Rancidity is associated with breaking the double bonds of polyunsaturated fats and the release of free radicals such as volatile aldehydes, ketones and epoxides
Oxidation is an autocatalytic process that occurs mostly in fats, due to the reaction of oxygen from the air with unsaturated fatty acids. That is the reason it is commonly known as lipid autoxidation.
As a result of this oxidation, free radicals peroxides are produced, which are the primary reaction products. In a second phase, lipid degradation and development of oxidative rancidity occurs.
This is characterized by the occurrence of undesirable odors and lower feed palatability.
→Several authors have observed a direct effect between levels of peroxides above 7.10 meq / kg in oxidized feed and growth depression in chicken (cabel et al., 1988; Inoue et al., 1984; Takahashi and Akiba, 1999) . This effect has also been described by other authors (Inoue et al., 1984).
In addition, high levels of oxidation feed are associated with the presence of encephalomalacia, exudative diathesis, muscular dystrophy and tissue necrosis in chickens by lack of vitamin E, very sensitive to oxidation. It also has been dsclaimed a decline in fertility and hatchability egg (Cabel et al., 1988).
Antioxidants effectiveness is based on their performance in the early stages of autoxidation
The use of antioxidants in food can prevent the oxidation of food for animals and the consequences of free radical ingestion by the animal
Figure 1&2. Effect of feed oxidation level on chicken growing (Cabel et al., 1988)
Also, antioxidants may also have an effect on the antioxidant activity in vivo, thus participating significantly in the stability and lifespan of the meat.
→The choice of an antioxidant is based on its effectiveness in preventing oxidation in raw materials for feed, in the manufactured feed, and at last in the intestinal tract of the animal and the meat carcases for consumptions.
More specifically, antioxidants incorporated into the animal's diet should meet the following requirements (FAO, 1978):
Must warn the deterioration of lipids of animal and vegetable origin, vitamins, carotenoids and other components susceptible of autoxidation.
Their use must be safe, with no toxic effects observed at the doses used.
Must be effective at low doses.
Must have a cost that makes feasible their use in practice.
The use of antioxidants should reduce the consumption of antioxidant nutrients such as selenium or vitamin E (tocopherols) or other nutrient deficiency susceptible to oxidation auch as vitamin A.
Classification of antioxidants
While there have suggested different types of classifications of antioxidants by their mode of action, in general terms we can speak of the following types:
Primary or preventative: They are those that prevent the formation of free radicals. Include metal chelators, which contribute to the effectiveness of secondary antioxidants or terminators.
Secondary or terminators: They intercept free radicals breaking the chain reaction of the oxidation propagation phase. They constitute the bulk of antioxidants in the industry. They are mainly phenolic compounds, electron donating or proton as hydrogen ions (H +), which react with newly formed free radicals to stabilize them.
In this second category it highlights the Ethoxyquin although other synthetic antioxidants are also used commercially, primarily BHT, gallates and BHA. Among the natural antioxidants the most popular are tocopherols, followed far behind by other aromatic plant extracts (e.g. rosemary extract).
Invariably, antioxidants base their effectiveness on lower performance in the early stages of autoxidation. In fact, to reverse the so call oxidation propagation phase, in which free radicals interact with other lipids and exponentially peroxides increase occurs, will require much higher levels of antioxidant. In addition, they can not avoid the organoleptic effects caused by lipids already oxidized and degraded.
Antioxidant protection in vivo and meat quality
The quality of a food product is determined by the set of characteristics that can influence the preference or consumer acceptability.
If we refer to sensory or organoleptic properties, important factors are the appearance, texture and flavor (Carreras, 2004). On this regard, the oxidation state and antioxidant capacity in animals tissues constitute a very important factor that affects sensory properties and meat lifespan (Morrissey et al., 1998).
In fact, lipid oxidation and the consequent oxidative rancidity are one of the main causes of deterioration of food for human consumption, along with microbial development.
→Lipid oxidation, apart from producing unwiseable odors is also responsible for alterations in flavor, texture, consistency and nutritional value (Fellenberg y Speisky, 2006).
Oxidative rancidity is especially critical in fresh meat products, due to the current trend of promoting an increase in polyunsaturated fat content (with multiple double bonds in the chains of fatty acids) because of their dietary benefit over saturated fats.
This problem is much more marked in the case of poultry because lipid oxidation of fatty acids of phospholipids of muscle tissue is influenced by a wide variety of specific species factors, such as decreased activity of the enzyme glutathione peroxidase or low levels of α-tocopherol (Daun y Akesson, 2004).
These factors make oxidation more important in birds than in pigs or ruminants, being in processed products of these species where the effects are more noticeable. Oxidation in vivo in the animal tissue is also affected by the composition of the diet. Thus, a high feed consumption with high content of oxidized lipids and polyunsaturated fatty acids , as well as prooxidant components or decreased consumption of dietary components involved in the antioxidant defense system, contribute to oxidation in vivo animal oxidation and post-mortem meat (Morrissey et al., 1998 o Stem et al., 2008).
In fact, the use of oxidized lipids in the diet can not manifest itself in terms of production in vivo but instead can lead to rusty channels of lower quality (Table 1) and also more perishable.
Table 1. Effect of Oxidation of oxidized oil in the diet on the values of metabolizable energy and level of oxidation in chicken fat (Racanicci et al., 2004).
→Also, the level of oxidative stress and lipid oxidation in vivo directly affect the quality of meat afterwards (Morrissey et al., 1998).
In this regard, it is well known antioxidant effect of vitamin E and selenium from the diet of the animal, which is explained by the α-tocopherol position within cell membranes. This membrane localization is not achieved with the addition of vitamin E in the flesh (meat), so only from the diet it has antioxidant effect.
However, it has also been indicated that a high unsaturation of fat as in the case of poultry products, decreases the α-tocopherol deposition in chicken meat (Table 2).
Table 2. Effect of level of polyunsaturated fatty acids (PUFAs) and vitamin E supplementation in the deposition of tocopherol in chicken meat (Cortinas, 2004).
The limitation of available vitamin E from the diet can be partly supplemented with the addition of other antioxidants. Thus, it has been observed that the combination of vitamin E with synthetic antioxidants as BHT or Ethoxyquin can increase the stability of the meat and fat in broilers carcasses (Table 3).
Table 3. Effect of α-tocopherol acetate (ATA), BHT and Ethoxyquin and a combination of both, in levels of α-tocopherol and oxidative stability in chickens carcase.
The oxidation of lipids in poultry production affects both the feed consumed by the animal, as in vivo, in the animal itself and / or later in the quality and lifespan of products of animal origin intended for human consumption.
The incorporation of antioxidants in the feed allows to reverse the deleterious effects of lipid oxidation in all these stages; their effectiveness depend on the type of antioxidant used and their interactions with other system antioxidant protection components of the animal.