| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
ANIMAL PRODUCTS |
* Nutreco Swine Research Centre, Boxmeer, The Netherlands and
and
Wageningen Institute of Animal Sciences, Wageningen University and Research Center, The Netherlands
Abstract
The present experiment investigated whether increasing chilling rate could improve meat quality in pigs exposed to either minimal or high stress immediately preslaughter. Pigs (n = 192) were offspring of halothane-free lines. On various days, four groups of 48 pigs were processed at a commercial plant. Within each group, half the pigs were exposed to either minimal or high preslaughter stress. Before entering the cooler at 45 min postmortem, carcasses of both minimal and high preslaughter stress treatments were allocated randomly to either conventional (+4°C for 22 h) or rapid (three-phase chilling tunnel: -15, -10, and -1°C for 15, 38, and 38 min, respectively, followed by storage at 4°C until 22 h postmortem) chilling. Temperature and pH were measured in the blood at exsanguination and in the longissimus lumborum (LL) and semimembranosus (SM) muscle at 0.5, 2.5, 4.5, 6.5, and 22 h postmortem. Meat quality attributes (water-holding capacity and objective color measurements) were assessed on the LL. Preslaughter stress level affected pH and temperature in both blood and muscle, with lower (P < 0.001) pH values and higher (P < 0.001) temperatures for pigs exposed to high vs. minimal stress. Rapid chilling led to a faster (P < 0.001) temperature decline regardless of preslaughter stress level. Rapid chilling did not (P > 0.05) influence the rate of pH decline in the LL muscle, but reduced (P = 0.061) pH decline in the SM. Rapid chilling, as opposed to conventional chilling, decreased (P < 0.05) electrical conductivity in the LL, regardless of preslaughter stress; however, it could not compensate for the detrimental effect (P < 0.05) of stress on drip loss, filter paper moisture absorption, and meat color (L* value). Results from the present study indicated that increasing chilling rate is not a suitable method to resolve pork quality problems caused by inadequate preslaughter handling.
Key Words: Chilling • Meat Quality • Pigs • Stress
Introduction
Pale color and high drip losses, two common problems in pork, are pH- and temperature-dependent phenomena. Exposure to high temperatures at a low pH causes denaturation of muscle proteins, which impairs water-holding capacity and meat color (Wismer-Pedersen, 1959
; van der Wal and Eikelenboom, 1984; Offer and Knight, 1988). There are various factors that can increase the rate of decline in pH and carcass temperature. Stress in the immediate preslaughter period is one important factor, playing a crucial role even with stress-resistant breeds (Warriss et al., 1994; van der Wal et al., 1999; Channon et al., 2000). Prevention of stressful events prior to slaughter is often difficult, and additional strategies to lessen the impact of preslaughter stress are needed.
Increasing chilling rate decreased drip loss and/or improved meat color in some studies (Milligan et al., 1998; Kerth et al., 2001) but not in others (Gigiel et al., 1989; Long and Tarrant, 1990; van der Wal et al., 1995). Meade and Miller (1990) reported a higher pH for highly stressed pigs that were hot-fat trimmed, facilitating a more rapid temperature decline in the carcass compared with highly stressed pigs that were not hot-fat trimmed; however, there was no difference observed between hot-fat and non-hot-fat-trimmed carcasses of unstressed pigs. Likewise, Kerth et al. (2001) observed a reduction in PSE meat in the loins and hams of stress-sensitive pigs after accelerated chilling but not in stress-resistant pigs. Based on these studies, it was hypothesized that increasing the rate of temperature decline to improve meat quality would be particularly effective in highly stressed pigs. Therefore, the aim of the present experiment was to compare the effect of two commercial chilling methods (conventional vs. rapid) on meat quality (color and drip loss) of pigs after either minimal or high preslaughter stress.
Materials and Methods
The experimental protocol was approved by the Animal Care and Ethics Committee of the University of Nijmegen, The Netherlands.
