Article pubs.acs.org/JAFC
Soy Soluble Polysaccharide as a Nanocarrier for Curcumin Fei-Ping Chen,† Shi-Yi Ou,§ Zhong Chen,*,† and Chuan-He Tang*,†,‡ †
Department of Food Science and Technology, South China University of Technology, Guangzhou 510640, People’s Republic of China § Department of Food Science and Engineering, Jinan University, Guangzhou 510632, People’s Republic of China ‡ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China ABSTRACT: The complexation between soy soluble polysaccharide (SSPS) and curcumin at pH 7.0 and 4.0, as well as some physicochemical characteristics of the resultant complexes, was investigated. The encapsulation efficiency and loading amount of curcumin in the complexes at pH 4.0 reached 67.3% and 4.49 μg/mg SSPS, respectively. Ethanol-induced denaturation and structural unfolding of the protein fraction in SSPS was essential for complex formation. The complexation with curcumin resulted in aggregation of SSPS and the subsequent formation of compacted nanoparticles with curcumin as the core. The complexation greatly improved the heat stability and in vitro bioaccessibility of curcumin. In general, the encapsulation efficiency, heat stability, and bioaccessibility of curcumin in the complexes at pH 4.0 were better than those at pH 7.0. The findings are of importance for the development of food grade nanovehicles for enhanced water solubility, stability, and bioaccessibility of hydrophobic bioactives. KEYWORDS: curcumin, soy soluble polysaccharide (SSPS), nanocarrier, complexation, bioaccessibility
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fraction).15 This structural characteristic imparts to this polysaccharide a good surface activity, and as a result, it can be used as an emulsifier or stabilizer for oil-in-water emulsions,13,14,18 as a thickening agent for acidic milk products,19 or as a microencapsulation wall materials for lipid-soluble bioactives.20,21 More importantly, SSPS can be a source of dietary fiber, thus exhibiting a great potential to be formed in the functional food formulations.22,23 In our recent work, it was reported that SSPS can be used to form core−shell soy protein nanoparticles as carriers for further improving stability and sustained-release behavior of encapsulated bioactives, for example, curcumin.24 To the best of our knowledge, no work is available reporting the potential of this polysaccharide alone to be applied as (nano)carriers for bioactives. The main objective of the current work was to investigate the complexation of SSPS and curcumin at two specific pH values of 7.0 and 4.0, as well as the potential of SSPS as a nanovehicle for curcumin. In the first part, the interaction between SSPS and curcumin was characterized using a fluorescence technique. Considering that the complexation between SSPS and curcumin was performed in the presence of an appropriate concentration (e.g., 40%, v/v) of ethanol, both untreated and ethanol-pretreated SSPSs were tested for their complexation with curcumin, with the aim of unraveling the importance of ethanol pretreatment to the interaction between SSPS and curcumin. Subsequently, the correspondingly formed SSPS− curcumin complexes were characterized in terms of particle size
INTRODUCTION In recent years, the development of food-grade (nano)carriers for hydrophobic bioactives, especially those with poor water solubility and low bioavailability, has attracted increasing interest in the functional food field. Curcumin (bis-α,βunsaturated β-diketone), is a polyphenolic compound extracted from the rhizomes of the turmeric plant, Curcuma longa, that possesses a number of biological activities, including antioxidant, anti-inflammatory,and anticancer activities.1 However, the extremely low water solubility and poor bioavailability of this compound greatly limits its application in formulations of functional foods. To date, many strategies or technologies have been proposed to overcome this limitation, for example, molecular complexation with proteins or cyclodextrins,2−8 emulsion formulation,9 or fabrication of (nano)particles.10−12 Of all these, molecular complexation seems to be one of the most promising encapsulation techniques for curcumin. In this regard, the potential of many food proteins to perform as effective (nano)vehicles for curcumin has been widely investigated. 