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Tenderness of pork muscles as influenced by chilling rate and altered carcass suspension
Meat Science 21 (1987] 275-286
Tenderness of Pork Muscles as Influenced by Chilling Rate and Altered Carcass Suspension
A. J. Moiler, E. Kirkegaard Royal Veterinary and Agricultural University, Institute of Meat Technology and Process Engineering, Howitzvej 11, DK 2000 Copenhagen F, Denmark
& T. Vestergaard National Institute of Animal Science, Postbox 39, DK 8833 Orum Sonderlyng, Denmark (Received 13 October 1987; revised version received 17 December 1987; accepted 18 December 1987)
A BS TRA CT Tenderness improvements in porcine muscles (M. longissimus dorsi, LD; M. sernimembranosus, SM; M. biceps femoris, BF) were evaluated in a total of 72 carcasses by using combinations of three different chilling rates (fast, delayed fast, slow) and two different suspension methods (Achilles tendon, pelvic bone). Tenderness was improved by fast chilling in LD, S M and BF by the pelvic suspension as compared to conventional suspension in the Achilles tendon (P < 0"05). The lengthening of the sarcomeres in S M and BF as produced by pelvic suspension exceeded those found in LD, without having proportional additional effect on the tenderness. While the pelvic-induced tenderization did not change significantly by delayed fast chilling, additional tenderization in BF and S M was obtained by combining pelvic suspension with slow chilling. In conventionally suspended sides, tenderness was unaffected by delayed fast chilling--with slow chilling, however, improvements were observed in LD and S M to a similar extent as obtained by the pelvic suspension. In the LD muscle, the tenderizing effect produced by treatments was largest in muscles having pH values 45 rain post stunning above 6"1 (P < 0"05). 275 Meat Science 0309-1740,/87/$03-50 © 1987 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain
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A. J. Moiler, E. Kirkegaard, T. Vestergaard
INTRODUCTION Prevention of muscle and sarcomere shortening during the onset of rigor mortis is effective in improvement of meat tenderness (Locker, 1960; Marsh & Leet, 1966). However, the relationship of toughness to sarcomere shortening has mainly been characterized on pre-rigor excised muscles (Marsh & Leet, 1966; Herring et al., 1967; Marsh & Carse, 1974). The extent of muscle shortening in a carcass depends on the physical restrictions imposed by their attachments to the skeleton. Reduced muscle shortening and improved tenderness of major beef muscles has been obtained by suspending carcasses from the pelvic bone as opposed to the Achilles tendon (Hostetler et al., 1970; Smith et al., 1971; Bouton & Harris, 1972). Also pork improves in tenderness by pelvic suspension, as shown recently for the M. longissimus dorsi (M~ller & Vestergaard, 1986). Sarcomere shortening due to cold contracture, i.e. the ability of pre-rigor excised muscles with high ATP levels to contract when exposed to temperatures below 10°C, was described by Locker & Hagyard (1963). Toughness due to cold-induced sarcomere shortening has been well characterized in beef (Marsh & Leet, 1966). In pork, however, cold shortening has been considered as a minor problem due to the more rapid postmortem glycolysis and the protection afforded to the muscles by the insulating layer of subcutaneous fat. In pre-rigor excised porcine muscles, cold contracture does occur (Galloway & Goll, 1967), but less forcefully as compared with beef (Bendall, 1975) and apparently at lower temperatures (Fischer et al., 1980). There are relatively few studies on the toughness of rapidly chilled pork, and the existing results are somewhat conflicting. Weiner et al. (1966) showed that freezing pork loins removed from carcasses at 1 h post mortem increased toughness. A study by Moore et al. (1966) using very rapid chilling rates on pork loins did not show any significant amount of toughening. Dransfield & Lockyer (1985) reported that toughness of excised longissimus dorsi muscles increased if cooling to 10°C took place within 3 h post stunning. James et al. (1983) showed that partially frozen loins during rapid chilling were more tender than those that had not frozen. Gigiel et al. (1984) showed that rapid chilling without ageing produced tougher pork than conventional chilling. Moller& Vestergaard (1987) found the potential of cold-induced toughening in the porcine longissimus dorsi muscle to be significantly influenced by the rate or extent of postmortem glycolysis, which also appeared to have a substantial effect on the relationship between Warner-Bratzler shear force and sarcomere length. The purpose of this study was to extend our previous study (Moiler & Vestergaard, 1986) by including muscles from the hind leg in order to
Tenderness of pork as influenced by chilling rate and carcass suspension
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investigate the potential of tenderness improvement by pelvic suspension separately and in combination with different chilling rates of carcasses showing low or high pH value in M. longissimus dorsi prior to chilling.
MATERIALS AND METHODS
Experimental design and sampling procedure The investigations were performed by using 72 carcasses of Danish Landrace or Yorkshire breeds with mean backfat thickness 18.1 m m (range 13 to 23mm). The pigs, reared at the same progeny testing station to approximately 90kg live weight, were stunned with carbon dioxide and slaughtered conventionally. At 45 min post stunning the pH was measured in M. longissimus dorsi at the 15th rib. This measure was used to allocate the 72 carcasses to two groups of 36 each, according to low pH value (5.7 < pH < 6.1) or high pH value (6-I < pH < 6-5) 45 min post stunning. The right sides of the carcasses from each pH group were randomly assigned to five treatment groups with different combination of suspension method (Achilles tendon or pelvic bone) and chilling procedure (fast, delayed fast or slow) as shown in Table 1. The left sides were used as control carcasses, i.e. they were normally suspended in the Achilles tendon and chilled by the fast procedure. In the pelvic suspension procedure a stainless steel hook was inserted through the obturator forarmen with the limbs hanging free. Fast chilling was obtained by transferring the sides immediately after dressing through a conveyerized chilling tunnel operating at - 1 8 ° C , 3-0m/s for 65rain, and then placed in a chilling r o o m at 2--4°C, 0-2 m/s, until 24 h post stunning. TABLE 1 Experimental Design and Number of Sidesa Carcass suspension
Achilles tendon Pelvic bone
Chilling procedure Fast
Delayed fast
Slow
72 (control) 24
12
12
12
12
Number of sides in each group of treatment were equally divided into two groups according to low (5.7 < pH < 6"1)or high (6.1_
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A.J. Moller, E. Kirkegaard, T. Vestergaard
Delayed fast chilling was obtained by inserting a 4-h delay time in the chilling room at 2-4°C before the sides were transferred to the chilling tunnel. Slow chilling was obtained by keeping the sides in the chilling room at 2-4°C throughout the 24-h chilling period. At 24 h post stunning, the following three muscles were excised from each carcass: M. longissimus dorsi (LD) at the position of the 12th to 15th rib, M. sernimembranosus (SM) and M. biceps femoris (BF). Subsamples from the excised muscles were cut from the same location within each of the muscles. The subsamples were individually vacuum packed and kept at - 2 0 ° C for 8-12 weeks until further analysis.
pH and temperature In addition to the initial pH measurement at 45 min post stunning, pH was measured also in the LD muscle at 1, 3, 5, 8 and 24 h post stunning using a Knick Digital model 653 and a direct insertion probe electrode (Ingold Lot 406-M3). At the same intervals, temperature was measured using a Technoterm 1500 inserted into the loin centre. In the SM and BF muscles, only temperature during the initial chilling period was registered.
