Maillard, reactive lysine, amino acid digestibility


Feed cost represents 70% of the total cost of poultry and pork production (Patience et al., 2015); therefore, a number of processing techniques have been developed to maximize utilization of nutrients in feed ingredients and diets for optimum animal growth performance. Oilseed meals are commonly exposed to varying degrees of heat to remove solvents used during oil extraction, increase nutrient digestibility, improve storage life, and to reduce anti-nutritional factors (Liener, 1994; Rehman and Shah, 2005; Goebel and Stein, 2011). Heat is also applied in the production of distillers dried grains with solubles (DDGS) from dry grind ethanol facilities during the liquefaction and drying stages (Bothast and Schlicher, 2005). Thus, heat treatment of many processed feed ingredients is a necessity. However, excessive heat results in reduced nutrient digestibility due to formation of Maillard reaction products (González-Vega et al., 2011; Pahm et al., 2008). Maillard reaction involves the reaction of reducing sugars with the terminal/Ɛ-amino group of amino acids (AA; particularly Lys), and products of the Maillard reaction can render AA in diets and/or feed ingredients unavailable (Finot and Magnenat, 1981). Factors affecting the rate of formation of Maillard reaction products include temperature, pH, substrate, and water activity, and each of these factors may affect the kinetics of the reactions in specific ways. Caramelization, which involves the oxidation of sugars and is another form of heat damage may also take place (Pahm et al., 2008). Formation of enzymatically resistant bonds between ε-amino groups of Lys and carboxyl groups of Glu and Asp can also occur, and these indigestible peptides can reduce protein digestibility and block AA transport across the intestinal wall (Dworschák, 1980). Other consequences of heat damage include conversion of some of the L-forms of AA into D-forms (racemization) and formation of lysinoalanine (Hirano et al., 1973; Meade et al., 2005). The objective of this contribution is to review the current understanding about heat damage, stages and products of Maillard reaction, and measurement of reactive Lys in diets and feed ingredients. Effects of heat damage in feed ingredients and diets on nutrient digestibility and growth performance of pigs will also be discussed.