Lipid powerpoint

General Information
• Definition
– Substances found in living tissues which are
generally insoluble in water and are soluble in
organic solvents (e.g. ether, chloroform,
hexane)
– Exceptions
• Some lipids (such as short chain fatty acids - < 4
carbons long) are water soluble
• Others are only soluble in a limited range of
organic solvents

General Information
• Use of fats and oils in body
– Source of energy
• For all cells except erythrocytes and cells of central
nervous system (which use carbohydrates for the
most part)
– Carriers of fat-soluble vitamins (A, D, E, K)
– Carrier of food flavors
• Most dietary lipids are triglycerides which are
relatively tasteless on their own

General Information
– Help provide food texture that increases
palatability (i.e. improves mouthfeel)
– Delays gastric emptying (which contributes to
satiety)
– Adipose tissue insulates and cushions organs
– Supplies essential fatty acids

General Information
• Metabolic energy from lipids
– calorie = the quantity of heat required to raise
the temperature of 1.0 g of water by 1 degree
Celsius (°C)
– 1000 calories = 1 kilocalorie (abbreviated as
kcal) = 1 Calorie (abbreviated as Cal)
– In general, lipids are 9 kcal/g (or 9 C/g)

Kcals – Lauric Acid

CH3(CH2)10COOH +17O2 12CO2 + 12 H2O

CH3(CH2)10COOH +17O2 12CO2 + 12 H2O
Bond Bond Energy Number of bonds Total Bond
(KJ/mol) Energy (KJ/mol)
C-C 347 11 3817
C=O 799 1 799
C-O 351 1 351
O-H 460 1 460
C-H 414 23 9522
O=O 499 17 8483
23432

CH3(CH2)10COOH +17O2 12CO2 + 12 H2O
Bond Bond Energy Number of
(KJ/mol) bonds
O-H 460 24
C=O 799 24

CH3(CH2)10COOH +17O2 12CO2 + 12 H2O
Bond Bond Energy Number of Total Bond
(KJ/mol) bonds Energy (KJ/mol)
O-H 460 24 11040
C=O 799 24 19176
30216

30216 KJ/mol - 23432 KJ/mol = 6784 KJ/mol

MWlauric acid: 200.32 g/mol

6784 KJ/mol = 33.92 KJ/g
200.32 g/mol

33.92 KJ/g * 1 kcal = 8.11kcal/g
4.18 KJ

Nomenclature – fatty acids
• Nomenclature of fatty acids requires both a
systematic approach and a knowledge of
trivial names

• Systematic
– Names of fatty acids are derived from the
appropriate parent hydrocarbon
– Remove terminal “e” from parent and add suffix
“oic”
– Example
• Hexane: CH3CH2CH2CH2CH2CH3
• Hexanoic acid: CH3(CH2)4COOH

– 2 main classes of fatty acids
• Saturated: no double bonds
• Unsaturated: double bonds present in carbon chain
– If 1 double bond, the parent alkene becomes
“enoic acid”
• Example
– 3-hexene: CH3CH2CH=CHCH2CH3
– 3-hexenoic acid: CH3CH2CH=CHCH2COOH
– If 2 double bonds, use the suffix “dienoic”
• Similarly for:
– 3 double bonds, use “trienoic”
– 4 double bonds, use “tetraenoic”
– Etc.

• Trivial names
– Names were selected prior to the identification of
the fatty acid’s chemical structure
– Name often identified the source of the fatty acid
– Examples:
• Saturated
– C12: Lauric
– C14: Myristic
– C16: Palmitic
– C18: Strearic
– C20: Arachidic

– Examples:
• Unsaturated
– C16:1: Palmitoleic
– C18:1: Oleic
– C18:2: Linoleic
– C18:3: Linolenic
– C20:4: Arachidonic

• Location of the double bond (several ways to
show this)
– Delta notation (δ or Δ): terminal COOH is #1
• Example
– Oleic acid: CH3(CH2)7CH=CH(CH2)7COOH
» 9, 10-octadecenoic acid
» Δ9-octadecenoic acid
» 18:1Δ-9
– Linoleic acid:
CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
» Δ9, 12-octadecadienoic acid
» 18:2Δ-9,12

– Omega notation (ω or η): terminal CH3 is #1
• Example
– Oleic acid: CH3(CH2)7CH=CH(CH2)7COOH
» ω9-octadecenoic acid
» 18:1ω-9
– Linoleic acid:
CH3(CH2)4CH=CHCH2CH=CH(CH2)7COOH
» ω6, 9-octadecadienoic acid
» 18:2-ω6, 9