R value The R value was determined in the LD muscle from 12 slow and fast chilled sides, respectively. The samples were removed at 1.5, 4 and 8 h post stunning and used as an indicator of the ATP breakdown in postmortem muscle. The R-value procedure (Honikel & Fischer, 1977) involved homogenization of 2 g of muscle in i0 ml of 1M perchloric acid. The homogenate was filtered and 0-1 ml of the filtrate was diluted into 4.9 ml 0"IM phosphate buffer, pH 7.0. The absorbance at 250 nm (IMP) and 260 nm (ATP) was measured, and R value calculated as the ratio 250/260.
Sareomere length A 1-cm thick cross-section of each muscle was cut from each subsample, and four 1-cm 3 cubes of meat were removed. Two sections from each cube were cut 100/~m thick with longitudinal orientation of the muscle fibres, placed on a glass slide and covered with a cover slip. A helium-neon laser with a wavelength of 632-8 nm was used for measurements of sarcomere lengths (Voyle, 1971). The average sarcomere length for each muscle was calculated on the basis of 20 measurements.
Tenderness of pork as influenced by chilling rate and carcass suspension
279
Warner-Bratzler (WB) shear force measurements Subsamples were thawed and four blocks (20 x 20 x 50 mm), with the length cut parallel to the muscle fibres, were removed. The blocks were individually placed in 100-ml glass tubes and a 0-9% NaC1 solution added to just cover the meat blocks. The tubes were heated in a waterbath at 80°C for 25 min. The meat blocks were then rinsed with cold tap water and immediately placed in the refrigerator at 2-4°C for no longer than 24 h. The weight of the blocks before and after heating was noted for calculating the cooking loss. Samples of rectangular cross-section (10 x 10ram) were cut from the cooked meat blocks and sheared at right angles to the fibre axis, using a Warner-BratzIer shear blade with a triangular slot angular cutting edge. An Instron Universal testing machine table model 1140 was used to measure the peak shear forces (kg/cm-'). Three shear force values were determined on each meat block, giving a total of 12 measurements on each muscle.
Statistical methods As the sides of each carcass have undergone the same pre-experimental treatment, they can be considered as paired observations. The effect of treatment and pH group on differences between treated and control sides can thus be analysed by means of the following statistical model: Yijk = tl "[- Tr i +
pHj + ( Tr x pH)/j + eij k
where Y~jk= the observed differences between right and left sides of the kth carcass in the jth pH group and the i TM treatment; T r i = the effect of the i TM treatment, i = 1, 2, 3,4, 5; pH~ = the effect of the jth group, j = 1, 2; ( T r x pH)ij = the effect of the 0"th interaction between T r and pH; and e~Sk = the random residual effect. RESULTS
Temperature, pH and R value The temperature decline in the LD muscle from 1 to 8 h post stunning is shown in Fig. I. The fast chilling procedure cools the LD below 10°C within 3 h post stunning while the temperature is only decreased to about 20°C by delayed fast or slow chilling treatments. The delayed fast procedure resulted in the lowest temperature (0-1°C) at 8 h post stunning. At 24 h post stunning, the temperature had equalized to 3-4°C in all sides. In the SM and BF muscles, temperature below 10°C was observed at approximately 8 h post stunning. For the fast, delayed fast and slow chilling procedure, the temperatures at 8 h post stunning were, respectively, 8.6 (0-18), 3.0 (0.58) and
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20 E 15 10
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I 3
I 5
[ 8
H post stunning Fig. 1. Temperature in M. longissimus dorsi during the period 1 to 8 h post stunning. Chilling procedure: • = fast, • = delayed fast and • = slow. (Within group of treatment. standard error varied between 0"19 and 0.55.)
High pH group 6.4
Low pH group
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6"2
1.3
6.4 -
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6.0
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5.6
~.9
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ov'
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,1 I 3 5 H post stunning
I 8
1
3 5 H post stunning
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Fig. 2. pH and R value in M./ongissimu~ dorsi during the period 1 to 8 h post stunning for carcasses belon~ng to the high (6"! < pH4s,,io < 6-5) respectively the low (5.7 < pHi,== < 6.1) pH group. Chilling procedure: • = fast, • = delayed fast and • = slow. Closed and open symbols are referring to pH and R value, respectively. (Within group of treatment, standard error varied between 0-025 and 0.090 for pH measurement; for R values between 0-016 and 0"073.)
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281
11-2 (0-32) in SM, and 9-6 (0-17), 6-7 (0-36) and 12-4 (0-31) in BF (parenthetical values are 1 standard error of the mean). The relationship of chilling procedure and rate of glycolysis is shown in Fig. 2. In both pH groups, the pH declines more rapidly at the delayed fast or slow chilling treatment as compared to the fast chilling. Accordingly, the R value is initially smaller during slow chilling, showing that the ATP content was higher during this period. Compared with the low pH group, the R value in the high pH group remains at a higher level through the initial chilling period. WB shear force
The effects of treatment and pH groups are analysed on differences in WB values between the treated and the control sides from each carcass according to the statistical model as described above. The analysis of variance of these data shows a significant effect of treatment in LD, SM and BF, whereas the effect ofpH group is only observed in the LD muscle (Table 2). Least-squares TABLE 2 Analysis of Variance for W B Shear Force, Sarcomere Length a n d C o o k i n g Loss as Indicated by Deviation from the C o n t r o l s
Muscle~source of variation
DF
Mean squares WB shear force
Sarcomere length
Cooking loss
3-915" 2.218* 1"381 "s 0-723
0-004 5 "s 0.036 9** 0"0266* 0.008 8
15"87 "2 1'6 l "~ 4"61 "S 4-82
1 4 4 62
0"294 "s 0-745* 0"071 ns 0-240
0-201 6** 0"538 6*** 0-098 0* 0"028 1
2"24~s 1-61 "~ 7-71 "s 6-02
I 4 4 62
0-440 "s 4.963*** 0-264 "s 0-605
0"565 3*** 0-834 5*** 0.107 5** 0-021 4
3"12n~ 8-56 ~" 4.02 "s 3"63
M, longissimus dorsi (LD) pH g r o u p (A) T r e a t m e n t (B) A x B Error
l 4 4 62
M. semimembranosus {SM) pH g r o u p (A) T r e a t m e n t (B) A x B Error
M. biceps femoris (BF) pH g r o u p (A) T r e a t m e n t (B) A x B Error
n s = n o t significant; *, ** a n d *** = s i g n i f i c a n t at 5%, 1% a n d 0.1% levels, respectively.
