surface erosion.

Figure 2 Granule morphology of encapsulated linalool on (a) native, (b) acetylated, (c) enzyme-acetylated, and (d) alcohol-enzyme acetylated tapioca starch (surface erosions are shown by yellow circle; agglomerate granules are shown by pink circle; leached materials are shown by blue arrow).
Figure 2

Granule morphology of encapsulated linalool on (a) native, (b) acetylated, (c) enzyme-acetylated, and (d) alcohol-enzyme acetylated tapioca starch (surface erosions are shown by yellow circle; agglomerate granules are shown by pink circle; leached materials are shown by blue arrow).

In Figure 2b, the encapsulated linalool in the acetylated starch granules shows a rough surface compared to the native starch, as influenced by the acetylation treatment. This might be due to the reaction of acetic anhydride as an acetylation reagent with the starch molecules, especially at weaker points and amorphous regions in starch granules. Moreover, the tapioca starch surface granules show the absence of cracks, allowing a greater reaction of acetic anhydride on the granule surface, resulting in rough surface granules. Some small leached materials on the surface were observed, suggesting the reaction of acetylated starch with linalool. The acetylated starch only absorbed the linalool compound on the surface. Some fused granules were detected due to the introduction of acetyl groups on the starch molecules (18). The acetyl groups formed on the starch molecules acted as binding sites, leading to linking of the linalool compound with starch molecules.

In the case of treatment by enzyme acetylation, the tapioca starch granules showed the formation of a rougher surface (Figure 2c). This is related to the enzymatic penetration mainly at the weaker points of starch granules (11,21). Moreover, the reaction of acetic anhydride on the surface could also be another contributing factor affecting the morphology. As shown in Figure 2c, starch granules coalescing together after acetylation treatment due to the introduction of hydrophilic groups resulted from the acetyl groups to the starch molecules. As the DS value increased, more O–H groups were replaced; therefore, starch molecules coalesced together resulting in more fused granules (18).

Based on Figure 2d, alcohol–enzyme acetylation treatment also affected the morphology of the granules, in which rough and deep surface erosion of granules were observed. It could be due to the reaction of alcohol reagent that led to a decrease in the amount of lipid content on the starch surface. Once the granules were disrupted, the enzyme molecules penetrated easily, thus the starch granules showed erosion on the surface (21). As is shown, the starch granules also exhibited rough surfaces and aggregation of a few granules which could also be due to the acetylation process (38).

However, the scanning electron micrographs of acetylated samples showed similar patterns, which had leached materials on the surface of the starch granules, suggesting the linalool compound after the encapsulation process via hydrogen bonding (18,19). This is an indication that linalool molecules are attached to the surface instead of encapsulating inside. Since the presence of oxygen atoms of carboxyl and hydroxyl molecules from acetyl groups increased the hydrophobicity of starch granules that allowed more entrapment of hydrophobic linalool molecules on the surface of the granules. Besides, the granules’ surface did not have a porous structure, and the linalool compounds were only absorbed on the granule surface (39). Among these treatments, the alcohol–enzyme-acetylated sample showed the most clumped structures, which could be attributed to the effect of high Ac% and DS values in starch molecules (40). The higher Ac% and DS values in the alcohol–enzyme-acetylated sample than other samples suggest that introduction of more acetyl groups leads to an increased number of oxygen atoms in starch molecules, so more linalool molecules can be entrapped in the granules, leading to an enhancement of linalool encapsulation.

