Lipids Definition of Lipids Naturally occurring biological
substances made primarily of C, H, and O of pronounced
hydrophobicity that are soluble in organic solvents but have
limited solubility in water Petroleum distillates (e.g. hexane)
Chloroform Ethers (e.g. diethyl ether) Alcohols Term Fat vs. Oil,
chemically identical (both lipid) but Fat = solid at room temp. Oil
= liquid at RT Special case the term fat can be used to refer to
oils from food sources to avoid confusion with other oils such as
petroleum oils Lipids have more C and H than carbohydrates, which
is why they are said to be energy dense, when utilized 2.25 times
more = 9 kcal/g vs. 4 kcal/g.
Slide 4
Lipids Biological roles A. Structural - found in membranes -
protective barriers B. Regulatory - steroids/prostaglandins
(hormones) - phospholipids C. Storage - triglyceride is an energy
storage molecule D. Vitamins - fat-soluble - precursor
molecules
Slide 5
Lipids Role in foods A. Calories (kcal) American Heart
Association recommends energy from fat < 25- 35 % of all
calories ideal - satiety B. Essential fatty acids (cant synth.) -
linoleic acid, linolenic acid C. Flavor - lipid soluble compounds
or off- flavors D. Texture mouth feel & appearance E. Color -
carotenoids F. Heat transfer medium (frying) v.s.
Slide 6
Fat Content of Some Selected Foods Food% FatFood% Fat Brazil
nuts67Hamburger20 Walnuts61Avocado16 Almonds54Ice Cream13
Peanuts50Tuna, canned8 Sunflower seeds 47Poultry, dark meat 7 Pork
roast30Salmon6 Cheese30Whole milk4 Beef roast25Poultry, light meat
4 Ham, cured22Shredded wheat cereal 2 6
Slide 7
Lipids Classification of lipids (structure) 1) Simple
lipidsSimple lipids Mono, Di, and Triacylglycerols Account for 98 %
lipids in foods Waxes 2) Compound lipids - some polarityCompound
lipids Phospholipids Glycolipids 3) Derived lipids often hydrolyzed
1&2Derived lipids Free fatty acids Sterol esters Tocopherol
(Vit-E) -carotene Triglyceride Glycerol backbone Fatty acids
Lipids Structure & properties of fatty acids Fatty acid are
composed of a hydrocarbon chain with methyl group (CH 3 ) on one
end and a carboxyl group (COOH) on the other. Basic properties
common to most fatty acids 1. Most are even carbon # 2. Most are
monocarboxylic acids 3. Most are part of triacylglycerides
(triglycerides)
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Lipids NOMENCLATURE 3 ways to name 1) Short - # of Carbons, #
double bonds 2) Common reflective of source, little if any
structural info 3) Systematic follows IUPAC rules 1. Number of
carbonsNumber of carbons C4-C24 most common E.g. C8 = octa C12 =
dodec, C18 = ? 2. Saturation = saturated with H bonds (no double
bonds) Unsaturated (double bonds) Mono (1 = bond) Poly (>1 =
bond)
Slide 11
Lipids 2. Saturation Systematic Naming No double bond = Anoic
E.g. C18:0 One double bond = Enoic E.g. C18:1 Two double bonds =
Dienoic E.g. C18:2 Three double bonds = Trienoic E.g. C18:3 3.
Geometric configuration of double bonds Cis vs. Trans Has an
influence on the fatty acid backbone structure
Slide 12
Lipids 4. Position of double bonds Delta ( ) system - count #
of carbons to the = bond from the COOH end E.g. 9-octadecenoic acid
Means: a) C18 = octadecenoic b) 1 double bond = octadecenoic c)
double bond is 9 carbons from the COOH end Omega ( ) system - count
# of carbons to the = bond from the CH 3 end used for abbreviations
of fatty acids E.g. 9-octadecenoic acid would be C18:1 9 What about
all-cis-9,12,15-octadecatrienoic? Delta = same as systematic =
9,12,15-ocatadecatrienoic acid Omega = C18:3 3 (1 st double bond is
at C16, 3 carbons from the methyl end) -3, -6 and -9 the most
common Methyl (CH 3 ) end dictates biological activity (more
commonly used in nutrition and food science) -3 essential b/c our
bodies are not able to synthesize fatty acids that have double
bonds between an existing double bond and the methyl end
Slide 13
Lipids Commonly Encountered Major fatty acids in foods
Saturated Palmitic (16:0) Stearic (18:0) Monoenoic Oleic (18:1 9)
Dienoic Linoleic (18:2 6) 9, 12 - common in plants; some in animal
Trienoic Linolenic (18:3 3) 9, 12, 15 Tetraenoic Arachidonic (20:4
6) - 5, 8, 11, 14 - part of membrane phospholipids
Slide 14
Lipids Structure/function properties of fatty acids 1.Length of
fatty acids Longer chain length leads to increase in melting point
and gives more stable fat crystals Classes: C4 C8 - liquid @ room
temperature (20-25C) These are water soluble good emulsifiers C10 -
C14 - viscous @ room temperature C16 - C26 - solid @ room
temperature For example: C6:0 MP = -2C C10:0 MP = 31.5C C16:0 MP =
63C
Slide 15
Lipids Factors affecting the properties of fatty acids 2.