• Most naturally occurring fatty acids have
unconjugated double bonds
– Double bonds are separated by one or more
single bonded C atoms
– In most cases, double bonds are methylene
interrupted Methylene group

• CH2-CH=CH-CH2-CH=CH-
• Conjugated double bonds
– Double bonds adjacent to one another
• CH2-CH=CH-CH=CH-

The Biochemical ω-system
• Animals cannot synthesize ω-3 or ω-6 fatty
acids themselves
– Animals lack the enzymes that catalyze
desaturation towards the methyl end
• Enzymes in the body cannot function that close to the
methyl end
• Can elongate and desaturate towards carboxyl end
– Plants and microorganisms can desaturate
towards the methyl end
Since animals cannot make 18:2ω-3 or 18:ω-6 ,
they are termed Essential Fatty Acids (EFA)

• Using the omega notation can give necessary
information in a brief way
– Example: What do we know from 18:2ω6?
» 18 carbons
» 2 double bonds
» 1st double bond is 6 carbons down from the methyl end
» Since we know that most double bonds are methylene
interrupted, the 2nd double bond is 9 carbons down from
the methyl end

CH3(CH2)4CH=CH-CH2-CH=CH(CH2)7COOH

• What is the omega nomenclature of this
fatty acid (please type into the chat
space).

• What is the delta nomenclature of this fatty
acid (please type into the chat space).
Include all double bond locations.

• What is the common (i.e. trivial) name of
this fatty acid (please type into the chat
space).

• Cis versus trans
– Most unsaturated fatty acids in nature will exist in
the cis configuration
• If the fatty acid is in the trans configuration, it will be
stated
– The two configurations will yield different
properties at room temperature

Oleic Acid: 18:1ω9
Elaidic Acid: 18:1ω9t

Liquid at Troom

Solid at Troom

EFA’s and deficiency
• Linoleic (18:2ω-6) deficiency
– Clinical symptoms
• Scaly skin, water loss through skin, extreme thirst,
poor wound healing, failure to gain weight, impaired
reproduction, death
• Linolenic (18:3ω-3) deficiency
– Noticed symptoms
• Blurred vision, neurological symptoms, tingling
extremities
– Linolenic is critical to prostaglandin formation

Omega 3 fatty acids
– 18:3ω-3 found in:
• Soybean oil (~7%)
• Canola oil (~10%) VERY susceptible to
oxidative rancidity =
• Linseed oil (~50-60%) degradation
• Green leafy vegetables
• Desaturated 20:5ω-3 (EPA) in marine oils
– Studied Eskimos in Greenland: found a decrease
in coronary heart disease compared to
populations consuming less marine foods

Omega 3 fatty acids
• Desaturated 20:5ω-3 (EPA) in marine oils
– Studied Eskimos in Greenland: found a decrease
in coronary heart disease compared to
populations consuming less marine foods

Other fatty acids of interest
• Vaccenic acid
– 18:1Δ-11trans (18:1ω-7trans); also 18:1Δ11
– Trans fatty acids that occur in milk and butterfat
due to biohydrogenation in rumen
• Ricinoleic acid
– 12-OH octadeca cis-9-enoic acid
– Oleic acid with an OH group on carbon 12 from
COOH end
– Castor oil
• Laxative

• Erucic acid
– 22:1ω-9 rapeseed and mustard oils
– Accumulates in heart tissue when fed to rats
– Genetic variant is canola oil, or low erucic acid
rapeseed (LEAR)
• Branched chain
– Iso: mainly even C’s
– Anteiso: mainly odd C’s
– Occur in waxes like wool wax

• Milk fat group
– Fats derived from milk of domesticated land
animals
• Characteristic shorter chain fatty acids (C4 – C12) in
milk fat
– Influences milk flavor and how milk is processed into dairy
products
• C4: butyric acid
– A fatty acid that is liquid at Troom although saturated
– Normally masked in foods
– If broken down via lipolysis, it will be very pungent
» Occurs due to extreme heat or agitation

• Lauric acid group
– C12 is present
– Coconut oil
• Vegetable butter group
– Cocoa butter
• Oleic-linoleic acid group
– MAJOR GROUP
– Vegetable oils
– Saturated acids >20%

