The oxidation in feed ingredients such as fats and oils, feed premixes and complete feeds can significantly decrease their dietary value. Oxidation is a "weak point" of almost all fats and vegetable oils, even those containing natural antioxidants. The end results of oxidation are loss of nutrient quality, impairment of animal health, reduction in animal performance, and increased production cost. The detrimental effects of the oxidation process can be prevented through the addition of antioxidant compounds and their synergists. Antioxidant protection helps guarantee that animals receive the quality feed formulated by the nutritionist for maximum growth and profitability.

In many countries, nutritionists are obliged to use fats and oils or fat rich ingredients, such as fishmeal, carcass meal, poultry by-product meal and vegetable-origin products to achieve a balanced least-cost formulation. The nutrient value of feed containing these ingredients can be extremely variable, as the fat content may deviate from tabular values and rancidity frequently degrades the energy content and the availability of amino acids. In addition, the formation of free radicals during peroxidation of fats destroys critical nutrients, such as vitamins A, D and E. The quality of products may be affected by the evolution of aldehydes and ketones, which impart off-flavor to poultry meat, and the pigmentation of broiler skin and egg yolks is invariably reduced.

AUTO-OXIDATION PROCESS?

Auto-oxidation is a spontaneous reaction of atmospheric oxygen with fatty acids, which leads to cmplex chemical changes that eventually manifest themselves in the development of off-flavours in food/feed. Oxidation process is divided nto three stages Induction, Propagation and Termination.

Heavy metals present at low levels in fats, oils and other fatty ingredients can promote autoxidation. In oxidation process oxygen reacts with unsaturated fatty acids giving rise to free radicals. Initially, peroxides are formed which in turn break down to hydrocarbons, ketones, aldehydes, and smaller amounts of epoxides and alcohols. The result of the oxidation of fats, oils and fat rich ingredients is the development of objectionable flavors and odors characteristic of the condition known as "oxidative rancidity."

LEVEL OF OXIDATION

Peroxide value is one of the most important criteria used to monitor the oxidation process. In the course of oxidation, the peroxide value first increases very slowly (the induction period). When this period ends, the peroxide value starts growing very rapidly, and signs of deterioration become evident. The induction period varies for different fats and oils depending on their chemical composition. Even for the same fats and oils, it can vary, depending on such pro-oxidative factors as light, temperature, metal traces, size of contact surface between air and fats, etc.
The efficacy of antioxidants can be expressed as a protective factor (PF):

This formula shows that the role of antioxidants is to extend the induction period and thus prolong the product's shelf life.

SIGNIFICANCE OF OXIDATION

Oxidation may be important at several different stages of raw material processing and feed production and the bird's performance. Extensive research has defined the mechanisms of oxidative rancidity and its deleterious effect on reproduction, feed conversion, livability and the profitability of intensively reared poultry.

Oxidation can lead to heating and pyrolysis during storage of fats / oils / premixes / complete feed that can severely reduce the nutritional value of feed.

The presence of highly reactive free radicals in feed will result in vitro degradation of nutrients such as the vitamins A and D, biotin and also the tocopherols (Vitamin E) that act as intracellular and biological antioxidants.

Long-lived free radicals in-vitro convert into ketones and aldehydes, which may impart an abnormal flavor to feed, depressing intake and growth rate.

In addition to the triad of conditions resulting from oxidative rancidity, consumption of feed containing free radicals will seriously impair the immune system.

Ultimately birds fed diets containing oxidized ingredients/feed results in poor performance which is due to both reduction in the nutritive value and the deleterious effects of peroxides on cells.

NEED FOR PREVENTION OF OXIDATION IN FEED

Under normal conditions, inherent biological antioxidant systems degrade free radicals. Superoxide dysmutase, enzymatically converts free radicals with hydrogen into hydrogen peroxide and water. Subsequently, other enzymes, including glutathione peroxidase and catalase, degrade hydrogen peroxide into water and molecular oxygen. The inactivation of free radicals by natural enzyme systems is usually adequate to prevent oxidation and generally a balance exists between evolution and destruction of radicals at the cellular level. Ingestion of rancid feeds containing exogenous free radicals result in in-vivo peroxidation, which overwhelms the natural mechanisms that inactivate free radicals.

