Availability of amino acids include digestibility, chemical integrity (i.e., absorbed in an utilizable form) and freedom from interference in metabolism (i.e., presence of specific dietary anti-nutritional factor limiting protein synthesis). The greatest source of variation in amino acid availability is usually digestibility. The most susceptible amino acid to heat damage is Lys which is involved via its ε-amino group in the Maillard reaction. Lysine can also react with other amino acid side chains such as the reaction with cysteine to form lysinoalanine. Analysis of reactive lysine allows accurate estimation of the available Lys content of heat-damaged ingredients although recent research has identified some inaccuracies in the reactive Lys assay. Met and Cys can be oxidized via their side chain sulphur atom upon heat treatments with some of the oxidized forms being poorly available. Heating of proteins can convert the L- to the D-enantiomer of an amino acid (racemization), the latter which is poorly available. Free asparagine, present in relatively large amounts in certain plant-based feedstuffs, can form acrylamide during heat processing.
Before any feed processing occurs, various feed ingredients used in animal nutrition have already been processed to a smaller or larger extent which often affects the bioavailability of amino acids. Examples include soybean meal, DDGS, rapeseed meal but also commonly used grains (e.g. corn and wheat). Further processing (pelleting, extrusion, storage) of compound feeds can further affect the availability of amino acids via various reactions with the matrix.
