Enhancing the nutritional value of meat 219 9.6.3 Polyunsaturated fatty acids The PUFAs have a structural role because they are found in the membrane phos pholipids and they are also involved in eicosanoid synthesis. There are two types of polyunsaturated fatty acids, the omega-3(n-3)and the omega-6(n-6).Meat and meat products supply 17% n-6 and 19%n-3 PUFA intake( Gregory et al, 1990). Linoleic acid(C18: 2 n-6)and a-linolenic acid (C18: 3n-3) are essential fatty acids as we cannot synthesise them ourselves, so we are dependent on diet to provide them. In the body these are further elongated and desaturated to longer chain derivatives, arachidonic acid(C20: 4n-6), docosapentaenoic acid(C22: 5n 6), eicosapentaenoic acid( C20: 5n-3) and docosahexaenoic acid(C22: 6n-3) These are found in useful quantities in meat. Over the past 30 years there has been a major shift in the intakes of the different fatty acids and the saturated fats have been replaced by the unsaturated fats. The increase in the unsaturated fatty acids was mainly due to an increase in n-6 fatty acids as a consequence of replac ing vegetable oils for animal fat Today, the usual Western diet contains 10-20 times more n-6 than n-3. For instance, in Britain, the n-6 PUFA intake is now responsible for 87.5% of total PUFA intake, the remainder being the n-3 PUFAS. However, evidence now indicates that it is the n-3 PUFAS which are cardioprotective, in particular, the very long chain n-3 PUFAS, eicosapentaenoic acid(C20: 5n-3) and docosahexae noic acid(C22: 6n-3). The GISsI trial showed that I g of eicosapentaenoic acid (C20: 5n-3)and docosahexaenoic acid(C22: 6n-3) daily reduced coronary heart disease deaths by 20%(GIssI, 1999). The exact mechanism for this effect is not clear but they may reduce blood cholesterol. Other beneficial effects of the very long chain n-3 PUFAS include anti-infammatory and anti-tumourigenic proper ties. Docosahexaenoic acid(C22: 6n-3)also plays a role in neuronal development cognitive function and visual acuity. It appears that newborn babies have a reduced ability to make the longer chain derivatives and docosahexaenoic acid (C22: 6n-3) is an essential fatty acid for the newborn. Meat and fish are the only significant sources of preformed very long chain n-3 PUFAS in the diet. The chief sources of n-3 PUFAS are oily fish and fish oils, however, only one third of the UK population consume oily fish weekly. It is rising then that in the Uk, meat and meat products supply more n-3 PUFAS (19%)than do fish nd fish dishes (14%)( Gregory et al, 1990). In a report on n-3 fatty acids the British Nutrition Foundation summarised this fact with the following statement: red meat is likely to rival fish as a source of n-3 PUFAS in many people's diet (BNF,1999) Animals can convert a-linolenic acid to 20- and 22-carbon n-3 PUFAs but plants cannot, hence, there are no long chain PUFAS in vegan diets. Diets, which exclude meat and fish, such as vegetarian diets, are practically devoid of very long chain n-3 PUFAS. Vegans rely solely on the endogenous synthesis of very long chain n-3 PUFA from a-linolenic acid. This fact is verified by studies that have shown that vegetarians have lower n-3 PUFA intake than their omnivore counterparts. This imbalance may have nutritional consequences for vegans and vegetarians. For instance, results from a recent observation study showed that the
9.6.3 Polyunsaturated fatty acids The PUFAs have a structural role because they are found in the membrane phospholipids and they are also involved in eicosanoid synthesis. There are two types of polyunsaturated fatty acids, the omega-3 (n-3) and the omega-6 (n-6). Meat and meat products supply 17% n-6 and 19% n-3 PUFA intake (Gregory et al, 1990). Linoleic acid (C18:2 n-6) and a-linolenic acid (C18:3n-3) are essential fatty acids as we cannot synthesise them ourselves, so we are dependent on diet to provide them. In the body these are further elongated and desaturated to longer chain derivatives, arachidonic acid (C20:4n-6), docosapentaenoic acid (C22:5n- 6), eicosapentaenoic acid (C20:5n-3) and docosahexaenoic acid (C22:6n-3). These are found in useful quantities in meat. Over the past 30 years there has been a major shift in the intakes of the different fatty acids and the saturated fats have been replaced by the unsaturated fats. The increase in the unsaturated fatty acids was mainly due to an increase in n-6 fatty acids as a consequence of replacing vegetable oils for animal fat. Today, the usual Western diet contains 10–20 times more n-6 than n-3. For instance, in Britain, the n-6 PUFA intake is now responsible for 87.5% of total PUFA intake, the remainder being the n-3 PUFAs. However, evidence