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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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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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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Growth curves and growth patterns

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

Serial Ultrasound Scanning

To obtain a good genetic selection base for intramuscular fat, subcutaneous fat thickness, and rib-eye area, serial ultrasound values need to be obtained at each growth stage, shown in Figs. 6.8 and 6.9. Iowa State University researchers under the leadership of Dr. Gene Rouse and Dr. Doyle Wilson developed experiments to establish a database to determine EPD values for carcass traits that included intramuscular fat, rib-eye area, and subcutaneous fat thickness. The bulls were scanned with a real-time ultrasound machine every 28 days for intramuscular fat, subcutaneous fat thickness, and rib-eye area. Live weight was also obtained every 28 days. Examples of the ultrasound images that were obtained are shown later. Image 1 of Fig. 6.32 shows the ultrasound scan at the rump area of the steer and Image 2 shows the ultrasound scan at the 12th–13th rib region for rib-eye area and subcutaneous fat thickness. Image 3 of Fig. 6.32 shows the intramuscular fat in the rib-eye muscle at the 12th–13th rib sections.

Fig. 6.32

Fig. 6.32. Images obtained from ultrasound scans of cattle. Image 1 is from the rump section, Image 2 is a cross section at the 12th and 13th rib, and Image 3 is a longitudinal scan of the rib eye showing intramuscular fat.

 Courtesy, Dr. Gene Rouse, Iowa State University.

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Use of meat quality information in breeding programmes

G. Simm , ... R. Roehe , in Improving the Sensory and Nutritional Quality of Fresh Meat, 2009

Marbling scores

Quartered beef carcasses are commonly visually assessed for marbling or IMF, using a subjective score awarded by trained assessors. Different scoring systems and anatomical assessment sites are used in various countries (e.g. USDA, AUS-MEAT, Ferguson, 2004). As well as predicting IMF content, marbling scores give information about distribution and size of IMF depots, which may affect the attractiveness of meat. Correlations between IMF and marbling score are generally moderate, but vary depending on the system used, amongst other factors. Nevertheless, EBVs for marbling in beef are available in USA and Australia for different breeds.

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Meat and Fat

Gerhard Feiner , in Salami, 2016

1.9 Fat

Fats, or lipids, are the most concentrated source of food energy and are necessary to health. Carcass fat contains around 80–85% triacylglycerol fat, 5–10% moisture, and around 10% connective tissue. For the context of this book, fat is seen as fatty tissue given that fatty tissue and fat are by definition not the same, as "fat" refers to the fat material only without water or connective tissue. Fat is a nonpolar molecule, and unlike water, which is polar, fat does not exhibit a negative and positive end (pole). Fat, or lipids, are therefore insoluble in water due to the presence of insoluble carbon–hydrogen components within the molecule. Food fats are carriers of fat-soluble vitamins and some essential unsaturated fatty acids.

Fat is generally colorless but exhibits occasionally a touch of yellow and is by nature extremely hydrophobic (lipophilic). Fat from cattle, fed fresh grass containing carotene, frequently shows a yellow touch, and fat itself carries some flavor but is an excellent solvent for countless other flavor and aroma components. Depending on the type of fatty acids present in meat, the flavor can vary dramatically. Pork fat produces on the impact of heat saturated as well as unsaturated aldehydes, which are typical for the pork flavor. Such components are hardly present in beef fat, as beef fat contains predominantly saturated fatty acids. Some branched fatty acids are found within fat originating from sheep, which are responsible for the pronounced sheep, or lamb, flavor. The difference in flavor and taste of different types of fat is not solely based on pure fat but more due to the heat treatment of fatty tissue overall. Fatty tissue, besides fat, also contains connective tissue as well as other amino acids, which contribute to a large extend to the various flavors originating from different types of fat. Fatty acids, such as oleic acid, show a positive impact toward the flavor of fat, while stearic and linolenic acid demonstrate a negative impact on the flavor of fat overall.

Fats are divided into three major groups:

1.

Intramuscular fat (fat between the muscle fiber bundles). It is also known as the marbling fat and as such plays a major role toward juiciness, flavor, and tenderness of meat. The world famous Kobe beef in Japan has extremely high levels of such marbling fat, which, beside other factors such as a very special diet and treatment of the animal overall, contributes to the very tender, juicy, and tasteful meat.

2.

Intermuscular fat (between individual muscles)

3.

