lipids

Question 1

what are the two methods for lipid nomenclature

Answer 1

1. Delta nomenclature. This names the double bond positions from the carboxyl end (C_18:0 is 18 carbons, 0 double bonds (fully saturated, called stearic acid), and C_18:3^delta9,12,15 is 18 carbons, double bonds starting at carbons 9, 12, and 15 (alpha linolenic acid).
2. Omega Nomenclature. this names double bond positions from the CH3 end. alpha line logic acid is C_18:3^omega3,6,9

Question 2

what is the relationship between carbon chain length and fat melting point?

Answer 2

longer the chain, higher the melting point. think it’s logical as more energy needed to excite the longer chain. This is a direct result of the dispersion forces holding neutral carbon chains together.

Question 3

how do cis and trans double bonds affect fatty acid melting temperatures?

Answer 3

Trans double bonds retain a linear structure, meaning that the fatty acids can be stored more tightly together, increasing the dispersion forces, and raising the melting temperature. Cis double bonds result in a kink in the chain, resulting in loose packing of the FAs, lowering the melting temp.

Question 4

how does the saturation impact fatty acid melting point

Answer 4

unsaturation results in lower melting point

Question 5

what is the general structure of a triacylglyceride? And what is the system for classifying the position of the fatty acids on it?

Answer 5

three fatty acids and the glycerol. it is a storage lipid (neutral). Its short name is TAG. Uses a stereospecific number system for determining how to name the fatty acids (chiral center, with position 1, 2 and 3). Then you name it 1-Palmitoyl-2-oleoyl-3-linolenoylglycerol (for fatty acids Palmitic acid, oleic acid, and linoleic acid. Hydrolysis occurs of TAGs by pancreatic lipase.

Question 6

what is the general structure of a glycerophospholipid?

Answer 6

glycerol, fatty acids, and a PO4 connected to an alcohol, with possibilities for other molecules being attached to an Oxygen. They are a membrane lipid (polar), and more specifically a phospholipid.

Question 7

what are essential fatty acids, and why are only 2 essential? Which are these 2?

Answer 7

fatty acids which can not be synthesised, and must be obtained by the diet. the 2 essential fatty acids are linoleate and alpha linolenate. They are essential as there is a general chain order to how FAs are synthesised, and only plants can do the steps leading to these two fatty acids, and animals can do all of the rest, but require a dietary intake of these 2 compounds.

Question 8

how is metabolism different for MCFAs compared to LCFAs?

Answer 8

Direct transport to liver versus incorporation into chylomicrons in the lymphatic system. Quick and direct entry into mitochondrial for metabolism in hapatocytes verse requirement for carnitine carrier (slower). faster rate of conversion into ketones for rapid energy production. Therefore MCFAs take more energy expenditure to metabolise, and greater satietyresulting in less body weight gain, and better weight control.

Question 9

how are fatty acids in fats analysed?

Answer 9

Gas Chromatography, whereby different fatty acids end up on different areas of the column, showing the makeup of the fats.

Question 10

How are fatty acids released from fats and converted into FAMEs (fatty acid methyl esters)?

Answer 10

They are hydrolysed at 100C and joined with sodium hydroxide (NaOH), before undergoing esterification where the Na+ is swapped for CH3, at 100C. This forms FAMEs.

Question 11

How are fatty acids released from fats and converted into FAMEs (fatty acid methyl esters)?

Answer 11

They are hydrolysed at 100C and joined with sodium hydroxide (NaOH), before undergoing esterification where the Na+ is swapped for CH3, at 100C. This forms FAMEs.

Question 12

what are amphipiles?

Answer 12

compounds which contain both hydrophilic and hydrophobic parts. this allows them to interact with both aqueous and lipid environments (e.g. detergents).

Question 13

what are micelles? Also, what are liposomes and bilayer sheets.

Answer 13

they are layers of amphiphilic molecules which stabilise emulsions by displaying their polar salt side to the water, and non-polar hydrocarbon chain to the oil. Bilayer sheets are where the hydrocarbon chains face each other, making two hydrophobic layers on the outside. A liposome is a spherical version of a bilayer sheet

Question 14

What are major food Glycerophospholypids (GPL)

Answer 14

GPLs are commonly called Lecithin. One major food GPLs is phosphatidylcholine (mainly found in egg yolks and soya beans. 2 fatty acids, glycerol, phosphate, choline(tertiary amine)).

Question 15

How are lecithin used in food science

Answer 15

used as an emulsifier, which reduces the surface tension between 2 immiscible liquids (oil/water) allowing them to mix, forming colloidal particles in the phase. Lecithin is very effective in emulsifying oils, and therefore used in foods as such.

Question 16

How does phosphatidylcholine stabilise emulsions?

Answer 16

The hydrophilic choline end faces the water, stabilising the surface tension, and the hydrophobic fatty acid chains face the oil, forming micelles which can integrate the oil in phase as an emulsion.

Question 17

what are some applications of lipid emulsions in foods?

Answer 17

the use of eggs in mayonaise, cakes, and salad dressings.

Question 18

how are bile salts important for lipid digestion?

