According to the Animal Nutrition Group at Wageningen University in the Netherlands, studies show drying pet food at 160°C (320°F) to 180°C (356°F) can significantly reduce its nutritional value.
In the smaller size kibble used in the study (4 mm or about .16 inch), a drying temperature of 200°C (392°F) lowered concentrations of the following amino acids:
- Total lysine
- Reactive lysine (lysine that has remained unchanged during processing)
The same size kibble dried at 120°C (248°F) had a higher ratio of reactive to total lysine than the kibble dried at the higher temperature.
The drying temp of 200°C also decreased concentrations of linolenic (omega-3) and linoleic (omega-6) fatty acids, and increased the concentration of oleic acid (omega-9 monounsaturated). The increase in oleic acid may point to lipid oxidation of the smaller kibbles during the drying process. Lipid oxidation can create off-flavors and aromas, as well as potentially toxic compounds.
Larger size kibble (8 mm or .31 inch) showed only a lowered concentration of reactive lysine, according to study results.