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Shortening of the Pectoralis Muscle and Meat Tenderness of Broiler Chickens
Shortening of the Pectoralis Muscle and Meat Tenderness of Broiler Chickens C. M. PAPA and C. E. LYON United States Department of Agriculture, Agricultural Research Service, Richard B. Russell Agricultural Research Center, P.O. Box 5677, College Station Road, Athens, Georgia 30613 (Received for publication August 8, 1988)
1989 Poultry Science 68:663-669 INTRODUCTION
The fiber architecture of m. pectoralis, the largest of the avian flight muscles, is complex. The origin of the muscle is extensive and the internal arrangement of the fibers "is not readily characterized in textbook terms" (Raikow, 1985). The fibers proceed craniodorsally from the sternal origin of the muscle and cranioventrally from the large caudolateral fasciculus (Hudson and Lanzillotti, 1964), converging onto an aponeurotic extension of the tendinous insertion attaching to the humerus. These general characteristics appear to give the muscle a modified bipennate form (Vanden Berge, 1979). The complex anatomy of the pectoralis muscle has been related, at least in part, to observations of postmortem complexity in the conversion of this muscle to meat. Smith et al. (1969) described the portion of the muscle that they used for the evaluation of temperature effects on shortening of the excised muscle. The location of their observations corresponded to the anterior (cranial) area sampled in the investigation by Papa and Fletcher (1988). This latter report dealt specifically with location-dependent differences during the development of rigor, and demonstrated an
earlier onset and greater extent of shortening in anterior compared with posterior (caudal) portions of the main belly. Another study has demonstrated differences in the diameter of the muscle fibers sampled from these selected locations (Smith and Fletcher, 1988). In a report by Dickens and Papa (1988), postmortem shortening of broiler pectoralis was analyzed along the total length of the whole muscle. The measurement was made approximately parallel to the line of the tendinous partition that separates the medial portion of the muscle from the lateral fasciculus, but was not parallel to the direction of muscle fiber convergence onto the tendinous partition. The amounts of shortening demonstrated by these authors were substantially less than those obtained by Smith et al. (1969). The objective of the present study was to compare the extent of shortening in this muscle obtained by selected measurements that differ in their orientation of the observation. Results included in this report for the measurement along the total length of the muscle have been published elsewhere (Dickens and Papa, 1988). An additional objective sought to relate these different measurements to an empirical model of postmortem shortening for the
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ABSTRACT The pectoralis muscles of broilers were either excised immediately after picking (hot-boned) or after 24 h of aging on the carcass. Gross muscle shortening along selected lengths, muscle area loss, and sarcomere shortening were calculated for the raw and cooked muscles of both treatments. Cooked meat tenderness was also evaluated at selected intramuscular locations. For hot-boned muscles, shortening after excision was about 10% along a shorter length confined and parallel to the region of clavicular origin, compared with 2% along a longer length parallel to the tendinous partition between the medial belly and lateral portion, and 16% when expressed as muscle area loss. After 24-h chill, hot-boned muscles had shortened 21% along the shorter length, 8% along the longer length, and 20% as area loss. After cooking, in the same comparison, the shortenings averaged 32% and 24%, and area loss 42%, respectively. The linear shortenings of the hot-boned muscles were related to area loss in an empirical model. Shear and texture values of the cooked meat indicated a difference due to location, which seemed consistent with the model of complex muscle shortening for this group. For muscles aged on the carcass, postchill (postexcision) and postcook shortenings, respectively, averaged 9 and 22% along the shorter length, 1 and 20% along the longer length, and 10 and 36% as area loss. Postcook decreases in sarcomere length averaged 6% for hot-boned muscles and 32% for those aged on the carcass. (Key words: broiler chicken, pectoralis muscle, muscle shortening, meat tenderness)
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muscle. The application of these results to the textural quality of the cooked meat was also considered. MATERIALS AND METHODS
Immediately after these markings were made, the pectoralis muscles of those carcas-
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Commercially reared broilers of mixed sex were obtained from a local processing plant and transferred for immediate slaughter in pilot facilities. Birds were stunned, 50 V AC for 10 s on a path from head to feet, and then killed by bleeding through a conventional outside neck cut. Carcasses were scalded at 55 C for 90 s and machine picked for 30 s. Skin covering the whole breast was then removed. The pectoralis muscles of each carcass, while still attached to skeletal restraints, were then subjected to a series of markings and a tracing. For the measurement referred to as "Line 1," two, 5-mL syringes containing indelible dye were fixed in a clamp holder to allow the marking of two points on the ventral surface of the muscle representing a line 7 cm in length. This imaginary line extended from a point in the shoulder joint, approximately 1 cm from the humeral insertion, to a point of muscle origin near to die carinal apex, i.e., roughly parallel to the line of clavicular origin (c/. Smith et al., 1969). For the measurement referred to as "Line 2," another pair of 5-mL syringes with dye were set 14 cm apart, to allow the marking of two points representing a line extending from the same cranial point used above (for Line 1) to a point directed toward the caudal end of the sternum, i.e., roughly parallel to the tendinous partition that separates the medial belly of the muscle from the lateral fasciculus. For the measurement of muscle area, clear plastic film was laid over the whole breast and the perimeters of bom muscles were traced. Outlined areas were later calculated by electronic integration. For the measurement of overall length, the muscle was dyed at two points that set an imaginary line representing the longest length of the whole muscle, i.e., one in the region of insertion at the shoulder joint and the other at me caudal end of the sternum. Locations of these extreme points were marked on the tracing made of the muscle perimeter, and the length was later measured between the markings. Figure 1 illustrates the approximate positions of Lines 1 and 2 on the muscle surface.