282
A. J. Moiler, 15. Kirkegaar~ T. Vestergaard TABLE 3 Effect o f T r e a t m e n t s o n W B S h e a r F o r c e (kg/cm-') as I n d i c a t e d by D e v i a t i o n f r o m the C o n t r o l s ~
Chilling procedure
Muscle~carcass suspension Fast
Delayed fast
Slow
--0"75 c
- 0,01 b -0-97 c
_ 1.03 c - 1.0V
-- 0"37 b'~
-0-09 h - 0-62 ~'a
- 0 . 3 9 b'a - 0"72 a
--0-60 a
0-42 b - 0 . 4 8 La
- 0 - 0 3 b'" - l'27 *
M. longissimus dorsi (LD) Achilles t e n d o n Pelvic b o n e
,~L semimembranosus (SM) Achilles t e n d o n Pelvic b o n e
M. biceps femoris (BF) Achilles t e n d o n Pelvic b o n e
L e a s t - s q u a r e s m e a n s e s t i m a t e d by the m o d e l (1). M e a n s w i t h i n muscle b e a r i n g different s u p e r s c r i p t s are significantly different ( P < 0 " 0 5 ) . U n d e r l i n e d figures are significantly different f r o m zero. M e a n s a n d s t a n d a r d d e v i a t i o n o f W B s h e a r values f r o m c o n t r o l sides for L D , S M a n d BF: 5 " 4 0 + 0 . 9 6 , 4-50_+0"67 a n d 5-17_+1'01, respectively.
means for the various groups of treatment are presented in Table 3, where underlined figures indicate significant deviation from zero at the level of 5%. Pelvic suspension, as opposed to conventional carcass suspension, improves tenderness regardless of chilling procedure (P < 0-05). The pelvic effect is of similar extent whether sides have been cooled by the fast or delayed fast chilling procedure. As compared to fast chilling, pelvic suspension combined with slow chilling caused further tenderization in SM and BF (P < 0.05). Based upon previous observations, differences in WB values above approximately 0.5 kg/cm 2 are likely to be marked by a trained taste panel. The Achilles tendon suspended sides were also unaffected in their WB values by using delayed fast chilling. However, the stow chilling procedure improved the WB values in LD and SM muscles from conventional fast chilled sides. This tenderization does not differ significantly from the tenderization observed in the pelvic suspended sides. Besides the effect of treatment, the main effects o f p H ~ o u p on WB values were analysed (Table 2). As shown, only WB values from LD were affected by the pH group (P < 0-05). Differences in WB values between treated and control sides were larger in the high pH group (1 s mean = - 1"04 kg/cm 2) as compared to the low pH group (i s mean = - 0.47 kg/cm2).
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283
TABLE 4 Effect of pH Group and Treatments on Sarcomere Length (/~m) as Indicated by Deviation from the Controls ~
Muscle/carcass suspension
Chilling procedure Fast
M. longisstmus dorsi (LD) Achilles tendon low pH group high pH group Pelvic bone low pH group high pH group M. sernimembranosus (SM) Achilles tendon low pH group high pH group Pelvic bone low pH group high pH group M. biceps femoris (BF) Achilles tendon low pH group high pH group Pelvic bone low pH group high pH group
Delayed fast
Slow
0.06 c.J _0.05 b
_ 0.002 b,c 0.09",J."
0"07¢'a
O"16d'~
0-08¢'a'~
0" 11 a.e
0 ' 0 9 ¢'d'~
0" 18"
O"10b'~ 0.002 b 0.2Y .d 0.53 e
0.28 c 0.62 a
0.33 d 0-35~
- 0"09b - 0.03 b 0.29 ~.a 0-36a
- 0.05 b
0.04 b
- 0.04 b
0.02 b
0.33 c 0-53J
0.30 ~ 0.52 d
Least-squares means estimated by the model (1). Means bearing different superscripts are significantly different (P < 0-05). Underlined figures are significantly different from zero. Means and standard deviation of sarcomere length from control sides for LD, SM and BF: 1.75 + 0"08, 1"77+ 0"09 and 1.77 +_0-08, respectively.
°
Sarcomere
length
Differences in s a r c o m e r e length b e t w e e n t r e a t e d a n d c o n t r o l sides are significantly affected b y p H g r o u p in the S M a n d B F muscle, a n d b y t r e a t m e n t in all t h r e e m u s c l e s (Table 2). As significant i n t e r a c t i o n s are evident, the l e a s t - s q u a r e s m e a n s o f t r e a t m e n t s f r o m e a c h p H g r o u p are s h o w n in T a b l e 4. I n c r e a s e d s a r c o m e r e l e n g t h is o b t a i n e d in all o f the pelvic s u s p e n d e d g r o u p s as c o m p a r e d to the c o n t r o l s ( P < 0-05), a n d this effect is a p p a r e n t l y n o t i n f l u e n c e d b y chilling t r e a t m e n t . T h e p e l v i c - i n d u c e d s a r c o m e r e l e n g t h e n i n g is largest in S M a n d BF, a n d t e n d e d to be larger f o r
284
A. J. Metier, E. Kirkegaard, T. Vestergaard
the muscles belonging to the high pH group. Neither delay fast nor slow chilling procedure affect sarcomere length in the SM and BF muscle from Achilles tendon suspended sides. However, the slow chilling procedure results in less sarcomere shortening in the LD muscles belonging to the high pH group.
Cooking loss As shown in Table 2, the cooking loss is unaffected by both pH group and treatment. Mean values of cooking loss in the LD, SM and BF muscles are 30.47% + 2.13, 30-03% + 1-99 and 31-98 + 3.06.
