10. the Biochemistry of Fat

John Thompson

Learning objectives

This lecture examines the biochemical pathways by which fat is digested, absorbed and deposited in both ruminants and monogastrics and the scope for manipulation by either management or genetic means.

At the end of this chapter you will:

• understand the importance of fat to the meat and food industries. • know the principles of digestion of fat in monogastrics and ruminants • understand the metabolic pathways for fat absorption, synthesis and catabolism in the body • understand the effect of genetic variation on fatty acid composition in ruminants (both between and within breed) • know the importance of environmental effects on fat composition in ruminants (diet, temperature and seasonal effects)

10.1 Introduction

Fatty acid composition in meat animals is important because of the impact it has the melting point of fat and therefore presentation in the display cabinet. Perhaps of greater importance is the influence that dietary fat has on human blood lipids and ultimately fatness and perhaps cardiovascular disease in humans. The 5 fatty acids shown below (Figure 10.1) all have the same carboxyl group at the start, but have different chain length and some have double bonds which impacts on the physical properties of the resultant fat depots (ie melting point)

10.2 Importance of fat composition

• In addition to being an energy storage depot for the animal, the total amount of fat and its composition is important for the following reasons; • Subcutaneous fat is important to stop dehydration in the carcase. • A good coverage of subcutaneous fat reduces the likelihood of cold-shortening in the carcase. • Fat hardness affects processing efficiency through workplace safety and repetitive strain injury. • Fat is prone to oxidation and therefore responsible for development of rancidity. • Fat quality (colour, hardness and texture) is important in some markets for desirable odour and flavour characteristics. • Approx. 85% of subcutaneous fat tissue is made up of triacylglycerols within fat cells. • The remainder comprises moisture (approx. 12%) and connective tissue (approx. 3%). • The collagen component, which affects the texture hardness, is largely a function of fatty acid composition and the molecular make-up of the triglyceride. • The position of the fatty acids on the triacylglycerol affects hardness. • Much of the characteristic species flavour associated with different types of meat originates from carbonyl compounds concentrated in the adipose tissue. In contrast the flavour from the muscle component is similar between lean meat from different species.

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©2009 The Australian Wool Education Trust licensee for educational activities University of New England Figure 10.1. The chemical formulae for several common classes of fatty acids. Source: Thompson, (2005).

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Triacylglycerols (formerly called triglycerides) are derived from 3 primary sources: the diet; de nova synthesis, particularly in the liver; and storage depots in adipocytes. That is, they are ingested, synthesised in the liver, or mobilised from storage. Triacylglycerols are made up of 3 fatty acids attached to a glycerol. Most triacylglycerols contain a mixture of fatty acids, which may be saturated or unsaturated.

Fatty acids are made up of a carboxylate group attached to one end of a hydrocarbon chain. Most of the naturally occurring fatty acids have an even number of carbon atoms. A saturated acid is one in which the carbons are saturated with hydrogen atoms. Many naturally occurring fatty acids are unsaturated – ie they contain one or more double bonds (one or more carbons which are not saturated with hydrogen atoms and therefore have a double bond between consecutive carbons). Nomenclature of fatty acids

Each fatty acid has a common name (eg stearic), and a systematic name (eg octadecanoic), and an abbreviation based on their structure (eg 18:.0) where the number before the colon indicates the number of carbons in the hydrocarbon chain, and the number after the colon the number of double bonds.

The most common fatty acids present in the adipose tissue of ruminants and non-ruminants, along with their carbon chain length and the number of double bonds, are shown in Table 10.1. The melting points are also given. Note the large decrease in melting point with increased unsaturation of the C18 acids.

Table 10.1 Fatty acid composition of beef adipose tissue Source: Thompson, (2005). Name Structure Percent Melting Point (oC) Myristic 14:0 2-4 54 Palmitic 16:0 23-27 63 Stearic 18:0 4-30 70 Oleic 18:1c9 38-45 16 Linoleic 18:2c9,c12 1-2 -5

Digestion of dietary lipids

The major problem that animals must cope with in the digestion, absorption and transport of dietary lipids is their insolubility in water. The action of bile salts secreted by the gall bladder is essential to the digestion of lipids and their absorption through the intestinal mucosa. The complexing of lipids with proteins to form lipoproteins enables transport through the blood and lymph. Monogastric

Pancreatic juices provide lipase, which hydrolyses triacylglycerols in the small intestine to form two free fatty acids and a monoglyceride, which are soluble in bile salts. The fatty acids and monoglyceride combine with bile salts form a micelle, which passes across the intestinal wall. The triglyceride is then resynthesised and combines with proteins to form lipoproteins (chylomicrons). This process solubilises the lipids and permits their transport through blood and lymph, and it is in this form that dietary fat is transported from the intestine to peripheral tissues, eg for storage in adipocytes.

