National Swine Improvement Federation Annual Meeting
December 1, 1995
R.L.J.M. van Laack, Dept. of Food Science, University of Tennessee,
Knoxville, TN, USA
R.G. Kauffman, Muscle Biology Laboratory, Dept. of Animal Sciences, University of Wisconsin, Madison, WI, USA
P. Polidori, Institute of Zoology, University of Milan, Milan, Italy
Prom a marketing perspective, variations in carcass quality reflect at least a 5300,000,000 annual cost to the US. Pork Industry (Meyer et al., 1996). Only when quality traits are easily, accurately, rapidly and cost-effectively assessed, can farmers expect to be appropriately compensated and encouraged to provide high quality pork. When this happens. and only then, can the industry expect poor quality pork to be minimized or completely eliminated. Until such time, the demand will decrease for pork which is inconsistent in quality. In contrast to composition pork quality includes the characteristics of wholesomeness, nutritive value, appearance, water-holding capacity (WHC), palatability and consistency, In pork, rapid chilling of pre-rigor carcasses may reduce the incidence of PSE. However, to date, final appearance of the meat, WHC and marbling must be assessed post-rigor if the information is to be reliable. With current packing plant technology and organization, it would be advantageous to determine traits related to quality immediately after slaughter for the purposes of sorting for further processing and to determine value for payment to farmers before identity is lost. Composition, wholesomeness and weight can presently be measured on the slaughter-floor. However pre-rigor muscle temperature, carcass stiffness, electrical conductivity and resistance, light scattering and reflectance, pH and ultrasonic are non-destructive methods that have been tested to predict ultimate post-rigor appearance, WHC and marbling. Of all approaches tested, separately or in combinations, the only one having predictive potential and application for pre-rigor musculature has been muscle pH45, but even this is not sufficiently accurate. However, invasive probes (such as pH, Hennessy Grading Probe, Danish MQM Probe, and Sigma Meat Check conductivity) appear to be most promising to quickly, accurately and economically measure post-rigor quality. They can predict the categories of PSE and DFD) with remarkable accuracy. To differentiate between RSE and RFN, conductivity appears to be the most reliable of the methods tested.
Currently significant variations occur between pork carcasses for the quality of their musculature. Unfortunately, in the past, few in the pork industry have either realized this or cared. If the pork industry became concerned, then it is likely the large existing variation could be reduced or even eliminated. It is conceivable that most seed stock producers know Very little about muscle quality, Their main concern is to market pounds of healthy, lean, live hogs. Packers transforming live hogs into pork have a reasonably good idea of leanness but less knowledge about quality. To date, they sort carcasses to meet certain cutting specifications and to market their product to processors that are conscious of leanness of boneless, defatted pork that they themselves convert to sausage and cured meat. However, quality of product is only beginning to be noticed! However, we recognize packers must worry about other matters too. Volume of production, over-capacity, government regulations, consistent live hog supplies, slaughtering efficiencies, labor costs, and condemnations often make value differentials due to quality seem trivial.
Therefore, assuming the above observations are reasonably accurate, the merits of desirable pork carcass value tend to "slip through the cracks"! It is our view that this is not by design but because the industry is so focused on short-run concerns that the importance of producing high quality products has been unintentionally ignored.
As with many industries, the pork industry is plagued with traditions that are not easily changed. Instead of relating everything to pounds of quality lean pork muscle, the use of live weights, dressing percentages, and carcass and wholesale cut weights arc emphasized. Hogs have been marketed on a live weight basis for centuries and it's easier that way. Furthermore, standardized methods that are practical, accurate and inexpensive to sort for variations and quality have not been readily available
Hence, it is no surprise that little progress has been made in the past several decades. While the rest of the world has improved the quality of pork as a food, the U.S. has hesitated. If the United States pork industry expects to maintain and improve its status as a provider of animal protein to its own consumer as well as to become a significant economic force in international trade, then it has little choice but to make efforts to improve the final product. It must eliminate extremes [pale, soft and exudative (PSE) and dark, firm and dry (DFD] in pork quality and be more concerned about to reddish pink, soft and exudative (RSE) pork which possesses undesirable textural and WHC, but is desirable in color.
How can this be achieved? Systems must be established to ensure that every hog marketed is individually identified and then assessed for quality. Carcasses can meet the demands of wholesomeness as outlined by the USDA, and they can be carefully and accurately examined for leanness and weight. However, quality is another matter, We have known for decades that marbling, color, texture and WHC are unquestionable virtues, but assessing them under packing plant conditions has been nearly impossible to achieve.
