Fluorometric determination of chemically available lysine: Adaptation, validation and application to different milk products

Abstract

A spectrophotometric method based on the reaction between available lysine and ortho-phthaldialdehyde (OPA) was adapted and validated for fluorometric determination of the chemically available lysine contents in milk matrices (UHT and conventional in-bottle sterilized cow milk, milk-based infant formulas and infant formula ingredients). The values of the analytical parameters show its usefulness as a routine method (linearity, r = 0.9992; detection limit, 0.0066 mg/mL assay; accuracy, 99-108%; precision, intra-day 2.1-5.9% and inter-day 3.5 10.2%). No statistically significant differences (p < 0.05) were found between the values obtained with the adapted method and those obtained applying the 1-fluoro-2,4-dinitrobenzene (FDNB) (Carpenter) technique. The OPA method was used to measure the chemically available lysine contents in UHT and sterilized milk marketed in Spain, to study the evolution of chemically available lysine during the shelf-life of UHT milks, and finally the quality of name- and store-brand UHT milks was also compared. No statistically significant differences (p < 0.05) were found between either the available lysine contents of the same type of UHT or sterilized milk or between store- and name-brand UHT milks. Statistically significant differences (p < 0.05) were found between the chemically available lysine contents in UHT and sterilized milk. Losses of chemically available lysine ranging from 2.7 to 29% were obtained during the shelf-life of UHT milk.

Biorefinery process for protein extraction from oriental mustard (Brassica juncea (L.) Czern.) using ethanol stillage

“…Lysine availability of extracted protein was measured using a fluorometric technique ( Ferrer et al 2003 ). A reconstituted protein sample (50 μl) containing 0.3-1.5 mg of protein was mixed with deionized water (950 μl), and then 1 ml of SDS solution (120 g/l) was added.…”

Available lysine content in human milk: Stability during manipulation prior to ingestion

“…Determination of available lysine: A fluorimetric method was used to measure available lysine by the method proposed by Goodno et al [32] with the modifications by Ferrer et al [33].…”

Impact of the spray drying conditions and residence time distribution on lysine loss in spray dried infant formula

“…Available lysine was measured using the o-phthaldialdeyhde method described by Ferrer et al (2003). At alkaline conditions and in the presence of βmercaptoethanol, o-phthaldialdehyde (OPA) reacts with free amino groups, e.g., the ε-amino group of lysine and a fluorescent isoindole is formed.…”