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Quality and microstructural changes in goat meat during heat treatment
Meat Science 86 (2010) 451–455
Contents lists available at ScienceDirect
Meat Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m e a t s c i
Quality and microstructural changes in goat meat during heat treatment M.S. Yarmand a,⁎, A. Homayouni b a b
Department of Food Science and Technology, Faculty of Biosystems Engineering, Agriculture and Natural Resources Campus, University of Tehran, Tehran, Islamic Republic of Iran Department of Food Science and Technology, Faculty of Health and Nutrition, Tabriz University of Medical Sciences, Tabriz, Islamic Republic of Iran
a r t i c l e
i n f o
Article history: Received 10 September 2009 Received in revised form 27 January 2010 Accepted 16 May 2010 Keywords: ESEM Goat Microstructure Conventional heating Microwave heating
a b s t r a c t Environmental scanning electron microscopy (ESEM) was used to study the structure of the semimembranous muscle of goat meat during heat treatment. The four treatments examined were raw (control), conventional heating, domestic microwave heating and industrial microwave heating. The ESEM showed shrinkage and denaturation in muscle fibers after cooking by the various methods. No surface damage was observed in any image for conventional heating. More structural damage was observed in microwave heating at both domestic (700 W) and industrial (12,000 W) levels. The pattern of distribution in microwave heating caused surface damage to muscle fiber with separation of some portions and denaturation of collagen. Crown Copyright © 2010 Published by Elsevier Ltd. on behalf of The American Meat Science Association. All rights reserved.
1. Introduction Quality control methods and microstructural evaluation were employed to study the qualitative parameters of meat and meat products (Arvanitoyannis & van Houwelingen-Koukaliaroglou, 2003; Papadima, Arvanitoyannis, & Bloukas, 1999; Tzouros & Arvanitoyannis, 2001; Yarmand & Homayouni, 2009). Among these methods, the environmental scanning electron microscopy (ESEM) has its own advantages for evaluation of microstructural changes in meat especially for comparing the effect of various heat treatments (Yarmand & Homayouni, 2009). Environmental scanning electron microscopy (ESEM) has been used for the examination of living and fresh botanical samples (Danilatos, 1981) including fungal mycelium and cross sections of stems from different plant sources. Danilatos and Postle (1982) studied common biological applications. A number of studies have demonstrated the use of ESEM for hydrated biological samples (Klose, Webb, & Teakle, 1992; Wallace, Uwins, & McChonchie, 1992). Some research compared unprocessed ESEM specimens and samples prepared by conventional methods (Danilatos, 1981; O'Brien, Webb, Uwins, Desmarchelier, & Imrie, 1992). Much research has been done on the study of industrial wool fiber, commencing with sheep breeding and concluding with the study of finished fabric. Investigations of many of these processes can be greatly enhanced by the use of ESEM techniques. Early results in this area have been briefly reported by Danilatos and Brooks (1987).
⁎ Corresponding author. Tel.: +98 261 2248804; fax: +98 261 2248804. E-mail address: [email protected] (M.S. Yarmand).
Fig. 1. Diagrammatic representation of a two-stage differential pumping system for an ESEM (Danilatos, 1991).
