LIPIDS: Fats and oils

LECTURE 4
Learning Outcomes
 Explain the composition of lipid and their importance
 Describe the chemical structures of edible lipids
 Discuss functional properties of lipids.
 Explain the processes involve in production of fats and oils
 Explain the development of rancidity.
 Trace the changes that occur in fats and oils as a consequence
of heating.
 Lipids
 Represents a broad group of substances that are insoluble in water but
soluble in organic solvent
 Due to the physical state at ambient temperature-
 Fats are solid (animal foods) –(fish oil –liquid)
 Oils are liquid (plants foods) – except palm oil (solid)
 Classified as polar (phospholipids) and nonpolar (e.g., triacylglycerol
and cholesterol) - solubility and functional properties
 Sources: Content vary widely
Animal: Meat, poultry, dairy products (-visible -white striations in bacon);
invisible –marbling in meat
Plant sources: Nuts, seeds, avocado, olives &coconut
 Contain more energy than carbohydrates ( 1g= 9kcal : 4kcal)
 Lipids ( Cont’d)
 Important in food quality – food attributes like texture, flavor,
nutrition, and caloric density,
 the manipulation of lipids has been a major emphasis in food product
development in food research:
 Focus: alteration of lipid composition to change the texture, alter the
fatty acid and cholesterol composition, decrease total fat, alter
bioavailability, make lipids more oxidatively stable
 The physical stability of lipids is important in food quality because
many lipids exist as dispersions/emulsions - thermodynamically
unstable.
 To ensure the production of high-quality foods : understanding of
the chemical and physical properties of lipids is critical
 Chemical composition and major Lipid
components

 Composed of carbon, hydrogen and oxygen

 Made up of fatty acids
 that contain an aliphatic chain with a carboxylic acid group
 Its chain may contain 4 carbons to 24 or more (even number)- due to
elongation process ( two carbons added at a time) but major ones have
14-24 carbons.
 Fatty Acids in Foods
 Do not exists in free form, are cytotoxic due to their ability to disrupt cell
membrane
 Be saturated or unsaturated
 Monosaturated (1 Dbond) or polyunsaturated more than 2 double bonds

 The specific fatty acids in a molecule of fat determine the physical

characteristics of the fat
 The rheological (flow and deformation) qualities of fats are of
particular interest when they are used in food preparation
 Identified by common name, systematic name and number of
carbons
oic-acid grp
Enoic- presence of double
bond
Configurations of fatty acids :Cis and Trans

Presentation of double bond ( h) affects rotation of carbon atom- and shape which consequently
affects melting point of triglycerides
Same FA-one with trans has higher melting point than cis configuration
Cis and trans Fatty acids
 Fatty acids carbon length
 Short chain vs medium chain and long chain
Short chain contain 4-9/10 carbon atoms eg butyric (4 C)
caproic (6) caprylic (8)
Medium chain contain 10-12 carbon atoms eg capric (10)
lauric (12),
Long chain contain >12 eg myristic (14), palmitic (16),
stearic , linoleic, linolenic (18), arachidic (20)
 Nutritional Significance
 Classified as essential and non essential ( what is the
difference?)

 Essential can not be synthesized by the body only three, have

to be provided for in the diet
 linoleic, linolenic, arachdonic acid
 Non essential can be synthesized by the body include the rest
 butyric, lauric, palmitic, stearic
Fatty Acid Composition
 Classification of Lipids
 Three major groups that are important in food and nutrition
 Triacylglycerol or triglycerides
 Phospholipids
 Sterols
 Waxes
 Acylglycerol/ Triglycerides
 Major class of lipid that is found in foods ( 90-95%)
 Made up of one molecule of glycerol and three fatty acids (Once fatty
acids are esterified onto glycerol, their surface activity decreases, as
does their cytotoxicity
 Glycerol molecule

 Fatty acids

 Length of chain (number of carbon atoms)

