Tackling oxidation

OXIDATION & ANTIOXIDANTS

Tackling oxidation

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 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.

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

Physical and Chemical Reactions during Deep-Fat Frying Source: ID&AFigure 1: Cycle of lipid oxidation

Figure 2: Breaking the cycle of lipid oxidation

Source: David B. Min, Ohio State University Figure 3: Physical and chemical reactions during deep-fat frying Source: David B Mn, Ohio State University Figure 4: Five stages of frying oil quality

occurs even at a very low temperature, such as in frozen meat or oily fish, which get rancid even when stored at –200C.

Pro-oxidants (metals): Copper is the most potent oxidation catalyst and, along with iron, should be kept to as low a level as possible. A chelating agent such as citric acid or phosphoric acid should be used for extra protection and iron, copper and bronze in systems should not come into contact with finished oils.

Light: Oil should be protected from exposure to light in closed vessels and single oxygen quenchers such as betacarotene and tocopherols can be added.

Time: Given sufficient time, any oil or fat will deteriorate even if handled under ideal conditions. The first in, first out (FIFO) principle should be applied in a strict way for oils and fats as well as finished products.

Improving oxidative stability

The strategies to improve the oxidative stability of oils and fats can be divided into three groups: 1. Modification of fatty acid profile 2. Additives 3. Surface management

Modification of the fatty acid profile includes choosing the right oil or fat for the application required. High oleic (HO) oils such as HO sunflowerseed, soyabean and canola oils are popular as they are more stable and still liquid. As trans fatty acids have been phased out, they are often replaced with saturates from palm oil, palm olein or palm oil fractions. Interesterified oils (or oleic and saturated fats) are developed for specific applications while in confectionery products, new cocoa butter equivalents (CBEs) and cocoa butter substitutes (CBSs) complement available products.

Additives fall into three categories. Metal chelators bind metal ions and include citric or phosphoric acids. Radical scavengers ‘absorb’ free radicals and include synthetic antioxidants (BHA, BHT), semi-natural antioxidants such as gallic acid and propyl gallate, and natural antioxidants such as tocopherols and rosemary extract.

Surface management of bulk oils and fats during storage includes filling tanks from the bottom; using nitrogen blanketing or sparging where possible; the use of dish or cone-bottom stainless steel tanks; applying the FIFO rule; eliminating residual oils as much as possible; washing equipment and tanks at least twice a year; and applying a maximum storage temperature of 300C for oils and 600C for fats.

The inert properties of nitrogen can be used to protect against oxidation when vegetable oils are stored. Sparging involves expelling any air entrained in the liquid by creating a saturated nitrogen level, preventing the uptake of oxygen in the oil. Blanketing using inert gas or padding protects liquid oils in storage tanks by filling the vapour space above the product.

Nitrogen (and nitrogen mixed with CO2 and oxygen) can be used in transport trucks and in modified atmosphere packaging (MAP) to extend the shelf life of packaged foods by preventing oxidation and moisture migration.

Conditions can also be adapted during

storage and transport to protect a product from air, light and temperature, depending on whether it is packaged in cans, cartons, glass or PET.

Oxidation during frying

Cooking can be divided into four main methods: • Roasting - with direct exposure to heat • Baking - with exposure to hot air • Boiling- with exposure to hot water • Frying – heating in oil or fat in the form of shallow pan frying or deep frying

Many physical and chemical reactions take place during deep-fat frying, producing different compounds (see Figure 3, previous page). These include hydrolysis and the formation of FFAs, monoacylglycerol (MG) and diacylglycerol (DG); oxidation producing aldehydes, ketones, alcohols, FFAs and oxidised triacylglycerol (TG); and thermal degradation, causing cistrans isomerisation, polymerisation, and producing cyclic and aromatic compounds.

The primary degradation compounds resulting from frying include FFAs, which can catalyse oil degradation and decrease the shelf stability of fried food. It is important to limit the smoke point of frying oil to a minimum of 1700C to tackle FFA formation.

Polar compounds are another degradation product and directly relate to the taste and quality of fried food, in which some oxidation and breakdown is necessary for the basic flavour of the fried product.

Surfactants rapidly speed up oil degradation and cause an oil to foam. Adding an anti-foaming agent such as poly-dimethyl siloxane (PDMS) acts as an oxygen barrier. PDMS at 100ppb is sufficient to form a continuous layer over the full surface of the oil. This reduces the oxygen concentration in, and at the oil surface, at cooking and frying oil temperatures. Polymers can also cause oil to foam and decrease heat exchanger efficiency; as well as increasing oil absorption in the fried product.

The Maillard reaction is a chemical reaction between amino acids and reducing sugars that gives browned or fried food its distinctive flavour. The reaction is needed for colour and crust development but can lead to increased acrylamide formation.

Industrial frying critical points

Industrial frying is different from home cooking as fresh oil is continuously added to the process. Heating should only be applied when needed and continuous filtration is recommended. The use of active absorbents to remove polar compounds can significantly increase the use time of frying oils in continuous processes.

Preferably, the oil leaves with the product after minimal use and fresh oil is fed continuously. Residual oils must be eliminated as much as possible and processing units washed on a regular basis.

Conclusion

The oxidation of oils and fats goes only one way. In processing, food manufacturers work towards the highest level of oil oxidation stability by selecting the right oil for the right application; using protecting additives and antioxidants; and by adopting surface protection. However, in cooking and frying, some controlled oxidation is necessary for the required taste and quality of a product. ● Dr Ignace Debruyne is a technical and market consultant for ID&A, Belgium. This article is based on a presentation he made at the Advanced Oils & Fats Processing and Application Technology Smart Short Course on 16-18 November 2021 www.smartshortcourses.com