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. 2024 Mar 11;22:101277. doi: 10.1016/j.fochx.2024.101277

Effect of sulfuric acid hydrolysis on the structure and Pickering emulsifying capacity of acorn starch

Changsheng Guo a,b, Sheng Geng b, Yuzhong Shi b, Chao Yuan a,⁎, Benguo Liu a,b,⁎
PMCID: PMC10955292  PMID: 38515830

Abstract

The acid-hydrolyzed acorn starch samples (HAS-1, HAS-2, HAS-3, and HAS-4) were prepared from natural acorn starch (NAS) at sulfuric acid concentrations of 1, 2, 3, and 4 mol/L for 2 d. The particle characteristics and structures of HAS were investigated, and Pickering high internal phase emulsions (HIPEs) based on HAS were constructed and characterized. The results showed that with an increase in sulfuric acid concentration, the size, yield, amylose content, molecular weight, and amylopectin chain length of HAS gradually decreased. HAS retained an A-type crystal structure, and its relative crystallinity and short-range order degree gradually increased with increasing sulfuric acid concentration. Acid hydrolysis treatment improved the wettability of NAS, and its effect was positively correlated with the sulfuric acid concentration. HAS-3 and HAS-4 could stabilize the Pickering HIPEs with an oil phase volume fraction of 80% at c ≥ 1.5%. The mechanical properties of the HIPEs were positively correlated with c.

Keywords: Acorn starch, Acid hydrolysis treatment, Structural characterization, Pickering high internal phase emulsions, Mechanical properties

Highlights

  • •

    The sulfuric acid-hydrolyzed acorn starch (HAS) was prepared for the first time.

  • •

    The size, yield, amylose content of HAS reduced with increasing sulfuric acid concentration.

  • •

    The molecular weight and amylopectin chain length of HAS also gradually decreased.

  • •

    The wettability of HAS was positively correlated with the sulfuric acid concentration.

  • •

    The acorn starch treated with 3 or 4 mol/L sulfuric acid had good emulsifying ability.

1. Introduction

Starch is a type of green molecule that is widely used in the food industry due to its affordability, accessibility, and biocompatibility (Burrell, 2003). With the advancement of food processing technology, the demands placed on starch processing properties are gradually increasing. High-temperature sterilization, mechanical mixing, and pump transport in modern food processing technology require that starch be heat-resistant and shear-resistant (Jeong, Han, Liu, & Chung, 2019; Zhang, Rempel, & Liu, 2014). Refrigerated foods require gelatinized starch that resists easy regeneration and coagulation, with strong hydrophilicity. Some foods also require that starch have certain special functions, such as film forming and smearing (Copeland, Blazek, Salman, & Tang, 2009; Wang, Sharp, & Copeland, 2011). However, native starch has several disadvantages, such as poor water solubility, high viscosity, low transparency, and easy regeneration. Therefore, starch must be modified to meet the demands of modern food processing (Miyazaki, Van Hung, Maeda, & Morita, 2006).

Natural starch comprises ordered crystallization regions and disordered amorphous regions. In the crystallization regions, amylose and amylopectin form microcrystalline structures closely bound by hydrogen bonds. Starch nanocrystals (SNCs) can be produced by removing the amorphous region from native starch granules using acid treatment (Kumari, Yadav, & Yadav, 2020). Due to their small size, SNCs will easily agglomerate after drying, typically forming aggregates with an average size of 4.4 μm (Angellier, Choisnard, Molina-Boisseau, Ozil, & Dufresne, 2004). Due to their strong mechanical properties and high surface energy, SNCs have certain properties not found in inorganic nanoparticles, such as bio-renewability, biocompatibility, and degradability. Nanocomposites prepared by blending SNCs with carboxymethyl chitosan, soy protein isolate, and gelatinized starch can significantly enhance the mechanical properties of materials (Duan, Sun, Wang, & Yang, 2011; García, Ribba, Dufresne, Aranguren, & Goyanes, 2011; Zheng, Ai, Chang, Huang, & Dufresne, 2009). Moreover, SNCs can serve as particle emulsifiers to stabilize food-grade Pickering emulsions due to their nanoscale properties. Azfaralariff et al. prepared SNCs from sago starch and found that they could be used to produce stable Pickering emulsions (Azfaralariff et al., 2021). López-Hernández et al. also fabricated a stable rapeseed oil-based Pickering emulsion using barley SNCs (BSNCs) as a stabilizer. The study showed that with increased BSNC content, the droplet size decreased while the apparent viscosity increased (López-Hernández et al., 2022).

The primary method for preparing SNCs is through acid hydrolysis, which is a straightforward and convenient process. SNCs can be prepared by using diluted acid to hydrolyze starch at 25 °C and 55 °C at different times (Le Corre, Bras, & Dufresne, 2010). The type of starch and acid used will also significantly influence the preparation process of SNCs. The effect of starch on SNC preparation through acid hydrolysis will be mainly reflected in the amylose content. Starch with higher amylose content exhibits increased tolerance to acid, thus reducing the efficiency and final yield of hydrolysis (Wei et al., 2013). Singh and Ali used sulfuric acid, phosphoric acid, hydrochloric acid, and nitric acid to hydrolyze corn, millet, wheat, mung bean, chickpea, cassava, and potato starches to prepare SNCs. The study observed the highest hydrolysis efficiency for hydrochloric acid and nitric acid, followed by sulfuric acid, while phosphoric acid had the lowest hydrolysis efficiency (Singh & Ali, 2008). Velásquez-Castillo et al. prepared quinoa SNCs through sulfuric acid hydrolysis and evaluated the effects of temperature on its yield, structure, and physicochemical properties. The results showed that with an increase in hydrolysis temperature, the crystallinity of quinoa SNCs remained stable, but the thermal stability decreased (Velásquez-Castillo, Leite, Ditchfield, do Amaral Sobral, & Moraes, 2020). However, current research on SNCs has mainly focused on their process optimization and structure characterization, with a lack of systematic research on the relationship between the acid hydrolysis process and the Pickering emulsifying ability of SNCs.