DISCUSSION The present findings that pelvic suspension significantly reduces WB shear values for LD and SM is in accordance with previous reports of improved tenderness of bovine and ovine muscles from utilization of this technique (Arrango et aL, 1970; Hostetler et al., 1971, 1972, 1973; Bouton & Harris, 1972; Bouton et aL, 1973; Joseph & Connolly, 1977; Jeremiah et aL, 1984). In this study also the BF muscle showed a substantial reduction in WB values, while BF from beef has shown only minor response to altered carcass suspension (Hostetler et al., 1975; Joseph & Connolly, 1977). Both tenderness and final state of muscle contraction are largely dependent upon the amount of strain on the muscle before and during rigor mortis (Locker, 1960). Results from the present study indicate that pelvic suspension, as opposed to conventional carcass suspension, increases the sarcomere lengths. However, the magnitude of the improvements in tenderness does not appear to have a simple relationship with sarcomere length changes. Thus, the sarcomere length changes due to pelvic suspension were much larger in SM and BF than in LD without producing proportional tenderness improvement. Regardless of suspension method, neither WB values nor sarcomere length were affected by using the delayed fast chilling procedure. When cooling is retarded even more, as in the slow chilling procedure, the LD maintained temperature above 10°C until 8 h post stunning. As a result of using the slow chilling, additional tenderness improvements were obtained in the BF and SM muscles from pelvic suspended sides (P < 0-05). Similarly, slow chilling improved tenderness in LD and SM from conventionally suspended sides (P<0-05), and this extent of tenderization did not differ from the pelvic effect. This result contrasts similar studies on beef, where pelvic suspension has been reported more effective in enhancing tenderness
Tenderness of pork as influenced by chilling rate and carcass suspension
285
than by using delayed or slow chilling rate (Hostetler et al., 1975; Joseph & Connolly, 1977; Jeremiah et al., 1984). The enhanced tenderness of slowly chilled meat has been attributed to the earlier onset of proteolytic breakdown of the myofibrillar structure (Smith et al., 1971; Parrish et al., 1973; Hostetler et al., 1975), but the LD muscle from the high pH group in this study did obtain an increase in sarcomere length (P < 0-05) when using the slow chilling rate for sides being suspended from the Achilles tendon. From earlier reports a value of pH 6"0 has been suggested as a useful level from where rapid cooling of a carcass can begin (Bendall et al., 1976). Pork muscles are well known to vary substantially in their pH decline post mortem, which consequently makes muscles showing a slow pH decline generally more prone to cold-induced toughening (Mailer & Vestergaard, 1987). This is supported from the present results, however, only for the LD muscle, which shows significantly larger effect of treatments on WB values in the high pH group as compared to LD muscles from the low pH group. Otherwise the pH group as a main effect did not affect tenderness in the various groups of treatments. It can be concluded from these results that slow chilling improves tenderness in the LD muscle from normal suspended sides and adds further tenderness improvements to SM and BF muscles from pelvic suspended sides. A slow chilling rate may not be suitable for factory use, owing to the risk of PSE development, higher bacterial counts and greater weight loss. Thus, pelvic suspension seems to be the most suitable method as improved tenderness can be obtained in a fast chilling system. But technically the pelvic method needs to be improved in order to be adopted by the industry. ACKNOWLEDGEMENTS This work was supported by funds from the Danish Agricultural and Veterinary Research Council. Appreciation is expressed to Mrs M. Neumann and Mrs A. Preusse for their skilled technical assistance. We thank the staff at Tulip Odense for their kind co-operation. REFERENCES Arrango, T. C., Smith, G. C., Carpenter, Z. L. & Cross, H. R. (1970). J. Anita. Sci., 31, 180 (Abstr.). Bendall, J. R. (1975). J. Sci. Food Agric., 26, 55. Bendall, J. R., Ketteridge, C. C. & George, A. R. (1976). J. Sci. FoodAgric., 27, 1123. Bouton, E E. & Harris, E V. (1972). J. Food Sci., 37, 539. Bouton, P. E., Harris, P. V., Shorthose, W. R. & Baxter, R. I. (1973). J. FoodSci., 38, 932.
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Dransfield, E. & Lockyer, D. K. (1985). Meat Sci., 13, 19. Fischer, C., Honikel, K. O. & Hamm, R. (1980). Fleischwirtsch., 60, 263. Galloway, D. E. & Goll, D. E. (1967). J. Anim. Sci., 26, 1302. Gigiel, A. J. & James, S. J. (1984). Meat Sci., 11, 1. Herring, H. K., Cassens, R. G., Suess, G. C., Brungardt, V. H. & Briskey, E. J. (1967). J. Food Sci., 32, 317. Honikel, K. O. & Fischer, C. (1977). J. Food Sci., 42, 1633. Hostetler, R. L., Landmann, W. A., Link, B. A. & Fitzhugh. H. A. Jr (1970). J. Anita. ScL, 31, 47. Hostetler, R. L., Link, B. A., Landmann, W. A. & Fitzhugh, H. A. Jr (1971). Beef Cattle Res. in Texas PR-2988, 80. Hostetler, R. L., Link, B. A., Landmann, W. A. & Fitzhugh, H. A. Jr (1972). J. Food Sci., 37, 132. Hostetler, R. L., Link, B. A., Landmann, W. A. & Fitzhugh, H. A. Jr (1973). J. Food Sci., 38, 264. Hostetler, R. L., Carpenter, Z. L., Smith, G. C. & Dutson, T. R. (1975). J. FoodSci., 40, 223. James, S. J., Gigiel, A. J. & Hudson, W. R. (1983). Meat Sci., 8, 63. Jeremiah, L. E., Martin, A. H. & Achtymichuk. G. (1984). J. Food Qual., 6, 259. Joseph, R. L. & Connolly, J. (1977). J. Food Technol., 12, 231. Locker, R. H. (1960). Food Res., 25, 304. Locker, R. H. & Hagyard, C. J. (1963). J. Sci. Food Agric., 14, 787. Marsh, B. B. & Carse, W. A. (1974). J. Food TechnoL, 9, 129. Marsh, B. B. & Leet, N. G. (1966). J. Food Sci.. 31, 450. Moore, R. E., Mandigo, R. W. & Henrickson, R. L. (1966). Food Technol., 29, 957. Maller, A. J. & Vestergaard, T. (1986). Meat Sci., 18, 77. Mailer, A. J. & Vestergaard, T. (1987). Meat Sci., 19, 27. Parrish, E C. Jr, Young, R. B., Miner, B. E. & Anderson, L. D. (1973). J. FoodSci., 38, 690. Smith, G. C., Arrango, T. C. & Carpenter, Z. k. (1971). J. Food Sci., 36, 445. Voyle, C. A. (1971). Proc. 17th European Meeting of Meat Res. Workers, Bristol. England, 95. Weiner, P. D., Kropf, D. M., Macintosh, D. L. & Koch, B. A. (1966). Food Technol., 20, 541.
