Fatty Acids

1
Fatty Acids

• fatty acids essential components of lipids are
  aliphatic carboxyl acids
• Two main groups
• Saturated
• Unsaturated
• Natural fatty acids contains, with extreme
  exception, even number of carbon atoms
• 4 28 in fats
• Higher number of carbon atoms are found in waxes
• Chain is usually straight, unbranched,
  unsubstituted

2
Saturated fatty acids

• (list of some saturated fatty acids in Tabel 3,
  p170)
• The common names of these acids indicate the
  specific source in which they are especially
  abundant
• Or from which they have been isolated
• Physical properties vary with the number of
  carbon atoms
• Acids with fewer than 12 carbon atoms are
  conventionally called the volatile fatty acids
  since they can steam distilled with relative ease

3
Saturated fatty acids

• Members with carbon atoms higher than 10 are
  solids at room temp
• Solubility in water decreases with the chain
  length
• Acids with more than 10 carbons are practically
  water-insoluble

4
Unsaturated fatty acids

• Majority of oils from plants sources
• Generally straight-chain fatty acids
• With an even number of carbons, C10 to C24
• Possibilities of isomers existing among them are
  largely due to
• The number of unsaturated double bonds
• Their position in the chain
• The possibility of cis or trans configurations
• Examples p171

5
Unsaturated fatty acids

• Oleic and linoleic acid, account for 34 and 29
  of all the edible oils produced by man annually
• Acids with two or more double bonds are known as
  poly-unsaturated acids
• Poly-unsaturated fatty acids perform certain
  important physiological functions, but they
  cannot be synthesized in the body fast enough and
  must be supplied in the food
• Referred to as essential fatty acids
• Linoleic acid is the most abundant member of this
  group

6
Unsaturated fatty acids

• Unsaturated fatty acids have considerably lower
  melting points than corresponding saturated fatty
  acids
• Thus oleic acid with 18 carbon atoms is a liquid
  at room temp
• The industrial hardening of fats by hydrogenation
  is based on the saturation of the double bonds in
  unsaturated fatty acid residues

7
Isomerism in fatty acids

• Three possible types of isomerism
• 1) Single isomerism of a straight chain versus
  branched chain
• 2) Isomerism caused by the position of the double
  bond in the chain of an unsaturated fatty acid
• In the case of more than one unsaturated double
  bond, this type of isomerization can give two
  distinct kinds of systems, conjugated and
  non-conjugated

8
Isomerism in fatty acids

• 3) cis-trans (geometrical) isomerism
• In nature most unsaturated fatty acids occur
  mainly in the cis-form

9
cis-trans
10
Fats Oils - Composition

• Fats and oils are glycerides
• Glycerol esters of fatty acids
• All the three hydroxyl groups of the glycerol
  molecule participate in ester bonds
• Hence the chemical name triglycerides
• Fats solidify at room temp
• Oils remain liquid
• The more saturated a fatty acid the higher the
  melting point of the fat

11
Glycerol
12
Structure of Fatty acid
13
Fats Oils Physical properties

• Pure triglycerides are colourless, tasteless,
  odorless and water insoluble substances
• Any colour, odor and taste in fats and oils are
  due to non-triglyceride components
• The solid-liquid transition of fats is of
  importance
• Triglycerides containing a large proportion of
  unsaturated fatty acids have lower melting
  points
• Therefore most vegetable oils are liquid at room
  temp

14
Fats Oils Physical properties

• Glycerides can exist in a number of different
  crystalline forms polymorphism
• Hence the phenomena of multiple phase transition
  (melting) points
• The crystalline characteristics and melting
  behavior of fats do not depend only on fatty acid
  composition, but also on the distribution of
  fatty acids among the triglyceride molecules

15
Fats Oils - Hydrolysis

• Glycerides are easily cleaved into fatty acids
  and glycerol by heating alkali
• The resulting alkaline salts of fatty acids are
  the well known soaps hence the name
  saponification given the hydrolytic cleavage
  of fats
• The de-esterification of triglycerides is also
  catalyzed by the enzyme lipase
• The enzyme is widespread in all lipid-containing
  tissues

