Determination of free amino acids in beers: A comparison of Czech and foreign brands

Characterization of aroma compounds and effects of amino acids on the release of esters in Laimao baijiu

BACKGROUND

Laimao baijiu is a typical soy-sauce aroma-type baijiu in China. Amino acids are non-volatile compounds in baijiu and are beneficial to human health. Aroma is one of the important indicators that are used to evaluate the quality of baijiu. The interaction between aroma-active compounds and non-volatile compounds can also affect the release of aroma compounds. In this study, we identified the active-aroma compounds and amino acids in Laimao baijiu by stir bar sorptive extraction (SBSE), gas chromatography-olfactometry (GC-O), gas chromatography-mass spectrometry (GC-MS), and ultra-performance liquid chromatography-mass spectrometry (UPLC-MS). The interaction between amino acids and key esters was investigated by sensory analysis and partition coefficients.

RESULTS

A total of 63 aroma compounds and 21 amino acids were identified. Twenty-one esters were identified from them as major aroma-active ester compounds with odor activity values ≥ 1. Finally, sensory analysis revealed that l-alanine had a significant effect on the strength of the aromas of esters, suggesting that low concentrations of amino acids were more likely to promote the release of esters and high concentrations were more likely to inhibit this. The partition coefficient can be a good explanation for this phenomenon.

CONCLUSION

l-Alanine can significantly affect the aroma intensity of key ester aroma compounds in Laimao baijiu, and the effects of different concentrations of amino acids are different. This work shows that amino acids, as non-volatile compounds, have a regulatory effect on the release of aroma compounds in alcoholic beverages, which may provide new technical support for the aroma modulation of alcoholic beverages. © 2022 Society of Chemical Industry.

Amino acids and glycation compounds in hot trub formed during wort boiling

The aim of this study was to investigate the amino acid composition and the amount of individual glycation compounds in hot trub formed during boiling of wort prepared from different malts. Compared to the initial amino acid composition of the used malts, some Maillard reaction products (namely MG-H1, pyrraline) and hydrophobic amino acids (leucine, isoleucine, valine, phenylalanine) accumulated in the hot trub, whereas hydrophilic amino acids remained in the boiled wort. For MG-H1, a threefold increase was observed during wort boiling, whereas the other Maillard reaction products, namely CML, CEL, pyrraline and maltosine increased only slightly (1.1–2-fold). Furosine as a hallmark for peptide-bound Amadori compounds showed a small decrease. The results suggest that mainly glycated amino acids derived from small dicarbonyl compounds such as methylglyoxal and glyoxal are formed during wort boiling. Furthermore, the studies indicate that the modification of the protein structure as a result of the Maillard reaction has an influence on the hydration of the denatured proteins during the wort boiling process, thus affecting the coagulation process and, therefore, precipitation of the hot trub. The work carried out contributes to the understanding of the chemical reactions influencing the amino acid and Maillard reaction product transfer from malt to beer.

Graphical abstract

Comparison of the ionisation mode in the determination of free amino acids in beers by Liquid Chromatography tandem mass spectrometry

A comparison of positive and negative ionization modes in LC-ESI-MS/MS was carried out for the analysis of derivatized amino acids in 15 different beer samples. 22 free amino acids were derivatized using Diethyl ethoxymethylenemalonate (DEEMM) and their content was determined. When using the DEEMM as derivatization reagent the negative ionization mode provided analytical performance equal to or in some cases even superior to the positive ionization mode. For 6 amino acids (Thr, β-Ala, α-Ala, Met, Val and Orn) the negative mode led to lower LoQ values, while the positive mode offered lower LoQ values for 5 amino acids (Arg, Asp, Glu, GABA, and Pro). The remaining 11 amino acids showed similar LoQ values in both modes. Because of this, negative ionization mode allowed to detect and quantify amino acids such as: β-Alanine, threonine, and ornithine whose concentrations were low in most of the analysed samples. The relative standard deviation (RSD) for the results in both modes were similar. The method's linearity was determined to be in the range of 1 to 130 ppb with r² > 0.99. Recoveries ranged from 93 to 112%. Negative mode was less affected by matrix effects the main effect was signal enhancement. In contrast, the positive ionization mode suffered from signal enhancement as well as signal suppression.