3.2.2 FTIR analysis

FTIR spectra of native and surface-modified tapioca starches after acetylation treatments are shown in Figure 3a. For the native tapioca starch, a strong absorption band around 3,000–3,600 cm−1 was detected, which correspond to the presence of O–H bonds in starch molecules. The absorption band at 2,929 cm−1 is attributed to the C–H2 stretching vibrations and that at 1,653 cm−1 to the bending vibrations of H2O in carbohydrate groups. The observed bands at 1,159 and 1,010 cm−1 are both assigned to the C–O stretching vibrations in starch molecules. These findings were similar to that of native tapioca starch as found by several researchers in the FTIR spectra (34,41,42). However, the spectra of acetylated and acetylated surface-modified tapioca starches show additional strong absorption bands at 1,230, 1,369, and 1,750 cm−1 that correspond to C–O, C–H, and C═O of acetyl groups, respectively (29,35). The appearance of these bands in the molecular structure of starch samples confirms the formation and presence of acetyl groups after acetylation treatment, which indicated that the hydrophilic native starch was converted into hydrophobic starch. Moreover, as shown in the spectra (Figure 3a), increasing intensities of those absorption bands correspond to a significant increase in the Ac% and DS values among the starch samples. In addition, the intensity of the absorption bands for O–H groups was reduced, indicating the substitution of O–H groups with acetyl groups in starch molecules due to the acetylation treatment.

Figure 3 (a) FTIR spectra of native and acetylated surface modified starch and (b) encapsulated linalool in native and acetylated surface modified tapioca starches.
Figure 3

(a) FTIR spectra of native and acetylated surface modified starch and (b) encapsulated linalool in native and acetylated surface modified tapioca starches.

The absorption band at 1,022 cm−1 (C–O stretching) is attributed to the amorphous region, while that at 1,047 cm−1 (C–O stretching) is related to the crystalline regions in the starch molecules (43). Thus, their wavenumber ratio of intensity (WR1047/1022) can be used to describe the proportion of crystalline regions to amorphous regions in the starch molecules and is reflected as the amount of ordered crystalline region in the starch granules (4,44). Based on Figure 3a, the WR1047/1022 value of samples was calculated and is summarized in Table 3. The WR1047/1022 value of native tapioca starch (1.31) showed significantly higher values compared to those of acetylated surface-modified tapioca starches (1.21–1.28). These findings are similar to those reported by Kong et al. (45) and Shah et al. (43), indicating that the molecular structures of starch samples were more affected with acetylation treatment. In addition, the increase in Ac% and DS values of the surface-modified starch sample results in loss of crystallinity regions in starch molecules. These results were in agreement with gelatinization enthalpy (ΔH) and the percentage of crystallinity results. It has been suggested that the acetylation treatment occurred mainly in the crystalline regions in the starch granules (18,32).

Figure 3b shows the FTIR spectra of the free linalool compound, linalool–native, linalool-acetylated, linalool–enzyme-acetylated, and linalool–alcohol–enzyme-acetylated samples. Most of the adsorption peaks of starch–linalool were similar to those of native and acetylated surface-modified tapioca starches (Figure 3a), indicating that the encapsulation process did not influence the molecular structure of tapioca starch. In the spectra of encapsulated samples, the fingerprint region of linalool was observed at 995, 1,450, 2,972, and 3,373 cm−1 (46), which correspond to the presence of stretching vibrations of C–O, C–C, C–H, and O–H groups, respectively, in the linalool compound. However, decreasing intensities of these absorption bands were observed in this study. This might be due to the interaction of the linalool with the starch molecules during the encapsulated process.

Besides, it can be seen that there is a wide peak at 995 cm−1 in the FTIR spectra of the linalool–native sample compared to other, suggesting that the lowest C–H stretching vibrations of linalool compound in the sample resulted from a little number of linalool molecules interacting with the native sample during the encapsulation process. Moreover, after the encapsulation process, the band at 2,970 cm−1 attributed to the C–H stretching vibrations of the methylene group in linalool compound disappeared from the spectrum. This is due to the presence of intermolecular H-bonds through the interaction of linalool with the starch molecules. In addition, it was found that the there is an absorption band of 2,922 cm−1 attributed to the reduction of C–H2 bonds. This band became narrower and stronger in the spectrum attributed to the hydrophobic groups of linalool molecules interacting with the starch granules. However, the broad band at peak 3,373 cm−1 turned narrower after the encapsulation process is related to the reduction of hydrogen bonds in the encapsulated linalool–starch molecules. The O–H molecules of the glucose units interacted with linalool and formed hydrogen bonds. Besides, this reduction was also due to the substitution of the O–H groups with acetyl groups in the starch molecules (47).

