Microstructural and mechanical properties of camel longissimus dorsi muscle during roasting, braising and microwave heating

Meat Science 95 (2013) 419–424

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Microstructural and mechanical properties of camel longissimus dorsi muscle during roasting, braising and microwave heating M.S. Yarmand a, P. Nikmaram a,⁎, Z. Emam Djomeh a, A. Homayouni b,⁎ a b

Department of Food Science, Engineering and Technology, Faculty of Agricultural Engineering and Technology, Agriculture and Natural Resources, University of Tehran, Tehran, Iran Department of Food Science and Technology, Faculty of Health and Nutrition, Tabriz University of Medical Sciences, Tabriz, Iran

a r t i c l e

i n f o

Article history: Received 9 June 2012 Received in revised form 11 May 2013 Accepted 13 May 2013 Keywords: Camel Roasting Braising Microwave heating Mechanical properties Microstructure

a b s t r a c t This study was conducted to investigate the effects of various heating methods, including roasting, braising and microwave heating, on mechanical properties and microstructure of longissimus dorsi (LD) muscle of the camel. Shear value and compression force increased during microwave heating more than roasting and braising. Results obtained from scanning electron microscopy (SEM) showed more damage from roasting than in either braising or microwave heating. Granulation and fragmentation were clear in muscle fibers after roasting. The perimysium membrane of connective tissue was damaged during braising, while roasting left the perimysium membrane largely intact. The mechanical properties and microstructure of muscle can be affected by changes in water content during cooking. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction A considerable amount of literature has been published on microstructure (Yarmand & Sarafis, 1997) and mechanical properties of beef but there is much less known about camel meat. The dromedary camel is one of the most important domestic animals in arid and semi-arid regions, as it produces high-quality food at comparatively low cost under extremely harsh conditions (Knoess, 1977; Yagil, 1982; Yousif & Babiker, 1989). Some of the most important sensory attributes of meat are appearance, juiciness, flavor and texture (Barton-Gade, Cross, Jones, & Winger, 1988). Texture values in meat mainly depend on the breed and age of animal (Huff & Parrish, 1993; Ouali, 1990), on anatomical characteristics such as type of muscle, or on cooking method (de Huidobro, Miguel, Blázquez, & Onega, 2005; Yarmand & Homayouni, 2010). The determination of meat texture can be made using a trained taste panel or by physical methods. Instrumental texture assessment of meat may be done by means of a texturometer, which determines tissue resistance to both shearing and compression (Bratzler, 1932; Warner, 1929). A large number of devices have been developed to evaluate mechanical tenderness. The most widely used is the Warner–Bratzler (WB) shear device (Harris & Shorthose, 1988; Lepetit & Culioli, 1994). As meat is usually cooked before being eaten, it is important to understand the physical changes in meat texture during heating. ⁎ Corresponding authors. Tel./fax: +98 261 2248804. E-mail addresses: [email protected] (P. Nikmaram), [email protected] (A. Homayouni). 0309-1740/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.meatsci.2013.05.018

Davey and Gilbert (1974) described cooking as the heating of meat to a sufficiently high temperature to denature proteins. The components of muscle that control toughness are the myofibrillar proteins (myosin and actin) and the connective-tissue proteins (mainly collagen, along with elastin), and intrafiber water (Offer, Restall, & Trinick, 1984). During heating, the different meat proteins denature, which causes structural changes in the meat, such as the damage of cell membranes, shrinkage of meat fibers, aggregation, gel formation of myofibrillar, sarcoplasmic proteins and shrinkage and solubilization of the connective tissue (Kong, Tang, Rasco, & Crapo, 2007; Tornberg, 2005). Heatinduced changes in connective tissue causes a tenderizing effect, while hardening of the myofibrillar proteins during cooking toughens the meat (Harris & Shorthose, 1988; Laakkonen, 1973). Meat toughness largely depends on contribution of connective tissue which is determined by collagen features, content and solubility. Shrinkage of connective tissue exerts pressure on the aqueous solution in the extra cellular void and expelled water, and the resulting cook loss is associated with loss of tenderness, as well as increased rigidity of tissue (Palka & Daun, 1999). Meat structure can be considered in its simplest form as a collection of parallel fibers, a myofibrillar structure, bound together by a connectivetissue network (Palka & Daun, 1999). Scanning electron microscopy (SEM) is very useful in revealing aspects of the structure of muscle fibers before and after treatments (Voyle, 1981). The effects of cooking on different types of meat have been studied by several authors. Roasting to an internal temperature of 50 °C slightly affects the structure of bovine meat. During roasting to an internal temperature of 60–90 °C, significant changes occur in both the myofibrils