Brewing and the Chemical Composition of Amine-Containing Compounds in Beer: A Review

As microbreweries have flourished and craft beer brewing has expanded into a multibillion-dollar industry, the ingredients and techniques used to brew beer have changed and diversified. New brewing ingredients and techniques have led to increased concern over biogenic amines in the final product. Biogenic amine composition and concentration in beer, as well as the changes to the protein and amino acid content when adjuncts are used, have received little attention. A complex biochemical mixture, the proteins, amino acids, and biogenic amines undergo a variety of enzymatic and non-enzymatic catabolic, proteolytic, and oxidative reactions during brewing. As biogenic amines in fermented food receive increased scrutiny, evaluating knowledge gaps in the evolution of these compounds in the beer brewing process is critical.

Analysis of chemical compounds and toxicological evaluation of Forsythia suspensa leaves tea

To determine the compositions of Forsythia suspensa leaves tea (FSLT) and its safety, the chemical compounds were analysed with some methods, and the toxicity was evaluated in Kunming mice and Wistar rats. The results showed that FSLT contained rich flavonoid, lignans, triperpene acids, amino acids, and mineral elements. In the acute toxicity study, none of the mice died, and no obvious poisoning symptoms were observed after 14 days in mice at the dose of 15 mg/g·body weight (bw) FSLT; in the sub-chronic toxicity, no abnormal or dead rat was found at the dose of 1, 3, and 10 mg/g·bw during 90 days feeding administration; there was no significant difference in bw and food consumption; no significant differences were found in each hematology and serum biochemistry parameter and organ/body weight ratio comparing with the control experimental group. The results revealed that the FSLT has low or no toxicity via oral administration. Therefore, FSLT is very suitable and safe to be used as a new resource food.

Umami potential of fermented beverages: Sake, wine, champagne, and beer

The free amino acid (FAA) contents of a special selection of fermented beverages have been measured by ultra-high performance liquid chromatography (UHPLC). The selection, which includes 8 sakes, 9 white, rosé, and sparkling wines, 9 genuine champagnes, as well as 5 types of beer, was made to uncover the umami potential of different types of fermented beverages, in particular whether long yeast contact and ageing may influence the contents of free glutamate that is known to elicit umami sensation. The data show that in particular sakes as well as some beers, wines and champagnes with long yeast contact contain appreciable amounts of free glutamate. The results are discussed in the context of food pairing where umami synergy can be achieved by combining fermented beverages with long yeast contact with food rich in free nucleotides.

Evaluation of craft beers through the direct determination of amino acids by capillary electrophoresis and principal component analysis

The composition of beer wort in terms of amino acid (AA) content affects the final product quality, once it is related to the vitality of yeast during the initial exponential growth phase and throughout fermentation. The objective of this work was the use of a capillary zone electrophoresis method with UV-vis detection in association with Principal Component (Data) Analysis for craft beer classification. Cysteine, histidine, phenylalanine, lysine, tryptophan and arginine were the monitored AAs in wort and finished beer, which were extracted through cation exchange resin. Good differentiation among samples according to their production was obtained, showing a profile of AAs (<LQ-99 µg mL^-1) for each wort, which can be used as indicator of the quality of a beer. One of the samples had the mashing step monitored, showing an increasing profile in the concentrations of AAs (6.5-55 µg mL^-1), which can be explained by the protein cleavage.

Determination of Free Tryptophan in Beer Samples by Capillary Isotachophoretic Method

Tryptophan is essential amino acid and precursor for many neurotramsmiters that must be obtained from dietary proteins. However, its free form is easily absorbed and could increase the availability of this amino acid to the brain. Because of free tryptophan interaction with human health simple, eco-friendliness and low-cost method of determination are still needed. In this study, new and simple procedure for free tryptophan determination using capillary isotachophoresis is discussed. The method validation pointed good linearity, satisfactory selectivity, accuracy (recoveries varied from 98.4 to 100.1%), intra- and inter-day precision (coefficent of variation was < 5% for each standard solution and < 6% for real samples) and no matrix effect. The proposed procedure was successfully applied to analyse free tryptophan in beer samples and found contents varied from not detected to 40.74 ± 0.27 mg L−1. The obtained results were compared with chromatographic determination after derivatization with 2-chloro-1,3-dinitro-5-(trifluoromethyl)benzene and pointed better selectivity and accuracy of isotachophoretic procedure with similar precision. Due to the simplicity and flexibility, the proposed procedure is suitable for tryptophan analysis in complex matrices.

Recent Applications of Mass Spectrometry at Clarkson University

Mass spectrometry (MS) is a powerful technique that has various applications including the identification and characterization of proteins, protein-protein interactions and protein post translational modifications, as well as other molecules (i.e. metabolites, lipids, nucleotides and polynucleotides). However, not too many undergraduate students within the USA and around the world have access to (and are trained in) MS. The undergraduate students in our department are taught to analyze proteomics and metabolomics data obtained from MS analysis, including de novo sequencing of peptides and to interpret the MS and MS/MS data acquired in positive and negative ionization modes. Here, we give some examples of MS data analyzed in the Biochemistry I class and then examples of some independent research projects performed by students over the years in the Biochemistry and Biotechnology laboratory, where MS is used for both proteins, peptides and metabolites analysis, thus demonstrating the applicability of MS analysis in diverse fields. The projects discussed include analysis of the protein content present in yogurt, beer, protein shakes, contact lenses, or milk of animal or vegetal origin.

Determination of free amino acids in tea by a novel method of reversed-phase high performance liquid chromatography applying 6-Aminoquinolyl-N-Hydroxysuccinimidyl carbamate reagent

We describe a novel analytical method for quantification of free amino acids in tea using variable mobile phase pH, elution gradient and column temperature of reversed-phase high-performance liquid chromatography (RP-HPLC). The study of mobile phase pH 5.7 was chosen to simultaneous quantification of 19 free amino acids in tea, while it improved maximum resolution of glutamine, histidine and theanine. Elution gradient was adapted for enhancing the solution of free amino acids, mainly because of adjustment of mobile phase A and B. The column temperature of 40 °C was conducive to separate free amino acids in tea. The limit of detection (LOD) and limit of quantitation (LOQ) of this method were in the range of 0.097-0.228 nmol/mL and 0.323-0.761 nmol/mL, respectively. The relative standard deviation of intraday and interday ranged in 0.099-1.909% and 3.231-7.025%, respectively, indicating that the method was reproducible and precise, while recovery ranged between 81.06-112.78%, showing that the method had an acceptable accuracy. This method was applied for the quantification of free amino acids in six types of tea. Multivariate analysis identified serine, glutamine, theanine and leucine as the most influencing factor for classify among analyzed sample.

