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Biochemical and Physicochemical Changes in Spent Hen Breast Meat During Postmortem Aging

National Research Centre on Meat, Chengicheria, P. B. No. 19, Uppal PO, Hyderabad, India

¹ Corresponding author: svaith{at}gmail.com

	ABSTRACT

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An experiment was conducted to study the biochemical and physicochemical changes with respect to improvement in tenderness of spent hen breast meat. Breast muscle obtained from freshly slaughtered spent hens (72 wk old) was divided into 5 equal lots and dipped in 1 mM NaN₃ before being packed in low-density poly-ethylene pouches under aerobic conditions and stored at refrigeration temperature (4°C). Lots were removed on 0, 7, 14, 21, and 28 d of storage and analyzed for pH, TBA reactive substances (TBARS), total sulfhydryl content, protein-bound sulfhydryl content, nonprotein-bound sulfhydryl content, perimysial fraction, collagen content, free OH-proline, N, nonprotein N, and proteolysis rate. Shear force value and penetrometer readings were also determined after making patties from the stored muscle samples. Results showed that pH values were gradually decreasing over the storage period. The TBARS values were increasing (P < 0.001), whereas the sulfhydryl content was decreasing (P < 0.001) over the storage period. The TBARS values were negatively (P < 0.05) correlated with total sulfhydryl content. This suggests that sulfhydryl content may prevent further higher oxidation of lipids. The soluble collagen content, collagen solubility, free OH-proline, and proteolysis rate were increasing (P < 0.001) during postmortem aging. These results suggest that collagen degradation into free amino acids occurs postmortem. A gradual decrease (P < 0.001) in shear force value and a gradual increase (P < 0.001) in penetrometer readings were recorded in the patties made from matured breast meat. Therefore, postmortem aging of spent hen breast meat resulted in 23% improvement in tenderness of minced patties on 14 d and 39% on 28 d as evidenced by biochemical and physicochemical changes.

Key Words: biochemical change • physicochemical change • spent hen • breast meat • postmortem

	INTRODUCTION

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Generally, the meat obtained from spent hens has poor quality attributes, resulting in a lower remunerative price for it. Tenderness is considered to be the most important organoleptic characteristic of meat (Lawrie, 1991). As the animal matures, fiber hypertrophy is accompanied by maturation of the endomysium, perimysial thickness, and the formation of nonreducible cross-links between the collagen molecules (Robins et al., 1973). The inferior quality, such as toughness in spent hen meat, is primarily due to increased cross-linking in the connective tissue of older animals (Bailey and Light, 1989). Many attempts have been made to tenderize spent hen meat (Kondaiah and Panda, 1992; Woods et al., 1997; Naveena and Mendiratta, 2001; Bhaskar et al., 2006).

The structural integrity of collagen has been considered to be unaffected during postmortem aging of meat (Bailey, 1985). However, changes in its mechanical (Nishimura et al., 1998), physicochemical (Stanton and Light, 1990), and ultrastructural (Liu et al., 1995; Nishimura et al., 1995) properties have been observed during postmortem aging. The collagen properties and myofibrillar protein changes of spent hens have been studied in relation to growth (Bannister and Burns, 1972), but little is known about the effect of postmortem aging on collagen degradation. In addition, myofibrillar proteins are important structural proteins implicated in tenderness and waterholding capacity. The information related to their denaturation is of great importance in meat technology as a whole. Therefore, with a view to monitor subtle changes in chemical and physical state of protein, meat protein hydrophobicity can be a suitable parameter to estimate protein denaturation (Chelh et al., 2006).

Cross-links formed between Lys residues of neighboring collagen molecules play an important physiological role in maintaining the mechanical stability of collagen fibers in live animals (McCormick, 1994). Previous studies have shown that breakdown products of lipid oxidation such as malonaldehyde can react with the -amino group of myosin and reduce the Lys availability (Tironi et al., 2004). Protein denaturation could be affected by malonaldehyde. However, no such report on the effect of malonaldehyde on collagen content in meat during postmortem aging has been reported.

