Chemical and Antioxidant Properties of Casein Peptide and Its

Article pubs.acs.org/JAFC

Chemical and Antioxidant Properties of Casein Peptide and Its Glucose Maillard Reaction Products in Fish Oil-in-Water Emulsions Shiyuan Dong,†,‡ Binbin Wei,† Bingcan Chen,‡ D. Julian Mcclements,‡ and Eric A. Decker*,‡ †

College of Food Science and Engineering, Ocean University of China, Qingdao, Shandong 266003 China Department of Food Science, University of Massachusetts, Amherst, Massachusetts 01003, United States

‡

ABSTRACT: Maillard reaction products (MRPs) were prepared by reacting casein peptides with different concentrations of glucose at 80 °C for up to 12 h. The chemical properties of MRPs and their effects on lipid oxidation in fish oil-in-water emulsions were investigated. Increasing browning development and absorbance in 294 nm in the MRPs caused an increase in DPPH radical scavenging, but a decrease in iron chelation, which could be related to the loss of free amino groups in the peptides. The MRPs produced with longer reaction time or higher glucose concentrations were less effective in inhibiting lipid oxidation in emulsions at pH 7.0 compared to casein peptides alone. However, the antioxidant activity of MRPs in emulsions at pH 3.0 was not decreased by prolonged heating. The bitterness of MRPs was less than that of the original casein peptides, and bitterness decreased with increasing heating time and glucose concentrations. Therefore, the Maillard reaction was a potential method to reduce the bitterness of casein peptides while not strongly decreasing their antioxidant activity. KEYWORDS: casein peptides, Maillard reaction products, emulsion, lipid oxidation, antioxidant activity, bitterness

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preferred over their synthetic counterparts because the latter impose potential health threats.11 Our previous studies have shown that casein hydrolysates could effectively decrease the oxidative rancidity of corn oil-inwater emulsions.12 However, the information regarding chemical properties of MRPs prepared from casein peptides and their application in oil-in-water emulsions is still limited. The objectives of this research were to investigate the chemical properties of MRPs from casein peptides and glucose and their effects on lipid oxidation in fish O/W emulsions.

INTRODUCTION

The Maillard reaction is involved in the formation of brown pigments via the condensation of a carbonyl group of reducing sugars, aldehydes, or ketones with the amine group of amino acids (such as amino acids, peptides, and proteins) or any nitrogenous compound.1 Maillard reaction products (MRPs) markedly contribute not only to the aroma, taste, and color but also the antioxidant potential of foods.2,3 Some research has shown that the Maillard reaction is a chemical reaction that improves the antioxidant activity of proteins or amino acids. 4−6 In recent years, several researchers have reported that MRPs from protein hydrolysate had strong antioxidant activity. MRPs derived from hydrolysates of mechanically deboned chicken residue exhibited good antioxidant activity as determined by their reducing power, DPPH radical scavenging activity, and ability to inhibit lipid oxidation in Cantonese sausage during storage.7 Liu et al.3 reported that MRPs from soy protein hydrolysate showed higher reducing power, DPPH radical scavenging, and Fe2+ chelating activity in comparison with the original hydrolysates. Guerard and Sumaya-Martinez also reported that the radical scavenging effect was improved by 75% when casein peptone and cod viscera hydrolysate were heated in the presence of glucose.8 Fish oils are rich in docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA), which are susceptible to oxidation because of the multiple double bonds in their carbon chain.9 The oxidative susceptibility of fish oil also depends on the presence of antioxidants and prooxidants found in the oil as well as the food matrix to which they are added.10 Oil-in-water (O/W) emulsions exist in many food products, including beverages, sauces, and soups. Oxidation is a common problem for these types of emulsion-containing foods. Various antioxidants are incorporated into O/W emulsions to enhance their oxidative stability, with natural antioxidants being © 2011 American Chemical Society

