Intramuscular Fat

Intramuscular fat (marbling) tends to dilute the connective tissue of elements in muscle in which it is deposited, and this may help explain the greater tenderness reported for beef from well-fed good-quality animals (Beard, 1924).

From: Lawrie's Meat Science (Seventh Edition) , 2006

Fat and fat cells in domestic animals

Steven M. Lonergan , ... Dennis N. Marple , in The Science of Animal Growth and Meat Technology (Second Edition), 2019

Intramuscular Fat (or Marbling)

Intramuscular fat ( Fig. 5.14) is located between and within muscle fibers (cells) and its greatest deposition is in the later stages of the growth process. Intramuscular fat is called marbling in the meat industry and marbling has a significant impact on marketing fresh meat, particularly beef and pork loin cuts. The higher grades receive higher prices. The degree of marbling is used in the USDA Beef Grading System. The USDA marbling specifications have the highest degree of marbling for the USDA Prime grade and lower amounts of marbling for the USDA Choice and Select grades (Fig. 5.15). An illustration of the amount of marbling within 6 marbling degrees of the USDA Quality Grades for beef carcasses is shown in Fig. 5.16. The Abundant Marbling and the Moderately Abundant Marbling represents the USDA Prime grade; the Moderate, Modest, and Small degree of marbling represents the USDA Choice grade; and the Slight Amount of marbling represents the USDA Select grade. The relationship between the degree of marbling and the percentage of intramuscular fat is presented in Table 5.6. Marbling is also important for export standards used for pork sold to Japan and other Asian nations. Fig. 5.17 shows marbling standards used by exporters of quality pork cuts from the United States. Japan importers of US pork will pay a premium for highly marbled cuts. The same marketing concepts for marbling apply to beef exported to Japan.

Fig. 5.14

Fig. 5.14. An example of intramuscular fat in the pork muscle from the loin region of the carcass.

Fig. 5.15

Fig. 5.15. An example of the three levels of intramuscular fat in beef rib-eye muscle: (12th–13th rib) moderately abundant (Prime), moderate (Choice), slight (Select).

Fig. 5.16

Fig. 5.16. Marbling standards used for the USDA Beef Quality grades. Left column (top down): slight, small, modest. Right column (top down): moderate, slightly abundant, moderately abundant.

 Courtesy of the USDA.

Table 5.6. Relationship between percentage of intramuscular fat, marbling score, and carcass quality grade in beef cattle

Grade Marbling score Percentage intramuscular fat
Prime + Abundant
Prime ° Moderately abundant 12.3 and higher
Prime − Slightly abundant 9.9–12.2
Choice + Moderate 7.7–9.8
Choice ° Modest 5.8–7.6
Choice − Small 4.0–5.7
Select + Slight + 3.1–3.9
Select − Slight − 2.3–3.0
Standard + Traces 2.2 and lower
Standard ° Practically devoid
Standard − Practically devoid −

From Gene Rouse, Courtesy of Animal Science Department, Iowa State University.

Fig. 5.17

Fig. 5.17. An example of marbling standards used for the selection of pork for export.

 Courtesy of the National Pork Board, Des Moines, IA.

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CHEMICAL AND PHYSICAL CHARACTERISTICS OF MEAT | Palatability

R.K. Miller , in Encyclopedia of Meat Sciences (Second Edition), 2014

Marbling or Intramuscular Fat as an Indirect Measure of Meat Tenderness

Intramuscular fat also has an indirect relationship to meat tenderness. As animals grow and develop, fat is deposited sequentially into five different fat depots – mesenteric fat; kidney, pelvic, and heart fat; subcutaneous fat; seam fat; and marbling or intramuscular fat. As marbing is the last fat depot to be deposited, it can be used as an indication of growth and nutritional status of animals. If animals are fed high energy-based diets, they grow rapidly or they have high rates of protein and lipid accretion. Therefore, these animals are heavier with higher levels of subcutaneous, seam, and intramuscular fat and greater muscle mass. These heavier, fatter, and more muscular carcasses chill slower and are less susceptible to cold-induced toughening. Meat from early postmortem muscle subjected to cold shortening or cold-induced toughening has shorter muscle contractile state that results in tougher meat. Additionally, animals fed energy-based diets that grow rapidly have higher collagen solubility. Meat that has greater collagen solubility will be more tender because, during cooking, more of the collagen matrix (the main component of connective tissue) will melt. As more collagen melts, the connective tissue within the muscle will not contribute toward meat toughness or the meat is more tender.

