Uijttenboogaart Consultancy,
Beekbergen
The Netherlands
Aging of poultry before deboning is required to obtain tender breast meat. For modern processing holding of carcasses during 6 to 24 hours to allow the aging process time to proceed is very costly. Acceleration of the process of aging therefore is required. Attempts have been made to use electrical stimulation during processing to increase aging speed.
Results are not very conclusive. In most situations electrical stimulation of carcasses results in an increased pH drop. However tenderness/shear force of breast meat from stimulated birds is not always improved. Most studies show some improvement in comparison with non-stimulated controls, but shear values mostly are much higher than from breast samples aged during 24 hours. Interactions with other processing methods may be the reason of the inconclusive results and there is not sufficient theoretical knowledge of the effects of electrical stimulation on the biochemical processes in the muscle and the development of tender meat.
For reasons of continuous processing an optimal process would be to apply an aging time of maximum 3 hours. Electrical stimulation, together with a good combination of other processes could be a means to reach that goal.
Introduction
To consumers meat tenderness is a primary quality characteristic of boneless meat. Tenderness of chicken breast meat depends on the moment of boning and the progress of the proteolytic breakdown of structural proteins. Pre-rigor boning causes muscle tissue to shorten and as a result to toughen (Papa and Fletcher, 1988; Papa and Lyon, 1989). This toughness is irreversible, which means it does not disappear during proteolysis of structural proteins of the meat. Toughness can become so severe that complaints of consumers can occur.
Shortening and hence toughening of meat is caused by contraction of muscle tissue in which the energy has not been depleted entirely at boning. This residual energy is present as adenosine tri phosphate (ATP).
During aging ATP is used for all the metabolic processes continuing to proceed in the muscle after slaughtering the animal. New ATP is synthesised by anaerobic glycolysis. As a result, lactate will start to accumulate, decreasing the pH of the muscle. At a pH of about 5.8 glycolysis in the chicken breast muscle will stop. From this moment on hardly any ATP will be formed. The moment the ATP concentration drops below 1 µmol per gram tissue, the muscle will no longer be able to contract. The muscle has gone into rigor mortis. Methods to follow the development of the aging processes in the muscle are to monitor the pH decline or the depletion of the ATP. A simple method to measure ATP depletion is to determine the R-value (Honikel and Fischer, 1977), a relative measure for the adenosin/inosin compounds in the cell.
For optimal processing the complete slaughter -, evisceration-, (air) chilling- and deboning process should be a continuous process. Space required for buffering carcasses is expensive. Also buffering of birds in a processing plant makes it more difficult to apply modern processing and quality procedures like tracking and tracing. Finally buffering increases processing time, which could be critical with respect to the shelf life of fresh poultry meat products.
Technically a process time of about 3 hours until deboning can be realised in a full continuous operation in which air chilling is applied. When carcasses are chilled, applying water immersion, processing time can be reduced even further (Sams, 2000). However, to guarantee full rigor development at deboning and, hence tender breast meat, a processing period of at least 4 to 6 hours before deboning is required (Dawson et al. 1987; Schreurs, 1999; Savenije, 2002). This means that carcasses have to be stored for several hours to avoid toughening because of pre rigor deboning. This makes it worthwhile to look for methods to accelerate the aging.
The biochemical processes that take place during conversion of muscle to meat depend on the condition of the animal at slaughter, as well as on processing. Regular processing affects the rate of post mortem biochemical processes of the muscles. Stunning, is known to affect aging. Electrical stunning, using a water bath, reduces the speed of aging (Veerkamp, et al, 1987, Papinaho and Fletcher, 1996). The effects of gas stunning depend on the gas mixtures used. Argon anoxia (Mohan Raj, et al., 992) has shown to accelerate aging, due to severe convulsions evoked by the sudden oxygen depletion. Other gas mixtures, like CO2/O2 mixtures, show a slight decrease in aging rate (Uijttenboogaart, 1997). Plucking, during which heavy massage of the carcasses occurs, accelerates aging through mechanical stimulation of the muscle tissue. Aging is temperature dependent, being a biochemical process. Therefore, the chilling rate will affect rigor development as well. All these processes can interact with each other and with the condition of the of the bird upon slaughter. This makes it very complex to find the optimum process for obtaining a tender product.
