The growing demand for protein-containing products for food and feed requires an efficient use of current protein sources and therefore accurate determination of protein quality. In animal feed, by-products are often used as ingredient after processing, for example to extract oil from oil seeds. Processing may reduce protein quality by inducing chemical reactions, such as binding of sugars to amino acids, which is currently not accounted for in pig feed formulation. Lysine is the most sensitive amino acid to react and also often the first limiting amino acid for body weight gain in pig feed. It is, therefore, important to determine the effect of processing on the amount of (non-)available lysine and subsequently its effect in pigs. This thesis describes a chemical method to determine the level of available lysine and addresses its. Moreover, processing effects on amino acid digestibility and utilization, after absorption, for protein synthesis in growing pigs are described.