Yeast Metabolites of Glycated Amino Acids in Beer

Glycation reactions (Maillard reactions) during the malting and brewing processes are important for the development of the characteristic color and flavor of beer. Recently, free and protein-bound Maillard reaction products (MRPs) such as pyrraline, formyline, and maltosine were found in beer. Furthermore, these amino acid derivatives are metabolized by Saccharomyces cerevisiae via the Ehrlich pathway. In this study, a method was developed for quantitation of individual Ehrlich intermediates derived from pyrraline, formyline, and maltosine. Following synthesis of the corresponding reference material, the MRP-derived new Ehrlich alcohols pyrralinol (up to 207 μg/L), formylinol (up to 50 μg/L), and maltosinol (up to 6.9 μg/L) were quantitated for the first time in commercial beer samples by reverse phase high performance liquid chromatography tandem mass spectrometry in the multiple reaction monitoring mode. This is equivalent to ca. 20-40% of the concentrations of the parent glycated amino acids. The metabolites were almost absent from alcohol-free beers and malt-based beverages. Two previously unknown valine-derived pyrrole derivatives were characterized and qualitatively identified in beer. The metabolites investigated represent new process-induced alkaloids that may influence brewing yeast performance due to structural similarities to quorum sensing and metal-binding molecules.

Enhancing the performance of brewing yeasts

Beer production is one of the oldest known traditional biotechnological processes, but is nowadays facing increasing demands not only for enhanced product quality, but also for improved production economics. Targeted genetic modification of a yeast strain is one way to increase beer quality and to improve the economics of beer production. In this review we will present current knowledge on traditional approaches for improving brewing strains and for rational metabolic engineering. These research efforts will, in the near future, lead to the development of a wider range of industrial strains that should increase the diversity of commercial beers.

Kinetic Models for the Role of Protein Thiols during Oxidation in Beer

Thiol-containing proteins have been suggested to have antioxidative properties in beer. A kinetic model has been setup for the reactivity of thiols during early stages of oxidative degradation of beer. Kinetic analysis based on the proposed reaction mechanism allowed evaluation of the relative reactivity of beer components, such as bitter acids from hops and polyphenols. The rate constants for the reaction of 1-hydroxyethyl radicals, which are generated during radical mediated oxidation of ethanol in beer, with hop bitter acids and thiols were very similar, and the concentration of these compounds in beer is therefore essential for the relative reactivity. For a standard pilsner beer with 35 international bitter units with typical concentrations of thiols and hop bitter acids, thiols were found to react with ca. 9% of 1-hydroxyethyl radicals, while bitter acids from hops accounted for ca. 88% of the reaction with 1-hydroxyethyl radicals. Polyphenols were not found to account for any major part of the reaction with 1-hydroxyethyl radicals due to low reaction rates and low concentrations in pilsner beer compared to the other components. The kinetic model suggests that the concentration of thiols has to be increased in order to contribute with any significant antioxidative protection and that the fate of thiols during oxidation must be considered since some thiol oxidation products may induce further damage.

Determination of underivatized amino acids to evaluate quality of beer by capillary electrophoresis with online sweeping technique

Capillary electrophoresis (CE) with ultraviolet detection was applied to determine underivatized amino acids in beer, based on the coordination interaction of copper ions and amino acids. An online sweeping technique was combined with CE to improve detection sensitivity. Using the United Nations Food Agriculture Organization/World Health Organization model of essential amino acid pattern and flavor of amino acids, the quality and taste in three kinds of beer were evaluated. It was found that Beer2 had higher quality than the other two kinds and the content of phenylalanine, proline, serine, and isoleucine was relatively large in all three kinds of beers with a great influence on beer flavor. Optimal conditions for separation were as follows: 50mM CuSO₄ at pH 4.40 as buffer; total length of fused silica capillary, 73 cm; effective length, 65 cm; separation voltage, 22.5 kV; and optimized sweeping condition, 70 seconds. In the appropriate range, linearity (r² > 0.9989), precision with a relative standard deviation < 8.05% (n = 5), limits of detection (0.13–0.25 μg/mL), limit of quantification (0.43–0.83 μg/mL), and recovery (80.5–115.8%) were measured. This method was shown to be applicable to the separation of amino acids in beer and to perform quantitative analysis directly without derivatization for the first time.