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MATERIALS AND METHODS

Materials. Deodorized, refined, and bleached fish (Menhaden) oil without added antioxidants was supplied by Omega Protein (Reedville, VA) and contained 10−17% of EPA and 7−12% of DHA. Oil was stored in the dark at −80 °C and thawed in cold tap water immediately before use. Casein sodium salt from bovine milk (sodium caseinate), Alcalase 2.4 L, D-glucose, ferrous sulfate, cumene hydroperoxide, 2,4,6trinitrobenzenesulfonic acid (TNBS), butylated hydroxytoluene (BHT), ammonium thiocyanate, 2,2-diphenyl-1-picrylhydrazyl (DPPH), Tween 20, 2-thiobarbituric acid (TBA), 3-(2-pyridyl)-5,6diphenyl-1,2,4-triazine-4′,4″-disulfonic acid sodium salt (Ferrozine), and 1,1,3,3-tetraethoxypropane (TEP) were from Sigma-Aldrich Co. (St. Louis, MO). All other reagents were of analytical grade or better. Preparation of Casein Peptides (CP). CP were prepared by the hydrolysis of sodium caseinate using the proteolytic enzyme Alcalase. Sodium caseinate (150 g) was dissolved in 2.5 L of deionized water to give a starting protein concentration of 6% (w/v). The solution was adjusted to pH 8.0 using 5 M NaOH and hydrolyzed by protease (0.3:100 enzyme/substrate ratio) at 55 °C in a stirred water bath. The pH of the mixture was maintained constant during hydrolysis using 5 M NaOH. After 3 h of hydrolysis, the solutions were heated at 90 °C Received: Revised: Accepted: Published: 13311

September 17, 2011 October 25, 2011 November 18, 2011 November 19, 2011 dx.doi.org/10.1021/jf203778z | J. Agric.Food Chem. 2011, 59, 13311−13317

Journal of Agricultural and Food Chemistry

Article

Preparation of Emulsions. Stock surfactant solutions for emulsion preparation were prepared by dispersing surfactant (1.0 wt % Tween 20) in phosphate buffer (10 mM) adjusted to pH 7.0 with 1 M HCl. Stock MRP solutions were prepared by dispersing 50 μg mL−1 of MRPs in 10 mM phosphate buffer (pH 7.0). An oil-in-water emulsion was prepared by homogenizing 10 wt % fish oil and 90 wt % stock Tween 20 surfactant solution using a highspeed blender (M133/128-0, Biospec Products, Inc., ESGC, Switzerland) for 2 min followed by further particle size reduction with a Microfluidizer (model M-110 L Microfluidizer Processor, Microfluidics, Newton, MA) for three passes at a pressure of 10 kpsi. The emulsions were kept in an ice container over the whole procedure to minimize oxidation. After homogenization, 0.02 wt % NaN 3 was added as an antibacterial agent, and the emulsion was adjusted to pH 7.0 by adding 1 M HCl. This emulsion was referred to as the stock emulsion. A portion of the stock emulsion was diluted with MRP solutions and/or buffer to achieve the final desired concentrations of 5 wt % menhaden oil, 0.5 wt % Tween 20, and MRP concentrations of 10 μg mL−1. The final emulsions were adjusted to pH 7.0 using 0.1 M HCl and were stored in the dark at 20 °C. The ζ-potential and particle size (z-average mean diameter) of emulsions were measured by dynamic light scattering measurements (Zetasizer Nano-ZS, model ZEN3600, Malvern Instruments, Worchester, U.K.), and creaming stability (visual observation) of the final emulsions were determined using experimental methods described in previous papers.19,20 Measurement of the Lipid Oxidation in Emulsions. Lipid hydroperoxides were measured as the primary oxidation products using a method adapted from Shanta and Decker.21 Emulsion (0.3 mL) was added to 1.5 mL of a mixture of isooctane/2-propanol (3:1, v/v) and vortexed three times for 10 s each, followed by centrifugation for 2 min at 10000g. The organic phase (≤0.2 mL according to the level of oxidation) was added to a mixture of methanol/butanol (2:1, v/v) followed by the addition of 15 μL of 3.94 M thiocyanate and 15 μL of 0.072 M Fe2+. The solution was vortexed, and after 20 min, the absorbance was measured at 510 nm using a Genesys 20 spectrophotometer (Thermo Spectronic, Waltham, MA). The concentration of hydroperoxides was calculated from a cumene hydroperoxide standard curve. Secondary oxidation products were monitored with the thiobarbituric acid reactive substances (TBARS) method using a procedure described elsewhere.22 TBARS were chosen for this study because of the high reactivity of omega-3 fatty acid oxidation products with TBA and the need to screen large numbers of samples. Briefly, at each incubation time, 1 mL of each sample was mixed with 2 mL of a TBA solution containing 20% trichloroacetic acid, 0.5% TBA, 0.2% EDTA, and 30 mM HCl in screw-capped tubes. Immediately afterward, 30 μL of 3% BHT in ethanol was added, and the tubes were then closed and vortexed. Samples were then heated in a boiling water bath for 15 min, cooled at room temperature, and centrifuged at 1600g for 20 min. The absorbance of the supernatant was measured at 532 nm using a Genesys 20 spectrophotometer (Thermo-Spectronic). Concentrations were determined from a MDA standard curve produced from 1,1,3,3tetrahydroxypropane. Statistical Analysis. Duplicate experiments were performed with freshly prepared emulsion except for the chemical stability experiments, which were done three times. All data shown represent the mean value ± standard deviation of triplicate measurements. Statistical analysis of the effect of MRPs on the DPPH scavenging and Fe2+ chelating capacity was performed using a one-way analysis of variance. A significance level of p < 0.05 between groups was accepted as being statistically difference. In all cases, comparisons of the means of the individual groups were performed using Duncan’s multiple-range tests. All calculations were performed using SPSS17 (http://www.spss.com; SPSS Inc., Chicago, IL).

