Lipids Chapter

FND300

Lipid solubility

• generally insoluble in water

• soluble in non polar solvents

Lipids are composed of

• glycerol esters of fatty acids

Types of lipids

• oils and fats

• oils are liquid at room temperature

• fats are solid at room temeprature

Sources of vegetabel oils

• soybeans, canola, palm, coconut, olive

Fat sources

• from animals

  • butter and lard

Lipid qualities

• carry fat soluble vitamins

• increase mouthfeel, texture and taste

• increase obesity and chronic disease

Fatty acids composition

• made of hydrocarbon with carboxylic group at one end (the alpha end)

  • this is acidic end

• omega end contains a methyl group

Forms of fatty acids

• saturated (no double bonds)

• unsaturated (containing one or more double bond within the hydrocarbon chain

Short chain fatty acids

• <10 carbons

• butyric acid (C4:0)

  • found in gut as a product of fibre fermentation

Medium chain fatty acids

• 12-16 carbons

• lauric acid = c12:0

  • found in coconut oil

Long chain fatty acids

• >18 carbons

• includes steric acid (C18:0)

  • oleic acid (C18:1)

  • linoleic acid (C18:2)

Gamma linolenic acid

• C18:3 w 6 with the double bond being introduced at the 6th carbon

alpha linolenic acid

• C18:3 w 3

• first souble bond introduced at the third carbon from the omega end

Cis formation

• 2 hydrogens are on the same side of the double bond

• most fats are normally found in this configuration

Trans fatty acids

• H is on opposite sides of the double bond

• found naturally in meat and dairy

• often formed due to hydrogenation of oils in processed foods

  • This is more common in the diet than the from meats

• creates a shape similar to saturated fatty acids

Non conjugated fatty acids

• alpha methylene group is between two double bonds

conjugated Fatty acid

• 1 single bond between a pair of double bonds

Melting point

• temperature at which a solid turns in to a liquid

• higher MP is more solid at room temperature

Short chain FA - MP

• lower MP than longer chain

• due to lower molecular weight

• will melt faster than long chain

Melting points of FAs from lowest to highest

• Lauric (C12) → Palmitc (C16) → Steric (C18)

Saturation and MP

• higher unsaturation results in a lower melting point

  • more liquid at room temperature

• saturated FAs have molecules packed more tightly together and therefore a higher MP

• unsaturated FA have double bonds which gives a bend and molecules cannot interact strongly enough and therefore have a lower MP

Order of melting points by saturation

• lowest = linolenic acid (18:2)

• Middle = oleic acid (C18:1)

• Highest = stearic acid (C18:0)

Cis FA vs Trans FA Melting points

• Trans have a higher MP because they are more tightly packed together

• Cis FA have the kink in them which prevents them from binding as closely

  • therefore there is a lower MP

Melting point order based on cis vs trans orientation

• Lower (oleic acid) - cis

• higher (elaidic acid) -trans

Melting points of saturated vs trans vs cis at similar chain length

• Lowest = Oleic acid (C18:1 - Cis)

• Middle = Elaidic acid (C18:1 - Trans)

• Highest = Stearic acid (C18:0) - saturated

• Melting point is highest in the saturated FA and lowest in the Cis unsaturated

Carotenoid FA composition

• mostly trans isomers and conjugated double bonds

• increase in the cis isomer due to isomerization of the trrans isomer during processing

PUFAs

• synthesized through a series of desaturation and elongation reactions

• Arachadonic acid, Docosapentaenoic acid, docosahexaeonic acid, eicosapentanoic acid are all essential

importance of AA, DPA, EPA, DHA

• cell membrane components

• precursors for eicosanoids and docosanoids which act as hormones

Omega-6 Desaturation and elongation

• Linoleic acid

  • D-6 desaturase

• Y-linolenic acid

  • Elongase

  • D-5 Desaturase

• Arachadonic acid (AA)

  • Elongase

  • Elongase

  • D-6 Desaturase

  • B-oxidation

• Docosapentaenoic acid (DPA n-6)

Linoleic acid

• becomes DPA and AA

• from veg oils (sunflower, grapeseed)

• cannot be synthesized so must be obtained in the diet

• makes Omega-6

Desaturation of Omega 3

• a-linolenic acid.

