Quality Characteristics of Broiler Chicken Meat on Salt at Different Temperatures

Abstract

The aim of this investigation was to compare the quality characteristics and muscle structure of broiler chicken meat stored at different temperatures in the retail market in Oman. The meat quality characteristics of broiler breast meat were analysed. Ten samples were randomly selected from each group of fresh, frozen, and chilled chicken meat. Colour L∗, a∗, b∗, pH, expressed juice, cooking loss, sarcomere length, W-B-shear force and muscle structure (using scanning electron microscopy) were determined. Fresh meat samples had significantly (P < 0.05) lower pH values and lightness L∗ than those of chilled and frozen samples. The chilled meat samples were significantly (P < 0.05) lighter and had lower shear force values than fresh and frozen samples. Frozen samples had significantly (P < 0.05) higher expressed juice and cooking loss values than either fresh or chilled samples. The pH values of fresh, chilled, and frozen breast samples were related to colour and expressed juice. Electron microscopy demonstrated that the changes in the physical properties of the chilled meat were related to breakdown of the muscle fiber bundles. The quality characteristics of broiler meat from different storage temperatures varied significantly.

Keywords:

• Broiler
• Meat quality
• Storage temperature
• Electron microscope
• Colour
• Shear force
• Expressed juice
• Ultimate pH
• Muscle bundle

INTRODUCTION

The poultry industry constitutes an integral part of the Sultanate of Oman's growing economy, and it is an important supplier of protein to its population. During the past decade, spectacular growth has been observed in the poultry industry in the Sultanate. Poultry meat and its products have a vast consumer market and are making a significant contribution to the requirement for quality protein. The increased consumption of broiler chicken meat is related to a variety of factors including poultry meat being promoted as a healthier meat option. Recommendations to reduce the risk of cardiovascular disease have been based on diet and lifestyle changes and include suggestions to reduce red meat consumption by increasing poultry and fish intake.[1] Studies have indicated that the consumption of poultry meat rather than red meat poses a lower risk for coronary heart disease[2,3] and for various cancers.[4] Also, outbreaks of bovine spongiform encephalopathy (BSE) in cattle have reduced consumer confidence in beef products resulting in a lower consumer demand for beef.[5]

Meat quality and safety have become important issues due to the increased awareness of consumers. A range of measures can be used to improve the safety of food but many of these are compromised if they are stored at unsafe temperatures, especially in tropical regions.[6] Appearance is the major criterion for purchase selection and initial evaluation of broiler meat quality.[7] Other quality attributes, such as tenderness, juiciness, cooking loss, ultimate pH and shelf-life are important to the consumer after purchasing the product, as well as to the processor when producing value-added meat products. Consumers usually prefer fresh poultry meat rather than frozen. Physical defects associated with the appearance of the product or quality problems may originate from a variety of factors at any stage along the production line, from the livestock on the farm through to the end product.[8] Such factors mainly affect consumer acceptance and product prices rather than health issues. As the functional muscle of the live animal changes to edible meat upon slaughtering, a number of complex metabolic, chemical and physical changes take place in the tissue.[9] Meat quality is influenced by a number of factors including stress conditions at death, rate of glycolysis in the muscle after slaughtering, simultaneous fall of ATP and pH, time, and temperature at the onset of “rigor,” and the level of muscle shortening.[10]

The quality characteristics of broiler chicken meat depend not only on the inherent properties of the birds, but are also influenced by the handling, processing, and storage conditions of the meat. Temperature has been shown to be a key factor in determining the shelf life of poultry.[11] Frequently, low-temperature methods such as freezing and chilling are used to extend the shelf life of poultry products. In addition to inhibiting microbial growth, low-temperature storage can also influence the physicochemical and biochemical properties of the meat, which will affect the meat quality characteristics.[12] Freezing and frozen storage induces various changes to the proteins, and these are reflected in a decrease of functional property. The functional and physicochemical properties of proteins are influenced by changes in macro structure of mince. The changes in the structure of mince on its proteins can be monitored by scanning electron microscopy.[13] Consequently, the aim of this investigation was to examine the effects of the storage temperature on meat structure and quality characteristics of broiler chicken meat.