Marbling or intramuscular fat positively affects meat flavor (Tables 1–3). As fat level increases, consumers tend to like the flavor of beef and pork. Fat has a characteristic flavor and is one of the major components of meat flavor. Many times it is not the predominant flavor in meat, but it does provide a balance with lean meat flavors. When meat contains very low levels of fat, the predominant flavors are associated with the lean, such as cooked beef lean, serumy, bloody, grainy, metallic, livery/organy, and brothy flavor aromatics. As the level of fat or marbling increases, the cooked fat aromatic or flavor increases in meat and this aromatic can assist in decreasing or masking flavor attributes associated with lean, thus providing a balance of meat flavors. The chemical basis of how adipose tissue and lipids contribute toward meat flavor will be discussed in the Section Lipids and Off-Flavor Development.

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Factors affecting the quality of raw meat

R.K. Miller , in Meat Processing, 2002

3.2.1 Fat component

Intramuscular fat content has been shown to affect flavor, juiciness, tenderness and visual characteristics of meat. Savell and Cross (1988) developed the Window of Acceptability to demonstrate the general relationship between the role of increased intramuscular fat on meat pork, lamb and beef palatability (Fig. 3.1). In general, as fat content increases, palatability increases; however, improvements in palatability with increasing fat percentage are not equal across all fatness levels. If fat content is less than 3%, palatability decreases markedly with each decrease in fat percentage. In fact, this is the steepest slope on the curve. As fat increases from 3% to about 6%, meat palatability improves, but not as dramatically as reported at the lower levels. As fat content exceeds 7.3%, fat is highly visible and has been identified as too fatty by health-conscious consumers. Too much visible fat has raised questions about consumption of fat in meat products and increased incidence of coronary heart disease, obesity or some forms of cancer in humans; these issues can affect consumers' perception of acceptability. Therefore, meat with fat content between 3 and 7.3% is generally considered acceptable. Diet/health-conscious consumers may be willing to sacrifice palatability for lower fat content.

Fig. 3.1. The Window of acceptability. Adapted with permission from Designing Foods: Animal Product Options in the Marketplace. Copyright 1988 by the National Academy of Sciences.

 Courtesy of the National Academy Press, Washington, DC.

How does intramuscular fat affect palatability? One way is through the relationship of intramuscular fat with meat juiciness. As intramuscular fat increases, humans perceive that the meat is juicier. During mastication or during the first bites, if fat is present, some of it is released and the salivary glands are stimulated. This results in a perception of juiciness, additionally, meat with a higher fat content may give a longer sustained perception of juiciness. Savell and Cross (1988) stated that 'fat may affect juiciness by enhancing the waterholding capacity of meat, by lubricating the muscle fibers during cooking, by increasing the tenderness of meat and thus the apparent sensation of juiciness, or by stimulating salivary flow during mastication'.

A second way that intramuscular fat affects palatability is through the relationship between fat content and tenderness. Interestingly, there is conflicting evidence as to the meat tenderness and fat relationship. Savell and Cross (1988) supported the relationship between increased intramuscular fat and meat tenderness by proposing four hypotheses. The first hypothesis, the Bulk Density Theory, states that as fat is lower in density than heat-denatured protein in cooked meat, as the fat percentage increases, the overall density of the meat decreases. As bulk density decreases within a given bite of meat, the meat is more tender. The second hypothesis is defined as the Lubrication Effect. Intramuscular fat is mainly triglycerides stored in adipose cells embedded in the perimysial connective tissue wall of the muscle. As meat is cooked, triglycerides melt and bathe the muscle fibers. As the meat is chewed, fat is released, salivation increases and the meat is perceived as juicy. Additionally, the muscle fibers give or slide more easily resulting in an increased perception of tenderness. The third hypothesis, the Insurance Theory, states that fat provides protection against the negative effects of over-cooking or high heat on protein denaturation. Meat proteins are involved in binding water in the muscle fiber. As meat is cooked, proteins denature and lose some of their ability to bind water. Fat can act to insulate the transfer of heat or slow down the heat transfer so that protein denaturation is less severe and less moisture is lost during cooking. The fourth theory or the Strain Theory relates to the weakening of the perimysial connective tissue surrounding muscle bundles. As marbling is deposited as adipose cells dispersed in perimysial connective tissue, development and an increased number of adipose cells weaken the connective tissue structure resulting in more tender meat.