Double bondsDouble bonds An increase in # of double bonds decreases
the melting point Example: 18:0 = 71.2C 18:1 = 16.3C 18:2 = -5C
18:3 = -11C
Slide 16
Lipids Factors affecting the properties of fatty acids 3. Cis
vs. TransCis vs. Trans Cis has lower melting point than Trans Cis
produces a kink in the fatty acid chain which creates a more open
fatty acid crystal structure Kink Melting Point 18:1c15C 18:1t44 C
18:2c-5 C 18:2t29 C 18:3c-11 C 18:3t71 C
Slide 17
Lipids >98% of fatty acids in food products are found as
triacylglycerols (also the largest group of neutral lipids)
Structure: Fatty acid esters of glycerol (three carbon alcohol)
Most triglycerides are mixed (i.e. contain different fatty acids)
Triacylglycerol Glycerol backbone Fatty acids The structure and
properties of triacylglycerols
Slide 18
Lipids We use stereochemical numbering system (sn) to indicate
the position of the fatty acids on the glycerol backbone If you
have 20 fatty acids to chose from then you have 20 3 (i.e. 8000)
possible numbers of different triacylglycerols Complete
randomization at positions seldom observed
Slide 19
Lipids Arrangement of fatty acids on triacylglycerides 1. Not
random (usually) 2. Specificity controlled 3. General pattern The
arrangement can significantly affect physical properties of fat LC
: Long chain; SC: Short chain PositionPlantMammalMilkBirdFish
1SSSSS-LC 2UUSUU 3ULCU or SCS or ULC
Slide 20
Lipids Solid fat index (SFI) Reflects percentage of oil that is
solid Therefore, the rest is liquid Curve shows that tallow has a
broader melting point while cocoa butter has a uniform sharp
melting point (desirable)
Slide 21
Lipids One can produce triacylglycerols with specific
properties Examples: 1. Medium chain triglycerides (MCTs)Medium
chain triglycerides (MCTs) C8:0 and C10:0 fatty acids (from palms,
coconuts, milk) Hydrolyze, fractionate, re-esterify with glycerol
Metabolized in the liver (not through the gut) and thus used more
for energy than for deposition as fat and thus have fewer kcal (8.3
kcal/g) Used as flavor, color and vitamin carriers in foods and
pharmaceuticals; also in reduced fat applications MCTs are
bland
Slide 22
Lipids 2. Salatrim (short and long chain triacylglyceride
molecule), Benefat (Danisco) Triacylglycerol made to contain a
short chain (C2:0, C4:0 or C6:0) and a long chain (C18:0) fatty
acid esterified to glycerol backbone Get only 5 kcal/g because: Get
less energy from short chain fatty acids Stearic acid (C18:0) is
incompletely absorbed Can be custom made to suit a variety of
applications but it is not suitable for frying 3. Caprenin (Proctor
and Gamble Co.) Has C8:0, C10:0 and C22:0 Only 5 kcal/g due to
partial absorbance of behenic acid (C22:0) Confectionary
applications
Slide 23
Lipids Important Compound Lipids 1. PhospholipidsPhospholipids
Make up cellular membranes Lipid molecules that contain a phosphate
group attached to a functional group Have both hydrophobic (fatty
acids) and hydrophilic (phosphate and functional group) portions
Good emulsifiers May have a protective effect against ulcers (milk
PL)
Slide 24
Lipids 2. GlycolipidsGlycolipids Contain at a minimum one sugar
Some may also have a phosphate amino group (glycosphingolipids)
Found in all tissues of animals Have same solubility
characteristics as regular lipids 3. Sterols Made of four fused
hydrophobic rings with a hydrophilic OH group Not so important as a
food ingredient but important for dietary reasons Cholesterol
mostly in animal foods Can contribute to coronary heart disease
(arteriosclerosis) 300 mg/day the recommended intake limit
Slide 25
Lipids 4. Fat substitutes Sucrose fatty acid polyesters
Olestra, brand name Olean (Procter and Gamble Co.) FDA approved for
use in frying oils (snacks) in 1996 6-8 fatty acids (>C12)
esterified to sucrose Caloric free due to its bulky structure and
because lipases cannot hydrolyze it May lead to loss of fat soluble
vitamins and can give diarrhea FDA now requires warning labels
Sucrose and polyol fatty acid esters 1-3 fatty acids esterified to
sucrose or a polyol (e.g. sorbitol) Have caloric value (polyol
fatty acid esters only about 1.5 kcal/g) Used as emulsifiers and
stabilizers
Slide 26
Lipids Functional Properties
Slide 27
Lipids Functional Properties Crystallization Solid fats are
most all in a crystal form Cooling liquid fat (oil) results in it
