LIPID PROFILE OF MEAT
Ambashree Dubey
Division of LPT, ICAR-IVRI, Izatnagar, Bareilly, UP-243122
Introduction
Meat has an important place in a healthy diet, providing protein with a good balance of amino acids,iron in a readily available form, vitamins, especially of the B group and other essential minerals such as zinc (Williamson et al., 2005). Meat also contributes a signi cant amount of fat to the human diet and it is this component that has been most under the spotlight in recent years in relation to the healthiness of people consuming meat. Meat contains relatively high amounts of saturated fatty acids (SFA) and ruminant meats (beef and lamb) are low in polyunsatura. This balance, if replicated in the whole diet, predisposes people to a range of diseases including cardiovascular disease (CVD).
However, research papers show that the fatty acid composition of meat can be greatly modi ed by production factors such as animal diet, age, weight, sex, and breed. Fatty acid composition also var-ies between species and tissue sites in the body. All these variations provide the meat industry with the tools to supply meat containing a healthy balance of fatty acids to the consumer, either in the form of fresh meat or meat products.
The fatty acids in meat are located mainly in adipose tissue, commonly termed “fat.” This has a role in product quality, contributing toward texture (tenderness and mouthfeel) and juiciness in both fresh meat and meat products. The softness/hardness of fat, which is greatly in uenced by fatty acid composition, affects various properties such as the sliceability of bacon and the stability of Meat has an important place in a healthy diet, providing protein with a good balance of amino acids, iron in a readily available form, vitamins, especially of the B group and other essential minerals such as zinc (Williamson et al., 2005). Meat also contributes a signi cant amount of fat to the human diet and it is this component that has been most under the spotlight in recent years in relation to the healthiness of people consuming meat. Meat contains relatively high amounts of saturated fatty acids (SFA) and ruminant meats (beef and lamb) are low in polyunsatura. This balance, if replicated in the whole diet, predisposes people to a range of diseases including cardiovascular disease (CVD).
However, research papers show that the fatty acid composition of meat can be greatly modi ed by production factors such as animal diet, age, weight, sex, and breed. Fatty acid composition also var- ies between species and tissue sites in the body. All these variations provide the meat industry with the tools to supply meat containing a healthy balance of fatty acids to the consumer, either in the form of fresh meat or meat products.
The fatty acids in meat are located mainly in adipose tissue, commonly termed “fat.” This has a role in product quality, contributing toward texture (tenderness and mouthfeel) and juiciness in both fresh meat and meat products. The softness/hardness of fat, which is greatly in uenced by fatty acid composition, affects various properties such as the sliceability of bacon and the stability of Meat has an important place in a healthy diet, providing protein with a good balance of amino acids, iron in a readily available form, vitamins, especially of the B group and other essential minerals such as zinc (Williamson et al., 2005). Meat also contributes a significant amount of fat to the human diet and it is this component that has been most under the spotlight in recent years in relation to the healthiness of people consuming meat. The fatty acids in meat are located mainly in adipose tissue, commonly termed as “fat.” This has a role in product quality, contributing towards texture and juiciness in both fresh meat and meat products. Fats are a very concentrated energy source. The energy value of fat is almost double that of carbohydrate or protein. Lipids are a very diverse group of substances chemically but are characterized by their relative insolubility in water and high solubility in organic solvents such as ethyl ether and chloroform. Oleic, palmitic and stearic acids are some of the commonest naturally occurring fatty acids. Linoleic, linolenic and arachidonic acids, cannot be synthesized by animals (although arachidonic acid can be synthesized from linoleic acid) and are therefore referred to as essential fatty acids and must be obtained from the diet. They are abundant in the oils found in plant seeds. For reasons of human health there is interest in trying to increase the proportion of polyunsaturated fatty acids to saturated fatty acids (the P:S ratio) in meat. It is not difficult to increase the proportion of unsaturated fatty acids in non ruminants like pigs and poultry because the fat laid down in their bodies closely reflects the characteristics of the dietary fat. So, feeding pigs diets high in linseed, rapeseed or fish oils results in softer, more unsaturated carcass fat. The process is more difficult in ruminants. Unsaturated fats in the diet are hydrogenated by the rumen microorganisms to much more saturated fats.