16
Fats Oils - Hydrolysis

• Lipases specificity may involve
• Selectivity towards different fatty acids
• Preference towards the position of the ester
  bonds on the glycerol backbone
• The enzyme responsible for the digestion of fats,
  pancreatic lipase, has some preference for the
  1,3 position and for shorter chain fatty acids
• As ester bonds are gradually broken, intermediate
  products, di- and monoglycerides are formed

17
Hydrolytic rancidity

• Lipases react in heterogenous systems such as
  emulsions of glycerides in aqueous media
• Action occurs at the interface between phases
• The most significant consequence of lipase
  activity in foods is the development of a harsh,
  acrid taste as a result of free fatty acid
  liberation
• The short chain volatile fatty acids (butyric
  acid) also contribute their characteristic odor
  to foods

18
Hydrolytic rancidity

• Quite common in olives, milk, cream, butter, nuts
• As lipase activity occurs on the interphase,
  hydrolytic rancidity is more rapid in more finely
  dispersed emulsions, homogenized milk/cream
• A high free fatty acid level in edible oils is
  objectionable and these must be removed in the
  process of refining

19
Fats Oils - Oxidation

• Tendency of fats and oils to become rancid is
  well known
• Serious rancidity results from oxidative
  reactions
• The susceptibility of fats and fatty acids to
  oxidation is associated with the presence of
  unsaturated bonds
• Spontaneous nonenzymic oxidation of lipids
  exposed to air is autoxidation
• Most frequent type of oxidative deterioration of
  lipids in manufactured foods
• Lipid oxidation is catalyzed by the enzyme
  lipoxidase

20
Phospholipids

• Lipids containing phosphoric acid
• Difficult to classify in view of its wide
  heterogeneity
• Phosphoglycerides is the most important
• (Structure p180)
• Two hydroxyls of the glycerol residue are
  esterfied with fatty acids
• The third hydroxyl is bound to phosphoric acid
  which in turn is ester-linked with X-OH, usually
  an amine alcohol

21
Phospholipid
22
Phospholipids

• The phosphoric acid end of the molecule is
  strongly polar hydrophylic
• The fatty acid tails are non-polar
• This dual structure (amphiphathic) makes the
  phosphoglycerides valuable surface active agents
  and emulsion stabilizers
• Phosphoglycerides are important cell wall
  constituents

23
Phosphoglycerides in cell wall
24
Phospholipids - Lecithin

• (structure p180)
• Lecithin contain different saturated or
  unsaturated fatty acids groups
• Amphoteric at pH 7 it forms a zwitterion
• A dipolar ion in which the negative charge on the
  phosphoric acid residue is neutralized by a
  positive charge on the quaternary nitrogen of
  choline
• The polar nature of the phosphoric acid-choline
  residue activates the entire molecule
• Lecithins, as well as other phospholipids, are
  easily oxidized or hydrolyzed and combine with a
  number of other substances, such as proteins and
  carbohydrates

25
Role of lipids in Foods

• Nutritionally main function to supply energy
• Nutritional caloric content of fats is very high
• 9 cal/g compared to 4 in carbohydrates/proteins
• Dietary fats are important as vehicles for fat
  soluble vitamins and as a source of essential
  fatty acids
• Fats are the preferred form of long-term storage
  fuel in living organisms
• Many lipids are important structural elements of
  biological membranes, because of their special
  surface properties

26
Role of lipids in Foods

• The role of lipids in sensory characteristics is
  mainly connected with texture and rheological
  properties
• Refined fats have no taste of their own
• The presence and physical form (dispersion) of
  fats, determine the taste sensation and mouth
  feel
• Flow properties of the food in the mouth are
  controlled by the fat fraction
• Spreadability, coating of the tongue, sensation
  of swallowing, viscosity

27

• Lipid Oxidation

28
Introduction

• Off flavours are generally described as
  rancidity in fat-containing foods
• The principal source of rancidity in foods is
  the autoxidation of lipid compounds
• Autoxidation Defined as the spontaneous
  oxidation of a substance in contact with
  molecular oxygen