Acorns, the fruit of oak trees, contain high starch content in their kernels, ranging from 50% to 60%, making them a promising starch resource Zhang et al., 2019). However, acorn starch contains a substantial amount of polyphenols, which negatively affects taste and restricts its application in food products (Kim, Lee, Kim, Lim, & Lim, 2012). In a previous study, we systematically assessed the physicochemical properties of acorn starch and reported on its Pickering emulsifying ability for the first time (Guo et al., 2023).

Building on this foundation, in this study, we prepared acid-hydrolyzed acorn starch samples (HAS-1, HAS-2, HAS-3, and HAS-4) by hydrolyzing natural acorn starch (NAS) at sulfuric acid concentrations of 1, 2, 3, and 4 mol/L for 2 d. The particle appearance, molecular structure, and wettability of HAS were investigated, and Pickering high internal phase emulsions (Pickering HIPEs) stabilized by HAS were constructed and characterized. Subsequently, we clarified the correlation of sulfuric acid concentration–HAS structure–HAS emulsifying ability. The findings from this research contribute to our understanding of starch modification and may promote the application of acorn starch in food industry.

2. Materials and methods

2.1. Chemicals

The acorn starch (starch content, 88.56 ± 0.22%) used in this study was produced by Yuefeng Agricultural Products Co., Ltd. (Xinyang, China), while Medium-chain triglyceride (MCT) was obtained from Shanghai Yuanye Biotechnology Co., Ltd. (Shanghai, China). Amylose and amylopectin were the products of Sigma-Aldrich (St. Louis, MO, USA). All other chemicals were of analytical grade.

2.2. Preparation of HSA

The NAS sample (75 g) was mixed with varying sulfuric acid solutions (500 mL) with different concentrations (1.0, 2.0, 3.0, and 4.0 mol/L). The resulting mixture was placed in a water bath shaker at 40 °C and oscillated at a speed of 150 r/min for 2 d. Then, the residue was repeatedly washed until neutral with distilled water and freeze-dried. The obtained samples were denoted as HAS-1, HAS-2, HAS-3, and HAS-4.

2.3. Observation of starch granules

After the samples were sprayed with gold, their appearance was observed by a Quanta 200 scanning electron microscope (FEI, Hillsboro, OR, USA) at 2000 times magnification. The birefringence of the samples was recorded using a polarizing microscope, and the particle size distribution was analyzed using a BT-9300H laser particle size analyzer (BETTER, Dandong, China).

2.4. Measurement of amylose content

The amylose contents of NAS and HAS were determined using the colorimetric method (Lv, Zhang, Zhen, Shi, & Liu, 2022). First, 2.5 mL of 1 mg/mL standard solution with different amylose and amylopectin ratios was mixed with 20 mL of distilled water, and its pH was adjusted to 3.0. Then, 0.5 mL of iodine reagent (0.2% I2, 2% KI) was added and diluted to 50 mL. After standing for 10 min, the absorbance of the mixture was measured at 620 nm, with the absorbance taken as the vertical coordinate, and the amylose and amylopectin ratio taken as the horizontal coordinate. Subsequently, the standard curve was plotted. The sample starch (50 mg) was moistened with a small amount of ethanol, then mixed with 10 mL of 0.5 mol/L sodium hydroxide, oscillated in a water bath at 95 °C for 15 min, and then diluted to 50 mL with distilled water. A 2.5 mL sample solution was siphoned into a 50 mL volumetric bottle, and the color was developed according to the previously outlined steps, with the absorbance determined at 620 nm. The amylose and amylopectin contents were calculated according to the standard curve.

2.5. Infrared spectrum measurement

The infrared spectra (IR) of the samples in the range of 500–4000 cm−1 was measured according to the KBr disk method on a Thermo Nicolet iS20 infrared spectrophotometer (Waltham, MA, USA) with 32 scans and a resolution of 4 cm−1.

2.6. Measurement of molecular weight

The molecular weight of the starch samples was analyzed using a GPC-RI-MALS system (Wang et al., 2023), which consisted of a U3000 HPLC (Thermo Fisher Scientific, Waltham, MA, USA), a Wyatt DAWN HELEOS II laser light scattering detector (Santa Barbara, CA, USA), and a Wyatt Optilab T-rEX differential detector (Santa Barbara, CA, USA). For determination, 5 mg of starch and 5 mL of DMSO were mixed and heated at 80 °C for 3 h until the starch was completely dissolved. The injection volume was set to 200 μL. Sample separation was performed at 60 °C on three tandem GPC columns (Ohpak SB-805 HQ, 300 × 8 mm; Ohpak SB-804 HQ, 300 × 8 mm; and Ohpak SB-803 HQ, 300 × 8 mm). DMSO served as the mobile phase with 0.5% LiBr. The flow rate was set at 0.3 mL/min, and the dn/dc value was 0.07 mL/g.

2.7. Determination of amylopectin chain length

The amylopectin chain length (CL) distribution of the sample was measured using ion chromatography (ICS5000, Thermo Fisher Scientific, Waltham, MA, USA) (Zhu & Cui, 2019). The sample was separated on a Dionex™ CarboPac™ PA200 column at 30 °C (250 × 4.0 mm, 10 μm) with an injection volume of 5 μL. The mobile phase consisted of 0.2 M NaOH (A) and 0.2 M NaOH/0.2 M NaAC (B). The elution procedure (A/B) was as follows: 9:1 at 0 min, 9:1 at 10 min, 4:6 at 30 min, 4:6 at 50 min, 9:1 at 50.1 min, and 9:1 at 60 min.