Therefore, fats, oils, premixes and complete feeds need to be protected against oxidation. This has lead to the development of variety of products known as ANTIOXIDANTS. These can help to overcome the problem associated with rancid fats/oils or feeds.

PREVENTION OF DETERIORATION OF FATS / OILS AND FEED

The following factors have to be kept in mind when we consider different methods of prevention

Commonly used additives to prevent the deterioration of fats/oils and feed are,

Antioxidants and preservatives act independently of each other. While antioxidants prevent the deterioration of feed by hindering the oxidation process, antimicrobials does the same by eliminating the microbes that are responsible for deterioration.

ANTIOXIDANTS - HOW THEY WORK?

ANTIOXIDANTS are those substances that extend the induction period of autoxidation process and thus help to reduce destruction of the fats/oils/fat rich ingredients.

Antioxidants delay rancidity by binding to oxygen molecules and therefore making them unavailable to free radicals. They also function by reacting with initiating and propagating free radicals and forming non-reactive substances that lay dormant within the substrate. The remarkable characteristic of antioxidants is that they have optimum effectiveness in low concentrations in oil. This can be attributed to the fact that when great quantities of antioxidants are added to an oil, they act as prooxidants.

Antioxidants can be classified into four groups: Primary, Synergistic, Secondary, and Miscellaneous. Primary antioxidants function by terminating the free-radical chain reaction. They donate hydrogen or electrons to free radicals and then convert them into more stable products. They may also function in reactions with lipid radicals, forming lipid-antioxidant complexes. Most primary antioxidants are chemical antioxidants, such as phenols (e.g. nordihydroguaiaretic acid), ‘hindered' phenols (e.g. BHA and BHT), and other miscellaneous primary antioxidants (e.g. ethoxyquin). Phenols are benzine rings, compounds that contain hydroxyl groups and that are highly reactive.

Synergistic antioxidants are broadly classified as oxygen scavengers and chelators. Synergists may act as hydrogen donors to free radicals, which regenerate the primary antioxidants. Synergists may also provide an acidic medium that improves the stability of primary antioxidants. Oxygen scavengers include ascorbic acid (vitamin C), ascorbyl palmitate, sulfites, and erythorbates. These react with free oxygen and remove it from a closed system. Chelators, such as citric acid, phosphates, and tartaric acid, are not true antioxidants but are highly effective as synergists with both primary antioxidants and oxygen scavengers. They react with prooxidant metals like iron and which raises the activation energy of initiation reactions. These compounds enhance the effectiveness of primary antioxidants or inhibit the effect of pro-oxidants.

Secondary, or preventive, antioxidants include esters (the product of a reaction between an alcohol and an acid to form a solid that is soluble in oil), which function by decomposing the lipid peroxides into stable end products. Ascorbic acid can be reacted with alcohol so that it forms a salt, or ester.

Miscellaneous antioxidants such as flavonoids (metabolites in plants that occur in almost all plant parts) and related compounds and amino acids function as both primary antioxidants and synergists.

Both Natural as well as Synthetic antioxidants have been studied for their antioxidant effect and are used in the feed.

NATURAL ANTIOXIDANTS

Natural antioxidants are those that exist naturally in the feed ingredients. Though there are a number of them, only few are used as additives commercially for combating the oxidation in the feed. Tocopherol and ascorbic acid or its salts are listed as allowed feed additives. Natural tocopherols in vegetable oils are mixtures of alpha-, beta-,gamma-, and delta-tocopherols. The antioxidant activity and stability of these varies with their structure. In-vivo, the vitamin E activity of the tocopherols decreases in the order alpha > beta > gamma > delta, but in oil stabilization studies, delta-tocopherol has often been quoted as the most effective antioxidant.

Delta-tocopherols consumed more slowly and are therefore active for a longer period of time, whereas œ-tocopherol may be more active initially but is consumed more rapidly. Ascorbic Acid acts as an antioxidant by removing oxygen and also by reducing free radicals. Despite having good antioxidant activity, their usage in the feed is limited by certain factors as describe below.

LIMITING FACTORS OF NATURAL ANTIOXIDANTS