2,3,5−8 In contrast, nonprotein vehicles for curcumin are much less investigated. Soy soluble polysaccharide (SSPS), an acidic polysaccharide composed of galacturonan and rhamnogalacturonan as the main backbone and arabinan and galactan as branches, is an important food ingredient isolated from the residual byproduct of soy processing. SSPS includes a ∼50 kDa protein moiety covalently bound to the carbohydrate backbone chain.13,14 The protein content of SSPS is in the range 1.2−8.2% (w/w), depending on the preparation process.15−17 Although the protein fraction in SSPS may contain some positively charged amino acid residues, SSPS is negatively charged within a pH range of 2−12, as a consequence of a much higher amount of negatively charged galacturoric acid (than that of the protein © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
November 13, 2016 February 10, 2017 February 10, 2017 February 10, 2017 DOI: 10.1021/acs.jafc.6b05087 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry
SSPS or its complexes) was diluted to a polysaccharide concentration of about 0.05% (w/v) with phosphate buffer (5 mM, pH 7.0) or citrate−phosphate buffer (5 mM, pH 4.0). All of the used buffers were filtered with a 0.22 μm HA Millipore membrane filter. DLS analysis was performed using a Zetasizer Nano-ZS instrument (Malvern Instruments, Worcestershire, UK) equipped with a 4 mW He−Ne laser (633 nm wavelength) at a fixed angle of 173° at 25 °C. Droplet sizing was performed at 10 s intervals in a particle-sizing cell using backscattering technology. The morphology of particles in different SSPS dispersions, before and after complexation with curcumin, was evaluated using atomic force microscopy (AFM). The AFM images were acquired in tapping mode using a Dimension 3000 microscope (Digital InstrumentsVeeco, Santa Barbara, CA, USA), equipped with a ‘G′ scanning head (maximum scan size = 10 μm) and driven by a Nanoscope III a controller. Various SSPS dispersions (5 mg/mL) were diluted with 5 mM phosphate buffer (pH 7.0) or citrate−phosphate buffer (pH 4.0) to a concentration of 5 μg/mL. A droplet (2 μL) of each diluted sample was immediately spread on a freshly cleaved mica disk in air overnight at ambient temperature. Single-beam uncoated silicon cantilevers (type OMCL-AC, Olympus, and RTESP, Veeco) were used for the imaging, and the drive frequency and scan rate were set at 300 kHz and 2 Hz, respectively. Heat Stability. The heat stability of curcumin, free or complexed with SSPS in water (pH 7.0 or 4.0), was evaluated by monitoring the degradation kinetics of curcumin at 80 °C. A total volume of 200 μL of samples was withdrawn at predetermined time intervals, and the curcumin extraction and determination were performed according to the same processes described above. Considering that the degradation of curcumin is dramatically affected by the pH, especially under neutral-basic conditions, where a slight change in pH may remarkably affect the degradation of curcumin,26 the dispersion containing free curcumin (22 μg/mL) was prepared by adding the stock curcumin solution (in ethanol) into 5 mM phosphate buffer (pH7.0) or citrate− phosphate (pH 4.0). The ethanol concentration in the dispersions was 95% was purchased from Gen-View Scientific, Inc. (China). Porcine bile extract (B8631), pancreatin (P1750, from porcine pancreas, 4×USP), pepsin (P7000, from porcine gastric mucosal, 975 units/mg of protein), and curcumin (purity > 98%) were purchased from Sigma Chemical Co. (St. Louis, MO, USA). All other chemicals were of analytical grade. Complexation of SSPS and Curcumin and Preparation of the Resulting Complexes. Stock SSPS solution (5.0 mg/mL) was prepared by dissolving SSPS powder in deionized water, whereas stock curcumin solution (1.0 mg/mL) was prepared in absolute ethyl alcohol. After storage of the stock SSPS solution overnight, absolute ethyl alcohol was added dropwise to reach a