Topics In This Section
1. Introduction
2. General Rules of Frying
3. Temperature Control
4. Preparation of Food
5. Fat Absorption and Topping Up
6. Choice of Frying Medium
7. The Spoilage of Fats and Oils
8. Cleaning
9. Frying Specific Foods
10. Recommended Frying Temperatures
11. The Basic Chemistry of Fats and Oils
12. The Chemistry of Fat Spoilage
13. The Visible Effects of Chemical Spoilage
14. The Role of the Oil Refiner
15. The Role of the Fryer
16. Fire Hazards
Reproduced with permission from Pura Food Products Limited
12 The Chemistry of Fat Spoilage
Introduction
With a basic idea of the simple chemistry of fats and oils, it is possible to explain how they can spoil. Fats and oils are natural products obtained from plants and animals, and no matter how well they have been selected and refined, they will begin to spoil as time goes by. In frying, an oil is forced to break down at a greater rate than normal because of the high temperatures which are involved.
Stages of spoilage
During frying, it is possible to define three different stages over which spoilage and breakdown can occur.
Reactions involved
There are a number of things which can break a fat down, and the type of reaction which takes place depends upon the particular period under consideration.
The main reactions which can occur are summarised in the following table.
Oxidation
During the storage period, the only chemical spoilage reaction which normally takes place is known as oxidation, attack by oxygen in the air, In fact, it is a special
type of oxidation in which the products that are formed tend to promote the reaction, making it take place at an ever increasing rate. If one buys old butter and the
surface of it tastes tallowy or meaty, it s because the butter has become oxidised. The oxygen in the air attacks the points of weakness in the fat that
were mentioned in Section 11.
Obviously, the more points of weakness there are in a particular fat, the more the effect will be. In the storage period, it is some time before the effect becomes
really noticeable.
During the standby and frying periods, the fat is heated to about 191C (375F). Once a fat is heated, the process of oxidation, which was going on during storage, becomes much faster.
As a result, the weak links in the fat are broken and new substances begin to be formed in the fryer. Unfortunately, most of these substances are evil smelling and are responsible
in part, for the characteristic odour in a well-used fat and for off-tastes in the fried food,
Certain metals such as iron and copper accelerate oxidation of fats and should be avoided. For this reason it is also advisable not to add salt before frying, because the
salt's sodium ions act as an oxidation catalyst. Salt also contains other impurities such as copper and iron ions. These impurities cause increased oxidation of the
frying medium.
Isomerisation
This is another reaction which occurs rapidly in the standby and frying periods. In Section 11, when molecules with two points of weakness were discussed, it was mentioned
that these points could be various distances apart; the closer together they are in the same molecule, the more unstable the molecule tends to be.
In the process of isomerisation, the molecule shuffles itself around and the points of weakness often end up closer together as a result. Isomerisation can therefore make
the fat more unstable and more sensitive to oxidation.
Polymerisation
This means, simply, that two or more molecules of the same material have joined together to make a larger one. For example, polystyrene ceiling tiles and coving are made
from a colourless liquid called styrene. The molecule of this liquid are made to join together - polymerise - to such an extent that they produce a solid. In frying, there
is a tendency, more marked with some oils than others, for a yellowish-brown resin-like substance to be formed on the surfaces of the fryer and baskets. This substance is
formed by the process of polymerisation.
Pyrolysis
This also occurs during the standby and frying period, although it proceeds at a slow rate. Pyrolysis is a sort of burning, where the fat simply breaks up into
smaller compounds.
Hydrolysis
When food is fried in hot oil, this further reaction is introduced. The water which is present in the form of steam (from the chips for example) attacks the oil
This is the reverse reaction to the way in which the triglyceride molecule was built up at the beginning of Section 11, and the parts broken off are fatty acids.
They are called 'free' fatty acids. The smoke point of an oil is related to the amount of free fatty acids present and their breakdown products have objectional smells.
Eventually, they cause the oil and fried food to develop off-flavours.
Caustic Soda and other alkalis used for cleaning tend to promote hydrolysis and should be well rinsed away after they have been used.
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