Answer 18

bile salts emulsify large fat globules into small fats droplets, making them accessible for enzyme digestion.

Question 19

What are some of the main functions of lipids

Answer 19

1. TAGs and GPLs are energy sources
2. TAG and GPL provide FAs for the generation of important FA derived mediators and signalling molecules in cells.
3. Omega 3-fatty acids alleviate inflammation (anti-inflammatory) as they compete with arachidonic acid for utilisation of enzymes which can produce inflammatory cytokines.
4. Omega 3 fatty acids regulate endothelial cell function, supporting regulation of blood flow, and reducing risk of vascular damage and atherosclerosis.
5. TAGs and GPLs and precursors of many flavourful fatty acids in foods.

Question 20

explain how TAGs and GPLs are energy sources

Answer 20

digestion occurs in the small intestine, with bile secreted from the gall bladder emulsifying the fats for absorption in the small intestine. Then they are packaged in chylomicrons inside intestinal epithelial cells, transported to the rest of the body and broken down generating ATP and acetyl co-a. extra lipids are stored as adipocytes as TAGs

Question 21

what is hydrogenation

Answer 21

a chemical reaction which adds hydrogen atoms at carbon double bonds of unsaturated fatty acids. This happens at high pressures and temperatures. In this process, partial hydrogenation also impacts the leftover double bonds, with isomerisation from cis to trans (as trans is thermodynamically preferred) which is very unhealthy. This results in a higher melting point, as SFAs and trans isomers are more tightly packed, resulting in room temperature solid fats.

Question 22

why are trans fats bad!

Answer 22

the link between trans fats and cardiovascular disease is strong. The main theory is that, they remain longer in the blood circulation, meaning they are more likely to be oxidised, increasing the risk for arterial deposition and plaque formation. Also they raise LDL levels (bad cholesterol) and lower HDL levels (good cholesterol) by a yet understood mechanism.

Question 23

what is rancidity, and what are the two processes which drive it?

Answer 23

Rancidity in food is a bad smell and taste due to lipid degradation. The two types are Oxidative rancidity (oxidative degradation forming a wide range of volatile compounds), and Hydrolytic rancidity (chemical or enzymatic hydrolysis of lipids releasing free fatty acids).

Question 24

what is the major cause of food (more specifically oil) instability

Answer 24

Lipid oxidation

Question 25

what is lipid oxidation?

Answer 25

oxidation of the fatty acids in fats and oils which causes food instability. The initial lipid hydroperoxidr product is relatively tasteless, but further reactions result in the rancid tastes of off food. consumer tolerance is very low.

Question 26

what are the types of lipid oxidation?

Answer 26

Auto-oxidation (no catalyst), Photo-oxidation and Photo-Sensitised Oxidation (catalysed by light), Enzyme catalysed oxidation, and metal catalysed oxidation

Question 27

Describe auto-oxidation (and what are the stages)

Answer 27

it is lipid oxidation in the absence of a metal catalyst. These reactions will speed up in the presence of free radicals. It involves a distinct initiation phase (lipids not breaking down), propagation (sharp increase in oxidation, and termination (all substrate consumed, and free radicals are not around). After initiation, a propagating radical is formed, which leads to oxidation (formation of lipid hydroperoxide ROOH), and the formation of more propagating radicals. This shows how the propagation phase builds in numbers rapidly until all lipids are consumed.

Question 28

what are the two mechanisms of lipid hydroperoxide breakdown?

Answer 28

there is bimolecular degradation (two particles colliding and producing other particles, forming a peroxy radical (propagating more oxidation), and alkoxy radical (results in the rancid taste and aroma)), and unimolecular degradation (the strains on the O-OH bond causes it to break, and produces a hydroxy radical (OH, propagates more oxidation), and alkoxy radical (results in the rancid taste and aroma)).

Question 29

what is the result of lipid hydroperoxide degradation

Answer 29

It generated peroxyl radicals (HCOO) and hydroxyl radicals (OH, dangerous), alongside alkaloxyl radicals. The degradation of the alkaloxyl reactions forms aldehyde compounds which contribute to the rancid flavour and aroma.

Question 30

why are PUFAs more liable to auto-oxidation?

Answer 30

the amount of double bonds increases the rate of oxidation as bond dissociation energy decreases. therefore, PUFAs are more liable.

Question 31

Describe photo-oxidation, and photo sensitised oxidation

Answer 31

Photo-oxidation is when oxygen electrons are excited, creating a singlet oxygen which is more reactive (as it has enough energy to be added onto a double bond, like in PUFA), forming lipid hydroperoxide. Photo sensitised oxidation is when a molecule called a sensitiser gets excited, and transfers that energy to an oxygen particle, which can then form lipid hydroperoxide. (oxygen can dissolve in sweat and form the singlet oxygen, damaging skin proteins, driving inflammation)

Question 32

How can photo oxidation reactions be prevented (additive)

Answer 32

Singlet oxygen quenchers absorb the energy from the oxygen, preventing the formation of the lipid hydroperoxide. examples of these include carotenoids, vitamins A and E.

