The application and testing of antioxidants in pet food
In order to preserve palatability, pet food producers should pay attention to controlling the lipid oxidation of fats to ensure the shelf life of pet food and susceptible ingredients.
What is lipid oxidation?
Feed can become rancid as the fats in the products undergo chemical as well as physical changes. Lipid oxidation is a common undesirable chain reaction consisting of three main phases: initiation, propagation and termination. During this oxidation process, unsaturated fatty acids are slowly oxidized. As a result, a series of breakdown products are produced, which can lead to a typical rancid flavour with less palatability.
What are antioxidants?
In order to extend the shelf life of pet food and ingredients, it is useful to add antioxidants to susceptible materials. Antioxidants are classified as feed additives (European Union Register of Feed Additives, Annex I of Regulation (EC) No. 1831/2003) and defined as substances that delay the oxidative degradation processes of feedstuffs and thus improve oxidative stability. In pet food, these primarily include propyl gallate, butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT) and ascorbyl palmitate as synthetic antioxidants, as well as tocopherol-rich extracts of natural origin as natural antioxidants. Plant extracts like rosemary are not approved as antioxidants, even though they partly display antioxidative effects. Moreover, chelators such as citric or phosphoric acid are added to bind positively-charged ions of transition metals and thus delay the spoilage process.
Causes of oxidative rancidity
The oxidative stability of fats is affected by several factors. Light, temperature, presence of transition metals and the oxygen in the air, as well as fatty acid composition, can increase the rate of lipid oxidation. For example, fats become more sensitive to oxidation as the level of unsaturation and the length of the fatty acid chain increase.
Measuring lipid oxidation
Selecting the optimum test is difficult due to the complexity of the lipid oxidation process. A variety of methods are available to measure the quality of fats and feed, e.g. oxidative stability tests or determination of primary (peroxides) and secondary oxidation products (aldehydes and other volatiles). It is generally difficult to predict the stability of fats or feed by using only one test, so method selection and data interpretation require careful cooperation between the laboratory and the pet food/fat producer.