During the last decades many studies have been conducted to use post-mortem electrical stimulation for acceleration of the aging process. Initially, research was focused on red meat. By the end of the eighties, chicken meat producers started to shorten the processing time by deboning muscle in a pre-rigor state. As a consequence, problems arise with respect to meat tenderness. This made it more urgent to look for systems to accelerate the aging process through electrical stimulation in poultry processing.
Electrical stimulation is predominantly applied by means of pulses. The muscles are stimulated to contract and relax. During these contraction and relaxation cycles energy is consumed in the muscle cells. The pH decline in the meat is accelerated, thus accelerating the aging process. As a result negative effects of early deboning on shear development could be prevented.
The results of application of electrical stimulation in poultry processing however were and still are inconclusive. The mechanisms of the process are not fully understood, but more and more the positive effects are better reproducible.
Electrical stimulation research
First attempts to accelerate aging in poultry by means of the application of electrical stimulation are made in the late eighties. Maki and Froning (1987) stimulated turkey carcasses at a high voltage (800V). They found an accelerated pH decline and somewhat lower shear values. Dawson, et al., (1988) found no differences in shear values in very early (5, 10 and 15 minutes post mortem) boned breast meat from broilers. Also the pH drop was very small. Chilling rate did not make any difference. Froning and Uijttenboogaart (1988) studied the effect of electrical stimulation (100 V) and boning time on the development of meat quality characteristics. In this study the pH was strongly affected by the stimulation treatment. However no or even a negative effect on shear values of the meat was found. Sams et al (1988) however found a positive effect of electrical stimulation on early harvested breast filets. They concluded from their study that holding time of birds could be reduced by 60%. Also Lyon, et al., (1989) found no effect of electrical stimulation on shear development. In this study 3 different levels of voltage were used (50, 200 and 350 V). Furthermore 2 different chilling regimes were applied and boning took place at 2 hours post mortem. Birds were held for 24 hours prior to cooking and shearing. The pH in this study was significant lower direct after stimulation for the 200 and 350 V group as compared to the unstimulated controls and the 50 V group. These differences disappeared after 2 hours post mortem at the time of deboning.
Uijttenboogaart and Reimert (1993) studied the effects of low voltage (100 V) electrical stimulation on shear development in broilers. The stimulation was applied at different stages during the process. In this study best results were obtained by applying electrical stimulation by a copper strip at the breast side of the bird. Furthermore L* colour values appeared much lower in stimulated birds, which means a darker colour. Uijttenboogaart and Reimert (1994) also studied the interaction between electrical stimulation and chilling. Two methods of stimulation were applied. Stimulation 1 is by a water bath set up, stimulation 2 is by the use of a copper strip at the breast side of the bird.
The results are given in Figure 1. Very fast chilling causes very tough meat. Effects of cold shortening could play a role to explain the very high shear. Furthermore the results show an effect of stimulation in the ice-chilled samples. Air chilled samples are not or negatively affected by stimulation. Dunn, et al., (1993) studied shortening of muscle tissue at temperatures between 0 and 40oC and 0 and 24 hours post mortem. Storage at 0oC gives sarcomere lengths in average of 1.20 µm. With increasing temperature (20oC) the average sarcomere length increased to values around 1.54 µm. At higher temperatures the sarcomere length decreases to 1.22 µm. As the rate muscle contraction and with that of sarcomere shortening is accounted responsible for an increased shear force this is not shown in this study. A quadratic relation has been found between shear force and percentage of muscle shortening with highest shear values at a percentage of about 25%.
In 1993 Li, et al., gave a review on the electrical stimulation with poultry. It is very hard to compare results from different studies. Due to differences in bird quality, experimental conditions during processing, methods of application of electrical stimulation, methods of sample preparation and measurements of quality characteristics possible interaction effects can vary very extreme. In the work of Li, et al., in 1994 a statistical analysis is conducted to estimate the effect of voltage on variations in shear. Three hierarchical nested factors, bird, side and sample, counted for respectively 20.5, 24.9 and 20.0 % of the variation in shear values. Voltage, which was applied in levels of 0, 20, 40 and 120 V, affected shear variation by 34.6 %. Walker, et al., (1994) studied high and low voltage electrical stimulation in combination with muscle tensioning and high temperature conditioning of 1 hour post mortem deboned muscle. All the samples were less tender than 24 hour deboned muscle. Most of the combinations gave shear values that were not different from the 1 hour deboned control. One of the treatments even gave samples with significant higher shear values.