2.8. Determination of X-ray diffraction patterns

The X-ray diffraction (XRD) patterns of the sample in the 2θ range of 5–50° were recorded using a Rigaku D/Max-2600 X-ray diffractometer (Tokyo, Japan) with a step size of 0.02 and a scanning speed of 8°/min (Wan, Li, Sun, Liu, & Xia, 2024). The relative crystallinity of the samples was analyzed using MDI Jade 6.0 software (Livermore, CA, USA).

2.9. Determination of contact angle and interfacial tension

The interfacial tension and contact angle measurements of the samples were determined using a Theta Lite optical contact angle meter (Biolin Scientific, Stockholm, Sweden) (Wei & Huang, 2019). The samples were pressed into a thin slice (diameter 2 cm, thickness 0.5 cm) using a tablet machine at a pressure of 15 MPa. The slices were then placed in a quartz vessel containing MCT, and 6 μL of ultra-pure water was dropped on the surface with a syringe. An image of the droplet was automatically recorded, and the corresponding contact angle was calculated using OneAttension software (Biolin Scientific, Stockholm, Sweden).

The excess sample was dispersed in water to prepare a saturated solution. Then, the pipette tip was inserted into MCT, extracting 25 μL of saturated sample solution, which was then emitted dropwise. The droplet morphology was continuously recorded, and the corresponding interfacial tension was calculated using OneAttension software.

2.10. Preparation of the Pickering HIPEs

A certain amount of sample (NAS, HAS-1, HAS-2, HAS-3, and HAS-4) was dispersed in water as the water phase and MCT as the oil phase. The above two phases were then mixed at an oil volume fraction (φ) of 80% and homogenized for 3 min at a speed of 15,000 r/min. Pickering HIPEs with a starch concentration (c) of 0.5–3.0% (w/w) were subsequently obtained. After standing for 24 h at 25 °C, the stability of the emulsion was compared using the inverted bottle method.

2.11. Microscopic observations of the Pickering HIPEs

The Pickering HIPEs were prepared at c = 2.5 and 3.0%, φ = 80%, using HAS-3 and HAS-4 as emulsifiers. Then, 20 μL of Pickering emulsion was dripped on the glass slide and covered with a cover glass. The droplet appearance was observed using a Hengping BH200P polarizing microscope (Shanghai, China), and the droplet size distribution was measured using a BETTER BT-9300HD particle size analyzer (Dandong, China).

2.12. Gel strength determination of the Pickering HIPEs

The Pickering HIPEs were prepared at c = 2.5 and 3.0%, φ = 80%, with HAS-3 and HAS-4 as emulsifiers. The cylindrical sample with a diameter of 1 cm and a height of 2 cm was used for gel strength measurement. The gel strength was determined using a TA-XT plus texture analyzer (Stable Micro Systems, UK) using a P/0.5 probe (Geng et al., 2021), with the following measurement parameters: probe speed = 1.0 mm/s, compression distance = 10 mm, and trigger force = 2.0 g.

2.13. Rheological property evaluation of the Pickering HIPEs

The Pickering HIPEs were prepared at c = 2.5 and 3.0%, φ = 80%, with HAS-3 and HAS-4 as emulsifiers, and their rheological properties were analyzed using a HAAKE MARS III rheometer (Waltham, Massachusetts Co., Ltd.) equipped with a parallel plate probe at 25 °C under a gap of 1 mm (Tripathi, Bhattacharya, Singh, & Tabor, 2017). In a certain range, the storage modulus (G') and loss modulus (G") of the sample do not change with the change of shear stress, which is called linear viscoelastic region (LVR). To determine the LVR, the shear stress scanning test was performed at a shear stress of 0.1–100 Pa and a frequency of 1 Hz. The frequency scanning test was conducted in the frequency range of 0.1–20 Hz and a shear stress of 1 Pa. Subsequently, the G' and G" values of the HIPEs were recorded. In addition, to investigate the change in apparent viscosity with the shear rate, a shear rate scanning test was performed at a shear rate of 0.1–100 s−1 and a shear stress of 1 Pa.

2.14. Statistical analysis

The measurement was based on three parallel tests and expressed as the mean ± standard deviation, and the statistical comparison was based on the Tukey method with a confidence level of 95%. IBM SPSS 21 software (Armonk, NY, USA) was used for statistical analysis. Origin 2021 software (Origin lab, Northampton, MA, USA) was used to generate corresponding graphs from the data.

3. Results and discussion

3.1. Appearance of starch granules

As shown in Fig. S1, the sulfuric acid concentration significantly affected the appearance and yield of NAS and HAS. The HAS-1, HAS-2, and HAS-3 samples showed little color difference from NAS and were all grayish white. The color of HAS-4 changed significantly and was yellowish brown. With an increase in sulfuric acid concentration, the corresponding starch yield gradually decreased from 85.45% for HAS-1 to 56.47% for HAS-3 and then rapidly decreased to 17.82% for HAS-4. The yield of HAS-4 was similar to that of waxy maize SNCs (15.7%) reported by Angellier et al. (Angellier et al., 2004).

