Discovery of Polyhydroxyalkyl Pyrazine Generation upon Coffee Roasting by In-Bean Labeling Experiments.

The major non-volatile reaction products formed from free amino acids during the early stage of coffee roasting were investigated using biomimetic in-bean experiments with labeled and unlabeled free amino acids. Comprehensive untargeted screening by ultra-high performance liquid chromatography-electrospray-ionization-quadrupole time-of-flight-tandem mass spectrometry (UHPLC-ESI-QToF-MS) in data-independent acquisition (DIA) mode was carried out and modeling by orthogonal partial least-squares discriminant analysis (OPLS-DA) helped in revealing 11 pyrazine structures identified in coffee for the first time. 2-(2',3',4'-Trihydroxybutyl)-(5/6)-methyl-pyrazine () and 2,(5/6)-bis(2',3',4'-trihydroxybutyl)-pyrazine () were the most prominent compounds, while 2-(3',4'-dihydroxybutyl)-(5/6)-methyl-pyrazine () and 2-(2',3',4'-trihydroxybutyl)-(5/6)-(2'-hydroxyethyl)-pyrazine () were not even previously identified in other food matrices.

Quantitative Validation of the In-Bean Approach in Coffee Roasting.

The representativeness of the so-called biomimetic "in-bean" technique was studied by following the formation of target aroma compounds during the roasting course (10 points from 0 to 400 s) in recombined coffee beans and non-treated green coffee reference beans. For this purpose, the water-soluble fraction was replaced by a biomimetic recombinate in reconstituted beans prior to roasting. The targeted analysis of key aroma compounds was performed by means of the stable isotope dilution assay and solid-phase microextraction-gas chromatography-mass spectrometry of roasted and ground coffee samples.

Generation of α-Diketones and 4-Hydroxy-2,5-dimethyl-3(2)-furanone upon Coffee Roasting-Impact of Roast Degree on Reaction Pathways.

The formation pathways of α-diketones (2,3-butanedione and 2,3-pentanedione) and 4-hydroxy-2,5-dimethyl-3(2)-furanone (HDMF) upon coffee roasting were investigated in a kinetic study applying labeled and unlabeled sucrose (CAMOLA approach) in biomimetic in-bean experiments. The results highlighted that not only did the contribution of sucrose to the level of α-diketones in roasted coffee change with the roasting degree but also the portion of the individual reaction pathways. At early roasting stages, 2,3-butanedione was formed from sucrose mainly via the intact sugar skeleton, whereas from the middle of the roasting course, the formation foremost occurred from sugar fragments, primarily by C/C and C/C recombinations.

Unexpected Potential of Iso-oligosaccharides in the Generation of Important Food Odorants.

The generation of selected Maillard-derived odorants from iso-oligosaccharides (IOSs), namely, from isomaltose, isomaltotriose, isomaltulose, and melibiose, was studied and compared with that from other oligosaccharides (maltose, lactose, and panose) and monosaccharides (glucose, galactose, and fructose). The study was carried out in binary mixtures of sugar and amino acids (glycine, proline, and cysteine) and upon wafer baking. The results indicate that IOSs induce browning and generation of the majority of the monitored odorants, in particular 4-hydroxy-2,5-dimethyl-3(2 H)-furanone, 2,3-butanedione, 2-acetyl-1-pyrroline, 2-propionyl-1-pyrroline, 2-acetylthiazole, and 2-acetyl-2-thiazoline, far more than the other oligosaccharides and to a higher or similar degree to that of the monosaccharides.

Preparation of activated flavor precursor DFG, N-(1-deoxy-1-fructosylglycine) by combination of vacuum evaporation and closed system heating steps.

Amadori rearrangement products are potent "activated flavor precursors". This study describes a stepwise optimization of DFG (1-deoxy-1-fructosylglycine) formation from glycine and glucose, varying temperature, water activity a, concentration, and ratio/state of precursor mix. In the solid state reaction at 50 °C for 16 h, yield of DFG increased with decreasing water activity with a peak at a = 0.

New Insight into the Role of Sucrose in the Generation of α-Diketones upon Coffee Roasting.

The origin and formation pathways of the buttery-smelling α-diketones 2,3-butanedione and 2,3-pentanedione upon coffee roasting were studied by means of biomimetic in-bean experiments combined with labeling experiments. For this purpose natural sucrose in the coffee bean was replaced by fully or partially C-labeled sucrose or by a mixture of unlabeled and fully C-labeled sucrose (CAMOLA approach). The obtained data point out that sucrose contributes to both α-diketones; however, its importance and reaction pathways clearly differ.