ses allocated to the hot-boning treatment were carefully excised. At this time (postexcision), lengths of Lines 1 and 2 were measured using a pair of outside calipers. An oudine of the muscle was traced onto waxed paper for the subsequent determination of area; overall length of the muscle was again measured from the oudine made for area determination. Muscles of the hot-boned group and carcasses that were allocated to the other treatment (muscle deboning after 24-h aging on the carcass) were placed in polypropylene bags. A large portion of the air in the bag was manually excluded, bags were sealed, and die samples were chilled in ice/water slush for 24 h. After chill, muscles aged on the carcasses were excised, and, togetiier with the previously hot-boned muscles, subjected to the same procedural series as mat described above for hot-boned muscles at the time postexcision. Both groups of the deboned muscles were men placed on foil-lined cookie sheets and baked uncovered at 177 C for 45 min in a rotary oven. After allowing the cooked meats to cool at room temperature for about 1 h, caliper measurements and outlines were made again. Corresponding to me same time of linear measurements and area tracings made on one pectoralis muscle, me muscle or cooked meat of the other side was sampled along the line of action for the muscle, as depicted by Raikow (1985). Sarcomere lengms of the samples were measured by the method of laser diffraction described by Cross et al. (1981), with some modification (Papa and Fletcher, 1988). The extent of sarcomere shortening was based on a mean "intact" value, 2.26 u., obtained at the beginning of each trial of the experiment by sampling the muscles deboned immediately postmortem from 10 birds receiving an intravenous injection of 200 mg sodium iodoacetate/kg bird (to arrest glycolysis) 3 to 5 min before slaughter (de Fremery, 1966). Texture of the cooked meat was objectively evaluated by both compression (Texture Profile Analysis, TPA) and shearing. Three locations of the muscle, designated cranial, middle, and caudal (c/., anterior to posterior, Papa and Fletcher, 1988) were evaluated for the TPA attributes of hardness, cohesiveness, and chewiness as described by Friedman et al. (1963). Cores of the cooked meats, 2.54 cm diameter, were removed from each location of the muscle and evaluated using die techniques
CHICKEN BREAST MUSCLE AND MEAT SHORTENING
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Downloaded from http://ps.oxfordjournals.org/ by guest on May 27, 2015 FIGURE 1. Empirically modeled figures of tiie pectoralis muscle of broiler chickens at different stages of processing: (A) represents the muscle attached to normal skeletal restraints on the carcass. The progressive shortening along selected lengths (Lines 1 and 2) and loss of area are illustrated for the muscle after deboning immediately postmortem (B), after chilling for 24 h in ice/water slush (C), and then after cooking (D). The loss of muscle area was related to the shortenings of Lines 1 and 2 by the use of certain assumptions.
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TABLE 1. Selected evaluations o/pectoralis muscle and meat shortening of muscle deboned from carcasses of broiler chickens either immediately postmortem (hot-boned) or after 24 h of aging on the carcass (intact) Intact muscle2
Hot-boned muscle.1 Measure
Postexcision
Line 1 10.3"b Line 2 1.9° 7.4b0 Muscle length Area 16.3' Sarcomere lengthsi 8.7'b
Postchill
Postcook
Postexcision
Postcook
21.0" 8.0b 11.9*b 19.7' 19.2'
31.7C 24.3d 26.4" 42.0* 36.4"
8.6b .8° 7.0b 9.7" 13.9"
22.2* 20.0° 24.3bc 36.5" 32.5""
'"''Means in the same column with no common superscripts are significantly different (P<.05). 'For each mean, n = 44 except those of sarcomere lengths (for which n = 15), at each time postmortem. ^ o r each mean at each time-postmortem, n = 25 except those of sarcomere lengths (for which n = 13 and n = 12, at the times postexcision and postcook, respectively).
RESULTS AND DISCUSSION
Results in Table 1 indicate that the extent of shortening observedpostexcision along Line 1,
10.3%, agreed well with the results reported by Smith et al. (1969), 10.4 and 11.8%. The use of Line 1 was a direct attempt in the present study to repeat the specific measurement made by those authors. The shortenings of hot-boned muscle determined for this sampling time were highly variable, as indicated by the general lack of significant difference between rather widely separated means. For all of the sampling times at which hotboned muscles were evaluated, die extent of shortening observed along Line 2 was significantly less than that observed along Line 1, as area loss, and as sarcomere contraction (Table 1). Because Line 2 was roughly parallel to the tendinous partition that separates the medial belly from the lateral fasciculus, it was not parallel to the longitudinal axis of the muscle fibers as they converge onto the partition. Line 1 was more nearly parallel to the direction of the longitudinal axis of the muscle fibers, and sarcomere contraction is inextricably linked to decreases in the length of that axis. The shortening of Line 2 was not significandy different from that observed as the change in overall length of the muscle at any of the sampling times. The measurement of overall length represented a slight displacement and extension of the measurement of Line 2 (Figure 1). Because of the displacement, the line representing the overall length had some shared aspect not only with Line 2, but also, to a lesser degree, with Line 1. Taking this into account might explain why the extent of shortening observed along the overall length was a value between those observed for Lines 1 and 2, at all of the sampling times. Furthermore, the difference in
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and conditions described by Lyon et al. (1984). Between the locations of the three removed cores, two strips of the meat were cut, 1.27 cm in width, and tested for WarnerBratzler shear value (Chatillon Model No. 1901-S, John Chatillon & Sons, New York, NY). No effort was made to account for the unequal heights of sample strips. The peak load required to shear each strip once was recorded in kilograms. Two trials of the experiment were conducted. The first used a total of 40 birds, 30 allocated to the hot-boning treatment and 10 to aging muscles on the carcass. The second trial used a total of 30 birds, IS allocated to each treatment. The percentage shortening of gross muscle lengths and area lost were calculated on the basis of the "intact" value measured for each muscle; in exception to this, sarcomere contraction was based on a common value, as described above. The resulting values (almost entirely between 0 and 40%) were subjected to arcsin transformation, for statistical analyses (Snedecor and Cochran, 1980). An analysis of variance with main effects of treatment (measure of shortening of location of muscle) and trial was conducted using the general linear model, as programmed by SAS (1982). Hypotheses were tested using the mean square between replications (i.e., birds) nested within trial as the error term. This same, conservative term for error was specified when separating means by Duncan's multiple range test.
