Tomas Davidek

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. Labeling studies indicated two major formation pathways for both furans: (i) from the intact sugar skeleton and (ii) by recombination of reactive C(2) and/or C(3) fragments. Under roasting conditions in the absence of amino acids, furan was mainly formed from the intact sugar skeleton. Formic and acetic acid were identified as byproducts of sugar degradation, indicating the split off of C(1) and/or C(2) units from hexoses. The presence of alanine, threonine, or serine promoted furan formation by the recombination of C(2) fragments, such as acetaldehyde and glycolaldehyde, which may originate from both sugars and amino acids. In aqueous solution, about half of furan was generated by the recombination of sugar fragments. 2-Methylfuran was preferably formed in the presence of amino acids by aldol-type reactions of C(2) and C(3) fragments with lactaldehyde as a key intermediate, the Strecker aldehyde of threonine. The total furan levels in cooked vegetables were increased by spiking with hexoses. However, in pumpkin puree, only about 20% of furan was formed from sugars, preferably from the intact carbon skeleton.

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. The biomimetic in-bean experiments ruled out the 2-furaldehyde route to FFT as suggested by model studies. Furthermore, no evidence was found for the incorporation of the arabinose C5 skeleton into FFT. Pathways proposed for the formation of alkylpyrazines and diketones were confirmed, and a new mechanism is suggested for the formation of 2-ethenyl-3-ethyl-5-methylpyrazine. The role of amino acids, for example, alanine, and free sugars was substantiated. The results underscore the potential of this methodology to provide better understanding of the formation pathways occurring in complex food systems, which may be different from those obtained in model experiments.

The Effect of Reaction Conditions on the Origin and Yields of Acetic Acid Generated by the Maillard Reaction

Abstract: 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 β‐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. Acetic acid was built up from all six carbon atoms of glucose. The relative distribution of acetic acid was independent of the reaction time. Temperature and pH had only small effects.

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°C, pH 6.0–8.0) containing equimolar concentrations of 13C‐labeled xylose and glycine. Acetic acid was quantified by gas chromatography–mass spectroscopy using an isotope dilution assay. Acetic acid was mainly formed from the C‐1/C‐2 carbon atoms of xylose (77–87%), while small amounts were also formed from the C‐4/C‐5 atoms of the pentose sugar (9–15%). Temperature and pH had only a small effect on the relative contribution of the sugar carbon atoms to acetic acid. These results support β‐dicarbonyl cleavage of 1‐deoxypento‐2,4‐diulose as a major pathway leading to acetic acid in pentose‐based Maillard reaction systems under food processing conditions. Acetic acid was confirmed as a major degradation product of pentoses at the early stage of the Maillard reaction, yielding 16 mol% and 28 mol% at pH 6.0 and pH 8.0, respectively.

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. The degradation of rhamnose was followed by high performance anion exchange chromatography and the formation of HDMF by solid phase extraction in combination with GC/MS. The study permitted the identification of critical reaction parameters that affect the formation of HDMF from rhamnose in aqueous systems. Although all studied parameters have some impact on the HDMF formation and rhamnose degradation kinetics, the effect of phosphate is by far the most important, followed by concentration of precursors and pH. The experimental design approach permitted us, with a limited number of experiments, to accurately model the effects of the four investigated reaction parameters on the kinetics of rhamnose degradation and HDMF formation (R(2)>0.93). Overall, the results indicate that rhamnose can be an excellent precursor of HDMF (yield >40 mol%), if the reaction conditions are well mastered.

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. 2-Acetyl-1-pyrroline is formed (i) by acylation of 1-pyrroline via C2 sugar fragments (major pathway) and (ii) via ring-opening of 1-pyrroline incorporating C3 sugar fragments (minor pathway), whereas 2-propionyl-1-pyrroline incorporates exclusively C3 sugar fragments. 2-Acetyl-1(or 3),4,5,6-tetrahydropyridine and the corresponding propionyl analogue incorporate C3 and C4 sugar fragments, respectively. In addition, it has been shown that the formation of 2-acetyl-1-pyrroline in low-moisture systems depends on the pH value of the reaction mixture.

Formation of styrene during the Maillard reaction is negligible.

The elucidation of chemical pathways and the identification of intermediates leading to vinylogous compounds such as acrylamide by the Maillard reaction have proven challenging. This study was conducted to assess the formation of styrene from L-phenylalanine, employing binary mixtures of the amino acid heated together with simple C3-sugar analogue (1-hydroxyacetone) or methylglyoxal. The formation of the corresponding vinylogous product, i.e. styrene, was measured under different moisture, pH, and temperature conditions. The formation of intermediates over time was monitored by gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS) together with the target compound styrene. Two intermediates, i.e. 1-phenethylaminopropan-2-one and 2-phenylethylamine, play a role in the formation of styrene, the latter of more importance in high-moisture systems, whilst the former favours the release of styrene in low-moisture systems. The model further showed that Strecker-type reactions are of less importance in the formation of styrene, as the yield from single immediate precursors was maximally 0.03 mol%. The low conversion rate of L-phenylalanine to the vinylogous product and existing data on the occurrence of free L-phenylalanine in food plants suggests that the amounts of styrene expected in foods subjected to thermal treatment are negligible.