Tenderness of Pork Muscles as Influenced by Chilling Rate and Altered Carcass Suspension
A. J. Moiler, E. Kirkegaard Royal Veterinary and Agricultural University, Institute of Meat Technology and Process Engineering, Howitzvej 11, DK 2000 Copenhagen F, Denmark
& T. Vestergaard National Institute of Animal Science, Postbox 39, DK 8833 Orum Sonderlyng, Denmark (Received 13 October 1987; revised version received 17 December 1987; accepted 18 December 1987)
A BS TRA CT Tenderness improvements in porcine muscles (M. longissimus dorsi, LD; M. sernimembranosus, SM; M. biceps femoris, BF) were evaluated in a total of 72 carcasses by using combinations of three different chilling rates (fast, delayed fast, slow) and two different suspension methods (Achilles tendon, pelvic bone). Tenderness was improved by fast chilling in LD, S M and BF by the pelvic suspension as compared to conventional suspension in the Achilles tendon (P < 0"05). The lengthening of the sarcomeres in S M and BF as produced by pelvic suspension exceeded those found in LD, without having proportional additional effect on the tenderness. While the pelvic-induced tenderization did not change significantly by delayed fast chilling, additional tenderization in BF and S M was obtained by combining pelvic suspension with slow chilling. In conventionally suspended sides, tenderness was unaffected by delayed fast chilling--with slow chilling, however, improvements were observed in LD and S M to a similar extent as obtained by the pelvic suspension. In the LD muscle, the tenderizing effect produced by treatments was largest in muscles having pH values 45 rain post stunning above 6"1 (P < 0"05). 275 Meat Science 0309-1740,/87/$03-50 © 1987 Elsevier Applied Science Publishers Ltd, England. Printed in Great Britain
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A. J. Moiler, E. Kirkegaard, T. Vestergaard
INTRODUCTION Prevention of muscle and sarcomere shortening during the onset of rigor mortis is effective in improvement of meat tenderness (Locker, 1960; Marsh & Leet, 1966). However, the relationship of toughness to sarcomere shortening has mainly been characterized on pre-rigor excised muscles (Marsh & Leet, 1966; Herring et al., 1967; Marsh & Carse, 1974). The extent of muscle shortening in a carcass depends on the physical restrictions imposed by their attachments to the skeleton. Reduced muscle shortening and improved tenderness of major beef muscles has been obtained by suspending carcasses from the pelvic bone as opposed to the Achilles tendon (Hostetler et al., 1970; Smith et al., 1971; Bouton & Harris, 1972). Also pork improves in tenderness by pelvic suspension, as shown recently for the M. longissimus dorsi (M~ller & Vestergaard, 1986). Sarcomere shortening due to cold contracture, i.e. the ability of pre-rigor excised muscles with high ATP levels to contract when exposed to temperatures below 10°C, was described by Locker & Hagyard (1963). Toughness due to cold-induced sarcomere shortening has been well characterized in beef (Marsh & Leet, 1966). In pork, however, cold shortening has been considered as a minor problem due to the more rapid postmortem glycolysis and the protection afforded to the muscles by the insulating layer of subcutaneous fat. In pre-rigor excised porcine muscles, cold contracture does occur (Galloway & Goll, 1967), but less forcefully as compared with beef (Bendall, 1975) and apparently at lower temperatures (Fischer et al., 1980). There are relatively few studies on the toughness of rapidly chilled pork, and the existing results are somewhat conflicting. Weiner et al. (1966) showed that freezing pork loins removed from carcasses at 1 h post mortem increased toughness. A study by Moore et al. (1966) using very rapid chilling rates on pork loins did not show any significant amount of toughening. Dransfield & Lockyer (1985) reported that toughness of excised longissimus dorsi muscles increased if cooling to 10°C took place within 3 h post stunning. James et al. (1983) showed that partially frozen loins during rapid chilling were more tender than those that had not frozen. Gigiel et al. (1984) showed that rapid chilling without ageing produced tougher pork than conventional chilling. Moller& Vestergaard (1987) found the potential of cold-induced toughening in the porcine longissimus dorsi muscle to be significantly influenced by the rate or extent of postmortem glycolysis, which also appeared to have a substantial effect on the relationship between Warner-Bratzler shear force and sarcomere length. The purpose of this study was to extend our previous study (Moiler & Vestergaard, 1986) by including muscles from the hind leg in order to
Tenderness of pork as influenced by chilling rate and carcass suspension
")
_7"7~
investigate the potential of tenderness improvement by pelvic suspension separately and in combination with different chilling rates of carcasses showing low or high pH value in M. longissimus dorsi prior to chilling.
MATERIALS AND METHODS
Experimental design and sampling procedure The investigations were performed by using 72 carcasses of Danish Landrace or Yorkshire breeds with mean backfat thickness 18.1 m m (range 13 to 23mm). The pigs, reared at the same progeny testing station to approximately 90kg live weight, were stunned with carbon dioxide and slaughtered conventionally. At 45 min post stunning the pH was measured in M. longissimus dorsi at the 15th rib. This measure was used to allocate the 72 carcasses to two groups of 36 each, according to low pH value (5.7 < pH < 6.1) or high pH value (6-I < pH < 6-5) 45 min post stunning. The right sides of the carcasses from each pH group were randomly assigned to five treatment groups with different combination of suspension method (Achilles tendon or pelvic bone) and chilling procedure (fast, delayed fast or slow) as shown in Table 1. The left sides were used as control carcasses, i.e. they were normally suspended in the Achilles tendon and chilled by the fast procedure. In the pelvic suspension procedure a stainless steel hook was inserted through the obturator forarmen with the limbs hanging free. Fast chilling was obtained by transferring the sides immediately after dressing through a conveyerized chilling tunnel operating at - 1 8 ° C , 3-0m/s for 65rain, and then placed in a chilling r o o m at 2--4°C, 0-2 m/s, until 24 h post stunning. TABLE 1 Experimental Design and Number of Sidesa Carcass suspension
Achilles tendon Pelvic bone
Chilling procedure Fast
Delayed fast
Slow
72 (control) 24
12
12
12
12
Number of sides in each group of treatment were equally divided into two groups according to low (5.7 < pH < 6"1)or high (6.1_
278
A.J. Moller, E. Kirkegaard, T. Vestergaard
Delayed fast chilling was obtained by inserting a 4-h delay time in the chilling room at 2-4°C before the sides were transferred to the chilling tunnel. Slow chilling was obtained by keeping the sides in the chilling room at 2-4°C throughout the 24-h chilling period. At 24 h post stunning, the following three muscles were excised from each carcass: M. longissimus dorsi (LD) at the position of the 12th to 15th rib, M. sernimembranosus (SM) and M. biceps femoris (BF). Subsamples from the excised muscles were cut from the same location within each of the muscles. The subsamples were individually vacuum packed and kept at - 2 0 ° C for 8-12 weeks until further analysis.
pH and temperature In addition to the initial pH measurement at 45 min post stunning, pH was measured also in the LD muscle at 1, 3, 5, 8 and 24 h post stunning using a Knick Digital model 653 and a direct insertion probe electrode (Ingold Lot 406-M3). At the same intervals, temperature was measured using a Technoterm 1500 inserted into the loin centre. In the SM and BF muscles, only temperature during the initial chilling period was registered.