Animals and Experimental Design
All pigs were commercial, halothane-free crossbreeds (gilts and barrows) with an average hot carcass weight of 89.8 kg and a lean percentage of 55.9% measured by the Hennessey Grading Probe. There were no differences (P > 0.05) in carcass weight or lean percentage between treatments. In a completely randomized design, pigs (n = 192) were assigned to one of four treatments in a 2 x 2 factorial arrangement, with two preslaughter stress levels (minimal or high preslaughter stress) and two chilling methods (conventional or rapid chilling). Every slaughter group originated from a different commercial farm. Four groups of 48 pigs (12 pigs per treatment combination) were used, resulting in a total of 48 pigs per treatment.
Preslaughter and Slaughter
All farms were located within close distance to the processing plant (<20 km). After transport, pigs were held in lairage for 3 to 8 h and slaughtered at 0630. Experimental stress treatments started approximately 5 min before slaughter. Pigs of the minimal stress group were guided to the stunning area without the use of electric goads and were handled as calmly as possible. Pigs of the high-stress group were forced by yells and electric goads to move four times back and forth in the corridor leading to the stunning area. Unpublished data from our laboratory has shown that this high-stress treatment led to an almost twofold increase in blood lactate levels and substantially higher blood cortisol levels when compared with the minimal stress treatment. Pigs were electrically stunned in a fully automated head-to-heart stunning system (MIDAS, Stork, The Netherlands).
Chilling
Shortly before entering the cold storage (45 min postmortem), pigs within the same preslaughter stress treatment were randomly allocated in groups of six to either the conventional or the rapid chilling system. Conventionally chilled carcasses entered the cooler and were kept at 4°C (air velocity of 0.5 m/s) until 22 h postmortem. Rapidly chilled carcasses passed through a three-phase chilling tunnel: 1) -15°C for 15 min (air velocity of 3 m/s); 2) -10°C for 38 min (air velocity of 2 m/s); and 3) -1°C for 38 min (air velocity of 2 m/s). After passing through the chilling tunnel, carcasses were held at 4°C until 22 h postmortem, similar to the manner in which conventionally chilled carcasses were handled.
Measurements
At exsanguination, pH and temperature were measured in blood using a portable pH meter (Portamess 911 pH; Knick Elektronische Messgeräte, Berlin, Germany) equipped with a probe-type glass electrode (LoT406; Mettler Toledo, Switzerland) and a portable thermometer (hand held digital thermometer, Stekon, Hoofddorp, The Netherlands). In muscle, pH and temperature were measured in the M. longissimus lumborum (LL) at the level of the third lumbar vertebra and in the M. semimembranosus (SM). Muscle pH and temperature in the SM were measured with the same instruments at 0.5, 2.5, 4.5, and 6.5 h, and pH was measured at 22 h postmortem. In the LL, temperature was recorded at 5-min intervals from 0.5 to 22 h postmortem with a data logger (Diligence EV N2002; Comark Instruments, Stevenage, U.K.) equipped with a food penetration probe (PX22L/C; Comark Instruments). Only data corresponding with the measurement times in the SM are presented (0.5, 2.5, 4.5, 6.5, and 22 h postmortem). The day after slaughter (22 h postmortem), final meat quality measurements were taken in the same region of the LL as muscle pH and temperature (third lumbar vertebra). Internal light scattering was measured using the fiber optic probe (TBL Fibres, Leeds, U.K.). Meat color was determined after a 10-min blooming period by measuring the L*, a*, and b* values with a Minolta Portable Chroma Meter (model CR 210; Osaka, Japan) equipped with a 50-mm aperture and using illuminant D65. Electrical conductivity was measured using the LF-Star (Ingenieurbüro Matthäus, Nobitz, Germany).
Water-holding capacity of the LL was measured by two different methods. A filter paper (45-mm diameter) was weighed, gently pressed on the caudal cut surface of the LL for 10 s, and subsequently reweighed to determine the absorbed moisture content. Additionally, samples of the LL were placed with a cut surface facing down on a metal grid that was placed in a closed plastic container. Drip loss was determined as percentage of weight loss after 1 and 2 d of storage at 4°C.