Free and Protein-Bound Maillard Reaction Products in Beer: Method Development and a Survey of Different Beer Types

The Maillard reaction is important for beer color and flavor, but little is known about the occurrence of individual glycated amino acids in beer. Therefore, seven Maillard reaction products (MRPs), namely, fructosyllysine, maltulosyllysine, pyrraline, formyline, maltosine, MG-H1, and argpyrimidine, were synthesized and quantitated in different types of beer (Pilsner, dark, bock, wheat, and nonalcoholic beers) by HPLC-ESI-MS/MS in the multiple reaction monitoring mode through application of the standard addition method. Free MRPs were analyzed directly. A high molecular weight fraction was isolated by dialysis and hydrolyzed enzymatically prior to analysis. Maltulosyllysine was quantitated for the first time in food. The most important free MRPs in beer are fructosyllysine (6.8-27.0 mg/L) and maltulosyllysine (3.7-21.8 mg/L). Beer contains comparatively high amounts of late-stage free MRPs such as pyrraline (0.2-1.6 mg/L) and MG-H1 (0.3-2.5 mg/L). Minor amounts of formyline (4-230 μg/L), maltosine (6-56 μg/L), and argpyrimidine (0.1-4.1 μg/L) were quantitated. Maltulosyllysine was the most significant protein-bound MRP, but both maltulosyllysine and fructosyllysine represent only 15-60% of the total protein-bound lysine-derived Amadori products. Differences in the patterns of protein-bound and free individual MRPs and the ratios between them were identified, which indicate differences in their chemical, biochemical, and microbiological stabilities during the brewing process.

Analysis of beers from an 1840s' shipwreck

Two bottles of beer from an about 170-year-old shipwreck (M1 Fö 403.3) near the Åland Islands in the Baltic Sea were analyzed. Hop components and their degradation compounds showed that the bottles contained two different beers, one more strongly hopped than the other. The hops used contained higher levels of β-acids than modern varieties and were added before the worts were boiled, converting α-acids to iso-α-acids and β-acids to hulupones. High levels of organic acids, carbonyl compounds, and glucose indicated extensive bacterial and enzyme activity during aging. However, concentrations of yeast-derived flavor compounds were similar to those of modern beers, except that 3-methylbutyl acetate was unusually low in both beers and 2-phenylethanol and possibly 2-phenylethyl acetate were unusually high in one beer. Concentrations of phenolic compounds were similar to those in modern lagers and ales.

Simultaneous determination of multiple amino acids in plasma in critical illness by high performance liquid chromatography with ultraviolet and fluorescence detection

There is increasing recognition that the host response to critical illness includes derangement of multiple amino acid pathways, including amino acids (AAs) central to metabolism and immune, endothelial and neurological function. To characterise concentration changes of these plasma amino acid we report the development and validation of a method for the quantification of AAs in small volumes of plasma (50μL) using HPLC with simultaneous UV and fluorescence (FL) detection. Protein precipitation and pre-column derivatization with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) is followed by reversed phase HPLC separation. Calibration curves were built with norleucine as an internal standard. Thirty-three (including the 20 proteinogenic) AAs, were selected as standards and their corresponding concentrations in the plasma of healthy human controls and patients with severe falciparum malaria were quantified. This method enables the detection of perturbations in arginine metabolism, aromatic amino acid pathways, methionine transsulfuration and transmethylation pathways and other metabolic pathways.

Application of UHPLC for the determination of free amino acids in different cheese varieties

A rapid ultra-high performance liquid chromatography (UHPLC) protocol for the determination of amino acids as their respective 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) derivatives was successfully applied for assessing free amino acid levels in commercial cheese samples representing typical product groups (ripening protocols) in cheesemaking. Based on the Waters AccQ.Tag™ method as a high performance liquid chromatography (HPLC) amino acid solution designed for hydrolyzate analyses, method adaptation onto UHPLC was performed, and detection of AQC derivatives was changed from former fluorescence (λ(Ex) 250 nm/λ(Em) 395 nm) to UV (254 nm). Compared to the original HPLC method, UHPLC proved to be superior by facilitating excellent separations of 18 amino acids within 12 min only, thus demonstrating significantly shortened runtimes (>35 min for HPLC) while retaining the original separation chemistry and amino acid elution pattern. Free amino acid levels of the analyzed cheese samples showed a high extent of variability depending on the cheese type, with highest total amounts found for original Italian extra-hard cheeses (up to 9,000 mg/100 g) and lowest for surface mold- or bacterial smear-ripened soft cheeses (200-600 mg/100 g). Despite the intrinsic variability in both total and specific concentrations, the established UHPLC method enabled reliable and interference-free amino acid profiling throughout all cheese types, thus demonstrating a valuable tool to generate high quality data for the characterization of cheese ripening.

