26/05/2017: Feed: The potential of NIR to measure reactive lysine

Figure 2: Correlation between autoclaving time and total and reactive lysine content of a soybean meal sample.

 Lysine
 Lysine is an essential amino acid for poultry and swine, meaning it cannot be produced via transamination and must be supplied to the animal through dietary intake. Lysine is usually a limiting amino acid if not supplemented in synthetic form, and due to its high concentration in tissue deposition, formulation of diets for amino acid requirements often relies upon ratios between lysine and all other amino acids. The total lysine that has been measured in the raw material or finished feed is not a measure of the bioavailability to the animal; this only comes with measuring the performance response to lysine in animal trials.

 Apparent ileal digestibility gives a measurement of the apparent disappearance of lysine in the small intestine. Standardised ileal digestibility, which accounts for basal endogenous losses, is widely accepted as the most advanced method for quantification of the dietary-origin amino acids digested and absorbed in the small intestine. Synthetic lysine is often added to diets, based on price, to supply animal requirements.  

 The Maillard Reaction

 Reducing sugars are able to bind to free amino groups found on amino acids, especially the ε-amino group found on lysine. This is known as the Maillard reaction and results in the production of modified lysine, such as Amadori and Maillard products, including furosine and carboxymethyl-lysine (see Figure 1). The Maillard reaction is an irreversible reaction, which makes the lysine unavailable for digestion. Reactive lysine, the lysine that has not undergone the Maillard reaction and is metabolisable, can be described as unmodified lysine which possesses a free side chain amino group and can be either free or protein bound.

 In the past, reactive lysine has been called a number of names including chemically reactive lysine, chemically available lysine, available lysine and total available lysine. Measuring total lysine in feeds involves the acid hydrolysis of the proteins present; during hydrolysis some of the damaged lysine can be released and analysed as lysine. However, as discussed above, this lysine is not available to be metabolised by the animal. To identify the lysine that can be both digested and metabolised by the animal, ileal digestible reactive lysine must be determined.

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