29
Introduction

• Consequences of lipid autoxidation
• Most significant rancidity
• Also flavour deterioration
• Colour is affected through accelerated browning
• Nutritional value is impaired
• Toxicity may occur
• Texture may change due to side reactions between
  proteins and the products of fat oxidation
• Oxidative deterioration of lipids may be
  considered as a spoilage factor affecting all the
  aspects of food acceptability

30
Introduction

• Lipid compounds most susceptible to autoxidation
  are the unsaturated fatty acids
• Especially those with more than one double bond
• Autoxidative deterioration of lipids resembles
  somewhat non-enzymic browning

31
Mechanism

• hydroperoxide
• Is proposed as the central mechanism of lipid
  autoxidation
• General course
• Reaction proceeds through a free radical
  mechanism consisting of the following steps

32
Mechanism

• STEP 1 Initiation
• STEP 2 Propagation
• STEP 3 Decomposition
• STEP 4 Termination

33
Mechanism

• In the first stage a few molecules of the lipid
  RH are sufficiently activated by heat, light or
  metal catalyst to decompose into the unstable
  free radicals R and H
• In the presence of molecular O2 the the
  possibilities of recombination include an
  encounter between the free radical and R and O2
  , resulting in the peroxide radical ROO
• This radical then reacts with a fresh molecule of
  lipid, RH, producing the hydroperoxide ROOH and a
  free radical R through which the chain reaction
  is propagated

34
Mechanism

• The reaction proceeds and more lipid molecules
  are transformed to hydroperoxides
• The reaction is terminated when free radicals
  combine with other free radicals or with free
  radical inactivates (X), to yield stable
  compounds which accumulate in the system
• The hydroperoxide enter a series of reactions
  leading to more free radicals and stable final
  products
• Final products include short chain carbonylic
  compounds responsible for the rancid flavour and
  for side reactions to overall deterioration

35
Hydroperoxide

• The various regions of the lipids are not
  equally susceptible to activation
• The methylenic group, adjacent to a double bond
  of a fatty acid, is particularly labile
• hydroperoxide is the primary products of lipid
  autoxidation
• They are non-volatile, odorless and tasteless
• Formation and accumulation are measured as the
  increase in the peroxide value
• Indicates the progress of autoxidation, but not
  necessarily the appearance of rancidity

36
Degradation of hydroperoxides

• Hydroperoxides are relatively unstable
• As their concentration in the system increases,
  they begin to decompose
• On possible reaction is the monomolecular
  decomposition of hydroperoxides into an alkoxy
  and hydroxy radical

37
Alkoxy radicals

• a) Aldehyde generation
• Short chain aldehyde is formed
• Oleate peroxides would produce C8 , C9 and C11
  aldehydes
• Aldehyde itself may be oxidized to an acid,
  reduced to an alcohol, react with amine groups

38
Alkoxy radicals

• b) Formation of Ketones
• This is a termination reaction
• Monomolecular decomposition of hydroperoxides to
  alkoxy and hydroxy radicals seem to be the
  predominant route
• At more advanced stages of the oxidation,
  bimolecular processes take over

39
Alkoxy radicals

• c) Reduction to an alcohol
• Here the alkoxy group react with another lipid
  molecule, generation alcohol and a free radical
• Which participates in the propagation of the chain

40
Polymerization

• Polymerization
• One of the consequences of lipid oxidation is the
  formation of viscous, gum-like or solid polymers
  (resins)
• Drying of highly unsaturated oils used in
  paints is the result of such polymerization
• May occur through direct contact with free
  radicals or through other reactions

41
Kinetic aspects

• The course of autoxidation in lipids is
  experimentally followed by measuring the
• Accumulation of peroxides
• Rate of oxygen uptake
• Concentration of secondary reaction products
• Organoleptic evaluation

42
Peroxide value

• One of the most widely used concepts in lipid
  chemistry
• It is the measure of the peroxide concentration
  of an oil
• expressed as milli-equivalents of peroxide
  oxygen per 100 g of fat

