Lipids Flashcards
Elemental Composition of Lipids
Carbon
Hydrogen
Oxygen
CHO
Chemical Structure of Lipids
- Lipids are made up of 3 fatty acids and a glycerol molecule (TRIGLYCERIDE)
- Glycerol is an alcohol with three hydroxyl groups (OH)
- A fatty acid molecule attaches itself to each hydroxyl group with the elimination of three water molecules (condensation)
- Fatty acids vary and determine the properties of a lipid
Classification of fatty acids
Fatty acids are long carbon chains with CH3 (methyl group) at one end and COOH (carboxyl group) at the other end.
The number of carbon atoms differ with each fatty acid (eg:) stearic acid in meat has 18.
Chemical Formula
CH3 (CH2)nCOOH
Three classes of Fatty acids
Saturated
Monounsaturated
Polyunsaturated
Saturated fatty acids
- Each carbon has a full quota of hydrogen atoms.
- There are no double bonds between the carbon atoms.
- Saturated fatty acids are solid at room temperature.
- Generally from animal sources (eg:) butyric acid (butter) and stearic acid (meat)
Monounsaturated
- Not all carbon atoms are saturated with hydrogen
- There is one double bond between the carbon atoms of a monounsaturated fatty acid
- Generally soft/liquid at room temperature
- Generally come from plant sources (eg:) oleic acid (corn oil, olive oil and avocados)
Polyunsaturated
- Not all carbon atoms are saturated with hydrogen
- There’s more than one double bond between the carbon atoms
- Generally soft/liquid at room temperature
- Generally plant sources (walnuts, seeds and flax oil) and fish (salmon, mackerel, and herring)
- Examples include the essential fatty acids: linoleic acid (2), linolenic (3) and archidonic (4)
Essential fatty acids
Essential fatty acids can’t be manufactured in the body - must be obtained from food.
Polyunsaturated fatty acids:
linoleic
linolenic
arachidonic
Why is linoleic considered to be the most important?
As both linolenic and arachidonic acids can be synthesised from linoleic acid
Functions of essential fatty acids
• These polyunsaturated fatty acids are necessary for the formation of cell membranes and therefore are essential for growth.
• They’re also thought to counteract the hardening effect of cholesterol on the arteries thereby reducing the risk of CHD
Omega-3 fatty acids
An omega-3 fatty acid is a particular unsaturated fatty acid with the first double bond between the 3rd and 4th carbon atoms
Sources of Omega-3
Oily fish: sardines, herring, mackerel, salmon
Nuts: walnuts
Seeds
Soya beans
Supplements
Benefits of Omega-3 fatty acids
- Reduce risk of heart attack and stroke
- Improve brain function, cognitive function and help fight age related mental decline
- Improve eye health
- Reduce risk of certain cancers
Cis fatty acids
Polyunsaturated fatty acids are called cis fatty acids when the hydrogen atoms on either side of the double bond are both above or below the carbon chain.
Cis fatty acids: Sources
Sunflower and olive oil
Nuts
Avocados
Fish Oils
Trans fatty acids
Polyunsaturated fatty acids are called trans fatty acids when the hydrogen atoms on either sides of the double bond are on opposite sides of the carbon chain
Trans fatty acids: Sources
Crisps
Cakes
Biscuits
Crackers
Significance of trans fatty acids in the diet
- Most unsaturated fatty acids are naturally cis fatty acids which are regarded as good as they help reduce cholesterol
- During processing they may be converted into trans fatty acids (eg:) by the addition of hydrogen during production of margarine
- When frying in oil, the high heat changes cis fatty acids to trans fatty acids
- Trans fatty acids are regarded as bad as they are thought to increase incidence of CHD
- Tests have shown that trans fatty acids increase level of low density lipoproteins which deposit cholesterol on walls of arteries
- TFA reduce HDLs which remove cholesterol from circulation
Lipids: Animal sources
Meat
Meat fats (eg:) lard
Butter
Cream
Cheese
Milk
Egg yolk
Lipids: Plant sources
Nuts
Nut oils
Vegetable oils
Margarine
Cereals
Soya beans
Avocados
Lipids: Marine sources
Salmon
Mackerel
Trout
Herring
Sardines
Tuna
Fish liver oil (eg:) cod liver oil
Properties of Lipids: Solubility
Lipids are insoluble in water but soluble in solvents, for example alcohol
Properties of Lipids: Affected by heat
Solid fats melt when heated to 30°-40°C. Lipids boil at extremely high temperature (175°-195°C)
Beyond these temperatures smoke points and eventually flashpoint can occur.
Properties of Lipids: Plasticity
This property relates to whether a lipid is solid, liquid or spreadable.
It is determined by the degree of saturation.
The more saturated fatty acids are present, the more solid the lipid.
As a result, many unsaturated margarines are spreadable and suitable for creaming
Properties of Lipids: Rancidity
The spoilage of lipids:
Oxidative and Hydrolytic
Properties of Lipids: Rancidity: Oxidative
More common
Occurs when oxygen combines with carbon atom at the double bond
Results in an unpleasant rancid smell and taste
Aided by light
Properties of Lipids: Rancidity: Hydrolytic
Occurs when enzymes and/or bacteria hydrolyse lipids, changing them to fatty acids and glycerol
Results in an unpleasant smell or taste
Can occur in the freezer due to enzymes
To prevent oxidative rancidity:
Store lipids in a cool, dark place
Wrap well
Antioxidants added during production prevent oxidative rancidity by combining with the oxygen making it unavailable to attach to the carbon atom
Properties of Lipids: Hydrogenation
An unsaturated fatty acid can have hydrogen added in the presence of a nickel catalyst, converting oil into fats.
