Article pubs.acs.org/JAFC
Contribution of Three Ionic Types of Polysaccharides to the Thermal Gelling Properties of Chicken Breast Myosin Yan-zi Zhou, Cong-gui Chen,* Xing Chen, Pei-jun Li, Fei Ma, and Qiu-hong Lu School of Biology and Food Engineering, Hefei University of Technology, Hefei 230009, Anhui Province, People’s Republic of China ABSTRACT: The effects of anionic (κ-carrageenan, KCG), neutral (locust bean gum, LBG), and cationic polysaccharides (water-soluble chitosan, WSC) on the water-holding capacity (WHC) and hardness of chicken myosin gels were investigated at 0−1.0% addition levels. The changes of gel properties were explained using different instrumental techniques. The results revealed that KCG and LBG at 0.5−1.0% could respectively cause significant increases of both WHC and hardness of corresponding heat-induced myosin−polysaccharide gels (P < 0.05). These increases could be ascribed to a slower relaxation, reinforced cross-linked extent, enhanced hydrogen bonding, and a fine-stranded gel network, according to the analysis of lowfield nuclear magnetic resonance, dynamic rheology, Fourier transform infrared spectroscopy, and scanning electron microscopy measurements. However, the weak molecular interaction within myosin−WSC gels induced an insignificant change of the WHC and hardness (P > 0.05). Therefore, it is interesting to search for the anionic polysaccharide and neutral polysaccharide for use as fat substitutes in the development of low-fat meat products. KEYWORDS: κ-carrageenan, water-soluble chitosan, locust bean gum, myosin, gelation
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κ-Carrageenan (KCG) is an anionic linear sulfated polysaccharide characterized by its repeating disaccharide units of 3-linked β-D-galactose 4-sulfate and 4-linked 3,6anhydro-α-D-galactose. The solution of KCG may form a gel upon cooling in the presence of cation; the mechanism of its gelation involves coil−helix transition of KCG molecules.20 In muscle protein gels, KCG likely forms an independent network, which supports the principal structure formed by proteins during gelation. KCG had been reported to interact with saltsoluble meat proteins (SSMP) to increase the compressive force of gels,21 and KCG with 2.5% NaCl could decrease the transition temperature of chicken thigh myosin head and actin.22 LBG is a branched neutral polysaccharide containing (1→4)β-D-mannopyranosyl backbone with the attachment of single (1→6)-α-D-galactose unit. It is not susceptible to ionic strength and does not form gels by itself. In meat research, LBG, generally added in the form of a complex, acted with complementary and/or synergistic effect, which produced better meat quality characteristics. For example, the mixture of KCG and LBG exhibited significantly higher penetration values and caused appreciable increases in hardness, cohesiveness, and water-holding capability (WHC) in blue whiting gels.23 Our previous work24 reported that there were certain interactions between LBG and SSMP, whereas the interactions and those effects on the gelling properties between LBG and specific protein in SSMP are not completely understood. Chitosan is a cationic linear polysaccharide consisting of β(1−4)-2-acetamido-2-deoxy-β-D-glucopyranose and 2-amino-2deoxy-β-D-glucopyranose. The polyelectrolytic character of
INTRODUCTION
Polysaccharides or hydrocolloids, derived from a variety of plants and microorganisms, are considered as the most effective fat substitutes for developing low-fat meat products.1 The polysaccharides such as carrageenan,2−4 chitosan,5,6 and locust bean gum (LBG)7,8 were widely used in the products for their ability to enhance gelling character, to retain water, and to provide a desirable texture. Functional properties of food proteins, such as viscosifying and gelling abilities and foaming and emulsifying abilities, are affected by the interaction with polysaccharide.9−11 Various papers have sought to explain the structures that could be formed by gums and how they could interact with the food protein.12−15 Different polysaccharides displayed various behaviors on the functional properties of meat products; for example, carrageenan could increase the hardness of low-fat beef sausages, but sausages containing alginate, LBG, and xanthan gum became softer, more deformable, crumbly, and slippery.8 The charge density of polysaccharides dominates the microstructure and large deformation properties of polysaccharide−whey protein gels.16 The anionic hydrocolloids (carragenaan, xanthan gum, etc) were mixed throughout the protein matrix of blue whiting myosystem, probably through the interaction with myofibrillar protein, whereas the neutral hydrocolloids (LBG and guar gum) were dispersed throughout the matrix but did not interact with myofibrillar protein and were located simply by inclusion.17 The different behaviors by which polysaccharides act on protein depend on the composition, distribution, physical state, volume fraction of polysaccharide, and interaction with the continuous protein matrix.17,18 The nonspecific protein−polysaccharide interactions are responsible for the attractive and repulsive forces that induce the complex formation or incompatibility of biopolymers.19 © 2014 American Chemical Society