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TABLE 2. Objective evaluations of the textural quality of cooked meat sampled from different intramuscular locations of the pectoralis muscle deboned from broiler chickens either immediately postmortem or after 24 h of aging on the carcass Texture profile analysis' Processing scheme
Muscle location
Hardness (h)
Cranial
(kg) 6.7*
(aA)
Deboned immediately postmortem and chilled (n=44) Intact through 24 hr chill and then deboned (n=25)
.47
(h x c) 3.1
Middle
6.1"
.49
3.0
Caudal Cranial
6.0b 5.4"
.49 .52
2.9 2.8"
Middle
6.1 ,b
.51
3.1"
Caudal
6.8"
.53
3.6"
Cohesiveness (c)
Chewiness
Warner-Bratzler shear value (kg) 8.9' 5.6"
3.4 3.5
orientation of a measurement may explain the disagreement between the lower results for muscle shortening presented by Dickens and Papa (1988), using an overall muscle length, and the higher results reported by Smith et al. (1969), using a measurement comparable to that used for Line 1. In order to quantify the full extent of postmortem shortening for poultry pectoralis muscle, measurement directed along the fiber length would seem to be necessary. For hot-boned samples, the muscle shortening observed postcook relative to that observed postchill demonstrated two-fold increases in the shortening of overall length and loss of muscle area and a three-fold increase in shortening along Line 2 (Table 1). The minimal shortening along Line 2 until cooking may involve a structural aspect of pectoralis muscle: the muscle is much thicker in the general area of the humeral insertion in comparison with the caudal areas associated with muscle origin along the carina of the sternum and the sternal ribs. Shrinkage due to cook would presumably register a greater effect in the thinner, caudal area, much of which is associated with the shortening along Line 2. To a large extent, shrinkage due to cook involved thermally induced denaturation of the myofibrillar proteins. However, to some extent, this final shrinkage also involved the networks of connective tissues in the muscle,
the most prominent being that of the tendinous partition, along which Line 2 was directed. Collagen shrinkage may be expected, at least in part, to effect some shortening in the length of the meat (Winegarden et al., 1952) when internal temperatures are elevated to 60 C and above (Machlik and Draudt, 1963). Shortening of muscle aged on the carcass (intact) followed a pattern similar to that of hot-boned muscle, with one notable exception. For intact muscles at the time postexcision (i.e., postchill also), the extent of sarcomere contraction was significantly greater than the shortening along Line 1 and area loss (Table 1). In contrast, for the hot-boned group, there was no significant difference between these parameters, and the numerical pattern was exactly opposite. Substantially shortened sarcomeres have been previously reported for broiler pectoralis muscle aged intact and sampled at an anterior (cranial) location (Papa and Fletcher, 1988). In the present study, the muscle was also shortened at the gross level, as evidenced by the shortening of Line 1 at this time. Objective evaluations of the textural quality of the cooked meats are presented in Table 2. Hardness values (TPA) for the cranial location of hot-boned muscle were significantly greater than those of the other locations. Values of cohesiveness and chewiness, however, were not significantly different for any locations in
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••"Means within the same column and processing scheme with no common superscripts are significantly different (P<.05). 'Hardness=the peak load required to initially compress the core to 70% of its original height (maximum height of the first compression curve); cohesiveness = the ratio of positive force areas under the first and second compression curves (a^a,); chewiness = the number resulting from the multiplication together of the values for hardness and cohesiveness.
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to be 50°; in a previous report (Papa and Fletcher, 1988), a figure of the muscle that was excised in a prerigor state, and drawn from a photograph, shows a similar angle. In Figure IB to D, the percentage shortening along Line 1 is imposed on the circles drawn in Figure 1(A) and the distance between their centers; shortening along Line 2 is imposed on the straight line of the figure's left side. The remaining lines of the figure are then determined, and the area is calculated and expressed on the basis of A = 100 cm2. Postexcision (B). When the muscle is deboned immediately postmortem, shortening along Line 1 averages nearly 10%, whereas that along Line 2 is only 2%, and the resulting area loss is still quite substantial, about 16% (Table 1). Compared with a circle with r = 5.64 cm and area = 100 cm2, a decrease in r by 10% results in about 20% loss of area. For the drawing of the muscle in Figure 1 (B), the radii and the distance between centers are decreased by 10%; the straight left side is decreased by only 2%. The result is a muscle width of 7.2 cm and area of 84 cm2, in close agreement with the experimental results. The dashed line surrounding B indicates the area of the intact muscle; in relation to the figure of B, it shows that shortening due to excision is represented in the model as an effect on almost all of the muscle, except for the observed general lack of an effect along Line 2. Postchill (C). In order for the shortening observed postchill along Lines 1 and 2 to be consistent with the loss of area, a whole muscle effect could not be considered. As drawn, the loss of area is imposed only on the left side, top and bottom of the figure; i.e., shortening in this instance proceeds only in a direction toward the insertion. As drawn, the radii of the circles are decreased by another 10% (the added shortening along Line 1), but the distance between centers is not decreased. Given the ordinary treatment of shortening along Line 2, the model shows a muscle of 7 cm in width and an area loss of 22%. The loss of area observed in the experiment, about 20%, is somewhat less than that shown in the model. Postcook (D). Shortening in this figure again assumed a "whole muscle" effect. The radii are shortened by about 30%. An overall effect is imposed equally on the top and both straight sides of the figure, resulting in a final muscle width of about 6.4 cm. The length of
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this treatment group. The Warner-Bratzler shear values for two locations in the muscle resulted in the same general pattern as that observed for values of hardness. For muscles aged intact, hardness and chewiness values of the caudal location were significantly greater than those of the cranial location, the reverse of the results obtained for the hot-boned group. The thin, caudal region of the pectoralis muscle might be expected to harden more on cooking, and thus produce these results observed for the group aged intact. The fact that an opposite pattern was displayed for the hotboned group is not consistent with this structural consideration, however. Smith et al. (1988) have conducted a more extensive investigation of locational differences in texture, and obtained results comparable to those reported here. The present study attempted to demonstrate that a rather wide range of results could be obtained when evaluating the postmortem shortening of poultry pectoralis muscle. The observed differences may have been due to a simple change in orienting the measurement, but this simple change would seem to involve the somewhat complex structure of the muscle. Because of this structural aspect, linear measurements and calculated shortenings for the hot-boned group were able to be related to the loss of muscle area by an empirical model, as shown in Figure 1. Intact (A). The beginning figure is drawn as a composite shape, consisting of two overlapping circles (in the area of Line 1), both with a radius (r) of 2.75 cm, and centers separated by 3.5 cm. The straight, vertical sides are set 8 cm apart; the straight line on the left side of the figure is 13 cm in length. The calculated area of the resulting figure is 107.4 cm2, but, for the sake of convenience and as the basis for subsequent expressions, a beginning area of 100 cm2 is assumed. The area actually observed for the intact muscles in this study was 97.8 ± 1.2 cm2 (x ± SEM, n = 69); the median area was 98.4 cm2. The figure of the intact muscle [1(A)] also assumes a 45° angle for the line of lateral departure from the caudal end of the sternum. When the muscle is deboned, especially in a prerigor state, the angle along this line increases, perhaps a result of mechanical contraction of the muscle (Jungk and Marion, 1970) or loss due to trimming or both. In Figure IB to D, the increased angle is assumed
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ACKNOWLEDGMENTS
The authors wish to thank R. Wilson for counseling in the areas of experimental design and statistical analyses, R. Vaughn for arranging the means by which muscle areas were measured, and F. Gillen for providing
very capable phases.