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©2009 The Australian Wool Education Trust licensee for educational activities University of New England Ruminants Figure 10.2 summarises the processes of digestion, absorption and synthesis of fat in the ruminant.

Figure 10.2. A diagrammatic representation of the pathways for digestion, absorption and synthesis of fat in the ruminant. Source: Kelly (1999).

The high intake of unsaturated fatty acids from plant materials is to a large degree hydrogenated in the rumen. Dietary lipids are hydrolysed by lipases produced by the rumen bacteria and the released free fatty acids are then hydrogenated (ie. hydrogen is incorporated into the double – unsaturated – bonds to produce more saturated fatty acids). Linoleic (C18:2) and linolenic (C18:3) acid, which are very common in plant material, are hydrogenated to form stearic acid (C18:0). The hydrogenation is not always complete. As a result, appreciable amounts of geometrical and positional isomers of octadecenoic and octadecadienoic acids are formed.

There is commonly more lipid in fluid that reaches the abomasum than that in the diet. This is mostly due to lipid synthesis by rumen bacteria and protozoa. Elongation of dietary fatty acids is the main mode of synthesis by bacteria and protozoans, as the process of building fatty acids from acetate is energetically expensive. Rumen bacteria and protozoans build more odd chain length fatty acids than are formed by plants and animals.

There is little alteration of fatty acids between the rumen and the small intestine. The physiological mechanisms of digestion in the intestines are similar for ruminants and non-ruminants. Fatty acids arrive into the small intestine attached to insoluble particulate matter. Any esterified fats that “ escaped” the rumen are lipolysed in the duodenum.

Free fatty acids are then emulsified by the action of bile to form micelles. These micelles are absorbed through the epithelial cells of the small intestine. Although the lipid content of ruminant digesta is generally low, the alimentary system has the capability to absorb considerable quantities of lipid. Absorption into the plasma is highly efficient with 82-92% of C16:0 and C18:0 absorbed respectively in cattle. Although the absorption is high there is some preferential absorption of oleic (C18:1) palmitic (C16:0) and stearic (C18:0). However, discrimination against the absorption of stearic acid may be hardly noticeable, since this is the predominant component of the digesta.

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©2009 The Australian Wool Education Trust licensee for educational activities University of New England Lipoprotein lipase is responsible for lipid uptake by adipose tissue. Exogenous free fatty acids are taken up readily. Triglycerides of plasma lipoproteins (VLDL and chylomicron) are the main source of exogenous fatty acids for adipose tissue. The rate of uptake by lipoprotein lipase is dependant on age, diet, pregnancy and lactation.

Exogenous fatty acids are rapidly incorporated into lipids. However the absorption of fatty acids may be differential. Studies have reported that palmitic (C16:0) was the preferred fatty acid for absorption, followed by oleic (C18:1c), linolenic (C18:3c), linoleic (C18:2c) then stearic (C18:0). As the amount of VLDL in the plasma is low, the lipoprotein lipase should incorporate any chain length into the adipocyte. In the plasma the major fatty acid is C18:0 which can constitute up to 47% of the total amount of lipid available for absorption

The fatty acid composition of ruminant fat is characterised by a high stearic content, the presence of positional and geometric isomers of unsaturated fatty acids and the existence of appreciable amounts of branched chain and odd numbered fatty acids.

The effect of the rumen on modification of the fatty acids is shown in Table 10.2. Major fatty acids in the diet and lymph triglycerides for pigs, sheep fed a hay and oats diet and sheep infused with maize oil via a duodenal fistula. Table 10.2. where there is little relationship between the fatty acid profile of dietary components and the lymph in the sheep. However in an animal fed through a duodenal fistula, or a monogastric, the fatty acid profile in the lymph largely reflects the profile in the dietary components.