concentration of approximately 35% (v/v), and the pH of the dispersion was adjusted to 7.0 or 4.0 with 0.1 M NaOH or 1 M HCl and then kept stirred for 20 min. Subsequently, the stock curcumin solution was added dropwise to the SSPS dispersion until a curcumin-to-SSPS weight ratio of 1:150 was reached. The ethanol volume fraction in the mixed dispersions was precisely adjusted to 0.4 (v/v) by the addition of absolute ethyl alcohol, and the resultant dispersions were further stirred at room temperature for 3 h (to complete the complexation between SSPS and curcumin). Finally, the ethanol in the dispersions was removed by a rotary evaporator under vacuum conditions at 40 °C, and any insoluble matter in these dispersions was removed by centrifugation at 10000g for 10 min. After the removal of ethanol, deionized water was added to maintain consistency with the initial volume of the dispersions. The controls without addition of curcumin, prepared according to the same process as above, were denoted the dispersions containing ethanol-pretreated SSPS. Encapsulation Efficiency (EE) and Loading Amount (LA) of Curcumin. The curcumin encapsulated in the complexes with SSPS was extracted with an organic solvent composed of ethyl acetate and ethanol at a volume ratio of 10:1. In brief, 3 mL of the organic solvent was added to 0.2 mL of the dispersions containing these SSPS− curcumin complexes. The resultant mixtures were then vortexed for 10 s and left for layering, and the supernatants (organic phase) were collected. The amount of curcumin in the supernatants was determined at 420 nm with a UV754N UV−vis spectrophotometer (Precision & Scientific Instrument, Shanghai, China), according to an established standard curve (R2 = 0.9999) of standard curcumin in the same solvent. The %EE and LA were calculated as follows (eqs 1 and 2): %EE =
curcumin encapsulated in SSPS × 100 total amount of added curcumin
(1)
curcumin encapsulated in SSPS SSPS amount
(2)
LA (μg/mg) =
Interaction between SSPS and Curcumin. The interaction between SSPS and curcumin was evaluated by steady state fluorescence technique, using an F4500 fluorescence spectrophotometer (Hitachi Co., Japan), according to the method previously described by Li and others,25 with a few modifications. The fluorescence determination was performed at a constant curcumin concentration of 10 μM and increasing concentrations (0−1.5 mg/ mL) of SSPS (native or ethanol-pretreated). The emission spectra were recorded from 450 to 700 nm with an excitation wavelength of 430 nm, and the excitation and emission slit widths used were both 5 nm. Characterization of SSPS−Curcumin Complexes. The particle size distribution profiles of native or ethanol-treated SSPS and SSPS− curcumin complexes, at pH 7.0 or 4.0, were evaluated using a dynamic light scattering (DLS) technique. Each tested dispersion (containing B
DOI: 10.1021/acs.jafc.6b05087 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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RESULTS AND DISCUSSION Formation of SSPS−Curcumin Complexes. The complexation of SSPS with curcumin was performed by directly mixing both the SSPS solution and the stock curcumin solution (in ethanol), at various volume ratios. One point to note is that the stability of SSPS is highly dependent on the ethanol concentration in the resultant mixtures. If the ethanol concentration was >40%, the solubility of SSPS would dramatically decrease, and as a consequence, the viscosity of the mixtures distinctly increased. Due to this consideration, all of the complexation experiments were carried out at a constant ethanol concentration of 40% (v/v) in the final mixtures. After 3 h of mixing, the ethanol in the dispersions containing SSPS and curcumin mixtures was evaporated under vacuum, and the dispersions were then centrifuged to remove any insoluble matter. As expected, free curcumin in water was completely insoluble (Figure 1A). In the presence of SSPS, it