Received: Revised: Accepted: Published: 2655
November 28, 2013 March 8, 2014 March 8, 2014 March 8, 2014 dx.doi.org/10.1021/jf405381z | J. Agric. Food Chem. 2014, 62, 2655−2662
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dialyzed against the 0.6 M KCl buffer (pH 6.5) described above. The resulting supernatant was regarded as the extraction solution of myosin. The protein composition of the extracted myosin was validated by SDS-PAGE with a gel imaging system (Universal Hood II, Bio-Rad Co., Hercules, CA, USA). The protein concentration in this myosin solution was measured using the biuret method, and the final concentration was adjusted to 20 mg/mL myosin using the 0.6 M KCl buffer (pH 6.5) before test. Preparation of Myosin−Polysaccharide Sols. Each polysaccharide (KCG, LBG, and WSC) was added to 80 g of myosin solution (myosin 20 mg/mL) at different concentrations (0, 0.25, 0.5, 0.75, and 1%, w/w). Deviations of weight of all ingredients were not in excess of ±0.5%. The mixture of myosin−polysaccharide was stirred for 30 min and homogenized for 1 min at 10000 rpm using a homogenizer (FA25 model, Fluko, China) in a 100 mL beaker and kept overnight (about 12 h) at 4 °C prior to treatment. Preparation of Myosin−Polysaccharide Gels. Each myosin− polysaccharide sol prepared as described above was added to a beaker of 27 mm in diameter and 35 mm in height. The beaker was heated for 30 min in an 80 °C water bath (an additional 25 min was needed to warm the system from 20 to 80 °C) to form a gel and then immediately cooled in a water bath (about 4−10 °C) and kept overnight (10−12 h) at 4 °C before the WHC, gel hardness, low-field nuclear magnetic resonance (LF-NMR), Fourier transform infrared (FT-IR) spectroscopy, and scanning electron microscopy (SEM) measurements. Gel Analysis. WHC. WHC was determined by a centrifugal method.34 Approximately 5 g of the gel was centrifuged at 1000g for 10 min at 4 °C. WHC was the percentage of the gel’s weight retained after centrifugation relative to its initial weight. The experiments were conducted in triplicate. Gel Hardness. The hardness of the gels was analyzed using a TAXT Plus Texture Analyzer (Stable Micro System Co., UK) at ambient temperatures (approximately 20 °C). The gel was subjected to a compression test using a cylindrical probe (P/0.5 in., aluminum) at a trigger type button with a 1.5 mm/s pretest speed, a 1.0 mm/s test speed, a 1.0 mm/s post-test speed, a 4.0 mm distance, and a 3 g trigger force. The maximum force of the sustained compression was the gel hardness. The experiments were conducted in six replicates. LF-NMR Spin−Spin Relaxation Time (T2) Measurements. The gel sample (2−4g) was placed in a ⌀ 25 mm glass tube. LF-NMR 1H T2 measurements were performed using the method described by Han et al.35 with slight modifications. The data were collected using a Niumag Benchtop Pulsed NMR Analyzer MiniMR-60 (Niumag Electric Corp., Shanghai, China) operating at 23.318 MHz (proton resonances) and a τ value (time between the 90° and 180° pulse) of 200 μs. The data from 12000 echoes were acquired as 32 scan repetitions with 6.5 s between each scan. T2 was measured in triplicate. The NMR T2 data were analyzed using Multi-Exp Inv Analysis Software (Niumag Electric Corp.). FT-IR Spectroscopy. The gel samples were dried for 32 h in an LGJ12 lyophilizer (Beijing Songyuan Huaxing Technology Development Co., Ltd., China). The spectra of the gels were obtained as previously described24 and recorded at ambient temperatures (20 °C) using 16 scans. Origin 7.5 software was used to analyze these spectra. SEM. The gel samples were dried for 15−16 h in an LGJ-12 lyophilizer (Beijing Songyuan Huaxing Technology Development Co., Ltd., China). The SEM observations were performed with a field emission scanning electron microscope (SU8020, Hitachi, Ltd., Japan), according to a reported method.24 Two fields from each treatment were examined, and one of the two is presented. Myosin−Polysaccharide Sol Analysis. Dynamic Rheological Measurements. The dynamic oscillatory measurements were carried out with a rheometer (Discovery HR-3, TA Instruments Co., USA) in oscillatory mode, as described by Verbeken et al.36 with a slight modification. A parallel steel plate geometry (60 mm) with a 550 μm gap was used. The samples were heated from 20 to 80 °C at 2 °C/min and cooled from 80 to 20 °C at 4 °C/min. A continuous 0.1 Hz