Volatile sulphur compounds and pathways of L-methionine catabolism in Williopsis yeasts

Volatile sulphur compounds (VSCs) are important to the food industry due to their high potency and presence in many foods. This study assessed for the first time VSC production and pathways of L: -methionine catabolism in yeasts from the genus Williopsis with a view to understanding VSC formation and their potential flavour impact. Five strains of Williopsis saturnus (var. saturnus, var. subsufficiens, var. suavolens, var. sargentensis and var. mrakii) were screened for VSC production in a synthetic medium supplemented with L: -methionine. A diverse range of VSCs were produced including dimethyl disulphide, dimethyl trisulphide, 3-(methylthio)-1-propanal (methional), 3-(methylthio)-1-propanol (methionol), 3-(methylthio)-1-propene, 3-(methylthio)-1-propyl acetate, 3-(methylthio)-1-propanoic acid (methionic acid) and ethyl 3-(methylthio)-1-propanoate, though the production of these VSCs varied between yeast strains. W. saturnus var. saturnus NCYC22 was selected for further studies due to its relatively high VSC production. VSC production was characterised step-wise with yeast strain NCYC22 in coconut cream at different L: -methionine concentrations (0.00-0.20%) and under various inorganic sulphate (0.00-0.20%) and nitrogen (ammonia) supplementation (0.00-0.20%), respectively. Optimal VSC production was obtained with 0.1% of L: -methionine, while supplementation of sulphate had no significant effect. Nitrogen supplementation showed a dramatic inhibitory effect on VSC production. Based on the production of VSCs, the study suggests that the Ehrlich pathway of L: -methionine catabolism is operative in W. saturnus yeasts and can be manipulated by adjusting certain nutrient parameters to control VSC production.

Characterization of amino acid profiles of culture media via pre-column 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate derivatization and ultra performance liquid chromatography

The AccQ.Tag™ method, as a well-established protocol for amino acid analysis combining derivatization procedure, dedicated HPLC separation and fluorescence detection, was properly transferred and accordingly optimized for the application on ultra performance liquid chromatography (UPLC™) and UV detection. Capitalizing on sub-2 μm particles, this newly established UV-UPLC™ technique facilitated efficient chromatographic separation of 21 amino acid derivatives within 12 min. In addition, UPLC™ demonstrated significant improvements due to superior performance and reduced run times compared with the former 35 min of the original HPLC protocol. Using UV instead of fluorescence detection, amino acid quantification after pre-column derivatization with 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) yielded appropriate sensitivities within the low pmol range with corresponding detection limits varying from 0.11 to 0.57 pmol per injection. Moreover, the UPLC™ method was applied to characterize changes in the free (FAA) as well as total amino acid (TAA) profiles specific to culture media at three distinctive stages of fermentation: fresh medium, fermentation broth after cell mass production prior to induction and after product expression at the end of fermentation. Amino acid profiles intrinsic to the fresh, sterilized medium indicated free, hence more bioavailable, amino acids at a FAA/TAA ratio of 40%, whereas ongoing fermentation implied a rather specific, successive decline in selective FAA concentrations. Thereby, the most distinctive variations in FAAs were highlighted by aspartic acid, serine and threonine, each exhibiting an almost complete uptake from the culture media (-96% to -99%), with remaining FAA/TAA ratios of 1%, 8%, and 1%, respectively. This indeed may indicate limitations and shortages within the nutrient broth. Thus, amino acid monitoring utilizing high-throughput chromatography, such as UPLC™, can be considered as a valuable tool to facilitate rapid adjustments of fermentation broths and to optimize culture media to specific requirements.