Subcutaneous or depot fat (under the skin)

The building blocks for fat (or simple lipids) are triglycerides, and fat is made of molecules of carbon, hydrogen, and oxygen. Other substances, known as complex lipids, also contain phosphorous, nitrogen, and sulfur, besides carbon, hydrogen, and oxygen. Triglycerides in fat are esters of the trihydric alcohol glycerol, and three fatty acids (three of the same type or three different ones) are bound to glycerol. An ester is a compound formed from the reaction between an alcohol and acid by the removal of water. The reaction is the following: Alcohol (glycerol)   +   acid (fatty acids)   ⇒   ester   +   water (see Fig. 1.17).

Figure 1.17. Molecule of triglyceride consisting of the alcohol glycerol and three fatty acids.

Glycerol (or 1,2,3-propane triol) is an alcohol showing three OH groups within its molecule. When triglycerides are solid at room temperature, they are called "fats." On the other hand, in case triglycerides are liquid at room temperature, they are called "oils."

Lipids include mono-, di-, and triglycerides; sterols; terpenes; phospholipids; fatty alcohols; and fatty acids. Phospholipids, such as lecithin, exhibits two fatty acids and a phosphoric component bound to glycerol. Cholesterol is the most well-known representative from the sterols group. Monoglycerides have one fatty acid bound to glycerol, while diglycerides demonstrate two fatty acids bound to glycerol. Triglycerides exhibit three fatty acids bound to the alcohol glycerol.

Fatty acids consist of long chains of hydrocarbon with a carboxyl group (single bondCOOH) at the end. The carboxyl group (single bondCOOH) is carbon 1 for the purposes of naming the fatty acid. This group is shown in a chemical formula mostly at the right-hand end, while the methyl (single bondCH3) end is generally shown on the left-hand end of a fatty acid. Counting of the total carbon atoms starts from the COOH carboxyl end. For example, the fatty acid C18:2 contains 18 carbon atoms, and two double bonds are present within the fatty acid. Numbers such as 9 and 12 are also shown in conjunction with the name of the fatty acid, and this indicates that two double bonds are present within this fatty acids, and those double bonds are located at carbon atom numbers 9 and 12, counted from the carboxyl end.

Stearic acid is a C18:0 saturated fatty acid exhibiting the carboxyl end as well as the methyl end. The term C18:0 means that the fatty acid contains 18 atoms of carbon, and no double bond is present. Hence, the α-carbon is the first, or the closest, carbon to the carboxyl group (COOH) and is theoretically the second carbon in the chain from the COOH group. As a result, stearic acid can also be expressed as C17H35COOH based on the fact that all carbon atoms are counted except the carbon belonging to the carboxyl group (COOH). Counting therefore starts at the α-carbon. The most common fatty acids also have a scientific name. Stearic acid (C18:0) is also known as octadecanoic acid, oleic acid (C18:1) is known as 9-octadecenoic acid, and linoleic acid (C18:2) is known as 9,12-octadecadienoic acid (see Fig. 1.18).

Figure 1.18. Stearic acid.

Double bonds between carbon atoms stabilize the structure of fatty acids by preventing the carbons from rotating around the bond axis. As a result, configurational isomers of the same fatty acid are obtained, and the arrangement of atoms within such fatty acids can only be changed by breaking the bonds.

This fact gives rise to either cis- or trans-configuration of fatty acids, and the Latin prefixes cis and trans demonstrate the location of the hydrogen atoms in respect of the double bond. The term trans indicates the other, or opposite, side, while cis means on the same side. Fatty acids generally exhibit cis-configuration, while the trans-configuration is more often present in nature overall (see Fig. 1.19).

Figure 1.19. The cis- or trans-configuration in fatty acids.

Fatty acids, demonstrating the same number of carbon atoms as well as number of double bonds at the same carbon number, become different fatty acids depending on whether cis- or trans-configuration is present.

The different types of fats are:

•

Saturated fats

•

trans-Fats (behave similar to saturated fats)

•

Monounsaturated fats

•

Polyunsaturated fats

The saturation of fat refers to the chemical structure of its fatty acids. Saturated fatty acids are of linear structure (nonbranched) and generally exhibit even numbers of carbon atoms within their molecule such as 16 or 18 carbons. Single-bond linkages are present in saturated fatty acids between carbon atoms, and no double bond is given. Such single-bond linkages are chemically not very active, and saturated fatty acids are commonly solid at room temperature. Animal fats are predominantly saturated fats or contain a high amount of saturated fatty acids. Important representatives of saturated fatty acids present in animal fat are stearic acid (C18:0) as well as palmitic acid (C16:0). The "0" indicates that no double bond is present within the fatty acid, which is made of 18 atoms of carbon. Beef fat contains a high level of saturated, long-chained fatty acids. The degree of saturation in fat decreases in the sequence beef   >   pork   >   poultry   >   fish (least saturated). Stearic acid is unique in a way that it does not raise blood cholesterol and unfortunately is very often associated with other saturated fatty acids, which do raise blood cholesterol. Major sources of stearic acid are chocolate, lard, tallow, and commercial fats and butter. Palm and coconut oils are also rich in saturated fatty acids.