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and the intramuscular connective tissue. Using SEM, (Wattanachant, Benjakul, & Ledward, 2005) studied the effect on broiler muscle fibers after cooking at 80 °C for 10 min. The broiler muscle fibers shrank more in a parallel than in a transverse direction to the fiber axis and expanded transversally after cooking, resulting in increasing muscle tenderness. The aim of this study was to assess the effects of different cooking methods on microstructure and mechanical properties of camel meat in an attempt to find the best method of cooking. Also the Differential Scanning Calorimetry (DSC) method was used for analyzing camel meat in this study. 2. Material and methods 2.1. Preparation of samples Samples were prepared from nine healthy male camels between one and three years old that were been held in a lairage for 1–2 h handled without any extra stress before slaughtering. All animals were exposed to routine pre-slaughter handling, including transportation. They were slaughtered and dressed following routine commercial slaughterhouse (Basimgosht, Tehran, Iran) procedures according to Halal methods (Kadim et al., 2006) under the supervision of an official veterinarian. The range of ambient temperatures was between 25 and 27 °C. Longissimus dorsi (LD) muscle was separated by razor blade from left side of each carcass between the 10th-12th ribs (they ranged between 800 and 1000 g). Samples were kept in zipped plastic bags and transported in an insulated box. They were then transferred to a chiller (1–3 °C) within about 2–2.5 h post-mortem for 24 h. Muscle samples were cut cylindrically parallel to muscle fiber direction (5 cm in diameter and 10 cm long). Any visible fat was removed from the muscle tissues and then were individually labeled and weighed. The samples were sealed in nylon/polyethylene bags. After heating, the samples were cooled to room temperature and stored at refrigerator temperature (4 °C) until analyzed.

speed was regulated at 200–250 mm/min and the angle of the WB blade was 60°. A cylindrical flat-ended plunger (with 1.13 cm diameter, 1 cm2 area) was used in compression testing. The plunger was driven (l00 mm/min) vertically 80% of the way through a 1 cm-thick meat sample cut so that the fiber axis was perpendicular to the direction of plunger penetration (Honikel, 1998).

2.5. Differential Scanning Calorimetry (DSC) Samples (9 mg) were transferred to an aluminum hermetic pan. To adjust for small natural variations in pH, ensure an excess of moisture and promote good thermal contact, 5 μl of 0.07 M Sorenson's phosphate buffer (pH 5.4) was included in the pan and sealed (Findlay, Parkin, & Stanley, 1986). A Perkin-Elmer (Model Pyris 6 DSC, Massachusetts, USA) was used to scan raw and cooked samples from 0 °C to 100 °C at endothermic heat flow of 10 °C/min under ambient pressure with a nitrogen flush of 20 ml/min. Differential heat flow was recorded at 0.5 s intervals for subsequent computerized analysis. Plots of heat flow versus temperature were obtained for each sample. Data analysis programs were applied to the thermal curves to provide heat flow and temperature.

2.6. Microstructure analysis 2 × 3 mm pieces of meat were excised from raw and cooked steaks and dehydrated in 25, 50, 70, 95% and absolute ethanol (three times), 10–15 min in each solution. The dried samples were placed on holders with aluminum cement and coated with gold under vacuum (0.5 mbar) using as sputter-coating/glow discharge (EMITECH K350 Attachment, Quorum Technologies Ltd, East Grinstead, UK,). The specimens were examined and photographed in a Scanning Electron Microscopy (SEM, VEGA II-TESCAN, Brno, Czech Republic).

2.2. Heat treatments 2.7. Statistical analysis Roasting at 100 °C was done in a convection oven for 100 min (Model FT420, W.C. HERAEUS HANAU, China). Braising was done in a water bath (W610B, Fater Ltd, Iran) at 100 °C for 70 min. Microwave treatment was done in a domestic microwave oven at 2450 MHz and 600 W for 3 min (Model Toshiba ER 766-ET, Tokyo, Japan). A mini thermocouple (Model Testo 0900 0525, Shortess-Rawson Inc., New Jersey, USA) was used for controlling the temperature inside the slices. The heat treatment was regulated to an internal temperature of 75 °C in all cases.

All experiments were replicated three times and the generated data were evaluated statistically by SAS software (9.1) (SAS, 2004) in a randomized complete block design. Duncan's multiple range tests were used for comparing the means. The least significant difference (p b 0.01) is reported.