• Linolenic acid group
– Contains 18:3
– Soybean oil ~8% 18:3
• Animal fat group
– 30-40% saturated
– ~60% unsaturated
– Lard, tallow
• Marine fat group
– Highly unsaturated

Major Lipid Components –
Acylglycerols
• The majority of fatty acids are esterified to
glycerol, making them acylglycerols
• Triacylglycerols are the most common in
foods
– Mono- and diacylglycerols do exist as food
additives

Acylglycerols
• Review of ester formation
– Ester linkage forms between the carbon atom
of carboxylic acid on the fatty acid chain and
the oxygen atom of the alcohol on the glycerol
backbone
O O
R-C-OH + R’OH R-C-OR’ + H2O
O
R : Fatty acid chain C-OH : Carboxylic acid OH : Alcohol

Acylglycerols
– Observe ester linkage
Glycerol Triacylglycerol = Triglyceride

H H
O
H-C-OH
H-C-O-C-R1
H-C-OH + 3 fatty acids O + 3 H2O

H-C-OH H-C-O-C-R2
O
H
H-C-O-C-R3 1 mol of water
is given off for
every mol of “R”
H participating in
the reaction

General Lipid Categories
• Simple lipids (neutral lipids)
– Esters of fatty acids and alcohols; lipids
derived from these by alkaline and acid
hydrolysis (derived lipids)
– Includes fatty acids, glycerides, fatty alcohols
– Examples
• Triacylglycerols (i.e. triglycerides)
– Esters of fatty acids and glycerol
• Waxes
– Esters of fatty acids with alcohols other than glycerol
– Normally: long chain alcohols (e.g. C24)
– Water insoluble

• Compound (complex) lipids
– Lipids containing other groups in addition to
an alcohol-fatty acid ester linkage
– Phospholipids
• Phosphoglycerides or glycerophospholipids
• Glycerol + fatty acids + phosphate + another group
• Example: phosphatidic acid (major component of
cell membranes)

• Example: cardiolipin (a phosphatidyl glycerol)
– An important component of the inner mitochondrial
membrane
– Important to the electron transport chain that produces ATP

• Example: Phosphatidyl-
Choline (component of lecithin) Ethanolamine (membrane lipid)

Serine (for NS cell functioning) Inositol (substrate for cell signaling
enzymes)

– Sphingolipids
• Associated with plant and animal membrane
components
• Play an important role in both signal transmission
and cell recognition
• Sphingosine + derived lipids + water soluble
products
– No glycerol backbone

• Examples
– Sphingomyelin

– Cerebrosides

– Sterols, other lipids and essential oils
• All contain the cyclopentanoperhydrophenanthrene
ring system

http://journals.iucr.org/a/issues/2006/02/00/xo5005/xo5005fig1.html

– Sterols, other lipids and essential oils
• Cholesterol: typical in animals
– Plants do have detectable levels, but VERY low
– Has the ability to undergo oxidation, leading to heart
disease and cancer
» Low density lipoprotein oxidation can lead to plaque

• Phytosterols: plant sterols
– Example: stigmasterol (in soybeans)
– May reduce the risk of CHD by lowering blood cholesterol
levels

• Essential oils
– Not “oils” in the real sense
• Actually terpenes ((C5H8)n hydrocarbons)
– Often mixed with other lipids in waxy coats or
located in special oil sacs in the skin of citrus
fruits
– Important to citrus-based flavor development

Natural fat and oil composition
• Fats of aquatic origin
– Number of carbons usually exceeds 20
• Mainly C14 – C24
– Major saturated acid is palmitic (16:0): 15-20%
by wt.
– Monoenoic acids are 16:1, 18:1, and 20:1
• Usually with double bond position at 9, but some have
double bond at carbon 1
– Many C16, C18, C20, and C22 polyenoic acids
• ω3 family: 18:3, 18:4, 20:4, 20:5, 22:5, and 22:6

– Other lipids present
• Glycerol ethers
• Waxes
– Occur occasionally as an oil of the sperm whale
– Ester of long chain alcohol and a fatty acid

• Milk fats
– Cow’s milk fat
• High in C4 – C10 fatty acids: 20 – 30% on a molar
basis
– Human milk fat
• Much lower in C4 – C10, but higher in C12 – C14

• Vegetable fats (usually oils)
– Present in all parts of plants but usually highest
in fleshy part of fruit or in seeds (seed oils)
– Oils and fats from different parts of plant differ in
composition