If pork carcass muscle quality is to be woven into an equation including composition, wholesomeness and weight to establish ultimate value, then both packers and producers must work together. They must cooperate in establishing: 1 ) standard and comparable guidelines for measuring value, 2) how a third party can maintain fairness if it is needed, 3) how the loss of value due to condemnations is fairly shared, and 4) how every hog processed can be identified so that hog farmers will know what they produce and how it compares to the industry. If progress is made, packers could sort products for efficient processing to the next level of distribution and utilization. It is predictable that realistic price premiums and discounts would evolve from those practices to effectively encourage farmers to use appropriate genetic and management practices. If such a revolution should occur, then the pork industry would surely survive and flourish in providing pork throughout the world. Perhaps pork could become the "meat of choice" as we enter the 21st century.
The purpose of this presentation to the 1995 NSIF meeting is to outline various approaches to estimate carcass quality which are important considerations for establishing value. Unlike composition, there are several practical problems associated with assessing pork muscle quality postmortem. The variables important in assessing ultimate pork quality arc WHC, color, pH, marbling content and possibly the extent of cold shortening or lack of it. Muscle temperature, as well as the rate and extent of pH fall in porcine muscles postmortem have significant effects on meat color and WHC. After the onset of rigor, the three traits most appropriate for detecting variations in color and WHC are ultimate pH. light absorbance and scattering, and a measure of free fluids in the interfascicular spaces. WHC is especially important because of the excessive shrink that can occur during processing storage and retail display of poor quality pork (PSE and RSE).
The appearance of meat is important because consumers reject pork that is either too pale or too dark. Even though color is not always related to WHC, the relationship is significant and if muscles are pale they are likely to be undesirable in WHC whereas if they are dark, they will almost always be desirable. If a muscle is inherently dark (i.e. DFD), it will have a high WHC, which is desirable for processing but it will still be objectionable because of its greater susceptibility to microbial spoilage at higher pH values (> 6.0), and because of its unattractive dark appearance.
Recently, a revised system for describing the quality of pork has been proposed to include pork which did not fit into the traditional quality categories of PSE, 'normal' (ideal) and DFD. The new system describes four quality categories: PSE, DFD, RFN (reddish pink, firm and non-exudative, i.e. 'normal') and RSE (Kauffman et al., 1992 Warner et al., 1 993).
Not all pork muscles are subject to wide variations in quality The majority of the shoulder muscles do not exhibit the PSE condition whereas some of the loin and ham muscles are considerably more susceptible (Warner et al., 1993). Our recent studies (Warner et al., 1993) clearly indicate that only about one-third of the musculature is affected by the PSE-RSE condition whereas all muscles may become DFD, and these conditions are not an all-or-none phenomenon but vary in degree that deviate considerately from RFN. Therefore, when assessing quality variations, it is essential that representative loin or ham muscles be examined.
The presence Or some marbling enhances juiciness and flavor, but there is little evidence that excessive amounts contribute proportionately to these quality traits. In fact, excessive marbling is likely to detract from the leanness characteristic, dilute the nutritive density of the product and be rejected at the market place. The correlation between marbling and carcass fatness is significant but far from perfect. Some breeds of swine such as the Duroc are more likely to possess marbling, and it is conceivable that some marbling can be deposited without excess subcutaneous and intermuscular fat. This has to yet be resolved through heritability studies. The discussions that follow explain the: principles of methods used to assess variations in ultimate pork quality.
SPECIFIC METHODS TO MEASURE QUALITY
Figure 1 has been included to summarize the various methods currently used or under consideration for use, that assess qualitative attributes of pork muscle. The summary briefly includes statements about accuracy, cost, speed and ease of application, a major advantage, a major limitation and special features of the technique.
A. Measurements Early Postmortem (Pre-rigor)
All available research on predicting pork quality from measurements made early postmortem show that it is not feasible. The reliability of pH45, and any other quality measurement proposed for slaughter-floor use as predictors of PSE, depend on the general relationship between the rate and extent of glycolysis. It was originally envisaged that early detection of meat quality was possible, but evidence of late-developing PSE and RSE carcasses (> 45 min postmortem) can be found in statistical data (Bendall and Swatland, 1988). The one exception to this is extreme PSE conditions which often can be detected by obvious stiffness and a low pH within minutes after bleeding.