Question 33

Describe Metal Catalysed Oxidation

Answer 33

Metals such as iron copper and zinc. Iron promotes degradation of relatively stable lipid hydroperoxide by undergoing redox cycling (acting as an electron donor or acceptor) generating free radicals (pro-oxidant effect). The oxidation of lipid hydroperoxide forms peroxy radical (ROO) that initiates auto oxidation, starting a chain effect of oxidisation of available lipids. The degradation (reduction) of lipid hydroperoxide forms alkoxy radical (RO) which generates rancid aroma compounds from its own degradation.

Question 34

describe enzyme catalysed oxidation

Answer 34

Lipoxygenase is an iron metallic enzyme which catalyses the formation of (lipid) hydroperoxide by directly inserting the ground state Oxygen without any electron excitement needed. It does this with specific recognition of the cis,cis-1,4-pentadiene group present in lineleic, linolenic, and arachidonic acid.

Question 35

Describe hydrolytic rancidity

Answer 35

Lipase breaks down triaglycerol into the glycerol and free fatty acids. If lipase breaks it down before we recognise, we taste a volatile bad taste, as we can taste free fatty acids, and it tastes off.

Question 36

What is lipolysis in milk? What can separating fats and liquid be used for?

Answer 36

Milk fats are colloidal oil globules of TAG stabilised by a lipoprotein bilayer membrane. Milk contains the lipoprotein lipase (LPL) enzyme which can cause lipolysis, hydrolysing the bilayer, however can only do this if the bilayer is damaged. This would also lead to separation of fats from the milk liquid, as the emulsifying qualities of the protein bilayer is compromised. A potential product of this is butter, as it deliberately separates the fats from the liquid, to keep the fat as butter.

Question 37

How can butter be kept from going off, while remaining room temperature?

Answer 37

Butter kept in the fridge works, but is hard to spread. Therefore, just keep the butter away from oxygen (using a butter bell, water creates an airtight seal to protect the taste of the butter)

Question 38

How can lipolysis be used for benefit?

Answer 38

Hydrolysis of triglycerides in cheese is a good example of how lipolysis can be used for good, as the extent of lipolysis increases with age, as more free fatty acids are formed, creating a (debatably) desired taste.

Question 39

What impacts the rate of auto-oxidation? what properties of the lipid and also the environment

Answer 39

Increases with:
1) FA unsaturation (ease of hydrogen abstraction by a radical from a fatty acid increases with unsaturation due to decrease in bond dissociation energy of the C-H bond next to C=C bond(s)).

2) Temperature (many reasons; increases reaction rate, promotes thermal decomposition of weak bonds (generates carbon centred radicals, initiating auto-oxidation), increases rate of decomposition of lipid hydroperoxide (generating more radicals proxy and hydroxy radicals)

3) oxidation of relatively stable oils increases dramatically with trace amounts of easily oxidisable PUFA such as linoleate or lonelenate (their auto-oxidation supplies the propagating radicals for auto-oxidation of the more stable fatty acids in fats)

4) Iron or copper presence (catalyse the degradation of lipid hydroperoxide, forming peroxy radicals which support auto-oxidation)

5) trace amount presence of organic plant materials (organic plant materials contain lipooxygenase which catalyzes the formation of lipid hydroperoxide, which when degrades (either slowly naturally, or quickly, catalysed by metals) generates peroxy radicals which initiate lipid auto-oxidation

Question 40

What control methods can be used to prevent lipid oxidation in foods?

Answer 40

1) store at low temperatures
2) reduce exposure to oxygen (to limit addition to form hydroperoxide)
3) reduce exposure to sunlight (as sunlight produces singlet oxygen which leads to oxidation)
4) limit metal exposure (metals catalyse oxidation and degradation of hydroperoxide, forming radicals. Can add metal chelators to remove metals)
5) minimise lipooxygenase and lipase enzymes (can preheat them to denature the enzymes)
6) formulation approach to enhancing lipid stability (reaction rate affected by viscosity and component concentrations).
7) add radical scavengers (antioxidants which reduce free radicals (herbs are a good example). But most have colour, so there is a trade off)

Question 41

How can carotenoids and vitamin A quench the reactivity of singlet oxygen?

Answer 41

When the oxygen is in its excited singlet form, the carotenoid accepts the energy, becoming excited, reducing the oxygen’s reactivity to triplet form. It then releases this energy as heat, therefore dissipating the oxidation threat of the singlet oxygen.

Question 42

What occurs with some phenolic “antioxidants”?

Answer 42

They donate or accept a proton, reducing the reactivity of the free radical, however, become a free radicsl themselves, not reducing the threat. This means they are not an antioxidant as they are unable to reduce the threat of the free radical.

Question 43

What are some other antioxidant mechanisms

Answer 43

Aromatic rings which when they donate or accept a proton are no longer free radicals, as they redistribute the hydrogen between their bonds (make a double bond to redistribute the hydrogen to the free radicals site).

Also, some free radicals bind together, removing their needs for a hydrogen by sharing electrons between them.

Question 44

which molecules can reduce singlet oxygen energy, preventing oxidation?

Answer 44

carotenoids and vitamin A