A holistic approach towards defined product attributes by Maillard-type food processing.

A fractional factorial experimental design was used to quantify the impact of process and recipe parameters on selected product attributes of extruded products (colour, viscosity, acrylamide, and the flavour marker 4-hydroxy-2,5-dimethyl-3(2H)-furanone, HDMF). The study has shown that recipe parameters (lysine, phosphate) can be used to modulate the HDMF level without changing the specific mechanical energy (SME) and consequently the texture of the product, while processing parameters (temperature, moisture) impact both HDMF and SME in parallel. Similarly, several parameters, including phosphate level, temperature and moisture, simultaneously impact both HDMF and acrylamide formation, while pH and addition of lysine showed different trends.

Study to elucidate formation pathways of selected roast-smelling odorants upon extrusion cooking.

The formation pathways of the N-containing roast-smelling compounds 2-acetyl-1-pyrroline, 2-acetyl-1(or 3),4,5,6-tetrahydropyridine, and their structural analogues 2-propionyl-1-pyrroline and 2-propionyl-1(or 3),4,5,6-tetrahydropyridine were studied upon extrusion cooking using the CAMOLA approach. The samples were produced under moderate extrusion conditions (135 °C, 20% moisture, 400 rpm) employing a rice-based model recipe enriched with flavor precursors ([U-(13)C6]-D-glucose, D-glucose, glycine, L-proline, and L-ornithine). The obtained data indicate that the formation of these compounds upon extrusion follows pathways similar to those reported for nonsheared model systems containing D-glucose and L-proline.

Study on the role of precursors in coffee flavor formation using in-bean experiments.

The formation of several key odorants, such as 2-furfurylthiol (FFT), alkylpyrazines, and diketones, was studied upon coffee roasting. The approach involved the incorporation of potential precursors in green coffee beans by means of biomimetic in-bean and spiking experiments. Both labeled and unlabeled precursor molecules were used, and the target analytes in the roasted coffee samples were characterized in terms of their isotope labeling pattern and abundance.

Generation of 4-hydroxy-2,5-dimethyl-3(2H)-furanone from rhamnose as affected by reaction parameters: experimental design approach.

The formation of 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF) was studied in aqueous model systems containing L-rhamnose and L-lysine. The approach consisted in systematically varying four reaction parameters (rhamnose concentration, rhamnose to lysine ratio, pH, and phosphate concentration) at 3 levels. A fractional factorial design was used to reduce the number of trials.

Origin and yields of acetic acid in pentose-based Maillard reaction systems.

The formation of acetic acid from pentoses was studied in aqueous buffered systems (90-120 degrees C, pH 6.0-8.0) containing equimolar concentrations of 13C-labeled xylose and glycine.

Formation of furan and methylfuran by maillard-type reactions in model systems and food.

The formation of furan and 2-methylfuran was studied in model systems based on sugars and selected amino acids. Both compounds were preferably formed under roasting conditions in closed systems yielding up to 330 micromol of furan and 260 micromol of 2-methylfuran per mol of precursor. The amounts obtained under pressure cooking conditions were much lower, usually below 20 micromol/mol, except for 2-furaldehyde, which yielded 70-100 micromol/mol of furan.

Sugar fragmentation in the maillard reaction cascade: formation of short-chain carboxylic acids by a new oxidative alpha-dicarbonyl cleavage pathway.

The formation of short-chain carboxylic acids was studied in Maillard model systems (90 degrees C, pH 6-10) with emphasis on the role of oxygen and water. The total amount of acetic acid formed did not depend on the reaction atmosphere. In the presence of labeled dioxygen or water (18O2, H2 17O), labeled oxygen was partially incorporated into acetic acid.

Sugar fragmentation in the maillard reaction cascade: isotope labeling studies on the formation of acetic acid by a hydrolytic beta-dicarbonyl cleavage mechanism.

The formation of acetic acid was elucidated based on volatile reaction products and related nonvolatile key intermediates. The origin and yield of acetic acid were determined under well-controlled conditions (90-120 degrees C, pH 6-8). Experiments with various 13C-labeled glucose isotopomers in the presence of glycine revealed all six carbon atoms being incorporated into acetic acid: C-1/C-2 ( approximately 70%), C-3/C-4 ( approximately 10%), and C-5/C-6 (approximately 20%).

The effect of reaction conditions on the origin and yields of acetic acid generated by the maillard reaction.