The microscopic morphologies of NAS and HAS were also observed using SEM (Fig. 1A). The majority of NAS granules appeared oval with smooth surfaces, although a few were irregular. After 2 d of 1–3 mol/L sulfuric acid hydrolysis, most of the HAS granules retained their original shapes but exhibited rough particle surfaces, and a small amount of particle fragments appeared. However, after 4 mol/L sulfuric acid hydrolysis, the morphology of the HAS particles changed significantly, and the oval particles completely decomposed into many irregular particles. SEM analysis indicated that sulfuric acid initially acted on the surface of acorn starch, and it could infiltrate into the starch with increasing sulfuric acid concentration, after breaking the surface. This resulted in the decomposition of the starch granule structure, which was consistent with the findings reported by Liu et al. (Liu, Li, Li, Zhang, and Li (2021), indicating that sulfuric acid hydrolyzed starch through external corrosion.

Fig. 1.

Fig. 1

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SEM images (A), polarizing microscope images (B), and particle size distributions (C) of NAS and hydrolyzed acorn starch samples (HAS-1, HAS-2, HAS-3, and HAS-4).

As shown in Fig. 1B, the NAS granules exhibited polarization crossover under a polarized light microscope, which confirmed the presence of a spherulite structure. After 1–3 mol/L sulfuric acid hydrolysis, a polarizing cross also appeared in the HAS granules, suggesting that the spherulite structure of starch was not destroyed, and the crystallization region of starch basically maintained its original state. However, when the concentration of sulfuric acid was increased to 4 mol/L, the polarization cross of the HAS-4 particles was not significant, indicating that the spherulite structure was seriously damaged. The particle size analysis results (Fig. 1C) were consistent with the microscopic observations, indicating that the particle size of acorn starch decreased linearly (r2 = 0.893) with increasing sulfuric acid concentration, from 8.50 μm for NAS to 6.86 μm for HAS-4.

3.2. Amylose content

As shown in Table S1, the amylose content of acorn starch declined rapidly with increasing sulfuric acid concentration, where the amylose content of NAS was 26.90%, while the amylose content of HAS-4 decreased to 2.89%. LeCorre et al. hydrolyzed high amylose corn starch, common corn starch, waxy corn starch, and potato starch with sulfuric acid (3.16 mol/L) for 5 d to prepare SNCs. The results showed that the amylose content decreased from 60 to 75%, 27%, 1%, and 21% to 11%, 1%, 0%, and 0%, respectively (LeCorre, Bras, & Dufresne, 2011). Chen et al. also found that when starch was subjected to acid hydrolysis modification, the amylose chain degraded very quickly and finally maintained a certain range, while the proportion of long amylopectin branches decreased (Chen, Xie, Zhao, Qiao, & Liu, 2017). Therefore, the results obtained in this work were consistent with these studies.

3.3. IR analysis

The change in absorption peak position and intensity in the IR spectrum reflected the change in starch molecular groups, as well as the short-range order degree of starch particles (Guo, Lin, Fan, Zhang, & Wei, 2018). In this study, the IR spectra of the obtained HAS were similar, and all demonstrated NAS characteristics (Fig. 2A), suggesting that the main chemical groups of starch were preserved during the hydrolysis process. In the deconvolution spectra of the samples (Fig. 2B), the peak intensity ratio of 1045/1022 cm−1 (R1045/1022) reflected the short-range order degree of starch, where the greater the ratio, the higher the order degree of starch. The R1045/1022 values of HAS-1, HAS-2, HAS-3, and HAS-4 were 0.836, 0.841, 0.941, and 1.045, respectively, which were all higher than the R1045/1022 value of NAS (0.818). Therefore, we inferred that the order degree of HAS increased with increasing sulfuric acid concentration.

Fig. 2.

Fig. 2

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Infrared spectra (A) and corresponding deconvolution spectra (B) of NAS and hydrolyzed acorn starch samples (HAS-1, HAS-2, HAS-3, and HAS-4).

3.4. SEC-MALLS-RI analysis

Fig. S2 presents the SEC-MALLS-RI spectra of NAS, HAS-1, HAS-2, HAS-3, and HAS-4, with the red curve representing the MALLS signal, which was proportional to the molecular size and weight of the substance. The blue curve was obtained from the RI signal, which was related to the concentration of the substance. The black blue represented the molecular weight fitted based on the MALLS and RI signals. The weight-average molecular weight (Mw) of NAS was 66,340.234 × 103 g/mol, while the Mw values of HAS-1, HAS-2, HAS-3, and HAS-4 were 160.438 × 103, 79.173 × 103, 38.638 × 103, and 37.392 × 103 g/mol, respectively. The effect of acid hydrolysis on the acorn starch particles was nonspecific, which randomly destroyed amylose and amylopectin in the amorphous starch regions, resulting in a decrease in molecular weight (Saibene & Seetharaman, 2010). The lowest concentration of sulfuric acid (1 mol/L) caused the acorn starch to rapidly hydrolyze; however, a further increase in sulfuric acid concentration showed limited improvement in the hydrolysis effect. Therefore, the MW values of HAS-3 and HAS-4 were similar.

3.5. Chain length distribution of amylopectin

Isoamylase can hydrolyze the α-1,6-glycosidic bond of amylopectin and subsequently hydrolyze amylopectin into glucans with different chain lengths (CLs). The content of glucan with different degrees of polymerization (DPs) could be determined using ion chromatography to infer the average DP (Lee & Lee, 2017). The CL distribution profiles of amylopectin for NAS and HAS are shown in Fig. 3, indicating that NAS had two obvious peaks. The first peak appeared near DP 12, while the other peak was located next to DP 42. After acid hydrolysis, HAS only contained one peak near DP 12. Amylopectin could be divided into chain A (fa, DP = 6–12), chain B1 (fb1, DP = 13–24), chain B2 (fb2, DP = 25–36), and chain B3 (fb3, DP ≥ 37) (Bertoft, 2004). Acid hydrolysis caused the amylopectin CL of acorn starch to gradually decrease, and the percentage sum of chains B2 and B3 declined significantly, while the percentage sum of chains A and B1 increased significantly. This indicates that acid hydrolysis destroyed the B2 and B3 chain structures of acorn starch and promoted the formation of chains A and B1. With increasing sulfuric acid concentration, the average CL of amylopectin also gradually became shorter, from an average DP value of 20.37 for NAS to an average DP value of 15.85 for HAS-4.