Moreover, as shown in Figure 3b, there is no new interaction found in the encapsulated linalool with the acetylated surface-modified tapioca starch. It can be proven that the linalool molecules were encapsulated well in the acetylated surface-modified starches as the major absorption peaks of linalool and starch molecules were still observed. In addition, the chemical structure of starch molecules remained unchanged during the encapsulation process. This suggests that the acetylated surface-modified tapioca starches can be used as carrier materials for linalool. Similar findings by Gao et al. (37), Luo et al. (48), Acevedo Guevara et al. (19), and Xiao et al. (1) indicate that starch bases are suitable to be used as carrier materials for active compounds. For instance, enzymatically modified starch was loaded with proanthocyanidins (6,49), resulting in improved effectiveness in adsorption during the encapsulation process. Nata et al. (17) and Acevedo Guevara et al. (19) reported that acetylated starch which had more hydrophobic properties due to the presence of acetyl groups have enhanced the EE, as compared to native starch.

3.2.3 EE

The EE of linalool in surface-modified tapioca starches was tested in SLC (%), TLC (%), and EE (%) (Table 4). As illustrated in Table 4, the SLC, TLC, and EE ranged 2.76% to 12.23%, 2.87% to 5.49%, and 3.63% to 66.76%, respectively. As can be seen from Table 4, the enzyme-acetylated sample was found to have the highest SLC value as compared to those of surface-modified tapioca starch samples. Meanwhile, the highest TLC and EE values were observed in acetylated and alcohol–enzyme-acetylated starch samples, respectively. The acetylation treatment could make starch a better carrier material for linalool compound compared to native starch.

Table 4

Summary of surface oil, total oil, and EE (%) of the native and acetylated surface-modified starches

Types of starches SLC% TLC% EE%
Native 2.76 ± 0.03a 2.87 ± 0.03a 3.63 ± 0.04a
Acetylation 9.22 ± 0.02b 5.49 ± 0.09d 40.50 ± 1.17b
Enzyme-acetylation 12.23 ± 0.14d 4.72 ± 0.04c 61.41 ± 0.19c
Alcohol-enzyme acetylation 11.63 ± 0.05c 3.86 ± 0.02b 66.76 ± 0.09d

Values are expressed as mean ± SD of triplicate samples; a, b, c, d represent significant differences among treatment at p < 0.05.

The enzyme-acetylated starch sample showed the highest SLC value compared to other samples. This result is likely to be related to the formation of deep surface erosion through enzyme penetration on the surface of the starch granules, which enhances the adsorption of the linalool compound on the surface granules. On the other hand, the increase in Ac% and DS values led to greater substitution by the acetyl groups into surface-modified starch, which entraps more linalool compounds. A previous study indicated that the acetylation treatment on potato starch molecules had changed the molecular structure of starch, thus causing greater encapsulating capacity of gallic acid due to the presence of acetyl groups (3). This greater attraction of acetylated surface-modified starches is due to the presence of binding sites on the surface granules; a significantly higher value of TLC was shown in acetylated modified starches compared to the native tapioca starch. This is in agreement with the DS, since there are higher number of acetyl groups in the starch granules, which make the acetylated and treated acetylated starches better carrier materials as they also had erosion on the surface of the granules which could entrap and protect linalool compounds.

According to Table 4, the values of EE of acetylated starch samples were between 40.50% and 66.76%, which are higher than encapsulated samples from native tapioca starch (3.63%). It has been assumed that more linalool compounds are encapsulated in this sample compared to the native tapioca starch. Generally, the presence of more oxygen atoms after the acetylation treatment on the starch molecules, which will hold more linalool compounds by hydrogen bonding (18). As can be seen from Table 1, the trends of EE results are in accordance with the trends in Ac% and DS results. It could lead to an increase in the Ac% and DS values, which significantly increased the presence of acetyl groups in the samples (18,19). Consequently, this leads to an increase of hydrogen bonding in starch molecules which allowed the interaction with linalool molecules and increases the EE.

