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
13 The Visible Effects of Chemical Spoilage
It is interesting at this stage to study the frying process in the light of these reactions. When an oil is heated and used for frying, a number of changes become apparent.
Distinctive odours and flavours develop
This is caused by the appearance of new compounds as the fat breaks down. Most of these new compounds have an objectionable smell and flavour but because they are relatively
light I weight, they tend to be driven off by the steams as they are formed. They can be smelt in the atmosphere or tasted in the first pieces of food that are fried but eventually
they can become more pronounced. They are mainly caused by the oxidation and hydrolysis of the fat (see Section 12).
The colour darkens
Most properly refined oils are pale in colour but as they are heated, they become darker until eventually they approach the colour of strong black coffee. This is mainly the
result of the oxidation process which yields substances by mechanisms which are still not fully understood.
There are also colour changes taking place in the frying food, the surfaces taking on colour by caramelisation as it is cooked. These colours can dissolve in oil and will tend
to darken the frying medium. Indeed, some of the dark colour is due to the presence of carbon in very fine form; so fine that filtration will not remove it.
The oil tends to foam
The larger the quantity of fatty acids present in the heated oil, the more it tends to smoke. As the oil begins to deteriorate under the effect of heat, more of these breakdown
products appear. Consequently smoke is formed at lower and lower temperatures as frying proceeds. As long as the frying areas is well ventilated, this may not be a serious
performance fault in itself, but the smoke is acrid and objectionable and it serves as an indication of the condition of the oil.
Moreover; all the associated properties are affected, and the flash points and fire points are reduced, increasing the risk of fire. (see Section 16 about flash point and fire point).
The oil becomes thicker
As an oil is heated, it becomes thicker and more viscous. This is mainly due to polymerisation, by which molecules join together to make larger ones. It is a serious performance
fault as it causes a number of problems. As the oil becomes more viscous, fat absorption increases because the oil does not drain from the food so readily. Also, the bubbles which
are formed at the surface are more stable, more long lasting. The amount of oil in contact with air at the surface is increased, and increased oxidation results.
The oil tends to foam
Some of the compounds which are formed when an oil is heated are chemically related to those which are actually made commercially for their foaming properties. They are formed by
oxidation and hydrolysis. At the same time, impurities can accelerate these reactions, particularly the presence of alkaline materials which might have been used
to clean the kettles.
These reactions, as well as making the oil foam, also tend to increase the penetration of the oil into the fried food, which results in higher fat absorption.
Summary
There are five important effects of chemical spoilage which become visible in a frying oil. They can be summarised in the next table:
By presenting this information in another way, the following table shows the effects of ignoring some of the basic rules of frying:
