Transcript Lipids chemical structure

LIPIDS
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The term Lipids or Fats includes both fats and oils.
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Fats tend to be solid at room temperature whereas oils are liquid.
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Fat supply the most concentrated source of energy to the diet = 37kJ (9kcal) of
energy per gram (more than double that provided by protein and carbohydrate).
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Eating fat is essential for a balanced diet and has many vital functions including
helping to keep the body warm and providing essential fatty acids.
Depending on their chemical structure, lipids are often classified into:
1. Saturated fatty acids/fats
2. Unsaturated fatty acids/fats – either in trans or cis form and divided into:
(a) Monounsaturated fatty acids
(b) Polyunsaturated fatty acids
3. Cholesterol
Fat is found in most food groups and fat-containing foods usually provide a range of different fatty acids.
So, even foods with a high saturated fat content, e.g. butter, also provide some monounsaturates and
polyunsaturates. But in simple terms, foods are often categorised by the dominant type of fatty acid.
For example, butter is often described as a ‘saturated fat’ because it has more saturated fatty acids
than unsaturated fatty acids.
Most vegetable oils are described as ‘unsaturated fats’ as they have more mono- and polyunsaturated
fatty acids than saturated.
CHEMICAL STRUCTURE
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Fat belongs to a group of substances called Esters, formed by the reaction between an
alcohol and organic acid – known as fatty acids
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Lipids are esters of glycerol and fatty acids. Chemically, fat is a Triglyceride which
consist of three fatty acids combined with one glycerol molecule. (Diglycerides combine
with two fatty acid molecules and monoglycerides combine with one).
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Glycerol is a complex alcohol because it has 3 hydroxyl groups therefore known as a
Trihydric Alcohol.
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Fatty acids have the general formula RCOOH where R represents a hydrocarbon chain.
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If all 3 fatty acids are the same, then Simple Triglyceride is formed. If fatty acids are
different then they are called Mixed Triglyceride.
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Different combinations of fatty acids make different types of fats and oils.
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There are about 40 different fatty acids found in foods and of these there are basically
two types: saturated fatty acids and unsaturated fatty acids.
1. SATURATED FATS
OCCUR WHEN EACH CARBON ATOM IS ATTACHED TO THE SURROUNDING ATOMS BY A
SINGLE BOND.
THE CARBON ATOMS ARE COMPLETELY SATURATED WITH HYDROGEN ATOMS AND HAVE
NO DOUBLE BONDS
• Saturated fatty acids include butyric (found in milk fat, butter),
palmitic (found in animal fats) and stearic (found in beef fat).
• Saturated fats have a melting point at about room temperature and
are therefore usually hard/solid
• Are usually from animal sources except Coconut and Palm Oil
• Sources: butter, meat fat, lard, cheese, milk, egg
2. UNSATURATED FATS
- OCCUR WHEN SOME OF THE CARBON ATOMS ARE JOINED TO THE SURROUNDING HYDROGEN
ATOMS BY A DOUBLE BOND BETWEEN 2 CARBON ATOMS IN A CHAIN.
- THE DOUBLE BOND PUTS A BEND IN THE CHAIN, ALLOWING THE MOLECULES MORE MOVEMENT AS
THEY CANNOT BE PACKED TOGETHER EASILY
- THE DOUBLE BOND IS ALSO AN AREA OF WEAKNESS IN THE MOLECULAR STRUCTURE OF OIL AND
CREATES AN OPPORTUNITY FOR OXIDATION TO OCCUR (WHERE SATURATED FATTY ACIDS ARE STABLE
AND UNLIKELY TO REACT CHEMICALLY)
- HAVE A MELTING POINT BELOW ROOM TEMPERATURE AND GENERALLY LIQUID AT ROOM
TEMPERATURE
THERE ARE TWO TYPES OF UNSATURATED FATS:
1. MONOUNSATURATED FATS HAVE ONE DOUBLE BOND AND INCLUDE OLEIC ACID FOUND IN OLIVE
OIL, AVOCADO, CASHEW NUTS, PEANUTS
2. POLYUNSATURATED FATS HAVE TWO OR MORE DOUBLE BOND AND INCLUDE LINOLEIC AND
LINOLENIC FOUND IN VEGETABLE (CORN, SOYA AND SUNFLOWER) AND FISH OILS.
POLYUNSATURATED FATS ARE EVEN MORE LIKELY TO REACT TO OXYGEN THAN MONOUNSATURATES BECAUSE OF THE EXTRA
DOUBLE BONDS AND TENDS TO GO RANCID WHEN EXPOSED TO AIR AND SUNLIGHT FOR LONG PERIODS
Saturated Fatty
Acid
Monounsaturat
ed Fatty Acid
Polyunsaturate
d Fatty Acid
Palmatic Acid
Oleic Acid
Linoleic Acid
Stearic Acid
Linolenic Acid
CIS AND TRANS FATTY ACIDS
Unsaturated fatty acids also exists in two different structures called
TRANS and CIS. These structures are identified by the position of the
hydrogen atoms to the double bonds.