Enzymatic and alcohol–enzymatic treatment significantly affected the EE values of the samples. The EE value increased up to 61.41% and 66.76%, respectively, which was observed due to the erosion and porous structures of the pre-treated starches granules. It is generally assumed that the modified granules allowed the access of linalool compound on the surface of the granules. Lei et al. (50) reported that the encapsulated molecules may diffuse into and entrap by the inner granules of the surface-modified starch. Additionally, the presence of electrostatic and hydrophobic interactions between the linalool molecule and starch chain may also contribute to increased EE values in the samples (17,37). Based on the DS values of both samples, it is suggested that more acetyl groups were present in the starch molecules, resulting in more hydrophobic groups in the starch molecules. Since there are high DS values in the sample, it might lead to the increase of binding sites and in turn increasing EE values after acetylation treatment.

3.2.4 Retention rate of linalool

The retention rates of linalool during storage with different starch samples are shown in Figure 4. The retention rate of linalool during storage was significantly different in terms of storage time and starch samples. A gradual reduction was observed for the linalool content in the starch samples as a function of storage time.

Figure 4 The effect of different acetylated surface modified tapioca starches on linalool retention during storage. A, B, C, D represents significant differences among the encapsulation sample within time; a, b, c represents significant differences among time within encapsulation sample at p < 0.05.
Figure 4

The effect of different acetylated surface modified tapioca starches on linalool retention during storage. A, B, C, D represents significant differences among the encapsulation sample within time; a, b, c represents significant differences among time within encapsulation sample at p < 0.05.

As compared with the native starch sample, the acetylated surface-modified starches had a significantly higher content of linalool. Among these acetylated surface-modified starches, the enzyme-acetylated starch showed the highest (5.09 ppm) linalool content. The reason could be the presence of surface erosion on granules with additional binding sites after acetylation treatments, thus allowing more linalool compounds to enter and fill the interstices of granules and also bind on the surface granule via acetyl groups. Moreover, the volatile compound in the interior of the granules is well protected from degradation (51), while additional binding sites on the surface lead to the trapping of more linalool compounds (3,18).

As expected, the linalool content in the starch sample was significantly reduced (approximately 50–90%) after 14 and 28 days of storage. Palma Rodríguez et al. (52) reported that about 76% of ascorbic acid content was lost after storage in native starch, modified starch, or gum Arabic over 12 weeks. The type of carrier material and storage time had effects on the retention rate during storage. The presence of porous structures and surface erosions generally allows the molecules to enter and fill them (7,51). In addition, there are more hydrogen bonding sites in the starch molecules for acetyl groups (19), allowing interactions with more linalool molecules. In our study, the presence of erosions and acetyl groups on the granular surface has been suggested as a possible mechanism for the entrapment of linalool molecules.

Increasing storage time of linalool in the native starch sample had decreased rapidly from 2.50 to 0.22 ppm, and this result presented the greatest reduction of linalool content (90.47%) during the 28 days of storage. The native starch (Figure 2a) presented a smooth surface granule, which provides a low surface area and may result in poor adsorption properties. Since the granules had a smooth surface, the volatile compounds adsorbed only onto the surface and diffused out faster during storage (53,54). As expected, the native tapioca starch showed the lowest stability toward linalool during storage.