Sulfhydryl (SH) content as a reactive group in meat may be affected during refrigerated storage. A significant reactive site can be easily oxidized by oxidizing agents generated during meat storage. Therefore, oxidation state is of great importance regarding biological activity (Batifoulier et al., 2002). During refrigerated storage, oxidative stress has long been known to cause lipid oxidation; however, the study of protein oxidation is a relatively new area, and the measurement of free SH is used in the determination of protein oxidation (Stadtman, 1990).

One of the proteolytic systems involved in collagen degradation is the matrix metalloproteinase system including collagenase, stromelysins, and gelatinase. This system degrades connective tissue proteins in all tissues. Collagen denatured by collagenases can be degraded into small peptides by gelatinase activity. The final step in the collagen degradation is the release of hydroxyproline, which is used as a specific indicator of collagen catabolism (Kivirikko, 1970). The products of catabolic activity (i.e., free OH-proline and peptides containing free OH-proline in meat during aging) have been very little studied (Feidt et al., 1998; Sylvestre et al., 2002). If tenderness of the spent hen meat could be improved by degradation of the collagen protein, it would be possible to expand the market for the spent hen meat and increase its value (Kondaiah and Panda, 1992). Therefore, the objective of this work is to study the biochemical and physicochemical changes of spent hen breast meat during postmortem aging. Patties made from spent hen breast meat stored at 4 ± 1°C for 28 d were used to study improvement in tenderness changes.

	MATERIALS AND METHODS

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Fifteen birds (72 wk old, White Leghorn spent hens) were obtained separately for each of 5 batches. Fifteen birds were used for each replication. A total of 75 birds were used for this study. Fresh breast meat was obtained from spent hens slaughtered by the traditional halal method and deboned manually in a commercial poultry processing unit. Breast muscles collected were dipped in 1 mM NaN₃, packed in low-density polyethylene pouches under aerobic conditions, and were stored in a household-type refrigerator (Whirlpool, FF-350 Elite, Whirlpool of India, New Delhi, India) at 4°C with digital display of internal temperature. Lots were removed on 0, 7, 14, 21, and 28 d of storage and kept at –20°C until further analysis. Samples were analyzed for pH, perimysial content, collagen content, TBA reactive substances (TBARS), total SH (T-SH) content, protein-bound SH (PB-SH) content, nonprotein-bound SH (NP-SH) content, free OH-proline, N, and nonprotein N (NPN). Shear force value and penetrometer readings were also determined after making patties from the muscles stored at 4 ± 1°C for 28 d.

Analytical Methods

pH. The pH of the samples was determined by blending 10 g of sample with 50 mL of distilled water for 60 s in a homogenizer (MICCRA D8-Si, ART-moderne Labortechnik, Mullheim, Germany). The pH values were measured using a standardized electrode attached to a digital pH meter (Thermo Orion model 420A+, Beverly, MA).

TBARS. The TBARS value (mg of malonaldehyde/kg) of muscle was determined by using the extraction method described by Witte et al. (1970) with slight modification, because slurry was centrifuged at 3,000 x g for 10 min (Refrigerated Centrifuge CPR24, Remi India, Mumbai, India) instead of filtration through Whatman filter paper No. 42 (Whatman International Ltd., Maidstone, UK).

SH Content. Sulfhydryl content was measured by the method as described by Sedlak and Lindsay (1968). Briefly, 800 mg of muscle sample in 16.0 mL of 0.02 M EDTA was homogenized in a homogenizer (MICCRA D8-Si, ART-moderne Labortechnik). The homogenate was kept in an ice bath until analyzed for T-SH content, protein-bound SH (PB-SH) content, and NP-SH content.