  • D-6 desaturase

• Steradonic acid

  • Elongase

  • D-5 desaturase

• Eicosapentaenoic acid (EPA)

  • elongase

• Docosapentaenoic acid (DPA n-3)

  • elongase

  • D-6 desaturase

  • B-oxidation

• Docosapentaenoic acid (DHA)

EPA and DHA

• humans can synthesize both from a-linolenic acid (ALA)

  • but this is very inefficient

  • only 9% is converted to DHA for women

  • <5% ALA conversion to DHA for men

• High concentrations are found in fatty fish

Health Benefits of w-3 FAs

• Fetal development of the brain and retina

• anti-inflammatory and anti-oxidative stress effects bring about improved cardiovascular function

• maintain brain function and prevent alzheimers disease

w-3 and w-6 FAs ingestion

• incorporated into cell membranes of tissues

• precursors for synthesizing signalling molecules important for

  • cell growth and development

  • inflammation

Overconsumption of w-6 FAs

• arachidonic acid become precursor for pro-inflammatory mediators like eicosanoids, prostaglandins, thromboxanes, leukotrienes

• overconsumption can lead to heart disease

• EPA and DHA produce anti-inflammatory cell mediators

• beneficial to consume less w-6 FAs

• Should aim for a ratio of 4:1 (omega-6:omega-3)

Hydrogenation

• addition of hydrogen to double bonds in FA chains

  • increases the saturation

Solid fat

• full saturation/hydrogenation

Semi-solid fat

• partial saturation/hydrogenation

Uses of hydrogenation

• margerine manufacturing

  • liquid fat is converted into a spreadable, plastic, semisolid fat

  • increased oxidative stability to lower rancidity and create a longer shelf life

Hydrogenation process

• oil mixed with nickel catalyst

• heated to 160-220 degrees celcius

• Exposed to H+ up to 60psig

• degree of saturation is monitored by a refractive index

• hydrogenated oil is cooled and the catalyst is removed via filtration

partial hydrogenation

• produces fats with firmness, plasticity and spreadability

• production of trans FA (amounts depending on the degree of hydrogenation)

trans fat effects

• increased low density lipoprotein (LDL) cholesterol

• Decreased high density lipoprotein (HDL) cholesterol

• increased risk of atherosclerosis and heart disease

use of partially hydrogenated fats

• baked goods, processed foods

• deep fried snacks

Lipid oxidation

• involves oxidation of unsaturated FA

• leads to oxidative rancidity in food (off flavours)

• reduces the nutritional value

• some oxidation in products may be toxic

• some oxidation products can be favourable

Desirable lipid oxidation

• flavour production in aged cheeses

Auto-oxidation

• occurs because of light, oxygen and emzymes

• reactions occur between unsaturated FAs (free or within triglycerides) and oxygen

3 steps of lipid oxidation

• initiation

• propagation

• termination

Initiation reaction

• X* + RH → R* + XH

Propagation Reaction

• R* + O2 → ROO*

• ROO* + RH → ROOH + R*

  • R* then loops back and the reaction occurs again

• 2ROOH → RO* + ROO* + H2O

Termination Reaction

• R*, RO*, ROO* → Stable, non propogating species

Initiation stage of oxidation

• presence of a catalysist is required to oxidize the unsaturated FA (RH)

  • usually reactive oxygen species known as a singlet oxygen (X*)

• H is abstracted from position alpha to FA double bonds to produce R* (fatty acid with free radical)

  • Easiest from methylenic carbon in non-conjugated FA

Singlet oxygen

• excited state of molecular oxygen with higher energy and reactivity

Propogation stage

• R* reacts with oxygn to produce peroxy radicals (ROO*)

• ROO* then abstract H from a-methylenic groups of other unsaturated FA (RH) to make hydroperoxides (ROOH) and new free radicals R*

• New R* reacts with oxygen and propogation is repeated

• hydroperoxides are stable and decompose immediately to form other free radicals, ROO* and RO* (alkoxy radicals) which continue propagation steps

Unsaturated FA in reactions

• RH

Singlet oxygen in reaction

• X*

Fatty acid with free radical in reaction

• R*

Peroxy radical in reaction

• ROO*

Hydroperoxides in reactions

• ROOH

Alkocy radical in reactions

• RO*

Termination in lipid oxidation

• propatation results in lots of free radicals which absorb oxygen

• as the concentration of the free radicals increases they cobine with other radicals to form non-radical products

Malondialdehyde (MDA)

• end product of lipid oxidation

MDA concentration

• determined by the thiobarbituric acid reactive substances (TBARS) assay

• MDA reacts with thiobarbituric acid (TBA) to form MDA-TBA2 adduct with its bright pink

• amount of pink chromogen is determined by measuring the amount of absorbance at 530-540nm and comparing this against a standard curve

Factors that influence lipid oxidation

• fatty acid composition

• oxygen concentration

• temperature

• surface area

• water activity

• pro-oxidants

• antioxidants

Fatty acid composition effect on lipid oxidation.