A recent study (Kauffman et al., 1993) has indicated that attempting to measure quality early postmortem, before the onset of rigor, is difficult if not impossible. Of all the various measurements made (conductivity, light scattering and absorbance, muscle rigidity, temperature, pH), only pH measured at 45 minutes postmortem (pH45) provided a reasonable prediction of final pork quality. When post-rigor (24 hr) muscles were categorized into the four quality groups (PSE, RSE:, RFN, and DFU), PH45 was accurate about 75% of the time when logistic discriminative analysis was used on a biased test population. If pH45 was 6.4 or higher, 95% of the museles were either RFNor DFU. When pH45 was 5.8 or lower, 81% of the muscles were PSE. However, the probability of accurately predicting the quality when the pH45 was between 5.8 and 6.4 was about 50%. Therefore, the authors concluded that it \vas not reasonable to determine the ultimate quality prior to the onset of rigor This is unfortunate because carcass composition and weight can be identified on the slaughter-floor when carcass identification is still intact, thus identifying quality early postmortem would have been idea. Furthermore, this would have provided an ideal time for packers to sort carcasses into quality groups prior to chilling and further processing.
Manufacturers of the reflectance grading probes (e.g. Hennessy-Chong Grading Probe- HGP, MQM) claim successful integration of the measurement of fat thickness (or lean meat percent)and the PSE condition on the slaughter-floor: Hansson et al. (1986) reported that the HGP system could be used to identify 50% of PSE carcasses. However the studies by Kauffman et al.(1993) indicate that the accuracy of such probes is even less.
B. Measurements Late Postmo~tem (Post-rigor)
1. Visual Assessment
Subjective assessment of muscle color, firmness-wetness and marbling is generally used as the reference for comparison to objective methods for measuring pork quality. Problems that can occur with subjective assessments include non-repeatability of assessments and the lack of a linear relationship with objective methods due to the complexity in the human response (Bendall and Swatland, 1988). The repeatability and accuracy of trained, experienced assessors can be high but is certainly improved by the use of reference standards such as the Japanese color standards (Nakai, 1991) or the photographic standards produced by the pork industry (NPPC, 1991), Visual assessment, preferably using reference standards is a step toward the ultimate goal of description of consumer-desirable quality traits. However, subjective assessment of quality is not acceptable because it lacks in objectivity and speed.
2. Physical Properties
WHC is usually most reliably confirmed by cutting a standard-sized sample from musculature at 24 hr postmortem and suspending it in a plastic bag at 2 C for 48 hours, removing it and blotting the excess surface fluids and measuring; the weight loss gravimetrically, However, this is a long procedure. Among those that are particularly accurate, simple, quick and practical for field tests, the filter paper technique has proved useful (Kauffman et al.. 1986). A cross-section of muscle is exposed to the air at 5 C for 10 min. A special filter paper, 45 mm in diameter, is quickly pressed on and then removed from the muscle surface and the resulting fluid absorbed is either scored or weighed. Centrifuging muscle samples is also effective but requires more time and more expensive equipment.
Firmness-texture of muscle is difficult to quantitate objectively. Some attempts have been made, but to date none have been effective. If firmness is to be assessed subjectively by using 'feel' scores as identified visually, then it is important to standardize the temperature, size and muscle location because they will influence the perceived firmness and textural properties.
3. Electrical Properties
Ultimate pH represents the negative logarithm of the hydrogen ion concentration, thus serving as a measure of acidity or lack of it in muscle tissue. It relies heavily on the standardization of glass, metal or plastic combination electrodes attached to digital meters that are either battery or main-line driven electrically. The measurement of ultimate pH (pHu) in the loin and ham musculature accurately identifies carcasses which are DFD (usually pHu > 6.0). When the pHu is <5.4, the muscle is usually PSE. However, between 5.4 and 6.0, it is difficult to separate RFN from RSE even though RSE is usually below 5.i and RFN is almost always > 5.6 (Kauffman et al., 1993), If electrode calibration, temperature and anatomical location can be monitored, it is conceivable that pHu alone can separate DFD, RFN, RSE, and PSE, but when only 0.1 to 0.2 pH units predict such extreme quality conditions, the use of pHu alone is not recommended. Nevertheless, pHu remains a valuable indicator of quality variations among carcasses and has become more practical for commercial plants with the advent of solid-state pH electrodes (Kress-Rogers, 1991 ). Metal or plastic electrodes using the isfet-refet principle of microchip detection of hydrogen ion concentration appears particularly attractive because they are more durable when used under field conditions. Also, most pH meters (battery and on-line) may be affected by low temperature conditions (5 C) that prevail in packing plant cutting rooms.