for 10 min to inactivate the enzymes, followed by centrifugation at 9000g for 15 min to remove insoluble materials. CP was then frozen, lyophilized, and kept at −18 °C before further analysis. Preparation of MRPs from a CP and Glucose. CP at a protein concentration of 50 mg mL−1 (as determined by the Lowry method13) was dissolved in 50 mM phosphate buffer solution (pH 8.0) and incubated with different concentrations of glucose (G). CP and G were mixed at the following ratios: CP/G at 1:0.19, 1:0.38, and 1:0.76 w/w (equivalent to approximately 1:0.5, 1:1, and 1:2 mol ratio of free amino group residues (as determined by TNBS method) to sugar carbonyl groups, respectively); these are named in the remainder of the paper as CP/G 1:0.5, CP/G 1:1,and CP/G 1:2, respectively. The CP/G solutions were kept in 25 mL screw-cap tubes and heated in a water bath at 80 °C for up to 12 h. Control experiments with CP (50 mg mL−1 of protein) heated without glucose, and phosphate buffer heated alone, were also conducted. Evaluation of the Chemical Properties of CP and MRPs. Absorbance Measurements. The absorbance of MRPs was measured using a Shimadzu UV-2500 spectrophotometer at 294 and 420 nm (browning intensity). Appropriate dilutions were prepared to obtain an absorbance value of 0.05). The loss of amino groups for casein heated alone probably originates largely from lysine residues and/or guanidino groups of arginine via cross-linking reactions.25,26 Casein peptides have more free amino groups and solventaccessible amino acids in comparison with casein, and heating possibly induced greater loss of their free amino groups. From the present results, the decreases in free amino group are in good agreement with the corresponding increase in browning and absorbance at A294 nm (Figure 1). Changes in DPPH Scavenging and Iron Chelating Activity. One of the major pathways to inhibit lipid oxidation is by free radical scavenging.27 DPPH can produce lipid-soluble radicals, and DPPH assays measure SET reactions from DPPH to antioxidant compounds.28 The ability of CP to scavenge DPPH was not affected by heating (p > 0.05) (Figure 3). After 3 h of heating, the DPPH scavenging activity of MRPs was higher than that of CP heated in the absence of glucose (p < 0.05). MRPs from CP/G 1:2 had stronger DPPH scavenging activity compared to CP/G ratios of 1:1 or 1:0.5. Other researchers have reported that browning intensity correlated well with DPPH scavenging activity.4,29 A similar phenomenon was observed in this study with DPPH scavenging activity

Figure 1. Changes in absorbance at 294 nm (a) and browning intensity (b) of the MRPs derived from CP/G 1:0.5, CP/G 1:1, CP/G 1:2, and CP alone (control) as a function of heating time. Bars indicate the standard deviation from triplicate determinations.

