Hydrolysis of Fats
Lesson 4
Hydrolysis of fats
Oils and fats develop an unpleasant (rancid) smell if
they are kept too long
Rancid food has a bad smell, taste, texture/appearance
Free fatty acids are generally absent in the fats of
living animal tissue, but can form by enzyme action
after the animal has died.
Hydrolysis is when a compound splits when reacted with
water.
Fats are hydrolyzed in heat and water
to their fatty acids. An example is
shown
Hydrolysis can happen quickly when
certain microorganisms [bacteria,
fungi, etc.] are present
The reaction is catalyzed by the
enzyme lipase [which helps the
reaction move along faster without
interfering in the reaction]
Interesting Facts
Fatty acids with 4 (butanoic), 6 (hexanoic) and 8
(octanoic) carbon atoms are released when the fats in
milk and butter are hydrolysed
Palmitic, stearic, and oleic acids are produced during
the hydrolysis of chocolate and give it an oily/fatty
flavour
Hydrolysis also happens during deep fat frying(when
water is produced from the food that is introduced to
high temperatures)
Oxidation
Alkenes are more reactive than alkanes
Similarly unsaturated fats are more reactive than
saturated fats. The carbon-carbon double bond in
unsaturated fats react with three things
a.
Oxygen (auto-oxidation)
b. Hydrogen (hydrogenation)
c.
Light (photo-oxidation)
Auto-Oxidation
The oxidation of unsaturated fats by
molecular oxygen, which occurs in air
in the absence of enzymes, is called
auto-oxidation
When fat molecules break down to form
unpleasant-tasting aldehydes and
carboxylic acids, the process is
known as oxidative rancidity
Photo - Oxidation
It is a free radical reaction which
can be initiated by light
Polyunsaturated oils contain a
greater number of C=C double bonds
and become rancid more quickly
Oily fish (mackerel and herring)
contain high percentages of
unsaturated fatty acids and are prone
to oxidation
However oxidation, like hydrolysis,
can be reduced by refrigeration
Hydrogenation
Hydrogenation can be added across the
carbon-carbon double bond to decrease
the level of unsaturation
This is an important reaction as it
increases the melting point, the
hardness and chemical stability of
the fat
This reaction is used commercially to
convert liquid oils into solid
margarine
Hydrogenation is carried out between
140 - 225 C in the presence of a
finely divided metal catalyst (Zn,
Cu, Ni)
The degree of saturation can be
controlled by varying the pressure of
the hydrogen and the nature of the
catalyst.
Advantages of Hydrogenation
Disadvantages of
Hydrogenation
Changes a liquid oil to a semi-solid or solid
to make the melting point of an unsaturated fat
more like that of a saturated fat
Mono- and poly-unsaturated fats are healthier
for the heart than saturated fats
Decreases the rate of oxidation (stability
increases with increasing saturation)
In partial hydrogenation, trans fatty acids can
form
Increases Hardness
Trans fatty acids are hard to metabolize,
accumulate in fatty issues, are difficult to
excrete from the body, increase levels of LDL
(bad) cholesterol and are a low quality energy
source
Controls the feel and plasticity (stiffness)