Electrical capacitance declines in muscle postmortem and the rate of decline is faster in PSE pork relative to normal pork. Postmortem changes in the electrical conductivity of muscle are probably due to changes in the distribution of electrolytes between intracellular and extracellular compartments (Swatland, 1980a). The MS-tester has been developed to measure the dielectric loss factor of pork using a 15 kHz frequency and is reported to differentiate between PSE and normal pork (Kleibel et al., 1983). Swatland (1980a) used a 100 kHz frequency to measure muscle capacitance. He used capacitance rather than resistance as it was found to be little affected by sample temperature compared to resistance and thus was the measurement of choice. Commercial testing of a probe has since shown that the error in detection of undesirable pork quality was unacceptably large for quality control measures (Bendall and Swatland, 1988). The coefficient of determination for several studies utilizing the Quality Meter, developed in Canada measure electrical conductivity of meat in intact carcasses, has been low (Swatland, 1980b).However, very recent studies indicate that the Meat Check instrument (Sigma Co., Erfurt, Germany) clearly differentiates pork muscle varying in WHC.
4. Optical Properties
The principle of using optics to measure pork quality is that the perceived color of an object will be determined by its reflectance of light, which depends on the amount of light absorbed and scattered and the ratio between the two. Four factors which must be considered when utilizing the optical properties of pork to measure its inherent quality are: wavelength of the incident light, angle of incident light to the muscle fibers, mode of measurement (scattering, absorbance or reflectance) and coupling efficiency at the meat-probe interface. These four factors significantly influence the predictive accuracy of probes developed for use in the industry.
Changes in light scattering will be caused by changes in the myofibrillar refractive index over a complete range of pH values and this principle is utilized in the design of optical probes. The degree of light scattering in meat will depend on the wavelength of light and on the angle of illumination, Wavelength-related effects are very important in optimizing the performance of meat probes. Probes have progressed from monochromatic ones, such as the fibre optic probe(FOP) developed in England (McDougall and Jones, 1975), to true spectrophotometers, such as the Colomlet developed in Canada (Swatland, 1986).
An important optical property of pork is that transmittance of low wavelength light is less than that of high wavelength light. Swatland and Irie (1992) recommended the use of a red laser at 633 nm for the clearest identification of PSE pork.
Elliott (1967) showed that the surface reflectance of meat is higher when the muscle fibers are parallel to the surface then when they are perpendicular. In addition, the angle of insertion of the probe can influence the readings and predictive accuracy.
The coupling efficiency between an optical probe and the pork interior can be influenced by the depth of penetration of light into the meat, reflection and scattering of light at the end of optical fibers, the refractive index of the medium immediately external to the probe, the wavelength of the light and the angle of illumination (Swatland, 1989). Until we have a better understanding of the optical properties of pork at a microstructural level, it will be difficult to improve the predictive accuracy of technology presently available.
We are currently studying the use of a combination of measurements of pH, light absorbance and free fluids simultaneously to categorize the quality of post-rigor musculature in pork carcasses. Either the longissimus thoracis et lunborum or the semimembranosus are proposed as the muscles for probe measurement as these muscles are accessible on the intact carcass and have been shown to represent the quality of the major muscles of the pork carcass (Warner et al., 1993), The four quality categories that are considered include PSE, RSE, DFD) and RFN. A recently completed survey (Kauffman et al., 1992) suggests that the U.S. industry produces about 15 % PSE, 10 % DFD, 15% RFN and the remainder as RSE. With this variation, the goal would be to identify the four categories for reporting to producers and for sorting carcassesga prior to further processing. We believe that if such emphasis is focused on pork quality, that the industry could correct (or at least minimize) the problem through genetic selection and management hy producers, and improved handling and processing conditions by packers.
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Manuscript No. 347, Muscle Biology Laboratory, University of Wisconsin - Madison. The authors are appreciative to the NSIF organization, the University of Wisconsin-Madison, and the National Pork Producers Council for helping make this publication possible.