Unsaturated fatty acids contain one or more double bond(s) between carbon linkages, and the double bonds in unsaturated fatty acids regularly show cis-configuration. Monounsaturated fat is a type of fat in which the fatty acid contains one double bond in its chemical structure. Such fatty acids are found in olive, canola, and peanut oil and avocados. Oleic acid is a C18:1 monounsaturated fatty acid, which exhibits one double bond after the ninth carbon from its carboxyl end (single bondCOOH) and shows 18 carbon atoms within it molecule (see Fig. 1.20).

Figure 1.20. Oleic acid.

Monounsaturated fats can lower the total cholesterol by replacing saturated fats and do not lower the level of the "healthy" high-density lipoprotein cholesterol. They are less prone to oxidation compared with polyunsaturated fats. Fats that contain monounsaturated fatty acids are normally liquid at room temperature, but many thicken up when placed under refrigeration. Monounsaturated fats are beneficial to health and may be better than polyunsaturated fats in preventing heart disease. The diet in countries such as Italy and Greece is high in monounsaturated fats coming from olive oil and is one explanation for the low rate of heart disease in those countries. Olive oil contains around 75% oleic acid (C18:1).

Polyunsaturated fatty acids exhibit two or more double bonds within their molecule and the two main types are:

1.

Omega-3 fatty acids such as alpha-linolenic acid (ALA), which is the starter fatty acid for the omega-3 series. This fatty acid is 18 carbons long and shows in total three double bonds placed after the third, sixth and ninth carbon from the methyl end (single bondCH3) within the molecule. Other representatives in this group are docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA).

2.

Omega-6 fatty acids such as linoleic acid, which is the main polyunsaturated fatty acids in vegetable oils, originate from canola, maize, sunflower, or peanut. Linoleic acid is the starter fatty acid for the omega-6 series, showing 18 carbon atoms as well as two double bonds placed after the carbon atom number six and nine from the methyl end within the molecule. Other members of this group are gamma-linoleic acid (GLA) and arachidonic acid (AA). Omega-9 fatty acids, such as palmitoleic acid, also exist (see Fig. 1.21).

Figure 1.21. Alpha-linolenic acid.

Omega-3 and omega-6 fatty acids are essential unsaturated fatty acids and have to be provided to the human body by eating foods containing those fatty acids given that the human body cannot build them out of other fatty acids. Polyunsaturated fatty acids are named by counting from the methyl (single bondCH3) end within the fatty acid. Letters from the Greek alphabet such as alpha, beta, gamma to omega are utilized in order to determine the location of double bonds within the polyunsaturated fatty acid. Omega is the last carbon in the chain of carbon atoms counted from the carboxyl COOH end as the letter omega is the last letter in the Greek alphabet. For example, linoleic acid is an omega-6 fatty acid and the first double bond, counted from the methyl end, is located six carbons away from the omega carbon. This omega-carbon is the same carbon as used within the methyl (CH3) group and carbon number 18 from the carboxyl end. Polyunsaturated fats, such as corn oil, are generally liquid at room temperature as well as under refrigeration (see Fig. 1.22).

Figure 1.22. Linoleic acid.

Different types of fat exhibit different melting points and as a result have a different impact on the mouth feel in meat products. Fats, containing a high number of saturated fatty acids (like kidney fat or lard) cause a greasy, smeary and sandy mouth feel while more unsaturated fats give a pleasant taste as well as smooth, nonsandy mouth feel. Generally, the "hardest" fat within a carcass, showing high levels of saturated fatty acids, are found in the center of the carcass and softer fats are placed toward the outside of a carcass.

Even in subcutaneous pork fat, such as pork back fat, the outer layer of pork back fat, directly connected with the skin, is softer compared to the inner layer. Hence, soft fat contains a higher amount of connective tissue within itself compared with hard fat and chicken fat, which is the softest fat (high amount of unsaturated fatty acids). On the other hand, beef fat, which is of hard consistency, exhibits the lowest level of connective tissue. Pork fat lies between chicken and beef fat in terms of the level of connective tissue within the fat itself. In summary, soft fat contains higher levels of connective tissue but fat molecules entrapped within connective tissue are of soft consistency (high degree of unsaturated fatty acids). Hard fat, on the other hand, contains less connective tissue but fat molecules covered by connective tissue are of hard consistency (high degree of saturated fatty acids).

