Determining Muscle Quality: Marbling, Color, Firmness, and Wetness

by Wayne F. Gipp

A. Introduction

Effective evaluation of meat quality requires 1) reference standards to keep evaluators on tract and consistent from one sample to the next and from day to day; 2) proper and consistent illumination of samples and reference standards; and 3) adequate training of evaluators for the ability to differentiate the traits being assessed.

Photographs depicting color and marbling standards are not included in this factsheet. These are available from NPPC either as a notebook chart ($1), wall chart ($10) or as a packet of laminated cards ($32.50)

B. Determination of Marbling (Intramuscular Fat)

Intramuscular fat or marbling is related to meat eating quality (defined as flavor, juiciness and/or tenderness of meat). Pork with a higher amount of intramuscular fat is expected to produce meat with more desirable and less variable eating quality than pork with less intramuscular fat. Some consumers prefer pork containing little or no marbling, in order to reduce calorie intake,while others desire the flavor and juiciness of moderately marbled meat.

  1. Subjective visual standards
    • Visual marbling is the congregation of adipose (fat) cells deposited between the muscle cells and bundles of lean tissue that can be observed by the human eye.
    • For the visual, subjective assessment of the amount of intramuscular fat (marbling) interspersed within the lean tissue, standardized pictures of lean with varying amounts of marbling are used for reference.
    • Pictures of pork containing low to high levels of intramuscular fat with standardized numerical marbling scores are presented below. The photos available from the National Pork Producers Council have been revised and standardized based on actual content of lipid within the muscle as determined by chemical analysis.
    • Note: The subjective photographic reference standards are designed to correlate directly with the percent intramuscular fat within a cut. Therefore, Marbling Score 1.0 = 1% intramuscular fat, Marbling Score 2.0 = 2% intramuscular fat, and so on up the scale at 1% increments.
  2. Recommended conditions for proper application of visual marbling standards:
    • It is very important to wait until the carcass has been chilled to at least 40 degrees Fahrenheit before trying to subjectively evaluate marbling. Warm fat will be semi-liquid and very difficult to see visually. It is generally recommended that marbling evaluation be conducted at the same time as color, wetness, and firmness assessment (no earlier than 24 hr after slaughter).
    • Trained evaluators subjectively evaluate the amount of marbling by comparing the chemically determined visual standard of the 10th rib loin muscle. The accuracy of visual evaluation of marbling will depend on the level of training of the evaluator and the evaluator’s ability to repetitively call the same marbling score for meat with similar intramuscular fat content.
    • These standards can be used on muscles other than the loin muscle. However, care must be taken to estimate the amount of intramuscular fat in the specified muscle using the NPPC marbling standards. It is very important to note that the relationship between visual marbling level and chemical lipid may not be the same when these standards are applied to other muscles because the entire chemical lipid is not visible to the eye.

C. Muscle Color Determination

Fresh meat color has a subtle but important impact on consumer decisions. Most consumers prefer a bright, reddish-pink color in fresh pork. Meat cuts that are too dark, too pale, or too variable in color within a package or within the display case, may lower the consumer’s perception of quality.

  1. Visual Fresh Pork Color Reference Standards
    • The National Pork Producers Council has developed a set of visual reference standards to be used in evaluating fresh pork color. The standards attempt to represent the full range of color encountered from very pale to very dark. Color evaluators should always have the reference standards available to maintain consistency when calling color scores. It may not be necessary to refer to the standard for every evaluation, however, it is recommended that evaluators use the standards at the beginning of each series of evaluations in any given day. When large numbers of samples are being evaluated, the standards should be referred to periodically in an effort to maintain consistent results.
    • The standards, illustrated below in black & white, are available as color cards from NPPC. For the most accurate depiction of the representative color standards, it is recommended that the actual color reference cards be used rather than the color photographs in the notebook or on wall charts.
  2. Recommended conditions for use of visual color standards.
    • The type and intensity of the illumination of the samples to be evaluated can have a major influence on the scores assigned. While it may not be possible to use a standardized illuminant, it is important to have the illuminant consistent within a group of carcasses. Under ideal conditions, a D65 light source is recommended.
    • Color evaluation should not be conducted until at least 24 hr. after slaughter. The temperature of the carcass should not exceed 40EF at the time of evaluation. Meat oxygenates (blooms) after exposure to air. For pork, a minimum bloom time of 10 minutes is recommended. Within any series of evaluations, bloom time should be kept constant.
    • In order to avoid photochemical effects and drying of surfaces (which may alter the perception of color) evaluation should be completed within 1 hour of cutting a fresh surface.

D. Muscle Firmness — Wetness Determination

Pork with a very low water holding capacity may exhibit a very coarse open texture accompanied by considerable fluid exudation on the cut surfaces of fresh meat. This is unattractive to the consumer, and also results in excessive losses and drying of the meat during cooking.

Firmness and wetness are evaluated as independent quality factors. Traditionally, these two variables have been evaluated together. However, pork is observed that may be firm and wet on the surface, instead of the soft and exudative condition always being linked. Therefore, it is recommended that the two be treated as independent variables.

  1. Subjective Assessment of Muscle Firmness
    • Three-point FIRMNESS evaluation scale.
      1. Soft — Cut surfacs distorteasily and are visibly soft.
      2. Firm — Cut surfaces tend to hold their shape
      3. Very Firm — Cut surfaces tend to be very smooth with no distortion of shape
      4. Subjective Assessment of Muscle Wetness
    • Three-point WETNESS evaluation scale
      1. Exudative — Excessive fluid pooling on cut surfaces or in packages
      2. Moist — Cut surfaces appear moist, with little or no free water.
      3. Dry — Cut surfaces exhibit no evidence of free water.
  2. Recommended Conditions for Proper Assessment of Firmness and Wetness
    • It is recommended that wetness and firmness of fresh meat be assessed under the same conditions as recommended for subjective color assessment. Light angle may be especially significant in evaluating these variables. Many times the evaluator may find that a change in viewing angle assists in making an accurate assessment.

E. Confounding Factors Affecting Subjective Evaluation of Marbling, Color, Firmness, and Wetness

Evaluators need to be constantly aware that color, firmness, wetness, and marbling are all viewed at the same time and can have a profound effect on the perception of each upon the other.

Large amounts of marbling may make it difficult to accurately assess the muscle color unless the evaluator concentrates on the lean portion.

Marbling in light colored pork is more difficult to see because the color contrast between marbling and lean is not as great as when the lean is darker. Care must be taken to adequately examine marbling content in light colored meat samples.

Size of marbling flecks may influence the marbling score by making it difficult to determine the overall area of marbling fat.

Meat surfaces that are wet with pink juices may mask some of the marbling.

Very soft tissue scatters the light tending to exaggerate paleness, while a firm surface may exaggerate darkness.

Variation of color on the lean surface of a given cut of meat is quite common. It becomes a judgement call to determine when a particularly pale area is sufficient to lower the color score.

Well trained and disciplined evaluators must concentrate on each factor being assessed and try to score them independently of other traits.

NOTE: The evaluation procedures, Fat Free Lean Equations, photos and Quality Standards in this factsheet were adapted with permission from "Pork Composition & Quality Assessment Procedures," published in 2000 by National Pork Producers Council, Des Moines, Iowa.

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