Elucidation of Chemical Pathways in the Maillard Reaction by 17O-NMR Spectroscopy

Abstract: 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 α‐dicabonyl was thermally treated in the presence of 17O‐enriched water under alkaline conditions. The reaction products were monitored by 17O‐NMR spectroscopy and their structures corroborated by complementary techniques. For the first time, evidence is shown for the direct formation of acetic acid from 1‐deoxy‐d‐erythro‐hexo‐2,3‐diulose by an oxidative α‐dicarbonyl cleavage and incorporation of a 17OH group into the acetic acid released as sugar fragment.

Analysis of Amadori compounds by high performance anion exchange chromatography–pulse amperometric detection

Abstract Rapid and sensitive detection of Amadori compounds was achieved by high performance anion exchange chromatography (HPAEC) coupled with a pulse amperometric detector (PAD). Several Amadori compounds were separated, such as fructosyl proline (Fru-Pro), fructosyl glycine (Fru-Gly), fructosyl valine (Fru-Val), fructosyl isoleucine (Fru-Ile), and fructosyl methionine (Fru-Met). The HPAEC–PAD method is suitable for high-throughput routine analysis of Amadori compounds in complex reaction systems such as the Maillard reaction. It was used to monitor the formation of Fru-Pro from glucose and proline in methanol resulting in a continuous increase with about 30 g Fru-Pro/mol glucose after a reaction period of 5 h. Another application included monitoring of Fru-Gly. The rate of Fru-Gly degradation increased with increasing pH. Phosphate accelerated the degradation of Fru-Gly, particularly in the pH range 5–7.

Elucidating the Secrets of the Maillard Reaction Cascade – The Role of Amadori Compounds

The reaction of reducing sugars with amino acids and proteins, referred to as the Maillard reaction or non-enzymatic browning, is the major source of color, taste and aroma that is characteristic for many heated foods, such as meat, bakery, and cocoa. For example the almost odorless green coffee beans are transformed into dark brown roasted coffee with delicious taste and aroma. Amadori compounds, 1-amino-1-deoxyketoses, play a pivotal role in the Maillard reaction cascade. It is also assumed that they participate in the formation of advanced glycated end products (AGEs) under physiological conditions. Pentose-based Amadori compounds are very unstable compared to their hexose-based analogues and have rarely been reported in the literature. It is rather challenging to obtain reliable data on the nature, amounts, and fate of Amadori compounds due to (i) the high complexity of Maillard systems and (ii) the high diversity in polarity of Amadori compounds. Modern analytical methods combine chromatographic separation efficiency and mass discrimination by tandem mass spectrometry (MS/MS). Separation by high-performance cation-exchange chromatography (HPCEC) followed by electrochemical detection (ECD) or tandem MS in the positive electrospray ionization (ECI+) mode turned out to be the most suitable approach. Alternatively, capillary electrophoresis coupled to MS/MS can be used. We have succeeded for the first time to monitor pentose-based Amadori compounds, such as N-(1-deoxy-D-xylulos-1-yl)glycine (Xyl-Gly). Reaction of D-xylose and glycine at 90 °C (pH 6) for 2 h showed rapid formation of Xyl-Gly (~12 mol %, 15 min) followed by slow decrease over time. Analysis of pentose-derived Amadori compounds represents a major breakthrough in studying occurrence, formation, and decomposition of these labile Maillard intermediates. Several hexose-based Amadori compounds could be identified and quantified in an aqueous extract of dried vegetables and fruits. Sample preparation consisted of maceration of dried tomatoes in water, homogenization, and filtration. N-(1-deoxy-D-fructos-1yl)-L-glutamic acid (Fru-Glu) was found as the major Amadori compound in dried tomatoes. About 1.5 g/100 g of Fru-Glu was determined, known for its umami taste properties that are also characteristic for dried tomatoes. Such data are now readily accessible to reveal taste-active constituents of natural products. Several other Amadori compounds could also be detected, i.e. Fru-Ala, Fru-Leu and Fru-Phe, indicating that no further clean-up was required. Simultaneous analysis of non-volatile Maillard reaction products such as Amadori compounds combined with a minimum of sample clean-up opens new avenues to study reaction mechanisms of the Maillard reaction cascade and elucidate new constituents in thermally processed foods and natural products with interesting sensory and other bio-active properties.