Fig. 3.

Fig. 3

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Chain length distribution profiles of amylopectin of NAS and hydrolyzed acorn starch samples (HAS-1, HAS-2, HAS-3, and HAS-4).

3.6. XRD analysis

Fig. 4A presents the XRD patterns of NAS and HAS. These starch samples exhibited strong diffraction peaks at 15°, 17°, and 23°, which were characteristic of A-type starch, indicating that their crystal type was not changed by acid hydrolysis. The relative crystallinity of NAS was 18.17%, while that of HAS increased with increasing sulfuric acid concentration (Fig. 4B). The relative crystallinity of HAS-4 reached a maximum value of 24.57%. Acid hydrolysis also removed the amorphous region of starch, leaving a dense crystallization region; therefore, the relative crystallinity of HAS was higher than that of NAS. de la Concha et al. hydrolyzed maize starch with 3.16 M sulfuric acid for 10 d The study found that the crystallinity increased by approximately 50%, and maize starch and its SNC showed a similar XRD pattern, which coincided with the results of this work (de la Concha et al., 2018). Amylopectin has been shown to play a decisive role in the ring layer size and crystal structure of starch particles. After acid hydrolysis, we observed that HAS-4 had the highest amylopectin content; therefore, it had the highest degree of crystallinity.

Fig. 4.

Fig. 4

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XRD patterns (A) and relative crystallinity values (B) of NAS and hydrolyzed acorn starch samples (HAS-1, HAS-2, HAS-3, and HAS-4).

3.7. Interfacial tension and contact angle

Starch-based Pickering emulsion offers significant application prospects in the field of solid fat substitution and delivery systems. Our previous report suggested that acorn starch had a Pickering emulsifying ability and could stabilize MCT-based Pickering HIPEs (φ = 80%). In this study, the effect of sulfuric acid hydrolysis on the interfacial tension and contact angle of acorn starch particles was investigated, providing potential insight into the connection between wettability and Pickering emulsifying ability (Geng et al., 2021). As shown in Fig. 5A, NAS could reduce the interfacial tension of the O/W interface (25.51 mN/m) compared with the control (32.06 mN/m), while HAS-1 (27.61 mN/m) and HAS-2 (27.55 mN/m) showed poor effects. However, HAS-3 (22.83 mN/m) and HAS-4 (20.70 mN/m) could significantly reduce the O/W interfacial tension, resulting in superior performance compared with that of NAS. The wettability of solid particles at the oil-water interface can be quantified by the contact angle, which determines the type of emulsion (O/W or W/O) and the thickness of the interface layer. Due to differences in surface composition and properties, different starch granules have different contact angles at the oil/water interface. Particles with a three-phase contact angle of 90° typically exhibit excellent Pickering emulsifying ability. Fig. 5B shows the contact angles of NAS and HAS. The contact angle of NAS was 117.56°, indicating high hydrophobicity, while the contact angles of HAS-1, HAS-2, HAS-3, and HAS-4 were 54.49°, 57.47°, 83.42°, and 92.43°, respectively. We subsequently concluded that sulfuric acid hydrolysis changed the interfacial properties of acorn starch particles. The contact angles of HAS-3 and HAS-4 were similar to the optimal theoretical value (90°) for Pickering emulsification; therefore, their Pickering emulsifying performance was expected to surpass that of NAS.

Fig. 5.

Fig. 5

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Interfacial tension (A) and contact angle (B) results of NAS and hydrolyzed acorn starch samples (HAS-1, HAS-2, HAS-3, and HAS-4).

3.8. Formation of the Pickering HIPEs

We evaluated the effect of acid hydrolysis on the Pickering emulsifying ability of acorn starch. In this study, MCT-based Pickering HIPEs stabilized by NAS, HAS-1, HAS-2, HAS-3, and HAS-4 were constructed at φ = 80% and c = 0.5–3% (Fig. S3). The results showed that NAS, HAS-1, and HAS-2 could only stabilize Pickering HIPEs at c = 3%. HAS-3 could build stable Pickering HIPEs at c ≥ 1.5%, while HAS-4 could stabilize Pickering HIPEs at c ≥ 1%. The Pickering emulsifying ability of acorn starch was not improved by low sulfuric acid concentrations (1 and 2 mol/L) but significantly increased with high sulfuric acid concentrations (3 and 4 mol/L). These observations were consistent with the contact angle and interfacial tension test results.

3.9. Microscopic observations of the Pickering HIPEs

The microscopy images of the MCT-based Pickering HIPEs stabilized by HAS-3 and HAS-4 at φ = 80% and c = 1.5–3% are shown in Fig. S4. With the same starch concentration, the droplet size of the emulsion constructed with HAS-4 was lower than that of the emulsion constructed with HAS-3, confirming its highest emulsifying ability, which coincided with the results of 3.7, 3.8. In addition, for the same starch, the droplet size of the HIPEs prepared at c = 3.0% was lower than that of the HIPEs prepared at c = 2.5%. This could be attributed to the fact that increasing the emulsifier concentration met the higher demand for oil-water interface area and subsequently led to a reduction in droplet size (Frelichowska, Bolzinger, & Chevalier, 2010).