Statistical Analysis
Data were analyzed by the mixed-model procedure (PROC MIXED) of SAS (version 8.02; SAS Inst., Inc., Cary, NC). Least squares means were generated by the LSMEANS statement. Tests of multiple comparisons of LSMEANS were adjusted according to the Tukey-Kramer method to ensure the overall significance level of P = 0.05. The model applied for the pH and temperature measurements in the LL and the SM included the fixed effects of stress level and chilling method, their interaction, the random effect of slaughter day, and the repeated effect of time with pig as subject. The model applied for pH and temperature measured in blood, as well as for all pork quality attributes measured in the LL, included the fixed effects of stress level, chilling method, their two-way interaction, and the random effect of slaughter day.
Results
Temperature Decline
Temperature declines in the LL and in the SM muscle are shown in Figure 1. Both stress level and chilling method affected (P < 0.001) the time course of temperature decline, with a (numerically) greater effect of chilling method as opposed to stress level. At exsanguination, highly stressed pigs had a 0.3°C higher (P < 0.05) blood temperature than minimally stressed pigs (40.0°C vs. 39.7°C). At 0.5 h postmortem, the temperature difference had increased (P < 0.05) to about 1°C for both the LL and the SM muscles. Temperatures for the LL from the highly and minimally stressed groups were 40.9 and 39.8°C, respectively, whereas SM temperatures were 42.3 and 41.4°C in pigs subjected to high and minimal levels of stress, respectively. In the LL, the difference between the two stress levels persisted until 6.5 h postmortem, even though pair-wise comparisons did not reveal statistically significant differences (P > 0.05). In the SM, there was no temperature difference between high and minimal stress for rapidly chilled carcasses, whereas high stress in combination with conventional chilling led to a higher (P < 0.05) temperature than the minimal stress and conventional chilling treatment combination at 6.5 h postmortem (stress level x chilling method interaction; P = 0.015)
|
. Pigs of the high-stress group had a lower (P < 0.05) blood pH at exsanguination than pigs in the minimal stress group (7.03 vs. 7.16, respectively). At 0.5 h postmortem, a greater contrast in pH was measured in both muscles (P < 0.05); in the LL, high stress led to a pH of 6.27 compared with 6.54 for the minimal stress pigs; similarly, in the SM, highly stressed pigs had a pH of 6.26 vs. 6.68 for minimally stressed pigs. The subsequent chilling did not (P = 0.255) affect pH decline in the LL, whereas rapid chilling led to a slower (P = 0.061) pH decline in the SM. Ultimate (22-h) pH was similar (P > 0.05) for most treatments (5.53 to 5.63 in the LL and 5.60 to 5.71 in the SM). Only in the SM, pair-wise comparisons revealed a difference (P < 0.05) between rapidly chilled carcasses of the high-stress treatment (pH 5.71) and conventionally chilled carcasses of the minimal-stress treatment (pH 5.60).
|
. High preslaughter stress increased (P < 0.05) internal light scattering, L* values, electrical conductivity, filter paper moisture, and both 24- and 48-h drip loss percentages compared with minimal preslaughter stress. Rapid, compared with conventional, chilling increased (P < 0.05) internal light scattering and decreased (P < 0.05) electrical conductivity.
|
High preslaughter stress has been shown to increase carcass temperature and the rate of pH decline (Warriss et al., 1994; van der Wal et al., 1999; Channon et al., 2000). These effects were also observed in the present study. However, the main objective of the present experiment was to study the combined effects of preslaughter stress and two different chilling systems on meat quality rather than just the effects of preslaughter stress on its own.