• Example: Erucic acid (22:1Δ13) in rapeseed plant
– No erucic acid in leaves
– In rapeseed oil, there’s approximately 40 – 50% erucic acid
– Problem
» Erucic acid accumulates in heart muscle, not in adipose
tissue, upon feeding
» Heart lacks enzyme for oxidizing erucic acid
» Could be a problem if an excess of one fatty acid
accumulates in heart muscle membranes
» The problem was avoided with the development of low
erucic acid rapeseed oil (LEAR)

• Example: castor plant
– Ricinoleic acid (12-OH 18:1)
– Leaves: no ricinoleic acid
– Castor oil: ~90 ricnioleic acid

• Depot fats of land animals
– Adipose tissue fat
• Fats are laid down in adipose tissue as triglycerides
• Two sources of fatty acids
– Endogenous supply from CHO and protein synthesis
– Dietary fat (this may be modified by the animal)
• Easy to distinguish from fish oils as they are solid or
semi-solid at Troom

• Characteristics
– Almost entirely C16 (32%) and C18 (62%) fatty acids
– C16 is almost all 16:0
– C18 is almost all 18:1
– Stearic-rich (18:0) fats come from ruminants such as sheep,
cattle, and deer
» Hydrogenation of fatty acids by rumen bacteria
» Also due to hydrogenation in rumen, small amounts of
trans fatty acids
» The fatty acid composition of adipose tissue can be
changed drastically by changing animal’s diet – pig feed
high in 18:2 (corn) can result in “soft-pork” problem (i.e.
soft adipose tissue = runny fat)

• Summary
– Animals do not generally produce linoleic and
linolenic acids (EFA), thus they must acquire it
from their diet
– Plants produce linoleic and linolenic, but carry
out less conversion to the longer, more
unsaturated fatty acids
– Microorganisms are versatile
• They can produce all kinds of fatty acids including
branched chain, hydroxyl, keto, and cyclic fatty acids

Lipid properties
Table 1. Physicochemical property comparison – Triolein oil vs. water

Property Oil Water
Molecular weight 885 18
Melting point (ºC) 5 0
Density (kg/m3) 910 998
Viscosity (mPa*s) ~50 1.002
Specific heat (J/kg*K) 1980 4182
Refractive index 1.46 1.33

Lipid properties
• Refractive index (RI)
– Used to determine what the fatty acid
composition might be
– A ratio of the velocity of light in air to the velocity
of light in the substance
– A function of the temperature and wavelength of
light employed

Lipid properties
– Extent of refraction depends on intermolecular
attractions
• Refraction

Lipid properties
– Bending of light as it passes from one medium to another
– The density of the medium impacts the speed of light through
the medium
» This causes the light to bend at different angles
– Extent of refraction is also impacted by intermolecular
attractions
– RI increases with increasing chain length and with increasing
unsaturation (i.e. number of double bonds)
– Use in industry
» As a control procedure during hydrogenation (a change
in RI results when the number of double bonds changes)

Lipid properties
• Iodine value
– The number of grams of iodine absorbed by 100
grams of fat
– Measure of the degree of UNsaturation
– Halogens (e.g. Cl, Br, I) react with double bonds
in fatty acids under mild conditions

Lipid properties
– The reaction results in addition to the double
bond

H H H H
I2
-C=C- -C - C-
I I

Lipid properties
Calculation of Iodine Value – Oleic Acid
CH3(CH2)7CH=CH(CH2)7COOH

1 mol of I2 adds across each double bond –
therefore, in the case of oleic acid, 1 mol of I2
will add across the 1 double bond in oleic
acid

Calculation of Iodine Value: Oleic Acid (18:1Δ-9)

For each mol of lipid, 1 mol of I2 will add across each double bond.
Therefore in the case of oleic acid, with one double bond, 1 mol of I2 will
add to 1 mol of oleic acid.
Since we know the molecular weights of both oleic acid (282g/mol) and I2
(254g/mol) we can establish a mass ratio on a per mol basis that we can
use to calculate how many grams of I2 will add across 100g of oleic acid.
In other words, if 1 mol of I2 adds to 1 mol of oleic acid, then 254g of I2 add
to 282 g of oleic acid. Thus, we can determine how many grams of I2
add to 100g of oleic using the following ratio:



254gI2 XgI2
=
282gOleic Acid 100gOleic Acid



254gI2 XgI2
=

(254gI2)(100gOleic Acid) = (XgI2)(282gOleic Acid)
X = 90gI2



254gI2 XgI2
=

X = 90gI2
meaning that 90g of I2 will add to 100g of oleic acid

Calculation of Iodine Value: Linoleic Acid (18:2Δ-9, 12)
Therefore in the case of linoleic acid, with two double bonds, 2 moles of
I2 will add to 1 mol of linoleic acid.