3.10. Mechanical properties of the Pickering HIPEs

We subsequently evaluated the gel strength and rheological properties of the MCT-based Pickering HIPEs stabilized by HAS-3 and HAS-4 at φ = 80% and c = 1.5–3% (Fig. S5). The gel strength values of the HIPEs increased linearly with increasing starch concentration (r2 = 0.998 and 0.983 for HAS-3 and HAS-4, respectively). Geng et al. also found a similar phenomenon when studying the formation mechanism of a procyanidin particle-stabilized Pickering emulsion gel (Geng et al., 2023). This was attributed to the viscosity effect of the starch, where the higher the starch concentration, the higher the viscosity of the system, the more difficult it was for oil droplets to merge, and the higher the strength of the system. With the same starch concentration, the gel strength values of the HIPEs formed by HAS-4 were higher, mainly due to the higher Pickering emulsifying ability of HAS-4, resulting in a smaller oil droplet size and tighter system. Liu et al. also reported that the gel strength of the Pickering HIPEs stabilized by β-cyclodextrin and beet pectin was positively correlated with the emulsifying ability of the particles (Liu, Li, Geng, Mo, & Liu, 2021).

Fig. 6 compares the rheological properties of the Pickering HIPEs stabilized by HAS-3 and HAS-4. The results showed that the storage modulus (G') and loss modulus (G") of all samples remained stable within a certain strain range (LVR) (Fig. 6A), and a shear stress of 1 Pa was selected for the follow-up experiments. As shown in Fig. 6B, the G' values of all samples were higher than the corresponding G" values in the experimental frequency range, which confirmed their gel-like properties. The G′ and G′′ values of the HIPEs developed using the same starch were positively correlated with the starch concentration, which was consistent with the results of Liu et al. for the preparation and characterization of β-cyclodextrin-stabilized Pickering HIPEs (Liu, Geng, Jiang, & Liu, 2020). Therefore, the higher the starch concentration (3%), the stronger the viscoelastic properties of the HIPEs. Under the same starch concentration, the performance of the HIPEs constructed with HAS-4 was slightly superior to that of HAS-3. In the shear rate scanning test, the apparent viscosity of all samples decreased with increasing shear rate (Fig. 6C), and with the same shear rate and starch concentration, the apparent viscosity of the HIPEs constructed with HAS-4 was higher than that of the HIPEs constructed with HAS-3. Moreover, the apparent viscosity of all samples was positively correlated with c, which was consistent with the gel concentration results.

Fig. 6.

Fig. 6

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Rheological properties of the Pickering HIPEs stabilized by HAS-3 and HAS-4 at c = 2.5 and 3%, φ = 80%: (A) strain sweep test; (B) frequency.

4. Conclusions

The results of this study showed that sulfuric acid concentration (1–4 mol/L) had a significant effect on the appearance, structure, and Pickering emulsifying ability of HAS. With increasing sulfuric acid concentration, the size, yield, amylose content, molecular weight, and amylopectin CL gradually decreased. HAS had an A-type crystal structure, and its relative crystallinity and order degree gradually increased with increasing sulfuric acid concentration. HAS could stabilize MCT-based Pickering HIPEs at φ = 80%, and the emulsifying performance of HAS was superior to that of NAS, which was positively correlated with the sulfuric acid concentration. In addition, although the emulsifying performance of HAS-3 was slightly inferior to that of HAS-4, a lower amount of sulfuric acid was used in the preparation of HAS-3, and the appearance and yield of HAS-3 were significantly superior to those of HAS-4. Therefore, HAS-3 could be used as a potential food-grade Pickering emulsifier.

CRediT authorship contribution statement

Changsheng Guo: Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Methodology, Investigation, Formal analysis, Data curation. Sheng Geng: Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Formal analysis, Data curation. Yuzhong Shi: Writing – review & editing, Writing – original draft, Methodology, Formal analysis, Data curation. Chao Yuan: Writing – review & editing, Writing – original draft, Supervision, Resources, Project administration, Funding acquisition, Conceptualization. Benguo Liu: Writing – review & editing, Writing – original draft, Visualization, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Foundation of State Key Laboratory of Biobased Material and Green Papermaking, Qilu University of Technology, Shandong Academy of Sciences (No. GZKF202303), the Natural Science Foundation of Henan Province of China (No. 212300410005) and the National Natural Science Foundation of China (No. 32072180).

Footnotes

Appendix A

Supplementary data to this article can be found online at https://doi.org/10.1016/j.fochx.2024.101277.

Contributor Information

Chao Yuan, Email: yuanchao@qlu.edu.cn.

Benguo Liu, Email: liubenguo@hist.edu.cn.

Appendix A. Supplementary data

Supplementary material

mmc1.docx (4.3MB, docx)

Data availability

Data will be made available on request.