Ruminant Fat
The major saturated fatty acids—palmitic and stearic acids—constitute 25% and 13%, respectively, of the total fatty acids. In the monounsaturated fatty acids category, oleic acid alone contributes 40% to the total fatty acids. Linoleic acid accounts for more than 5% of the total fatty acids. An important animal-origin fatty acid, arachidonic acid, constitutes only 0.8% of the fatty acid composition in beef. As marbling increases, the concentration (milligrams per gram of muscle) of almost all fatty acids in beef also increases; however, this increase is not the same for all fatty acids because saturated and monounsaturated fatty acids are deposited in adipose tissues at a greater rate than polyunsaturated fatty acids (Dinh et al., 2010). There is inverse relationship between PUFA/SFA ratio and intramuscular fat content. This phenomenon has been found across all livestock species because lipids stored in adipose tissues are usually neutral lipids rich in triglycerides, which in turn contain more saturated and monounsaturated fatty acids than polyunsaturated fatty acids in their structure Triglycerides in animal fats tend to have PUFA in position 2, whereas palmitic and oleic acids occupy positions 1 and 3 of the triglyceride structure. Moreover, C16 and C18 SFA and MUFA are natural end-products of fatty acid de novo synthesis (De Smet et al., 2004). In beef and other ruminant meats, biohydrogenation in the rumen contributes to the saturation of fatty acids; therefore, the meats from other ruminants have a very similar fatty acid composition to that of beef. Goat meat is typically very lean—with 2% to 3% intramuscular fat unless the animals are specifically fattened—regardless of anatomical location, sex, breed, and dietary influences. Across various meat cuts, goat meat has 30% to 40% SFA, more than 50% MUFA, and less than 10% PUFA (Banskalieva et al., 2000). Feeding castrated male Kiko goats with deoiled distillers dried grains with solubles led to a decrease in PUFA concentration in the fat. The goat meat had 44% SFA, 54% MUFA, and only 2% PUFA (Camareno et al., 2016). A similar phenomenon of the shifting between SFA and PUFA was reported by Brassard et al. (2017) on concentrate-fed Boer kids. The goat muscle had 40% SFA, 54% MUFA, and 6% PUFA. These findings again indicate that PUFA can bypass the rumen and, if naturally occurring (forage) or supplemented (distillers grain) more in feeds, will be deposited more in the adipose tissues. The MUFA seems to be unaffected, fluctuating in a narrow range of 50% to 54%. Interestingly, in beef cattle, grass-finished beef has up to 4% less MUFA than grain-finished beef (Leheska et al., 2008). This decrease was translated into an increase in SFA and PUFA proportions because these two categories become more predominant in the total fatty acid composition. The fatty acid composition of lamb includes 42% SFA, 47% MUFA, and 11% PUFA. Although, antemortem factors such as sex, breed, and diet also influence lamb fatty acid composition. The predominant fatty acids in SFA, MUFA, and PUFA categories in lamb are the same as in beef, although the percentages may vary up to 5%. The inclusion of more PUFA in the diets also increase PUFA percentage in lamb, similar to other ruminant species. Compared with beef, lamb has up to 5% more SFA, less MUFA, and approximately 0.5% greater arachidonic acid. This is significant for flavor development because thermal oxidation of MUFA contributes to the formation of a different, more desirable flavor profile than SFA and PUFA. In addition, the autoxidation of arachidonic acid is one of the primary sources of off-odors in meat (Pegg and Shahidi, 2012). Lamb has up to 3% volatile branched-chain fatty acids (BravoLamas et al., 2018), which are precursors of characteristic lamb and mutton odors. These odor producing fatty acids concentrate more in subcutaneous fat than in muscle (Brennand and Lindsay, 1992). Longer branched-chain fatty acids (14–16 carbons) have been found in beef; however, they do not contribute to characteristic odors in beef.