MATERIALS AND METHODS

Meat Samples

Physical parameters related to meat quality were determined using 30 poultry samples collected from food retailers. For each broiler chicken sample the shop storage temperature and the physical status of the sample in terms of the presence of any physical defects were recorded. The 30 samples consisted of ten fresh broiler chicken carcasses randomly obtained from on-site slaughtering shops; in addition, 10 frozen carcasses and 10 chilled samples that were also randomly collected from food retail outlets on the fourth day after chilling. In the region where the study took place, it is common practice to purchase poultry from small shops that deal with live birds only. They are kept in the shop until selected by a customer and then slaughtered, defeathered, washed, and placed into plastic bags on demand. Therefore, these samples have had no low-temperature exposure prior to receipt by the customer. The customer can select whether the skin is removed or not. Immediately after purchase, samples were transferred to the laboratory in a cool box and analyzed immediately. Frozen samples were maintained frozen at –18^oC until analyzed. Prior to analysis, frozen samples were allowed to thaw for about 2 hours, so that ice crystals did not interfere with parameters determined.

Meat Quality Evaluation

The breast muscle (M. pectoralis) was dissected from each broiler chicken sample and evaluated for meat quality characteristics. M. pectoralis was selected for the current study due to its greater size, as the major muscle in broiler chicken carcasses it enabled sufficient muscle material for quality measurements. Meat quality measurements including ultimate pH, expressed juice, cooking loss, Warner-Bratzler shear force, sarcomere length, and colour L∗, a∗ b∗ were determined.

Ultimate pH

The ultimate pH (pHu) was assessed in homogenates (prepared using an Ultra Turrax T25 homogeniser at ¼ speed with 3 × 5 second bursts) of duplicate samples containing 1.5–2.0 g muscle tissue in 10 ml of neutralized 5-mM sodium iodoacetate (pH 7.0, 150 mM KCl). The pH of the slurry was measured using a Metrohm pH meter (Model No. 744) with a glass electrode.[14]

Sarcomere Length

A small bundle of fibres was dissected from the centre of each breast muscle sample and laid out on a glass slide prior to covering with a drop of buffered (0.05M Tris, pH 7.6) 0.25 M sucrose solution.[15] Care was taken to ensure that single bundles were teased out and covered with a coverslip. Sarcomere length was determined using a laser diffraction method (Spectra-physics helium-neon laser, 2 mW 0.49 mm diameter beam, with a wavelength of 632.8 nm), described by Cross et al.[16] A ruler was used to make 12 measurements for each sample and the average was calculated. Sarcomere length was calculated based on the following formula [17]:

(1)

where n is the diffraction band; λ is the wavelength of 632.8 nm; S is sarcomere length; and θ is the diffraction angle.

Warner-Bratzler Shear Values

Triplicate 25 mm-thick slices were cut from each muscle at the time of breast muscle preparation. The slices were weighed and stored at chiller temperature (2–4°C) in plastic bags until cooked by immersing the bags in a water bath at 70°C for 90 min. The cooked meat was kept at 2-3°C overnight in the cooler.[14] After cooling, 12 cores (13 mm × 13 mm in cross-section) were cut from the centre of each muscle sample. Cores were prepared to ensure that shears were made parallel to the fibres rather than across them. Each core was then sheared perpendicularly to the fibres in 2 places, using a digital Dillon Warner-Bratzler (WB) shear device. This machine measured the maximum force required to cut across the muscle fibres. The Warner-Bratzler shear values are expressed as kilograms of shear force (kg) required.

Cooking Loss

Cooking loss was determined in meat samples (about 150 g in weight and 25 mm thick) placed inside polyethylene bags in a water bath at 70°C. Samples were cooked for 90 min and then cooled overnight in cooler. They were carefully dried with tissues to remove excess surface moisture and re-weighed to determine cooking losses. The cooking loss (%) was calculated as follows:

(2)

Expressed Juice

Determinations of the water-holding capacity (in terms of expressed juice values) were based on measuring the water liberated when pressure was applied to the muscle tissue. Expressed juice was assessed, using a filter paper method, as the total wetted area less the meat area (cm²) relative to the weight of the sample (g).[18] A cube of 500 ± 20 mg of meat from the inside of each muscle was placed on a tarred filter paper (Whatman Nº 1, 11.0 cm diameter, Qualitative), stored previously in a desiccator over saturated KCl between screw tightened Perspex plates for exactly 5 min. The wet and meat areas were measured with a planmeter. Expressed juice values were calculated based on the following formula:

(3)

Duplicate measures of expressed juice were made for each sample.

Colour CIE L∗, a∗, b∗

Approximately 60 min after exposing the fresh surface of M. pectoralis CIE L∗, a∗, b∗ light reflectance coordinates of the muscle surface were measured at room temperature (25 ± 2°C) using a Minolta Chroma Meter CR-300 (Minolta Co., Ltd., Japan), with a colour measuring area 1.1 cm in diameter.[14] The L∗ value relates to Lightness; the a∗ value to Red-Green hue where a positive value relates to the red intensity; and the b∗ value to the Yellow-Blue where a positive value relates to yellow. The average of two measurements from each sample was recorded as the colour coordinate value of the sample.