References

  1. Angellier H., Choisnard L., Molina-Boisseau S., Ozil P., Dufresne A. Optimization of the preparation of aqueous suspensions of waxy maize starch nanocrystals using a response surface methodology. Biomacromolecules. 2004;5(4):1545–1551. doi: 10.1021/bm049914u. [DOI] [PubMed] [Google Scholar]
  2. Azfaralariff A., Farahfaiqah F., Joe L.S., Fazry S., Mohamed M., Nazar M.F., Lazim A.M. Sago starch nanocrystal-stabilized Pickering emulsions: Stability and rheological behavior. International Journal of Biological Macromolecules. 2021;182:197–206. doi: 10.1016/j.ijbiomac.2021.03.132. [DOI] [PubMed] [Google Scholar]
  3. Bertoft E. On the nature of categories of chains in amylopectin and their connection to the super helix model. Carbohydrate Polymers. 2004;57(2):211–224. doi: 10.1016/j.carbpol.2004.04.015. [DOI] [Google Scholar]
  4. Burrell M.M. Starch: The need for improved quality or quantity—An overview. Journal of Experimental Botany. 2003;54(382):451–456. doi: 10.1093/jxb/erg049. [DOI] [PubMed] [Google Scholar]
  5. Chen P., Xie F., Zhao L., Qiao Q., Liu X. Effect of acid hydrolysis on the multi-scale structure change of starch with different amylose content. Food Hydrocolloids. 2017;69:359–368. doi: 10.1016/j.foodhyd.2017.03.003. [DOI] [Google Scholar]
  6. de la Concha B.B.S., Agama-Acevedo E., Nuñez-Santiago M.C., Bello-Perez L.A., Garcia H.S., Alvarez-Ramirez J. Acid hydrolysis of waxy starches with different granule size for nanocrystal production. Journal of Cereal Science. 2018;79:193–200. doi: 10.1016/j.jcs.2017.10.018. [DOI] [Google Scholar]
  7. Copeland L., Blazek J., Salman H., Tang M.C. Form and functionality of starch. Food Hydrocolloids. 2009;23(6):1527–1534. doi: 10.1016/j.foodhyd.2008.09.016. [DOI] [Google Scholar]
  8. Duan B., Sun P., Wang X., Yang C. Preparation and properties of starch nanocrystals/carboxymethyl chitosan nanocomposite films. Starch-Stärke. 2011;63(9):528–535. doi: 10.1002/star.201000136. [DOI] [Google Scholar]
  9. Frelichowska J., Bolzinger M.A., Chevalier Y. Effects of solid particle content on properties of o/w Pickering emulsions. Journal of Colloid and Interface Science. 2010;351(2):348–356. doi: 10.1016/j.jcis.2010.08.019. [DOI] [PubMed] [Google Scholar]
  10. García N.L., Ribba L., Dufresne A., Aranguren M., Goyanes S. Effect of glycerol on the morphology of nanocomposites made from thermoplastic starch and starch nanocrystals. Carbohydrate Polymers. 2011;84(1):203–210. doi: 10.1016/j.carbpol.2010.11.024. [DOI] [Google Scholar]
  11. Geng S., Han F., Lv X., Zhang S., Ma H., Liu B. Formation mechanism of Pickering emulsion gels stabilized by proanthocyanidin particles: Experimental and molecular dynamics studies. Food Chemistry. 2023;418 doi: 10.1016/j.foodchem.2023.135904. [DOI] [PubMed] [Google Scholar]
  12. Geng S., Jiang Z., Ma H., Pu P., Liu B., Liang G. Fabrication and characterization of novel edible Pickering emulsion gels stabilized by dihydromyricetin. Food Chemistry. 2021;343 doi: 10.1016/j.foodchem.2020.128486. [DOI] [PubMed] [Google Scholar]
  13. Guo C., Han F., Geng S., Shi Y., Ma H., Liu B. The physicochemical properties and Pickering emulsifying capacity of acorn starch. International Journal of Biological Macromolecules. 2023;239 doi: 10.1016/j.ijbiomac.2023.124289. [DOI] [PubMed] [Google Scholar]
  14. Guo K., Lin L., Fan X., Zhang L., Wei C. Comparison of structural and functional properties of starches from five fruit kernels. Food Chemistry. 2018;257:75–82. doi: 10.1016/j.foodchem.2018.03.004. [DOI] [PubMed] [Google Scholar]
  15. Jeong D., Han J.A., Liu Q., Chung H.J. Effect of processing, storage, and modification on in vitro starch digestion characteristics of food legumes: A review. Food Hydrocolloids. 2019;90:367–376. doi: 10.1016/j.foodhyd.2018.12.039. [DOI] [Google Scholar]
  16. Kim H.Y., Lee J.H., Kim J.Y., Lim W.J., Lim S.T. Characterization of nanoparticles prepared by acid hydrolysis of various starches. Starch-Stärke. 2012;64:367–373. doi: 10.1002/star.201100105. [DOI] [Google Scholar]
  17. Kumari S., Yadav B.S., Yadav R.B. Synthesis and modification approaches for starch nanoparticles for their emerging food industrial applications: A review. Food Research International. 2020;128 doi: 10.1016/j.foodres.2019.108765. [DOI] [PubMed] [Google Scholar]
  18. Le Corre D., Bras J., Dufresne A. Starch nanoparticles: A review. Biomacromolecules. 2010;11(5):1139–1153. doi: 10.1021/bm901428y. [DOI] [PubMed] [Google Scholar]
  19. LeCorre D., Bras J., Dufresne A. Influence of botanic origin and amylose content on the morphology of starch nanocrystals. Journal of Nanoparticle Research. 2011;13(12):7193–7208. doi: 10.1007/s11051-011-0634-2. [DOI] [Google Scholar]
  20. Lee B.H., Lee Y.T. Physicochemical and structural properties of different colored sweet potato starches. Starch-Stärke. 2017;69(3–4):1600001. doi: 10.1002/star.201600001. [DOI] [Google Scholar]
  21. Liu C., Li K., Li X., Zhang M., Li J. Formation and structural evolution of starch nanocrystals from waxy maize starch and waxy potato starch. International Journal of Biological Macromolecules. 2021;180:625–632. doi: 10.1016/j.ijbiomac.2021.03.115. [DOI] [PubMed] [Google Scholar]