loosing heat and molecular motion is decreased Fat molecules come
in close contact and the non-polar nature of the fatty acids align
via strong hydrophobic interactions (attractive forces) and a
crystal is formed Simple triacylglycerides (identical FA on the
glycerol) strong bond and tightly packed crystals Mixed
triacylglycerides (the FA are different) Weaker bonds and packing
is less = weaker and more numerous crystals Most fats fall into
this category The actual nature and thus the functionality of the
crystals formed is highly influenced by the type of fatty acids
that are on the glycerol backbone
Slide 28
Lipids Functional Properties -crystals Formed on rapid cooling
Randomly associated triglycerides Size < 1m Very delicate,
needle like, smooth, shiny and fine-grained texture Unstable due to
their disorder Heating transforms this form to the two other higher
stability forms ( or )
Slide 29
Lipids Functional Properties -crystal Close packing Crystal
axes alternate Intermediate of and -crystals Many food fats are
processed in this manner to produce a fine-grained texture (more
grainy than but less than ) The desired form for many processed
fats Shortenings & Margarine Ideal texture Good at
incorporating air
Slide 30
Lipids Functional Properties -crystals Have more order and
greater packing than the -crystals Most stable Very large coarse
crystals with grainy texture Size = 25-45 m This crystal is the
ideal form for some fats Chocolate fat (cocoa butter ) Mp = 35-36 C
Brittle/firm until eaten Develops a glossy sheen fat bloom Improper
storage leads to chocolate bloom (change in crystal
structure)chocolate bloom
Slide 31
Lipids Functional Properties Most of the crystal forms are
interconvertible Polymorphism Determined by: Fatty acid type
(length, unsaturation and cis vs. trans) Distribution of fatty acid
on glycerol backbone Rate of cooling Agitation Storage conditions
LIQ. '
Slide 32
Lipids Functional Properties Crystal structure and melting
point relationship With longer fatty acids there is more packing
and stronger crystals result MP Fatty acid heterogeneity
(nonuniformity) = more packing and MP Fatty acid heterogeneity =
less packing and MP double bonds (cis) kink in the fatty acid =
less packing (bulkier crystal) and MP (trans form = MP) For the
same fatty acid the melting point follows this order: >
>
Slide 33
Lipid crystal forms summary 33 random weakest association
between chains = some order most association between chains = most
order some association between chains =
Slide 34
Lipids Functional Properties Hydrogenation Very important
chemical modification method The objective is to chemically reduce
unsaturated fatty acids into saturated fatty acids (i.e. remove
double bonds) The result Functional properties of the oil/fat is
modified to ones desired Liquid oil solid (vegetable oil margarine)
Higher melting point (due to less double bonds and trans fatty
acids) By proper control of the reaction one can get a range of
textures (SFI profiles) Main reasons to perform hydrogenation 1.
Impart specific physical or chemical property 2. A cheaper oil/fat
source can be converted to mimic a more expensive oil/fatcheaper
oil/fat source (e.g. using cottonseed oil and hydrogenate to a
product similar to cocoa butter) 3. Give product unique functional
characteristics 4. Increased stability of oil/fat = shelf life
Fewer double bonds and more trans fatty acids lead to less
oxidation problems Reduced nutritional value More saturation and
more trans fatty acids Most hydrogenated oils/fats are partially
hydrogenated (trans, cis, removed double bonds) Canola and Soybean
oil blend
Slide 35
The reaction factors: Oil source type of FA and their position
has a dramatic effect on the final oil/fat properties High
temperature H 2 gas bubbled into oil under pressure Agitation
Catalyst usually Ni on an inert silica support removed by
centrifugation or filtration Possible reactions (example with
linolenic acid) 18:3 18:2 18:1 18:0 Lipids Functional Properties
These will have a great impact on which of the double bonds in a
fatty acid are hydrogenated and which will migrate in the fatty
acid
Slide 36
The hydrogenation process: A.Oleic acid (18:1) (letters
correspond to figure) B.Nickel catalyst adsorbs onto the double
bond C.Hydrogen atom binds to a carbon leaving one Ni bond At this
point the reaction can go in several possible directions: D.1.