chitosan, due to a high degree of deacetylation, allows it to react efficiently with negatively charged groups of proteins.25 The addition of chitosan resulted in significant inhibition of microbial growth in Greek-style fresh pork sausages6 and could also improve thermal gelling properties of grass carp gels and delay lipid oxidation.26 However, the high molecular weights and high viscosity may restrict its uses;27 the oligomers of chitosan have received considerable attention because they may have a greater biological activity than chitosan and are also water-soluble.28 Myosin, as one component of the myofibrillar protein matrix, is a primary gelling protein, which contributes to desirable texture and stabilization of fat and water in processed meat products. The gelation of a single myosin has been studied in detail.29−31 Many papers have discussed the interaction between myofibrillar protein and polysaccharide,13,17,19 but the myofibrillar protein system is complex (containing a variety of proteins such as myosin, actin, actomyosin), and it is not clear that the certain protein of the system interacts with polysaccharide. To our knowledge, no investigation has so far been reported regarding the interactions of different ionic types of polysaccharide (KCG, LBG, and water-soluble chitosan (WSC)) with myosin and those interaction-induced effects. The objective of this study was to investigate the effects of three ionic types of polysaccharide (anionic polysaccharide KCG, neutral polysaccharide LBG, and cationic polysaccharide WSC) on the WHC and hardness of poultry myosin gels. And the changes in the properties of the gels were explained using different instrumental techniques.
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MATERIALS AND METHODS
Materials. KCG and LBG were provided by Danisco (China) Co., Ltd. The viscosities of a 1.5% KCG water solution (w/w) and a 1% LBG water solution (w/w) were above 10 and 2400 mPa·S, respectively. WSC was provided by HeFei XinDe Biotechnology Co., Ltd., and the viscosity of 1% water solution (w/w) was 55 mPa·S. The chicken breast meat was purchased from a local supermarket of the Carrefour Group, and the preparations were commenced within 30 min after purchase. Extraction of Myosin. Myosin was extracted from chicken breast muscle using a modified procedure originally reported by Wang et al.32 and Hayakawa et al.33 First, 150 g of minced chicken breast muscle was mixed with 400 mL of a cold solution (0.1 M KCl, 20 mM potassium phosphate, 2 mM MgCl2, 1 mM EGTA, pH 7.0) containing 1 mM dithiothreitol and subsequently ground with a blender (DS-1, Shanghai Yueci Electronic Technology Co., Ltd., China). The mixture was treated with 1% Triton X-100 and homogenized for 3 min at 10000 rpm using a homogenizer (FA25 model, Fluko, China); the material was subsequently centrifuged at 8000g for 12 min at 4 °C in a refrigerated centrifuge (CT14RD, Techcomp Co., Ltd., Shanghai, China). The sediment was extracted with 3 volumes of a modified Guba−Straub solution (0.3 M KC1, 0.1 M KH2PO4, 50 mM K2HPO4, 1 mM EGTA, 4 mM sodium pyrophosphate, 20 mM MgCl2, pH 6.5) and stirred for 15 min at 4 °C with an agitator. The extraction was centrifuged at 5000g for 12 min at 4 °C. The supernatant was diluted with 14 volumes of cold 1 mM EDTA−2Na solution and stored for 90 min at 4 °C. After the floating material was removed via siphon, the precipitated protein was collected by centrifugation at 10000g for 6 min at 4 °C and suspended using 1 volume of 0.6 M KCl buffer (0.6 M KCl, 150 mM sodium phosphate, pH 6.5). Magnesium chloride and sodium pyrophosphate were added into the protein, reaching final concentrations of 5 mM. The mixture was stirred vigorously for 30 min without foaming at 4 °C and centrifuged at 10000g for 20 min at 4 °C. The supernatant was rediluted with 9 volumes of cold distilled water and precipitated for approximately 12 h. The floating material and precipitated protein were re-treated as above. The solution was 2656
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oscillation and a 2% strain were applied to monitor the storage modulus (G′) and tan δ. All experiments were conducted in triplicate. Statistical Analysis. The analyses of variances, means, and standard errors were determined using Excel 2003 (Microsoft Office Excel 2003 for Windows). A significance level (P < 0.05) was used to determine differences between the treatments.