UPLC analysis of free amino acids in wines: profiling of on-lees aged wines

The evolution of free amino acid (FAA) profiles intrinsic to on-lees aged white wines was determined by ultra performance liquid chromatography (UPLC™). On basis of the AccQ.Tag™ method as a commercialized amino acid analysis solution for HPLC, a new protocol for dedicated amino acid analysis using 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC) for pre-column derivatization was established by method transfer onto UPLC™ conditions. Since AQC derivatives enable both fluorescence (AccQ.Tag™ method) and UV detection, the performed method transfer additionally included changing to a more versatile UV detection. Emphasizing enhanced performance of UPLC™, the newly established protocol facilitated rapid and reliable separations of 24 amino acids within 23 min, hence proved to be superior compared to the original HPLC protocol due to significant improvements in resolution and reduced runtime. Applying UV detection enabled adequate quantifications of AQC amino acid derivatives at μM level (LOQs from 0.12 to 1.10 μM), thus proved sufficient sensitivity for amino acid profiling in wine samples. Moreover, this compiled methodology was successfully applied to monitor the changes of FAA concentrations in four distinct sets of on-lees aged white wines (fermented with different yeasts) at three progressing ripening periods, each (control, 3 and 6 months aging). For the control wines, the applied winery yeast significantly affected total FAA amounts (1450-1740 mg L(-1)). During maturation, the proceeding yeast autolysis implied a rather complex impact on FAAs, yielding total FAA excretions up to 360 mg L(-1). However, the magnitude for increases of specific FAAs (up to +200%) highly depended on the individual amino acids as well as on the applied fermenting yeast. Given the overall complexity of yeast autolysis in winemaking, the application of efficient LC techniques such as UPLC™ may indeed contribute as a valuable tool in wine research for product monitoring and characterization of intrinsic developments during wine maturation.

High-performance liquid chromatographic method for determination of amino acids by precolumn derivatization with 4-chloro-3,5-dinitrobenzotrifluoride

This work presents an high-performance liquid chromatography method for the determination of amino acids after precolumn derivatization with 4-chloro-3,5-dinitrobenzotrifluoride (CNBF) which can readily react with both primary and secondary amines. The precolumn derivatization conditions, including the CNBF concentration, reaction pH, temperature and reaction time were investigated for method optimization. In pH 9.0 borate buffer, the reaction of amino acids with CNBF was carried out at 60 degrees C for 30min, the optimized concentration of CNBF was 70mmol L(-1) and the molar ratio of amino acids to CNBF was 1:5.25. The chromatographic separation of 19 amino acids derivatives was performed on a Kromasil ODS C(18) column (250mm x 4.6mm, 5microm) with good reproducibility, and ultraviolet detection was applied at 260 nm. The mobile phase was a mixture of phase A (acetonitrile) and phase B (acetate buffer, acetonitrile, triethylamine; 82.8:17:0.2, pH 4.9), and the flow rate was 0.4mL min(-1). The separation of all the labeled amino acids was achieved within 45min at room temperature by gradient elution mode. The method linearity, calculated for each amino acid, had a correlation coefficient higher than 0.9979, in concentrations ranging from 9.60 to 3330.00 micromol L(-1). The detection limits of amino acids were 2.40-6.50micromol L(-1), at a signal-to-noise ratio of 3. The proposed method was applied for the determination of amino acids in beer with recoveries of 97.0-103.9% and relative standard deviations of 2.62-4.22%, respectively. This method showed good accuracy and repeatability that can be used for the quantification of amino acids in real samples.