The effect of the reaction conditions on the origin and yields of acetic acid from glucose was studied in the system containing equimolar concentrations of 13C-labeled glucose and glycine. Acetic acid was quantified by GC-MS using isotope dilution assay. The beta-dicarbonyl cleavage of 1-deoxyhexo-2,4-diulose is proposed to be a major pathway leading to the formation of acetic acid in the glucose-based Maillard reaction systems under food processing conditions.

Elucidation of chemical pathways in the maillard reaction by 17O-NMR spectroscopy.

17O-NMR spectroscopy was employed as an innovative method to help understand mechanistic pathways in sugar fragmentation. Elucidation of reaction mechanisms to final Maillard end products was achieved by starting from specific intermediates obtained by synthesis, such as 1-deoxy-D-erythro-hexo-2,3-diulose. This alpha-dicarbonyl was thermally treated in the presence of 17O-enriched water under alkaline conditions.

Analysis of Amadori compounds by high-performance cation exchange chromatography coupled to tandem mass spectrometry.

High-performance cation exchange chromatography coupled to tandem mass spectrometry or electrochemical detection was found to be an efficient tool for analyzing Amadori compounds derived from hexose and pentose sugars. The method allows rapid separation and identification of Amadori compounds, while benefiting from the well-known advantages of mass spectrometry, such as specificity and sensitivity. Glucose- and xylose-derived Amadori compounds of several amino acids, such as glycine, alanine, valine, leucine/isoleucine, methionine, proline, phenylalanine, and glutamic acid, were separated or discriminated using this new method.

In-depth mechanistic study on the formation of acrylamide and other vinylogous compounds by the maillard reaction.

The formation of acrylamide was studied in low-moisture Maillard model systems (180 degrees C, 5 min) based on asparagine, reducing sugars, Maillard intermediates, and sugar degradation products. We show evidence that certain glycoconjugates play a major role in acrylamide formation. The N-glycosyl of asparagine generated about 2.

Alpha-mercaptoketone formation during the maillard reaction of cysteine and [1-(13)C]ribose.

The volatiles formed from [1-(13)C]-ribose and cysteine during 4 h at 95 degrees C in aqueous phosphate buffer (pH 5) were analyzed by headspace SPME in combination with GC-MS. The extent and position of the labeling were determined using MS data. The identified volatiles comprised sulfur compounds such as 2-[(13)C]methyl-3-furanthiol, 2-[(13)CH(2)]furfurylthiol, [1-(13)C]-3-mercaptopentan-2-one, [1-(13)C]-3-mercaptobutan-2-one, [4-(13)C]-3-mercaptobutan-2-one, and 3-mercaptobutan-2-one.

Simultaneous quantitative analysis of maillard reaction precursors and products by high-performance anion exchange chromatography.

A new analytical setup allowing the simultaneous analysis of precursors and products of the Maillard reaction is described. It is based on high-performance anion exchange chromatography with electrochemical (ECD) and diode array detectors (DAD) coupled in series. Chromatography and detection were optimized to permit simultaneous monitoring of compounds relevant to the Maillard reaction, such as the sugar, the amino acid, and the corresponding Amadori compound as well as the cyclic intermediates 5-(hydroxymethyl)-2-furaldehyde, maltol, and 2,3-dihydro-3,5-dihydroxy-6-methyl-4(H)-pyran-4-one.

Formation of aroma compounds from ribose and cysteine during the Maillard reaction.

The headspace volatiles produced from a phosphate-buffered solution (pH 5) of cysteine and a 1 + 1 mixture of ribose and [(13)C(5)]ribose, heated at 95 degrees C for 4 h, were examined by headspace SPME in combination with GC-MS. MS data indicated that fragmentation of ribose did not play a significant role in the formation of the sulfur aroma compounds 2-methyl-3-furanthiol, 2-furfurylthiol, and 3-mercapto-2-pentanone in which the carbon skeleton of ribose remained intact. The methylfuran moiety of 2-methyl-3-(methylthio)furan originated from ribose, whereas the methylthio carbon atoms came partly from ribose and partly from cysteine.

Degradation of the Amadori compound N-(1-deoxy-D-fructos-1-yl)glycine in aqueous model systems.

The fate of the Amadori compound N-(1-deoxy-D-fructos-1-yl)glycine (DFG) was studied in aqueous model systems as a function of time and pH. The samples were reacted at 90 degrees C for up to 7 h while maintaining the pH constant at 5, 6, 7, or 8. Special attention was paid to the effect of phosphate on the formation of glycine and the parent sugars glucose and mannose, as well as formic and acetic acid.