Preparation of Organogel with Tea Polyphenols Complex for

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Preparation of Organogel with Tea Polyphenols Complex for Enhancing the Antioxidation Properties of Edible Oil Rong Shi,†,∥ Qiuyue Zhang,†,∥ Frank Vriesekoop,§ Qipeng Yuan,† and Hao Liang*,† †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, People’s Republic of China § Department of Food Science, Harper Adams University, Newport, Shropshire TF10 8NB, England ABSTRACT: Food-grade organogels are semisolid systems with immobilized liquid edible oil in a three-dimensional network of self-assembled gelators, and they are supposed to have a broad range of potential applications in food industries. In this work, an edible organogel with tea polyphenols was developed, which possesses a highly effective antioxidative function. To enhance the dispersibility of the tea polyphenols in the oil phase, a solid lipid−surfactant−tea polyphenols complex (organogel complex) was first prepared according to a novel method. Then, a food-grade organogel was prepared by mixing this organogel complex with fresh peanut oil. Compared with adding free tea polyphenols, the organogel complex could be more homogeneously distributed in the prepared organogel system, especially under heating condition. Furthermore, the organogel loading of tea polyphenols performed a 2.5-fold higher antioxidation compared with other chemically synthesized antioxidants (butylated hydroxytoluene and propyl gallate) by evaluating the peroxide value of the fresh peanut oil based organogel in accelerated oxidation conditions. KEYWORDS: organogel, tea polyphenols, antioxidation

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INTRODUCTION Oxidative deterioration of foods remains a very difficult and real problem while we attempt to maintain minimum quality standards. One of the principal causes of food quality deterioration is lipid peroxidation.1,2 The oxidation of lipids is a free radical driven, chain reaction process, which can cause many deleterious changes including loss of nutritional values and formation of toxic compounds.3,4 Most of these toxic compounds are known to damage DNA or cause cancers5 and as a key phenomenon in chronic diseases.6,7 At present, the principal method to minimize oxidative deterioration in lipidcontaining foods and edible oils is the application of antioxidants. Antioxidants terminate the lipid peroxidation chain reactions by removing free radical intermediates and inhibit other oxidation reactions.8 Synthetic antioxidants, such as butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), and propyl gallate (PG), are widely used in the food and cosmetics industries9,10 due to their high efficiency and relatively low costs. However, the safety of synthetic antioxidants with regard to human consumption has long been questioned.11−14 In response, natural antioxidants are identified and used as safer and alternative sources of antioxidants in the prevention of oxidative deterioration of foods.15,16 Tea polyphenols, which include a wide range of chemical compounds, such as flavonoids, tannins, and catechins, possess outstanding antioxidative properties.16,17 The main mechanisms for inhibition of oxidative reactions by tea polyphenols include scavenging reactive oxygen species, chelating redox-active transition metal ions, and inhibition of “pro-oxidant” enzymes.18 However, tea polyphenols are severely restricted to direct use in foods with high lipid content, due to their low solubility in oil phase. © XXXX American Chemical Society

Organogel-based delivery systems for functional ingredients are relatively new in food science and drug delivery.19−21 Organogels are semisolid systems containing liquid oil trapped within a three-dimensional networked structure, which is formed by the self-assembly of a relatively low concentration of organogelator molecules.19,22 This structured network appears to be based on the orientational ordering of the hydrocarbon chains,23 which can facilitate an improved dispersibility of aliphatic functional compounds in the oil phase.24 Meanwhile, the structure also influences the oxidative stability of the oil entrapped in the system.25 In recent years, these semisolid products have gained much importance in the food and nutraceutical industries due to applications including the restriction of oil mobility and their ability to control the rate of nutraceutical release.19,26 In this paper, we describe the development of an edible oil organogel laden with tea polyphenols, which has a thermal reversibility. To enhance the dispersibility of the tea polyphenols in the oil phase, a solid lipid−surfactant−tea polyphenols complex (organogel complex) was prepared using a novel method. The resultant organogel displayed highly effective antioxidative properties. A novel method to produce the organogel complex formed of stearic acid, surfactant, and tea polyphenols was developed. The food-grade organogel was prepared by mixing this complex with fresh peanut oil. Microscopy, Fourier transform infrared (FTIR), and X-ray diffraction (XRD) were used to characterize the structure of the organogel complex. Furthermore, the antioxidative properties of the organogel Received: April 1, 2014 Revised: July 26, 2014 Accepted: August 4, 2014