The MW of linoleic acid is 280g/mol and the MW of I2 is 254g/mol. We can
establish the mass ratio to calculate how many grams of I2 will add
across 100g of linoleic acid. Remember that 2 moles of I2 add to 1 mol
of linoleic acid, so (254g/mol * 2 moles) of I2 add to (282 g/mol * 1 mol)
of linoleic acid.


of linoleic acid.

508gI2 XgI2
=
280gLinoleic Acid 100gLinoleic Acid


of linoleic acid.

508gI2 XgI2
=

X = 181gI2


of linoleic acid.

508gI2 XgI2
=

X = 181gI2
meaning that 181g of I2 will add to 100g of linoleic acid

Lipid properties
• Saponification value
– Number of mg of potassium hydroxide required
to saponify with 1 gram of fat or oil
– 3 moles of KOH react with on mol of
triacylglycerol

Lipid properties
– If the triglyceride contains low molecular weight
fatty acids, the number of molecules present in a
1 gram sample of the fat will be greater than if
the fatty acids have long carbon chains and high
molecular weights
• The fat with the lower molecular weight fatty acids will
consequently have a higher saponification value
• Butter, for example, with a high percentage of butyric
acid, has a high saponification value

Lipid properties
Table 1. Examples saponification and iodine numbers
Fat or oil Saponification # Iodine #
Beef tallow 194 – 200 34 – 43
Cocoa butter 192 – 198 32 – 42
Coconut oil 245 – 262 6 – 10
Cottonseed oil 192 – 196 103 – 112
Lard 193 – 200 50 – 80
Milk fat 210 – 233 26 – 35
Peanut oil 186 – 194 89 – 98

Modification of fats and oils
• Fats have the ability to enhance the
palatability of foods
• Because of this there is a great emphasis on
the crystallization and melting behavior of
fats
• Unique fatty acid distribution of some natural
fats makes them undesirable for certain
applications

– Physical characteristics are influenced by:
• Carbon chain length
– Increased chain length = increased melting point
• Degree of unsaturation
– The more unsaturated a fatty acid is, the more liquid it will be
at Troom
• Distribution of fatty acids on glycerol
monoglyceride diglyceride triglyceride

• Modified in order to change the solid fat
content (SFC) of lipids
– The fraction or percentage of a lipid that is solid
at a given temperature
– Enables less expensive lipids to be used
– Can reduce unsaturation, reducing susceptibility
to oxidation
– Can also increase unsaturation, potentially
increasing nutritional quality

• Common processes for modifying lipids
– Blending
– Dietary interventions
– Genetic manipulation
– Fractionation
– Interesterification
– Hydrogenation

• Interesterification
– Process used to improve the consistency of
some natural fats to enhance their usefulness
– Alteration of the original fatty acid distribution on
the glycerol backbone
• Affects melting and crystallization properties
• Rearrangement at random

– Process
• Rearrangement of fatty acids so that they become
distributed RANDOMLY among the TAG molecules
• Mixing of 2 esters resulting in the exchange of “R”
groups

O O
R1C – O – CH3 + R2C – O – C2H5
O O
R1C – O – C2H5 + R2C – O – CH3

• Occurs within TAG’s or between TAG’s
• Heat fat at high temperatures
• Use a catalyst to speed up the reaction
– Most popular: NaOCH3 (sodium methoxide)
– Alteration of physical properties of fats and oils
• Example: cocoa butter (mp 28 – 30ºC)
– Cocoa butter has a characteristic fatty acid composition and
distribution
» “Melts in your mouth, not in your hands”
– Once cocoa butter undergoes radomization by
interesterification, it no longer melts at the same temperature

Table 2. MP changes due to interesterification
Lipid MP (ºF) – Before MP (ºF) – After
Soybean oil 19.4 41.9
Cottonseed oil 50.9 93.2
Coconut oil 78.8 82.8
Palm oil 103.7 116.6
Lard 109.5 109.5
Tallow 115.2 112.3
40% hydrog. cottonseed oil 136.0 106.0
23% hydrog. palm oil 122.3 104.5

• Hydrogenation
– Very important to the oil industry
• Need to modify natural liquid oils to make fats with a
wide range of properties
– Soft and greasy to hard and brittle
• Usually only partial hydrogenation occurs

– Simple reaction
H H
- C = C - + H2 -C-C-
H H H H

• Addition of H2 across double bonds makes
compounds saturated
• Alters:
– Molecular configuration
– Number, geometry and location of double bonds

Most importantly, it can result in the formation of trans fatty
acids!!!