In their work in 1995 Walker, et al., studied cathepsin- and calpain activities the formation of a 30 kDa component during proteolytic breakdown of the muscles. The reduction of sarcomere shortening and an increased myofibrillar fragmentation that occurs in a specific combination of low voltage stimulation, muscle tensioning and high temperature conditioning is said to give a tenderizing effect. In this study no tenderness measurements have been made.
Other achievements
Effect of voltage during electrical and stunning and stunning method
A work by the group of Sams suggests that aging time can be reduced up to 50% when high voltage electrical stimulation is used (Skarovsky and Sams, 1999) in combination with air chilling. The same effect is found by Zocchi and Sams (1999). They compared 2 h deboned stimulated breast fillets with 4 h deboned fillets from conventionally processed birds. The shear values were at the same level.
Craig, et al., (1999) found an interaction between stunning and the application of high voltage (440V) electrical stimulation on quality characteristics. In their experiment very early deboning times were applied. They found a very significant toughening effect of high current stunning without an electrical stimulation treatment. If an electrical stimulation treatment was applied the shear values were decreased to low values. The values obtained in the breast meat after 2 hours deboning were still corresponding to a moderate toughness. Sams and Dzuik (1999) studied the effect of gas stunning, electrical stimulation and chilling method on the development of shear in chicken pectoralis major. They found an effect of the stimulation treatment that was not enhanced by the way of stunning. Gas stunning without electrical stimulation showed intermediate shear values when compared to controls and stimulation.
Results like these suggest an interaction between processing treatments like method of stunning, way of chilling, with the application of electrical stimulation.
According to Sams, there appears to be an important interaction between voltage at which stimulation is applied and the rate of carcass chilling. According to Takahashi (see Sams) and Geesink (personal communications) suggests that electrical stimulation at a low voltage in combination with high muscle temperatures in beef accelerates pH decline and calpain activation through partial autolysis. Activated calpain is highly susceptible to denaturation due to a fast acidification of the tissue and a low pH. Because calpains are neutral proteases, a low pH will inhibit the enzyme activity. Therefore, stimulation at a low voltage that accelerates pH decline, probably inhibits proteolytic breakdown of the cytoskeleton and therefore increases toughness.
In our studies acceleration of the post mortem glycolyses and ATP depletion had an effect on the breakdown of the cytoskeletal protein desmin of a parental strain not primarily selected for high growth rates. There was, however, no effect of stimulation on the desmin proteolosysis of a parental strain selected for fast growing. Differences in shear measured at the moment of consumption of the meat (72 hr post mortem), therefore are not likely the result of an impaired proteolysis due to electrical stimulation.
According to Sams, acceleration of the rigor development, through low amperage stunning, is insufficient to reduce the degree of toughening due to pre rigor boning. Stimulation at a high current (>400 mA), induces severe muscle contraction, hence disrupting muscle structures. In combination with an accelerated post mortem metabolism this will sufficiently reduce toughness to produce meat deboned at less than two hours after slaughter.
Upon pre rigor deboning, muscles are no longer restrained and therefore can contract and shorten upon excision. Additionally, excised muscle will cool down more rapidly. As a result the temperature may decrease below a critical value, thus causing leakage of the membrane of the sarcoplasma. The subsequent increase of the intracellular calcium concentration causes cold shortening, and hence toughening of the meat (Sams, 2001). However, in contrast to red meat poultry meat seems not to shorten at temperatures below 10°C. According to this temperature has to drop to about the freezing point for cold shortening to occur.
Chilling
Dickens and Lyon (1995) studied the effect of electrical stimulation and extended chilling times on Warner Bratzler shear values. They found an effect after a 2 h chill treatment between control and stimulated birds. The breast meat however was relatively tough with shear values around 3.2 and 4.5 kg. In their experiment they used the typical US way of chilling by putting the birds in an immersion chiller containing slush ice. This can cause cold shortening. Owens and Sams (1998) studied the effect of electrical stimulation on breast fillets, which were harvested 1 h post mortem, and subsequently individually quick frozen or aged on ice. They found shear values of the ice-chilled samples of the same level as the 4 h controls.