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heated in a temperature-controlled water bath as the temperature was varied from 20 to 80 °C at an increment of 2 °C. The samples were equilibrated for 5 min at the previous temperature for 5 min before the temperature increment was made. Sol−gel phase transition temperature (TSG) and gel−sol phase transition temperature (TGS) are defined as the midpoint of the temperature range above which gelation changes.33 Sol−gel phase transition temperature was measured by cooling the sample stepwise to 20 °C, following the same procedure. Preparation of Different Organogel Samples. Generally, the organogels with tea polyphenols complex (5, 10, 20, or 30%) were prepared to add organogel complex (1, 2, 4, or 6 g) in 20 g of peanut oil. Then, all of the samples were heated at 80 °C and stirred at 500 rpm to obtain the homogeneous solutions. The samples were cooled to room temperature and crystallized under static conditions. The control organogel was obtained in the same manner with the control organogel complex. Organogel complex (0.04 g) and stearic acid (0.06 g) were added into 20 g of peanut oil and stirred at 500 rpm to obtain the homogeneous solution. The solution was cooled to room temperature and crystallized under static conditions. Then the organogel with tea polyphenols complex (2%) was prepared. The organogels with free tea polyphenols, BHT, or PG (0.02%) were prepared by adding free tea polyphenols, BHT, or PG (0.004 g) into 20 g of peanut oil, respectively. The concentrations of BHT, PG, and free tea polyphenols were 0.02% (w/w) in peanut oil, which were the same as the content of tea polyphenols in the organogel containing 5% organogel complex. Then, all of the samples were heated at 80 °C and stirred at 500 rpm. The samples were cooled to room temperature and crystallized under static conditions. Characterizations of the Organogel Complex and Organogel Containing Tea Polyphenols. Microscopic Analysis. A compound optical microscope (Olympus, Japan) was used for analyzing the microstructure of the organogels. Attempts were made to understand the mechanism of formation of the organogels through the variation of the proportions of organogel complex in the organogels. FTIR Spectra Analysis. FTIR spectra were investigated to detect the functional groups of compounds through a TENSOR 27 FTIR spectrometer (Bruker, Germany). Stearic acid and representative complexes were prepared for scanning in the range from 4000 to 500 cm−1 to examine the interactions among the components of the organogel complex. X-ray Diffractometry. XRD analyses of the samples were carried out with an XRD-600 (Shimadzu, Japan). Samples were scanned in the range from 5° to 95°/2θ, and the scanning rate was 2°/min. Measurements were operated at a voltage of 40 kV, 40 mA. Accelerated Oxidation Conditions and Determination of the Peroxide Value (PV). A range of organogel samples with various antioxidant substances were placed in open transparent glass beakers. The range of organogel samples was prepared by the way of organogel preparation and contained different compositions (see details in Table 3). The range of samples contained various controls and was designed to investigate the effectiveness of tea polyphenols as an antioxidant in an organogel. To artificially induce an accelerated oxidation of the lipids, all samples were marked and stored in an oven at 90 °C for 84 h. During this process of oxidation, the PVs of samples were determined every 12 h. The PV was evaluated following the NP-904 (1987) method. This method consists of a reaction carried out in darkness of a mixture of oil and isooctane/acetic acid 2:3 (v/v) with a saturated potassium iodide solution. Autoxidation causes the formation of peroxides, which in turn causes the release of iodine. The free iodine formed is then titrated with a sodium thiosulfate solution. The peroxide value was expressed as milliequivalents of active oxygen per kilogram of oil (mequiv/kg). The experiment was conducted in triplicate, and at least three determinations were performed for each sample.

containing tea polyphenols was also tested and analyzed and were compared with those of stearic acid based organogels with free tea polyphenols and chemically synthesized antioxidants.

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EXPERIMENTAL PROCEDURES

Materials and Methods. Tea polyphenols were obtained from Jiangsu Dehe Biotechnology Co., Ltd., China. The composition of these tea polyphenols was determined by using UPLC-MS (XEVOG2QTOF, Waters, USA). The result is shown in Table 1. Stearic acid

Table 1. Composition of the Tea Polyphenols (PLC-MS) peak

retention time (min)

[M − H]− (m/z)

1 2 3 4

3.29 3.52 3.72 3.94

305 305 289 457

5

3.99

457

6

4.22

471

7

4.33

441

compound

area

%

gallocatechin (GC) epigallocatechin (EGC) catechin (C) epigallocatechin-3gallate (EGCG) gallocatechin-3-gallate (GCG) epigallocatechin-3-(3″O-methyl)gallate (EGCmetG) epicatechin-3-gallate (ECG)