Monogastric Fat
Monogastric animals such as pigs cannot hydrogenate fatty acids; therefore, their muscle fatty acids are more similar to the fatty acid composition of their diets. On an average, pork has 38% SFA, 50% MUFA, and 12% PUFA. In the SFA category, palmitic and stearic acids account for 25% and 12% of the total fatty acids, respectively. Oleic acid in pork fat is slightly more than that in beef fat, contributing 45% to the total fatty acids. Linoleic acid constitutes approximately 10% of the total fatty acids. The arachidonic acid proportion in pork fat is slightly greater than that in beef fat, at 1.4%. The contribution of each fatty acid to flavor development may not be different among livestock species, except for the fatty acids that provide distinct species odors. However, the fatty acid composition of pork can be easily manipulated by altering the dietary composition, such as fatty acids (Jiang et al., 2017; Komprda et al., 2020) or amino acids (Wang et al., 2018). By supplementing the pigs’ diet with soybean oil and linseed oil, SFA decreases by 6% to 7% and MUFA by 12%. De Tonnac et al. (2018) reported that pigs that were fed microalgae rich in n-3 fatty acids and contained less SFA and MUFA in muscle but more n-3 fatty acids. Interestingly, n-3 fatty acid deposition varied by fat depots. This depot-specific phenomenon has been found in other monogastric species such as horses, a monogastric species which grazes on forage rich in linoleic and linolenic acids (Belaunzaran et al., 2017). Wang et al. (2018) altered the lysine content in the diet of pigs and were able to change oleic acid in intramuscular fat by approximately 3%. It was observed that the activity of stearoyl-coenzyme A desaturase was increased with a lysine-deficient diet. This enzyme desaturates stearic acid to produce more oleic acid in muscle tissues. Although horse meat is rarely consumed in the US, it is a popular meat in various countries around the world. In one of the very few studies on the fatty acid composition of horse meat, Belaunzaran et al. (2017) reported 36% to 37% SFA, 32% to 35% MUFA, and 23% to 27% PUFA. Moreover, similar to lamb, horse meat has approximately 0.4% branch-chained fatty acids. The predominant fatty acids in the SFA and MUFA categories are the same as other species. However, the predominant PUFA is linolenic acid at 17%, in comparison with 12% linoleic acid. This reflects the fact that linolenic acid is the predominant fatty acid in most grasses (Khan et al., 2012) and that horse is a monogastric animal typically grazing on grasses. However, grasses only have 3.4% oleic acid, which reveals the significant role of fatty acid de novo synthesis, including elongation and Δ9 -desaturation in livestock adipose tissues since oleic acid is still one of the predominant fatty acids in horse meat. The distribution of fatty acids, especially unsaturated fatty acids, vary by tissue location. Abdominal fat is most saturated (37%–38%), whereas cardiac fat has more PUFA (37%– 38%)—although cardiac fat also has palmitic acid (25%–26%) as the predominant fatty acid instead of oleic acid (22%–24%) as in other fat depots (Ferjak et al., 2019). This phenomenon is similar to what has been reported in pork meat, another monogastric species. A high percentage of PUFA—especially the predominance of linolenic acid (15%–17%)—may contribute to lipid oxidation that contributes to the development of negative flavors in horse meat.
Types of fatty acids in meat
The fatty acids in meat are mainly of medium to long chain length, that is, they have 12- to 22-carbon atoms in the molecule, with a basic structure of CH3–(CH2)n–COOH. Small amounts of shorter chain length fatty acids, C8–C10, are present in lamb fat. About 40% of fatty acids are saturated (SFA), that is, each carbon has two hydrogen atoms attached, about 40% have one double bond (monounsaturated fatty acid, MUFA) where adjacent carbon atoms are attached to only one hydrogen atom each and a smaller proportion, about 2%–25% have more than one double bond (polyunsaturated fatty acid, PUFA). Fatty acids are commonly labeled according to carbon chain length and the number of double bonds, for example, linoleic acid is labeled as 18:2, being 18 carbons in length and containing two double bonds. Double bonds are either of the more common cis-type, in which the hydrogen atoms point in the same direction, or of the trans-type, in which they point in opposite directions resulting in a straighter molecular configuration. Oleic acid (18:1 cis-9) is the major fatty acid in all meats, contributing over 30% of total fatty acids. The length, degree of unsaturation, and confi guration of the fatty acid molecule influence physical properties such as melting point. The longer the chain length and the fewer the number of double bonds present in the molecule, the higher the melting point. Saturated and trans fatty acids have a higher melting point than unsaturated and cis fatty acids. A high proportion of SFA therefore causes hard rather than soft fat tissue. The fatty acids in ruminant tissues are more complex than those in non ruminants, containing higher proportions of trans fatty acids, fatty acids with an odd number of carbon atoms (arising from rumen-derived proprionic acid rather than acetate as a precursor for fatty acid synthesis, e.g., C15 and C17), fatty acids with branched chains (derived from the amino acids, leucine, valine, and isoleucine, i.e., 4-methyl octanoic acid, C8:0 and 4-methyl nonanoic acid, C9:0) and fatty acids with conjugated double bonds (i.e., the bonds are on adjacent carbon atoms rather than being separated by a CH2 group). These variations are the result of the actions of enzymes present in microorganisms in the rumen that degrade plant structures and dietary fatty acids, producing a wide range of products, some of which are absorbed in the small intestine and incorporated into tissue lipids. An important group of fatty acids in ruminants are the conjugated linoleic acids (CLAs) with 18 carbons and 2 conjugated double bonds. These have been shown to have a range of physiological actions in the body of the animal and consumers of meat.