 Degree of saturation (number of double bonds)
 Formation of Triglyceride

•Ester linkage formed after loss of water molecule

•Can be formed with same type of fatty acid, more mixed: two alike and one different
Formation of Triglyceride
 Acylglycerol/ Triglycerides
 Acylglycerols can exist as mono-, di-, and triesters, known as
monoacylglycerols, diacylglycerols, and triacylglycerols,
 Triacylglycerols are the most common of the three in foods,
 The mono- and diesters- as food additives (e.g., emulsiers).
 The central glycerol carbon of a triacylglycerol exhibits chirality
if different fatty acids are present at the terminal carbons of the
glycerol.
 the three carbons on the glycerol portion of the triacylglycerol can
be differentiated by stereospecic numbering (sn)
 Triacylglycerols can be named by several different systems
 Often named using the common names of the fatty acids
 If the triacylglycerol contains only one fatty acid (e.g., stearic acid,
abbreviated as St) it could be named tristearin, tristearate, glycerol
tristearate, tristearoylglycerol, StStSt, or 18:0-18:0-18:0.

 Triacylglycerols that contain different fatty acids are named

 differently depending on whether the stereospecic location of each
fatty acid is known.
 The nomenclature for these heterogeneous triacylglycerols replaces
the -ic at the end of the fatty acid name with -oyl.
 If the stereospecic location is not known, a triacylglycerol
containing palmitic acid, stearic acid, and oleic acid would be named
palmitoyl-oleoyl-stearoyl-glycero
 Heterogeneous triacylglycerols can also be named using fatty acid
abbreviations such as in PStO or 16:0-18:0-18:1
 (sterospecic location unknown) or sn-PStO, or sn-16:0-18:0-18:1
 (sterospecic location known) for 1-palmitoyl-2-oleoyl-3-stearoyl-
sn-glycerol
 Phospholipids
 Also known as polar lipids
 Like triglycerides but differ number of FA, because it has phosphate
group + choline (nitrogen base)
 Attached to a glycerol molecule are two fatty acids (e. g.oleic & palmitic)

 Found in membranes of plant and animal tissue

 Emulsifying agent
 Double nature: acts as bridge between water and oil

 Egg yolk good source of phospholipid- Lecithin

 Food industry- mayonnaise, candy bars, margarine
 Other foods : liver, soya beans, wheat germ and peanuts
 In the body component of cell water –helps in movement of
fat-soluble vitamins
 Sterols
 A large intricate molecule of interconnected rings of carbon

 Important in maintaining human body (cholesterol, bile, hormones

 In foods -cholesterol is widely known and found in animal foods only
 Meat, fish, poultry, egg yolks, milk , kidney etc
 Plant sterol
 Small amounts in fruits, vegetables, nuts, seeds, cereals, legumes
Pigments and Waxes
 Pigments
 Certain pigments are associated with food lipids and occurs as esters of
fatty acids or crystals in liquid oils
 These include carotenoids ( beta-carotene and lycopene, anthocyanins,
betalanins (betanin) & chlorophyll
 Waxes
 Esters of fatty acids and even-numbered carbon long chain alcohols e.g
stearly alcohol
 Occur as low melting point solids that coat plants leaves and fruits eg
beeswax, carnauba wax-from palm tree leaves ( GRAS substance)
 In food industry used as protective coatings-as fat acts as barrier to
moisture escape
 Carnauba is use as coating in chewing gum, sauces, fruits and confections
 Chemical reactions in lipids
 Hydrogenation

 Interesterification

 Hydrolysis

 oxidation

 Polymerization
 Hydrogenation
 Technique of adding hydrogen to unsaturated fatty acids
 Hydrogenation is done as a batch or continuous process at
temperatures ranging from 250°C to 300°C.
 Occurs in a reactor where hydrogen gas is pumped through the liquid
oil in the presence of a catalyst
 Catalyst: Reduced nickel is added at 0.01%–0.02%, later removed
 Characteristics changes: show /poses different crystallization
behavior (compositionally homogeneous) and more oxidatively stable;
raises the fat’s melting point
 Can be controlled to retain high content of linolenic and oleic acids
 Hydrogenation (Cont’d)
 Products of hydrogenation: shortenings and partially hydrogenated
oils that have improved oxidative stability
 Manufacture of margarine, butter from vegetable oils
 Decreases tendency of fats such as frying oil (corn or peanut) to
oxidize due heat or oxygen
 Such oils have high smoke point of 440 ºF > 350-400 ºF
 Hydrogenation leads to bleaching of oils because the destruction of
double bonds in compounds like carotenoids lead to lose color
 Side effect- change percentage of cis-unsaturated to trans-unsaturated
 Low hydrogen concentrations, the process is selective targeting
polyunsaturated FA more rapidly than monounsaturated ones
 Process results in new configuration or re-formation of the double
bond and thus production of geometric and positional isomers- trans
FA

