The oxidation of fats is one of the main causes of deterioration of food along with the action of microorganisms. It results in alterations of aroma and flavour, colour, loss of certain nutrients and the formation of potentially harmful substances, which leads to a reduction in the shelf life of the food. This process has important economic repercussions because the food becomes unfit for consumption.
The oxidative process of fats causes a decrease in the nutritional capacity of the food by the destruction of fat-soluble vitamins and the degradation of polyunsaturated acids.
Food safety can also be affected, since the products originating from the oxidation can present toxicity, some volatile products, peroxides, oxy acids etc.
This article aims to describe the oxidative process of lipids and therefore the importance of the use of antioxidant substances. To do this, we will focus on the oxidation process of fats and the main factors that influence the oxidation of fats
Oxidation of fats
The main oxidation form of lipids occurs through a free radical chain propagation reaction, in which peroxides and hydroperoxides are formed from fatty acids and oxygen, which is known as the auto-oxidation process. These compounds are quite unstable, so they can be broken, giving rise to more free radicals and generating a chain reaction.
Historically, it was believed that adding oxygen to the fatty acid molecule formed a four-atom ring as the primary reaction product. This molecule is called “peroxide”. However, what is actually formed is a hydroperoxide due to the addition of oxygen to the alpha carbon of the double bond. Despite this, the inaccurate idea still persists and peroxides still refer to the products of the degradation of a fat.
Auto-oxidation is an irreversible oxidation process of fats. It is impossible to avoid it in its entirety, but it can be delayed due to the addition of antioxidants.
The oxidation mechanism consists of three phases:
- Induction: The initiators are usually energy (light, heat, etc.), traces of heavy metals, and radical peroxides that cause active free radicals to occur. The action of the energy causes the decomposition of the fatty acid in a free radical and hydrogen ion, as shown in the following figure:
- Propagation: With the oxidation of free radicals in combination with other fatty acids, hydroperoxides and more free radicals are formed, which re-enter the oxidation chain. On the other hand, hydroperoxides with the incidence of energy, form oxydryl groups and the oxidized form of free radicals, which together with other fatty acids give rise to more hydroperoxides and new free radicals. Finally, the oxydryl groups along with other fatty acids release water and new free radicals exposed to a new oxidation.
- End: The amount of reactive compounds is huge and begin to interact with each other. The concentration of peroxide radicals falls as the formation of deteriorated products begins to stabilise. Given the stability of the products originating in the termination reactions, the oxidation activity is terminated. The auto-oxidation of fats continues to be affected by a free radical and the action of temperature and light. The formation of new chains favor the acceleration of the overall reaction, in addition the reaction capacity is favored in the presence of certain metals like copper and iron.
After the complete destruction of the fatty acids, secondary products of oxidation are generated, which are responsible for the occurrence of rancidity.
To delay or prevent the oxidation of fats and the rancidity of foods, it is necessary to know how to prevent the formation of the first free radicals or first hydroperoxides, since this can only be achieved during the first stage of the oxidative process, during initiation. On the contrary, once the oxidative process reaches the propagation phase, said process can no longer be delayed or stopped.
Factors that influence oxidation
The factors that influence oxidation may be intrinsic and / or extrinsic to the food, that is to say, they can be regarding the product and the applied technology. Some of the most relevant are mentioned below (Dr. Paucar Penacho, Deterioration of Agroindustrial Products – 2014):
- Temperature; The rate of auto-oxidation increases with temperature. It can affect not only the auto-oxidation rate, but also the reaction mechanisms.
- Light: Fatty acids and their peroxides are colorless substances that do not absorb visible light. Thus, unless an accessory sensitiser is present, it can be assumed that the effect of visible light on autoxidation is not of greater importance. However, ultraviolet light is strongly absorbed in the unsaturated compounds.
- Oxygen: The rate of auto-oxidation increases with increasing oxygen pressure, until it reaches a constant rate of reaction.
- Humidity: The effect of water activity on the oxidation rate of lipids is very complex. Rancidity develops rapidly at both very high and very low moisture levels. Maximum stability is observed at intermediate moisture levels corresponding to monolayer values (protective effect of water in the form of a monolayer).
- Ionizing Radiation: One of the most notable effects of high energy irradiation on food is a marked increase in the susceptibility of oxidative rancidity.
- Catalysts: Heavy metal ions are powerful catalysts for the oxidation of lipids, decrease the induction period and increase the rate of reaction.
As a conclusion we indicate that to inhibit, reduce or delay the oxidation of lipids, it is necessary to act against one or more of the factors that encourage its development.
In a broad sense, it is considered as an antioxidant any substance or process of action that helps to limit the speed and / or extension of oxidative processes. According to this concept, three types of antioxidants could be considered, depending on their mechanism of action. The first two types are associated with the addition of chemical compounds, while the third owes its action to modifications of certain factors in the food and / or in its processing.
In the next articles we will analyse in detail the classification and action of the antioxidants for use in food.