0309-1740/$ – see front matter. Crown Copyright © 2010 Published by Elsevier Ltd. on behalf of The American Meat Science Association. All rights reserved. doi:10.1016/j.meatsci.2010.05.033
452
M.S. Yarmand, A. Homayouni / Meat Science 86 (2010) 451–455
Image analysis has been used for the qualitative evaluation of lamb meat (Russ, 2005; Yarmand & Homayouni, 2009; Yarmand & Sarafis, 1997). Preliminary conclusions on a variety of novel ESEM applications were recorded by Bolon, Roberstson, & Lifshin (1989). ESEM techniques have also been demonstrated by Baumgarten (1990) and Peters (1990). Lee (1992) studied the effect of different microwave power levels on the chemical, physical and histological characteristics of beef roasts
and found that the use of variable power for microwave heating of beef roasts does not result in different chemical or physical characteristics. Histological results of the ultrastructure of muscle and collagen revealed changes in sarcomere appearance and size that were related to the internal endpoint temperature (Lepetit, 2007). Mechanical and microstructure characteristics of meat dough heated by a continuous process in a radio-frequency field or conventionally in a water bath were investigated by Roon, Van Houben, Koolmees, and Van Vliet (1994). Purchas, Rutherfurd, Pearce, Vather, and Willkinson (2004) studied the influence of final cooked temperature on the forms of iron present and the levels of several other compounds of beef semitendinosus muscle. Changes in the form of iron with cooking generally took place more rapidly in surface samples than inner samples. On a dry matter basis, the concentration of taurine, carnosine, coenzyme Q10 and creatine all decreased with cooking, with the greatest decreases noted for taurine and creatine. Pawar, Khan, and Agarkar (2000, 2002) studied the quality of chevon patties as influenced by different methods of cooking and demonstrated that oven-cooked chevon patties were of high quality and were most stable against oxidative changes (Bejerholm & Aaslyng, 2003; Raj, Sahoo, Hooda, & Karwasra, 2005). In the present study, ESEM was applied to study the microstructure of goat semi-membranous (SM) muscle using raw muscle as a control and comparing it to different heat treatments, including conventional and microwave heating.
Fig. 2. (A) ESEM of structure of raw goat SM muscle, (B) ESEM of structure of raw goat SM muscle, (C) structure of SM muscle using Miller stain showing bright collagen network, (D) cross section view of SM.
Fig. 3. (A) ESEM of domestic microwave-heated (700 W) goat SM muscle. Surface damage indicated by arrows in cross section view; (B) ESEM of domestic microwaveheated goat SM muscle.
M.S. Yarmand, A. Homayouni / Meat Science 86 (2010) 451–455
2. Materials and methods 2.1. Preparation of samples Semi-membranous muscle was removed from a goat carcass at room temperature. Thirty samples were used for each treatment. A Glendale fan-forced model oven was used for conventional heating (roasting) of the meat at an oven temperature of 163 °C to an internal meat temperature of 70 °C. Domestic and industrial microwave heating was applied at 2450 MHz frequency. The wattages for domestic and industrial microwaves were 700 W and 12,000 W, respectively. The internal temperature was regulated at 70 °C in all heat treatments. Small samples of 2 × 3 × 3 mm were taken from SM muscle and examined using an environmental scanning electron microscope (model E-3) to study their microstructure. The samples were placed in the gun chamber of the microscope. Miller stain was used for better visibility (Miller, 1994) by increasing contrast of the metal ions (Fe). 2.2. Characteristics of ESEM ESEM is an interesting new development in the field of electron microscopy. The ESEM and higher resolution micrographs can build an image in the presence of a gas (Danilatos, 1989). ESEM has been described as a technique that retains a minimum water vapor pressure (at least 609 Pa) in the chamber specimen. This creates the possibility of testing the natural state of a sample without any initial
453
preparation required for the samples (Uwins, 1994). A depiction of ESEM is shown in Fig. 1. The two main parts of the instrument include an electron gun chamber or electron optics column and a specimen chamber. The gun chamber is located at the top part of instrument and provides a flow of electrons by heating a tungsten filament, lanthanum hexaboride filament or field emission source. The specimen chamber is capable of working in a very poor vacuum, unlike other types of ESEM that require a high vacuum. Thus, the specimens will not dry out and ESEM can observe the specimens in the fresh state (Uwins, 1994). Some important advantages have been shown for the presence of gas around samples in the specimen chamber. The accumulation of a charge on the insulating samples can be caused by gases inside the specimen chamber (Crawford, 1979; Moncrieff, Robinson & Harris, 1978; Parsons, Matriccardi, Moretz, & Turner, 1974). Danilatos (1983, 1986, 1990) discovered that the gas itself can be employed as a detector in the microscope system, which is another advantage of ESEM. In ESEM, it is important to consider the possibility of dehydration of the fresh tissues occurring in the sample chamber. This may also occur from the electron beam on a sample if the operating system is not precisely set and adjusted prior to imaging. To prevent dehydration and localized beam damage in the specimen, it should be maintained in a semi-wet condition by monitoring the system at a lower voltage (2–5 kV) or at a lower temperature by applying a cooling step (− 20 to −30 °C) and higher gas pressure of up to 20 Torr (Uwins, 1994).