R value The R value was determined in the LD muscle from 12 slow and fast chilled sides, respectively. The samples were removed at 1.5, 4 and 8 h post stunning and used as an indicator of the ATP breakdown in postmortem muscle. The R-value procedure (Honikel & Fischer, 1977) involved homogenization of 2 g of muscle in i0 ml of 1M perchloric acid. The homogenate was filtered and 0-1 ml of the filtrate was diluted into 4.9 ml 0"IM phosphate buffer, pH 7.0. The absorbance at 250 nm (IMP) and 260 nm (ATP) was measured, and R value calculated as the ratio 250/260.
Sareomere length A 1-cm thick cross-section of each muscle was cut from each subsample, and four 1-cm 3 cubes of meat were removed. Two sections from each cube were cut 100/~m thick with longitudinal orientation of the muscle fibres, placed on a glass slide and covered with a cover slip. A helium-neon laser with a wavelength of 632-8 nm was used for measurements of sarcomere lengths (Voyle, 1971). The average sarcomere length for each muscle was calculated on the basis of 20 measurements.
Tenderness of pork as influenced by chilling rate and carcass suspension
279
Warner-Bratzler (WB) shear force measurements Subsamples were thawed and four blocks (20 x 20 x 50 mm), with the length cut parallel to the muscle fibres, were removed. The blocks were individually placed in 100-ml glass tubes and a 0-9% NaC1 solution added to just cover the meat blocks. The tubes were heated in a waterbath at 80°C for 25 min. The meat blocks were then rinsed with cold tap water and immediately placed in the refrigerator at 2-4°C for no longer than 24 h. The weight of the blocks before and after heating was noted for calculating the cooking loss. Samples of rectangular cross-section (10 x 10ram) were cut from the cooked meat blocks and sheared at right angles to the fibre axis, using a Warner-BratzIer shear blade with a triangular slot angular cutting edge. An Instron Universal testing machine table model 1140 was used to measure the peak shear forces (kg/cm-'). Three shear force values were determined on each meat block, giving a total of 12 measurements on each muscle.
Statistical methods As the sides of each carcass have undergone the same pre-experimental treatment, they can be considered as paired observations. The effect of treatment and pH group on differences between treated and control sides can thus be analysed by means of the following statistical model: Yijk = tl "[- Tr i +
pHj + ( Tr x pH)/j + eij k
where Y~jk= the observed differences between right and left sides of the kth carcass in the jth pH group and the i TM treatment; T r i = the effect of the i TM treatment, i = 1, 2, 3,4, 5; pH~ = the effect of the jth group, j = 1, 2; ( T r x pH)ij = the effect of the 0"th interaction between T r and pH; and e~Sk = the random residual effect. RESULTS
Temperature, pH and R value The temperature decline in the LD muscle from 1 to 8 h post stunning is shown in Fig. I. The fast chilling procedure cools the LD below 10°C within 3 h post stunning while the temperature is only decreased to about 20°C by delayed fast or slow chilling treatments. The delayed fast procedure resulted in the lowest temperature (0-1°C) at 8 h post stunning. At 24 h post stunning, the temperature had equalized to 3-4°C in all sides. In the SM and BF muscles, temperature below 10°C was observed at approximately 8 h post stunning. For the fast, delayed fast and slow chilling procedure, the temperatures at 8 h post stunning were, respectively, 8.6 (0-18), 3.0 (0.58) and
4035
3O (J !
*25
20 E 15 10
m
,q I 1
I 3
I 5
[ 8
H post stunning Fig. 1. Temperature in M. longissimus dorsi during the period 1 to 8 h post stunning. Chilling procedure: • = fast, • = delayed fast and • = slow. (Within group of treatment. standard error varied between 0"19 and 0.55.)
High pH group 6.4
Low pH group
~
6"2
1.3
6.4 -
1'2
6,2--
1.3
/~"~ / /
/ "/~'~>0 6.C
/
"1'2
//D" //
- 1.1
6.0
o//
1.1
,.o
,_
,.o
o.=,
5.6
~.9
// 5.e
ov'
I 1
,1 I 3 5 H post stunning
I 8
1
3 5 H post stunning
8
Fig. 2. pH and R value in M./ongissimu~ dorsi during the period 1 to 8 h post stunning for carcasses belon~ng to the high (6"! < pH4s,,io < 6-5) respectively the low (5.7 < pHi,== < 6.1) pH group. Chilling procedure: • = fast, • = delayed fast and • = slow. Closed and open symbols are referring to pH and R value, respectively. (Within group of treatment, standard error varied between 0-025 and 0.090 for pH measurement; for R values between 0-016 and 0"073.)
Tenderness of pork as influenced by chilling rate and carcass suspension
281
11-2 (0-32) in SM, and 9-6 (0-17), 6-7 (0-36) and 12-4 (0-31) in BF (parenthetical values are 1 standard error of the mean). The relationship of chilling procedure and rate of glycolysis is shown in Fig. 2. In both pH groups, the pH declines more rapidly at the delayed fast or slow chilling treatment as compared to the fast chilling. Accordingly, the R value is initially smaller during slow chilling, showing that the ATP content was higher during this period. Compared with the low pH group, the R value in the high pH group remains at a higher level through the initial chilling period. WB shear force
The effects of treatment and pH groups are analysed on differences in WB values between the treated and the control sides from each carcass according to the statistical model as described above. The analysis of variance of these data shows a significant effect of treatment in LD, SM and BF, whereas the effect ofpH group is only observed in the LD muscle (Table 2). Least-squares TABLE 2 Analysis of Variance for W B Shear Force, Sarcomere Length a n d C o o k i n g Loss as Indicated by Deviation from the C o n t r o l s
Muscle~source of variation
DF
Mean squares WB shear force
Sarcomere length
Cooking loss
3-915" 2.218* 1"381 "s 0-723
0-004 5 "s 0.036 9** 0"0266* 0.008 8
15"87 "2 1'6 l "~ 4"61 "S 4-82
1 4 4 62
0"294 "s 0-745* 0"071 ns 0-240
0-201 6** 0"538 6*** 0-098 0* 0"028 1
2"24~s 1-61 "~ 7-71 "s 6-02
I 4 4 62
0-440 "s 4.963*** 0-264 "s 0-605
0"565 3*** 0-834 5*** 0.107 5** 0-021 4
3"12n~ 8-56 ~" 4.02 "s 3"63
M, longissimus dorsi (LD) pH g r o u p (A) T r e a t m e n t (B) A x B Error
l 4 4 62
M. semimembranosus {SM) pH g r o u p (A) T r e a t m e n t (B) A x B Error
M. biceps femoris (BF) pH g r o u p (A) T r e a t m e n t (B) A x B Error
n s = n o t significant; *, ** a n d *** = s i g n i f i c a n t at 5%, 1% a n d 0.1% levels, respectively.