Table 10.2. Major fatty acids in the diet and lymph triglycerides for pigs, sheep fed a hay and oats diet and sheep infused with maize oil via a duodenal fistula. Source: Thompson, (2005). Pig Sheep Sheep with a Duodenal Fistula Lymph Diet Lymph Diet Lymph Diet C16:0 21.8 28.0 28.1 19.8 16.4 14.0 C18:0 8.4 5.9 40.0 2.3 19.6 2.6 C18:1 28.2 23.7 14.8 24.0 22.7 28.0 C18:2 34.7 40.9 2.8 36.4 37.5 52.6 C18:3 1.2 0.9 1.3 16.3 0.9 1.5

10.4 De Nova synthesis of lipids

Anabolism Excess dietary energy can be stored as fat. Many tissues are capable of synthesising fatty acid from acetyl-CoA. This is called de nova synthesis. The pathway for accretion and breakdown of fat is shown in Figure 10.3. In non-ruminant species glucose or hexose is the major precursor for fat accretion, whereas in ruminant species glucose is conserved and the major carbon donor is acetate steps in fatty acid metabolism (see Figure 10.3)

I. Acetyl-CoA is condensed to form citrate. II. Citrate can transverse the mitochrondrial membrane. Citrate is cleaved to form malonyl- CoA which is a 2 carbon unit. III. Malonyl-CoA is the carbon donor which provides the 2 carbon unit to form long chain fatty acids. IV. The final product of fatty acid synthesis is a usually saturated 16 carbon fatty acid called palmitate (C16:0).

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©2009 The Australian Wool Education Trust licensee for educational activities University of New England Figure 10.3. Anabolic and catabolic pathways of adipose tissue metabolism. Glucose is metabolised to pyruvate, which enters the mitochrondia and is decarboxylated to a 2 carbon structure called Acetyl-CoA. Source: Thompson, (2005).

Fatty acid composition of endogenous fat in pigs In pig diets containing 2−4% fat, 73−82% of the fat deposited in the body comes from de nova synthesis. Endogenous fat consists mainly of C16 (30%) and C18 fatty acid, with only minor amounts of fatty acids with shorter or longer chain lengths. The C18 fatty acids comprise almost all stearic acid (C18:0, 11−18%) and oleic acid (C18:1, 48−57%) formed by desaturation of stearic acid. Negligible amounts of linoleic (C18:2) and linolenoic (C18:3) acid are found, as these are not synthesised by mammalian tissue.

Essential fatty acids The so-called essential fatty acids include the polyunsaturated fatty acids (PUFAS), linoleic (C18:2, omega-6), linolenic (C18:3, omega-3), and arachidonic acid (C20:4, omega-6). These acids are termed ‘essential’ because animals cannot synthesise double bonds in the omega-3 and -6 positions. The PUFAS are vital components of cell membranes throughout the body, and one of their major roles is to generate a group of highly active local hormones, the eicosanoids. The synthesis of these of these 20 carbon compounds (prostaglandins etc.) originates from PUFAS, mostly via arachidonic acid.

Catabolism of fat Lipolysis in adipose tissue is initiated by hormone sensitive lipase and when completed results in

1. free fatty acids, 2. glycerol

The fatty acids can enter the adipocyte fatty acid pool and can be re-esterified or they can be transported to the plasma and be broken down as in Figure 10.2. In fat depots of fasting animals there is a reduction in the activity of clearing-factor lipase involved in the uptake of lipids, a depression in fatty acid synthesis and an increase in lipolysis, resulting in the release of free fatty acids. In fed animals there is an increase in clearing-factor lipase and active lipid deposition, with the result that fat synthesis occurs.

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Monogastrics When the diet of pigs is high in unsaturated fat, the deposited fat may become soft and commercially unacceptable as a result of its lowered melting point.

The fatty acids synthesised from carbohydrates (i.e. through de nova synthesis) are unsaturated in character (i.e. they have double bonds and a low melting point). The faster a pig deposits fat, the more it eats the greater the amount of fat derived from the diet. This fat is generally more saturated and firmer.

Restricted feeding will result in a slower rate of fat deposition and the fat will contain a higher proportion of unsaturated fatty acids. The addition of copper to the diet will result in the formation of softer fat because of a change in the ratio of saturated/unsaturated fatty acids, and a redistribution of fatty acids within the triglyceride molecule. High dietary intake of copper can reduce fat melting point by 10ºC. Some of the effect may arise from direct stimulation of fatty acyl desaturase. However, it is also likely that copper affects distribution of fatty acids in the triglyceride molecule.

Ruminants In ruminants unsaturated fat in the diet has only a minor effect on the composition of depot fat, due to the hydrogenation of these in the rumen. Figure 10.4 provides a summary of the production factors that affect fatty acid composition and the flow on effects to meat quality and processing.

Figure 10.4. An overview of factors affecting fatty acid composition in ruminants. The heaver solid lines denote the magnitude of the effect. Dotted lines indicate increased variation in the results. Source: Kelly (1999).