now indicates that it is the n-3 PUFAs which are cardioprotective, in particular, the very long chain n-3 PUFAs, eicosapentaenoic acid (C20:5n-3) and docosahexaenoic acid (C22:6n-3). The GISSI trial showed that 1 g of eicosapentaenoic acid (C20:5n-3) and docosahexaenoic acid (C22:6n-3) daily reduced coronary heart disease deaths by 20% (GISSI, 1999). The exact mechanism for this effect is not clear but they may reduce blood cholesterol. Other beneficial effects of the very long chain n-3 PUFAs include anti-inflammatory and anti-tumourigenic properties. Docosahexaenoic acid (C22:6n-3) also plays a role in neuronal development, cognitive function and visual acuity. It appears that newborn babies have a reduced ability to make the longer chain derivatives and docosahexaenoic acid (C22:6n-3) is an essential fatty acid for the newborn. Meat and fish are the only significant sources of preformed very long chain n-3 PUFAs in the diet. The chief sources of n-3 PUFAs are oily fish and fish oils, however, only one third of the UK population consume oily fish weekly. It is unsurprising, then that in the UK, meat and meat products supply more n-3 PUFAs (19%) than do fish and fish dishes (14%) (Gregory et al, 1990). In a report on n-3 fatty acids the British Nutrition Foundation summarised this fact with the following statement: ‘red meat is likely to rival fish as a source of n-3 PUFAs in many people’s diet’ (BNF, 1999). Animals can convert a–linolenic acid to 20- and 22-carbon n-3 PUFAs but plants cannot, hence, there are no long chain PUFAs in vegan diets. Diets, which exclude meat and fish, such as vegetarian diets, are practically devoid of very long chain n-3 PUFAs. Vegans rely solely on the endogenous synthesis of very long chain n-3 PUFA from a–linolenic acid. This fact is verified by studies that have shown that vegetarians have lower n-3 PUFA intake than their omnivore counterparts. This imbalance may have nutritional consequences for vegans and vegetarians. For instance, results from a recent observation study showed that the Enhancing the nutritional value of meat 219
220 The nutrition handbook for food processors n-3: n-6 ratio in plasma phospholipids was significantly lower among ovo lac tovegetarians and vegans compared with meat eaters and this may be respon- ible for an increased platelet aggregation tendency among vegetarians, which is a risk factor for cardiovascular disease (Li et al, 1999). Meat is already a valuable source of n-3 PUFAS among omnivores, thus any further increase in the n-3 PUFA content of meat will make useful contribut their overall intakes. Nowadays, researchers are looking at ways to enhance the n-3 PUFA content of meat. Feeding trials of cattle, pigs and sheep have shown dietary modification to be successful in raising n-3 PUFA content of their meats The n-3 PUFA content of meat can be enhanced by increasing the amount of n- 3 PUFAS in the diet of the animal. For instance, grass is rich in a-linolenic acid (C18: 3n-3)and grass-fed meat has a higher n-3 fatty acid content than has grain fed meat(Enser et al, 1998). Similarly, experiments have shown that including fish oil. marine algae. oils and oilseeds, such as linseed. which are rich sources of n-3 PUFAS, in the animals' diet can enhance favourably the n-3 content of the resultant meat. enhancing the n-3 PUFA content of meat is much easier to achieve in monogastrics, such as pigs and poultry, than in ruminants. In the rumen, the dietary unsaturated fatty acids are susceptible to biohydrogenation. Biohydro genation is a process that occurs in the rumen where the dietary unsaturated fatty acids are hydrogenated by ruminant microorganisms to more saturated end prod- ucts. Evidence indicates that some unsaturated fatty acids appear to be more resis- tant to biohydrogenation than others. Example s Include the very long chain n-3 PUFAS. However, more research is required to clarify this issue. Researchers are looking at ways to overcome biohydrogenation in ruminants by protecting the n- 3 PUFA. Altering the fatty acid composition of meat can have negative impacts on the meat quality, its shelf-life, colour and flavour. Therefore animal scientists food technologists and nutritionists are looking at ways to improve the nutritional quality of meat by enhancing its n-3 PUFA content without causing any adverse sensory qualities or negatively affecting its shelf-life The Department of Health(1994b)has issued guidelines regarding the rec ommended intake of saturated and polyunsaturated fats. The current recommen- dation for the polyunsaturated saturated ratio(P: S ratio) is about 0. 