The fatty acids in meat are found in two main lipid classes, neutral triacylglycerol (storage role) and more polar glycerophospholipid (structural and metabolic role). The former is the main lipid component (>90%) of adipose tissue in mature animals (visible fat) and the latter, a constituent of cell membranes, contributes between 10% and 40% of the total fatty acids in muscle. Phospholipid has a much higher concentration of PUFA than triacylglycerol. As meat animals grow toward the point of slaughter they deposit increasing amounts of fat in the carcass including that within the muscle (marbling fat).
Other Important Lipids
Phospholipids are found as a major structural component of cell membranes but include the lecithins also found in blood plasma. Phospholipids are esters of glycerophosphoric acid. The fatty aids they contain are unsaturated. Sphingomyelins are an important constituent of nerves and glycolipids are also found in cell membranes. Steroids have a characteristic ring structure. The commonest is cholesterol, found in cell membranes and as a precursor of steroid hormones such as oestrogens and corticosteroids. Beef fat contains about 0.1% cholesterol. As well as being esterified with glycerol in triglycerides fatty acids can exist in the un-esterified form [non-esterified (NEFA) or free fatty acids (FFA)] and are the principal way in which body fat stores are mobilized and carried in the blood.
Lipid oxidation
Unsaturated fatty acids are very prone to oxidation. This is why linseed oil was so useful in the manufacture of paints: the oil oxidized in air to hard waterproof substances. Even in meat in which most of the fat is saturated the cell membranes contain phospholipids. The polyunsaturated fatty acids present in these can react with oxygen to form fatty acid hydroperoxides. These are unstable and break down into various compounds including aldehydes, ketones and carboxyl compounds, which can produce off-flavours. The process is relatively rapid, often occurring within 1–2 days in meat that has been cooked then kept refrigerated. This leads to the rather stale, rancid flavour referred to as ‘warmed-over flavour’ or WOF (Kerler and Grosch, 1996). However, off-flavours caused by lipid oxidation can also occur in uncooked meat. Generally, the propensity of meats to show the problem is directly related to their content of unsaturated fat. Therefore fish is more prone than poultry meat which in turn is more susceptible than the red meats. Of these, pork is the worst, followed by beef then lamb. Lipid oxidation is promoted by processes that damage the muscle structure, such as mincing or comminuting. This exposes the fatty acids to oxygen and catalysing factors such as iron and haem. Oxidation is also promoted by sodium chloride. The processes used to make products such as sausages, which contain minced meat and salt, therefore provide ideal conditions for oxidation. However, nitrite, often also added in cured products, and chelating agents such as citrates and phosphates, which sequester, or mop up, free metal ions, inhibit it. Other factors that affect the propensity of fat to undergo oxidation in meat are the haem pigment concentration, the storage temperature and availability of oxygen. Redder muscles, containing higher concentrations of haem pigments, are oxidation prone. Storage at high temperatures increases the rate of oxidation while freezing reduces it. Reducing the availability of oxygen by packing meat in, for example, nitrogen, or by vacuum packing it, will inhibit oxidation. So, to a degree, will keeping it in larger pieces where the ratio of the surface area to volume is minimized. Although all unsaturated fatty acids are susceptible to oxidation, the problem is greater the larger the number of double bonds since this is where breakdown of the molecule is initiated. Thus, both linoleic and linolenic acids have the same chain length (18 carbon atoms), but linolenic acid has three double bonds, compared with the two of linoleic acid. Its rate of oxidation is correspondingly much greater. In the living animal, natural antioxidants reduce the level of lipid oxidation, so also limiting the production of dangerous free radicals which could damage other molecules. Several enzymes (glutathione peroxidase, catalase and superoxide dismutase) act to prevent oxidation.
Natural antioxidants include vitamin C (ascorbic acid) and vitamin E. Vitamin E is especially important. This can be taken advantage of by supplementing animal diets with the vitamin so the levels present in the meat post mortem are enhanced, sometimes up to eightfold those of unsupplemented animals. Vitamin E is lipid soluble and so accumulates in fats, particularly in cell membranes where it is most effective.