Fig. 4. (A) ESEM of effect of roasted goat SM muscle with arrows indicating transverse breakage, (B) ESEM of transverse and internal breakage (ib) of individual muscle fiber, (C) cross section of roasted goat SM muscle, (D) individual muscle fiber in roasted goat SM muscle showing granulation at surface of muscle.
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M.S. Yarmand, A. Homayouni / Meat Science 86 (2010) 451–455
3. Results and discussion ESEM showed bundles of muscle fibers in raw SM muscle running parallel to one another (Fig. 2A). The collagen fibers surrounding the muscle fiber are not clear in some figures but, in Fig. 2B, the collagen fibers are clearer and the myofibril surfaces seem normal. To observe collagen more clearly, Miller stain (Miller, 1994) was used. This technique increased contrast by increasing the conductivity of the material. As shown in Fig. 2C, the collagenous fibers are clearly seen as a white network with covering bundles of SM muscle fiber. A sample cross section of SM muscle is shown in Fig. 2D. Damage was observed in the microstructure of SM muscle from domestic microwave heating, which resulted in more shrinkage and breakdown in the SM muscle. The myofibril surface did not appear to be normal, but presented little evidence of tissue damage. A cross section of SM (Fig. 3A) illustrates the damage at the surface of the specimen. Further application of ESEM illustrated that the domestic microwave heating caused more physical damage to the connective tissue and myofibril elements compared to conventional oven heating (Fig. 3B). Domestic microwave heating caused little hydrolysis in connective tissue. This is in agreement with Hsieh, Cornforth, Pearson, and Hooper (1980). Therefore, it can be said that domestic microwave cooking causes more damage to the structure of both the connective tissue and the myofibrillar elements than roasting. The mechanism of microwave heating that causes a transfer of heat is the vibration and friction of the water molecules (Hui, 1992). The time employed for domestic microwave cooking (3–4 min) was less than that for conventional cooking (30–45 min). Thus, the structure of goat muscle, which is 70% water, will be damaged by vibration or rotation of water molecules by the electromagnetic field at a frequency of 2450 MHz. In contrast to microwave heating, in conventional roasting, heat transfer occurs by conduction from the exterior to the interior of the muscle. The longer cooking time allows the muscle more time to be heated. As a result, structural changes in the SM muscle occur gradually and shrinkage and denaturation of protein occurs more slowly (Kadim, Mahgoub, & Purchas, 2008; Thomas, Anjaneyulu, Gadekar, Pragati, & Kondaiah, 2007; Tornberg, 2005). However, some breakage can still be observed in Fig. 4A. A magnification of 2000× was used to observe breakage in conventional heating (Fig. 4A). The figure shows internal breakage of individual muscle fibers and denaturation and coagulation of protein. Transverse fractures of the myofibrils could be seen in the cooked fibers, which is in agreement with the findings of Paul (1963). Fig. 4B reveals the amount of distortion of an individual muscle fiber stretching across a crack, revealing that the formation of this fracture was not a passive phenomenon but required external force. No surface damage was observed any image taken for conventional heating (Fig. 4A–D). Although industrial microwave heating uses a similar frequency (2450 MHz), the wattage can be eleven times higher (12,000 W) than that of the domestic microwave (700 W). This means there is an increase in applied energy and the transfer of heat occurs more quickly than in the domestic microwave, resulting in less time for the process and more damage. Because of difference in wattage used, the penetration of the electromagnetic field to SM muscle may be higher in industrial microwave heating (Fig. 5A–C). A rapid increase of heat in 2–3 min of the industrial microwave thermal process not only causes shrinkage and disruption, but also granulation and separation of some parts of the muscle fiber, as shown in Fig. 5A–C. The influence of the rate of heating on the disintegration of muscle fibers suggests that it is possible that heat penetration affects the type and extent of disintegration of muscle fibers, affecting the tenderness of the muscle (Hearne, 1976). 4. Conclusions The present study demonstrates that it is possible to identify and characterize the fine structure of semi-membranous goat muscle and
Fig. 5. (A) ESEM of effect of industrial microwave heating (12,000 W), (B) ESEM of industrial microwave-heated goat SM muscle with surface damage (g: granulation), (C) industrial microwave-heated goat SM muscle with surface damage.
to study and compare the effect of various heat treatments, such as conventional and microwave heating at two wattage levels (700 W; 12,000 W) on the structure of this muscle. Results show that microwave heating causes more structural damage at both levels than conventional heating. It was also shown that distribution patterns in microwave heating are responsible for surface damage to muscle fiber, separation of some parts and denaturation of collagen.