43
Rate of oxygen uptake

• More meaningful measure of the rate of oxidation
• method to determine it involves the reation of
  the oxidized fat with thiobarbituric acid (TBA)
• TBA reacts with oxidized fats to give a red
  coloured complex which can be measured
  spectrophotometrically
• TBA test correlates well with the degree of
  rancidity

44
Rate of oxygen uptake

• When the peroxide value (or oxygen uptake) of an
  autoxidizing lipid is followed, a curve are drawn
  (FIG 25, p224)
• At first the PV increases slowly, uniform rate
• As soon as the PV reaches critical value, a
  sudden and drastic increase in rate is recorded
• The first slow phase is termed induction period
• The autocatalytic nature of this course is
  explained on the basis of the free radical chain
  mechanism explained

45
Rate of oxygen uptake

• During the induction period initiation and
  propagation occur
• Since for each free radical which is transformed
  to a hydroperoxide one new free radical is formed
• The reaction proceeds at a slow uniform rate
• As the concentration of of hydroperoxides
  increases, hydroperoxide decomposition reactions
  take place ant in increasing rate
• These reactions generate more free radicals than
  needed for the propagation of the chain reaction
  at a constant rate - Autocatalytic

46
Effect of environmental factors

• Temperature
• Light
• Oxygen
• Moisture
• Ionizing radiations
• Catalysts
• Antioxidants

47
Effect of environmental factors

• Ions of heavy metals are powerful catalysts of
  lipid oxidation
• Shorten the induction period and increases the
  reaction rate
• Most effective are metals that can exist in two
  or more states of oxidation and can easily pass
  from one state to another
• Iron, copper, manganese

48
Catalysts

• The main effect of these trace metals is to
  increase the rate of hydroperoxide decomposition,
  and hence the rate of free radical generation
• Hydroperoxides are decomposed and free radicals
  RO and ROO are formed as the metal oscillates
  between its two oxidation states

49
Catalysts

• The source of heavy metal ions in foods may be
  contamination
• Equipment, piping, packaging materials,
  environmental contaminants
• Or natural food components
• Most important metal-containing natural food
  components are the metallo-porphyrin substances
  (hematin compounds)
• Hemoglobin, myoglobin

50
Catalysts

• Another important catalyst of lipid oxidation in
  some foods is the enzyme lipoxidase
• Lipoxidase catalyzes specifically the direct
  oxidation of poly-unsaturated fatty acids
  containing a cis-cis 1,4-pentadiene group
  (linoleic and linolenic acid)
• This enzyme is found in oilseeds, legumes,
  cereals, leaves
• If it is not inactivated by heat (blanching),
  lipoxidase may cause rapid development of
  off-flavours in frozen and dehydrated veggies

51
Antioxidants

• Antioxidants are substances that retard
  autoxidation
• A substance may act as an antioxidant in a
  variety of ways
• Competitive binding of oxygen
• Retardation of free radicals
• Inhibition of catalysts
• Stabilization of hydroperoxides
• All these mechanisms are found in food systems
  the most important seems the be blockage of
  propagation

52
Antioxidants Blockage of propagation

• The antioxidant AH acts as a hydrogen donor to a
  free fradical such as ROO or R
• The antioxidant free radical A is inactive, it
  does not start a chain propagation process, but
  rather enter some termination reaction such as
  p228
• The antioxidant may also be regenerated if a
  secondary hydrogen donor BH is present, and if
  oxidation-reduction potential of the following
  reaction is favorable p228

53
Antioxidants

• Antioxidation action increases the length of the
  induction period Fig 26, p229
• The induction period increment is roughly
  proportional to the concentration of antioxidant
  up to a certain level
• Excess concentration of antioxidant is
  ineffective or may even cause reversion of the
  protective affect

54
Antioxidants

• One of the principal classes of antioxidants are
  the natural or synthetic phenolic compounds
• Synthetic phenolic antioxidants approved for food
  use include butylated hydroxyanisol (BHA)
  butylated hydroxytoluene (BHT) and propyl gallate
  (PG)
• BHT and BHA are quite volatile at frying temp
• PG forms dark compounds with iron ions