4. Pork has a higher P: S ratio whereas the P: S ratios of lamb and beef are lower(Table 9.1), as a consequence of biohydrogenation. The Department of Health(1994b)has also issued an index regarding the ratio of n-6: n-3 PUFAS. The recommended value for this ratio(n-6: n-3)is less than 4. The n-6: n-3 ratios of trimmed beef, lamb and pork are approximately 2.2, 1.3 and 7.5, respectively (Table 9.1). There- fore, both beef and lamb have acceptable n-6: n-3 ratios whereas that for pork needs to be reduced to reach acceptable values. The high n-6: n-3 ratio in pork is due to significant amounts of linoleic acid(C18: 2 n-6) present in its adipose tissue(Enser et al, 1996). In summary, researchers are focusing on ways of nhancing the n-3 PUFA content of meat and meat products. However, when increasing the n-3 fatty acid composition of ruminant meats such as beef and lamb, they are focusing on ways to increase the P: S ratio whilst retaining the positive n-6: n-3 ratio On the other hand, for monogastric meat, such as pork
n-3 :n-6 ratio in plasma phospholipids was significantly lower among ovo lactovegetarians and vegans compared with meat eaters and this may be responsible for an increased platelet aggregation tendency among vegetarians, which is a risk factor for cardiovascular disease (Li et al, 1999). Meat is already a valuable source of n-3 PUFAs among omnivores, thus any further increase in the n-3 PUFA content of meat will make useful contributions to their overall intakes. Nowadays, researchers are looking at ways to enhance the n-3 PUFA content of meat. Feeding trials of cattle, pigs and sheep have shown dietary modification to be successful in raising n-3 PUFA content of their meats. The n-3 PUFA content of meat can be enhanced by increasing the amount of n- 3 PUFAs in the diet of the animal. For instance, grass is rich in a-linolenic acid (C18:3n-3) and grass-fed meat has a higher n-3 fatty acid content than has grainfed meat (Enser et al, 1998). Similarly, experiments have shown that including fish oil, marine algae, oils and oilseeds, such as linseed, which are rich sources of n-3 PUFAs, in the animals’ diet can enhance favourably the n-3 content of the resultant meat. Enhancing the n-3 PUFA content of meat is much easier to achieve in monogastrics, such as pigs and poultry, than in ruminants. In the rumen, the dietary unsaturated fatty acids are susceptible to biohydrogenation. Biohydrogenation is a process that occurs in the rumen where the dietary unsaturated fatty acids are hydrogenated by ruminant microorganisms to more saturated end products. Evidence indicates that some unsaturated fatty acids appear to be more resistant to biohydrogenation than others. Examples include the very long chain n-3 PUFAs. However, more research is required to clarify this issue. Researchers are looking at ways to overcome biohydrogenation in ruminants by protecting the n- 3 PUFA. Altering the fatty acid composition of meat can have negative impacts on the meat quality, its shelf-life, colour and flavour. Therefore animal scientists, food technologists and nutritionists are looking at ways to improve the nutritional quality of meat by enhancing its n-3 PUFA content without causing any adverse sensory qualities or negatively affecting its shelf-life. The Department of Health (1994b) has issued guidelines regarding the recommended intake of saturated and polyunsaturated fats. The current recommendation for the polyunsaturated :saturated ratio (P :S ratio) is about 0.4. Pork has a higher P :S ratio whereas the P :S ratios of lamb and beef are lower (Table 9.1), as a consequence of biohydrogenation. The Department of Health (1994b) has also issued an index regarding the ratio of n-6 :n-3 PUFAs. The recommended value for this ratio (n-6 :n-3) is less than 4. The n-6 :n-3 ratios of trimmed beef, lamb and pork are approximately 2.2, 1.3 and 7.5, respectively (Table 9.1). Therefore, both beef and lamb have acceptable n-6 :n-3 ratios whereas that for pork needs to be reduced to reach acceptable values. The high n-6 :n-3 ratio in pork is due to significant amounts of linoleic acid (C18:2 n-6) present in its adipose tissue (Enser et al, 1996). In summary, researchers are focusing on ways of enhancing the n-3 PUFA content of meat and meat products. However, when increasing the n-3 fatty acid composition of ruminant meats such as beef and lamb, they are focusing on ways to increase the P :S ratio whilst retaining the positive n-6 :n-3 ratio. On the other hand, for monogastric meat, such as pork, 220 The nutrition handbook for food processors