2.3. Measuring cook loss 50

cook loss ¼

weight of raw sample−weight of cooked sample  100 ð1Þ weight of raw sample

45

a

40

cook loss(%)

After cooking, steaks were cooled to room temperature, surface dried with filter paper and reweighed using an analytical balance (Model GE 812, Sartorius AG Germany). Cook losses were calculated from differences in raw and cooked weight (Eq. (1)).

35

b

b

roasting

braising

30 25 20 15

2.4. Texture analysis An Instron (Model Testometric M350-10CT, Rochdale, England) was used for measuring the shear force and compression of raw and cooked meat. Meat samples were cut by a cylindrical blade to make a core with a 1.32 cm radius and 2 cm length. The cores were cut by a Warner–Bratzler (WB) blade perpendicular to direction of the fiber,

10 5 0 microwave

Fig. 1. The comparison of cook loss (%) of camel LD muscle processed by microwave, roasting and braising; (significant difference at p b 0.01).

M.S. Yarmand et al. / Meat Science 95 (2013) 419–424 Table 1 Mechanical properties in camel LD muscle for various heating methods (mean ± standard error). Treatment

Mean Compression(N)

Raw Microwave Roasting Braising

36.00 86.67 61.33 62.17

± ± ± ±

b

2.08 13.38a 6.00ab 4.34ab

Shear force(N) 12.587 54.197 38.697 34.934

± ± ± ±

2.57b 0.53ab 9.35b 3.83ab

For each treatment the letters within each column denote a statistically significant difference.

3. Results and discussion 3.1. Cook loss The percent of cook loss during various processing for camel LD muscle were demonstrated in Fig. 1. A statistical analysis revealed significant differences between cooking treatments (p b 0.01). The overall percentage of cook loss for samples treated by microwave was more than that from conventional methods; these results agreed with previous studies (Cipra, Bowers, & Hooper, 1971; El-Shimi, 1992; Janicki & Appledorf, 1974; Nikmaram, Yarmand, & Emamjomeh, 2011; Ruyack & Paul, 1972). From a nutritional point of view, cook loss led to loss of soluble proteins, vitamins and other nutrients.

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It is likely that the high electromagnetic field, high power and short time associated with microwaving cause protein denaturation, disintegration of the texture matrix, rapid protein destruction caused by heat shock to the proteins and, finally, liberalization of large amounts of water and fat (Yarmand & Homayouni, 2009). 3.2. Mechanical properties Mechanical properties of cooked meat samples depend on the myofibrillar mechanical properties and connective tissue network (Bouton, Harris, & Ratcliff, 1981). Toughness was increased by cooking; this increase has been ascribed in different studies to the denaturation of intramuscular collagen or changes in the myofibrillar structure. Heat solubilizes the connective tissue, leading to meat tenderization, while denaturation of myofibrillar proteins leads to meat toughening (Laakkonen, Wellington, & Sherbon, 1970). Since perimysium was damaged by heat during braising, the muscle fiber bundles collapse, and resistance to shear force was less than that in roasting. But the limited time required for microwave heating probably protects connective tissue (Nikmaram, Yarmand, Emamjomeh, & Darehabi, 2011), and shear force increased to 54.19 N to shear the sample (Table 1). Generally shear force is not sensitive to changes in connective tissue, while compression of specimens seems to be very sensitive to such changes (Bouton & Harris, 1972). Compression force for raw camel LD muscle reached 36 N (Table 1) and was increased after applying various heat treatments.

Fig. 2. DSC thermal curve of LD muscle sample of camel A — raw, B — cooked by microwave method, C — cooked by roasting method and D — cooked by braising method. Endotherm and — is base line.

is DSC

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Fig. 3. Scanning electron microscopy of raw camel muscle; bundles of muscle fibers are parallel.

Results for roasting and braising were 61.33 and 62.17 N, respectively. But there was a significant difference in the result of compression after microwave heating, which increased to 86.67 N. This may arise from more solubilization of collagen in roasting and braising methods, which results in less compression. In contrast, less solubilization of collagen in microwave heating results in more compression force. 3.3. Differential Scanning Calorimetry (DSC) measurements There are three major endotherms when a raw sample of muscle tissue is heated in a differential scanning calorimeter. The observed events may be attributed to the thermal myosin denaturation of muscle proteins (Findlay et al., 1986). The first transition reached its maximum between 54 °C and 58 °C, which has been attributed to myosin denaturation (Martens & Vold, 1976; Wright, Leach, & Wilding, 1977). The second transition, which occurs between 65 °C and 67 °C, has been attributed to changes in collagen structure (Martens & Vold, 1976; Stabursvik & Martens, 1980) and to sarcoplasmic proteins (Wright et al., 1977). The third transition, which has been consistently attributed to changes in actin structure, was found between 80 °C and 83 °C (Wright et al., 1977). However the thermal curve of uncooked samples (Fig. 2A) was similar to that of heated samples (Fig. 2B, C and D). Initial thermal flow at

