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The reactions of the fat

It is important to know the three chemical reactions which fat is often exposed to, since they have an effect on the quality of fats.


1.     Oxidation

Oxidation is responsible for the ageing of the fat due to the transfer of oxygen from the air. It is already happening before the cooking fat is heated. For every 10 °C increase in temperature, the rate of oxidation is doubled.*

(* This is only an assumption. The rate may differ from this figure in reality. )

For example, if two radicals are formed at room temperature (25 °C), there will be 16 radicals at 55 °C and 16,384 radicals at a temperature of 155 °C. For the fat, this means that the more radicals that are present, the faster the fat is broken down into its individual parts, in other words the faster it ages. Apart from temperature, light also has a considerable impact on decomposition. Light consists among other things of ultraviolet (UV) rays which create favourable conditions for triggering oxidation.

Fats are organic substances which can oxidise, and in fact all the more easily the more double bonds are contained in the fatty acids of the fat. Cold-pressed olive oil, for example, has a shelf life of approximately just six months at room temperature due to its large number of unsaturated fatty acids. In addition to degradation products with an intense taste such as fatty acids, oxidation also produces mono- glycerides and diglycerides. During the deep frying process, the water evaporates from the product being deep fried and a crust is formed. This stops the fat permeating the product too deeply. After a certain time, the majority of the water is evaporated and the cooling effect at the crust stops. The desired browning of the product being deep fried now begins as a result of the high temperature. As the proportion of polar materials in the fat increases, the water can evaporate through the fat more easily and quickly. The formation of the crust progresses more slowly in relation to the evaporation, but at the same time the rate of browning is quicker as the outer layer of the product is no longer being cooled so effectively. In the case of chips, this means that they become hollow inside. In the case of fats with a higher proportion of polar materials, more fat can permeate the product due to the faster evaporation.

The decomposing process in oxidation is divided into several phases.The "induction phase" triggers the oxidation. The products of oxidation as a result of effects such as heat, light or heavy metals (Cu, Fe) include free radicals (R*, R = fatty acid radical) which react with oxygen (O2) in the air to form oxygen-bonded radicals (ROO*) (see Fig. 10).

Note: Radicals are identified by means of an asterisk .


Induction phase
Figure 10: Induction phase

In the chain growth phase, the fatty acid peroxide radical ROO* gains a hydrogen atom H from another fatty acid and becomes a fatty acid peroxide molecule** (ROOH).

(** Hydrogen peroxide (H2O2) is a strong oxidant and is used in heavily diluted form to bleach hair, for example.)

The attacked fatty acid thus becomes a new radical and in turn reacts with the oxygen present (see Fig. 11).


Chain growth phase
Figure 11: Chain growth phase

The unstable fatty acid peroxide molecule (ROOH) is largely broken down into various radical products (RO* and *OH) and reacts with the oxygen present or with the surrounding bonded fatty acids (chain branching reaction) (see Fig. 12).


Chain branching reaction
Figure 12: Chain branching reaction

The more radicals that are formed, the greater the probability that the radicals will collide. When radicals collide, the two free radicals form a bond and there is a chain termination reaction. The radicals are "trapped" and can no longer catch hydrogen molecules (see Fig. 13).


Chain termination reaction
Figure 13: Chain termination reaction

Radical catchers (antioxidants) such as Vitamin E or C make use of this mechanism. They attract the radicals like "magnets" and prevent or delay the chain reaction by catching radicals. The antioxidant is used itself when the radicals are caught.


HollaOils Actions of radical catchers
Figure 14: Actions of radical catchers

2.      Polymerisation

This is a chemical reaction in which the unsaturated fatty acids present in the cooking fat, under the influence of heat, light or metals (Cu, Fe) and by breaking down the multiple bond, react to form first dimers (two connected fat molecules) and then polymeric (large number of connected molecules) triglycerides.The oil becomes more viscous as a result of the chain formation of the molecules. As a result, it is harder for the water to evaporate from the oil, which means that, as with fresh fat, the heat cannot get to the food properly, no browning reaction can take place and the food becomes dried out and shrivelled. At the same time, the fat has a greater tendency to stick to the food when it is removed from the deep fat fryer, which in turn results in greater fat loss in the deep fat fryer than with fresh fat. Following polymerisation, the quantity of volatile substances across the fat is reduced. Smoke formation is therefore lower in very old fats.

Apart from the change in colour, cooking fats with a high proportion of polymers are characterised by a high degree of fine-pored foaming.

As with oxidation, the first step is induction. A radical (R*) is produced as a result of the effect of light, heat or heavy metals (Cu, Fe). However, instead of now reacting with oxygen, the radical attacks the double bond of a fatty acid which constitutes part of the fat molecule. After the reaction, the entire fat molecule has become a radical (see Fig. 15).


Holla Oils Initial phase of polymerisation
Figure 15: Initial phase of polymerisation

If the fat molecule radical attacks another fat molecule with a double bond, the double bond breaks down and the fat molecule radical attaches itself. In this first step, chains of two fat molecules are produced which can grow during polymerisation to form a chain of many hundreds of fat molecules (polymers) (see Fig. 16).


HollaOils Chain growth
Figure 16: Chain growth

If two of these fat molecule radicals collide, the chain is terminated. The two radicals bond (green) and do not attack any further fat molecules (see Fig. 17).

It can sometimes happen that a fat radical attacks the double bond of one of its own fatty acids. This causes a ring closure within the molecule. The product of such a reaction is called a "cyclic bond".


Hollaoils Chain termination reaction
Figure 17: Chain termination reaction

3. Hydrolysis

Hydrolysis is primarily triggered by the permeation of water from the product being deep fried and is encouraged by certain substances such as baking powder. Hydrolysis is a controversial subject of discussion in specialist literature. Opinions of researchers differ with regard to whether the permeation of water does not also have positive effects on the fat. It is known, for example, that the evaporating water extracts volatile degradation products such as short-chain fatty acids or alcohols together with fat and thus helps to purify and stabilise the fat.

The water (H2O) content is evaporated across the cooking fat and leaves behind monoglycerides and diglycerides and free fatty acids.

In hydrolysis, the water attacks the bond between the glycerol and fatty acid and is then itself split into two parts. The one part (an H atom, red) attaches itself to the glycerol radical and the second part (OH radical, blue/ turquoise) remains attached to the fatty acid radical (see Fig. 18).


HollaOils Hydrolysis reaction
Figure 18: Hydrolysis reaction

The smoke point of the fat is lowered as a result of the decomposition of the fat molecule and the fat takes on a different taste due to the changed molecules.

If baking powder (alkaline) is added to the fat via the product being deep fried, soap is produced from the fatty acids. This is why hydrolysis is also known as "saponification". One ingredient of baking powder is sodium. If the baking powder reacts with the fatty acid, very small amounts of curd soap are produced.

The HollaOils company, in its activities, has a systematic and scientific basis. An individual approach to the client and the specifics of his activities allows us to build the most productive partnerships and obtain the desired result.

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