Scanning Electron Microscopy (SEM)

Meat sections of M. pectoralis cut into small pieces were frozen at −80°C, and then freeze dried using a JEOL-JFD-310 freeze drier for three days. Muscle specimens were then fragmented under a stereomicroscope to yield longitudinal and cross section orientations. Fragments of muscle specimens were orientated and adhered onto 10 mm silver stubs and coated with carbon discs under a stereomicroscope to expose the different sides of the specimens for examination. Stubs of specimens were then sputter coated with gold particles using Bio-Rad SEM Coating System for 135 seconds. Samples were examined using a Jeol JSM-5600LV scanning electron microscope operated at 10 kV. Secondary electron images of muscle fiber ultrastructures were recorded.

Statistical Analysis

Statistical analysis was carried out using analysis of variance procedures[19] to evaluate the effect of storage temperature on the quality of broiler chicken breast meat fillets. Significant differences between treatment means were assessed using Tukey least-significant-difference test applied by SPSS (Ver. 10).

RESULTS AND DISCUSSION

Meat Quality

The physical parameters that are used as indicators of the meat quality were studied for broiler chicken meat samples from different storage temperatures. These included ultimate pH, expressed juice, cooking loss, shear force value, sarcomere length and colour L∗, a∗, b∗ (Table 1). The ultimate pH of muscle is a major determinant of meat quality[20] and is related to the depletion of glycogen and liberation of lactic acid pre- and post-slaughter. The ultimate pH of the M pectoralis in fresh carcasses was significantly (P < 0.05) lower (5.91) than that of either chilled (6.13) or frozen samples (6.17). The lower ultimate pH in fresh samples indicated that slaughtering birds in the local market at high ambient temperatures (30–35^oC) accelerated the process of rigor mortis, possibly by the metabolic channeling mechanism.[21] Glycolytic enzymes are bound to the myofibrillar protein actin in vivo and the proportion of each glycolytic enzyme that is bound into the correct spatial arrangement may increase on stimulation of glycolysis (e.g., with high temperature) and decrease when such stimulation ceases or is absent.[21]

Table 1 Standard devision for meat quality characteristics of broiler chicken breast muscles stored at different temperarures

Parameter	Treatment (10 samples each)
	Fresh	Chilled	Frozen
Ultimate pH	5.91 ± 0.12 ^b	6.13 ± 0.17 ^a	6.17 ± 0.19 ^a
Colour L∗ (lightness)	42.43 ± 3.49 ^a	52.12 ± 2.51 ^c	46.93 ± 4.63 ^b
Colour a∗ (redness)	10.12 ± 1.2 ^a	11.55 ± 1.24 ^b	12.29 ± 1.08 ^b
Colour b∗ (yellowness)	8.11 ± 1.10 ^a	9.39 ± 2.11 ^a	12.50 ± 1.70 ^b
Sarcomere length (μm)	1.89 ± 0.11 ^a	1.84 ± 0.20 ^a	1.52 ± 0.10 ^b
Expressed juice^1	19.68 ± 6.87 ^a	20.02 ± 5.24 ^a	27.52 ± 5.72 ^b
Cooking loss %	17.56 ± 1.30 ^a	17.41 ± 2.66 ^a	24.52 ± 3.99 ^b
W-B share force (kg)	7.22 ± 1.94 ^a	2.83 ± 0.94 ^b	7.70 ± 2.79 ^a
^a,b,cValues in the same row followed by different letters are significantly different (P < 0.05). ^1Expressed juice = moist area (cm^2)/sample weight (g).

Colour is an important quality attribute that influences consumer acceptance of poultry meat. Several researchers have demonstrated a significant relationship between raw breast meat colour and ultimate pH.[22,23,24,25,26] The relationship between poultry meat colour and pH has been demonstrated in the present study (Fig. 1). Both chilled and frozen breast fillets had higher ultimate pH values than fresh breast fillets (Table 1). The correlation between L∗ and ultimate pH was negative (Fig. 1), which is in agreement with the findings of studies by Barbut,[27] Yang and Chen,[28] and Allen et al.[24]

Figure 1 Relationships between ultimate pH and color (A) (l∗), (B) a∗ b∗, and(C) expressed juice of the broiler breast muscle treated as fresh, chilled or frozen.