56 °C leads to the disappearance of the Tl transition as well as heating until 70 °C leads to the disappearance of the Tl and T2 transitions. Heating until 82 °C leads to the disappearance of all three transitions. This may indicate a combination that takes place in Tl which allows the camel meat to tolerate some degree of denaturation and causes a peak to remain to some extent. All the peaks were observed in the endothermic profile of sample of raw LD muscle of camel (Fig. 2A), as in previous studies. The first peak, at 58.17 °C, related to the myosin; the second peak, at 68.48 °C, related to the collagen and sarcoplasmic proteins; and the third peak, at 84.16 °C, related to the actin. In the endothermic profile of the sample cooked by microwave, peaks related to the T2 and T3 transitions were apparent (Fig. 2B), although the peak surfaces as well as the energy required were lower than those for the raw sample. This suggests that the proteins of the connective tissue were not quite denatured, which reduced fragility compared to the other two methods. Fig. 2(C) and (D) show the endothermic profile of the samples cooked by roasting and braising, respectively. No peaks at the original site of the transitions appeared; this suggests that the internal temperature of these samples uniformly reached the desired temperature, and was an expression of the complete denaturation of the meat proteins.

gr g

s

t A

B

Fig. 4. Roasted camel LD muscle; (A): separation (s) of individual muscle fibers and gaps (g) between fibers and endomysial tube are clear. (B): coagulation and denaturation of collagen are clear. Transverse breakage (t) and granulation (gr) are also evident.

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423

g

s

A

B

Fig. 5. Scanning electron micrograph of braised camel LD muscle; (A): evidence of gaps (g) between individual muscle fibers due to denaturation of the perimysium network. (B): shrinkage (s) of individual muscle fibers is clear.

3.4. Microstructural studies The microstructure of camel LD muscle was studied by SEM. Images of raw muscle showed bundles of parallel muscle fibers (Fig. 3). Collagen fibers surrounding the muscle fibers were not evident. There were great differences between raw and cooked camel LD muscle similar to that of previous studies (Palka, 1999; Rowe, 1989). Irregular organization of the muscle structure was observed during roasting (Fig. 4A). Fragmentation was clear in the muscle fibers, also within the heated muscle fibers, myofibril separation (s) was apparent. Gaps (g) appeared between heated muscle fibers and endomysial tube at the surface of the muscle during roasting. Impressions of denatured collagen fiber were clearly evident. Damage in the endomysium was also clear. Some parts of heated muscle fibers were separated from the structure (Fig. 4B). Granulation (gr) and erosion at the edge of muscle fibers could be observed. Perimysium membrane of connective tissue was damaged during braising, and individual myofibrils were separated, as was shown in Fig. 5A. In contrast, the perimysium membrane was largely intact after roasting. Destruction of muscle fibers in braising was more evident than with the other heating methods. This may be due to the fluid bed of water in braising, which along with heating, leads to the destruction of perimysium. Solubilization of perimysium was associated with

A

lower cook loss in braised samples, compared to microwave heated samples. As shown in Fig. 5B, the amount of distortion along the individual muscle fiber revealed that the formation of these fractures was not a passive phenomenon, but required an external force. In this study, damage to the microstructure of camel LD arising from microwave heating was evident (Fig. 6). This resulted in more shrinkage and breakdown in the muscle. The myofibril surface did not seem to be normal, but illustrated little evidence of tissue damage at the surface of the specimen. Results showed that domestic microwave heating caused little hydrolysis in connective tissue. This is in agreement with (Hsieh, Cornforth, Pearson, & Hooper, 1980). Regarding the rotation of water molecules to the electromagnetic field with the frequency of 2450 MHz, generation of heat was much faster than in roasting and braising. This could cause greater damage in the microstructure of the LD muscle. The degree of influence on the disintegration of muscle fibers suggest that heat penetration affects the type and extent of disintegration of muscle fibers, which affects the tenderness of the muscle (Hearne, 1976). The changes in toughness during cooking may be determined by measuring the mechanical rigidity of perimysium in the endomysium-perimysium space, while the shortening of endomysium supposes water loss from the muscle (Palka, 1999). Both phenomena contribute to the texture of heated LD muscle of camel.

B

Fig. 6. Microwave heating of camel LD muscle. Some evidence of granulation and separation of muscle fibers is clear.

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