Dunn, et al. (2000) studied the effect of air chilling (0oC) and fast air chilling (-12oC) in combination with electrical stimulation treatments on shortening induced toughness. However in the experimental set up the breast muscles were deboned after 24 h. Some minor but significant effects have been found, but shear values of all the stimulation groups ranged between 2 to 8 kg/cm2.
Kranen, et al. (2000) studied the effect of chilled storage time (0 until 72 hours post mortem) and the changes in shear force during that period related to an electrical stimulation treatment. Electrical stimulation accelerated post mortem pH decline and adenosin breakdown expressed as R-value. Electrical stimulation made it possible to reduce aging time before deboning until 3 hours.
Proteolysis
Veeramuthu and Sams (2000) also studied pH decline and myofibrillar fragmentation in muscle from electrical stimulated carcasses that also their muscles tensioned during rigor mortis development. They found a more rapid pH decline and less myofibrillar fragmentation in the stimulated carcasses. Post mortem proteolysis should be prevented by the inhibition of calpain during a faster pH decline in the M. pectoralis major. In this study no shear data were given. However in the work of Alvarado ans Sams (2000) an increased myofibrillar fragmentation has been found in breast meat from stimulated broilers. In this work an effect of electrical stimulation on shear decline in broilers has been demonstrated. With ducks no effect of electrical stimulation has been found.
Kranen, et al. (2000) measured shear force of breast meat samples from control and electrical stimulated birds, heated at the moment of deboning and heated after storage at 0-2°C during 72 hours. They found a strong improvement of the shear values of the meat, probably due to proteolytic breakdown of the cytoskeletal proteins. The stimulated samples showed low shear values, comparable with 24 h aged and deboned breast fillets.
Marination
Young and Lyon (1997) combined an electrical stimulation treatment with chilling of the early harvested muscles (30 – 120 min) in a CaCl2 and a NaCl solution. They found a somewhat higher decline in shear values in muscles that were stimulated. There was no effect of chilling medium composition on shear, but a higher cooking loss in the CaCl2 treated muscles.
Lyon and Lyon (2000) studied the effects of electrical stimulation, deboning time and marination on sensory properties of broiler breast meat. They found a significant tenderizing effect of stimulation and deboning. In this experiment birds were ice chilled until deboning.
After deboning the samples were quick frozen until measurement of sensory characteristics. Then the samples were thaw-tempered at 36°F for 24 hours and heated in sealed bags at 185°F during 25 minutes. Warner Bratzler shear values have been found ranging from 4 till 10 kg (40 till 100 N). Forty N can be seen as relatively tough meat. With this set-up and the US way of chilling extreme toughening can take place. It is possible that in this situation cold shortening of the muscles, due to the very fast chilling regime, occurs
Other species
In the work of Alvarado and Sams (2000) an effect of electrical stimulation on shear decline in broilers has been demonstrated. With ducks no effect of electrical stimulation has been found.
Owens and Sams (1997) studied the effects of electrical stimulation on quality characteristics of turkey meat, deboned at 2, 4 and 24 h post mortem. They found significant differences in pH and R-value and some of the sarcomere of some of the groups. No differences have been found in shear values, expressible moisture, cooking loss, gravimetric fragmentation index and L* and a* colour values. They conclude that electrical stimulation does not work for turkey.
Conclusions
- Tenderness problems in broilers only occur in early (pre-rigor) deboned breast fillets.
- The results of the research in the area of application of electrical stimulation to improve tenderness are still inconclusive. The rate of pH decline is increased by stimulation. Most studies show an improvement of the shear, but shear values very often are higher than in 24 h controls.
- There are many factors playing a role in the development of shear. Interactions of these processing factors with electrical stimulation are not known sufficiently.
- It is very difficult to compare results from literature due to differences in experimental set up. Most important differences are:
- Way of application of electrical stimulation (high-low voltage, duration, # of pulses, wave form, etc.)
- Chilling method used. US: ice chilling, Europe: air chilling
- Time of deboning and time of heating the samples for shear measurement
References are available on request
From Proceedings of 11th European Poultry Conferen
