A free radical is a molecule with a missing electron. When a fat molecule loses an electron, it becomes a fat free radical. An antioxidant donates one of its electrons to such a free radical. This stops a free-radical chain reaction that can lead to objectionable odors and flavors.
Selecting the appropriate antioxidant and dosage level can be a complex decision that involves evaluating several parameters, such as:
Antioxidants are generally categorized into two classes: primary and secondary. Primary antioxidants are capable of interrupting and terminating the free radical propagation step. The most common synthetic primary antioxidants are butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT), tert-butyl hydroquinone (TBHQ), propyl gallate and ethoxyquin.
The most common natural primary antioxidants are mixed-tocopherols and rosemary extract. Mixed-tocopherols are composed of four homologs: alpha-tocopherol (vitamin E), beta-tocopherol, gamma-tocopherol and delta-tocopherol. It is well recognized that the order of in vitro antioxidant activity is delta > gamma > beta > alpha. In addition, delta-tocopherol also has superior process stability and carry-through to gamma-tocopherol. In biological systems ( in vivo ), the alpha-tocopherol is most active.
Tocopherols have no legal limit, but can act as pro-oxidants if added at excessive levels. Tocopherols are expensive compared to synthetic antioxidants, and more importantly, they have less antioxidant activity compared to synthetics. Petfoods stabilized with tocopherols, as with all natural antioxidants, have a reduced shelf life compared to those stabilized with synthetic antioxidants. Other natural anioxidants are vitamin C, carotenoids, flavenoids, sulfides and thiocyanates.
Secondary antioxidants are chemicals that can prevent free radical formation. The main secondary antioxidants are metal chelators (citrates and phosphates) and reducing agents (ascorbates and sulfites).
Following are commonly used antioxidants, along with some of their pros and cons.
BHA. Pros: More effective than BHT, more heat stable than BHT and good oil solubility. Solubility in propylene glycol allows it to be mixed with high levels of propyl gallate and citric acid for oils that are more unsaturated. Cons: It is synthetic and it's susceptible to losses due to steam distillation.
BHT. Pros: Economical, good synergy with BHA and good oil solubility. Cons: It is synthetic and less effective than BHA. It's susceptible to losses due to steam distillation.
TBHQ. Pros: It is very effective in highly unsaturated fats such as fish and vegetable oils and it has excellent process carry-through. Cons: It is synthetic and lacks broad global regulatory approval. Can't be used in combination with propyl gallate.
Propyl gallate. Pros: Good alternative in highly unsaturated fats where TBHQ is not a desired or legal option. Good synergy with BHA and citric acid. Cons: It is synthetic and can form colored complexes with copper and iron ions. Poor process carry-through and poor oil solubility.
Ethoxyquin. Pros: Very effective antioxidant. Regulations allow higher dosage rates than other synthetics. Cons: It is synthetic and has a negative consumer perception. In 1997, the FDA requested a voluntary reduction of ethoxyquin in dog food from 150 ppm to 75 ppm. Usage has steadily decreased since the mid-1990s, to the point it's scarcely used.
Mixed-tocopherols. Pros: It is natural with good consumer acceptance. High heat stability, good process carry-through, low volatility and excellent solubility in fats. Cons: Increased diet formulation costs and increased application rate necessary. May require increased time to develop and maintain a unique supply chain.
Rosemary extract. Pros: It is natural with good consumer appeal. Effective in stabilizing unsaturated oils, such as fish and vegetable oils. Cons: Poor carry-through in extrusion processing, only regulatory approval is as a flavor or spice and there is a high level of variability amongst suppliers. Increased diet formulation costs and increased application rate necessary. May require increased time to develop and maintain a unique supply chain.
Increased usage of omega fatty acids has resulted in products more difficult to stabilize. Another challenge is a growing demand for natural products.
Oxidation is an irreversible process, so antioxidants should be added as early in the process as possible. Development of a quality supply chain and proper analytical testing procedures can assist in ensuring that oxidation does not impact product quality or performance.
By Lindsay Beaton
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By Lindsay Beaton