– Reasons for hydrogenation
• Convert liquid fats into plastic fats (suitable for
manufacture of shortenings and margarine)
• Improve resistance of fats and oils to deterioration
through oxidation or flavor reversion
• Convert soft fats into firmer fats
• Improve color

– General mechanism for hydrogenation
• Requires a catalyst
– Technically, it will happen naturally, however, the reaction
will take place VERY slowly
– Usually nickel
– Heterogeneous
» In other words, the catalyst is in a different chemical state
(typically solid when hydrogenating a liquid oil)

• Mechanism
CH2-CH=CH-CH2-
Absorption of fatty acid onto catalyst

Ni Ni
CH2-CH-CH-CH2- Double bond is broken and 2C-Ni bonds form

Reaction with absorbed H goes to partially hydrogenated
states
Ni Ni
CH2-CH2-CH-CH2- + CH2-CH-CH2 -CH2-
These may then go either of two ways

H desorption from catalyst Loses H from a C atom adjacent to a C-Ni bond

CH2-CH2-CH=CH- Double bond will
CH2-CH2-CH2-CH2- be cis or trans
Fully hydrogenated CH=CH-CH2-CH2- These fatty acids
can go back into
CH2-CH=CH-CH2- the cycle

– Rate of reaction depends on:
• Nature of substance being hydrogenated
– The greater the number of double bonds, the faster the
reaction
• Nature and concentration of the catalyst
• Concentration of H2
• Temperature, pressure and degree of agitation
– Increasing the temperature, pressure of H2 and degree of
agitation will all speed up the reaction

– If unlimited H2 at catalyst surface:
• Hydrogenation will be non-selective
– Selectivity: the tendency for more unsaturated fatty acids to
be reduced before those fatty acids that are more saturated
» Example: 18:3 are hydrogenated before 18:2 which are
hydrogenated before 18:1
• Any factor influencing the amount of H2 at the catalyst
surface will influence the rate and selectivity
• Control of selectivity
– Increase selectivity by reducing H2 at the catalyst surface
» Increase T, decrease P, increase amount of catalyst

– Partial hydrogenation (e.g. in soybean or
vegetable oil)
• In practice, partial hydrogenation is carried out in
vessels known as “converters”
• Closed, pressurized vessels with a capacity of
~60,000 pounds
• Agitation, heating, cooling and H2 inlet/vent systems
• Temperature ~ 175ºC
• Typical catalyst is Ni (0.01 – 0.02% of oil)
• H2 at 5 – 50 psi
• After partial hydrogenation, the oil is cooled, drained
and the catalyst is removed by filtration

– Testing partially hydrogenated oils
• Samples are analyzed for
– Iodine value
– Refractive index (AOCS method)
– Melting point
– Infrared spectroscopy (IR)

Polling question - hydrogenation
Which fatty acid would hydrogenate faster:
one with an IV of 103 or one with an IV
of 80?

Deterioration reactions
• Autoxidation
– General description
• Atmospheric oxidation of fats and oils
– General reaction characteristics
• Autocatalytic
• Has an induction point
• Accelerated by metals, light, and temperature
• Surface dependent
• Unsaturation dependent
• Produces a variety of oxidation products

Polling question - Autoxidation
Which of the following fatty acids is the most
susceptible to autoxidation:

A. Arachidic
B. Arachidonic
C. Palmitoleic
D. Myristic

– Mechanism of lipid oxidation
• Free radical chain mechanism
• Initiation
Alkyl radical

RH R· + H·

– Removal of a H atom from a C adjacent to a double bond
– H atom is usually from the methylene group
– Example:

R-CH=CH-CH2-R’ R-CH=CH-C·H-R’

Methylene group

• Propagation
– Alkyl radical (i.e. fatty acid free radical) combines with O2 to
first form peroxy radical

R· + O2 ROO· Peroxy radical: initial product during propagation

– Peroxy radical then combines with fatty acid to form
hydroperoxide and another alkyl radical