2106 7113 4563 49175

2.25 7.62 4.88 52.65

3651

3.91

26787

28.68

(98%, w/w) was purchased from Aladdin Co. (Shanghai, China). Sucrose erucate (ER290) was obtained from Mitsubishi-Kagaku Foods Corp. (Tokyo, Japan). HPLC-grade water was from Alfa Aesar (Lawrence, KS, USA). Glacial acetic acid, isooctane, potassium iodide, sodium thiosulfate, and starch were of analytical reagent grade. We chose peanut oil as the sample oil to prepare the organogel. Peanut oil possesses a well-balanced fatty acid profile and has been credited with cardioprotective and anti-inflammatory properties.27 Fresh peanut oil was squeezed out using a small edible oil press (6YL95A, SRS, China). During the process, the peanuts were mildly toasted to remove the internal water and then pressed to obtain fresh peanut oil. Procedure for Preparation of a Stearic Acid−Surfactant−Tea Polyphenols Complex (Organogel Complex). One hundred milligrams of tea polyphenols dissolved in 2 mL of water was used as aqueous phase. Stearic acid (10 g) containing sucrose erucate ER290 (5%w/w) was placed in an 80 °C water bath and stirred at 500 rpm until a homogeneous clear solution was obtained. Then the aqueous phase and the oil phase were homogenized (HENC homogenizer, Shanghai, China) at 26000 rpm for 2 min to form a stable W/O emulsion. The aqueous and oil phases were homogenized at 80 °C to ensure that the oil phase remained in a liquid state during the homogenization process. The resulting emulsion was frozen rapidly in liquid nitrogen and subsequently lyophilized (FD-1C-50, Beijing, China). The control organogel complex was prepared in the same way only without tea polyphenols in aqueous phase. Determination of Critical Gelation Concentration (CGC) and Gel−Sol Transition Temperature. Gelation was considered successful when the gels failed to flow under gravity on inversion of the vials.28,29 The CGC is the lowest concentration of gelator (the organogel complex) that induces gelation of edible oil.30 When determining the CGC, various amounts of the organogel complex were added to peanut oil, following which the mixture was carefully heated to 80 °C and stirred at 500 rpm and subsequently allowed to cool at room temperature under static conditions for 3 h before it was inverted. The lowest concentration of added organogel complex that induced gelation determined the CGC. Meanwhile, the homogeneity of tea polyphenols in the organogel could be visually inspected. Gel (nonflow)−sol (flow) transition was determined using the inverting tube method. The inverting tube method used in our study was similar to the one used by Motulsky, Behare, and co-workers.31,32 The organogels with tea polyphenols complex (5, 10, 20, or 30%) were B

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RESULTS AND DISCUSSION Preparation and Characterization of Organogel Complex. In recent years, there has been a great deal of interest in studying gelling of edible oils with fatty acid derivatives.34−36 However, saturated fatty alcohols showed lower efficiencies compared to saturated fatty acids of the same length in their ability to form oleogels with various oils.34 Furthermore, the melting point of stearic acid is lower than that of (R)-12-hydroxystearic acid. The solubility of stearic acid in edible oil is higher than that of (R)-12-hydroxystearic acid. Thus, stearic acid was chosen as the organogelator to form an organogel complex in this work. The addition of tea polyphenols to stearic acid resulted in a stable homogeneous emulsion when the tea polyphenols were added up to a concentration of 5% (w/w). At any concentration in excess of 5% (w/w) tea polyphenols, a precipitate would occur. Unless otherwise indicated, all experiments were carried out with a tea polyphenols loading of 1% (w/w). To assess the molecular interactions due to the presence of tea polyphenols, we undertook FTIR spectroscopy, which included samples of pure stearic acid, control organogel complex (without tea polyphenols), and an organogel complex (with tea polyphenols). We observed that the spectra of the three samples were similar except the different intensity of the broad peaks in the range from 3700 to 3300 cm −1 corresponding to O−H stretching (Figure 1). The intensity

XRD measurements were employed. A control organogel complex and organogel complexes containing 1 and 2% (w/w) tea polyphenols were taken as representative samples. The organogel complexes containing tea polyphenols displayed the same angle diffraction peaks compared to the control organogel complex (Figure 2). These peaks were the result of the

Figure 2. X-ray diffraction patterns of (a) control organogel complex, (b) organogel complex containing 1% (w/w) of tea polyphenols, and (c) organogel complex containing 2% (w/w) of tea polyphenols.