M.S. Yarmand, A. Homayouni / Meat Science 86 (2010) 451–455
Further research is required to study the fine structure of semimembranous or other muscles in different animals. Acknowledgements The authors would like to express their thanks to Professor V. Sarafis of The Centre for Biostructural and Biomolecuar Research, Faculty of Science and Technology, University of Western Sydney, Hawkesbury, Australia for his assistance. References Arvanitoyannis, I. S., & van Houwelingen-Koukaliaroglou, M. (2003). Implementation of chemometrics for quality control and authentication of meat and meat products. Critical Reviews in Food Science and Nutrition, 43, 173−218. Baumgarten, N. (1990). Introduction to the environmental scanning electron microscope. Scanning, 12, I-36−I-37. Bejerholm, C., & Aaslyng, M. D. (2003). The influence of cooking technique and core temperature on results of a sensory analysis of pork, depending on the raw meat quality. Food Quality and Preference, 15, 19−30. Bolon, R. B., Roberstson, C. D., & Lifshin, E. (1989). The environmental SEM: A new way to look at insulators. In P. E. Russel (Ed.), Microbeam analysis (pp. 449−452). San Francisco: San Francisco Press. Crawford, C. K. (1979). Charge neutralization using very low energy ions. Scanning Electron Microscopy, 2, 31−46. Danilatos, G. D. (1981). The examination of fresh or living plant material in an environmental scanning electron microscope. Journal of Microscopy, 121, 235−238. Danilatos, G. D. (1983). A gaseous detector device for an environmental SEM. Micron and Microscopica Acta, 14, 307−318. Danilatos, G. D. (1986). Environmental SEM; A new instrument, a new dimension. Proceedings of the Institute of Physics Electron Microscopy and Royal Microscopy Society Conference, 98(1), 455−458. Danilatos, G. D. (1989). Environmental SEM; A new instrument, a new dimension. Proceedings of the Institute of Physics Electron Microscopy and Royal Microscopy Society Conference, 98(1). (pp. 455−458). Danilatos, G. D. (1990). Theory of the gaseous detector device in the ESEM. Advances in Electronics and Electron Physics, 78, 1−102. Danilatos, G. D. (1991). Review and outline of environmental SEM at present. Journal of Microscopy, 162, 391−402. Danilatos, G. D., & Brooks, J. H. (1987). Environmental SEM in wool research: Present state of the art. In M. Sakamoto (Ed.), Proceedings 7th international wool textile research conference, 1985, Tokyo (pp. 263−272). Danilatos, G. D., & Postle, R. (1982). The environmental scanning electron microscope and its application. Scanning Electron Microscopy, 1, 1−16. Hearne, L.E. (1976). Tenderness and structural changes in beef semitendinosus muscles heated at two rates to four end point temperatures. PhD thesis, The University of Tennessee. Hsieh, Y. P. C., Cornforth, D. P., Pearson, A. M., & Hooper, G. R. (1980). Ultrastructural changes in pre- and post-rigor beef muscle caused by conventional and microwave cookery. Meat Science, 4, 299−311. Hui, Y. H. (1992). Encyclopedia of food science and technology, Vol. 3,USA: Wiley Interscience Publications. Kadim, I. T., Mahgoub, O., & Purchas, R. W. (2008). A review of the growth and of the carcass and meat quality characteristics of the one-humped camel (Camelus dromedaries). Meat Science, 80, 555−569. Klose, M. J., Webb, R. I., & Teakle, D. S. (1992). Studies on the virus pollen association of tobacco streak virus using ESEM and MDD techniques. Journal of Computer Assisted Microscopy, 4, 213−220.