Another hydrogen atom can bind to the second carbon and give a
saturated fatty acid (1 in figure) 2. A H-atom may be lost from the
carbon giving 2 unsaturated trans (or cis) isomers (2 & 4 in
figure) 3. The original H-atom may be lost and a new H-atom can
come in to form a trans double bond (3 in figure) 4. The fatty acid
may detach from the catalyst and thus lead to no change in its
structure (A in figure) 12341234
Slide 37
Lipids Functional Properties Hydrolytic rancidity Happens
faster with extracted fats/oils General mechanism involves the
cleavage of the fatty acids from the glycerol backbone (is a
hydrolysis = cleavage & addition of water) Usually only the
fatty acids at position 1 and 3 (outside fatty acids) Indication of
quality loss in foods and fats/oils Free fatty acids are volatile
(especially short chain) and can exert an unfavorable odor and
flavor Free fatty acids are more prone to lipid oxidation reactions
(i.e. to become rancid) Free fatty acids may react with other food
components, e.g. can make proteins lose their functionality
Slide 38
Lipids Functional Properties A. Chemical hydrolysis Catalyzed
primarily by heat (225-280 C) Deep fat frying Viscosity increases
Foaming increases Fat degradation products polymerize Color darkens
Off odors/flavors form Smoke point decreases % FFA SMOKE PT ACID
VALUE 0.01450F 0.02 1 320F 1.9 10260F 19 100200F 190 EXAMPLE
Slide 39
Lipids Functional Properties B. Enzymatic hydrolysis Caused by
enzymes that are naturally present in the foods native Lipases
Lipases can also be introduced via contaminating microorganisms
(can be intentional ex. cheese making) Thermal processing can be
used to inactivate Lipases (generally above 60 C but time &
temp factor in) Grains, flour a w FFA loaf volume Fish frozen
storage FFA (due to phospholipases) toughening water holding ($$)
flavor and color problems (rancidity) Dairy Lipases specific for
sn-3 Agitation, pumping, etc favors the reaction Inactivated by
high heat Problems 1. Off-flavors 2. Poor churning (mono and
diglycerides - emulsifiers) 3. Poor cheese (FFA inhibit the enzyme
Rennin)
Slide 40
Lipids Functional Properties Oxidative rancidity Oxidative
rancidity is the result of chemical reactions usually involving O 2
and lipid Referred to as autoxidation since it is a autocatalytic
process which reaction rate increases as reaction proceeds Leads to
major quality problems in foods 1. Off-flavors 2. Color change
(browning, loss of pigments) 3. Degradation of nutrients Essential
fatty acids Essential amino acids Vitamins 4. Toxicity?
Slide 41
Lipids Functional Properties Rate of oxidation reaction
affected by Fatty acid composition (saturated vs. unsaturated)
Degree of unsaturation Rxn rate increases with degree of
unsaturation (18:0, 18:1, 18:2, 18:3) except for conjugated series
Presence of pro- and antioxidants Pro-oxidants catalyze oxidation
Metals, light Antioxidants delay oxidation Synthetic BHA, BHT,
polyphenols Partial pressure O 2 Low pressure minimizes oxidation,
less O 2 ! (Vacuum packaging, flushing) Storage conditions
Temperature Light Water activity pH
Slide 42
Lipids Functional Properties The three steps of autoxidation A
Hydrogen atom is abstracted from the fatty acid (R) by an initiator
and a fatty acid free radical (missing an electron) is formed O2O2
A peroxyl free radical (ROO) is formed in the presence of O 2
Hydroperoxide (ROOH) is formed in the presence of another FA. Rxn
repeats rapidly! The propagation step is terminated by the reaction
between two radicals
Slide 43
Lipids Functional Properties Initiation Induced by: Light
(Visible, UV-rays, -radiation) Chlorophyll (sensitizer) 1 O 2 =
singlet oxygen Heme compounds (hemoglobin and myoglobin in muscle
foods) Metal compounds Only low concentration needed (0.1 ppm) From
soil (plants), animal (needed nutrient), metallic processing &
storage equipment M + n+ RHM [n-1] + R H+H+ ++ Cu 2+ Fe 3+ Cu + Fe
2+ Direct rxn of a metal with substrate (RH)
Slide 44
Molecular mechanism of initiation 4 isomers are formed C H 2 C
H 2 C H C H C H 2 C H 2 Attack is adjacent to the double bond CHC H
2 C H C H CHC H 2.. Radical can potentially form at either
site
ROOH (primary products) can be very unstable and decompose to
form secondary oxidation products: Acids Alcohols Aldehydes
(acetaldehyde) Carbonyls Ketones These are responsible for the
rancid odor/flavor of oxidized fat Propagation of linoleic acid
(18:2) hydroperoxide formation (OOH)
Slide 48
You can follow the progress of lipid oxidation chemically and
sensorially Time Sensory detection Hydroperoxides Aldehydes
Slide 49
Lipids Functional Properties For the exam your should be able
to predict how many fatty acid radicals can form for oleic,
linoleic and linolenic acid and know where they would be located
Where would the first attack be located? Where else could attack
occur? How many radicals can form per lipid? Which radical would be
the most important? Stability?