ROO· + RH ROOH + R·
Hydroperoxide

• Termination
– Reaction of 2 radicals, resulting in a non-propagating product

R· + R· RR

ROO· + ROO· ROOR + O2

RO· + R· ROR

ROO· + R· ROOR

2RO· + 2ROO· 2ROOR + O2

– Primary product = hydroperoxide (peroxide)
• Measurement: peroxide value
– Problem: hydroperoxide decomposition
» Example: break down product = hexanal
Concentration

Hexanal

Time

» When measuring PV, the value rapidly
increases after lag period, but then decreases
as hydoperoxide decomposes

– Induction period (i.e. lag period)
• No visible signs of oxidation occurring
– Doesn’t mean that oxidation isn’t occurring, though
• During the initiation phase
– Symbolizes reactants coming together

– Antioxidants
• Function to interrupt the free-radical mechanism
– Extends the induction period
– Delays the onset of oxidative rancidity
• Limit on the amount of an antioxidant that can be used
– 0.02% of the weight of the fat
• Must be added at the beginning of a process to be
most effective

– Initiation reaction
• Subject of great interest
– Common investigations: site of attack, energy requirements
• H atom adjacent to double bond is most susceptible
– Easy to remove because of neighboring double bond
• Unsure where 1st radical comes from in foods
– Perhaps singlet oxygen
– Trace metals may initiate the reaction (e.g. Cu, Fe)

– Oxidation of monoenoic acids
• C8 and C11 are most likely sites for hydrogen removal
• They then react with O2 and attack another RH
resulting in hydroperoxide formation
• Hydroperoxide decomposes to aldehydes, alcohols
and ketones
• 4 free radicals/4 hydroperoxides
– Decompose
» Aldehyde production common
» Trace metals, temperature and light accelerate
hydroperoxide decomposition

Oxidation of linoleic acid
9 10 11 12 13
-CH=CH-CH2-CH=CH-
Loss of proton

9 10 11 12 13
-CH=CH-CH-CH=CH-
Double bond shift for
isomerization
9 10 11 12 13 9 10 11 12 13
-CH-CH=CH-CH=CH- -CH=CH-CH=CH-CH-
O2, RH O2, RH
9 10 11 12 13 9 10 11 12 13
-CH-CH=CH-CH=CH- -CH=CH-CH=CH-CH-
Decomposition of
OOH Decomposition of hydroperoxide
OOH
O hydroperoxide O
10 11 12 13 13 14 15 16 17 18
9
C-CH=CH-CH=CH-(CH2)4-CH3 C-CH2-CH2-CH2-CH2-CH3
H 2, 4-decadienal H Hexanal

– Aldehydes produced from various unsaturated fatty acids
• Oleic acid
Hydroperoxide Aldehyde formed
C8 2-undecenal
C9 2-decenal
C10 n-nonanol
C11 n-octanol

• Linoleic acid

C9 2, 4-decadienal
C11 2-octenal
C13 n-hexanal

• Linolenic
C9 2, 4, 7-decatrienal
C11 2, 5-octadienal
C12 2, 4-heptadienal
C13 3-hexenal
C14 2-pentenal
C16 propanol

– Nutritional implications of autoxidation
• Loss of β-carotene (provitamin A)
• Loss of fat-soluble vitamins (A, D, E, K)
• Loss of essential fatty acids
• Possible build up of polymeric material
• Loss of protein quality
– Free radicals will react with protein
– Carbonyl-amine reactions (i.e. Maillard browning reaction)
– Other implications
• Loss of color and flavor = shelf life limitations
• Warmed over flavors in refrigerated foods

Other deteriorative reactions
• Lipoxygenase reactions
– Enzyme catalyzed lipid oxidation
– Not the same mechanism as autoxidation
– Common reaction in soybeans

• Lipolysis
– Hydrolysis reaction
• Water is involved
• Ester linkages can be broken by re-addition of water
produced when ester linkage is created
– Can occur due to enzymes, thermal stresses
(e.g. heat, moisture)
– Known as:
• Lipolysis, lipolytic rancidity, hydrolysis, hydrolytic
rancidity

DO NOT CONFUSE WITH OXIDATIVE RANCIDITY!!!