Within breed variation in fatty acid composition Preliminary analysis of genetic variation in fatty acid composition suggests that selection targeting changes in fatty acid composition is possible (Kelly 1999). The key fatty acids in terms of fat hardness are heritable with the major fatty acids which impact on fat hardness having heritabilities of the order of 0.3 to 0.5 (Table 10.3). Genetic correlations indicate that selection for increased C18:1c9 would also lead to corresponding decreases in C16:0 and C18:0 (Kelly 1999). Both C16:0 and C18:0 contribute substantially to fat hardness.

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©2009 The Australian Wool Education Trust licensee for educational activities University of New England Table 10.3. Genetic correlations and heritabilities for individual fatty acids in the subcutaneous depot. Heritabilities are listed on the shaded diagonal. Source: Kelly (1999). Fatty Acid C14:0 C14:1c C16:0 C16:1c C18:0 C18:1c C14:0 C18:1c (11) C18:2c C14:0 0.56 0.45 0.59 0.35 -0.08 -0.63 0.06 -0.21 -0.25 C14:1c 0.47 0.28 0.61 -0.65 0.09 -0.53 0.3 -0.36 C16:0 0.34 0.23 -0.37 -0.71 0.13 -0.34 -0.33 C16:1c 0.51 -0.7 -0.03 -0.78 0.39 -0.16 C18:0 0.43 -0.48 0.83 -0.7 0.33 C18:1c 0.24 -0.58 0.85 0.02 C18:1t 0.15 0.53 C18:1c (11) 0.17 -0.08 C18:2c 0.11

A number of factors may impede the inclusion of fatty acid composition in breeding programs. The genetic relationships between fatness and fatty acid composition are not well established. In general, increased fatness is associated with increased C18:1c9. Conversely, it is not known whether selection for increased C18:1c9 would result in increased fatness. This is an important issue, as current and future breeding objectives negatively weight fatness as they strive to increase lean meat production. Additionally, C16:0 consumption may increase cholesterol levels in humans. C18:1c9 and C18:0 consumption tends to have little effect however. It is important to note that attenuating overall fat intake is a more important human health issue than changing fatty acid composition.

Between breed variation in fatty acid composition Several studies have shown that Brahman or Brahman cross cattle have softer fat, with a higher proportion of unsaturated acids than Bos taurus cattle (eg Perry et al. 1998). There does not appear to be much difference between Bos taurus breeds.

Table 10.4. Predicted means for different genotype in the subcutaneous fat and intermuscular fat depots. Source: Tume and Thompson, unpublished data. Fatty Acid Percentage Braham 0% 25% 50% 75% 87% 100% Saturated Fat 16:0 26.22 25.40 25.21 24.26 25.10 24.93 18:0 15.95 13.66 12.26 13.75 14.06 13.81 18:1 c9 38.96 40.67 41.08 40.77 38.73 39.35 Saturation indicies SFA 48.04 44.98 43.43 44.03 45.67 45.33 MUFA 49.25 51.90 53.23 52.51 50.74 51.07 PUFA 0.96 1.04 1.11 1.16 1.22 1.07 USR 1.05 1.19 1.26 1.23 1.14 1.16

The terms SFA, MUFA, PUFA and USR refer to ways of summarising the fatty acid composition in a way that reflects its chemical or physical composition. A recent study in South Australia (Siebert et al 1999). has compared fatty acid composition in intramuscular fat in a range of breeds (Table 10.5)

Table 10.5. Intramuscular fat content and fatty acid concentration and the melting point of subcutaneous fat. *Sum of the cis mono unsaturated fatty acids. Source: Siebert et al (1999). Sire Breed IM Fat % C16:1 (9c) C18:0 C18:1(9c Cis* MUFA Melting ) Point Jersey 4.8 5.8 12.2 41.1 50.8 36.3 Angus 4.7 4.5 13.3 41.2 49.5 38.8 Waygu 4.7 4.9 12.6 42.2 50.9 36.6 Hereford 4.2 4.5 14.2 40.5 49.6 39.1 South Devon 3.9 4.6 14.0 40.6 48.8 39.7 Limousin 3.2 4.8 13.6 40.8 49.1 40.0 Belgium Blue 3.2 4.9 13.3 40.8 49.3 38.5

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©2009 The Australian Wool Education Trust licensee for educational activities University of New England Jersey and Waygu breeds had the highest levels of unsaturation in the intramuscular fat and similarly the lowest melting point of subcutaneous fat. These differences are not as extreme as those reported for Waygu cattle in Japan, but indicate that under Australian production conditions genetic differences between breeds do exist. Also the Jersey and Waygu breeds possess the ability to desaturate fatty acids to a greater degree than the other breeds (Table 10.5.)