technical
assistance
in
all
REFERENCES Cross, H. R., R. L. West, and T. R. Dutson, 1981. Comparison of methods for measuring sarcomere length in beef semitendinosus muscle. Meat Sci. 5:261-266. de Fremery, D., 1966. Relationship between chemical properties and tenderness of poultry muscle. J. Agric. Food Chem. 14:214-217. Dickens, i. A., and C. M. Papa, 1988. A procedure for measuring excised muscle contraction. J. Food Sci. 53: 1056-1057, 1061. Friedman, H. H., J. E. Whitney, and A. S. Szczesniak, 1963. The texturometer-a new instrument for objective texture measurement. J. Food Sci. 28:390-396. Hudson, G. E., and P. J. LanziUotti, 1964. Muscles of the pectoral limb in galliform birds. Am. Midi. Nat. 71: 1-113. Jungk, R. A., and W. W. Marion, 1970. Post-mortem isometric tension changes and shortening in turkey muscle strips held at various temperatures. J. Food Sci. 35:143-145. Lyon, C. E., D. Hamm, J. P. Hudspeth, and F. H. Benoff, 1984. Effects of age and sex on the texture profile of hot stripped broiler breast meat. Poultry Sci. 63: 2508-2510. Machlik, S. M., and H. N. Draudt, 1963. The effect of hearing time and temperature on the shear of beef semitendinosus muscle. J. Food Sci. 28:711-718. Papa, C. M., and D. L. Fletcher, 1988. Pectoralis muscle shortening and rigor development at different locations within the broiler breast. Poultry Sci. 67:635-640. Raikow, R. J., 1985. Locomotor system. Pages 57-147 in: Form and Function in Birds. Vol. 3. A. S. King and J. McClelland, ed. Academic Press, London, England, U.K. SAS, 1982. SAS User's Guide: Statistics, 1982 ed. SAS Inst. Inc., Cary, NC. Smith, D. P., and D. L. Fletcher, 1988. Chicken breast muscle fiber type and diameter as influenced by age and intramuscular location. Poultry Sci. 67:908-913. Smith, D. P., C. E. Lyon, and D. L. Fletcher, 1988. Comparison of the Allo-Kramer shear and texture profile methods of broiler breast meat texture analysis. Poultry Sci. 67:1549-1556. Smith, M. C , M. D. Judge, and W. J. Stadelman, 1969. A "cold shortening" effect in avian muscle. J. Food Sci. 34:42-46. Snedecor, G. W., and W. G. Cochran, 1980. Chapter 15. Failures in the assumptions. Pages 274-297 in: Statistical Methods. The Iowa State Univ. Press, Ames, IA. Vanden Berge, J. C , 1979. Myologia. Pages 175-219 in: Nomina Anatomica Avium. J. J. Baumel, A. S. King, A. M. Lucas, J. E. Breazile, and H. E. Evans, ed. Academic Press, London, England, U.K. Winegarden, M. W., B. Lowe, J. Kastelic, E. A. Kline, A. R. Plagge, and P. S. Shearer, 1952. Physical changes of connective tissues of beef during heating. Food Res. 17:172-184.
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Line 2 reflects the 20% observed for its shortening in the experiment, the contribution to area loss now becoming a major effect. The area loss suggested by the model, about 41%, is somewhat greater than that actually observed, about 36%. The general relationship depicted in these figures between the shortenings of Lines 1 and 2 and loss of muscle area can be considered the direct result of the complex structure of m. pectoralis. Area loss would seem to be substantial because of the muscle having such an extensive origin, an arrangement that makes a major contribution to the general proportions of the body, and is probably functionally important to the mode of flight (Raikow, 1985). The fact that different lines of measurement give different results for the extent of postmortem shortening emphasizes that aspect of the muscle characterized as a modified bipennate form, with different orientations of the muscle fibers as they converge onto the tendinous partition and toward the insertion. In order to quantify the full extent of postmortem shortening when using the whole muscle, the present study has demonstrated that gross measurements must be directed along the length of the fibers' longitudinal axis (c/. Smith et al., 1969)-the common procedure when evaluating muscle strips. Measurements directed along the length of the longitudinal axis for the whole muscle (c/. Dickens and Papa, 1988) cannot quantify the full extent of pectoralis muscle shortening, and therefore the results of such measurements would seem to be potentially misleading. No effort was given to constructing a model for the shortening of the muscle aged intact on the carcass; that model would involve only the results of shortening observed as a function of "isometric" contractions during aging, and then thermally induced shrinkage due to cook. That is, such a model would ignore, by necessity, effects related to the case of isotonic contractions of prerigor-excised muscle.