282
A. J. Moiler, 15. Kirkegaar~ T. Vestergaard TABLE 3 Effect o f T r e a t m e n t s o n W B S h e a r F o r c e (kg/cm-') as I n d i c a t e d by D e v i a t i o n f r o m the C o n t r o l s ~
Chilling procedure
Muscle~carcass suspension Fast
Delayed fast
Slow
--0"75 c
- 0,01 b -0-97 c
_ 1.03 c - 1.0V
-- 0"37 b'~
-0-09 h - 0-62 ~'a
- 0 . 3 9 b'a - 0"72 a
--0-60 a
0-42 b - 0 . 4 8 La
- 0 - 0 3 b'" - l'27 *
M. longissimus dorsi (LD) Achilles t e n d o n Pelvic b o n e
,~L semimembranosus (SM) Achilles t e n d o n Pelvic b o n e
M. biceps femoris (BF) Achilles t e n d o n Pelvic b o n e
L e a s t - s q u a r e s m e a n s e s t i m a t e d by the m o d e l (1). M e a n s w i t h i n muscle b e a r i n g different s u p e r s c r i p t s are significantly different ( P < 0 " 0 5 ) . U n d e r l i n e d figures are significantly different f r o m zero. M e a n s a n d s t a n d a r d d e v i a t i o n o f W B s h e a r values f r o m c o n t r o l sides for L D , S M a n d BF: 5 " 4 0 + 0 . 9 6 , 4-50_+0"67 a n d 5-17_+1'01, respectively.
means for the various groups of treatment are presented in Table 3, where underlined figures indicate significant deviation from zero at the level of 5%. Pelvic suspension, as opposed to conventional carcass suspension, improves tenderness regardless of chilling procedure (P < 0-05). The pelvic effect is of similar extent whether sides have been cooled by the fast or delayed fast chilling procedure. As compared to fast chilling, pelvic suspension combined with slow chilling caused further tenderization in SM and BF (P < 0.05). Based upon previous observations, differences in WB values above approximately 0.5 kg/cm 2 are likely to be marked by a trained taste panel. The Achilles tendon suspended sides were also unaffected in their WB values by using delayed fast chilling. However, the stow chilling procedure improved the WB values in LD and SM muscles from conventional fast chilled sides. This tenderization does not differ significantly from the tenderization observed in the pelvic suspended sides. Besides the effect of treatment, the main effects o f p H ~ o u p on WB values were analysed (Table 2). As shown, only WB values from LD were affected by the pH group (P < 0-05). Differences in WB values between treated and control sides were larger in the high pH group (1 s mean = - 1"04 kg/cm 2) as compared to the low pH group (i s mean = - 0.47 kg/cm2).
Tenderness of pork as influenced by chilling rate and carcass suspension
283
TABLE 4 Effect of pH Group and Treatments on Sarcomere Length (/~m) as Indicated by Deviation from the Controls ~
Muscle/carcass suspension
Chilling procedure Fast
M. longisstmus dorsi (LD) Achilles tendon low pH group high pH group Pelvic bone low pH group high pH group M. sernimembranosus (SM) Achilles tendon low pH group high pH group Pelvic bone low pH group high pH group M. biceps femoris (BF) Achilles tendon low pH group high pH group Pelvic bone low pH group high pH group
Delayed fast
Slow
0.06 c.J _0.05 b
_ 0.002 b,c 0.09",J."
0"07¢'a
O"16d'~
0-08¢'a'~
0" 11 a.e
0 ' 0 9 ¢'d'~
0" 18"
O"10b'~ 0.002 b 0.2Y .d 0.53 e
0.28 c 0.62 a
0.33 d 0-35~
- 0"09b - 0.03 b 0.29 ~.a 0-36a
- 0.05 b
0.04 b
- 0.04 b
0.02 b
0.33 c 0-53J
0.30 ~ 0.52 d
Least-squares means estimated by the model (1). Means bearing different superscripts are significantly different (P < 0-05). Underlined figures are significantly different from zero. Means and standard deviation of sarcomere length from control sides for LD, SM and BF: 1.75 + 0"08, 1"77+ 0"09 and 1.77 +_0-08, respectively.
°
Sarcomere
length
Differences in s a r c o m e r e length b e t w e e n t r e a t e d a n d c o n t r o l sides are significantly affected b y p H g r o u p in the S M a n d B F muscle, a n d b y t r e a t m e n t in all t h r e e m u s c l e s (Table 2). As significant i n t e r a c t i o n s are evident, the l e a s t - s q u a r e s m e a n s o f t r e a t m e n t s f r o m e a c h p H g r o u p are s h o w n in T a b l e 4. I n c r e a s e d s a r c o m e r e l e n g t h is o b t a i n e d in all o f the pelvic s u s p e n d e d g r o u p s as c o m p a r e d to the c o n t r o l s ( P < 0-05), a n d this effect is a p p a r e n t l y n o t i n f l u e n c e d b y chilling t r e a t m e n t . T h e p e l v i c - i n d u c e d s a r c o m e r e l e n g t h e n i n g is largest in S M a n d BF, a n d t e n d e d to be larger f o r
284
A. J. Metier, E. Kirkegaard, T. Vestergaard
the muscles belonging to the high pH group. Neither delay fast nor slow chilling procedure affect sarcomere length in the SM and BF muscle from Achilles tendon suspended sides. However, the slow chilling procedure results in less sarcomere shortening in the LD muscles belonging to the high pH group.
Cooking loss As shown in Table 2, the cooking loss is unaffected by both pH group and treatment. Mean values of cooking loss in the LD, SM and BF muscles are 30.47% + 2.13, 30-03% + 1-99 and 31-98 + 3.06.