T-SH Content:
Aliquots of 0.5 mL of homogenate were mixed with 1.5 mL of 0.2 M Tris pH 8.2, 0.1 mL of 0.01 M 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), and 7.9 mL of 0.5% wt/vol SDS. They were mixed well and allowed to stand for 30 min with occasional shaking before filtering through Whatman filter paper No. 113. The absorbance of clear filtrate was taken at 412 nm and was measured against a blank of DTNB at the same concentration without proteins.

NP-SH Content:
Aliquots of 5.0 mL of homogenate were mixed with 4.0 mL of distilled water and 1.0 mL of 50% trichloroacetic acid and were allowed to stand for 15 min at room temperature with intermittent shaking before filtering through Whatman filter paper No. 113. Two milliliters of filtrate was mixed with 4.0 mL of 0.4 M Tris pH 8.9 and 0.1 mL of 0.01 M DTNB. They were mixed well, and absorbance was measured within 5 min at 412 nm against a blank of DTNB at the same concentration.

The SH content was calculated by application of an absorption coefficient of 13.6 mM^–1cm^–1 in both T-SH and NP-SH. The PB-SH content was calculated by subtracting the NP-SH from T-SH.

Perimysial Fraction. Perimysial fraction was prepared from muscles by the method as described by Light and Champion (1984) with slight modifications. Each 50-g sample was homogenized in 200 mL of ice-cold 50 mM CaCl₂ for 30 s at 10,000 rpm in a homogenizer (MICCRA D8-Si, ART-moderne Labortechnik). The homogenate was filtered through a stainless steel grid (1-mm sieve; Jayant Scientific Industries, Mumbai, India), and the materials retained on the filter were rehomogenized in 50 mM CaCl₂ and refiltered. This process was repeated twice. The retained material was washed with 50 mM Tris HCl pH 7.4 and then washed with distilled water thrice. This fraction was termed as perimysial fraction, and the yield of fraction was determined by measuring the amount of hydroxyproline (HP) according to the method of Newman and Lohan (1950). Collagen concentration was calculated by multiplying HP with 7.25 (Cross et al., 1973).

Collagen. The stored muscle was analyzed for insoluble collagen (IC) content and soluble collagen (SC) content. The IC and SC content in the muscle were determined as described by Kristensen et al. (2002). Briefly, 6 g of muscle was cut into small pieces and kept in 20 mL of water in a 50-mL glass beaker before being heat-treated for 2 h in a water bath at 90°C. The pieces were cooled to room temperature and homogenized in a homogenizer (MICCRA D8-Si, ART-moderne Labortechnik) for 1 min at 10,000 rpm, which afterwards was flushed with 10.0 mL of distilled water. The homogenate was filtered through Whatman No.1 filter paper, and then 30.0 mL of 6N HCl was added to the filtrate, whereas 50.0 mL of 6N HCl was added to the residue. Both were then hydrolyzed overnight in an oven at 105°C. The concentration of HP was determined according to the method of Newman and Lohan (1950). The amount of SC was calculated from HP concentration in the filtrate, and IC was calculated from the HP concentration in the residue. The IC concentration was calculated by multiplying the residue HP concentration with 7.25, and SC concentration was calculated by multiplying filtrate HP concentration with 7.52 (Cross et al., 1973). Collagen solubility in each sample was expressed by the ratio SC/(IC + SC).

Free OH-Proline. Free OH-proline in the muscle was determined as described by Sylvestre et al. (2001). Briefly, 12 g of muscle was homogenized in a homogenizer (MIC-CRA D8-Si, ART-moderne Labortechnik) in 69 mL of 3.26% perchloric acid (Merck, Mumbai, India) and centrifuged for 20 min at 4,000 x g (Refrigerated Centrifuge CPR24, Remi India). The supernatant was neutralized with 2M K₂CO₃, filtered, centrifuged for 20 min at 12,000 x g at 4°C (Refrigerated Centrifuge CPR24, Remi India), and then free OH-proline in the extract was determined as described by Newman and Lohan (1950).