55
BHA, BHT, PG

• BHA, BHT and PG are recognized as safe food
  additives in most countries
• They are used at concentrations of up to 200 ppm
  on the basis of fat
• The tertiary butyl groups attached to BHA and BHT
  somewhat enhance the stability of the
  corresponding phenoxy radical by introducing
  steric hindrance which prevents interaction with
  lipid molecules RH (propagation)

56
Primary Antioxidants

• The antioxidants discussed so far primary
  antioxidants
• They interfere directly with the free radical
  propagation process and they block the chain
  reaction
• A number of other substances which have little
  direct effect of the autoxidation of lipids, but
  are able to enhance considerably the action of
  primary antioxidants
• These substances are termed synergists
• One of the best known and widely used synergists
  is citric acid

57
Citric acid

• Its direct action is believed to be due to its
  ability to form stable complexes with pro-oxidant
  metal ions
• Poly-carboxylic and hydroxylic structure
• Citric acid is a potent metal chelating agent
• Its direct effect on phenolic primary
  antioxidants is probably non-specific, and due to
  its acidic characteristic

58
Lipid autoxidation in Food Systems Oxidized
flavors

• Immediately recognizable effect of lipid
  oxidation in foods is the development of
  undesirable odors and off-flavors
• rancid products of lipid oxidation are largely
  short-chain carbonylic compounds formed as a
  result of peroxide decomposition
• The overall organoleptic nature of rancidity
  depends somewhat on the system

59
Oxidized flavors

• The rancidity in low-moisture foods is usually
  described as old oil or tallow-like
• Lipid oxidation in water-rich foods such as milk
  resutls in cardboard-like off-flavors, known as
  oxidized milk flavor

60
Oxidized flavors

• Flavor Reversion
• Oxidative deterioration process of great
  importance in some vegetable oils soybean oil
• Freshly refined soybean oil is practically
  tasteless
• Upon storage under improper conditions
• Extensive exposure to air, high temp
• Off-flavors ranging from beany to fish-like
  are quickly formed
• The term inversion implies that the refined oil
  reverts to its raw, unrefined form incorrect
• The reversed flavor is due to newly formed
  compounds unrelated to the flavor-bearing
  components of the raw oil

61
Oxidized flavors

• Flavor reversion is usually due to the
  autoxidation of linoleic acid
• It is characteristic of oils with a relatively
  high poly-unsaturated acid content (linseed,
  soybean, rapeseed)
• The reverted flavors are due to unsaturated
  aldehydes

62
Effect of Colour

• Lipid oxidation may affect indirectly the colour
  of foods
• In carotenoids, propagation of the lipid
  oxidation chain through free radicals may cause
  oxidative destruction of the carotenoid pigments
• This type of deterioration is important in
  dehydrated vegetables and usually involves the3
  catalytic action of lipoxidases

63
Effect on Texture

• The interaction between proteins and the products
  of lipid oxidation may result in changes of
  texture
• The mechanism of interaction involves propagation
  of the free radical chain to the protein system
• Various groups in the protein moleculae are
  capable of converting to free radicals by losing
  a hydrogen atom to a free radical of lipid origin
• The protein free radicals thus formed tend to
  combine by cross-linkages

64
Lipid Oxidation at High Temp

• Of interest in connection with food processing
  operations involving high temp Toasting,
  roasting, baking, frying
• Most important characteristics of heated oils
  are
• Despite the accelerated rate of oxidation,
  peroxide values are usually very low, due to the
  rapid decomposition of the peroxides formed
• The flavor of heated oils is not rancid (unlike
  fats oxidised at low temp). Their taste and odor
  are accepted, due to the elimination of the
  volatile breakdown products by evaporation
  steam distillation effect

65
Lipid Oxidation at High Temp

• Polymerization is one of the predominant
  termination processes. The viscosity of oils
  increases considerably in the process of heating
• The degree of unsaturation, measured as the
  iodine value decreases sensibly, indicating
  direct saturation of the double bonds.
  Poly-unsaturated acids are affected first
• Hydrolysis of the fat occurs and fatty acids are
  liberated, especially in the process of frying