scattering associated with stearic acid and showed an elevated crystallinity with sharp peaks. A layer thickness of 13.21 Å can be observed in all of these samples. The results indicated that the crystals were well developed in this direction and cause clear diffraction intensity. All of these samples showed short spacings around 4.13 and 3.68 Å, which are characteristic for an orthorhombic packing, and were similar to the β′ packing of triacylglycerols.38 As the concentration of tea polyphenols increased, a peak at 11° with higher intensity could be observed; on the contrary, the short spacings around 4.13 and 3.68 Å showed a weaker intensity (Figure 2), indicating that the corresponding crystals were much thinner in the direction of the unit cell. These results indicate that an increase in tea polyphenols affects the crystal structure and the texturing properties of the organogel complex. Preparation and Characterizations of Peanut Oil Based Organogels with Tea Polyphenols. The dissolution of the organogel complex in peanut oil at 80 °C resulted in the formation of a homogeneous solution. When the temperature was reduced, a change in the solubility parameter of the organogel complex resulted in the crystallization of the stearic acid molecules in the oil continuous phase and formed selfassembled structures. Depending on the concentration of the organogel complex in peanut oil, the solution either remained cloudy and liquid or formed a semisolid-like structure. Through the inverse experiments, the CGC of the organogel was found to be 5% (w/w) (Figure 3); hence, further analyses of the organogels were performed using gels having complex concentration ≥5% (w/w). The gel−sol and sol−gel transition temperatures were determined by using the inverting tube method. It is an important parameter for gel stability and shelf life.39 TGS and TSG were very close, because of the same concentration of organogel complex (shown in Table 2). The temperature at which transition occurred increased with organogel complex

Figure 1. FTIR analyses of (a) pure stearic acid, (b) control organogel complex (no tea polyphenols), and (c) organogel complex with 1% (w/w) tea polyphenols.

of the peak corresponding to the O−H stretching vibration increased in the organogel complex containing 1% (w/w) tea polyphenols. This may have been due to the presence of a phenolic hydroxyl group in the tea polyphenol molecules. The characteristic functional groups stretches indicate the presence of alkane (3001 and 2934 cm−1), N-methyl (2867 cm−1), and C−O stretch (1156 cm−1) in the samples.37 There was no significant difference in the rest of the spectra, which indicates that the chemical functionality of the components within the organogel complex was conserved. To further investigate the molecular arrangements of the structure forming those crystals within the organogel complex, C

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Figure 3. CGC of organogel loading of tea polyphenols: S-1, organogel containing 4% (w/w) organogel complex; S-2, organogel containing 5% (w/w) organogel complex.

Table 2. Results of Sol−Gel Phase and Gel−Sol Transition organogel sample organogel organogel organogel organogel

c

(5% complex) (10% complex)d (20% complex)e (30% complex)f

TSGa (°C)

TGSb (°C)

48.0 50.0 54.0 57.0

51.0 53.0 57.0 60.0

a

Sol−gel phase transition temperature (TSG). bGel−sol phase transition temperature (TGS). cOrganogel contains 5% tea polyphenols organogel complex. dOrganogel contains 10% tea polyphenols organogel complex. eOrganogel contains 20% tea polyphenols organogel complex. fOrganogel contains 30% tea polyphenols organogel complex.

concentration. Transitions were fully reversible, and the organogel had a thermal reversibility. The phenomenon of transition temperature was similar to reported results.38,39 Microstructure analysis showed the presence of needleshaped crystals of stearic acid in peanut oil when 5% (w/w) organogel complex concentration was used (Figure 4A). As the concentration of organogel complex was increased, these clusters aggregated to form fiber-like structures (Figure 4B,C). The density of these fiber-like structures increased with the increase of the organogel complex concentration, and these fiber-like structures were found to form a networked skeleton, which helped in the immobilization of the peanut oil. To study the effect of incorporation of tea polyphenols on the crystallinity of the peanut oil based organogel, XRD measurements was also employed to test the representative samples (Figure 5). As the concentration of organogel complex increased from 20 to 30% (w/w), the organogels showed a higher crystallinity, which can be observed from the larger peaks in Figure 5. The larger peaks are indicative of a greater presence of stearic acid in the peanut oil, causing a more pronounced crystallinity in the organogels, and the broader peaks that can be evidenced in Figure 5 correspond to the peanut oil in the organogel. Determination of the Antioxidation of the Organogel Loading of Tea Polyphenols. To test the effectiveness of the presence of tea polyphenols as antioxidants in the organogel, the generation of peroxides was monitored by a modified oven test at 90 °C for up to 84 h. Compared with the peanut oil (blank), the addition of stearic acid alone to peanut oil (control) did not reduce the kinetics of peroxide formation at the early stages of the oven test (Figure 6), but it did reduce the formation of peroxides by about 14% over 84 h, which seems to indicate that the structural characteristics of the organogel systems influence the oxidative reaction.23 The addition of 5% organogel complex to fresh peanut oil (5% complex) caused a 60% reduction in the formation of peroxides (Figure 6). These