N and NPN. Nitrogen content of the muscle was determined in 5 g of sample by the Kjeldahl method. Nonprotein N in the muscle was determined as described by Sylvestre et al. (2001). Briefly, 12 g of muscle was homogenized in a homogenizer (MICCRA D8-Si, ART-moderne Labortechnik) in 69 mL of 3.26% perchloric acid (Merck) and centrifuged for 20 min at 4,000 x g (Refrigerated Centrifuge CPR24, Remi India). The supernatant was neutralized with 2M K₂CO₃, filtered, centrifuged for 20 min at 12,000 x g at 4°C (Refrigerated Centrifuge CPR24, Remi India), and then N content was determined by the Kjeldahl method and was taken to be the NPN of the muscles.

Myofibril Hydrophobicity. Myofibril hydrophobicity was determined as described by Chelh et al. (2006). Briefly 10 g of muscle was homogenized in a homogenizer (MIC-CRA D8-Si, ART-moderne Labortechnik) in 100 mL of a solution at pH 6.5 containing 150 mM NaCl, 2.5 mM KCl, 3 mM MgCl₂, and 4 mM EDTA to which 1 mM phenyl methylsulfonyl fluoride, Manhattan, KS) has been added. Homogenate was filtered through a stainless steel sieve 1-mm hole (Jayant Scientific Industries). The filtrate was stirred for 30 min in ice and centrifuged at 2,000 x g for 15 min at 4°C. The pellet was washed twice with 100 mL of 50 mM KCl pH 6.4 and once with 20 mM phosphate buffer, and the protein concentration was adjusted to 5 mg/mL by the biuret method (Gornall et al., 1949).

Hydrophobicity of the myofibrils was determined using bromophenol blue (BPB). To 1.0 mL of myofibril suspension, 0.2 mL of 1 mg/mL of BPB in distilled water was added and mixed well. A control without myofibril consisted of the addition of 0.2 mL of 1 mg/mL of BPB to 1.0 mL of 20 mM phosphate buffer. Samples and control were kept under agitation at room temperature during 10 min and then centrifuged for 15 min at 2,000 x g. The absorbance of supernatant (diluted 1/10) was measured at 595 nm against a blank of phosphate buffer. The amount of BPB bound is determined by the following formula:

where A = absorbance at 595 nm.

Patty Making and Cooking. Breast muscles removed on each interval period were minced for 30 s in a home mixer (Preeti Chef Pro Plus, Chennai, India) to make a uniform ground meat, and common salt (NaCl) was added at 1.5% wt/wt and mixed thoroughly by hand. The ground meat was then formed into patties (100 g each) using sterile 15 x 90 mm petri dishes. After molding, patties were cooked in a convection-type microwave oven (LG Electronics Pvt. Ltd., Greater Noida, India) with 900 W of power operated at 2450 MHz. Each patty was placed in the center of the oven on a microwave-safe plastic container without a lid until the center of the patty reached an internal temperature of 80°C (4 min). Only 1 patty was cooked at a time. The container was rotated inside the microwave chamber during the cooking period. In addition, the patty was turned upside down to avoid uneven cooking. Preliminary time-temperature trials were conducted to determine the duration of cooking time needed to reach the designated internal temperature. The internal temperature of the cooked patty was checked by inserting an iron-constant thermocouple probe into the geometric center of the patty. The patties were cooled down to room temperature (25°C) and analyzed for Warner-Bratzler shear force value and penetrometer readings (Naveena et al., 2006). All the analysis was done in duplicate with a minimum of 10 readings for each.

Shear Force Value. Cooked patties were equilibrated to room temperature for 1 h before sampling. From each patty, cores 1 cm in diameter were taken and sheared with a Warner-Bratzler shear press (81031307, GR Electric Manufacturing Co., Manhattan, KS). The force required to shear the sample was recorded as kilograms per squared centimeter (Naveena et al., 2006).