Diet/ration effects on fatty acid composition Sheep and cattle fed diets high in grain may have a greater proportion of unsaturated fat (particularly C18:1) than animals fed a roughage diet. It has been suggested that this is due to a rate of passage effect and modification of the microbial population in the rumen.

In sheep, barley- or wheat-based diets will produce softer fat due to the formation of odd numbered and branched chain fatty acids (from the increased propionic acid production in the rumen). This effect does not occur in cattle.

In Australia, the feeding of cattle on grain tends to produce a similar hardness to grass fed cattle. This contrasts with the US where grain feeding produces softer fat than grass fed cattle. Nutrition is clearly important and US rations are generally based on maize, whereas Australian diets are based on wheat or sorghum.

Milk-fed ruminants which have diets high in unsaturated fats can have softer fat if the oesophageal groove protects the fat from hydrogenation. Cotton seed contains cyclopropeniod fatty acids (0.6- 2.1%). Although these acids are unsaturated they tend to increase saturation up to 2.5 fold when fed to rats. This increase in saturation is due to inhibition of fatty acids to unsaturated fatty acids (i.e. C18:0 to C18:1). The effect is not as great in ruminants as in non-ruminants, as these acids are hydrogenated in the rumen. Rumentek technology is basically protecting monounsaturates using a formaldehyde/casein coating. This can manipulate fatty acid composition so that they resemble monogastrics.

Fatty samples from Japanese and Australian cattle show the large difference in fatty acid composition largely due to different C18:0 and C16:0 ratios (Figure 10.5) It is not clear whether this difference is due to nutritional or genetic factors.

Effect of temperature on fatty acid composition Subcutaneous fat is softer in cattle raised in cooler environments. Therefore, fat hardness is likely to be a greater problem in tropical environments.

Exposure to low temperatures results in more unsaturated fat. Deer provide one example of this principle. The fatty acid composition of deer bone marrow changes between summer and winter. This may be explained in evolutionary terms, as deer often stand in snow and freezing water. If the level of unsaturation did not increase in winter, it is likely that the fat would freeze.

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©2009 The Australian Wool Education Trust licensee for educational activities University of New England Figure 10.5. Relationship between stearic acid content and (a) palmitoleic acid content (b) subjective hardness score for subcutaneous fat. Subjective hardness was scored as 1, very soft; 2, soft; 3, medium; 4, hard; and 5, very hard. Points with error bars represent the mean (sd) stearic acid content for each hardness score (adapted from Yang et al. 1999. Printed with permission from Elsevier).

Seasonal effects on fatty acid composition Environmental factors that oscillate in a seasonal pattern, such as feed quality and ambient temperature, appear to have an effect fatty acid composition in cattle. In a study by Kelly et al (1999) seasonal variation in subcutaneous fatty acid composition was modelled by fitting a sine curve to day of kill for 1051 southern CRC core cattle. Figure 10.6 shows the seasonal variation in C18:1c9 and C18:0 of grain and pasture finished cattle. There were differing seasonal fluctuations in grain and pasture finished cattle. Proportions of C18:0 and C18:1t11 in pasture finished cattle peaked mid year. These peaks were largely offset by reductions in C18:1c9, C16:1c and C16:0. Significant seasonal variation occurred in all fatty acids except C18:0 and C16:0 in grain finished cattle (P<0.05), peaking at similar times and amplitudes as pasture finished cattle. Seasonal oscillations present in pasture fed C18:0 and C16:0 were minimised by grain finishing. This infers that in pasture fed cattle, oscillations were due to fluctuations in pasture quality.

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©2009 The Australian Wool Education Trust licensee for educational activities University of New England Figure 10.6. The fitted sine curve for C18:0 and C18:1c9 of grain and pasture finished cattle and Julian day starting 1st January 1994 (Seasons are denoted as S - summer, A - autumn, W - winter, SP − spring). Source: Kelly et al (1999).

By contrasting seasonal patterns in grain and pasture finished cattle in the same environment, differences were a function of both climatic and nutritional effects. This suggested that fat hardness changed in a seasonal pattern.