Differences in final meat quality were mainly attributable to the effect of preslaughter stress (Figure 3). High preslaughter stress led to an approximately 50% increase in drip loss, filter paper moisture, and electrical conductivity compared with minimally stressed pigs. Conversely, rapid chilling could improve electrical conductivity by no more than 10%. Fiberoptic-measured light scattering within the LL was the exception because it was impaired rather than improved by rapid chilling. This latter effect is not confirmed by other studies testing systems with an even higher chilling rate (Gigiel and James, 1984; Long and Tarrant, 1990). In the present study, meat color was, to a lesser extent, affected by preslaughter stress than water-holding properties, whereas chilling system did not affect meat color. According to Offer and Knight (1988), alterations in initial pH hardly affect light scattering when the initial pH is relatively high (45-min pH > 6.1). Conversely, the same authors state that alterations in initial pH in those higher ranges affect drip losses. In the present experiment, initial pH (measured at 30 min postmortem) was well above the limit of 6.1, regardless of stress level. This may explain the observation that preslaughter stress affected water-holding properties more than meat color. However, it does not explain why the (albeit small) positive effect of rapid chilling on electrical conductivity, a trait related to water-holding properties, for both high and minimal stress did not correspond with a concomitant effect on pH decline in the LL muscle.
|
reported that blast-chilling did not decrease protein denaturation. Nevertheless, in contrast to the results of the present study, Jones et al. (1993) and Milligan et al. (1998) reported a darker muscle color after blast chilling. Muscle color may not only be influenced by protein denaturation but also by the absorption characteristics of myoglobin, as well as reduced light scattering of the meat surface, typically associated with high ultimate pH values (Lawrie, 1998). Indeed, in both experiments (Jones et al., 1993; Milligan et al., 1998), the blast chilling treatments were associated with a higher ultimate pH compared with the conventional chilling treatments. In the present study, ultimate pH in the loin was not affected by chilling system, and in agreement, no effect on color was observed.
Although increasing the rate of chilling leads to a more rapid temperature decline in the carcass and often to a slower pH decline (Long and Tarrant, 1990; Jones et al., 1993; Milligan et al., 1998), chilling does not necessarily induce a significant decrease in protein denaturation. This may be due to the fact that chilling affects pH mainly in the period between 3 to 4 h postmortem (Long and Tarrant, 1990; Milligan et al., 1998), which is in accordance with the results of the present study. It is, however, the period immediately postmortem, when carcass temperatures are above 30°C, that is most critical. If muscle pH is low during the first hour postmortem, protein denaturation occurs with negative consequences for meat quality (Wismer-Pedersen, 1959; van der Wal and Eikelenboom, 1984; Offer, 1991). It is thus postulated that chilling starts too late to repair damage that has been caused at an earlier stage in the pork chain. According to Offer (1991), this is especially true in carcasses with a very rapid pH decline because, in extreme cases, these carcasses may have completed postmortem glycolysis before entering the cooler. This was not true even for the highly stressed pigs in the present study; however, our initial hypothesis that increasing chilling rate would be particularly effective in carcasses of previously stressed pigs could not be confirmed by the outcomes of the present study.
Using higher chilling rates than those used in the present experiment may increase the effect of chilling on meat quality. However, Gigiel and James (1984), Gigiel et al. (1989), and van der Wal et al. (1995) studied various chilling systems operating at temperatures between -20 and -40°C and failed to note any effects on meat color or water-holding capacity. Moreover, it is emphasized that increasing chilling rate is not likely to fully overcome the detrimental effects of preslaughter stress.
Implications
The rapid chilling system applied in the present study could not compensate for the detrimental effect that preslaughter stress had on meat color and water-holding capacity of fresh pork. Even though increasing the chilling rate may have produced greater effects on pork quality, benefits are expected to be limited because severe preslaughter stress is very likely to have a greater effect than chilling rate. Moreover, the greatest damage likely occurs during the inevitable time lag between slaughter and start of chilling when carcass temperatures are highest. Investing in methods that help reduce stress in the immediate preslaughter period is recommended.
1 Correspondence: P.O. Box 240, NL-5830 AE Boxmeer (phone: +31-(0)-485-581-878; fax: +31-(0)-485-577-311; e-mail: Ellen.Hambrecht{at}nutreco.com).
Received for publication February 19, 2003. Accepted for publication October 16, 2003.
Literature Cited
Channon, H. A., A. M. Payne, and R. D. Warner. 2000. Halothane genotype, pre-slaughter handling and stunning method all influence pork quality. Meat Sci. 56:291–299.