The melting point of a fatty acid depends largely on the length of the fatty acid itself as well as the number of double bonds present. Saturated fatty acids generally show a higher melting point compared with unsaturated fatty acids. Double bonds (and therefore less hydrogen within a fatty acid), present in unsaturated fatty acids, lower the melting point and unsaturated fatty acids show generally a lower melting point compared to saturated fatty acids as a result. An increased number of double bonds within a fatty acid lowers the melting point once again. Increased length of a fatty acid, containing a higher number of carbon atoms, causes an increase in melting point. For example, stearic acid (18   C atoms) has a melting point around 70°C, while capric acid (10   C atoms) shows a melting point around 30°C. Overall, the melting point of beef fat is around 43–47°C, pork fat around 38–44°C, and chicken fat around 31–37°C. Hence, fat containing cis-shaped double bonds within the fat molecule exhibits a lower melting point compared to fat containing trans-double bonds.

The consistency of fat is largely depended on the saturation of the fatty acids.

A higher number of unsaturated fatty acids leads to "softer" fat. Pork fat contains a relatively high amount of unsaturated fatty acids and is "soft" as a result. Beef fat, on the other hand, contains predominantly saturated fatty acids and therefore is of a "hard" consistency. The level of saturated fatty acids within fats varies and is for beef around 55–60%, for pork around 42–44%, and for chicken only 30%. This explains the "hardness" of fat in the sequence beef   ⇒   pork   ⇒   chicken, with chicken being the softest. Lamb and mutton are similar to beef in regard to the content of saturated fatty acids. For the production of meat products, pork fat showing a low number of unsaturated fatty acids, such as fat from loin and neck, is the preferred choice over soft pork fat, coming from leg and shoulder, showing a higher number of unsaturated fatty acids. Such "soft" fat is best used for emulsified sausages and not recommended for products such as salami, where "hard" fat is needed as it can be cleanly cut and the tendency toward rancidity is reduced as well.

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Current feeding strategies to improve pork intramuscular fat content and its nutritional quality

C.M. Alfaia , ... J.A.M. Prates , in Advances in Food and Nutrition Research, 2019

5 Concluding remarks and challenges

Feeding strategies for the production of pork with higher amounts of IMF, and thus with an improved sensory quality, were presented in this chapter. Dietary protein reduction, alone or combined with some ingredients, contributes to satisfy consumer's requirements and enhance the competitiveness of meat industry through higher pork quality and lower production costs. RPD have an important effect on fat partitioning improvement, leading to increased IMF content. This increased IMF can improve pork quality traits, mainly sensory traits.

The mechanisms regulating fat deposition in pigs are genotype- and tissue-specific. It was found that RPD improve fat partitioning and meat sensory score due to lysine limitation, in all muscle types of lean pig genotypes, but only in red muscles of fatty pig genotypes. It was suggested that the increased IMF is mediated by shifting the metabolic properties of fibers from glycolytic to oxidative, by the up-regulation of key lipogenic enzymes and related transcription factors.

Current nutritional approaches for the improvement of pork fatty acid profile, with enrichment in the beneficial n-3 PUFA (mainly EPA and DHA), were also presented here. Feeding sources of n-3 fatty acids to pigs increased their content in pork, but results have been highly variable. The differences found across studies might be attributed to several distinct factors, of which stand out, the source and species of algae, amount and type of n-3 fatty acids fed, duration of experimental feeding, type of feed, weight, gender, age and strain of pigs. However, in PUFA-enriching diets, the susceptibility of these unsaturated fatty acids to oxidation should be taken in consideration, by using protecting levels of antioxidants (selenium and alpha-tocopherol).

The use of microalgae and seaweeds is a major nutritional challenge in the near future. Overall, the inclusion of algae in feed represents a promising approach for the maintenance and development of the livestock sector, as an environmental friendly alternative to balance food-feed-biofuel industries. The use of exogenous CAZymes is a very promising cost-effective strategy to degrade the recalcitrant polysaccharides of alga cell walls and to improve the nutritional value of microalgae and seaweeds for pigs. In addition, the degradation of alga cell walls by CAZymes provides oligosaccharides (prebiotics) that may substitute antibiotics, mainly in weaned piglets. Therefore, further in-depth studies on these aspects are warranted.

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