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RESULTS AND DISCUSSION WHC. As shown in Figure 1, all three polysaccharides could improve the WHC of myosin gel, and the improvement by
Figure 2. Effects of KCG, LBG, and WSC on the hardness of myosin gels. Different lower case letters (a−d) in the same curve indicate significant difference (P < 0.05). Different capital letters (A−C) among the three polysaccharides at the same concentration indicate significant difference (P < 0.05).
concentrations, respectively. For the myosin−WSC gel, the change of the gel hardness was insignificant (P > 0.05) within the range of 0.25−1.0% addition, except the addition of 0.5% WSC (Figure 2). The increasing effect of KCG in gel hardness was the greatest among the three polysaccharides. Synergistic effects have been previously observed between carrageenan and proteins, which resulted in much stronger gels compared to protein alone. DeFreitas et al.21 interpreted the positive influence of carrageenan on the strength of meat protein/carrageenan gels as resulting from the interaction between them. However, Verbeken et al.36 inferred that KCG did not seem to interact with meat protein in the network formation. Hunt et al.38 deemed that the increase in gel strength might be caused by electrostatic interactions between sulfate groups of carrageenan molecules, resulting in increased aggregation of the molecular chains of carrageenan. LBG is a nongelling polysaccharide, and Xiong et al.8 reported that a high hardness of low-fat beef sausages containing LBG was obtained at pH 6.2. In the presence of chitosan, protein− polysaccharide conjugation may be formed between the reactive amino group of glucosamine and the glutaminyl residue of myofibrillar proteins.26,37 The penetration force of SSMP gels increased proportionally with increasing amount of chitosan incorporated in gels.37 The breaking force of kamaboko gels increased proportionally with the amount increase of chitosan below 1% addition and then decreased significantly when a higher amount (>1%) of chitosan was added. It was postulated that a higher concentration of chitosan could disrupt the polymerization and aggregation of myofibrils, resulting in a reduced breaking force.26 Some researchers reported that KCG, LBG, and WSC would be entrapped as filler ingredients to influence the formation of the continuous gel matrix, modifying the physicochemical properties of the aqueous phase and/or influencing the texture of the final product.26,36,39 The interactions between polysaccharides and proteins in food systems are largely ascribed to electrostatic association or repulsion. Because myosin would have a negative net charge in myosin/polysaccharide systems at pH 6.5 (isoelectric pH 5.0 of myosin), it was speculated that electrostatic repulsion between the myosin and KCG with the
Figure 1. Effects of KCG, LBG, and WSC on the WHC of myosin gels. Different lower case letters (a−c) in the same curve indicate significant difference (P < 0.05). Different capital letters (A, B) among the three polysaccharides at the same concentration indicate significant difference (P < 0.05).