Trans
 Hydrogen atoms on geometrically opposite sides of the double bond.
The prefix ‘trans’ means ‘across’.
 This gives the molecule a more linear appearance like saturated fats
Trans fats have high melting points, and act a lot like saturated fats.
CIS
 2 hydrogen atoms on the same side of a double bond.
 This causes the molecule look like it is bent and curved.
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Most unsaturated fatty acids exists naturally in the cis form, but during food
manufacturing processes the cis fatty acids may be changed to trans fatty acids.
Trans fatty acids are created by a process called HYDROGENATION (see below)
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Trans fats can form when unsaturated fats are heated. As the heat makes the
molecules shake and vibrate, some of the bonds get changed from the cis to the
trans position. As the fat continues to cook, the bonds may change back and forth
many times, and the result is a fat that has about half of the bonds in the cis and
half in the trans positions.
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In the making of vegetable shortening, vegetable oils containing unsaturated fats
are heated with a catalyst and some hydrogen, so that some of the double bonds
gain a hydrogen and convert to saturated single bonds. But the process requires
heat, and the heating produces undesirable trans bonds.
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Trans fats have been shown to increase the risk of coronary heart disease
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Shortening was invented as a way to make solid fat from cheap vegetable oils,
which could be substituted for the more expensive lard in cooking. It was only
recently that the health problems associated with trans fats were discovered.
Originally, vegetable shortening and margarine were sold as healthier alternatives
to lard and butter.
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These days, shortening is made by continuing to saturate the oils, so that there are
almost no double bonds left, and then mixing the product with unsaturated oils until
there is less than a gram of trans fat in a tablespoon of shortening. This allows the
manufacturer to claim that there are zero grams of trans fat in the shortening.
HYDROGENATION
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Hydrogenation (hardening) is undertaken to remove some of the double bonds
in the fatty acids and effectively to make them more saturated.
It turns a liquid into a solid by adding hydrogen across the double bonds in the
unsaturated fatty acid molecules.
Hard fats are more hydrogenated (i.e. saturated) than soft fats.
Some oils are so unsaturated in the natural state that they need to be treated to
make them useful in food. A portion of the vegetable fat used in margarine is
hardened by hydrogenation to produce the required plasticity in the final product
before it is blended with the water.
It is carried out by heating the oil in large sealed vessels under pressure.
Hydrogen is bubbled into the oil with finely divided nickel (which acts as a
catalyst and is subsequently removed by filtration). The oil blend is mixed with
the water phase, which is skimmed milk, soured under controlled conditions to
give the right flavour to the product. Salt, vitamins A and D and artificial colouring
are then added. The mixture is then mixed to achieve an emulsion and cooled.
Partially hydrogenated oil is widely used in baking and processed foods like fat
spreads, cakes, biscuits and some ready meals
Hydrogenated fat makes TRANS fats which increases likelihood of cancer and
free radicals in the body. Like saturated fats, artificially created trans fats have
also been related to coronary heart disease. Several food manufacturers use
“free from hydrogenated fats” as a marketing claim, due to concerns about the
potential harmful effects of a high intake of trans fatty acids.
ESENTIAL FATTY ACIDS
POLYUNSATURATED FATS CAN BE SUB-DIVIDED FURTHER INTO A OMEGA GROUP FOLLOWED BY A
NUMBER (N-3 AND N-6). THE NUMBER INDICATES THE LOCATION OF THE FIRST DOUBLE BOND AT ONE
END OF THE CARBON CHAIN.
ESSENTIAL FATTY ACIDS MUST BE CONSUMED IN THE DIET AS THE BODY CANNOT MANUFACTURE
THEM
TWO WELL KNOWN POLYUNSATURATED FATS ARE ESSENTIAL FOR HEALTH: OMEGA 3 AND OMEGA 6
FATS.
SOURCES:
OMEGA 3 = THE RICHEST DIETARY SOURCE IS OILY FISH (SARDINES, SALMON, TUNA) THEREFORE A
WEEKLY SERVING IS RECOMMENDED. THEY ARE ALSO PRESENT IN THE MEAT OR EGGS OF ANIMALS
FED WITH DIETS ENRICHED WITH OMEGA 3 FATTY ACIDS, BUT IN SMALLER AMOUNTS COMPARED WITH
OILY FISH. RAPESEED OIL (CANOLA), MUSTARD SEEDS, PUMPKIN SEEDS, LINSEED AND LINSEED OIL,
SOYA BEAN, WALNUTS, GREEN LEAFY VEGETABLES
OMEGA 6 = THE MAIN SOURCES ARE VEGETABLE OILS AND SPREADS MADE FROM THEM: SUNFLOWER
OIL, CORN SOYA , PUMPKIN , WHEAT GERM
FUNCTIONS:
- REGULATE BODY CHOLESTEROL METABOLISM
- USED TO CREATE MANY HORMONES THAT CONTROL THE IMMUNE SYSTEM AND OTHER FUNCTIONS
- PROPER FUNCTION OF THE BRAIN
- ASSIST IN THE DEVELOPMENT OF THE NERVOUS SYSTEM OF THE FOETUS BEFORE BIRTH AND
VISUAL ABILITIES DURING THE FIRST SIX MONTHS OF LIFE
- IMPORTANT FOR PROPER NERVE FUNCTION
3. CHOLESTEROL
Cholesterol is a waxy substance and type of fat, but it does not produce any energy
75% of your cholesterol is made in your liver, the rest is absorbed from food
It is found in all animal cells and tissues, but not in vegetables or plants
Cholesterol is transported by complex proteins molecules around the body called lipoproteins
There are two main types of protein and cholesterol molecules:
High-Density Lipoproteins (HDL) and Low-Density Lipoproteins (LDL)
Where does cholesterol come from?