• increase in the number of a-methylene groups = increased oxidation rate

• FFAs are more reactive than triglycerides

  • faster when not esterfied

• cis double bonds are more reactive than trans double bonds

Oxygen concentration effect on lipid oxidation

• O2 is abundant: the rate of oxidation is independent of O2 concentration

• O2 is low: rate of oxidation is proportional to O2 levels

Temperature effect on lipid oxidaition

• high T results in an increase in oxidation rate

• as temperature increases O2 solubility decreases so O2 effect becomes negligible

Surface area effect on lipid oxidation

• oxidation rate increases according to surface area of the lipid exposed ot the air

• smaller surface areas have less interaction with O2

• modified atmosphere packaging replaces O2 with N2 gas

water activity effect on lipid oxidation

• Aw <0.1 = high oxidation rate because the foods are exposed to the O2 in air

• Aw 0.3 = oxidation is inhibited since small amounts of water reduces the catalytic activity of metal catalysts, quenches free radicals and minmizes the access of O2 to the lipid

• Aw 0.55-0.85 = high oxidation rate due to greater mobility of reactants such as O2 and catalysts

Pro-oxidants

• enhance or promote oxidation

• transition metal ions (Cu2+, 1+, Fe3+, 2+)

  • heavy metals in plant oils, animal tissues, egg, milk

  • introduced by soil, processing or storage equipment

  • directly interacts with the unsaturated FA

  • activates O2 to give singlet oxygen and a peroxy radical

• Radient energy (UV, Sinlight)

Pro oxidant mechanism

• accellerates the hydroperoxide breakdown by direct interaction with unsaturated FAs

• activates molecular oxygen to give singlet oxygen and a peroxy radical

Anti-oxidants role

• delay onset or reduce the rate of oxidation by

  • Inhibiting or inactivating the formation of free radicals

  • interrupting propagation

Antioxidants inbibiting or inactivating free radical formation

• aim ot prevent oxidation before it starts

• eg. B-carotene, lycopene, other caratenoids

• reacts with singlet oxygen to make an unreactive triplet oxygen

  • the carotenoid gets excited but it converts back to normal and releases heat

Carotenoids

• red, orange and yellow pigments found in chloroplasts and chromopolasts

• basic structure of conjugated double bonds

  • single and double bonds alternate on the backbone

• lipid soluble

• xanthophyll contains oxygen but carotene contains no oxygen

• precursors of vitamin A

  • especially B-carotene (growth, vision, development)

B-carotene

• precursor to vitamin A

• red-orange pigment

• singlet oxygen quencher

• carrots, squash, sweet potato, dark leafy greens, cantaloupe, watermelon

Beta-cryptoxanthin

• xanthophyll carotenoid

• orange-yellow pigment

• present in orange, tangerine, papaya, peaches, mango, guava

• hydroxylated derivative of B-carotene

• precursor of vitamin A

lycopene

• red carotene pigment

• present in tomatoes, pink grapefruit, apricots, papaya

• content increases significantly during tomato ripening and continues after harvest with heat treatment

• trans to cis isomerization induced by heating

Nutritional importance of the trans to cis isimerization induced by heating

• breaks down cell membranes so lycopene can be released

• cis are more soluble and can be absorbed more efficiently in the intestinal lumen

• cooking tomatoes in oil makes them more bioavailable because H it is lipid soluble

Inhibitory propagation

• inhibit the propogation step

• compete with the unsaturated FA and react with the peroxy radical instead

• the antioxidant radical (A*) is stable

• slows or stops the oxidation after it has started

Butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT)

• soluble in oils

• used along side main antioxidants

• good H+ donors and are stabilized by resonance with no positions reacting with molecular oxygens

• antioxidants that act by inhibitory propagation

a-tocopherol (TH2) function

• Reacts with the ROO* to create ROOH + TH*

• TH* is stable and can quench another peroxy radical

  • can also react with itself to regenerate TH2

• effective antioxidants at low concentration but at high concentration it is a pro-oxidant

a-tocopherol

• most biologically available form of vitamin E

• fat soluble and found in most vegetable oils

  • olive oil, coconut oil, almonds, avocados

  • animal fats contain small concentrations that comes from vegetables in their diet

Vitamin E content

• extraction of oil helps to preserve it

• levels are depleted when it is exposed to conditions that promote lipid oxidation

Food processing that leads to a-tocopherol loss

• drum drying seeds causes losses through exposure to air and heat which uses its vitamin E content

• Retorting does not affect a-tocopherol activity because it is an anaerobic process and therefore the a-tocopherol does not use its antioxidant activity when being processed

Seed oil extracting and processing

• uses hexane solvent but most of it is recovered and removed but small amounts remain in the product

• can government has determined that consumer exposure to the solvent is not harmful to human health at the current levels of exposure

Omega-6 balance

• we currently consume too many omega 6 and need to balance through increasing omega 3

• repeating uses of the same oil leads to changes in the oil which can lead to inflammation through creation of pro-oxidants

  • peroxides are created

UPFs

• cookies, cakes, donuts, chips etc

• UPF contain seed oils, sugar, salt, additives and preservitives

• over consumption of UPF is associated with poor health outcomes