1989 Poultry Science 68:663-669 INTRODUCTION
The fiber architecture of m. pectoralis, the largest of the avian flight muscles, is complex. The origin of the muscle is extensive and the internal arrangement of the fibers "is not readily characterized in textbook terms" (Raikow, 1985). The fibers proceed craniodorsally from the sternal origin of the muscle and cranioventrally from the large caudolateral fasciculus (Hudson and Lanzillotti, 1964), converging onto an aponeurotic extension of the tendinous insertion attaching to the humerus. These general characteristics appear to give the muscle a modified bipennate form (Vanden Berge, 1979). The complex anatomy of the pectoralis muscle has been related, at least in part, to observations of postmortem complexity in the conversion of this muscle to meat. Smith et al. (1969) described the portion of the muscle that they used for the evaluation of temperature effects on shortening of the excised muscle. The location of their observations corresponded to the anterior (cranial) area sampled in the investigation by Papa and Fletcher (1988). This latter report dealt specifically with location-dependent differences during the development of rigor, and demonstrated an
earlier onset and greater extent of shortening in anterior compared with posterior (caudal) portions of the main belly. Another study has demonstrated differences in the diameter of the muscle fibers sampled from these selected locations (Smith and Fletcher, 1988). In a report by Dickens and Papa (1988), postmortem shortening of broiler pectoralis was analyzed along the total length of the whole muscle. The measurement was made approximately parallel to the line of the tendinous partition that separates the medial portion of the muscle from the lateral fasciculus, but was not parallel to the direction of muscle fiber convergence onto the tendinous partition. The amounts of shortening demonstrated by these authors were substantially less than those obtained by Smith et al. (1969). The objective of the present study was to compare the extent of shortening in this muscle obtained by selected measurements that differ in their orientation of the observation. Results included in this report for the measurement along the total length of the muscle have been published elsewhere (Dickens and Papa, 1988). An additional objective sought to relate these different measurements to an empirical model of postmortem shortening for the
663
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ABSTRACT The pectoralis muscles of broilers were either excised immediately after picking (hot-boned) or after 24 h of aging on the carcass. Gross muscle shortening along selected lengths, muscle area loss, and sarcomere shortening were calculated for the raw and cooked muscles of both treatments. Cooked meat tenderness was also evaluated at selected intramuscular locations. For hot-boned muscles, shortening after excision was about 10% along a shorter length confined and parallel to the region of clavicular origin, compared with 2% along a longer length parallel to the tendinous partition between the medial belly and lateral portion, and 16% when expressed as muscle area loss. After 24-h chill, hot-boned muscles had shortened 21% along the shorter length, 8% along the longer length, and 20% as area loss. After cooking, in the same comparison, the shortenings averaged 32% and 24%, and area loss 42%, respectively. The linear shortenings of the hot-boned muscles were related to area loss in an empirical model. Shear and texture values of the cooked meat indicated a difference due to location, which seemed consistent with the model of complex muscle shortening for this group. For muscles aged on the carcass, postchill (postexcision) and postcook shortenings, respectively, averaged 9 and 22% along the shorter length, 1 and 20% along the longer length, and 10 and 36% as area loss. Postcook decreases in sarcomere length averaged 6% for hot-boned muscles and 32% for those aged on the carcass. (Key words: broiler chicken, pectoralis muscle, muscle shortening, meat tenderness)
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muscle. The application of these results to the textural quality of the cooked meat was also considered. MATERIALS AND METHODS
Immediately after these markings were made, the pectoralis muscles of those carcas-
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Commercially reared broilers of mixed sex were obtained from a local processing plant and transferred for immediate slaughter in pilot facilities. Birds were stunned, 50 V AC for 10 s on a path from head to feet, and then killed by bleeding through a conventional outside neck cut. Carcasses were scalded at 55 C for 90 s and machine picked for 30 s. Skin covering the whole breast was then removed. The pectoralis muscles of each carcass, while still attached to skeletal restraints, were then subjected to a series of markings and a tracing. For the measurement referred to as "Line 1," two, 5-mL syringes containing indelible dye were fixed in a clamp holder to allow the marking of two points on the ventral surface of the muscle representing a line 7 cm in length. This imaginary line extended from a point in the shoulder joint, approximately 1 cm from the humeral insertion, to a point of muscle origin near to die carinal apex, i.e., roughly parallel to the line of clavicular origin (c/. Smith et al., 1969). For the measurement referred to as "Line 2," another pair of 5-mL syringes with dye were set 14 cm apart, to allow the marking of two points representing a line extending from the same cranial point used above (for Line 1) to a point directed toward the caudal end of the sternum, i.e., roughly parallel to the tendinous partition that separates the medial belly of the muscle from the lateral fasciculus. For the measurement of muscle area, clear plastic film was laid over the whole breast and the perimeters of bom muscles were traced. Outlined areas were later calculated by electronic integration. For the measurement of overall length, the muscle was dyed at two points that set an imaginary line representing the longest length of the whole muscle, i.e., one in the region of insertion at the shoulder joint and the other at me caudal end of the sternum. Locations of these extreme points were marked on the tracing made of the muscle perimeter, and the length was later measured between the markings. Figure 1 illustrates the approximate positions of Lines 1 and 2 on the muscle surface.