Slide 50
Lipids Functional Properties Prevention/retardation of
autoxidation Remove oxygen Vacuum or modified atmosphere packing
Reduce light E.g. use opaque packaging, filters, cans Remove
catalysts (e.g. metals) Chelators (EDTA; citric acid; phosphoric
acid) Avoid high temperatures Use less unsaturated fatty acids or
use saturated fatty acids No double bonds far less to none
oxidation Hydrogenation fewer double bonds Use antioxidants EDTA Cu
2+ EDTA in soda http://en.wikipedia.org/ wiki/Vault_%28soft_drin
k%29 http://www.squirtsoda.c om/
Slide 51
Lipids Functional Properties Antioxidants Can be very effective
in slowing down lipid oxidation They function by
inhibiting/delaying the propagation chain reaction by scavenging
the free radical intermediates Some foods have natural antioxidants
but to stabilize them even further we add both synthetic and
natural antioxidants to them Propagation rxn Antioxidants Versus
Propagation rxn
Slide 52
Lipids Functional Properties Common synthetic antioxidants
Vitamin E (tocopherol) - A natural antioxidant - Common
antioxidants Vitamin C (ascorbic acid) - A natural antioxidant
-
Slide 53
Lipids Functional Properties "Quench free radicals" - therefore
prolong induction period - will eventually oxidize Induction
period
Slide 54
Lipids Functional Properties Emulsions Consist of 2 immiscible
phases Dispersed phase (also called discontinuous phase) Continuous
phase Oil and water These phases do not like each other and strive
to separate Oil in water (o/w) - milk, salad dressing Water in oil
(w/o) - butter, margarine
Slide 55
Lipids Functional Properties Macro-emulsion Particle size 0.5 -
100 m Milky due to light scattering Micro emulsion Particle size
0.01 - 0.5 m Clear Can have high viscosity - higher than equivalent
volume of o/w or w/o
Slide 56
Lipids Functional Properties Emulsion formation To form an
emulsion the dispersed phase needs to be divided into small
particles and then needs to be stabilized You need input of energy
to form the emulsion Without stabilizers (emulsifiers) the emulsion
will rapidly break down since the small droplets will coalesce and
form larger droplets High shear (more work) Large droplets (salad
dressing) Small droplets (milk) http://www.youtube.co
m/watch?v=oBbWJX EZoRQ - put sound off
Slide 57
Lipids Functional Properties Emulsion stabilization To provide
the emulsion with long term stability one needs to employ
emulsifiers along with high energy input Emulsifiers decrease the
tension (surface tension) between the two phases They can do this
by having a hydrophobic and hydrophilic character at the same
time
Slide 58
Lipids Functional Properties Common emulsifiers Mono and
diacylglycerides Detergents (Tween) Phospholipids (lecithin)
Proteins All have polar (e.g. OH) groups and non polar (e.g. fatty
acid or hydrophobic amino acids) groups OH Oil droplet
Monoglyceride H 2 O phase
Slide 59
Lipids Functional Properties The effectiveness and function of
an emulsifier is defined by its HYDROPHILIC - LIPOPHILIC balance
(HLB) HLB = % weight of hydrophilic portion of emulsifier 5 Scale
goes from 1-20 1 = entirely non-polar (hydrophobic) 20 = entirely
polar (hydrophilic)