The Liver = Makes Cholesterol and secretes it into the blood
Regulated by receptors in the liver
Genetics = genetic predisposition for familial hypercholesterolemia
Diet = High saturated fat intake
Types of cholesterol
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HDL considered the 'good'
cholesterol
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LDL considered the ‘bad'
cholesterol
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Contains a higher level of protein
than cholesterol
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Higher level of cholesterol
than protein
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HDLs transport cholesterol from
the tissues of the body to the
liver, so the cholesterol can be
eliminated in the bile.
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LDLs tend to stick to
damaged or inflamed areas of
blood vessel walls and form
plaque buildup
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The higher the LDL
cholesterol level, the higher
the risk of coronary artery
disease.
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The higher the HDL cholesterol
level, the lower the risk of
coronary artery disease.
Why do we need cholesterol?
Your body uses cholesterol to make:
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Vitamin D
 sex hormones (testosterone, estrogen, progesterone)
 Stress hormones (cortisol)
 bile acids for digestion
 Contributes to normal membrane function of nerve cells in the brain
 Serves as an antioxidant (Neuroprotective)
How can we control cholesterol?
Diet:
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Minimize sweets and refined carbohydrates (i.e.: white bread, candy). Excess sugar increases
triglyceride levels contributing to cholesterol production.
Increase consumption of healthy fats - replace saturated fat with poly and mono unsaturated fat –
this means replacing fat from animals in meat and dairy products with oils and fish.
Consume carbohydrates with proteins and fats to slow breakdown. Increases satiety, reduces
insulin response
Reduce portion sizes to control calorie intake. Helps maintain normal weight and sugar
metabolism
Increase fiber intake
Exercise:
 Physical activity increases cellular energy needs
 Increases ration of good to bad cholesterol
Stress Reduction:
 Reduces stress hormone (cortisol) response, reducing blood pressure and blood sugar.
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Medication and/or supplements
RANCIDITY
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Term used to describe the spoilage of fats and oils. Fat which is rancid will have
an unpleasant smell and flavour. Rancidity is used to describe two entirely
different processes:
• Oxidative rancidity is a condition caused by fat oxidation
• Hydrolytic rancidity is a condition caused by fat hydrolysis
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Oxidative Rancidity
Refers to the undesirable odours and flavours which develop when such products are
exposed to the oxygen in the air.
Products containing these fats, including but not limited to food products such as fish, poultry,
meat, frozen vegetables and dry milk can become rancid as the fats in the products react
to air.
The poly-unsaturated fatty acid portions of these foods react with oxygen to form peroxides.
The peroxides decompose to yield a complex of mixtures, including aldehydes, ketones,
and other volatile products. These products are responsible for "rancid" odours and
flavours.
It is important to note that fish contain highly unsaturated (poly-unsaturated and monounsaturated) fatty acids which make some fish products particularly susceptible to
oxidative deterioration. Highly saturated products, such as butter, are not as prone to
oxidative rancidity due to the absence of poly-unsaturated fatty acid compounds.
Reaction is accelerated by heat, light and traces of metals e.g. copper/ iron.
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Hydrolytic Rancidity
Refers to the odour that develops when triglycerides are hydrolysed and free fatty acids are
released.
Enzymes known as lipase hydrolyse fats, breaking them down into glycerol and fatty acids.
This reaction of lipid with water sometimes requires a catalyst, but results in the formation
free fatty acids and salts from free fatty acids (soaps). In particular, short chain fatty acids,
such as common butter fats, are odorous.
Rancidity in foods may be very slight, indicated by a loss of freshness to very severe,
indicated by objectionable odours and/or flavours. Slight degrees of rancidity are much more
common in foods than severe rancidity, yet slight rancidity is a much more a practical
concern. A slight degree of rancidity may not be objectionable to consumers, but products
which do not seem fresh will not attract repeat purchases. If customers do not return to a
product, the long-term effects of a slight degree of rancidity can be very serious.
Even though meat is held under refrigeration or in a frozen state, the poly-unsaturated fat
will continue to oxidize and slowly become rancid. The fat oxidation process, potentially
resulting in rancidity, begins immediately after the animal is slaughtered and the muscle,
intra-muscular, inter-muscular and surface fat becomes exposed to oxygen of the air. This
chemical process continues during frozen storage, though more slowly at lower
temperature.
Air tight packaging will slow rancidity development.