To understand if marbling or intramuscular fat affected consumer acceptance and the subsequent relationship with trained sensory responses, the Beef Customer Satisfaction study was conducted (Lorenzen et al., 1999; Neely et al., 1998; Savell et al., 1999) in the United States. Beef top loin steaks from four USDA Quality Grade classifications were selected to represent four Quality Grade classifications where Low Select would contain beef top loin steaks with Slight00 to Slight50 degrees of marbling that would equate to about 3 to 3.5% chemical lipid; High Select steaks had Slight51 to Slight100 degrees of marbling or about 3.5 to 4.0% chemical lipid; Low Choice steaks had a small degree of marbling or about 4 to 5% chemical lipid; and Top Choice consisted of steaks with modest and moderate degrees of marbling or about 6 to 7% chemical lipid. Chemical lipid approximations were projected from Savell and Cross (1988). Steaks were evaluated by 300 households in four cities where each household contained two adult consumers who ate beef three or more times per week. Four top loin steaks from each carcass was served to four consumers in each city and one steak was evaluated by a trained meat descriptive attribute panel and Warner-Braztler shear force was conducted as a mechanical measurement of tenderness as described by AMSA (1995). Consumers rated Top Choice steaks highest for overall like and juiciness (Table 3.1). They liked the tenderness and flavor of Choice (Top Choice and Low Choice) steaks compared to Select steaks and they indicated that the Choice steaks had a higher intensity of flavor than Select steaks. Trained sensory panels also indicated that as marbling score increased, cooked beef top loin steaks were juicier, more tender, more intense in flavor and they had higher levels of beef flavor and beef fat flavor (Table 3.1). Warner-Bratzler shear force values decreased as marbling score increased (Table 3.1). In this same study, top sirloin and top round steaks also were evaluated. These steaks had slightly lower fat content than top loin steaks and the marbling to palatability relationship was not as strong.

Table 3.1. Least squares means of top loin steaks from US Beef Customer Satisfaction Study for consumer sensory attributes,a trained meat descriptive sensory attributes and Warner-Bratzler shear force (kg) as effected by USDA quality grade

Quality attribute USDA quality grade Root mean square error P-value
Top choice Low choice High select Low select
Consumer sensory attributes a
Overall like/dislike 19.2 c 19.1 c 18.8 c 18.7 c 3.06 0.0004
Juiciness 18.5 c 18.5 c 18.3 d 18.0 c 3.57 0.0006
Tenderness like/dislike 19.0 cd 19.2 d 18.6 cd 18.6 c 3.28 0.0001
Flavor intensity 19.1 c 19.2 d 18.9 cd 18.9 c 2.87 0.0009
Flavor like/dislike 19.3 cd 19.3 d 19.0 cd 18.9 c 2.88 0.0002
Trained meat descriptive sensory attribute b
Juiciness 5.8 d 5.6 c 5.5 c 5.4 c 0.58 0.0001
Muscle fiber tenderness 6.7 d 6.6 cd 6.5 c 6.5 c 0.58 0.01
Connective tissue amount 6.8 c 6.9 d 6.9 c 6.9 c 0.45 0.55
Overall tenderness 6.6 d 6.6 cd 6.5 c 6.5 c 0.56 0.06
Flavor intensity 5.7 d 5.7 d 5.6 c 5.6 c 0.31 0.002
Beef flavor intensity 3.5 d 3.5 d 3.3 c 3.3 c 0.32 0.0001
Beef fat flavor intensity 2.1 e 2.0 d 1.8 c 1.8 c 0.23 0.000a
Mechanical tenderness measurement b
Warner-Bratzler shear force, kg 2.70 d 2.75 d 3.00 c 2.95 c 0.71 0.0002
a
Values from Neely et al. (1998) and Lorenzen et al. (1999). Values differ from those reported as models differed slightly in order to generate these least squares means. Consumers' sensory attributes were rated as 1 = dislike extremely, not at all juicy, not at all tender, dislike extremely, and no flavor at all, respectively and 23 = like extremely, extremely tender, extremely juicy, like extremely, and an extreme amount of flavor, respectively.
b
Values are unpublished data, but they were derived from the same data set as published by Neely et al. (1998) and Lorenzen et al. (1999).
cde
Least squares means within a row and a cut lacking a common superscript differ (P < 0.05).

In pork, a similar study was conducted in three cities with pork consumers in the United States. Pork loin chops were selected to vary in pH, lipid content and tenderness as determined by Warner-Bratzler shear force value (Table 3.2). Pork consumers in the US did not rate pork loin chops differently based on lipid content. However, when a similar study was conducted with Japanese consumers (Table 3.3), Japanese consumers rated pork loin chops with higher National Pork Producer Council (NPPC) marbling score (NPPC marbling scores are a visual assessment of intramuscular fat and they are related to a chemical lipid value) as juicier, they liked the flavor and taste, they liked the color and they tended to like the amount of fat and visual appearance. Pork loin chops with the highest level of lipid tended not to be preferred by Japanese consumers most likely due to too much visible fat. In summary, there is a marbling to meat palatability relationship, but this relationship may vary across meat species and across consumer populations. While this relationship is not strong across all meat species, increased marbling or intramuscular fat assists in improving the eating quality of meat.