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Lee, K. C. (1992). Effects of different microwave power levels on chemical, physical and histological characteristics of beef roasts. Dissertation Abstracts International, 52(7), 3364. Lepetit, J. (2007). A theoretical approach of the relationships between collagen content, collagen cross-links and meat tenderness. Meat Science, 76, 147−159. Miller, R. K. (1994). Quality characteristics in muscle foods. In D. M. Kinsman (Ed.), (pp. 301–305). New York: Chapman and Hall. Moncrieff, D. A., Robinson, V. N. E., & Harris, L. B. (1978). Charge neutralization of insulating surfaces in the SEM by gas ionization. Journal of Physics D: Applied Physics, 11(17), 2315−2325. O'Brien, G. P., Webb, R. K., Uwins, P. J. R., Desmarchelier, P. M., & Imrie, B. (1992). Suitability of the environmental scanning electron microscope for studies of bacteria on Mung bean seeds. Journal of Computer Assisted Microscopy, 4, 225−229. Papadima, S. N., Arvanitoyannis, I. S., & Bloukas, J. G. (1999). Chemometric model for describing Greek traditional sausages. Meat Science, 51, 271−277. Parsons, D. F., Matriccardi, V. R., Moretz, R. C., & Turner, J. N. (1974). Electron microscopy and diffraction of wet unstained and unfixed biological objects. Advances in Biological and Medical Physics, 15, 161−271. Paul, P. (1963). Influence of methods of cooking on meat tenderness. Proceeding of Meat Tenderness Symposium, 225−242. Pawar, V. D., Khan, F. A., & Agarkar, B. S. (2000). Quality of chevon patties as influenced by different methods of cooking. Journal of Food Science and Technology, 37, 545−548. Pawar, V. D., Khan, F. A., & Agarkar, B. S. (2002). Effect of fat/whey protein concentrate levels and cooking methods on textural characteristics of chevon patties. Journal of Food Science and Technology, 39, 429−431. Peters, K. R. (1990). Introduction to the technique of environmental scanning electron microscopy. Scanning 90 abstracts (pp. 71−72). FACMS Inc. Purchas, R. W., Rutherfurd, S. M., Pearce, P. D., Vather, R., & Willkinson, B. H. P. (2004). Cooking temperature effects on the forms of iron and levels of several other compounds in beef semitendinosus muscle. Meat Science, 68, 201−207. Raj, R., Sahoo, J., Hooda, S., & Karwasra, R. K. (2005). Effect of ginger extract and cooking methods on oxidative stability and physico-chemical quality of vacuum packaged cooked chevon patties during storage. Journal of Food Science and TechnologyMysore, 42, 275−278. Roon, P. S., Van Houben, J. H., Koolmees, P. A., & Van Vliet, T. (1994). Mechanical and microstructural characteristics of meat doughs, either heated by a continuous process in a radio-frequency field or conventionally in water bath. Meat Science, 38, 103−116. Russ, J. C. (2005). Image analysis of food microstructure. Boca Raton: CRC Press. Thomas, R., Anjaneyulu, A. S. R., Gadekar, Y. P., Pragati, H., & Kondaiah, N. (2007). Effect of comminution temperature on the quality and shelf life of buffalo meat nuggets. Food Chemistry, 103, 787−794. Tornberg, E. (2005). Effects of heat on meat proteins: implications on structure and quality of meat products. Meat Science, 70, 493−508. Tzouros, N. E., & Arvanitoyannis, I. S. (2001). Agricultural produces: Synopsis of employed quality control methods for the authentication of foods and application of chemometrics for the classification of foods according to their variety or geographical origin. Critical Rreviews in Food Science and Nutrition, 41, 287−319. Uwins, P. J. R. (1994). Environmental scanning electron microscopy. Materials Forum, 18, 51−75. Wallace, H. M., Uwins, P. J. R., & McChonchie, C. A. (1992). Investigation of stigma interaction in Macadamia and Grevillea using ESEM. Journal of Computer Assisted Microscopy, 4, 231−234. Yarmand, M. S., & Homayouni, A. (2009). Effect of microwave cooking on the microstructure and quality of meat in goat and lamb. Food Chemistry, 112, 782−785. Yarmand, M. S., & Sarafis, V. (1997). An improved maceration technique with polarised light microscopy in the study of muscle. Egyptian Journal of Food Science, 25, 425−431.