Penetrometer Readings. A penetrometer (AIMIL, Associated Instruments Manufacture Pvt. Ltd., Bombay, India) equipped with a total of 100 g of load weight was used to evaluate patties for hardness. Depth of the puncture was determined to 1/10 mm for each patty. Texture of a patty is determined by the depth of penetration (i.e., the greater the depth of penetration, the softer the patty).

Statistical Analysis

Data on pH, TBARS, SH content, collagen content, NPN, NPN-total N ratio, free OH-proline, myofibril hydrophobicity, shear force value, and penetrometer readings variables were analyzed with storage day as the main effect using the mathematical model given below in a 1-way ANOVA procedure of SPSS 10.0 (SPSS Inc., Chicago, IL). If a value of P < 0.05 was detected, differences among means were tested separately with a Bonferroni test:

where µ = general mean, D_i = effect of ith storage day, and E_ijk = random error.

Data on TBARS and SC content and TBARS and T-SH content were separately analyzed for correlation coefficient and regression analysis using SPSS 10.0.

	RESULTS

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Results of various biochemical and physicochemical parameters analyzed are presented in Table 1. The pH values gradually decreased (P < 0.001) from 5.73 on 0 d to 5.30 on 28 d postmortem. The TBARS values increased (P < 0.001) from 0.55 on 0 d to 1.92 (mg of malonaldehyde/ kg of muscle) on 28 d postmortem. The rapid increase of 1.4 times the initial value was observed from 0 to 7 d, and thereafter there was reduction in the increment of TBARS values. Although T-SH, PB-SH, and NP-SH decreased (P < 0.001) during postmortem aging, their values varied from 1.55 to 5.37, 1.44 to 4.45, and 0.11 to 0.92 (10^–3 mM/g of muscle; Figure 1). Further, it was also observed that there was an inverse (P < 0.001) relationship between TBARS values and T-SH content (Figure 2).

View larger version (16K): [in this window] [in a new window]   Figure 1. Effect on sulfhydryl (SH) concentration in spent hen breast meat under aerobically packed refrigerated storage (4 ± 1°C).

View larger version (13K): [in this window] [in a new window]   Figure 2. Relationship between TBA reactive substances (TBARS) and total sulfhydryl content in spent hen breast meat under aerobically packed refrigerated storage (4 ± 1°C).

View larger version (14K): [in this window] [in a new window]   Figure 3. Relationship between TBA reactive substances (TBARS) and soluble collagen in spent hen breast meat under aerobically packed refrigerated storage (4 ± 1°C).

	DISCUSSION

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The toughness of spent hen meat is an important area to study and to explore the potential of innate mechanisms to improve the tenderness, which is of great significance to the fast-growing poultry industry. The storage of meat at refrigeration temperature (4°C) causes many reactions, including the enzymatic reactions, to produce peptides and free amino acids (Sylvestre et al., 2001). The foremost is the pH, which decreased due to the accumulation of metabolites inside the cell. Postmortem accumulation of metabolites resulted in the gradual decline of pH. Nagaraj et al. (2006) have also reported similar results in goat meat during postmortem aging. Normal postrigor meat has a pH of 5.7 to 5.8 (Savell et al., 2005).

Lipid oxidation is a spontaneous and unavoidable process taking place during postmortem aging. In the present experiment, the highest amount of TBARS (1.92 mg/kg) observed was on 28 d of storage, which was lower than the value of 2.93 (mg/kg) observed in cooked turkey thigh meat by Du and Ahn (2002). Lipid oxidation products are involved in the production of off-odor in the stored meat. Apart from producing off-odor, malonaldehyde is also implicated in the structural modification of myofibrillar proteins of sea salmon and in the reduction of available Lys content (Tironi et al., 2004). In a similar way, malonaldehyde as a reactive group may react with Lys residues through the -amino group in the collagen molecules and may possibly disrupt its structure, resulting in the release of collagen from the action of other unknown factors. So, to view any relationship between the values of TBARS and SC, data on these values were analyzed and a positive significant (P < 0.01) relationship was found, suggesting a possible involvement in the disruption of collagen molecules.