Table 3.2. Least squares means for pork consumer sensory traits a as affected by predetermined categories of lipid, Warner-Bratzler shear force, and pH from loin chops from the US Pork Consumer Sensory Study.

Trait n Juiciness Tenderness Flavor Overall like
pH category 0.04 0.0165 0.06 0.03
  Low 648 3.3 d 3.3 d 3.2 3.2 d
  Medium 620 3.3 d 3.3 d 3.2 3.2 d
  High 498 3.5 e 3.4 e 3.4 3.4 e
RSD c 1.13 1.08 1.10 1.03
Lipid category 0.20 0.19 0.09 0.18
  Low 427 3.4 3.3 3.3 3.2
  Medium 857 3.3 3.3 3.2 3.2
  High 482 3.4 3.4 3.4 3.3
RSD c 1.3 1.08 1.05 1.03
Shear category 0.0004 0.0001 0.0004 0.0001
  High 379 3.2 d 3.1 d 3.1 d 3.0 d
  Medium 844 3.4 d 3.3 e 3.3 e 3.3 e
  Low 520 3.5 e 3.5 f 3.4 e 3.4 e
RSD c 1.12 1.07 1.05 1.03

b P-value from the Analysis of Variance table.

a
Consumer attributes were evaluated using a 5-point hedonic, end-anchored sensory scale where 1 = dislike extremely and 5 = like extremely.
c
RSD = Residual Standard Deviation from the Analysis of Variance table.
def
Least squares means within a column and a trait lacking a common superscript differ (P < 0.05).

Adapted from Miller et al. (2000).

Table 3.3. Least squares means for consumer sensory scores of pork loin chops from the Japanese Pork Consumer Study that vary by NPPC marbling scores determined at the 10th rib in the Longissimus muscle.

Consumer attribute Marbling score c
1 2 3 4 5 6 P Value
Aroma like/dislike a 3.20 3.11 3.16 3.27 3.87 3.00 0.13
Juiciness like/dislike a 3.09 de 3.00 d 3.01 de 3.13 de 4.12 e 3.36 de 0.048
Tenderness like/dislike a 3.34 3.29 3.25 3.39 4.25 3.82 0.07
Flavor like/dislike a 3.15 d 3.19 d 3.14 d 3.29 d 4.12 e 3.64 de 0.04
Overall taste like/dislike a 3.15 d 3.16 d 3.12 d 3.34 de 4.25 f 3.82 ef 0.006
Appearance like/dislike a 3.01 d 3.11 de 3.19 de 3.32 de 3.75 e 2.82 d 0.02
Color like/dislike a 3.07 d 3.17 d 3.23 de 3.28 de 3.87 e 2.82 d 0.04
Color intensity b 3.16 d 3.36 de 3.15 d 3.25 a 3.87 e 2.91 d 0.02
Amount of fat like/dislike a 3.06 d 3.19 de 3.26 e 3.36 e 3.75 e 3.09 de 0.02
Overall visual like/dislike a 3.00 d 3.13 d 3.23 d 3.34 d 3.50 d 2.82 d 0.009
a
Consumer attributes were evaluated using a 5-point scale where 1 = dislike extremely and 5 = like extremely.
b
Consumer attributes were evaluated using a 5-point scale where 1   = light and 5 = dark.
c
National Pork Producers Council new fresh meat marbling scores where 1<1% lipid, 2 = 2% lipid; 3 = 3% lipid, 4 = 4% lipid, 5 = 5% lipid and 6 >   6% lipid.
def
Least squares means within a row lacking a common superscript differ (P < 0.05).

Adapted from Miller et al. (2000)

Intramuscular fat also has an indirect relationship to meat tenderness. As animals grow and develop, fat is deposited sequentially and marbling is the last fat depot to fill. Marbling therefore is an indication of growth and nutritional status of animals. If animals are fed high-energy-based diets they grow rapidly or they have high rates of protein and lipid accretion. The end result is heavier animals with higher levels of subcutaneous, seam and intramuscular fat and greater muscle mass. These heavier, fatter and more muscular carcasses chill slower and are less susceptible to cold-induced toughening (see discussion in 3.2.2). Additionally, animals fed energy-based diets, that grow rapidly have higher collagen solubility (see discussion in 3.2.3) that improves meat tenderness. It becomes apparent that interrelationships between the connective tissue, muscle fiber and fat component are involved in understanding meat palatability.