summarized in Table 1. The results indicated that the %EE and LA at pH 4.0 were significantly higher than those at pH 7.0 (67 vs 51% for %EE; 4.49 vs 3.41 μg/mg SSPS for LA), suggesting that the acidic environment was favorable for nanocomplex formation. Although the LA value (0.34−0.45%) is considerably lower than that for spray-dried casein nanocapsules (16.7%)27 or zein colloidal particles formed by an emulsification and evaporation method (1.6−4.1%),11 it is within the LA range reported for soy protein isolate (SPI)−curcumin complexes (0.1−10%, w/w).2,3,7 When the dispersions containing SSPS−curcumin nanocomplexes were freeze-dried and reconstituted in water, there was no noticeable change in transparency between the initial and dried/reconstituted dispersions (Figure 1). However, the yellow color became relatively weak, especially for the dispersion at pH 7.0 (Figure 1C), indicating degradation of curcumin. For the dispersion at pH 7.0, about 66.6% of curcumin in the nanocomplexes was degraded after the freezedrying and reconstitution, whereas for the dispersion at pH 4.0, it was only 37.9% (Table 1). The observation showed that besides the higher LA, the nanocomplexes at pH 4.0 provided a higher stability to their encapsulated curcumin than that at pH 7.0. One percent (w/v) of the SSPS−curcumin complex powder (pH 4.0) in water would give 27.9 μg/mL solubilized curcumin (2500-fold as compared to that in water),28 which is higher than that (1670-fold) of hydrophobically modified starch (1670-fold).29 Interaction between Curcumin and SSPS. To confirm the occurrence of complexation between SSPS and curcumin, the intrinsic fluorescence emission spectra of curcumin as affected by increasing concentrations of SSPS were determined, as displayed in Figure 2. When free curcumin alone was excited at 430 nm, it showed a low-intensity emission fluorescence peak at around 578 nm. In the first set of experiments, the interaction between curcumin and native SSPS, in the absence of ethanol, was characterized. It can be observed that the emission fluorescence spectrum of curcumin was slightly affected by the presence of increasing concentrations (0.1− 1.5 mg/mL) of SSPS (Figure 2A), indicating weak interaction between curcumin and native SSPS. A similar phenomenon has been observed for curcumin when mixed with another kind of polysaccharide, ι-carrageenan.8 In the current work, the complexation between curcumin and SSPS was performed at an ethanol concentration of 40% (v/v), at which their interaction might be remarkably strengthened. To confirm the strong interaction between curcumin and SSPS in this environment, SSPS was first pretreated at an ethanol concentration of 40% (v/v) and then added to the curcumin dispersion. Interestingly, it can be observed that, at both pH 7.0 and 4.0, the SSPS addition with increasing concentrations of
Figure 1. Visual observations of free curcumin, as well as original and dried/reconstituted SSPS−curcumin complex dispersions in water, at pH 7.0 or 4.0L (A) free curcumin in water; (B) original dispersion containing SSPS−curcumin complex; (C) dried/reconstituted dispersion containing SSPS−curcumin complex.
can be interestingly observed that the resultant dispersions containing SSPS and curcumin mixtures (without ethanol) at both pH 7.0 and 4.0 were completely transparent and yellow in appearance (Figure 1B), indicating the formation of SSPS− curcumin nanocomplexes in the dispersions, which remarkably improved the water solubility of curcumin in the aqueous phase. The encapsulation efficiency (%EE) and loading amount (LA) of curcumin in these nanocomplexes at pH 7.0 or 4.0 are
Table 1. Encapsulation Efficiency (%EE) and Loading Amount (LA) of Curcumin in the Complexes with SSPS at pH 7.0 and 4.0a original
dried/reconstituted
item
pH 7.0
pH 4.0
pH 7.0
pH 4.0
%EE LA (μg/mg SSPS) increasing foldb
51.1 ± 0.8b 3.41 ± 0.05b 3100
67.3 ± 2.5a 4.49 ± 0.16a 4081
1.14 ± 0.14d 1036
2.79 ± 0.33c 2536
a Data presented are the means and standard deviation error bars of triplicates. Different letters (a−d) represent significant difference at p < 0.05 level within the same row. bImprovement of solubility of curcumin for SSPS−curcumin complexes (at a total solid concentration of 1.0%, w/v) as compared with that of free curcumin in water (11 ng/mL; 26).