The decrease in protein SH content is a good index of protein oxidation (Soszynsky and Bartosz, 1997). It was observed in our study that SH content significantly decreased (40 to 50%) on 21 and 28 d of storage. Similar results have also been reported in turkey meat during refrigerated storage (Mercier et al., 2001). The SH groups of proteins have been assimilated to antioxidants, because they can trap free radicals generated during storage. Tanenaka et al. (1991) indicated that the protein surface thiols may compete with tocopherol for trapping radicals and spare tocopherol to some extent. Similarly, in our experiment, the SH may have spared and offered a cushioning effect by trapping free radicals and preventing further greater oxidation of lipids. This was reflected in the negative significant (P < 0.001) correlation with SH content with TBARS values.

The perimysial fraction, which constitutes 85% of the total collagen, significantly (P < 0.001) decreased over the 28-d storage period. But, the decrease was inconsistent. Nishimura et al. (1995) reported that perimysium remained unchanged up to 14 d postmortem and decreased gradually thereafter. It was suggested that due to the slow process of structural weakening of i.m. connective tissue, the gradual decrease in the perimysial fraction occurred in beef during postmortem aging. However, the SC content was significantly higher (P < 0.001) during postmortem aging than the 0 d. This might possibly be due to the slow process of structural weakening of connective tissue. Collagen solubility also supported this slow weakening of connective tissue proteins in breast muscles of spent hens during postmortem aging. In addition, there was structural disintegration of myofibrillar proteins taking place slowly throughout the postmortem aging as reflected by the myofibrillar hydrophobicity. Jaczynski and Park (2004) suggested that denaturation and degradation of protein exposes hydrophobicity patches that are normally buried in the interior of protein structure. Consequently, the increased hydrophobicity increases the negative entropy of the protein system, which is thermodynamically unstable. Moreover the free OH-proline values and the proteolysis rate during postmortem aging suggest that the structural disintegration of connective tissue proteins and myofibrillar proteins resulted in higher (P < 0.05) levels of proteolysis. Similar results have also been reported in skeletal muscle of lambs (Sylvestre et al., 2001, 2002).

The spent hen meat can be best utilized by processing it into ground meat products. The texture of cooked breast meat patties was evaluated by shear force value and penetrometer readings. Measurement of the shear force value of cooked meat patties of breast muscle of spent hens showed a gradual decrease (P < 0.001) in shear force value, whereas there was a gradual increase (P < 0.001) in penetrometer readings, indicating increased tenderness as a result of postmortem aging. The tenderization process observed was due to chemical and biochemical changes in structural proteins of meat during postmortem aging. The tenderization was 23% on 14 d, which almost doubled to 39% on 28 d of initial value due to postmortem aging of breast muscle of spent hens. Nishimura et al. (1995) have reported 41 and 29% of postmortem tenderization of uncooked beef sample kept at 4°C for 10 and 14 d. Our results suggest that tenderization of meat patties made from spent hen breast muscle proceeds slowly under refrigerated storage.

In conclusion, postmortem aging of spent hen breast meat revealed various changes in the structural proteins degradation and tenderization of minced patties. The slow process of structural disintegration of connective tissue proteins and myofibrillar proteins resulted in increased SC content and proteolysis. These changes in the structural proteins improved the tenderness of minced/ salted patties made from spent hen breast meat stored at 4 ± 1°C for 28 d. The increase in tenderness was 23% on 14 d and 39% on 28 d of postmortem aging. This improvement in tenderness through postmortem aging and preparation of ground meat patties may be considered as an option to enhance the market value of spent hen breast muscle.

	ACKNOWLEDGMENTS

 
We are grateful to the director of the National Research Center on Meat (Hyderabad, Andhra Pradesh, India) for providing necessary facilities to carry out this experiment.

Received for publication February 9, 2007. Accepted for publication September 13, 2007.

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