Marbling has been shown to affect consumer and trained sensory panel meat flavor attributes (Tables 3.1, 3.2, 3.3). As fat level increases, consumers tend to like the flavor of beef and pork. Fat has a characteristic flavor and has been identified as one of the major components of the meat flavor lexicon (Johnsen and Civille, 1986). Whereas fat is not the predominant flavor in meat, it provides a balance between lean and fat flavors. When meat contains very low levels of fat, the predominant flavors are those associated with the lean such as cooked beef lean, serumy, bloody, grainy, metallic, livery/organy, and brothy (Johnsen and Civille, 1986; Lyon, 1987). As the level of fat or marbling increases, the cooked fat aromatic or flavor increases in meat and this aromatic can assist in decreasing or masking flavor attributes associated with lean, providing a balance of flavors.

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Real-time ultrasound (RTU) imaging methods for quality control of meats

S.R. Silva , V.P. Cadavez , in Computer Vision Technology in the Food and Beverage Industries, 2012

11.6.1 Using RTU image analysis for IMF prediction

The IMF is primarily determined by the distribution pattern of fat flecks in a cross-section of the LTL muscle, usually between the 12th and the 13th thoracic vertebrae (Fig. 11.5a). Although IMF is present in other muscles, the assessment generally is performed on a LTL muscle section. The IMF consists of deposits that occur within the muscle, which are irregular either in form or in their dispersal. These deposits represent a cluster of IMF cells. Individual cells can be very small (40–60   μm) and are not visible to the human eye (Anon., 2004). The rough surface and small size of IMF deposits cause sound waves to scatter (Brethour, 1990; Whittaker et al., 1992), producing spots on RTU images that are referred to as speckles (Fig. 11.5b). This is why ultrasound techniques have the potential to predict IMF in vivo after RTU image analysis (Brethour, 1990; Whittaker et al., 1992).

Fig. 11.5. (a) Image from a cattle lumbar cut section showing LTL muscle and intramuscular fat flecks and (b) RTU image of the LTL muscle showing speckle originated from IMF.

The RTU image analysis for predicting IMF or marbling has been carried out in a number of ways over the years. Early studies were conducted to predict marbling scores from a subjective analysis of the RTU image features (coherent speckle, attenuating and reverberation) from which a speckle score was obtained (Harada and Kumazaki, 1979; Brethour, 1990). Speckle scores were estimated visually and corresponded subjectively to a point classification scheme. This procedure had the benefit of allowing an immediate estimation of the marbling score and, thanks to the portability of the ultrasound equipment portability, could be used for farm animals (Brethour, 1990). However, it is subjective, and dependent on beam geometry and machine calibration. Furthermore, an understanding of the classification scheme and calculation of the score can be difficult for a technician to acquire (Brethour, 1990). These negative aspects led Brethour (1990) to observe that ultrasound speckle was a 'quick and dirty' way to estimate the marbling score of a carcass and that, consequently, further improvements were necessary to reduce the subjectivity of RTU images. Although a skilled ultrasound technician can visually interpret an RTU image and estimate marbling in a live animal with fair accuracy (Brethour, 1990, 1994), it was recognized that research using mathematical models for RTU image analysis was imperative (Amin et al., 1993; Kim et al., 1998).

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Supplementing selenium and zinc nanoparticles in ruminants for improving their bioavailability meat

J. Efrén Ramírez Bribiesca , ... Atmir Romero Pérez , in Nutrient Delivery, 2017

5 Selenium and Zinc in Muscle

Intramuscular fat content and composition of fatty acids are important in meat quality. Fat is susceptible to oxidative degradation due to the natural turnover, oxidation of lipids and membrane phospholipids ( Combs and Regenstein, 1980). Selenium acts on selenoenzyme (Papp et al., 2007), prevents or delays the oxidative reactions. Little research has been done showing a relationship between Se and meat quality. Furthermore, the shelf life of packaged meat is a major marketing problem due to deterioration in color and microbiological growth. Selenium and vitamin E are the main compounds used to improve the color stability and lipid muscle. It has been shown that the addition of Se and vitamin E reduces lipid oxidation (Combs and Regenstein, 1980). In reference to Zn, the ideal is to use sources of Zn or vehicles that can improve absorption and reduce fecal excretion and Zn deficient consumption, causes absorption coefficients in the small intestine of between 3% and 38%. Zinc absorption, as mentioned, appears to be regulated by the synthesis of a protein termed intestinal metallothionein (Liuzzi and Cousins, 2004). Zn then passes to portal circulation through the Zn transport protein-1 (ZnTP-1), reaching the liver and other tissues, such as muscle. Approximately 70% of the Zn is bound to circulating albumin. No specific anatomical site function as reserve Zn and consequently, no conventional reserves in tissues that can be released or stored; however, products of animal origin have a high Zn content, while legumes contain very low levels of Zn. The Zn replacement in the body is slow, with a maximum biological life of 250 days. Zinc body reserves are small and have a rapid turnover rate. Therefore, the continuous supply of Zn in ruminants necessary, this can be achieved with intraruminal bolus slow release or administration of nanoparticles, so that it can maintain suitable or high amounts in muscle tissue (Munday et al., 2001).