significant increase in A294 nm and browning intensity with heating time could be detected (p > 0.05). Heating the different concentrations of glucose alone resulted in slight changes in A294 nm and browning intensity (data not shown). These color changes are likely due to the caramelization of glucose and were found to account for 1−5% of browning intensity of the heated CP/G combination. At 1 h of heating, MRPs from CP/G 1:2 (mole ratio of free amino groups to sugar carbonyl groups) showed significantly higher A294 nm than those of CP/G 1:1 and CP/G 2:1 (p < 0.05). A steady increase in A294 nm and browning intensity of all MRP samples was observed for up to 12 h of heating. After 12 h of heating, the MRPs derived from CP/G 1:2 exhibited the highest increase in absorbance at A294 nm and browning intensity, followed by those of CP/G 1:1 and 1:0.5. From the present results, Maillard reaction from casein peptides increased faster as glucose concentrations increased. In addition, a positive correlation between A294 nm and browning intensity of 13313

dx.doi.org/10.1021/jf203778z | J. Agric.Food Chem. 2011, 59, 13311−13317


Article

Figure 3. DPPH radical scavenging activity for MRPs derived from CP/G 1:0.5, CP/G 1:1, CP/G 1:2, and CP alone as a function of heating time. Bars indicate the standard deviation from triplicate determinations.

Table 1. Iron Chelating Activity for MRPs Derived from CP/G 1:0.5; CP/G 1:1; CP/G 1:2, and CP Alone as a Function of Heating Timea heating time (h) 0 1 3 6 12

CP/G 1:0.5 46.70 46.29 47.51 42.09 42.63

± ± ± ± ±

0.58 aA 1.15 aA 0.96 aA 0.92 bA 1.34 bA

CP/G 1:1 47.79 45.89 44.71 44.12 40.46

± ± ± ± ±

1.35 aA 1.24 abA 0.39 bC 1.15 bA 1.49 cA

CP/G 2:1 46.61 45.31 45.21 43.17 41.00

± ± ± ± ±

0.57 aA 0.58 aA 1.05 aB 0.93 bA 0.57 cA

CP 43.58 41.27 43.31 43.44 42.36

± ± ± ± ±

1.15 aB 1.73 aB 0.38 aC 0.19 aA 1.22 aA

a

Each value is expressed as the mean ± SD (n = 3). Different capital letters show significant difference (p < 0.05) among samples with different glucose concentrations at the same heating time. Different lower case letters show significant differences (p < 0.05) for samples with the same glucose concentration but different heating times.

correlating well with browning intensity (r2 = 0.811) and absorbance at 294 nm (r2 = 0.867). MRPs can also inhibit lipid oxidation by chelating prooxidative metals.3 Iron chelating activity of MRPs and CP heated is shown in Table 1. The iron chelating activity of CP alone was not affected by heating (p > 0.05), but the iron chelating activity of unheated CP alone was significantly lower than that of unheated CP/G (p < 0.05), suggesting that some of the iron chelation was due to glucose. Overall, iron chelating activity in the CP/G samples decreased with increased heating time with all CP/G ratios showing significantly less iron chelating after 6 h of heating (p < 0.05). This result was in agreement with Zeng et al.,23 who reported that fructose−lysine and psicose−lysine mixtures have lower iron chelating activities than their substrates before the Maillard reaction. Ruzi-Roca and others30 thought that the significant decrease in iron chelating activity of the MRPs could be partially due to the loss of free amine groups during thermal treatment. However, MRPs prepared from xylose−soy protein hydrolysate by heating at 120 °C for 2.0 h showed higher iron chelating activity than their substrates before the Maillard reaction.3 The Maillard reaction is a chemical method that has been proposed to improve the free radical scavenging activity, reducing power, of protein hydrolysates.3,7 In this study, heating casein peptides with glucose resulted in an increase in browning and absorbance at 294 nm and a decrease in free amino group, indicating that MRPs were formed. Formation of Maillard products increased free radical scavenging as determined by the DPPH assay but decreased the ability of the casein peptides to chelate iron.

