Analysis of Maillard reaction products
Analysis of Lysine
Lysine is one of the most essential amino acids and fairly limited in a lot of food and feed stuff and the first limited amino acid in diets for production animals. Lysine possesses a reactive amino group at the end of its side chain (e-amino group) which can react with other compounds to form nutritionally unavailable products. When reducing sugars are involved Maillard reaction products can be formed.
Other amino acids are also thought to undergo Maillard type reactions, for example, proline, tryptophan and arginine but these reactions are less well studied. Maillard reactions products are named after their discoverer: L.C. Maillard who in 1912 reported about color formation in aqueous solution with sugar and amino acids, also known as non-enzymatic browning, which should not be mistaken for caramelisation, another type of non-enymatic browning. Since further acknowledgement of Maillards findings in the fifties, the importance of Maillard chemistry has been widely shown in food science, nutrition and medicine.
In human nutrition, most protein sources have been processed in a factory and are probably heat treated, which facilitates the formation of Maillard reactions. Although these reactions occur at any temperature, the rate is significantly accelerated at higher temperatures.
Lysine that reacts with other compounds is no longer available for digestion in the organism. Basically the occurrence of Maillard reactions lead to:
- Loss of nutritional quality, due to the availability of lysine.
- Loss of safety. Toxic and physiological effects have been reported about the presence of Maillard reaction products. But also compounds with anti oxidative and anti mutagenic effects been reported in milk and milk products.
The scheme of Maillard reaction is quite complex and consists in general of 3 stages:
- Early stage, formation of an (unstable) Amadori compound: fructosyl-lysine (from glucose) or lactulosyl-lysine (from lactose) or maltulosyl-lysine (from maltose).
- Advanced stage, breakdown of an Amadori compound, resulting in a variety of products which formation is pH dependent. One significant compound to be formed at lower pH is hydroxymethylfurfural (HMF). At higher pH it’s thought that the “Strecker degradation” will occur where di-carbonyl compounds react with amino acids to yield aldehydes and carbondioxide eventually. In addition, it’s thought that 1,2-dicarbonyls targets the guanidine side chain of arginine, resulting in numerous possible compounds.
- Final stage, formation of melanoidins from reaction products and amino compounds resulting in a typical brown colour and flavor enhancement.