LBG was the greatest. The increase of KCG or LBG concentrations in the range of 0−1% caused a clear improvement for the WHC of myosin gel, whereas the concentration of WSC in the range of 0−0.75% had no distinctive impact (P > 0.05). The difference between the effect of KCG and that of LBG on the WHC was insignificant (P > 0.05) at equal concentrations from 0.5 to 1%; however, the addition of KCG and LBG could respectively cause significant increases (P < 0.05) of WHC value compared to the WSC at equal concentration. The increase of WHC indicated that more favorable physical entrapment of water occurred in the protein−gum matrices.13 The results suggested that myosin− KCG or myosin−LBG matrices had a greater ability to entrap water than myosin−WSC matrices. The effects of KCG, LBG, and WSC on WHC properties shown in our study were consistent with the results of other studies.7,8,21,36,37 It is well-known that water-binding capacity involves electrostatic, hydrophobic, and hydrogen bonds, etc. LBG has the ability to form hydrogen bonds with water.7,8 It was found that KCG was present in the interstitial spaces of the protein network, where it bound water and might form gel fragments upon cooling,36 and thus more water was preserved in the gel network. The WHC improvement of WSC samples was due to the interaction between chitosan and protein.37 The result suggested that the ability to effectively immobilize water increased in the order myosin−LBG, myosin−KCG, and myosin−WSC. Gel Hardness. As shown in Figure 2, the hardness of myosin gels increased with the elevated KCG and LBG 2657
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The positions of T22 and T23 clearly shifted toward lower relaxation times with polysaccharide added at 1% concentration (Figure 3). A statistically significant effect was found on the T22 and T23 (P < 0.05) with the addition of gum to myosin (Table 1); the T22 and T23 decreased in the order myosin−LBG, myosin−KCG, and myosin−WSC, respectively, which is in accordance with the result of WHC. The decrease in nuclear relaxation times indicates an overall decrease of water mobility, and the results are similar to the effect of flaxseed gum added into myofibrillar protein.42 Hills et al.43 showed that protein aggregation reduced the water proton relaxation time. This implied that the polysaccharide could stimulate the aggregation of myosin, which was also consistent with the subsequent increase of the storage modulus (G′) induced by the addition of the three polysaccharides. Dynamic Rheological Measurements. The thermodynamic viscoelastic behavior of myosin (Figure 4A) showed that
same charges could be enhanced and thus lead to the changes of the WHC and hardness of myosin−KCG gel. On the other hand, the electrostatic attraction between the protein and WSC with positively charged groups could also be strengthened, and the gel hardness of myosin−WSC should theoretically be increased. However, the present result (Figure 2) revealed that the increase of the myosin−WSC gel hardness was insignificant (P > 0.05). Many researchers ascribed the gel hardness to the polymerization of myofibrils inter- or intramolecularly, and the polymerization was relative to the deacetylation degree of chitosan.37 This insignificant increase might be attributed to the deacetylation degree of WSC, but this mechanism is worth further investigation. The WHC of myosin gel with LBG was greatest among the three myosin−polysaccharide systems (Figure1), whereas the gel hardness of myosin with LBG was smaller than that with KCG (Figure2). The results were similar to the result of the KCG−/LBG−blue whiting muscle system described by PérezMateos et al.40 The WHC of myosin gel with KCG was less than that with LBG, because KCG is quite difficult to hydrate within the myosystem.40 This discrepancy could also be due to the different types of interaction (electrostatic, ionic, etc) between protein and polysaccharide and/or the differences in ability to solubilize or disperse through the protein matrix. LF-NMR. The distributions of T2 relaxation times on the gels with myosin−KCG, myosin−LBG, and myosin−WSC are shown in Figure 3. Three relaxation populations of myosin
Figure 3. Effects of KCG, LBG, and WSC on the distribution of the T2 relaxation times of myosin gels.
were centered at approximately 3.05−14.17 ms (T21), 151.99− 464.15 ms (T22), and 1232.85−1873.82 ms (T23), revealing bound, immobilized, and free water, respectively, according to the report of Bertram et al.41 In addition, a broadening of the populations centered at 151.99−464.15 ms (T22) was significant.
Figure 4. Effects of KCG, LBG, and WSC on the storage modulus (G′) of myosin sols during heating (A) and cooling (B).