• Lipolysis in heated fats – deep fat frying
– Usually at temperatures > 180ºC
– Results when oil is reused
– Moisture from food can cause hydrolysis (i.e.
lipolysis)
– Causes color changes (i.e. darkening) , an
increase in viscosity, a decrease in smoke
point, and potentially toxic products

– Glycerol dehydration to acrolein (acrylaldehyde)
• Moisture from food escapes and causes oil to
hydrolyze into glycerol and free fatty acid(s)
Glycerol Acrolein
H H O

H-C-OH C
-H2O
H-C-OH C-H
Heat
H-C-OH C-H

H H

• Responsible for puffs of smoke
– Very pungent – choking irritating odor
• Results in smoke point depression

• Thermal polymerization
– When a fat/oil is heated to a high temperature
(> 250ºC) in the absence of oxygen
– Also occurs during deep fat frying

– Diels-Alder reaction
• A conjugate addition reaction of a conjugated
diene to an alkene (the dienophile) to produce a
cyclohexene
+

Conjugated Dienophile Cyclic
diene adduct
http://www.chem.ucalgary.ca/courses/351/Carey/Ch10/ch10-5.html

1, 3-butadiene Acrolein 1, 2, 3, 6 –tetrahydro benzaldehyde

CH2 H O
•Can cause
HC C CHO color and
+ viscosity
C-H changes
HC
•Also, can be
CH2 C-H carcinogenic

H

Antioxidants
• General definition
– Substances that slow or prevent oxidative
reactions that would result in undesirable
changes
• Examples: the development of off-flavors,
discoloration, and loss of nutritive value
– Antioxidants
– Synergists
– Oxygen displacers (e.g. inert gases)
– Protective coatings

Antioxidants
• Better definition
– Compound which prevents rapid oxidation of
food products by extending or prolonging the
induction period

Antioxidants
• Protection factor
– Ratio:
induction period protected
induction period unprotected

Antioxidants
• Mechanism of antioxidant action
– Type I
• Primary antioxidants
• “Free radical chain stoppers”
– Interacts with free radicals produced during the initiation
phase (e.g. R· or ROO·)
• Normally phenolic (e.g. BHA, BHT)
OH H atom interacts with R· or ROO· to form RH or ROOH

Antioxidants
– Type II
• Inhibitors of free radical production in foods
• Examples: EDTA, citric acid, phosphates, and
phosphoric acid
– Tie up metal catalysts

Antioxidants
– Type III
• Elimination of environmental factors
• Examples:
– Lowering oxygen partial pressure in a package
» Vacuum, inert gas, airtight containers
– Lowering temperatures
» -12 to 20ºC
– Exclusion of light
– Prevention of contamination by catalytic, prooxidative metals

Antioxidants
• Example mechanisms
R· + AH RH + A·

RO· + AH ROH + A·

ROO· + AH ROOH + A·

R· + A· RA

RO· + A· ROA

Antioxidants
• Competition between inhibitory reaction

ROO· + AH ROOH + A·

and the chain propagating reaction

ROO· + RH ROOH + R·

Antioxidants
• Structures of synthetic antioxidants
OH

OCH3 OCH3

anisole hydroxyanisole

OH

CH3 CH3

toluene hydroxytoluene

Antioxidants
• Major antioxidants used in foods

2 and 3-tert-butyl-4-hydroxyanisole butylated hydroxytoluene

propyl gallate

Antioxidants
• Growing interest in natural antioxidants
– Examples
• Tocopherols
– Principal antioxidant in vegetable oils
» Most widely distributed antioxidants in nature
– Example: vitamin E

Antioxidants
• Ascorbic acid (i.e. vitamin C)
– Works synergistically with vitamin E by regenerating it

Antioxidants
• Chelating agents
– Tie up metals
– Examples:

EDTA
Citric acid

Antioxidants
• Plant extracts
– Rosemary
» Fresh
» Not as effective as vitamin E, BHA, etc
– Soybean
– Honey

Antioxidants
• Popular misconceptions of antioxidants
– Improve flavor of poor quality fats and oils
– Improve oil in which oxidative ancidity has
developed
– Prevent microbial decay
– Prevent hydrolytic rancidity

References
Gunstone F. 1999. Fatty Acid and Lipid Chemistry.
Gaithersburg: Aspen Publishers, Inc.

McClements DJ and Decker EA. 2007. Lipids. In:
Fennema's Food Chemistry (4th Edition).
Damodaran S, Parkin KL, Fennema OR eds. Boca
Raton: CRC Press. P 155-212.

Nawar WW. 1996. Lipids. In: Food Chemistry (3rd
edition) Fennema OR, editor. New York: Marcel
Dekker, Inc. p 225-320.

Lipid powerpoint

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