The fresh breast fillets averaged L∗ = 42.43, a∗ = 10.12 and b∗ = 8.11, chilled breast fillets L∗ = 52.12, a∗ = 11.55 and b∗ = 9.39 and frozen breast fillets L∗ = 46.93, a∗ = 12.29 and b∗ = 12.50. Fresh, chilled, and frozen samples demonstrated significantly different (P < 0.05) colour values (Table 1). Muscle colour is affected by several factors the most important of which are; age, sex, intramuscular fat, moisture content, pre-slaughtering conditions, processing, presence of muscle pigments, [29] and storage time.[30] Redness (a∗) values, of chilled and frozen samples were very similar to one another but were significantly higher (P < 0.05), than for fresh samples. The frozen samples had the highest yellowness intensity with mean b∗ value significantly higher than that of fresh and chilled samples. The latter values did not significantly differ from each other. These results are in agreement with previous reports for poultry meat.[26,31]

The shear values for the chilled breast fillets were significantly (P < 0.05) lower (2.83 kg) than those for the fresh (7.22 kg) and frozen (7.70 kg) breast fillets, which were not statistically different from each other. Similarly, Lyon et al.[32] reported that shear force values of broiler breast meat decreased as chill time increased. Increasing post-chill deboning time to 1 hr resulted in more tender cooked meat. However, the shear values were still too high to be acceptable based on research that established the relationship between shear values and sensory tenderness.[33] According to the sensory studies of Lyon and Lyon[33] chilled meat falls into the moderately tender range.

Sarcomere is the contractile unit of the myofibril.[34] In the present study the longest sarcomeres (mean = 1.89μm) were observed in fresh meat followed by chilled samples with an average value of 1.84 μm. Frozen samples had significantly (P < 0.05) shorter sarcomeres, with a mean length of 1.52 μm (Table 1). This large difference in sarcomere length indicates that there was probably still sufficient muscle ATP to induce contraction of the sarcomeres and subsequent lower shear force values.

Expressed juice and cooking loss collectively provide an indication of meat tenderness[35] and are measured to obtain an overall assessment of the water binding properties of meat. Expressed juice is a measure of the ability of meat to hold all or part of its own or added water,[18] which is a critical factor in differing meat texture, tenderness, and juiciness.[36] A variety of methods are available for determination of expressed juice some require the application of external forces such as drip loss and thawing loss, others involve external mechanical forces such as expressible juice, whilst others need the application of heat such as cooking loss.[36] The mean volume of expressed juice for the frozen samples (27.5 cm²/mg) was significantly (P < 0.05) higher than for the fresh (19.68 cm²/mg) and chilled samples (17.4 cm²/mg). These findings are similar to those reported by Barbut,[27] who found that breast muscle samples with low lightness values had lower expressed juice values. In the present study, there was a negative correlation between sarcomere length and expressed juice (Fig. 1C). In the same samples, cooking loss values of frozen carcasses were significantly (P < 0.05) higher (24.5%) than chilled (17.4%) and fresh (17.6%) carcasses.

Scanning Electron Microscopy (SEM)

Scanning electron micrographs of the fresh samples show muscle fibres are intact without any disturbance on the surface (Fig. 2B), whereas chilled samples were distorted, ruptured and fractured (Fig. 2C). Comparing SEM micrographs of fibers of fresh and frozen breast muscle samples, slight differences were observed (Figures 2A,B). The predominant mechanism by which chilling temperature is able to irreversibly alter skeletal tissue is presumed to involve proteolytic enzymes such as calpain.[37] The rupture of the biological tissues at cellular level (nuclear and plasma membranes) had been attributed to shear stress associated with microstreaming around the microbubbles.[38] The findings of the present study indicated that chilling temperature may negatively affect meat texture and firmness, and could demonstrate structural alterations.

Figure 2 Scanning electron micrographs of longitudinal sections of breast skeletal muscle from (A) frozen, (B) fresh and (C) five day chilling at 4^oC, extensive damage is evident in the muscle fibre bundles, with rupture and exposure of individual muscle fibres. m: fibre bundle; mf: myofibrils.

CONCLUSIONS

Overall, the results showed that chilled broiler samples were the most tender. Freezing meat, on the other hand, generally resulted in toughness with a high percentage cooking loss. Fresh meat could be placed in between these two groups. Overall, chill storage is a useful practice for good quality broiler chicken meat.

ACKNOWLEDGMENT

The technical assistance provided by Mr. S.A Al-Lawati is acknowledged with gratitude.