Figure 4. containing containing containing

Micrographs of organogels. 5% organogel, organogel 5% (w/w) organogel complex; 10% organogel, organogel 10% (w/w) organogel complex; 20% organogel, organogel 20% (w/w) organogel complex.

results indicate that the organogel loading of tea polyphenols performs an obvious antioxidative effect. To compare the function of different antioxidants, and according to the existing food additive regulations published by the U.S. Food and Drug Administration, BHA and PG can be used individually or in combination at a maximum accumulative level of 0.02%. Stearic acid based organogel samples containing 0.02% (w/w) of BHT, PG, and free tea polyphenols were also prepared (Table 3), and their ability to delay the onset of oxidative rancidity was determined (Figure 7). It is clear that the organogel (2% complex) has a much greater antioxidative effect than organogel with free tea polyphenols (0.02% free TPs), and both of them have the same weight of tea polyphenols, which indicates the organogel complex could improve the dispersibility of tea polyphenols in the oil phase. The addition of either BHT or PG at 0.02% in a stearic acid based organogel only slightly improves the antioxidative effect of organogel (Figures 6 and 7). The addition of 0.02% free tea D

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Figure 5. X-ray diffraction patterns of organogels. 20% organogel, organogel containing 20% (w/w) organogel complex; 30% organogel, organogel containing 30% (w/w) organogel complex.

Figure 7. Influence of a range of antioxidants on delaying the onset of rancidity as measured by changes in peroxide value of organogels as a function of storage time at 90 °C. The “2% complex” organogel (■) contains 2% organogel complex plus 3% stearic acid in peanut oil; “0.02% free TPs” organogel (●) contains 5% stearic acid plus 0.02% tea polyphenols in peanut oil; “0.02% BHT” organogel (▲) contains 5% stearic acid plus 0.02% BHT in peanut oil; and “0.02% PG” organogel (▼) contains 5% stearic acid plus 0.02% PG in peanut oil.

polyphenols in a stearic acid based organogel yielded a slight improvement in antioxidative ability compared to either BHT or PG (Figure 7). In general, after the oven test for 84 h, the organogel 2% complex, which contained 0.02% tea polyphenols, reduced the formation of peroxides by about 45%, and the stearic acid based organogels, 0.02% TPs, 0.02% PG, and 0.02% BHT, separately caused 27, 18, and 15% reductions in the formation of peroxides. The organogel loading of tea polyphenols performed a 2.5-fold higher antioxidation compared with other chemically synthesized antioxidants (BHT, PG) by evaluating the peroxide value of the fresh peanut oil based organogel in the oven test. In summary, a novel method was developed to prepare an organogel loaded with tea polyphenols, which significantly improved the dispersibility of tea polyphenols in the organogel system. Furthermore, the organogel loading of a stearic acid− tea polyphenols complex greatly enhanced the ability to delay the onset of oxidative rancidity. More importantly, the method developed here allows the addition of water-soluble, natural active ingredients into lipid-rich foods such as fried foods, cocoa butter, and cream products to improve the shelf life of the food.

Figure 6. Influence of tea polyphenols on delaying the onset of rancidity as measured by changes in peroxide value of organogels as a function of storage time at 90 °C. The “blank” (□) contains pure peanut oil; the “control” (○) contains 5% stearic acid and no tea polyphenols in peanut oil; and “5% complex” organogel (△) contains no stearic acid and 5% organogel complex in peanut oil.

Table 3. Composition of the Organogel Samples Used for Antioxidation Analysis organogel sample code

a

fresh peanut oil (g)

stearic acid (%)

blank control

20 20

0 0

5% complex

20

0

2% complex

20

3

0.02% free TPsb

20

5

0.02% BHT 0.02% PG

20 20

5 5

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antioxidant

AUTHOR INFORMATION

Corresponding Author

*(H.L.) Phone: +86 10 6443 1557. Fax: +86 10 6443 7610. Email: [email protected].

control organogel complex (1000 mg) organogel complexa (1000 mg) organogel complexa (400 mg) free tea polyphenols (4 mg) BHT (4 mg) PG (4 mg)

Author Contributions ∥

R.S. and Q.Z. contributed equally to this work.

Funding

We acknowledge financial support from the Natural Science Foundation of China (20806005), the Beijing Higher Education Young Elite Teacher Project (YETP0520), and the National High Technology Research and Development Program of China (863 Program, Grants 2012AA021403 and 2014AA021705).

Organogel complex contains 1% tea polyphenols. bTea polyphenols.

E

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Notes

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The authors declare no competing financial interest.

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