Enhancing the nutritional value of meat 221 Table 9.1 Fatty acid ratios related to healthy nutrition Source of meat Sample P:Sn-6:n-3 Muscle 112.11 dipose tissue 0.05 Beef 0.07 Lamb Muscle Adipose tissue 02332 Muscle 0.58722 Pork Adipose tissue 617.64 Pork 0.61 7.57 Values for steaks and chops calculated for whole cut as purchased. Adapted from Enser et al(1996)Fatty acid content and composi- tion of English beef, lamb and pork at retail. Meat Science 42(4) 443-56. the n-3 PUFA content should be increased, whilst maintaining its positive P: S ratio. Many of the results to date are promising; for instance, beef and lamb liver from animals raised on grass are particularly good sources of n-3 PUFAS wit the n-6: n-3 being 0. 46(Enser et al, 1998). Such data highlights the potential carcase meat with improved fatty acid composition as a highly acceptable and effective vehicle for providing optimal fatty acid intake for the consumer. 9.6.4 Conjugated Linoleic Acid (CLA) Another emerging dietary benefit for meat, in particular ruminant meat, is the existence within it of conjugated linoleic acid (CLA). CLA is a fatty acid that occurs naturally in ruminant meats such as beef and lamb. The acronym CLa is collective term used to describe a mixture of positional(7, 9-,8, 10-:9, 11 10, 12-or 11, 13-)and geometrical (C, C-;c, t-; t, t- or t, c-) isomers of linoleic acid 9c, 12c-18: 2). CLA has the same chain length as linoleic acid (18C), but in CLa the double bonds are conjugated Conjugated double bonds are separated by only one single carbon bond. The c9-t11-18: 2 isomer(rumenic acid) is the predomi nant isomer of CLA(Kramer et al, 1998 ). This isomer has been shown to account for at least 60% of total CLA in beef(Shantha et al, 1994: O"Shea et al, 1998) Factors influencing the CLA content of meat include the breed, age and diet of the animal (O'Shea et al, 1998; Mulvihill, 2001). As well as having a high n-3 PUFA content, grass-fed meat also has higher CLA content( Shantha et al, 1994) Since, CLA is formed predominately in the rumen, the CLA content of ruminant meat, beef and lamb, is much higher than non-ruminant meat such as pork, chicken and game( Chin et al, 1992). The best natural dietary sources of CLa are ruminant products such as beef and lamb(Ma et al, 1999). Meat and meat products supply approximately a quarter of dietary Cla in Germany(Fritsche
the n-3 PUFA content should be increased, whilst maintaining its positive P :S ratio. Many of the results to date are promising; for instance, beef and lamb liver from animals raised on grass are particularly good sources of n-3 PUFAs with the n-6 :n-3 being 0.46 (Enser et al, 1998). Such data highlights the potential for carcase meat with improved fatty acid composition as a highly acceptable and effective vehicle for providing optimal fatty acid intake for the consumer. 9.6.4 Conjugated Linoleic Acid (CLA) Another emerging dietary benefit for meat, in particular ruminant meat, is the existence within it of conjugated linoleic acid (CLA). CLA is a fatty acid that occurs naturally in ruminant meats such as beef and lamb. The acronym CLA is a collective term used to describe a mixture of positional (7,9-; 8,10-; 9,11-; 10,12- or 11,13-) and geometrical (c,c-; c,t-; t,t- or t,c-) isomers of linoleic acid (9c,12c-18:2). CLA has the same chain length as linoleic acid (18C), but in CLA the double bonds are conjugated. Conjugated double bonds are separated by only one single carbon bond. The c9-t11-18:2 isomer (rumenic acid) is the predominant isomer of CLA (Kramer et al, 1998). This isomer has been shown to account for at least 60% of total CLA in beef (Shantha et al, 1994; O’Shea et al, 1998). Factors influencing the CLA content of meat include the breed, age and diet of the animal (O’Shea et al, 1998; Mulvihill, 2001). As well as having a high n-3 PUFA content, grass-fed meat also has higher CLA content (Shantha et al, 1994). Since, CLA is formed predominately in the rumen, the CLA content of ruminant meat, beef and lamb, is much higher than non-ruminant meat such as pork, chicken and game (Chin et al, 1992). The best natural dietary sources of CLA are ruminant products such as beef and lamb (Ma et al, 1999). Meat and meat products supply approximately a quarter of dietary CLA in Germany (Fritsche and Steinhart, 1998). Enhancing the nutritional value of meat 221 Table 9.1 Fatty acid ratios related to healthy nutrition Source of meat Sample P :S n-6 :n-3 Beef Muscle 0.11 2.11 Beef Adipose tissue 0.05 2.30 Beef Steak 0.07 2.22 Lamb Muscle 0.15 1.32 Lamb Adipose tissue 0.09 1.37 Lamb Chop 0.09 1.28 Pork Muscle 0.58 7.22 Pork Adipose tissue 0.61 7.64 Pork Chop 0.61 7.57 Values for steaks and chops calculated for whole cut as purchased. Adapted from Enser et al (1996) ‘Fatty acid content and composition of English beef, lamb and pork at retail.’ Meat Science 42(4): 443–56