Oxidation and Rancidity of Fats and Oils
Kemin Europa N.V.
Fats and oils together with other lipid materials are important and relatively expensive components of many animal feeds. Lipid materials include fat soluble vitamins A and E, xanthophyll pigmentors for poultry and fish, essential oils used in flavours, and carotenes in lucerne hay and in silages.
There are three separate phases in the overall theory of autoxidation (Figure 1). In the first phase, initiation, free radicals are gradually produced. During this phase, there are no obvious gross changes in the composition of the fat. The second phase, propagation, occurs when a critical level of free radicals have been produced and a relatively rapid chain reaction starts. The rate of oxidation accelerates which indicates the autocatalytic nature of the reaction. There is a rapid absorption of oxygen and the generation of peroxides. The third phase, termination, comprises the recombination of various species of free radicals and has the effect of slowing down oxidation.
Figure 1. Three phases of autoxidation.
The degree of oxidation of a fat can be influenced by several factors such as environment, presence of metals and enzymes.
If fat contains more than 0.1% moisture, hydrolysis of triglycerides can occur generating free fatty acids which can be readily oxidised.
Metals such as copper and iron care very active in promoting free radical formulation.
Light provides a source of energy which may promote the oxidation of fats. Ultra violet light is the most effective. Photo-oxidation is much more rapid than classical autoxidation.
As with all chemical reactions, increasing temperature increases the rate of oxidation. However, cooling below ambient temperature will not necessarily prevent oxidation as it can occur at low temperature.
Contact with Air
A peroxide value of 20 meq 02/kg fat can be generated by the fat absorbing only 0.016% of its own weight of oxygen. Consequently, exposure to air should be kept to the minimum.
Oxidation may be promoted by lipase enzymes which split free fatty acids from triglycerides. Lipoxygenase enzymes catalyse the direct addition of oxygen to fatty acid molecules producing hydroperoxides.
Both lipases and lipoxygenases occur in may plant seeds and in by-products such as soyabean and rice bran. Lipoxygenase may also catalyse the co-oxidation of carotenes which are important constituents of forages.
The first products of autoxidation of a fat are odourless, tasteless, peroxides and hydroperoxides (Figure 1). Oils and fats from different sources oxidise at greatly different rates (Table 1). This makes it very difficult to predict the oxidative stability of a specific fat or fat blend.
Oxidation of Different Fats
At ambient temperature however, hydroperoxides break down and produce a variety of hydrocarbons, aldehydes, ketones and alcohols. The production of these end products is the chemical manifestation of rancidity. The undesirable flavours in rancid materials can be caused by very small quantities of only a few ppm of aldehydes and ketones. Therefore, the amount of lipid in a feed mixture is actually less important than its nature and susceptibility to oxidation.
EFFECT OF OXIDATION AND RANCIDITY
Oxidative rancidity decreases the nutritional quality of feeds and may generate toxic materials. In particular oxidised sterols, aromatic hydrocarbons and carious carbonyl compounds such as malondialdehyde may have toxic properties.
Lipid peroxides and the free radicals, they destroy fat soluble vitamins A and E. They react with sulphydryl bonds in proteins leading to a decrease in protein quality.
Oxidised fats can have a negative influence on the deposition of xanthophylls in egg yolks. Oertel and Hartfiel showed that there was a direct relationship between the peroxide value of feed fat and the amount of xanthophylls deposited in egg yolks. The more oxidised the fat, the less the deposition of xanthophylls.
An antioxidant works by reacting with free radicals as they are formed (Figure) and converting them back to the original substrate. Free radicals of antioxidant molecules are formed in this process. However, these free radicals are relatively stable and do not have enough energy to react with the fat to form new free radicals. This type of antioxidant molecules are known as free radical scavengers. They include natural materials like tocopherols and the conventional antioxidants such as propyl gallate, BHA, BHT and ethoxyquin. Compounds such as ascorbic acid and ascorbyl palmitate can exert a antioxidant effect by reacting with oxygen. These have been termed oxygen scavengers.
FIGURE 2. Effect of Various Antioxidants on Peroxide Values Active Oxygen Method: Beef & Tallow From IOWA (200 ppm) Antioxidant
An antioxidant will only be really effective if it is added to the fat during the initiation phase (Figure 1). Therefore the antioxidant should be added to the fat as early as possible to produce the maximum effect.
Antioxidants will not prevent the formation of free fatty acids which are produced by the chemical hydrolysis of fats. Antioxidants will not improve the flavour of fat already oxidised.
Antioxidants can be used to treat many different raw materials. They can be added to fats and oil, incorporated into meat meals and fishmeal, incorporated into vitamin and pigmentor products and into premixes. Antioxidants are used during the tanning leather, to preserve carotenes in forages. Many human food and pet food products also frequently contain antioxidants.
BAROX Liquid has been extensively studied in a variety of different fats. It has given good efficacy in poultry fat, beef tallow, bone fats, fish oil and fishmeal.