Sex effects on fatty acid composition In a more recent study by Kelly et al (2000) the effect of sex on fatty acid composition of the subcutaneous fat depot was examined in samples collected from 577 cattle from the Beef CRC northern program. There was a significant sex x finish interaction on fatty acid composition and melting point. Heifers had higher proportions of C18:1c9 than steers. When finished on grain both sexes had similar proportions of C18:0, but when finished on pasture heifers had 2% less C18:0 than steers. It was predicted that fat hardness was similar in grain finished steers and heifers, while pasture finished heifers would have softer fat than pasture finished steers

Ewes tend to have softer fat than wethers. In pigs, unsaturation of the backfat decreases from boars to gilts to barrows. Rams have less stearic acid (C18:0) and more C18:2 and C18:3 than wethers. There is no difference in saturation between bulls and steers.

Effect of depots Subcutaneous fat is softer than internal fat in pigs, cattle and sheep. There is often a gradient in saturation within the depots.

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©2009 The Australian Wool Education Trust licensee for educational activities University of New England Table 10.6. Proportions of the major fatty acids in brisket fat and kidney fat in a 10-year old Jersey cow Source: Thompson, (2005). Brisket Fat Kidney Fat 1mm 50mm 80mm 125mm C16:0 19.4 24.0 21.4 20.3 31.5 C16:1 18.1 13.6 11.9 7.6 2.7 C18:0 2.9 3.9 4.3 6.4 17.0 C18:1 47.8 43.7 48.2 54.4 36.5

Stage of development The fat of newborn sheep and cattle contains less than 10% stearic acid (C18:0). This proportion rises as the rumen develops. However as age increases the proportion of stearic acid declines in both sheep and cattle. The decrease in stearic is compensated by an increase in C18:1 in kidney fat and C16:1 in subcutaneous fat (Figure 10.7).

Figure 10.7. Variation with age in percentage of stearic acid (C18:0) in kidney fat from (a) sheep, and (b) Jersey cattle. Source: Thompson, (2005).

In non-ruminant species glucose or hexose is the major precursor for fat accretion, where in ruminant species glucose is conserved and the major carbon donor is acetate steps in fatty acid metabolism (see Figure 10.3).

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Summary Summary Slides are available on CD · Fatty acid composition determines the hardness of fat. This can be affected by species, breed, sex, nutrition and environment (particularly temperature).

References Kelly, M.J. (1999). Environmental and genetic effects upon the fatty acid composition of subcutaneous beef fat. Master of Science Thesis. University of New England Armidale. Kelly, M.J., Tume, R.K. and Thompson., J.M. (1999). Seasonal patterns in fatty acid composition of beef subcutaneous fat. Proceedings of the 45th International Congress of Meat Science and Technology. Yokohama, Japan, vol 45 Kelly, M.J., Tume, R.K. Newman, S. and Thompson, J.M. (2000). Effect of sex and finishing regime on fatty acid composition of subcutaneous fat in tropically adapted cattle. Asian- Australasian Journal of Animal Sciences. Supplement July 2000, vol B.13 pp 141-144. Seibert BD, Pitchford WS, Malau-Aduli, AEO, Deland MPB and Bottema CDK (1999). Breed and sire effects on fatty acid composition of beef fat. Australian Association for the Advancement of Animal Breeding and Genetics, vol 13 pp 389-392. Perry, D., Nichols P.J. and Thompson J.M. (1998). The effect of sire breed on the melting point and fatty acid composition of subcutaneous fat in steers. Journal of Animal Science, vol 76 pp 87-95. Tume, R.K. and Thompson, J.M. unpublished data. Yang A, Larsen TW, Powell VH and Tume RK. (1999). A comparison of fat composition of Japanese and long-term grain-fed Australian steers. Meat Science, vol 51 pp 1-9.

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Glossary of terms Anabolism involves the building of substances from different or less complex precursors into products that are more complex – this usually consumes energy, and the stored products may be important as energy reserves, eg fat stores in adipose tissue. Catabolism involves the breakdown of substances toward their end products, usually with the release of energy that either is used immediately in other biochemical reactions, is stored, or is dissipated as heat Hydrogenation the conversion of double (or unsaturated) bonds between carbon atoms to saturated bonds by the incorporation of hydrogen – it is a form of reduction. Hydrolysis a chemical reaction or process in which a molecule is split into two parts by reacting with a molecule of water. One of the parts gets an OH from the water molecule and the other part gets an H from the water Chylomicra a lipoprotein - colloidal particles which are complex mixtures of triglycerides, phospholipids, cholesterol and proteins. They are formed mainly in the wall of the intestine. Most of the absorbed fats from the guts are in this form

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