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DATE: 2006
CITATION: Pamela A. Williams, Suzanne M. Hodgkinson, Shane M. Rutherfurd, Wouter H. Hendriks - The Journal of Nutrition, Volume 136, Issue 7, 1 July 2006, Pages 1998S–2000S
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The protein quality of a food is the product of its amino acid content and the nutritional availability of these amino acids. Heat processing has been shown to have a negative impact on the nutritional value of the amino acids in animal feeds (1,2). In foods that are heat processed or stored for long periods of time, the free ε-amino group of lysine can react with the carbonyl group of other compounds such as reducing sugars. The complex that is formed (Maillard complex) may be partially absorbed but cannot be utilized by the animal (3). Moreover, a proportion of the complexes formed can revert back to lysine during conventional amino acid analysis, which does not occur in the digestive tract of the animal. This results in an overestimation of the amount of lysine that is available to the animal if it is determined using conventional methods. A method in which the lysine content with a free ε-amino group in foods is measured [such as the O-methylisourea (OMIU)-reactive lysine assay] provides a superior estimate of the lysine available to the animal (4).
The production of commercial pet foods can involve heat treatments including sterilization, extrusion, cooking, and baking. During these heat treatments and the subsequent storage of the food, Maillard complexes may form, thereby reducing the availability of lysine for the animal. In addition, most pet food manufacturers use by-products of the meat, fish, and milling industries as primary ingredients, which, because of the processing required in their manufacture, can include lysine with a blocked ε-amino group. Although some pet foods are formulated using large proportions of non-heat-treated ingredients such as meat, lamb, rice, and corn, the manufacture of pet foods still promotes a reaction between the ε-amino group of lysine and other compounds. Although pet foods are extensively heat treated, little research has been conducted into the effects of the various heat processes on the nutritional value of pet foods.
The objectives of this study were to determine the total and reactive lysine content of commercial dog foods and to determine whether a relation exists between lysine availability and product price.