UNDERSTANDING PORK QUALITY

Introduction

 

Pork quality is one of the main issues of concern for the pork industry in the U.S. For pork to be the meat of choice, meat quality problems associated with Porcine Stress Syndrome (PSS) need to be addressed. Sonka et al. (1994) reported in the Pork Chain Quality Audit that total cost per pig from color and Pale Soft and Exudative meat (PSE) related problems were $1.05. Of this amount, $0.79 per pig was reported to be directly controllable by pig producers. In an industry that produces 88 million swine annually, this amounts to a total of $69.5 million in losses due to pork quality problems. These numbers may be an underestimate considering that some pork processing facilities reported that up to 40% of the pigs they process have PSE related problems during the summer (Morgan et al., 1994). The main quality concerns identified by all members of the pork marketing chain are poor color and inadequate waterholding capacity (Cannon et al., 1995). Therefore, addressing these quality traits is of great priority to increase the market value of pork and to provide consumers with a desirable product.

 

Pork Quality Measurements

 

Pork color is mainly determined by the chemical state of the muscle pigment myoglobin. Myoglobin consists of a heamatin portion which contains a central iron atom and a globulin type protein. The status of the iron (ferrous or ferric) in the heamatin portion and the status of the globin will determine the color of the pigment. In fresh pork, the myoglobin exists in an oxygenated form (oxymyoglobin) which results in a bright red color. A purplish-red color is associated with myoglobin that is not oxygenated. Oxidation of myoglobin or oxymyoglobin to metmyoglobin is responsible for a brown color.

 

The color of pork can be significantly influenced by the pH of the meat and the chilling process after slaughter. In the case of a high ultimate pH, residual enzyme activity will be high, resulting in removal of oxygen from the oxymyoglobin molecule. Muscle fibers will also be packed tightly together because the waterholding capacity of proteins above the iso-electric point (as is the case with high pH) is greater than below the iso-electric point. This results in dark and firm meat (DFD). In the case of PSE, a fast drop in pH or a low ultimate pH favor the oxidation of myoglobin to metmyoglobin, which results in a pale appearance (Lawrie, 1985). The importance of pH in the development of PSE, DFD, and other quality categories of pork is illustrated in Figure 1.

Figure 1. Effect of initial and ultimate pH on pork quality (from PIC, 1997)

Post-mortem glycolysis is responsible for the drop in pH. Glycogen stores in muscle are converted into glucose which is subsequently converted to lactic acid in the absence of oxygen. If glycogen stores are high and the temperature of the carcass is relatively high, glycolysis will proceed at a fast rate and the ultimate pH will be low. Glycolysis will cease either because glycogen stores are depleted or the pH has reached a point at which enzymes become inactivated, typically at a pH of about 5.5 (Lawrie, 1985).

 

The color of meat can be scored subjectively by comparing the meat color to a standardized color chart. The NPPC (1999) has recently changed their 5 color standard system to a 6 standard system, which concurs with the Japanese scale. The optimal color is 3 to 4, which represents a reddish pink to a dark reddish pink pork color.

 

More objectively, color can be measured using a Minolta Chromameter. This method measures the brightness (L* value), the redness (a* value), and the yellowness (b* value) of the sample. The NPPC (1999) suggested that the L values for their range of light color quality pork (color standard 1) to dark quality pork (color standard 6) should be approximately 61, 55, 49, 43, 37, and 31, respectively. Thus, the ideal L* value would appear to be between 43 and 49.

 

Drip loss from fresh pork is a result of shrinkage of muscle proteins (actin and myosin) and the subsequent expressing of fluids from the meat. At a low pH, the ability of proteins to bind water is diminished which will increase drip loss. In addition, a fast drop in pH will cause the actin-myosin complex that is formed during rigor mortis to contract and expel more fluid from the meat (Lawrie, 1985). The importance of pH in the  water holding capacity, and the development of dry or exudative pork is illustrated in Figure 1.

 

Drip loss can be measured by placing a pre-weighed filter paper on the muscle surface for a few seconds after which water uptake by the filter paper is determined. True drip loss can be measured by hanging portions of pork muscle of known weight suspended above a surface for approximately 48 hours at 4 °C. The sections of pork muscle are then weighted and the loss in weight as a fraction of the initial weight is a measure of drip loss.

 

The oxidative status of meat is often measured by determining the thiobarbituric acid reactive substances (TBARS). Malonaldehyde, a breakdown product of oxidized fatty acids is measured through its reaction with TBA (2-thiobarbituric acid) which results in a primarily red product. In addition, TBA can react with other substances and can thus be useful in determining oxidative spoilage from compounds other than fatty acids to give an estimate of overall oxidative stability. A level of approximately 1.0 mg MDA per kg tissue has been suggested as the threshold level in which rancidity or warmed-over flavor could be detected by a taste panel (Gray and Pearson, 1987). Standard measurements of meat quality, their ranges, and the optimum scores for each measurement are summarized in Table 1. The resulting pork quality categories most commonly encountered are listed in Table 2.

Table 1. Measures of meat quality (adapted from PIC, 1997)

Figure 2. Categories of pork quality (adpated from PIC, 1997)

 

Literature Cited

 

Cannon, J. E., J. B. Morgan, F. K. McKeith, G. C. Smith, S. Sonka, J. Heavner, and D. L. Meeker. 1995. Pork chain quality audit packer survey: Quantification of pork quality characteristics. J. Muscle Foods: 6:369.

Gray, J. I., and A. M. Pearson. 1987. Rancidity and warmed-over flavour. Adv. Meat Res. 3:221.

Lawrie, R. A. 1985. Meat Science. 4^th Edition. Pergamon Press, Oxford, UK.

Morgan, J. B., G. C. Smith, J. Cannon, F. McKeith, and J. Heavner. 1994. Pork distribution channel audit report. Pork Chain Quality Audit, Progress Report, April 6, 1994. p. 30.

NPPC, 1999. Pork Quality Standards. National Pork Producers Council, Des Moines, IA.

PIC, 1997. Meat Quality. Understanding industry measurements and guidelines. PIC, Spring, 1997, Franklin, KY.

Sonka, S., T. Doehring, S. Hofing. 1994. An economic assessment of enhancing quality management in the pork industry. Pork Chain Quality Audit, Progress Report, April 6,1994. p. 170.

 

Eric van Heugten