66
Toxicity of Oxidized Fats

• Massive ingestion of highly oxidized fats or
  concentrated fractions containing peroxides, or
  their decomposition products, has been reported
  to cause disturbances ranging from
• Growth inhibition to carcinogenesis
• In most cases, however, the levels of intake
  necessary to cause such disturbances was
  unrealistically high, as to the expected level of
  voluntary intake of rancid foods

67
Lipid nature of Carotenoids

• Carotenoids
• Large group of pigments
• Widely distributed in the plant and animal
  kingdoms
• Yellow-orange to purple in colour
• Insoluble in water
• Soluble in fats and organic solvents
• Classed as Lipochrome pigments
• Food products of animal origin such as milk,
  butter, egg yolk, some fish and shellfish,
  contain carotenoids dispersed in the lipid
  components

68
Lipid nature of Carotenoids

• Carotenoids Structure
• Belong to the class of polyenes
• Long chains of conjugated double bonds
• The presence of many conjugated unsaturated bonds
  explains the intense colour of carotenoids
  ranging from yellow to red and purple
• Isoprenic nature carotenoids are built of
  isoprene units (structure p186)

69
Lipid nature of Carotenoids

• Breakdown of Carotenoids
• Because of their highly unsaturated nature, they
  oxidize very quickly, particularly at the double
  bonds
• As double bonds are saturated and finally broken
  down, the characteristic color of carotenoids is
  bleaches
• Carotenoids are much more stable to oxidation in
  their natural form than in pure systems.
• Crystalline pure lycopene or a solution of
  lycopene n chloroform fades in a matter of a few
  hours when exposed to air, while the same pigment
  in its natural form in tomatoes is quite stable

70
Lipid nature of Carotenoids

• The oxidation of carotenoids and the autoxidation
  of fats have may points in common and are often
  interrelated in food systems
• Free radicals formed in the course of fat
  oxidation may participate in the oxidative attack
  on carotenoids
• The enzyme lipoxidase, which is important in the
  oxidative degradation of fats in grains and
  vegetables, may also take part in carotenoid
  oxidation

71
Lipid nature of Carotenoids

• The most important factor in the oxidation of
  carotenoids is the presence of oxygen or strongly
  oxidizing agents
• The destruction is more rapid at high temp
• The effect of temperature may accelerate
  oxidation directly, but it may also render the
  carotenoid more susceptible to breakdown by
  denaturing the protective protein
• In the absence of air, carotenoids can withstand
  relatively high temp
• Bleaching is more rapid in the absence of water
• Moisture content levels can have a protective
  effect on carotenoids

72
Functions of Carotenoids

• Carotenoids contribute to photosynthesis through
  the ability to transmit the accumulated light
  energy to the chlorophyll
• In the absence of the carotenoid pigments, the
  photosynthetic apparatus is rapidly destroyed by
  chlorophyll-catalyzed photo-oxidation
• One of the main biological functions of
  carotenoids seems to be photoprotection
• Protection of cells and tissues against harmful
  effects of light

73
Carotenoids in Food Systems

• Mixtures of natural carotenoids extracted form
  plant tissues and synthetic B-carotene are
  commercially available as food colorants
• Both in the oil-soluble and water-dispersible
  forms
• Chemical unstability of carotenoids
• Loss of carotenoids is a problem in fat rich
  systems (butter/margarine), and in low moisture
  foods (dehydrated veggies)
• Loss of characteristic colour
• Formation of undesirable odors due to breakdown
  products

74
Carotenoids in Food Systems

• Veggies opaque containers, package under
  nitrogen prevents destruction
• Carrots coating of starch can be sprayed on
  before dehydration
• The bleached colour of the carotenoids, caused by
  oxidation, is often a very important indication
  of deterioration in food product
• Citrus essential oils
• Colour of carotenoids can be undersirable
• Bleaching of wheat flour during storage in air