Gigiel, A., F. Butler, and B. Hudson. 1989. Alternative methods of pig chilling. Meat Sci. 26:67–83.
Gigiel, A. J., and S. J. James. 1984. Electrical stimulation and ultra-rapid chilling of pork. Meat Sci. 11:1–12.
Jones, S. D. M., L. E. Jeremiah, and W. M. Robertson. 1993. The effects of spray and blast-chilling on carcass shrinkage and pork muscle quality. Meat Sci. 34:351–362.
Kerth, C. R., M. A. Carr, C. B. Ramsey, J. C. Brooks, R. C. Johnson, J. E. Cannon, and M. F. Miller. 2001. Vitamin-mineral supplementation and accelerated chilling effects on quality of pork from pigs that are monomutant or noncarriers of the halothane gene. J. Anim. Sci. 79:2346–2355.
Lawrie, R. A. 1998. Meat Science. 6th ed.Woodhead Publishing Ltd., Abington, England.
Long, V. P., and P. V. Tarrant. 1990. The effect of pre-slaughter showering and post-slaughter rapid chilling on meat quality in intact pork sides. Meat Sci. 27:181–195.
Meade, M. K., and M. F. Miller. 1990. The use of rapid chilling to reduce pale, soft and exudative pork from highly stressed market hogs. J. Anim. Sci. 69(Suppl. 1):351. (Abstr.)
Milligan, S. D., C. B. Ramsey, M. F. Miller, C. S. Kaster, and L. D. Thompson. 1998. Resting pigs and hot-fat trimming and accelerated chilling of carcasses to improve pork quality. J. Anim. Sci. 76:74–86.
Offer, G. 1991. Modelling of the formation of pale, soft and exudative meat: effects of chilling regime and rate and extent of glycolysis. Meat Sci. 30:157–184.
Offer, G., and P. Knight. 1988. The structural basis of water-holding in meat. Pages 63–243 in Developments in Meat Science—4, R. Lawrie, ed. Elsevier Applied Sciences, London.
Van der Wal, P. G., and G. Eikelenboom. 1984. Effect of muscle temperature after slaughter on pork quality: a pilot study. Neth. J. Agric. Sci. 32:245–247.
Van der Wal, P. G., B. Engel, G. van Beek, and C. H. Veerkamp. 1995. Chilling of pig carcasses: Effect on temperature, weight loss and ultimate meat quality. Meat Sci. 40:193–202.
Van der Wal, P. G., B. Engel, and H. G. M. Reimert. 1999. The effect of stress, applied immediately before stunning, on pork quality. Meat Sci. 53:101–106.
Warriss, P. D., S. N. Brown, and S. J. M. Adams. 1994. Relationships between subjective and objective assessments of stress at slaughter and meat quality in pigs. Meat Sci. 38:329–340.
Wismer-Pedersen, J. 1959. Quality of pork in relation to rate of pH change post mortem. Food Res. 24:711–727.
This article has been cited by other articles:
|
|
 |
|
  G. Bee, C. Biolley, G. Guex, W. Herzog, S. M. Lonergan, and E. Huff-Lonergan Effects of available dietary carbohydrate and preslaughter treatment on glycolytic potential, protein degradation, and quality traits of pig muscles J Anim Sci, January 1, 2006; 84(1): 191 - 203. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. Hambrecht, J. J. Eissen, D. J. Newman, C. H. M. Smits, M. W. A. Verstegen, and L. A. den Hartog Preslaughter handling effects on pork quality and glycolytic potential in two muscles differing in fiber type composition J Anim Sci, April 1, 2005; 83(4): 900 - 907. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
E. Hambrecht, J. J. Eissen, D. J. Newman, C. H. M. Smits, L. A. den Hartog, and M. W. A. Verstegen Negative effects of stress immediately before slaughter on pork quality are aggravated by suboptimal transport and lairage conditions J Anim Sci, February 1, 2005; 83(2): 440 - 448. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
