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Meat Quality Traits and Genetic Selection
Tom J. Baas, Ph.D., Iowa State University
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Ideal quality in fresh pork can be defined as a combination of traits that include appearance, taste, nutritional value and wholesomeness. All of these characteristics will affect continued acceptance of pork by the consumer.
Four measurements of loin quality (color, water holding capacity, ultimate pH and intramuscular fat) have been identified as key traits to consider in evaluation of meat quality. These traits are important because they are related to attractiveness, palatability, and loss during processing and storage, and therefore, account for a large portion of the economic loss associated with poor quality pork. A description of each trait follows.
Color. Ideal color in fresh pork is described as reddish pink. Consumers object to pork that is either too pale or too dark. Abnormally pale muscles quickly turn gray in the retail display case and often undergo considerable shrink, resulting in economic losses during processing and dry tasting products after cooking. Extremely dark pork will generally have a shorter shelf life because it is less acidic and therefore supports bacterial growth.
Color can be measured objectively using a Minolta Chromameter on the cut surface of the loin muscle or other available muscle of the carcass after the surface has been allowed to bloom. Color may be expressed as Minolta (normal range from 17-33) or Hunter L* values (normal range from 30-60). Lower values are darker and higher numbers are paler, lighter colored meat. Consumers prefer medium to darker colored meat.
Water Holding Capacity (WHC). Water holding capacity is the ability of meat to retain its water during further processing, including cutting, heating, grinding and pressing. It is the amount of exudate or moisture on the cut loin surface and can be estimated using the filter paper method. A pre-weighed piece of filter paper is placed on the loin muscle and allowed to absorb moisture on the surface. It is then reweighed and the difference in weight is the WHC estimate. Lower numbers are more desirable due to their association with higher value for all segments of the industry.
Ultimate pH. A measurement that is highly correlated to water holding capacity or drip loss is pH, which is a measure of the acidity of the meat. Ultimate pH is measured in the cooler 24 hours after slaughter by inserting a pH probe into the pork muscle. Lower pH values are related to greater drip losses during further processing. Higher pH values are more desirable because they are associated with lower drip loss, darker color, more firmness, and increased tenderness of the loin chop — all positive attributes.
Intramuscular Fat (IMF). This trait may also be called marbling or lipid content. It is estimated using laboratory analysis of total lipid content of a loin muscle sample. Marbling is necessary for a juicy and flavorful cooked product. On the other hand, pork with large amounts of marbling supplies excess calories and is visually objectionable to some consumers. Research results indicate a minimum of 2.5-3.5 percent is needed for desirable eating quality. Visual marbling scores range from 1-10 (see chart) and correspond to percent intramuscular lipid content. Scores of 2-4 are considered desirable in most situations.
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| Visual color scores based on the scale of 1-6 on the Pork Quality Standards chart may also be used to estimate color: 1=Pale, pinkish gray to white These visual color scores correspond to Hunter L* values (1=61, 2=55, 3=49, 4=43, 5=37, 6=31). |
The hams pictured at the top of the adjoining chart offer an excellent contrast of the extremes in meat quality. The ham in the center (RFN) is considered ideal because it is reddish pink in color, firm and non-exudative (good water holding capacity). The PSE ham on the left is very pale in color, very soft, and has a great deal of moisture loss. This type of ham is undesirable in appearance and shrinks excessively during processing and cooking. Pigs that carry the stress gene are likely to produce PSE hams due to rapid decline of the pH of the carcass immediately after slaughter. The DFD ham on the right is very dark in color, very firm and dry. It has good water holding capacity, but its extremely dark color makes it undesirable for most consumers.
Trends in swine marketing over the past two decades have changed dramatically. Pricing systems have changed from being based solely on weight of the market pig to emphasis and premiums for lean value of the carcass, with an optimum weight range for each carcass that must be met to avoid discounts. Premiums are given to selected producers who can deliver large quantities of lean hogs in regularly scheduled shipments directly to the packer. Since the genetics that produce the highest lean percentage can also lead to poorer meat quality, swine producers must use genetic sources that offer a combination of high lean percentage with good meat quality.
For producers to make genetic improvement in meat quality traits, they must first realize that both the sire and the dam contribute equally to the genetic merit for meat quality. Next, they must purchase from a seedstock source that measures meat quality and has lines that have been tested and proven to be superior for meat quality. Selection decisions that must be made include which population to purchase from, line within the chosen population, and individual animals within the selected line. These decisions must be guided by the individual performance testing programs offered by the seedstock source.
The National Swine Registry has been a world leader in performance-testing programs and genetic-evaluation systems. The progress that has been made in backfat, lean percentage, and growth through the STAGES program is well documented. Recognizing the importance of meat quality traits, U.S. purebred breeders have moved beyond selection for leanness and have implemented testing programs to also emphasize meat-quality traits. Breeders are conducting unbiased progeny testing programs and regularly evaluate meat-quality traits to ensure their customers have access to lines that will improve these important traits. Breeding values (EPDs) are now being estimated to assist producers in making selection decisions. In addition, breeders have eliminated the stress gene from their herds to further improve the meat quality of their genetic lines.