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Quality indicators for raw meat

M.D. Aaslyng , in Meat Processing, 2002

8.3.1 Intramuscular fat and fatty acid composition

The content of intramuscular fat or the degree of fat marbling has a great influence on the eating quality beginning when the consumers choose the meat in the supermarket. Many consumers will reject buying meat with a medium or high amount of visual fat marbling both in beef and pork even though they find it more palatable when eaten without knowing the amount of fat ( Grunert, 1997; Bredahl et al, 1998; Brewer et al., 2001a), Bligaard, 2002, Pers. comm.).

There are conflicting results on the influence of IMF on tenderness. It is said to increase both the tenderness of the meat (DeVol et al., 1988; Cameron and Enser, 1991; Gwartney et al., 1996; Fernandez et al., 1999; Candek-Potokar et al., 1999; Laack et al, 2001; Brewer et al, 2001a; D'Souza and Mullan, 2002) and to have no effect or even a negative effect (Göransson et al., 1992; Kipfmuller et al., 2000). The reason for these results could be, that variations in IMF are always confounded with other variations, which also have significance for tenderness. The age of slaughter and the slaughter weight influence the content of IMF (Johnson et al., 1969; Candek-Potokar et al., 1999) but can also influence factors like the content and strength of connective tissue and in this way influence the tenderness. Feeding strategy not only influences the IMF content (Blanchard et al., 1999) but also the growth rate and thereby the proteolytic activity that is of significance for the tenderisation of the meat during ageing (Therkildsen et al., 2002). The genetic background also contributes to variations in IMF. In pork some breeds like the Chinese breeds and Berkshire have an extremely high fat content. In the more commercial breeds Duroc especially is known to have a higher content of IMF compared to the white breeds like Landrace and Large White. It has been shown however, that the correlation between IMF and sensory quality depends on breed (Fjelkner-Modig and Persson, 1986). The fatty acid composition can also influence the effect of IMF on tenderness. In pork the saturated and monounsaturated fatty acids are positively correlated to tenderness where polyunsaturated fatty acids are negatively correlated to tenderness (Cameron and Enser, 1991; Eikelenboom et al., 1996). The fatty acid composition is dependent on both breed (Garcia et al., 1986; Tejeda et al., 2001) and feed (Engel et al, 2001).

It has also been said that a high content of IMF would improve the robustness of the meat against a non-optimal cooking. This was shown in beef by Cummings et al., (1999) who found a decline in tenderness in meat with a low IMF content when cooked to 80   °C while meat with a high IMF content was still tender at this end-point temperature. The difference between the two groups at 70   °C end-point temperature was only small. It was not possible to find a similar effect in another study (Rymill et al., 1997) and the effect of IMF on the robustness of the meat might therefore interact with other matters, as the direct effect of IMF on tenderness is said to do.

Juiciness is the feeling of moisture in the mouth during chewing. It is a dynamic attribute changing during the chewing process. The content of IMF is positively correlated to juiciness (Savell and Cross, 1988; Gwartney et al. 1996; Flores et al., 1999; Cummings et al., 1999; Brewer et al. 2001a). Some investigations indicate especially that the sustained juiciness experienced during the last part of the chewing process, is increased by increasing amount of IMF (Savell and Cross, 1988; Aaslyng et al., 2002). An increasing amount of IMF also implies a decrease in cooking loss (Aaslyng et al., 2002). Juiciness is to some extent negatively correlated to cooking loss (Tornberg and Goransson, 1994; Toscas et al., 1999; Aaslyng et al., 2002) and the decreased cooking loss could explain part of the effect of IMF on juiciness.

The content of IMF also influences the flavour of meat (Candek-Potokar et al., 1998; Fernandez et al. 1999). This might be due to production of a volatile component as the fatty acid composition is important to the flavour. In pork the content of polyunsaturated fatty acids is correlated with abnormal flavour while monounsatuated and saturated fatty acids are correlated with pork flavour and overall liking (Cameron and Enser, 1991; Cameron et al., 2000). In beef it has been found that the meaty aroma was due to phospholipids and not to such a great extent to triglycerides (Mottram and Edwards, 1983). Flavour is a composition of volatile and nonvolatile components. It is not investigated how IMF influences the nonvolatile flavour components but part of the unspecified effect on flavour could be due to facilitating the contact between the flavour components and the taste buds.