DISCUSSION The present findings that pelvic suspension significantly reduces WB shear values for LD and SM is in accordance with previous reports of improved tenderness of bovine and ovine muscles from utilization of this technique (Arrango et aL, 1970; Hostetler et al., 1971, 1972, 1973; Bouton & Harris, 1972; Bouton et aL, 1973; Joseph & Connolly, 1977; Jeremiah et aL, 1984). In this study also the BF muscle showed a substantial reduction in WB values, while BF from beef has shown only minor response to altered carcass suspension (Hostetler et al., 1975; Joseph & Connolly, 1977). Both tenderness and final state of muscle contraction are largely dependent upon the amount of strain on the muscle before and during rigor mortis (Locker, 1960). Results from the present study indicate that pelvic suspension, as opposed to conventional carcass suspension, increases the sarcomere lengths. However, the magnitude of the improvements in tenderness does not appear to have a simple relationship with sarcomere length changes. Thus, the sarcomere length changes due to pelvic suspension were much larger in SM and BF than in LD without producing proportional tenderness improvement. Regardless of suspension method, neither WB values nor sarcomere length were affected by using the delayed fast chilling procedure. When cooling is retarded even more, as in the slow chilling procedure, the LD maintained temperature above 10°C until 8 h post stunning. As a result of using the slow chilling, additional tenderness improvements were obtained in the BF and SM muscles from pelvic suspended sides (P < 0-05). Similarly, slow chilling improved tenderness in LD and SM from conventionally suspended sides (P<0-05), and this extent of tenderization did not differ from the pelvic effect. This result contrasts similar studies on beef, where pelvic suspension has been reported more effective in enhancing tenderness
Tenderness of pork as influenced by chilling rate and carcass suspension
285
than by using delayed or slow chilling rate (Hostetler et al., 1975; Joseph & Connolly, 1977; Jeremiah et al., 1984). The enhanced tenderness of slowly chilled meat has been attributed to the earlier onset of proteolytic breakdown of the myofibrillar structure (Smith et al., 1971; Parrish et al., 1973; Hostetler et al., 1975), but the LD muscle from the high pH group in this study did obtain an increase in sarcomere length (P < 0-05) when using the slow chilling rate for sides being suspended from the Achilles tendon. From earlier reports a value of pH 6"0 has been suggested as a useful level from where rapid cooling of a carcass can begin (Bendall et al., 1976). Pork muscles are well known to vary substantially in their pH decline post mortem, which consequently makes muscles showing a slow pH decline generally more prone to cold-induced toughening (Mailer & Vestergaard, 1987). This is supported from the present results, however, only for the LD muscle, which shows significantly larger effect of treatments on WB values in the high pH group as compared to LD muscles from the low pH group. Otherwise the pH group as a main effect did not affect tenderness in the various groups of treatments. It can be concluded from these results that slow chilling improves tenderness in the LD muscle from normal suspended sides and adds further tenderness improvements to SM and BF muscles from pelvic suspended sides. A slow chilling rate may not be suitable for factory use, owing to the risk of PSE development, higher bacterial counts and greater weight loss. Thus, pelvic suspension seems to be the most suitable method as improved tenderness can be obtained in a fast chilling system. But technically the pelvic method needs to be improved in order to be adopted by the industry. ACKNOWLEDGEMENTS This work was supported by funds from the Danish Agricultural and Veterinary Research Council. Appreciation is expressed to Mrs M. Neumann and Mrs A. Preusse for their skilled technical assistance. We thank the staff at Tulip Odense for their kind co-operation. REFERENCES Arrango, T. C., Smith, G. C., Carpenter, Z. L. & Cross, H. R. (1970). J. Anita. Sci., 31, 180 (Abstr.). Bendall, J. R. (1975). J. Sci. Food Agric., 26, 55. Bendall, J. R., Ketteridge, C. C. & George, A. R. (1976). J. Sci. FoodAgric., 27, 1123. Bouton, E E. & Harris, E V. (1972). J. Food Sci., 37, 539. Bouton, P. E., Harris, P. V., Shorthose, W. R. & Baxter, R. I. (1973). J. FoodSci., 38, 932.
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Dransfield, E. & Lockyer, D. K. (1985). Meat Sci., 13, 19. Fischer, C., Honikel, K. O. & Hamm, R. (1980). Fleischwirtsch., 60, 263. Galloway, D. E. & Goll, D. E. (1967). J. Anim. Sci., 26, 1302. Gigiel, A. J. & James, S. J. (1984). Meat Sci., 11, 1. Herring, H. K., Cassens, R. G., Suess, G. C., Brungardt, V. H. & Briskey, E. J. (1967). J. Food Sci., 32, 317. Honikel, K. O. & Fischer, C. (1977). J. Food Sci., 42, 1633. Hostetler, R. L., Landmann, W. A., Link, B. A. & Fitzhugh. H. A. Jr (1970). J. Anita. ScL, 31, 47. Hostetler, R. L., Link, B. A., Landmann, W. A. & Fitzhugh, H. A. Jr (1971). Beef Cattle Res. in Texas PR-2988, 80. Hostetler, R. L., Link, B. A., Landmann, W. A. & Fitzhugh, H. A. Jr (1972). J. Food Sci., 37, 132. Hostetler, R. L., Link, B. A., Landmann, W. A. & Fitzhugh, H. A. Jr (1973). J. Food Sci., 38, 264. Hostetler, R. L., Carpenter, Z. L., Smith, G. C. & Dutson, T. R. (1975). J. FoodSci., 40, 223. James, S. J., Gigiel, A. J. & Hudson, W. R. (1983). Meat Sci., 8, 63. Jeremiah, L. E., Martin, A. H. & Achtymichuk. G. (1984). J. Food Qual., 6, 259. Joseph, R. L. & Connolly, J. (1977). J. Food Technol., 12, 231. Locker, R. H. (1960). Food Res., 25, 304. Locker, R. H. & Hagyard, C. J. (1963). J. Sci. Food Agric., 14, 787. Marsh, B. B. & Carse, W. A. (1974). J. Food TechnoL, 9, 129. Marsh, B. B. & Leet, N. G. (1966). J. Food Sci.. 31, 450. Moore, R. E., Mandigo, R. W. & Henrickson, R. L. (1966). Food Technol., 29, 957. Maller, A. J. & Vestergaard, T. (1986). Meat Sci., 18, 77. Mailer, A. J. & Vestergaard, T. (1987). Meat Sci., 19, 27. Parrish, E C. Jr, Young, R. B., Miner, B. E. & Anderson, L. D. (1973). J. FoodSci., 38, 690. Smith, G. C., Arrango, T. C. & Carpenter, Z. k. (1971). J. Food Sci., 36, 445. Voyle, C. A. (1971). Proc. 17th European Meeting of Meat Res. Workers, Bristol. England, 95. Weiner, P. D., Kropf, D. M., Macintosh, D. L. & Koch, B. A. (1966). Food Technol., 20, 541.