C
DOI: 10.1021/acs.jafc.6b05087 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Figure 2. Fluorescence emission spectra of curcumin (at a constant concentration of 10 μM) in buffer solution at pH 7.0 (A, B) and pH 4.0 (C) in the presence of increasing concentrations (0−1.5 mg/mL) of SSPS: (A) native SSPS (pH 7.0); (B) ethanol-pretreated SSPS at pH 7.0; (C) ethanolpretreated SSPS at pH 4.0.
Figure 2B,C). The differences of the changes might be attributed to the differences in intermolecular associations between the curcumin-loaded SSPS molecules at these two pH values. Particle Size and Morphology of SSPS−Curcumin (Nano)complexes. To confirm the differences in intermolecular associations between pH 7.0 and 4.0, the size and morphology of SSPS−curcumin complexes formed at these two pH values were characterized by DLS and AFM observations. Figure 3 shows the representative particle size distribution profiles of SSPS in the absence or presence of 40% ethanol, as well as SSPS−curcumin complexes at pH 7.0 and 4.0. In the absence of ethanol, the SSPS at pH 7.0 had a monomodal particle distribution with sizes ranging from 3 to 12 nm (Figure 3). At this pH, the presence of 30% (v/v) ethanol led to a marked shift of the distribution peak to larger sizes; for example, the particle distribution peak shifted from 4.5 to 9.8 nm (Figure 3). The increase in size by the presence of 40% (v/ v) ethanol might result from two reasons. One is that the presence of ethanol would induce denaturation and/or unfolding of the protein fraction in SSPS, subsequently leading to the increase in flexibility of polysaccharide chains. The other is that in the presence of 40% (v/v) ethanol, SSPS molecules would interact to form aggregates. To determine which reason would play a dominant role, the morphology of SSPS at pH 7.0 in the absence or presence of ethanol was observed by AFM, as shown in Figure 4. As expected, native SSPS (without ethanol)
0.1−1.5 mg/mL resulted in a progressive and remarkable increase in fluorescence peak intensity of curcumin and, concurrently, a progressive blue-shift of the emission maximum (of curcumin; λm) (Figure 2B,C). The observations clearly indicated that the chromophores of curcumin shifted to a more nonpolar environment when ethanol-pretreated SSPS was present, confirming that the complexation between curcumin and this SSPS was mainly driven by the interactions of hydrophobic nature. The differences in reactivity between native and ethanol-pretreated SSPS may be related to the ethanol-induced denaturation and unfolding of its protein fraction. The complexation between curcumin and proteins, mainly driven by hydrophobic interactions, has been well recognized in the literature.5,7 The improvement of complexation between curcumin and proteins by denaturing the proteins has been reported for heated casein micelles,30 high hydrostatic pressure-treated milk proteins,31 ultrasonic-treated SPI,3 and glycosylated α-lactalbumin.32 Another interesting noteworthy point is that, in the presence of 40% (v/v) ethanol, the changing extents of the maximal fluorescence intensity and λm upon the addition of SSPS with increasing concentrations were different at pH 7.0 and 4.0 (Figure 2B,C). For example, the maximal fluorescence intensity at pH 7.0 progressively increased from 325 to 1580 as the SSPS concentration increased from 0 to 1.5 mg/mL, whereas at pH 4.0, it increased to 2600. On the other hand, the blue-shift of λm was much more distinct at pH 7.0 than at pH 4.0 (74 vs 44 nm; D
DOI: 10.1021/acs.jafc.6b05087 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Figure 3. Typical particle size distribution profiles of SSPS in the absence (native) or presence of 40% (v/v) ethanol, as well as SSPS− curcumin complexes (in the presence of 40%, v/v), at pH 7.0 or 4.0.