Contents lists available at ScienceDirect
Meat Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m e a t s c i
Quality and microstructural changes in goat meat during heat treatment M.S. Yarmand a,⁎, A. Homayouni b a b
Department of Food Science and Technology, Faculty of Biosystems Engineering, Agriculture and Natural Resources Campus, University of Tehran, Tehran, Islamic Republic of Iran Department of Food Science and Technology, Faculty of Health and Nutrition, Tabriz University of Medical Sciences, Tabriz, Islamic Republic of Iran
a r t i c l e
i n f o
Article history: Received 10 September 2009 Received in revised form 27 January 2010 Accepted 16 May 2010 Keywords: ESEM Goat Microstructure Conventional heating Microwave heating
a b s t r a c t Environmental scanning electron microscopy (ESEM) was used to study the structure of the semimembranous muscle of goat meat during heat treatment. The four treatments examined were raw (control), conventional heating, domestic microwave heating and industrial microwave heating. The ESEM showed shrinkage and denaturation in muscle fibers after cooking by the various methods. No surface damage was observed in any image for conventional heating. More structural damage was observed in microwave heating at both domestic (700 W) and industrial (12,000 W) levels. The pattern of distribution in microwave heating caused surface damage to muscle fiber with separation of some portions and denaturation of collagen. Crown Copyright © 2010 Published by Elsevier Ltd. on behalf of The American Meat Science Association. All rights reserved.
1. Introduction Quality control methods and microstructural evaluation were employed to study the qualitative parameters of meat and meat products (Arvanitoyannis & van Houwelingen-Koukaliaroglou, 2003; Papadima, Arvanitoyannis, & Bloukas, 1999; Tzouros & Arvanitoyannis, 2001; Yarmand & Homayouni, 2009). Among these methods, the environmental scanning electron microscopy (ESEM) has its own advantages for evaluation of microstructural changes in meat especially for comparing the effect of various heat treatments (Yarmand & Homayouni, 2009). Environmental scanning electron microscopy (ESEM) has been used for the examination of living and fresh botanical samples (Danilatos, 1981) including fungal mycelium and cross sections of stems from different plant sources. Danilatos and Postle (1982) studied common biological applications. A number of studies have demonstrated the use of ESEM for hydrated biological samples (Klose, Webb, & Teakle, 1992; Wallace, Uwins, & McChonchie, 1992). Some research compared unprocessed ESEM specimens and samples prepared by conventional methods (Danilatos, 1981; O'Brien, Webb, Uwins, Desmarchelier, & Imrie, 1992). Much research has been done on the study of industrial wool fiber, commencing with sheep breeding and concluding with the study of finished fabric. Investigations of many of these processes can be greatly enhanced by the use of ESEM techniques. Early results in this area have been briefly reported by Danilatos and Brooks (1987).
⁎ Corresponding author. Tel.: +98 261 2248804; fax: +98 261 2248804. E-mail address: [email protected] (M.S. Yarmand).
Fig. 1. Diagrammatic representation of a two-stage differential pumping system for an ESEM (Danilatos, 1991).
0309-1740/$ – see front matter. Crown Copyright © 2010 Published by Elsevier Ltd. on behalf of The American Meat Science Association. All rights reserved. doi:10.1016/j.meatsci.2010.05.033
452
M.S. Yarmand, A. Homayouni / Meat Science 86 (2010) 451–455
Image analysis has been used for the qualitative evaluation of lamb meat (Russ, 2005; Yarmand & Homayouni, 2009; Yarmand & Sarafis, 1997). Preliminary conclusions on a variety of novel ESEM applications were recorded by Bolon, Roberstson, & Lifshin (1989). ESEM techniques have also been demonstrated by Baumgarten (1990) and Peters (1990). Lee (1992) studied the effect of different microwave power levels on the chemical, physical and histological characteristics of beef roasts
and found that the use of variable power for microwave heating of beef roasts does not result in different chemical or physical characteristics. Histological results of the ultrastructure of muscle and collagen revealed changes in sarcomere appearance and size that were related to the internal endpoint temperature (Lepetit, 2007). Mechanical and microstructure characteristics of meat dough heated by a continuous process in a radio-frequency field or conventionally in a water bath were investigated by Roon, Van Houben, Koolmees, and Van Vliet (1994). Purchas, Rutherfurd, Pearce, Vather, and Willkinson (2004) studied the influence of final cooked temperature on the forms of iron present and the levels of several other compounds of beef semitendinosus muscle. Changes in the form of iron with cooking generally took place more rapidly in surface samples than inner samples. On a dry matter basis, the concentration of taurine, carnosine, coenzyme Q10 and creatine all decreased with cooking, with the greatest decreases noted for taurine and creatine. Pawar, Khan, and Agarkar (2000, 2002) studied the quality of chevon patties as influenced by different methods of cooking and demonstrated that oven-cooked chevon patties were of high quality and were most stable against oxidative changes (Bejerholm & Aaslyng, 2003; Raj, Sahoo, Hooda, & Karwasra, 2005). In the present study, ESEM was applied to study the microstructure of goat semi-membranous (SM) muscle using raw muscle as a control and comparing it to different heat treatments, including conventional and microwave heating.