Table 1. Effects of Three Ionic Types of Polysaccharide on the Spin−Spin Relaxation Times (T22 and T23) of Myosin Gelsa
a
sample
myosin gel
myosin + 1% KCG gel
myosin + 1% LBG gel
myosin + 1% WSC gel
T22 T23
265.60 ± 17.37d 1629.75 ± 53.05d
174.75 ± 13.14b 811.13 ± 46.14b
132.19 ± 0.93a 705.48 ± 0a
231.01 ± 0c 1417.47 ± 60.9c
Data are expressed as the mean ± SD, n = 3. Different letters (a−d) in the same row indicate statistically significant differences at P < 0.05. 2658
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the G′ of the myosin suspension, reflecting the elastic portion of the gel, began to increase from 44 °C and reached a maximum value at 48 °C before declining sharply to a minimum value at 54 °C, after which an ascending curve was exhibited. This rheological transition could be ascribed to the initial association of myosin head as a result of denaturation of the S1 subfragment (42−48 °C) and the disruption of the temporary protein network due to subsequent unfolding of the myosin tail (50−55 °C).44 Moreover, a similar G′ curve was reported during heating of chicken myosin in 0.6 M NaCl at pH 7.0.45 The G′ of myosin−polysaccharide increased with the increase of temperature, showing differences between different systems (Figure 4A). However, the G′ did not decrease during heating at 48 °C. This phenomenon was similar to the effect of β-lactoglobulin added to myosin,45 and the authors suggested that the relative importance of the various chemical bonds forming the mixed protein gel network might be different from those bonds forming the myosin gel network. The finding in our experiments suggested that the effects of the three polysaccharides were the same as the effect of β-lactoglobulin. The G′ values of myosin−KCG mixtures were significantly higher than those of myosin−LBG or −WSC at the same temperature, which meant that the myosin−KCG interaction could lead to a higher elasticity of the sol/gel. These results corresponded with the gel hardness of myosin−KCG, which tended to be higher compared to the gels containing LBG or WSC. The mixtures of myosin with LBG and WSC showed similar G′ evolution with heating up to 60 °C, but the myosin− WSC above 60 °C showed higher values of G′. The G′ increase of myosin−polysaccharide suggested the presence of fine gel structure containing polysaccharide sample, because G′ is relative to the degree of three-dimensional network formation. Previous observations on protein−KCG systems showed synergistic effects between the two biopolymers on gelation properties at pH above the isoelectric point of proteins.36,38,46 The main cause of these synergistic effects seems to be thermodynamic incompatibility between the different biopolymers in solution. At low pH values, sulfated polysaccharides and proteins seem to be completely compatible. When the pH increases above the isoelectric point of the protein, the net charge of the protein becomes negative. As a result, electrostatic repulsive forces between protein molecules and sulfated polysaccharides increase, promoting excluded volume effects. This may lead to a mutual concentration of both biopolymers in separate microphases and favor the gelation of the hydrocolloid.39 The tan δ of myosin gradually increased until 44 °C and then decreased in subsequent heating. However, the tan δ is 0.05) on the final elasticity of myosin gels during cooling. This observation suggested the
Figure 5. Effects of KCG, LBG, and WSC on the tan δ of myosin sols during heating.
anionic polysaccharide KCG and the neutral polysaccharide LBG could strengthen the cross-linked extent of myosin gel network when the thermal gelling of the myosin was irreversible, whereas the cationic polysaccharide WSC could not. In addition, the G′ value is relative to the aggregation of protein49,50 and the gel strength. Although the changes in G′ of myosin−LBG were similar to those of myosin−WSC (Figure 4), the change of gel hardness is not entirely consistent with that of G′. The hardness could be also affected by other factors, such as interactions between protein and polysaccharide, 12 hydrogen bonding, and the microstructure of gels. The different extents of these influences on both hardness and G′ might be why the gel hardness of myosin−LBG was much higher than that with WSC (Figure 2). FT-IR. As shown in Table 2, myosin gel revealed transmittance bands at 3439 cm−1 (amide A, N−H or O−H Table 2. FT-IR Spectra Data for Three Kinds of Myosin− Polysaccaride Gels FT-IR spectra numbers (cm−1) treatment A B C D
myosin myosin myosin myosin
gel + 1% KCG gel + 1% LBG gel + 1% WSC gel
PK1 3439 3358 3366 3439
PK2
PK3
PK4
3006 3006
1655 1655 1655 1655
1106 1131 1106 1106
stretching, PK1),51 1655 cm−1 (amide I, CO and CN stretching, PK3), and 1106 cm−1 (C−O and C−C stretching, PK4).52 Among these bands, the amide A band between 3000 and 3500 cm−1 is known as the “water region” to assess and estimate the interaction of protein molecules with water,51 and the amide I band between 1600 and 1700 cm−1 is commonly used for secondary structure analysis of proteins.53 The spectra of myosin and myosin−polysaccharide gels indicated similar patterns, which suggested that there was no major change in the functional groups of the myosin with the addition of polysaccharide. The addition of KCG and LBG caused the shift of NH-stretching left peak (PK1) to lower number and the appearance of the NH-stretching right peak (PK2). The shift of PK1 to lower number implied the 2659
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Figure 6. SEM images of (A) myosin gel, (B) myosin + 1% KCG gel, (C) myosin + 1% LBG gel, and (D) myosin + 1% WSC gel.