Hydrogenation is used in the production of margarine.
Properties of Lipids: Emulsification
Lipids have the ability to form an emulsion.
Can be Water-in-oil or Oil-in-water
Water in oil emulsion
Water droplets are dispersed in the oil
(eg:) butter, margarine
Oil in water emulsion
Oil droplets are dispersed in the water
(eg:) milk, mayonnaise
Temporary emulsions
Temporary emulsions are formed by vigorous shaking
(eg:) oil and vinegar in a vinaigrette dressing
Permanent emulsion
Are formed with the help of an emulsifier (eg:) lecithin in egg yolk acts as an emulsifier in mayonnaise preventing the water in the eggs and vinegar form separating from the oil
Maintaining emulsions:
Emulsifier/ Emulsifying agents
An emulsifier is a molecule that has a hydrophilic head and a hydrophobic tail
The hydrophilic head attaches itself to the water molecule and the hydrophobic tail attaches itself to the oil molecule, preventing the two substances from separating.
Example of an Emulsifier/ emulsifying agent
Lecithin in egg yolk
Glycerol monostearate (GMS) (commercially)
Maintaining emulsions: Stabilisers
Stabilisers maintain the emulsion preventing separation
Examples of Stabilisers
Pectin, Gelatine and gum are stabilisers used in the production of many foods (eg:) ice cream and mayonaise
Effect of heat on lipids: Melting Point
Solid fats melt at 30°-40°C
Effect of heat on lipids: Boiling Point
Lipids boil at high temperatures (175°-195°C)
Effect of heat on lipids: Smoke point
Fats and oils start to decompose if overheated
The glycerol separates from the fatty acids and changes to acrolein, producing a blue haze (vapour) or smoke while emitting an acrid smell.
Smoke point occurs at 200°C (fats)
Smoke point occurs at 250°C (oils)
The smoke point is lowered if lipids contain impurities such as food particles
Effect of heat on lipids: Flash Point
Extreme overheating causes the vapour to spontaneously ignite.
Flash point occurs at 310° (fats)
Flash point occurs at 325° (oils)
Biological functions of lipids
- Lipids supply the body with heat and are a concentrated source of energy
- Excess lipids are stored as adipose tissue, insulating the body and acting as an energy reserve
- Lipids protect the delicate organs of the body (kidneys) and insulate nerve fibres
- Foods containing lipids supply fat soluble vitamins A D E K
Energy value of Lipids
Lipids supply more energy than carbohydrates or protein when oxidised in the body
less than 20% overall energy intake
To reduce fat in the diet
- By lean cuts of meat and remove excess fat before cooking
- Choose low-fat methods of cooking, such as grilling, steaming or baking food instead of frying
- Use low fat spread and low fat varieties of milk and yogurt
- Avoid high-fat snacks such as crisps and pasties (hidden fats)
- Increase consumption of high-fibre foods (whole grains, fruits, veg) that decrease the portion size of food containing fat (cheese, meat)
Digestion of Lipids:
Organ/Glad: Pancreas
Secretion: Pancreatic juice
Enzyme: Lipase
Substrate: Lipid
Product: Fatty acids and glycerol
Digestion of Lipids:
Organ/Gland: Liver
Secretion: Bile
Enzyme: ————
Substrate: Lipids
Product: Emulsified lipid
Digestion of Lipids:
Organ/Gland: Small intestine
Secretion: Intestinal juice
Enzyme: Lipase
Substrate: Lipids
Product: Fatty acids and glycerol
Absorption of Lipids
Absorbed through the lacteals villi of the small intestine.
Lymh system carries digested lipids and deposites them into the bloodstream at the subclavian vein at the base of the neck.
Utilisation of Lipids
To produce heat and energy
In the formation of cell membranes (essential fatty acids)
Excess stored as adipose tissue: insulating and protecting delicate organs and acting as an energy reserve.
Coronary heart disease
CHD occurs when cholesterol accumulated on the walls of the arteries causing narrowing.
Gradual blockage of the arteries is called atherosclerosis
Effects of CHD: Angina
If blood supply is restricted , the lack of oxygen causes shortness of breath and chest pain
Angina attacks are brought on by exertion and emotional stress
Effects of CHD: Heart attack: Coronary thrombosis
blood clot can develop in a narrowed artery, cutting off the oxygen to part of the heart causing a heart attack
Effects of CHD: Death
Sudden death can occur
Cholesterol
Soft wax like substance found in every cell of the body- essential for production of bile, cell membranes, hormones and vitamin D
Low density lipoproteins
Known as Bad cholesterol
LDL are associated with the build up of cholesterol in the arteries, damaging blood vessels and causing heart disease.
Saturated fats and trans fats raise production of LDLs
High density lipoproteins
Known as Good cholesterol.
HDLs remove cholesterol from circulation thereby reducing the risk of heart disease.
Unsaturated fats raise production of HDLs