  22. Liu Z., Geng S., Jiang Z., Liu B. Fabrication and characterization of food-grade Pickering high internal emulsions stabilized with β-cyclodextrin. LWT- Food Science and Technology. 2020;134 doi: 10.1016/j.lwt.2020.110134. [DOI] [Google Scholar]
  23. Liu Z., Li Y., Geng S., Mo H., Liu B. Fabrication of food-grade Pickering high internal phase emulsions stabilized by the mixture of β-cyclodextrin and sugar beet pectin. International Journal of Biological Macromolecules. 2021;182:252–263. doi: 10.1016/j.ijbiomac.2021.04.002. [DOI] [PubMed] [Google Scholar]
  24. López-Hernández R.E., García-Solís S.E., Monroy-Rodríguez I., Cornejo-Mazón M., Calderón-Domínguez G., Alamilla-Beltrán L.…Gutiérrez-López G.F. Preparation and characterization of canola oil-in-water Pickering emulsions stabilized by barley starch nanocrystals. Journal of Food Engineering. 2022;326 doi: 10.1016/j.jfoodeng.2022.111037. [DOI] [Google Scholar]
  25. Lv X., Zhang S., Zhen S., Shi Y., Liu B. Physicochemical properties of tigernut (Cyperus esculentus) tuber starch and its application in steamed bread. Journal of Food Processing and Preservation. 2022;46(8) doi: 10.1111/jfpp.16792. [DOI] [Google Scholar]
  26. Miyazaki M., Van Hung P., Maeda T., Morita N. Recent advances in application of modified starches for breadmaking. Trends in Food Science & Technology. 2006;17(11):591–599. doi: 10.1016/j.tifs.2006.05.002. [DOI] [Google Scholar]
  27. Saibene D., Seetharaman K. Amylose involvement in the amylopectin clusters of potato starch granules. Carbohydrate Polymers. 2010;82(2):376–383. doi: 10.1016/j.carbpol.2010.04.070. [DOI] [Google Scholar]
  28. Singh V., Ali S.Z. Properties of starches modified by different acids. International Journal of Food Properties. 2008;11(3):495–507. doi: 10.1080/10942910802083774. [DOI] [Google Scholar]
  29. Tripathi S., Bhattacharya A., Singh R., Tabor R.F. Rheological behavior of high internal phase water-in-oil emulsions: Effects of droplet size, phase mass fractions, salt concentration and aging. Chemical Engineering Science. 2017;174:290–301. doi: 10.1016/j.ces.2017.09.016. [DOI] [Google Scholar]
  30. Velásquez-Castillo L.E., Leite M.A., Ditchfield C., do Amaral Sobral P.J., Moraes I.C.F. Quinoa starch nanocrystals production by acid hydrolysis: Kinetics and properties. International Journal of Biological Macromolecules. 2020;143:93–101. doi: 10.1016/j.ijbiomac.2019.12.011. [DOI] [PubMed] [Google Scholar]
  31. Wan W., Li W., Sun L., Liu H., Xia X. Effects of freeze-thaw cycles on in-vitro digestive properties of myofibrillar protein in mirror carp (Cyprinus carpio L.), based on protein degradation, oxidation, and structural properties. Food Chemistry. 2024;436 doi: 10.1016/j.foodchem.2023.137662. [DOI] [PubMed] [Google Scholar]
  32. Wang B., Zhang G., Yan S., Xu X., Wang D., Cui B., Abd El-Aty A.M. Correlation between chain structures of corn starch and properties of its film prepared at different degrees of disorganization. International Journal of Biological Macromolecules. 2023;226:580–587. doi: 10.1016/j.ijbiomac.2022.12.084. [DOI] [PubMed] [Google Scholar]
  33. Wang S., Sharp P., Copeland L. Structural and functional properties of starches from field peas. Food Chemistry. 2011;126(4):1546–1552. doi: 10.1016/j.foodchem.2010.11.154. [DOI] [PubMed] [Google Scholar]
  34. Wei B., Hu X., Zhang B., Li H., Xu X., Jin Z., Tian Y. Effect of defatting on acid hydrolysis rate of maize starch with different amylose contents. International Journal of Biological Macromolecules. 2013;62:652–656. doi: 10.1016/j.ijbiomac.2013.10.008. [DOI] [PubMed] [Google Scholar]
  35. Wei Z., Huang Q. Edible Pickering emulsions stabilized by ovotransferrin–gum arabic particles. Food Hydrocolloids. 2019;89:590–601. doi: 10.1016/j.foodhyd.2018.11.037. [DOI] [Google Scholar]
  36. Zhang N., Jiang J.C., Yang J., Wei M., Zhao J., Xu H.…Yu L. Citric acid production from acorn starch by tannin tolerance mutant aspergillus Niger AA120. Applied Biochemistry and Biotechnology. 2019;188:1–11. doi: 10.1007/s12010-018-2902-4. [DOI] [PubMed] [Google Scholar]
  37. Zhang Y., Rempel C., Liu Q. Thermoplastic starch processing and characteristics—A review. Critical Reviews in Food Science and Nutrition. 2014;54(10):1353–1370. doi: 10.1080/10408398.2011.636156. [DOI] [PubMed] [Google Scholar]
  38. Zheng H., Ai F., Chang P.R., Huang J., Dufresne A. Structure and properties of starch nanocrystal-reinforced soy protein plastics. Polymer Composites. 2009;30(4):474–480. doi: 10.1002/pc.20612. [DOI] [Google Scholar]
  39. Zhu F., Cui R. Comparison of molecular structure of oca (Oxalis tuberosa), potato, and maize starches. Food Chemistry. 2019;296:116–122. doi: 10.1016/j.foodchem.2019.05.192. [DOI] [PubMed] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supplementary material

mmc1.docx (4.3MB, docx)

Data Availability Statement

Data will be made available on request.

Articles from Food Chemistry: X are provided here courtesy of Elsevier

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