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Saturated fat reduction in butchered meat

K.R. Matthews , in Reducing Saturated Fats in Foods, 2011

10.5.1 Role of fat in the tenderness of meat

It is mainly the intramuscular fat that is considered important for the tenderness of meat. This is the fat within the muscle itself, which at low levels is invisible and at higher levels becomes visible as 'marbling'. This might be thought to have an effect on tenderness in a number of ways. Fat tissue within the muscle might substitute for muscle that, within a given piece, is diluted with softer tissue, thus reducing the overall force required to bite through the meat. This might be considered to be a likely effect at higher levels of intramuscular fat. Alternatively, or in combination, the fat might weaken the structural integrity of muscle, perhaps preventing cross links forming between connective tissue of muscle fibre proteins, thus enabling the muscle to be broken up more readily in the mouth. A further possible effect in the mouth is the potential for fat to lubricate during chewing, reducing resistance to the teeth through reduced friction. Studies to evaluate these effects in the mouth are very difficult to conduct and generally sufficient useful information is obtained by the use of trained sensory panels assessing the overall tenderness (or toughness) of the meat. The complex nature of chewing, however, means that care should be taken in the interpretation of results from instrumental measures of toughness. These should only be relied upon as a guide to the sensory perception of quality.

Where an effect of fatness on the tenderness of meat has been observed it is usually positive. Across the range of fat contents seen normally in British red meat, however, the effect is generally small, such that even a doubling of the fat content would have only a very small impact on the sensory perception of tenderness. Having said that, the literature is consistent, with a decline in tenderness for meat from those animals at the very leanest end of the scale, suggesting that a minimum level of intramuscular fat is required to prevent damaging tenderness. Below about 2.5% intramuscular fat beef tenderness has been seen to decline sharply, but above that there is very little effect of intramuscular fat (Buchter, 1986). Similarly, research at the Meat and Livestock Commission's Stotfold Pig Development Unit found that P2 fat depths below 8   mm were associated with tougher meat (MLC, unpublished data).

A further benefit of fat in meat tenderness is the insulating effect it has on the carcase immediately post slaughter. The muscle in fatter carcases cools more slowly post slaughter. When muscle chilling is too rapid a toughening effect, called cold shortening, can occur. The slower cooling of fatter carcases can reduce this effect, resulting in apparently more tender meat. If chilling is considerate, however, this advantage to fatter carcases disappears. This effect is particularly apparent in smaller lamb carcases, which cool more rapidly.

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The role of lipids in food quality

Z.E. Sikorski , G. Sikorska-Wiśniewska , in Improving the Fat Content of Foods, 2006

9.4.1 The role of lipids in the texture of meat and meat products

The amount and distribution of intramuscular fat have been regarded as important characteristics of meat quality and are recognized as one of several criteria in establishing beef carcass quality grades. The degree of marbling is determined visually in cross-sections of the longissimus dorsi muscle. Among some professionals there is a belief that marbling contributes to meat tenderness. However, no unequivocal published evidence has been found showing high positive correlation between the contents of intramuscular fat and meat tenderness. The beneficial effect of marbling on meat quality may be due to the lubricating action of the fat layers during chewing and swallowing, which may be perceived as increased tenderness of tough meat samples. Intramuscular fat uniformly distributed on the cross-section of the meat cut, in limited amount, improves the flavour and juiciness, while meat with almost no marbling may be dry and deficient in flavour. The effect of fat on the tenderness of meat is also treated by Moloney in Chapter 13.

The consistency of the fatty tissues in meats depends on the FA composition of the fat, which in turn is affected by the characteristics of the fats contained in the feed given to the slaughter animals. This is especially pronounced in pork. Owing to solidification caused by chilling, the subcutaneous fat and marbling increase the firmness of the carcass and retail cuts and contribute to retaining the characteristic shape during handling and processing of meat.

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Transcriptomics of Meat Quality

B. Guo , B.P. Dalrymple , in New Aspects of Meat Quality, 2017

2.2.6 Why the Broad Range of Different Genes Implicated?

The relationship between gene expression and IMF% is confusing, with limited overlap of gene lists between many of the publications on the subject even within a single species (Table 11.2). But despite the variation in results from the broad range of investigations, the expression of genes involved in the synthesis and storage of LCFAs and TAG, which is biologically sensible, appears to be the most consistently related to IMF% in ruminants. There are a number of explanations for the wide variation in genes identified and the apparent lack of congruence between many of the results:

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There are fundamental differences in the mechanism of LCFA and TAG synthesis under different conditions, or in different muscles, or at different ages.