 Trans fatty acids are not metabolised in the body and increase amount
of plasma cholesterol which causes heart attack
 Margarine contains some trans fatty acids which is a risk to heart
problems
Hydrogenation
 Interesterification
 Removal of fatty acids from the glycerol and rearrangement or
recombination, in different forms to produce different triacylglycerol
types
 It can be random or targeted
 Pure triglycerides contain 3 identical FA, O-O-O and P-P-P
 With Random interesterification
 make different configuration with improved characteristics
 O-P-O ; P-O-O; P-P-O;P-O-P etc
 Changes the melting profile of lipids without changing fatty acid
composition (random)
 Alters the crystallization behavior-hence makes it difficult for the lipid
to form the most stable crystal type – due to heterogeneity
 Interesterification
 For example -mixes a fat with a high-temperature melting range and
an oil with a low-temperature melting range.
 Blending –melting –stair-step but in interesterification have a gradual one

 In directed interesterication
 Reaction temperature is low enough - highly saturated triacylglycerols are
produced, crystallize and are removed in the reaction-a liquid phase-
more unsaturated and a solid phase- more saturated than the parent lipid
 Catalyzed by lipases-be specific for different stereospecific locations on
the triacylglycerol or for different fatty acids
 produced with changes in fatty acid composition or triacylglycerol type-
such have superior nutritional or physical properties.
 Hydrylosis and Oxidation
 Hydrolysis
 This requires heat and water and separates FA from the glycerol portion
 The glycerol is further changed to a substance called acrolein, that
produces odorous and irritating fumes in the smoke of overheated fats

 Triglyceride + 3H2O + heat 3 FA+glycerol +heat 3FA+acrolein + 2H2O

 Polymerization
 hydrolysed FA link to form a large FA polymers- causes increase in viscosity of
cooking oils and occurs at smoke point of fats ( gives off smoke)
 Oxidation
 Oxygen will react with the double bond to produce small organic compounds
that generate undesirable odor “rancid odor”
 This affects the nutritional value and quality of the fats
Deterioration of fat and its control
 Common type of spoilage in fats and fatty foods is rancidity
 It may develop on storage particularly if the fats are highly unsaturated
and the environment where all conditions are appropriate for initiating
the reaction (oxygen/presence of enzyme)
 Two types of rancidity
 Hydrolytic ( in dairy science)
 Oxidative
 Hydrolytic Rancidity
 A chemical reaction –where free fatty acids are split from glycerol
molecule –process called lipolysis.
 Catalysed by enzyme lipase or heat leading to uptake of water
 Hydrolysis involves breaking chemical bonds and adding the elements of water
(hydrogen and oxygen)