Fig. 2. (A) ESEM of structure of raw goat SM muscle, (B) ESEM of structure of raw goat SM muscle, (C) structure of SM muscle using Miller stain showing bright collagen network, (D) cross section view of SM.
Fig. 3. (A) ESEM of domestic microwave-heated (700 W) goat SM muscle. Surface damage indicated by arrows in cross section view; (B) ESEM of domestic microwaveheated goat SM muscle.
M.S. Yarmand, A. Homayouni / Meat Science 86 (2010) 451–455
2. Materials and methods 2.1. Preparation of samples Semi-membranous muscle was removed from a goat carcass at room temperature. Thirty samples were used for each treatment. A Glendale fan-forced model oven was used for conventional heating (roasting) of the meat at an oven temperature of 163 °C to an internal meat temperature of 70 °C. Domestic and industrial microwave heating was applied at 2450 MHz frequency. The wattages for domestic and industrial microwaves were 700 W and 12,000 W, respectively. The internal temperature was regulated at 70 °C in all heat treatments. Small samples of 2 × 3 × 3 mm were taken from SM muscle and examined using an environmental scanning electron microscope (model E-3) to study their microstructure. The samples were placed in the gun chamber of the microscope. Miller stain was used for better visibility (Miller, 1994) by increasing contrast of the metal ions (Fe). 2.2. Characteristics of ESEM ESEM is an interesting new development in the field of electron microscopy. The ESEM and higher resolution micrographs can build an image in the presence of a gas (Danilatos, 1989). ESEM has been described as a technique that retains a minimum water vapor pressure (at least 609 Pa) in the chamber specimen. This creates the possibility of testing the natural state of a sample without any initial
453
preparation required for the samples (Uwins, 1994). A depiction of ESEM is shown in Fig. 1. The two main parts of the instrument include an electron gun chamber or electron optics column and a specimen chamber. The gun chamber is located at the top part of instrument and provides a flow of electrons by heating a tungsten filament, lanthanum hexaboride filament or field emission source. The specimen chamber is capable of working in a very poor vacuum, unlike other types of ESEM that require a high vacuum. Thus, the specimens will not dry out and ESEM can observe the specimens in the fresh state (Uwins, 1994). Some important advantages have been shown for the presence of gas around samples in the specimen chamber. The accumulation of a charge on the insulating samples can be caused by gases inside the specimen chamber (Crawford, 1979; Moncrieff, Robinson & Harris, 1978; Parsons, Matriccardi, Moretz, & Turner, 1974). Danilatos (1983, 1986, 1990) discovered that the gas itself can be employed as a detector in the microscope system, which is another advantage of ESEM. In ESEM, it is important to consider the possibility of dehydration of the fresh tissues occurring in the sample chamber. This may also occur from the electron beam on a sample if the operating system is not precisely set and adjusted prior to imaging. To prevent dehydration and localized beam damage in the specimen, it should be maintained in a semi-wet condition by monitoring the system at a lower voltage (2–5 kV) or at a lower temperature by applying a cooling step (− 20 to −30 °C) and higher gas pressure of up to 20 Torr (Uwins, 1994).
Fig. 4. (A) ESEM of effect of roasted goat SM muscle with arrows indicating transverse breakage, (B) ESEM of transverse and internal breakage (ib) of individual muscle fiber, (C) cross section of roasted goat SM muscle, (D) individual muscle fiber in roasted goat SM muscle showing granulation at surface of muscle.