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Given the conserved nature of the pathways for LCFA and TAG synthesis in adipose tissues in mammals, this appears quite unlikely.

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The transcriptomic data are unreliable and inherently noisy.

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Early microarray platforms and qPCR-based approaches are not sufficiently comprehensive.

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Biological, and probably to a lesser extent technical, noise will be a significant contributor to variation between experiments, particularly with small numbers of samples and where the differences in expression of the responsible genes between the groups are small.

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It is likely to lead to partial identification of sets of important genes as individual genes miss the cutoffs.

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GAPDH, a widely used reference gene for qPCR, has been shown to be unsuitable for studies on IMF (Zhu et al., 2015).

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Differences in the rate of utilization of stored lipids have also been proposed to affect IMF% and could be a confounding factor in the relationship between gene expression and IMF%.

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The expression of genes encoding lipolytic enzymes has been reported to be both lower (Hamill et al., 2012; Zhao et al., 2009) and higher (Jeong et al., 2013; Lim et al., 2015) in high-IMF% animals than in low-IMF% animals. In one study Jeong et al. (2012) demonstrated that the expression level of the TAG synthesis gene glycerol-3-phosphate acyltransferase, mitochondrial (GPAM, aka GPAT1) explained more than half the variation in IMF, with the next largest effect gene being the TAG degradation gene patatin-like phospholipase domain containing 2 (PNPLA2, aka ATGL). While expression of GPAM was correlated with IMF% in our analysis, its expression was not highly correlated with the expression of the other genes in TAG synthesis and storage gene set across development (De Jager et al., 2013). In our analysis the expression of genes encoding lipolytic enzymes was not positively or negatively correlated with IMF% (De Jager et al., 2013; Guo et al., 2014).

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Some of the experiments with young animals have been undertaken when IMF% is decreasing; see, for example, in goats (Wang et al., 2015a). Under these conditions the signals from IMF deposition (if this is occurring at this time) and utilization are likely to be mixed and to represent different processes.

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The expression of the LCFA and TAG genes reflects current IMF% deposition rate, while the measurement of IMF% itself reflects the entire history of the individual.

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The current rate and the history are not always congruent, especially in sick, young, and old animals. Thus animals on different growth trajectories and at different stages of their growth are difficult to compare.

Since many of the analyses were conducted on animals in which the deposition of IMF was likely to be reaching maturity (Fig. 11.2), the last two explanations, and the partial representation of pathways, appear to be the most likely explanations for the lack of significant overlaps in many cases. The conflicting results in genes identified in very young animals are likely to reflect the balance of IMF synthesis and utilization under the particular conditions of the experiment. It appears likely that expression of FABP3 may be associated with the use of LCFAs (including IMF) as an energy source in very young animals, explaining the correlation of the expression of FABP3 with IMF% in these animals, but also the inconsistent results depending on the age and metabolic status of the animals. The expression of FABP3 may be a useful marker for the utilization of IMF and the ratio of expression of deposition genes to FABP3 may have more utility for estimating net deposition of IMF than the deposition genes alone.

The reason for the absence of a strong association between LCFA and TAG synthesis and storage genes and IMF% in the majority of investigations is not so clear for pigs and chickens, but, in addition to the factors mentioned previously, may also reflect the low IMF% (in pigs) and the relative roles of circulating and de novo synthesized LCFAs in deposited TAG.

With the exception of PPARG and CEBPA, there is even less consistency in the genes identified as potential regulators of lipogenesis. It appears unlikely that investigating animals reaching maturity in IMF deposition will readily identify regulators of IMF deposition. Such experiments should be undertaken during the earlier stages of IMF deposition when the genes encoding proteins involved in the synthesis and storage of LCFAs and TAG are correlated with IMF% (and most likely also IMF deposition rate).

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Chemical and Biochemical Constitution of Muscle

Clemente López-Bote , in Lawrie´s Meat Science (Eighth Edition), 2017

4.3.3 Sex

In general, males have less IMF than females, whereas the castrated members of each sex have more IMF than the corresponding sexually entire animals and higher concentration of saturated fat (Wood et al., 2008).

As assessed from a representative sample of UK animals, entire pigs were found to have a greater concentration of heme pigment (myoglobin plus hemoglobin) in their Longissimus dorsi muscles than castrates (Warriss, 1990). Because the meat of mature boars can be associated with an unpleasant odor, it is important to be able to detect the presence of meat from male pigs in various products.

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