OXIDATION & ANTIOXIDANTS
Tackling oxidation
The oxidation of vegetable oils is an undesirable series of chemical reactions involving oxygen, which degrades the quality of the oil. Oil oxidation is one of the main causes of food deterioration and results in alterations of aroma, flavour and colour; loss of certain nutrients; and the formation of potentially harmful substances, leading to a reduction in the shelf life of a food product. Oxidation eventually leads to rancidity in oil, with accompanying off-flavours and smells.
The oxidation process
Oxidation occurs through a free radical chain propagation reaction in which peroxides and hydroperoxides are formed from fatty acids and oxygen. This 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 (see Figure 1, following page). The process is one-way and irreversible but can be delayed with the addition of antioxidants. The oxidation process occurs in three phases: In the initiation phase, light, heat, traces of heavy metals and radical peroxides cause active free radicals to occur. In the propagation phase, the oxidation of free radicals in combination with other fatty acids forms hydroperoxides and more free radicals, which re-enter the oxidation chain. The high number of reactive compounds begin to interact with each other. Finally, the concentration of peroxide radicals falls as the formation of deteriorated products begins to stabilise and oxidation activity is terminated in the third phase. 26 OFI – FEBRUARY 2022
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Oil oxidation involves a series of chemical reactions that degrade the quality of an oil. Mitigating steps include selecting the right oil for the right application, and using protecting additives and antioxidants Ignace Debruyne
The primary degradation compounds resulting from frying include free fatty acids, which can catalyse oil degradation and decrease the shelf stability of fried food
After the complete destruction of fatty acids, secondary products of oxidation are generated, which are responsible for rancidity occurring. To delay or prevent the oxidation of oils and fats, the formation of the first free radicals or hydroperoxides must be prevented, which can only be achieved in the first phase. Once the oxidative process reaches the propagation phase, the process cannot be delayed or stopped.
Factors that influence oxidation
The critical factors that affect oil oxidation can be internal and related to the oil itself, or external and processing-related. Internal factors contributing to the oxidative deterioration of a finished oil are the presence of oxygen or air, with the rate of auto-oxidation rising with increasing oxygen levels. Temperature is another factor, with the auto-oxidation rate increasing – as any other chemical reaction – as temperature rises. Prooxidants such as (heavy) metal ions are powerful catalysts for oxidation, decreasing the induction period and raising the rate of reaction. Light and time also influence the oxidation rate. External critical factors in oxidation include the processing set-up and
conditions, product specifications, and packaging and storage conditions.
Tackling critical factors
In order to tackle oil oxidation, various steps can be taken to address the critical factors which contribute to the process. Oxygen or air: Oxidation can be initiated at a very low oxygen level and steps should therefore be taken to avoid oil exposure to air during processing. Handlers should also avoid spraying in air during filling and emptying of storage or holding tanks. Proper agitation systems should be used in holding/storage tanks. Leakages at joints, fittings or faulty pump seals should be avoided, vacuum should be maintained where possible; the blowing of lines with air should be avoided or eliminated and nitrogen used instead; refined oil should be protected with nitrogen blanketing or sparging and antioxidants should be used where possible. Heat: Because oxidation accelerates with increasing temperature, it is important to keep an oil no warmer than necessary. Local overheating should be avoided by agitating the oil when it is heated and storage temperatures should be kept as low as possible. Oxidation
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08/02/2022 11:09:37