Ability of CP and MRPs To Inhibit Lipid Oxidation in Oil-in-Water Emulsions. Physical Stability of Fish Oil-inWater Emulsions. The droplet size of the emulsions was measured immediately after emulsion preparation and every 24 h throughout storage. Emulsion droplet size ranged from 175 to 185 nm and did not change during the course of the experiments (data not shown). The stability of the emulsions was also confirmed by no visual observation of creaming during storage (data not shown). These indicated that the emulsions were stable to droplet aggregation, flocculation, or coalescence.19 Effects of MRPs on the Oxidative Stability of Fish Oil-inWater Emulsions. The oxidative stability of fish O/W emulsions can be evaluated by the length of the lag phase of oxidation. In this study, the lag phase was defined as the first data point statistically greater than the level of oxidation products at day 0.10 The lag phase for the formation of lipid hydroperoxides and TBARS in fish O/W emulsions (pH 7.0) containing samples was investigated. The data for the formation of lipid hydroperoxides in fish O/W emulsions was not shown because their trends were similar to those of the formation of TBARS. Glucose heated alone did not have the ability to inhibit the formation of TBARS in fish O/W emulsions (data not shown). The lag phase of the formation of TBARS in the control, consisting only of fish oil and Tween 20, was less than 1 day (Figure 4). The lag phase for the formation of TBARS in fish O/W emulsions containing CP alone was 8 days for 0, 1, and 3 h of heating time and 6 days for 6 and 12 h of heating. As for MRPs from CP/G 1:0.5 treatment, the lag phase for the formation of TBARS was 10 days for 3 h of heating, which 13314



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Figure 4. Formation of TBARS in fish oil-in-water emulsions (pH 7.0 and 20 °C) stabilized with Tween 20 in the absence (control) and presence of MRPs derived from CP alone (a), CP/G 1:0.5 (b), CP/G 1:1 (c), and CP/G 1:2 (d) as a function of heating time. Bars indicate the standard deviation from triplicate determinations.

Both CP and their MRPs were able to inhibit lipid oxidation in fish O/W emulsions at pH 7.0 and 3.0. However, formation of MRPs did not increase antioxidant activity in the oil-in-water emulsions and, in some instances, prolonged heating times decreased their ability to inhibit lipid oxidation even though these same heating conditions increased the ability of MRPs to scavenge DPPH radicals. These apparently conflicting data could be due to CP inhibiting lipid oxidation by both free radical scavenging and metal chelation. Whereas forming MRPs from CP increased their free radical scavenging activity, it decreased their ability to chelate metals, and the net effect resulted in no dramatic changes in antioxidant activity in oil-inwater emulsions. Bitterness Evaluation. A major disadvantage of using protein hydrolysates as food antioxidants is the presence of bitter-tasting peptides. The bitter taste of protein hydrolysates can limit their potential use and effectiveness, as bitter hydrolysates must be added to foods at concentrations below their bitter flavor threshold.18 The bitterness of MRPs and CP heated is shown in Table 2. When CP were heated alone, no significant decrease in bitterness could be detected (p > 0.05). After 3 h of heating, the bitterness scores of all MRPs were significantly lower than those of CP (p < 0.05). At the end of

showed the longest lag phase for the formation of TBARS among all samples. Overall, the MRPs had similar antioxidant activities after 6 h of heating regardless of glucose concentration. After 12 h of heating, the ability of all the MRPs to inhibit TBARS formation decreased, compared to the 6 h heating time. These data suggest that MRPs produced with longer reaction time will be less effective in inhibiting lipid oxidation. Because there are many antioxidant applications in acid foods (e.g., salad dressings), we investigated the ability of MRPs from the CP/G 1:0.5 treatment to inhibit lipid hydroperoxide and TBARS formation in fish O/W emulsion at pH 3.0 (Figure 5). The CP/G 1:0.5 treatment was chosen because its 3 h heating treatment showed the strongest antioxidative activity at pH 7.0. Overall, MRPs from CP/G 1:0.5 had lower antioxidant activity in oil-in-water emulsions at pH 3.0 than at pH 7.0. The lag phase for the formation of lipid hydroperoxides was 3 days for nonheated and 1 h heated MRPs and 4 days for MRPs heated for 3, 6, and 12 h, respectively. The lag phase for the formation of TBARS was 4 days for nonheated and 1 h heated MRPs and 6 days for MRPs heated for 3, 6, and 12 h, respectively. Unlike the results at pH 7.0, the antioxidant activity of MRPs was not decreased by prolonged heating. 13315