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3. Results and discussion ESEM showed bundles of muscle fibers in raw SM muscle running parallel to one another (Fig. 2A). The collagen fibers surrounding the muscle fiber are not clear in some figures but, in Fig. 2B, the collagen fibers are clearer and the myofibril surfaces seem normal. To observe collagen more clearly, Miller stain (Miller, 1994) was used. This technique increased contrast by increasing the conductivity of the material. As shown in Fig. 2C, the collagenous fibers are clearly seen as a white network with covering bundles of SM muscle fiber. A sample cross section of SM muscle is shown in Fig. 2D. Damage was observed in the microstructure of SM muscle from domestic microwave heating, which resulted in more shrinkage and breakdown in the SM muscle. The myofibril surface did not appear to be normal, but presented little evidence of tissue damage. A cross section of SM (Fig. 3A) illustrates the damage at the surface of the specimen. Further application of ESEM illustrated that the domestic microwave heating caused more physical damage to the connective tissue and myofibril elements compared to conventional oven heating (Fig. 3B). Domestic microwave heating caused little hydrolysis in connective tissue. This is in agreement with Hsieh, Cornforth, Pearson, and Hooper (1980). Therefore, it can be said that domestic microwave cooking causes more damage to the structure of both the connective tissue and the myofibrillar elements than roasting. The mechanism of microwave heating that causes a transfer of heat is the vibration and friction of the water molecules (Hui, 1992). The time employed for domestic microwave cooking (3–4 min) was less than that for conventional cooking (30–45 min). Thus, the structure of goat muscle, which is 70% water, will be damaged by vibration or rotation of water molecules by the electromagnetic field at a frequency of 2450 MHz. In contrast to microwave heating, in conventional roasting, heat transfer occurs by conduction from the exterior to the interior of the muscle. The longer cooking time allows the muscle more time to be heated. As a result, structural changes in the SM muscle occur gradually and shrinkage and denaturation of protein occurs more slowly (Kadim, Mahgoub, & Purchas, 2008; Thomas, Anjaneyulu, Gadekar, Pragati, & Kondaiah, 2007; Tornberg, 2005). However, some breakage can still be observed in Fig. 4A. A magnification of 2000× was used to observe breakage in conventional heating (Fig. 4A). The figure shows internal breakage of individual muscle fibers and denaturation and coagulation of protein. Transverse fractures of the myofibrils could be seen in the cooked fibers, which is in agreement with the findings of Paul (1963). Fig. 4B reveals the amount of distortion of an individual muscle fiber stretching across a crack, revealing that the formation of this fracture was not a passive phenomenon but required external force. No surface damage was observed any image taken for conventional heating (Fig. 4A–D). Although industrial microwave heating uses a similar frequency (2450 MHz), the wattage can be eleven times higher (12,000 W) than that of the domestic microwave (700 W). This means there is an increase in applied energy and the transfer of heat occurs more quickly than in the domestic microwave, resulting in less time for the process and more damage. Because of difference in wattage used, the penetration of the electromagnetic field to SM muscle may be higher in industrial microwave heating (Fig. 5A–C). A rapid increase of heat in 2–3 min of the industrial microwave thermal process not only causes shrinkage and disruption, but also granulation and separation of some parts of the muscle fiber, as shown in Fig. 5A–C. The influence of the rate of heating on the disintegration of muscle fibers suggests that it is possible that heat penetration affects the type and extent of disintegration of muscle fibers, affecting the tenderness of the muscle (Hearne, 1976). 4. Conclusions The present study demonstrates that it is possible to identify and characterize the fine structure of semi-membranous goat muscle and
Fig. 5. (A) ESEM of effect of industrial microwave heating (12,000 W), (B) ESEM of industrial microwave-heated goat SM muscle with surface damage (g: granulation), (C) industrial microwave-heated goat SM muscle with surface damage.
to study and compare the effect of various heat treatments, such as conventional and microwave heating at two wattage levels (700 W; 12,000 W) on the structure of this muscle. Results show that microwave heating causes more structural damage at both levels than conventional heating. It was also shown that distribution patterns in microwave heating are responsible for surface damage to muscle fiber, separation of some parts and denaturation of collagen.
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