In addition, the amount of meat proteins interacting with polysaccharide (κ-carrageenan) to form complexes/gels increased with the concentration of meat proteins,54 and the interactions between protein and hydrocolloid were the reason for the increased gel strength; higher protein concentration then affected the interaction by increasing gel strength.12 These implied that the present results of low myosin concentration (20 mg/mL) could be applicable to the gelation of myosin with higher concentration in meat. In conclusion, the additions of three polysaccharides could decrease the mobility (reducing the water relaxation time T22 and T23) of inner water in myosin gel. The anionic polysaccharide KCG and the neutral polysaccharide LBG could strengthen the cross-linked extent of myosin gel network, thus inducing the increase of G′ and forming a compact gel network with fine strands when the thermal gelling of the myosin irreversibly occurred. Moreover, KCG could enhance hydrogen bonding and electrostatic force, and LBG could enhance hydrogen bonding. These efficiencies resulted in the improvement of the WHC and hardness of myosin−KCG and myosin−LBG gels at 0.5−1.0% levels (P < 0.05). However, the weak molecular interaction between cationic polysaccharide WSC and myosin had an insignificant effect on the WHC and hardness of myosin−WSC gels (P > 0.05). Therefore, it is interesting to search for the anionic polysaccharide and neutral polysaccharide that can substitute for fat in the development of low-fat meat products.
enhancement of hydrogen bonding within myosin molecules. A similar shift of this peak caused by interrupting intramolecular hydrogen bonding was also observed by Ma et al.24 and Garcı ́aGarcı ́a et al.7 The enhancement of hydrogen bonding in gels might cause the increase of WHC (Figure 1) and gel hardness (Figure 2), but the effect of WSC on the spectral patterns of myosin gels was not obvious because the spectra did not change (Table 2). Microstructure. The three-dimensional network structure of gels is an important determiner of sensory texture, rheological properties, and WHC. As shown in Figure 6, distinctive differences among gels were observed, which corresponded to the differences in gel hardness and WHC. The control myosin gels (Figure 6A) appeared slightly porous and consisted of protein aggregates. Cross-linking with KCG− myosin (Figure 6B) or LBG−myosin (Figure 6C) reduced the empty spaces and changed the aggregate gel structure into a fine-stranded gel network. Baeza et al.39 also suggested that electrostatic cross-links between proteins and KCG might cause a reinforcement of the network structure. The structural difference between the control (myosin gel) and myosin− KCG/myosin−LBG gel may explain why the latter was harder (Figure 2) and more elastic (Figure 4A). On the other hand, Sun et al.42 proposed that the finer gel network with smaller pore size held the majority of water in myofibrillar structures, and it could be concluded that the increases of the WHC in myosin−KCG and myosin−LBG might be related to a compact and dense myosin gel matrix. Although the added WSC could also cause a compact and dense myosin gel structure (Figure 6D), this addition could induce a more concavo-convex microstructure with larger cavities, a severer phase separation, and a bigger aggregation of myosin, which resulted in the weak gel hardness (Figure 2).
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AUTHOR INFORMATION
Corresponding Author
*(C. Chen) Phone: +86-551-62919387. Fax: +86-55162901539. E-mail:
[email protected]. 2660
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Funding
This work was financially supported by the National Natural Science Foundation of China (No. 31271893) and the Special Foundation of Anhui Testing Area (12z0102062). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Shi Ke-fu and Wu Shuang-shuang for their help during this experiment.
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