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be more polar due to the addition of glucose derivatives and thus would exhibit reduced bitterness. From the present results, forming MRPs partially decreased the bitterness of the CP. Whereas the formation of MRPs did not dramatically alter the antioxidant activity of casein peptides, it does have an advantage in that it decreased their bitterness. This reduction in bitterness likely due to the MRPs having reduced hydrophobicity. Because MRPs have less bitterness than but antioxidant activity similar to the original CP, they could be added to foods at higher concentrations, at which they would be more effective at inhibiting lipid oxidation and extending shelf life. However, it should be recognized that the color and potential flavor of MRPs would not be compatible with all foods. In foods for which these color and flavor problems are not an issue, MRPs could be an effective strategy for inhibiting oxidative rancidity. MRPs could be a useful way to mask the bitterness of antioxidant peptides, but the potential cytotoxic effect of MRPs from CP should not be ignored. Generally, MRPs obtained from the mild heat treatment do not produce cytotoxic substances. Chevalier and others 32 determined that βlactoglobulin glycated with several sugars at 60 °C for up to 72 h was not mutagenic. Jing and Kitts33 also reported that MRPs from casein and glucose, fructose, or ribose produced at 55 °C and pH 7.0 for up to 28 days showed no toxicity to Caco-2 cell at both low and high concentrations. Future research should determine if the conditions to produce MRPs with low bitterness and high antioxidant activity produce compounds with potential cytotoxic effects.

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AUTHOR INFORMATION

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REFERENCES

Corresponding Author *Phone: (413) 545-1026. Fax: (413)545-1262. E-mail: [email protected]. Funding This research was partially funded by Grant 2010-30901121 from the National Natural Science Foundation of China.

Figure 5. Formation of lipid hydroperoxides (a) and TBARS (b) in fish oil-in-water emulsions (pH 3.0 and 20 °C) stabilized with Tween 20 in the absence (control) and presence of MRPs derived from CP/G 1:0.5 as a function of heating time. Bars indicate the standard deviation from triplicate determinations.

(1) Kim, J. S.; Lee, Y. S. Study of Maillard reaction products derived from aqueous model systems with different peptide chain lengths. Food Chem. 2009, 116, 846−853. (2) Jing, H.; Melissa, Y.; Wong, P. Y. Y.; Kitts, D. D. Comparison of physicochemical and antioxidant properties of egg-white proteins and fructose and inulin Maillard reaction products Food Bioprocess Technol. 2009, DOI: 10.1007/s11947-009-0279-7. (3) Liu, P.; Huang, M.; Song, S.; Hayat, K.; Zhang, X.; Xia, S.; Jia, C. Sensory characteristics and antioxidant activities of Maillard reaction products from Soy protein hydrolysates with different molecular

the heating period, the bitterness scores of MRPs from CP/G 1:0.5, CP/G 1:1, and CP/G 1:2 were decreased by 29.5, 43.5, and 43.6%, respectively, compared to that of CP heated for 12 h. Liu et al.3 reported that Maillard reaction at 120 °C heating for 2 h could reduce the bitterness of soy protein hydrolysates. Peptides induce bitter sensory perceptions when they are hydrophobic.31 The formation of MRPs would be expected to

Table 2. Mean Bitterness Score (%) for MRPs Derived from CP/G 1:0.5; CP/G 1:1; CP/G 1:2, and CP Alone as a Function of Heating Timea heating time (h) 0 1 3 6 12

CP/G 1:0.5 70.5 70.3 60.0 54.7 50.0

± ± ± ± ±

4.6 aA 4.1 aA 4.5 bB 4.3 cB 3.4 cB

CP/G 1:1 70.7 62.5 51.7 44.0 40.1

± ± ± ± ±

4.8 aA 5.2 bB 3.7 cC 3.4 dC 4.0 dC

CP/G 1:2 68.8 60.3 50.8 45.8 40.0

± ± ± ± ±

4.9 aA 3.2 bB 4.8 cC 3.8 dC 3.4 eC

CP 72.4 69.8 68.0 69.8 71.0

± ± ± ± ±

4.8 aA 3.6 aA 5.3 aA 5.2 aA 4.9 aA

a Each value is expressed as the mean ± SD (n = 9). Different capital letters show significant difference (p < 0.05) among samples with different glucose concentrations at the same heating time. Different lower case letters show significant differences (p < 0.05) for samples with the same glucose concentration but different heating times.

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Chemical and Antioxidant Properties of Casein Peptide and Its

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