ses allocated to the hot-boning treatment were carefully excised. At this time (postexcision), lengths of Lines 1 and 2 were measured using a pair of outside calipers. An oudine of the muscle was traced onto waxed paper for the subsequent determination of area; overall length of the muscle was again measured from the oudine made for area determination. Muscles of the hot-boned group and carcasses that were allocated to the other treatment (muscle deboning after 24-h aging on the carcass) were placed in polypropylene bags. A large portion of the air in the bag was manually excluded, bags were sealed, and die samples were chilled in ice/water slush for 24 h. After chill, muscles aged on the carcasses were excised, and, togetiier with the previously hot-boned muscles, subjected to the same procedural series as mat described above for hot-boned muscles at the time postexcision. Both groups of the deboned muscles were men placed on foil-lined cookie sheets and baked uncovered at 177 C for 45 min in a rotary oven. After allowing the cooked meats to cool at room temperature for about 1 h, caliper measurements and outlines were made again. Corresponding to me same time of linear measurements and area tracings made on one pectoralis muscle, me muscle or cooked meat of the other side was sampled along the line of action for the muscle, as depicted by Raikow (1985). Sarcomere lengms of the samples were measured by the method of laser diffraction described by Cross et al. (1981), with some modification (Papa and Fletcher, 1988). The extent of sarcomere shortening was based on a mean "intact" value, 2.26 u., obtained at the beginning of each trial of the experiment by sampling the muscles deboned immediately postmortem from 10 birds receiving an intravenous injection of 200 mg sodium iodoacetate/kg bird (to arrest glycolysis) 3 to 5 min before slaughter (de Fremery, 1966). Texture of the cooked meat was objectively evaluated by both compression (Texture Profile Analysis, TPA) and shearing. Three locations of the muscle, designated cranial, middle, and caudal (c/., anterior to posterior, Papa and Fletcher, 1988) were evaluated for the TPA attributes of hardness, cohesiveness, and chewiness as described by Friedman et al. (1963). Cores of the cooked meats, 2.54 cm diameter, were removed from each location of the muscle and evaluated using die techniques
CHICKEN BREAST MUSCLE AND MEAT SHORTENING
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Downloaded from http://ps.oxfordjournals.org/ by guest on May 27, 2015 FIGURE 1. Empirically modeled figures of tiie pectoralis muscle of broiler chickens at different stages of processing: (A) represents the muscle attached to normal skeletal restraints on the carcass. The progressive shortening along selected lengths (Lines 1 and 2) and loss of area are illustrated for the muscle after deboning immediately postmortem (B), after chilling for 24 h in ice/water slush (C), and then after cooking (D). The loss of muscle area was related to the shortenings of Lines 1 and 2 by the use of certain assumptions.
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TABLE 1. Selected evaluations o/pectoralis muscle and meat shortening of muscle deboned from carcasses of broiler chickens either immediately postmortem (hot-boned) or after 24 h of aging on the carcass (intact) Intact muscle2
Hot-boned muscle.1 Measure
Postexcision
Line 1 10.3"b Line 2 1.9° 7.4b0 Muscle length Area 16.3' Sarcomere lengthsi 8.7'b
Postchill
Postcook
Postexcision
Postcook
21.0" 8.0b 11.9*b 19.7' 19.2'
31.7C 24.3d 26.4" 42.0* 36.4"
8.6b .8° 7.0b 9.7" 13.9"
22.2* 20.0° 24.3bc 36.5" 32.5""
'"''Means in the same column with no common superscripts are significantly different (P<.05). 'For each mean, n = 44 except those of sarcomere lengths (for which n = 15), at each time postmortem. ^ o r each mean at each time-postmortem, n = 25 except those of sarcomere lengths (for which n = 13 and n = 12, at the times postexcision and postcook, respectively).
RESULTS AND DISCUSSION
Results in Table 1 indicate that the extent of shortening observedpostexcision along Line 1,
10.3%, agreed well with the results reported by Smith et al. (1969), 10.4 and 11.8%. The use of Line 1 was a direct attempt in the present study to repeat the specific measurement made by those authors. The shortenings of hot-boned muscle determined for this sampling time were highly variable, as indicated by the general lack of significant difference between rather widely separated means. For all of the sampling times at which hotboned muscles were evaluated, die extent of shortening observed along Line 2 was significantly less than that observed along Line 1, as area loss, and as sarcomere contraction (Table 1). Because Line 2 was roughly parallel to the tendinous partition that separates the medial belly from the lateral fasciculus, it was not parallel to the longitudinal axis of the muscle fibers as they converge onto the partition. Line 1 was more nearly parallel to the direction of the longitudinal axis of the muscle fibers, and sarcomere contraction is inextricably linked to decreases in the length of that axis. The shortening of Line 2 was not significandy different from that observed as the change in overall length of the muscle at any of the sampling times. The measurement of overall length represented a slight displacement and extension of the measurement of Line 2 (Figure 1). Because of the displacement, the line representing the overall length had some shared aspect not only with Line 2, but also, to a lesser degree, with Line 1. Taking this into account might explain why the extent of shortening observed along the overall length was a value between those observed for Lines 1 and 2, at all of the sampling times. Furthermore, the difference in
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and conditions described by Lyon et al. (1984). Between the locations of the three removed cores, two strips of the meat were cut, 1.27 cm in width, and tested for WarnerBratzler shear value (Chatillon Model No. 1901-S, John Chatillon & Sons, New York, NY). No effort was made to account for the unequal heights of sample strips. The peak load required to shear each strip once was recorded in kilograms. Two trials of the experiment were conducted. The first used a total of 40 birds, 30 allocated to the hot-boning treatment and 10 to aging muscles on the carcass. The second trial used a total of 30 birds, IS allocated to each treatment. The percentage shortening of gross muscle lengths and area lost were calculated on the basis of the "intact" value measured for each muscle; in exception to this, sarcomere contraction was based on a common value, as described above. The resulting values (almost entirely between 0 and 40%) were subjected to arcsin transformation, for statistical analyses (Snedecor and Cochran, 1980). An analysis of variance with main effects of treatment (measure of shortening of location of muscle) and trial was conducted using the general linear model, as programmed by SAS (1982). Hypotheses were tested using the mean square between replications (i.e., birds) nested within trial as the error term. This same, conservative term for error was specified when separating means by Duncan's multiple range test.