could not be easily observed by AFM, due to its flexible conformation of polysaccharide chains, and in this case, only a limited number of particles with heights of 1−2 nm were observed (Figure 4A). The morphology of SSPS was nearly unaffected by the presence of 40% (v/v) ethanol (Figure 4A,B). The observations clearly excluded the possibility that the presence of 40% (v/v) ethanol led to aggregation of SSPS molecules alone. This means that the increased hydrodynamic size of SSPS by the presence of 40% (v/v) ethanol was largely attributed to the ethanol-induced unfolding or increase in flexibility of SSPS molecules. When the pH was changed from 7.0 to 4.0, the particle sizes of SSPS became a bit less, as evidenced by a shift of the distribution peak toward smaller sizes (e.g., from 12 to 9.8 nm; Figure 3) and slight decreases in particle heights (Figure 4B,C). This observation suggested that the pH change (from 7.0 to 4.0) might result in the formation of a more compacted conformation of SSPS, in the presence of 40% (w/v) ethanol. This is in accordance with the fact that SSPS at pH 7.0 exhibits a higher magnitude of ξ-potential than at pH 4.0 (−43 vs −32 mV; data not shown) because a more negative surface charge would lead to stronger electrostatic repulsion between the SSPS molecules. When in the presence of 40% (v/v) ethanol, the SSPS at both pH 7.0 and 4.0 was complexed with curcumin; it can be interestingly observed that the complexation with curcumin produced a significant influence on the size and morphology of SSPS molecules (Figures 3 and 4). DLS results indicated that the SSPS−curcumin complexes had sizes ranging from 18 to 70 nm (pH 7.0) or from 20 to 80 nm (pH 4.0) (Figure 3). AFM observations showed that as compared with the SSPS molecules themselves, the heights and contour sizes of SSPS−curcumin complexes (at both pH 7.0 and 4.0) were much greater (Figure 4). These observations clearly indicated that the complexation with curcumin resulted in aggregation of SSPS into larger particles. In the complextion of SPI with curcumin, the binding of curcumin to the SPI particles also led to the formation of larger sizes of aggregated particles,3 where the bound curcumin was considered to act as “bridges” in the formation of the macro-aggregated particles. One interesting noteworthy point is that, in the AFM images of SSPS−curcumin complexes, most of the particles were in a regularly spherical form, whereas for the SSPS alone, the particles were irregular (Figure 4). This observation suggested that the SSPS and curcumin complexation occurred in a self-assembly way, in which the curcumin
Figure 4. Typical 2-D AFM height images of particles in native SSPS (pH 7.0) (A), ethanol-pretreated SSPS (B,C), or SSPS−curcumin complexes (D, E). Panels B and D indicate the images of ethanolpretreated SSPS and SSPS−curcumin complexes at pH 7.0, respectively; panels C and E indicate the images of ethanol-pretreated SSPS and SSPS−curcumin complexes at pH 4.0, respectively. Height profiles of selected particles as a function of section length (with the arrows as the indicators) are displayed at the right side of the corresponding AFM height images.
molecules might act as the nuclei for the SSPS−curcumin assemblies. This assembly seemed to more easily occur at pH 4.0 than at pH 7.0, as evidenced by the higher loading amount of curcumin and larger sizes of particles (Table 1 and Figure 3), as well as the stronger fluorescence intensity (Figure 2B,C). This assembly process is illustrated in Figure 5. E
DOI: 10.1021/acs.jafc.6b05087 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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curcumin in the SPI−curcumin complexes remained after a heating at 85 °C for 3 h. The difference might be due to the difference in polarity of the environment for curcumin-binding sites between SPI and SSPS. In Vitro Bioaccessibility and Stability during Digestion. The in vitro bioaccessibility of curcumin in its complexes with SSPS at pH 7.0 or 4.0 was evaluated by a simulated in vitro gastric (60 min) and intestinal (120 min) digestion (SGF+SIF), which was defined as the percentage of curcumin transferred to the aqueous phase (relative to the initial total amount of curcumin) at the end of digestion. Figure 7A shows the in vitro
Figure 5. Schematic illustration for the formation of SSPS−curcumin complexes.