 McWilliams (2008) the free fatty acids alter aroma and fats
 Bennion (2004) free fatty acids does not produce odors + fats unless
they occur in the presence of short chain fatty acids
 Free fatty acids does not produce undesirable odors and flavours in
fats unless they are in the presence of short chain fatty acids such as
butyric acid (4c) and caproic acid (6c)
 Caproic acid has the unpleasant smell and together with Butyric
acids are predominant in butter
 are responsible for unpleasant flavour of rancid butter
 Long chain free fatty acids such as stearic (18), palmitic (16), oleic
(18) do not usually produce bad flavours unless other changes such as
oxidation also occurs
 Oxidative Rancidity
 May be caused by the action of enzyme called lipoxygenase
 However most often it results from chemical reaction that
is self perpetuating called chain reaction
 Unsaturated fatty acid portions of triglycerides are
susceptible to oxidative changes
 The chemical oxidation of fat is initiated when hydrogen
atom is lost from a carbon which is located next to a double
bond in a fatty acid chain, goes through 3 steps : initiation,
propagation and termination
 This loss leaves the carbon atom as a free radical which is a
highly reactive chemical group
 This free radical reacts with a molecule of oxygen from the
environment to produce a peroxide free radical which is still
reactive
 To propagate the chain reaction the peroxide free radical (O-O-)
pulls a hydrogen atom from an adjacent fatty acid thus leaving a
free radical on the carbon atom of the adjacent fatty acid
 This free radical reacts with oxygen and continues the chain
reaction
 Hydroperoxides themselves do not appear to have unpleasant
rancid flavours but molecules further readily break into pieces
producing volatile substances that give characteristic odours of
rancid fat
 This type of rancidity is responsible for most of the spoilage of
fats and fatty foods
 Oxidative Rancidity
 Its action involves the uptake of oxygen at a double bond in an unsaturated fatty
acid in a fat
 When fats are exposed to oxygen, the double bond can be broken so that oxygen
can then become a part of the molecule
 Begins when a free radical form which often is initiated in a polyunsaturated fatty
acid, such as linoleic acid.
 Its structure is able to form a free radical at any of three carbons (9, 11, or 13).
The free radical then combines with two oxygen atoms to form a peroxide.
Subsequently, one hydrogen atom is removed from another unsaturated fatty acid,
and that hydrogen is added to complete the formation of the hydroperoxide on
the first fatty acid
 Another new free radical is formed when that hydrogen atom is removed from
the second fatty acid
 Then two oxygen atoms are added to this second free radical; now yet another
fatty acid is stripped of one hydrogen atom to finish formation of the second
hydroperoxide, leaving yet another free radical
 This process is autocatalytic- self-perpetuating
Ways of preventing/ reducing rancidity
 Store fats at refrigeration temperature with the exclusion of light,
moisture and air
 Store fats in containers that can absorb active rays which can catalyze
oxidation of fats (green, yellow, transparent wrappers)
 Vacuum packaging of fats in containers to exclude air
 Use of antioxidants
 These act as interceptors in the oxidative process by providing a
hydrogen atom to satisfy the peroxide free radical thereby breaking
the chain reaction
 Antioxidants include
 vitamin C, E, beta carotene
 chemicals such as butylated hydroxyanisole (BHA), butylated
hydroxytoluene (BHT), tertiary butyl hydroquinone, propyl gallate
(effective in vegetable oils used in combination with BHA/BHT)
 Lipid Refining
 Triacylglycerols are extracted from both plant and animal sources
 To release the triacylglycerols from animal by products and oil-laden
underutilized fish species - rendering
 Rendering- is a thermal processing operation that breaks down cellular
structures to release the oil
 Plant triacylglycerols are isolated by pressing (olives) or solvent
extraction (oilseeds), or a combination of the two
 This produces crude oils and fats that contain not only
triacylglycerols but also lipids such as free fatty acids, phospholipids,
lipid-soluble off-flavors and carotenoids, and nonlipid materials such
as proteins and carbohydrates
 Lipid Refining (Cont’d)
 To produce oils and fats with the desired color, flavor, and shelf-
life the other components must be removed
 Hence the need for refining which has 4 steps
 Degumming
 Neutralisaztion
 Bleaching
 Deodorization
 Degumming
 Phospholipids will cause the formation of water-in-oil (W/O) emulsions
in fats and oils- making the oil cloudy, and the water can present a hazard
when the oils are heated to temperatures above 100°C (spattering and
foaming).
 Phospholipids also contain amines that can interact with carbonyls to form
browning products during thermal processing and storage
 Removal of phospholipids is done by the addition of 1%–3% water at
60°C–80°C for 30–60_min and small amounts of acid are often added to
the water to increase the phospholipids’ solubility. Settling, filtering, or
centrifugation is then used to remove the coalesced gums. This process is
called degumming.
 With oils such as soybean, the phospholipids are recovered and sold as
lecithin
 Neutralization
 Presence of free fatty acids in crude oils can cause off-flavors,
decrease smoke point, accelerate lipid oxidation, leading to foaming,
and may interfere with hydrogenation and interesterification
operations
 Neutralization is achieved by reacting the oil with a solution of
caustic soda and then removing the water containing the soaps of the
free fatty acids.
 The amount of caustic soda used is dependent on the free fatty acid
concentrations in the crude oil.
 The soap stock produced can be used as animal feed or to produce
surfactants and detergents
 Bleaching
 Crude oils will contain pigments that produce undesirable colors
(carotenoids, gossypol, etc.) and can promote lipid oxidation
(chlorophyll).
 Removal of pigments is accomplished by mixing the hot oil (80°C–
110°C) with absorbents such as neutral clays, synthetic silicates,
activated carbon, or activated earths under vacuum to minimise lipid
oxidation
 The absorbent is latter removed by filtration
 An added benefit of bleaching is the removal of residual free fatty
acids and phospholipids and the breakdown of lipid hydroperoxides.
 Deodorization
 Undesirable aroma compounds such as aldehydes, ketones, and
alcohols that occur naturally in the oil or are produced from lipid
oxidation reactions that occur during extraction and refining of crude
oils
 These volatile compounds are removed by subjecting the oil to steam
distillation at high temperatures (180°C–270°C) at low pressures
 Deodorization processes can also breakdown lipid hydroperoxides to
increase the oxidative stability of the oil but can also result in the
formation of trans fatty acids.
 why most lipid-containing foods are not free of trans fatty acids
 Deodorization
 Oils can also be physically refined to remove both free fatty acids and
off-flavors, by skipping the neutralization step.
 This process requires higher temperatures, increases yield, but
increases trans fatty acid formation
 After deodorization is complete, citric acid (0.005%–0.01%) is
added to inactivate prooxidant metals
 The deodorizer distillate will contain tocopherols and sterols, which
can be recovered and used as antioxidants and functional food
ingredients (phytosterols)
 Physical properties of lipids