75
Lipid nature of Vitamin A

• Vitamin A is now termed Retinol
• In relationship with carotenoids
• ß-carotenoids consists of two molecules of vit A,
  bound tail to tail
• ß -carotene is converted to vit A, with the help
  of enzymes present in the intestinal mucosa of
  animals
• The name provitamin A given to B-carotene
• The conversion involves oxidation at the middle
  point, by a specific enzyme
• ß-carotene-15, 15-oxygenase

76
Lipid nature of Vitamin A

• A peroxide is formed
• Cleavage of the peroxide yields two molecules of
  retinal (the aldehyde form of vit A)
• The majority of retinal is reduced to retinol by
  a non-specific enzyme
• Retinol is carried into the blood stream and any
  amount in excess of the required level in blood
  is stored in the liver, in the form of fatty-acid
  esters
• In the blood retinol is carried by a very
  specific protein, retinol binding protein

77
Lipid nature of Vitamin A

• Best known function of vit a is connected with
  vision
• In the eye, retinol, is oxidizes to retinal
• Retinal combines with certain proteins termed
  opsins, to form so-called visual pigments of the
  retina

78
Lipid nature of Vitamin A

• Vit A and its precursor, B-carotene, are soluble
  in fats and oils, and there is always a danger
  that when the oils become rancid (due to
  oxidation) the vitamin will suffer considerable
  losses
• This is true to a large extent of such food
  products as butter or vit-enriched margarines,
  subjected to prolonged storage
• Vitamin can also be destroyed to some extent by
  the action of light
• The effect of packaging is therefore an important
  factor in the retention of vit A

79
Terpenes, Essential oils

• Essential oils are widely distributed in many
  different parts of the same plant
• Roots, stem, leaves, flowers, fruits
• The aromatic material may be actually dissolved
  in the juice, or the essential oils are secreted
  in numerous oils sacs or glands located in the
  epicarp, adjacent to the chromoplast

80
Terpenes, Essential oils

• Essential oils are a mixture of various volatile
  organic substances along with some non-volatile
  waxy materials
• The term oil implies that these substances are
  insoluble in water but soluble in non-polar
  solvent
• The greater part of essential oils consists of
  terpenoids and their derivatives
• Terpenoids are naturally occuring isoprenoid
  hydrocarbons (terpenes) and their oxygenated
  derivatives

81
Terpenes, Essential oils

• Structure and Nomencalture
• Terpenes may be classified according to the
  number of isoprene units in their molecule
• Monoterpenes 2 isoprenes (p196)
• By this classification, vit A, is a diterene and
  carotenoids, which all have 40 carbon atoms,
  would be tetraterpenes

82
Terpenes, Essential oils

• Chemistry of Food Flavors
• Flavor is a complex sensation arising from the
  simultaneous perception of odour and taste
• Odors are sensed when molecules of volatile
  substances reach the olfactory receptors at the
  top of the nasal cavity
• The accurate and complete quantitative analysis
  of food volatiles was made possible by the
  development of gas chromatography and mass
  spectrograhy
• The odors of food is seldom due to one or few
  chemical substances

83
Terpenes, Essential oils

• These analytical methods have allowed us to
  follow the changes in food volatiles as a result
  of processing and storage, or to evaluate
  quantitatively the loss of flavors by
  evaporation, in concentration or dehydration
  processes
• (Look at aroma recovery p207)

84
Lipid Nature of Cholesterol

• Cholesterol is the most abundant sterol (p209) of
  the animal kingdom
• It occurs as a structural element of cell
  membranes of many tissues in conjunction with
  phospholipids
• Cholesterol, as most other sterols, also occurs
  in ester combination with fatty acids
• Closely related to cholesterol is lanosterol,
  found in the fatty component of wool lanosterol
  is an effective fat-water binding agent, hence
  the use of lanolin in moisturizing creams

85
Lipid Nature of Cholesterol

• Lanosterol is an intermediate in the biosynthesis
  of cholesterol
• In addition to its function as a structural
  element in the cell membrane, cholesterol serves
  as a precursor in the biosynthesis of ergosterol
  (vit D) and steroid hormones and bile acids
• Bile acids emulsify the fats in the intestinal
  tract and thus facilitate their digestion and
  absorption. The bile acids are synthesized in
  the liver, from cholesterol