Heat Stability. Figure 6 shows the degradation kinetics of curcumin, free or complexed with SSPS (at both pH 7.0 and
Figure 6. Degradation kinetics of free curcumin and curcumin in the complexes with SSPS in water at pH 7.0 and 4.0 as a function of storage time up to 2.5 h at 80 °C. Data presented are means and standard deviation error bars of triplicates. Figure 7. Percentage of curcumin remaining in the aqueous phase (A) and percentage of curcumin in the whole digesta (B) for free curcumin and SSPS−curcumin complexes after 180 min of the whole simulated gastric and intestinal digestion (SGF+SIF). Data presented are means and standard deviation error bars of triplicates. Different letters (a−d) above the bars represent significant difference at p < 0.05 level among different samples.
4.0), when heated at 80 °C for an incubation period up to 2.5 h. As expected, the curcumin at pH 7.0 remarkably degraded during the initial heating period of 30 min, and after that, the rate of degradation gradually declined. In contrast, the heatinduced degradation of curcumin at pH 4.0 was much slower than that at pH 7.0 (Figure 6). For example, about 98% of curcumin degraded at pH 7.0 after the heating for to 2.5 h, whereas it was only 36% at pH 4.0. The observations confirmed the consensus that curcumin is susceptible to degradation in an aqueous environment and that the degradation is faster at neutral pH values than at acidic pH values.26 On the other hand, it can be seen that at both pH 7.0 and 4.0, the complexation with SSPS significantly improved the heat stability of curcumin (Figure 6). At pH 7.0, the retention of curcumin after the heating of 2.5 h was increased from about 2 to 10% by the complexation, whereas at pH 4.0, it was from approximately 64 to 80%. The improvement of heat stability by complexation with SSPS at pH 7.0 is poorer than that by complexation with SPI,3 where approximately 35−45% of
bioaccessibility of curcumin, free or complexed with SSPS at both 7.0 and 4.0. For free curcumin, the bioaccessibilities at pH 7.0 and 4.0 were 24.8 and 34.8%, respectively (Figure 7A). The bioaccessibility at pH 4.0 is basically comparable to that reported in our previous work.2 As expected, the complexation with SSPS at both tested pH values remarkably improved the bioaccessibility of curcumin (Figure 7A). The bioaccessibility (76.8−82.8%) of curcumin in the complexes with SSPS is similar to the highest value (about 85%) reported for curcumin in the complexes with SPI.2 The results indicated that SSPS could perform as an effective vehicle for enhancing the bioaccessibility of curcumin, especially at acidic conditions. F
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On the other hand, it can be seen that the complexation with SSPS at both tested pH values could improve the stability of curcumin against the whole digestion (Figure 7B). For free curcumin, about 25−30% of curcumin degraded after the whole digestion, whereas for the SSPS−curcumin complexes, the loss was only 4−15% (Figure 7B). Relative to the undegraded curcumin, the bioaccessibility of free curcumin at pH 7.0 and 4.0 can reach about 35 and 45.7%, whereas that for the curcumin in the complexes reached 90.3 and 87%, respectively. This suggested that regardless of the pH, the complexation with SSPS could remarkably improve the bioaccessibility of curcumin. In summary, the present work confirmed that SSPS can perform as an effective nanocarrier for improving the water solubility, stability, and in vitro bioaccessibility of curcumin. The improvement was largely due to the formation of nanocomplexes between the protein fraction (of SSPS) and curcumin, through hydrophobic interactions. The complexation occurred only between ethanol-pretreated SSPS and curcumin, which was more favorable at pH 4.0 than at pH 7.0. More importantly, the curcumin itself was much more stable against degradation at pH 4.0 than at pH 7.0. The findings of this work are of interest for the development of SSPS ingredients enriched with hydrophobic bioactives, which holds great potential in the formulations of functional foods. However, in vivo experiments are needed to further confirm the improved bioavailability of curcumin encapsulated in complexes with SSPS.
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AUTHOR INFORMATION
Corresponding Authors
*(Z.C.) Phone: +86 20 87114262. Fax: +86 20 87114263. Email:
[email protected]. *(C.H.T.) E-mail:
[email protected], ORCID
Chuan-He Tang: 0000-0002-8769-2040 Funding
This work was supported by the NNSF of China (serial no. 31471695). Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.jafc.6b05087 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.jafc.6b05087 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