 Solubility
 Melting
 Crystallization
 Plasticity
 Solubility and Melting points
 Solubility
 Insoluble in water and do not mix with readily with water
 Can dissolve in benzene, chloroform, petroleum ether and acetone
 Melting points and smoke points
 Melting point
 Relatively high melting point, which is influenced by type if fatty acids (
saturated vs unsaturated.
 High melting point with high proportion of saturated e.g palmitic/stearic/trans
fatty acids
 Length of chain – butyric acid (4C) –low vs stearic acid (18C)
 Low melting point with high proportion of monounsaturated and
polyunsaturated
 The fatty acid combinations on triacylglycerols impact the liquid–solid
phase transitions of the lipid since each triacylglycerol type has a
different melting point.
 This means that food triacylglycerols do not typically have a sharp
melting point, but, instead, they melt over a wide temperature range.
 Crystallization
 Many food fats are in solid form at refrigerator and room
temperatures
 Although these fats appear to be a solid mass, they actually are a
mixture of crystals of fat in oil. Fats contain solid fats crystals in small
quantities –crystals are formed by the different arrangements of
triglycerides in the oil
 When melted fats cool, the molecules gradually, occasionally, a
molecule links to another by means of van der Waals forces, and
crystals begin to form
 The tuning fork configuration is well suited to alignment of
molecules to form a crystalline matrix.
 Four Forms : alpha (A), beta prime (B′), intermediate, and beta (B).
 alpha (A) and beta prime (B′),

