Transcript Fats and Oils

Fats & Oils
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AL Chemistry
Structure & Properties
Hydrolysis of Fats & Oils
Iodine Value
Hardening of Vegetable Oil
Hydrolytic & Oxidative Rancidity
p. 1
Structure & Properties (1)
AL Chemistry
Fats and oils = glycerol + fatty acids
Propane-1,2,3-triol
if the fatty acid
part has many
C=C bonds
unsaturated
p. 2
Structure & Properties (2)
AL Chemistry
forms fats
(or oils)
with glycerol
p. 3
p. 4
p. 5
Structure & Properties (3)
AL Chemistry
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Unsaturated Oils usually have a lower
melting point and exist as a liquid.
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Hydrogenation of some of the C=C
bonds converts to solid fats.
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e.g. Margarine
p. 6
Hydrolysis of Fats & Oils
AL Chemistry
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Fats and oils are hydrolysed into
carboxylic acid and glycerol in
human body (acidic medium).
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In lab, hydrolysis can be carried out
in alkaline medium more effectively,
to give carboxylate and glycerol.
(saponification)
soap!
p. 7
Iodine Value (1)
AL Chemistry
► To measure the degree of unsaturation of a
fat or vegetable oil.
► It is defined as the number of grams of iodine
that reacts with 100 grams of fats/oils.
► Vegetable has a high iodine value than fats.
p. 8
Iodine Value (2)
# higher iodine value  higher degree of unsaturation
Fats / Oils
Animal fats
Vegetable oils
Iodine values
Butter
25-30
Dripping (肉汁)
35-65
Lard (豬油)
45-65
Coconut oil (椰子油)
8-10
Soya oil (大豆油)
80-140
Ground-nut oil (花生油)
85-105
Olive oil (橄欖油)
80-90
Almond oil (杏仁油)
90-110
Corn oil (粟米油)
115-130
p. 9
Examples of Unsaturated Fatty Acid:
Chemical Names and Descriptions of some Common Fatty Acids
Carbon
Atoms
Double
Bonds
Oleic Acid
18
1
9-octadecenoic acid
olive oil
Linoleic Acid
18
2
9,12-octadecadienoic acid
corn oil
g-Linolenic Acid (GLA)
18
3
6,9,12-octadecatrienoic acid
borage oil
Gadoleic Acid
20
1
9-eicosenoic acid
fish oil
EPA
20
5
5,8,11,14,17-eicosapentaenoic acid
fish oil
DHA
22
6
4,7,10,13,16,19-docosahexaenoic acid
fish oil
Common Name
Scientific Name
Sources
p. 10
Hardening of Vegetable Oil
AL Chemistry
 Animal fats ---- usually saturated
 regular packing
higher melting point, therefore solid state at rm temp.
catalytic (Ni)
hydrogenation
But the vegetable oil should not be
completely hydrogenated, otherwise,
the solid will be too hard!
 Vegetable oil ---- usually unsaturated
 irregular packing (due to presence of cis-isomers)
lower melting point, therefore liquid state at rm temp.
p. 11
Disadvantage of Animal Fats?
AL Chemistry
They contain larger amount of “Cholesterol”.
p. 12
Hydrolytic and Oxidative
Rancidity (酸敗) of Fats & Oils:
AL Chemistry
 Hydrolytic and Oxidative Reactions of triglyceride molecules
give out unpleasant odour…….
due to volatile and foul smelling
RCHO & Fatty Acids
 Hydrolytic Rancidity occurs due to the presence of
moisture in the oils.
p. 13
Oxidative Rancidity (1) :
AL Chemistry
 Oxidative Rancidity occurs under exposure to air
(oxygen)
 Very common for those with high iodine value
► high degree of unsaturation
--- more susceptible to oxidation!
 free radical mechanism!!
p. 14
“Autoxidation”
Oxidative Rancidity (2) :
H
H
C
C
H
H
C
C
H
H
O 2 i n the air
H
H
H
H
C
C
C
C
HO
O
H
A uto xid a tio n
s e g m e n t o f c ar b o xylic a cid in fa t/o il
volatile
aldehyde,
ketones ….
AL Chemistry
h ydr o p ero xid e
R-O-O
R-O-O-H
[hydroperoxide
free radical]
[hydroperoxide]
p. 15
Oxidative Rancidity (3) :
AL Chemistry
 Oxidative Rancidity can be prevented by using
antioxidants (BHA or BHT)
R-O-O
R-O-O-H
i.e. the hydroperoxide free radical is “killed” by the H
atom given by BHT  chain reaction stops.
p. 16
Oxidative Rancidity (4) :
AL Chemistry
 If the triglyceride molecules do not contain any
unsaturation carboxylic acid chain …..
 no hydroperoxide will be generated,
and therefore oxidative rancidity does not occur.
p. 17
HKALE ….
AL Chemistry
1997:
Vegetable oil
(iodine value, hydrogenation and saponification)
1998:
Animal Fats and Vegetable oil
(iodine value, hardening, saponification and rancidity)
2001:
Vegetable oil [unsaturated oil]
(principle of anti-rancidity by BHT)
p. 18