Abstract
Although free amino acids (FAAs) represent a significant component of ripened cheeses and can provide useful information for their characterization, no inter-laboratory validated analytical method exists which allows a reliable comparison of data obtained by different laboratories and the adoption of quality control schemes based on FAA pattern. The objective of the present work was to test the effectiveness of an analytical protocol for the determination of the FAA composition of cheese and to verify the adequateness of this type of analysis for quality control procedures of Grana Padano PDO cheese as well as for research purposes. After an initial test to compare performances of ion-exchange chromatography (IEC) and HPLC techniques, an inter-laboratory collaborative study (seven laboratories, four samples) was organized to validate an IEC method with post-column ninhydrin derivatization and using l-norleucine as an internal standard. Determined amounts of individual FAA ranged from 8 to over 1380 mg/100 g cheese, with relative standard deviation of repeatability (RSDr) ranging from 0.5 to 4.6%, and relative standard deviation of reproducibility (RSDR) ranging from 1.3 to 9.9% for FAA concentrations over 100 mg/100 g. For lower concentrations, RSDr and RSDR were about thrice as high. On the basis of the results of this investigation, at present, the validated method is adopted as the official method for the determination of FAA patterns in the quality control of Grana Padano PDO cheese.
Highlights
Free amino acids (FAAs) are usually considered minor cheese constituents, they have been shown to contribute to sensory properties (Toelstede et al 2009; Zhao et al 2016), nutritional characteristics (Bottesini et al 2013), and physiological functions (San Gabriel and Uneyama 2013) of several cheese varieties
The mean values of the total content of the 17 free amino acids (FAAs) determined in the four test samples were comparable between the two techniques, but variability was much higher for HPLC data (Fig. 1)
Contents of individual FAAs approximately ranged from 50 mg/100 g cheese
Summary
Free amino acids (FAAs) are usually considered minor cheese constituents, they have been shown to contribute to sensory properties (Toelstede et al 2009; Zhao et al 2016), nutritional characteristics (Bottesini et al 2013), and physiological functions (San Gabriel and Uneyama 2013) of several cheese varieties. Protein is progressively degraded by a number of proteolytic enzymes including (1) chymosin, (2) indigenous milk proteases, and (3) proteases and peptidases from both starter (LAB) and non-starter lactic acid bacteria (NSLAB), mainly released after cell lysis (Borsting et al 2012; Gatti et al 2014). According to the manufacturing process and ripening period, up to 20–25% of the cheese protein may be split into FAAs, which can represent over 50% of the soluble N fraction (Sousa et al 2001; Pellegrino et al 2013). FAA patterns have been investigated as a possible tool for characterizing the ripening process. Some non-protein amino acids (AAs), principally ornithine, citrulline, and γ-aminobutyric acid, are formed that may represent characteristic traits of certain cheeses
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