 The α crystals are very fine and extremely unstable

 quickly melt and recrystallize into the next larger crystalline form, the
β′ form.
 Fat crystals are in the β′ form, the fat has an extremely smooth
surface, seen in margarines and hydrogenated vegetable shortening of
high quality.
 The β′ crystals are considerably more stable than are the α crystals; in
fact, they are stable enough to survive the marketing process unless
subjected to high temperatures during storage.
 For baking with solid fats, β′ crystals are the desirable form. Their
presence aids in promoting a fine texture in the finished product
 intermediate and beta (B).
 Intermediate crystals give a somewhat grainy appearance to a fat
and are not recommended for use.
 They may form if a fat is stored at a warm temperature. Under this
condition, β′ crystals melt gradually and then recrystallize into the
larger, coarser intermediate form
 The coarsest crystalline form is the extremely stable β form.
 When a fat melts completely and then recrystallizes without being
disturbed, beta crystals form.
 This process can be seen when a small amount of fat is melted and
cooled slowly without being stirred
 The gradual change from transparent melted fat to the opaque solid
fat. This increase in opacity is the consequence of the crystals forming
and refracting the light differently.
 Plasticity
 Plasticity
 Fats that are solid at room temp posses both solid fat crystals and liguid
oil
 Liquid is held in network of the small crystals-makes it unigue so it can be
molded or pressed without breaking
 Though solid, fats have liquid oils with network of solid fats crystal
that hold it together allowing fats to be molded yet holding its shape
 Enables fats to be spread on bread; used in preparation of pastries,
icings and confectioneries
 Unsaturated fats are more plastic compared to saturated ones;
temperature determines plasticity i.e, hard fats spread well when
warmed
 Type and size of crystals influence its performance
 Functional Properties of fats

 Heat Transfer
 Emulsification
 Flavour
 Mouthfeel/texture
 Appearance
 Heat transfer
 Medium of heat transfer (different amount are used, shallow or deep
frying
 In deep-frying food is cooked through stages: fat transfer heat, moisture
escapes, part of oil is absorbed, interior cooking and crust formation
 Increase in amount of fats in fat cooked foods
Emulsions
 Are a combination of water and oil/fats in food systems. Its
viscosity varies, from more liquid to thick
 Two types
 oil in water e.g milk, cream, egg yolks;, gravies, salad dressing, sauces,
cream soups and puddings
 Water in oil e.g. Butter and margarine, mayonnaise
 This water and oil are mixed and held by an emulsifier
Stability can be
Temporary - separate on
standing
Semi-permanent- stabilizer are
added to hold it. French
dressing
Permanent-are viscous and
stable e.g mayonnaise
Flavour and Texture
 Flavour-
 observed in fried foods versus boiled of same food
 Absorbs other flavour compounds in foods and release this flavour
compounds

 Texture
 Consider texture of different types of fats in pastries, crispy fired
foods, chocolate & ice cream
 Provide tenderness, volume, structure and freshness
 Higher fats the smoother & creamy mouthfeel
Appearance and Satiety
 Appearance
 Natural based pigments in milk give it its creamy-yellowish
colour-butter
 Fried foods are brown

 Satiety
 Fried foods give more sense of filling (fullness) hence delay
onset of hunger
 Fats take long to digest compared to CHO and protein
 Delay process of emptying the stomach
 Winterization
 Some oil become cloudy under refrigeration – because of presence of
triglycerides that crystallize or solidify at refrigeration temperature
 Ensures that the oil intended for making oil for salad dressings is crystal
free when stored under refrigeration temperature
 Winterization process is used

 The temperature of the oil is lowered to a point that higher melting

triglycerides crystallize
 The oil is filtered to remove the crystallized triglycerides
 The remaining oils is called salad oil

 has lower melting point and does not crystallize at refrigeration

temperature
 Churning
 Mechanical beating of liquid milk with paddles
 In the process the protein film that surrounds the fat globules
ruptures and the fats coalesce forming a continuous network rather
than dispersed
 The oil in water emulsion is broken (lipoprotein component of fat
membrane is more water soluble than fat soluble)
 The liquid that is removed is called butter milk and contains water,
proteins, lactose, calcium, vitamin B
 The fatty material is further squeezed to remove some water and
remain with 15% water, 81.1 % fats, 0.85% protein, vitamin A, D,
sodium, calcium
 The product is butter which is water in oil emulsion (more fat
soluble)
 Butter
 80% milk fat, no more than 16% water & 4% milk solids.
 Made from cream that forms on milk when you stand it for long time
 Changes from oil in water emulsion to water-in-oil emulsion

 Granules are washed, churned slowly to produce smooth homogenous

paste
 Emulsifier in butter is proteins
 Before divided into blocks the butter is sometimes mixed with salt to
increase its keeping properties