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TABLE 2. Objective evaluations of the textural quality of cooked meat sampled from different intramuscular locations of the pectoralis muscle deboned from broiler chickens either immediately postmortem or after 24 h of aging on the carcass Texture profile analysis' Processing scheme
Muscle location
Hardness (h)
Cranial
(kg) 6.7*
(aA)
Deboned immediately postmortem and chilled (n=44) Intact through 24 hr chill and then deboned (n=25)
.47
(h x c) 3.1
Middle
6.1"
.49
3.0
Caudal Cranial
6.0b 5.4"
.49 .52
2.9 2.8"
Middle
6.1 ,b
.51
3.1"
Caudal
6.8"
.53
3.6"
Cohesiveness (c)
Chewiness
Warner-Bratzler shear value (kg) 8.9' 5.6"
3.4 3.5
orientation of a measurement may explain the disagreement between the lower results for muscle shortening presented by Dickens and Papa (1988), using an overall muscle length, and the higher results reported by Smith et al. (1969), using a measurement comparable to that used for Line 1. In order to quantify the full extent of postmortem shortening for poultry pectoralis muscle, measurement directed along the fiber length would seem to be necessary. For hot-boned samples, the muscle shortening observed postcook relative to that observed postchill demonstrated two-fold increases in the shortening of overall length and loss of muscle area and a three-fold increase in shortening along Line 2 (Table 1). The minimal shortening along Line 2 until cooking may involve a structural aspect of pectoralis muscle: the muscle is much thicker in the general area of the humeral insertion in comparison with the caudal areas associated with muscle origin along the carina of the sternum and the sternal ribs. Shrinkage due to cook would presumably register a greater effect in the thinner, caudal area, much of which is associated with the shortening along Line 2. To a large extent, shrinkage due to cook involved thermally induced denaturation of the myofibrillar proteins. However, to some extent, this final shrinkage also involved the networks of connective tissues in the muscle,
the most prominent being that of the tendinous partition, along which Line 2 was directed. Collagen shrinkage may be expected, at least in part, to effect some shortening in the length of the meat (Winegarden et al., 1952) when internal temperatures are elevated to 60 C and above (Machlik and Draudt, 1963). Shortening of muscle aged on the carcass (intact) followed a pattern similar to that of hot-boned muscle, with one notable exception. For intact muscles at the time postexcision (i.e., postchill also), the extent of sarcomere contraction was significantly greater than the shortening along Line 1 and area loss (Table 1). In contrast, for the hot-boned group, there was no significant difference between these parameters, and the numerical pattern was exactly opposite. Substantially shortened sarcomeres have been previously reported for broiler pectoralis muscle aged intact and sampled at an anterior (cranial) location (Papa and Fletcher, 1988). In the present study, the muscle was also shortened at the gross level, as evidenced by the shortening of Line 1 at this time. Objective evaluations of the textural quality of the cooked meats are presented in Table 2. Hardness values (TPA) for the cranial location of hot-boned muscle were significantly greater than those of the other locations. Values of cohesiveness and chewiness, however, were not significantly different for any locations in
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••"Means within the same column and processing scheme with no common superscripts are significantly different (P<.05). 'Hardness=the peak load required to initially compress the core to 70% of its original height (maximum height of the first compression curve); cohesiveness = the ratio of positive force areas under the first and second compression curves (a^a,); chewiness = the number resulting from the multiplication together of the values for hardness and cohesiveness.
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PAPA AND LYON
to be 50°; in a previous report (Papa and Fletcher, 1988), a figure of the muscle that was excised in a prerigor state, and drawn from a photograph, shows a similar angle. In Figure IB to D, the percentage shortening along Line 1 is imposed on the circles drawn in Figure 1(A) and the distance between their centers; shortening along Line 2 is imposed on the straight line of the figure's left side. The remaining lines of the figure are then determined, and the area is calculated and expressed on the basis of A = 100 cm2. Postexcision (B). When the muscle is deboned immediately postmortem, shortening along Line 1 averages nearly 10%, whereas that along Line 2 is only 2%, and the resulting area loss is still quite substantial, about 16% (Table 1). Compared with a circle with r = 5.64 cm and area = 100 cm2, a decrease in r by 10% results in about 20% loss of area. For the drawing of the muscle in Figure 1 (B), the radii and the distance between centers are decreased by 10%; the straight left side is decreased by only 2%. The result is a muscle width of 7.2 cm and area of 84 cm2, in close agreement with the experimental results. The dashed line surrounding B indicates the area of the intact muscle; in relation to the figure of B, it shows that shortening due to excision is represented in the model as an effect on almost all of the muscle, except for the observed general lack of an effect along Line 2. Postchill (C). In order for the shortening observed postchill along Lines 1 and 2 to be consistent with the loss of area, a whole muscle effect could not be considered. As drawn, the loss of area is imposed only on the left side, top and bottom of the figure; i.e., shortening in this instance proceeds only in a direction toward the insertion. As drawn, the radii of the circles are decreased by another 10% (the added shortening along Line 1), but the distance between centers is not decreased. Given the ordinary treatment of shortening along Line 2, the model shows a muscle of 7 cm in width and an area loss of 22%. The loss of area observed in the experiment, about 20%, is somewhat less than that shown in the model. Postcook (D). Shortening in this figure again assumed a "whole muscle" effect. The radii are shortened by about 30%. An overall effect is imposed equally on the top and both straight sides of the figure, resulting in a final muscle width of about 6.4 cm. The length of
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this treatment group. The Warner-Bratzler shear values for two locations in the muscle resulted in the same general pattern as that observed for values of hardness. For muscles aged intact, hardness and chewiness values of the caudal location were significantly greater than those of the cranial location, the reverse of the results obtained for the hot-boned group. The thin, caudal region of the pectoralis muscle might be expected to harden more on cooking, and thus produce these results observed for the group aged intact. The fact that an opposite pattern was displayed for the hotboned group is not consistent with this structural consideration, however. Smith et al. (1988) have conducted a more extensive investigation of locational differences in texture, and obtained results comparable to those reported here. The present study attempted to demonstrate that a rather wide range of results could be obtained when evaluating the postmortem shortening of poultry pectoralis muscle. The observed differences may have been due to a simple change in orienting the measurement, but this simple change would seem to involve the somewhat complex structure of the muscle. Because of this structural aspect, linear measurements and calculated shortenings for the hot-boned group were able to be related to the loss of muscle area by an empirical model, as shown in Figure 1. Intact (A). The beginning figure is drawn as a composite shape, consisting of two overlapping circles (in the area of Line 1), both with a radius (r) of 2.75 cm, and centers separated by 3.5 cm. The straight, vertical sides are set 8 cm apart; the straight line on the left side of the figure is 13 cm in length. The calculated area of the resulting figure is 107.4 cm2, but, for the sake of convenience and as the basis for subsequent expressions, a beginning area of 100 cm2 is assumed. The area actually observed for the intact muscles in this study was 97.8 ± 1.2 cm2 (x ± SEM, n = 69); the median area was 98.4 cm2. The figure of the intact muscle [1(A)] also assumes a 45° angle for the line of lateral departure from the caudal end of the sternum. When the muscle is deboned, especially in a prerigor state, the angle along this line increases, perhaps a result of mechanical contraction of the muscle (Jungk and Marion, 1970) or loss due to trimming or both. In Figure IB to D, the increased angle is assumed
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ACKNOWLEDGMENTS
The authors wish to thank R. Wilson for counseling in the areas of experimental design and statistical analyses, R. Vaughn for arranging the means by which muscle areas were measured, and F. Gillen for providing
very capable phases.