During storage time, a reduction of linalool compound in the acetylated surface-modified starch samples was observed. As shown in Figure 4, with increasing storage time, enzyme-acetylated and alcohol–enzyme-acetylated starch showed a gradual decrease from 6 to 0.65 ppm and from 4.81 to 0.53 ppm, respectively, due to an improvement in the protective effect of the starch samples compared to that of acetylated samples. In addition, these treatments presented the greatest retention of linalool compound, which was 89% at the end of the storage time. This might be due to the eroded granular surface, which improved the surface area and adsorption capacity (48). This behavior allowed the linalool molecules to cause erosion (7,51) and acted as the second barrier (55). Moreover, higher EE values in both surface-modified starches, as shown in Section 3.2.3, suggest that more linalool molecules were present on the surface of the granules. Besides, the combination of acetylation treatment suggested more binding sites on the granular surface and improved the accommodation of molecules during the encapsulation process (19). It seems that the combination of treatments caused more protective effects than single treatments during storage.

However, after 28 days of storage, no significant differences were obtained among the samples. This observation was slightly different from studies that reported the stability of olive oil encapsulated with porous starch (50). Based on the results, the erosions occurred on the acetylated surface-modified starch granules (Figure 2a–d), were not homogenous and unstable, and only a few starch granules had deep surface erosion. Besides, higher SLC values in acetylated surface-modified starches suggest that more linalool molecules were located on the granular surface instead of entrapping into the erosion (50). Theoretically, the volatile compound which adsorbed onto the granular surface carrier material diffuses out easily than that entrapped inside the porous structure. Based on this observation, the encapsulated linalool of acetylated surface-modified tapioca starch was well protected from degradation at 14 days of storage, which could be related to the morphology of the granular surface that can trap the molecules in the starch matrix.

For this reason, the knowledge of structures formed after the modification of starch could be taken as the basis for the selection of a carrier material. The Ac% and DS values after acetylation treatment may be used for modulating the starch morphology. Therefore, the erosion of the surface-modified starches can be improved as it can trap more volatile compounds inside the granules, providing effective protection. Meanwhile, the low DS values of acetylated surface-modified starches in the present work (0.02–0.06) can be increased within the allowed ranges, as suggested by the FDA for food applications. Further increment in the hydrophobic acetyl sides in starch chains improved the volatile compound retention and protected against the surrounding environment making them good alternatives for the encapsulation of flavor in the food industry.

4 Conclusions

In this study, acetylated surface-modified starches from tapioca were prepared and used as carrier materials for linalool encapsulation. The enzyme and alcohol–enzyme treatment could significantly increase both the Ac% and DS values of the samples as compared to those with only acetylation treatment. The introduction of acetyl groups into starch granules decreased the percentage of crystallinity, WR1047/1022 value, and ΔH value in the following order: native > acetylated > enzyme-acetylated > alcohol–enzyme-acetylated, due to the breakage of double helix structures during acetylation treatment. Moreover, a significantly higher EE% (66.76%) was obtained with high Ac% (1.54 g/100 g) and DS (0.06) values. Among these treatments, higher linalool molecules were accommodated in the alcohol–enzyme-acetylated starch sample, which could be due to the presence of erosions and more binding sites of acetyl groups on starch granules. However, these triple treatments did not improve linalool reduction within storage time due to the limited ability of the surface erosions and acetyl groups on the surface of the granules to trap and protect the linalool compound. Further study involving the mechanism of linalool release is needed to investigate an effective encapsulation system and to control the linalool release profile in the surface-modified starch during storage that could be applied in food applications.

  1. Funding information: This work was supported by a Universiti Sains Malaysia, Bridging with Project No: R501-LR-RND003-0000000920-0000.

  2. Author contributions: Syuzeliana Shaari: writing – original draft, writing – review and editing, methodology, and formal analysis; Hayati Samsudin: writing – review and editing; Uthumporn Utra: writing – review and editing and project administration.

  3. Conflict of interest: The authors state no conflict of interest.

  4. Data availability statement: The data presented in this study are included in this published article.

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Received: 2023-10-09
Revised: 2024-02-21
Accepted: 2024-02-22
Published Online: 2024-12-18

© 2024 the author(s), published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.

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https://doi.org/10.1515/epoly-2023-0160
Keywords for this article
acetylation; acetyl groups; degree of substitution; encapsulation; surface-modified tapioca starches
Creative Commons
BY 4.0

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