Meat fatty acids and human health
Some SFA, that is, those with less than 18-carbon atoms chain length, raise blood levels of low-density lipoprotein (LDL) cholesterol, which increases the risk of atherosclerosis leading to CVD in man (Williamson et al., 2005). On the other hand, MUFA and PUFA lower blood levels of LDL-cholesterol. As a result of these findings, health bodies around the world have issued dietary guidelines for the fatty acid composition of the human diet. The World Health Organization (2003) has said that total fat should constitute not more than 15%–30% of total energy in the diet, SFA around 10%, n-6 PUFA around 5%–8%, and n-3 PUFA 1%–2%. Several studies agree that n-3 PUFA levels are too low in Western diets and that these fatty acids reduce the risk of CVD are necessary for proper brain and visual development in the fetus, and have a role in reducing various cancers. Trans fatty acids have potentially more potent effects on LDL-cholesterol than SFA, although trans fatty acids are generally low in meat and there is some evidence that the trans fatty acids in meat and milk are less damaging to human health than those in other processed fatty foods (Williamson et al., 2005). Trans-11 18:1 (trans vaccenic acid) is the precursor in tissues of the major CLA isomer, cis-9, trans-11 CLA, which is recognized to have several positive health benefits including inhibition of carcinogenesis and atherosclerosis and enhancement of the immune response.
Effects of fatty acids on meat quality
The total amount of fat in meat and its fatty acid composition affect many aspects of meat quality. In pigs, the fatty acid composition of subcutaneous fat affects its firmness, which is a factor in product quality. High concentrations of PUFA in fat tissue, caused by a high level in the diet, produce soft, oily fat with low visual and handling quality. The two fatty acids having the biggest effect on firmness are 18:0 and 18:2(n-6) and ratio between these provided the best prediction of firmness measured by using a penetrometer. The reduced “fat quality” of pigs with thin as opposed to thick subcutaneous fat, which includes separation between fat layers and a dull color as well as softness, is closely related to changes in the proportions of these two fatty acids as well as water and lipid. An increase in fat thickness indicates a more mature tissue with larger fat cells, a higher proportion of lipid, and a corresponding reduction in the proportion of water. Fatty acids contribute to the meat flavors generated during cooking, either directly through the lipid oxidation products, some of which are odorous, or indirectly by reactions between lipid oxidation products and proteins and carbohydrates present in the meat. High levels of 20:5(n-3) and 22:6(n-3) in lamb, created by feeding fish oils or marine algae, produce high taste panel scores for “rancid” and “fishy” and low scores for “overall liking”(Nute et al., 2007). On the other hand, an increased concentration of 18:3(n-3) from linseed elicited the highest scores for “lamb flavor” and “overall liking.” Campo et al. (2006) observed that beef sirloin steaks displayed in “modified atmosphere” packs containing a high concentration of oxygen showed increased lipid oxidation as the display period progressed. Samples with high concentrations of PUFA had the highest oxidation and they received the lowest taste panel scores for “beef flavor.” They scored highest for “abnormal flavor” and “rancid” and there were strong negative correlations between TBARS (thiobarbituric acid reactive substances, the standard test for meat lipid oxidation) and “beef flavor” and between “beef flavor” and “rancid.” These results show that fatty acid composition is a major factor in beef flavor.
Conclusion
Fatty acid composition of meat in pigs, cattle, and sheep is affected by diet and breed, and differs between tissues. The levels of fat tissue in the carcass and meat are important underlying factors because fatty acid composition changes as fat is deposited. One important consequence of increasing fat deposition is that the proportion of phospholipid is reduced, leading to a decrease in the proportion of the major PUFA, 18:2(n-6). The fatty acid composition of pork can be readily modified by diet since fatty acids are deposited unchanged by digestion. In ruminants, the rumen is a barrier to the incorporation of PUFAs into meat although the effect of grass diets in increasing proportions of n-3 PUFA and possibly conjugated linoleic acid is an interesting area of current research, leading to more desirable meat products for the consumer. The fatty acid composition of meat is important for human health reasons and also has crucial effects on meat quality, for example, fat tissue firmness, color, shelf life, and flavor. Changes in production that affect fatty acid composition should therefore only be introduced after examining the impact on meat quality.
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