technical
assistance
in
all
REFERENCES Cross, H. R., R. L. West, and T. R. Dutson, 1981. Comparison of methods for measuring sarcomere length in beef semitendinosus muscle. Meat Sci. 5:261-266. de Fremery, D., 1966. Relationship between chemical properties and tenderness of poultry muscle. J. Agric. Food Chem. 14:214-217. Dickens, i. A., and C. M. Papa, 1988. A procedure for measuring excised muscle contraction. J. Food Sci. 53: 1056-1057, 1061. Friedman, H. H., J. E. Whitney, and A. S. Szczesniak, 1963. The texturometer-a new instrument for objective texture measurement. J. Food Sci. 28:390-396. Hudson, G. E., and P. J. LanziUotti, 1964. Muscles of the pectoral limb in galliform birds. Am. Midi. Nat. 71: 1-113. Jungk, R. A., and W. W. Marion, 1970. Post-mortem isometric tension changes and shortening in turkey muscle strips held at various temperatures. J. Food Sci. 35:143-145. Lyon, C. E., D. Hamm, J. P. Hudspeth, and F. H. Benoff, 1984. Effects of age and sex on the texture profile of hot stripped broiler breast meat. Poultry Sci. 63: 2508-2510. Machlik, S. M., and H. N. Draudt, 1963. The effect of hearing time and temperature on the shear of beef semitendinosus muscle. J. Food Sci. 28:711-718. Papa, C. M., and D. L. Fletcher, 1988. Pectoralis muscle shortening and rigor development at different locations within the broiler breast. Poultry Sci. 67:635-640. Raikow, R. J., 1985. Locomotor system. Pages 57-147 in: Form and Function in Birds. Vol. 3. A. S. King and J. McClelland, ed. Academic Press, London, England, U.K. SAS, 1982. SAS User's Guide: Statistics, 1982 ed. SAS Inst. Inc., Cary, NC. Smith, D. P., and D. L. Fletcher, 1988. Chicken breast muscle fiber type and diameter as influenced by age and intramuscular location. Poultry Sci. 67:908-913. Smith, D. P., C. E. Lyon, and D. L. Fletcher, 1988. Comparison of the Allo-Kramer shear and texture profile methods of broiler breast meat texture analysis. Poultry Sci. 67:1549-1556. Smith, M. C , M. D. Judge, and W. J. Stadelman, 1969. A "cold shortening" effect in avian muscle. J. Food Sci. 34:42-46. Snedecor, G. W., and W. G. Cochran, 1980. Chapter 15. Failures in the assumptions. Pages 274-297 in: Statistical Methods. The Iowa State Univ. Press, Ames, IA. Vanden Berge, J. C , 1979. Myologia. Pages 175-219 in: Nomina Anatomica Avium. J. J. Baumel, A. S. King, A. M. Lucas, J. E. Breazile, and H. E. Evans, ed. Academic Press, London, England, U.K. Winegarden, M. W., B. Lowe, J. Kastelic, E. A. Kline, A. R. Plagge, and P. S. Shearer, 1952. Physical changes of connective tissues of beef during heating. Food Res. 17:172-184.
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Line 2 reflects the 20% observed for its shortening in the experiment, the contribution to area loss now becoming a major effect. The area loss suggested by the model, about 41%, is somewhat greater than that actually observed, about 36%. The general relationship depicted in these figures between the shortenings of Lines 1 and 2 and loss of muscle area can be considered the direct result of the complex structure of m. pectoralis. Area loss would seem to be substantial because of the muscle having such an extensive origin, an arrangement that makes a major contribution to the general proportions of the body, and is probably functionally important to the mode of flight (Raikow, 1985). The fact that different lines of measurement give different results for the extent of postmortem shortening emphasizes that aspect of the muscle characterized as a modified bipennate form, with different orientations of the muscle fibers as they converge onto the tendinous partition and toward the insertion. In order to quantify the full extent of postmortem shortening when using the whole muscle, the present study has demonstrated that gross measurements must be directed along the length of the fibers' longitudinal axis (c/. Smith et al., 1969)-the common procedure when evaluating muscle strips. Measurements directed along the length of the longitudinal axis for the whole muscle (c/. Dickens and Papa, 1988) cannot quantify the full extent of pectoralis muscle shortening, and therefore the results of such measurements would seem to be potentially misleading. No effort was given to constructing a model for the shortening of the muscle aged intact on the carcass; that model would involve only the results of shortening observed as a function of "isometric" contractions during aging, and then thermally induced shrinkage due to cook. That is, such a model would ignore, by necessity, effects related to the case of isotonic contractions of prerigor-excised muscle.