Unexpected Potential of Iso-oligosaccharides in the Generation of

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Cite This: J. Agric. Food Chem. XXXX, XXX, XXX−XXX

Unexpected Potential of Iso-oligosaccharides in the Generation of Important Food Odorants Ondrej Novotny, Thierry Dufosse,́ and Tomas Davidek*

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Nestlé Research and Development Orbe, Nestec LTD., Route de Chavornay 3, CH-1350 Orbe, Switzerland ABSTRACT: 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(2H)-furanone, 2,3-butanedione, 2acetyl-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. Plausible mechanisms, consistent with the yields obtained from individual sugars, were proposed for the formation of the studied compounds. This newly obtained data brought for the first time evidence about the extraordinary potential of IOSs in the formation of several potent food odorants. KEYWORDS: iso-oligosaccharide, Maillard reaction, odorant, browning

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Maillard type reactions. Kato et al.14 reported that the IOSs isomaltose and melibiose mixed with ovalbumin strongly induced brown colorization, production of fluorescent compounds, and protein polymerization, whereas maltose, cellobiose, and lactose, disaccharides with α(1→4) linkages, did so very weakly. The baking of cakes formulated with isomaltulose showed much darker brown top-surface color and crumb color, compared with cakes made with sucrose or glucose syrups.15 Isomaltulose was also reported to be an unusually strong reducing sugar that revealed a reduction potential even stronger than that of fructose.16 The aim of the present study was to investigate the generation of selected Maillard-derived odorants from IOSs and compare them with those from other oligosaccharides and monosaccharides.

INTRODUCTION Iso-oligosaccharides (IOSs), sugar oligomers with (1→6) glycosidic bonds, are found naturally in some foods, as well as being manufactured commercially. Various IOSs such as isomalto-oligosaccharides (IMOs), galacto-oligosaccharides (GOSs), and isomaltulose are gaining increasing importance because of multiple health benefits. Low glycemic indices and prebiotic, anticariogenic, and digestion-resistant properties, as well as other physiological properties such as impacts on hormone production, lipid and carbohydrate metabolism, and immune response, are widely reported.1−3 IMOs occur naturally in fermented foods, such as rice miso,4 soy sauce,5 and sake,6 and also in honey.7 Commercial IMOs are produced enzymatically from starch processed from cereal crops and are a well-established functional food, especially in Asia.1 GOSs, made through the enzymatic conversion of lactose from bovine milk, are an emerging class of prebiotics used in food and beverages, as well as in dietary supplements. The (1→6) glycosidic bond is mostly favored by the transgalactosylation reaction under β-galactosidase, which is used in GOS production.8 Recent research has shown that GOSs can also have a positive impact on immunity, calcium absorption, and markers of metabolic syndrome.3 Isomaltulose, a disaccharide composed of α-1,6-linked glucose and fructose, is present in honey9−11 and sugar-cane juice, including products derived therefrom, such as treacles or food-grade molasses.11 Commercially produced isomaltulose is known by the trade name Palatinose, and it is manufactured from foodgrade sucrose by enzymatic rearrangement (isomerization) of the glycosidic linkage. It is particularly suitable as a noncariogenic sucrose replacement and is favorable mainly in products for diabetics and prediabetic dispositions. Several IOSs, such as isomaltose, gentiobiose, and isomaltotriose, were identified in caramel.12,13 Although the physiological properties of IOSs have been extensively investigated, little is known about their reactivity in © XXXX American Chemical Society

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MATERIALS AND METHODS

Chemicals. The following chemicals were commercially available: D-glucose, D-galactose, D-fructose, D-maltose monohydrate, D-lactose monohydrate, melibiose, isomaltose, isomaltotriose, D-panose, glycine, L-proline, L-cysteine, sodium dihydrogen phosphate, disodium hydrogen phosphate, 2,3-butanedione, 2,3-pentanedione, 2-acetylthiazole, 2-acetyl-2-thiazoline, 4-hydroxy-2,5-dimethyl-3(2H)-furanone, maltol, methanol, and sodium chloride (Sigma-Aldrich, Buchs, Switzerland). The following chemicals were obtained upon customized synthesis: [13C4]-2,3-butanedione, [2H5]-2,3-pentanedione, [13C2]-2-acetyl-1pyrroline, [2H5]-2-propionyl-1-pyrroline, [13C2]-2-acetyl-1-thiazole, [13C2]-2-acetyl-1-thiazoline, [13C2]-4-hydroxy-2,5-dimethyl-3(2H)furanone, [2H3]-maltol, 2-acetyl-1-pyrroline (AtlanChim Pharma, Saint-Herblain, France), and 2-propionyl-1-pyrroline (aromaLAB GmbH, Planegg, Germany). Materials. The following food-grade materials were commercially available: PalatinoseTM (Beneo, Mannheim, Germany), wheat flour Received: December 19, 2018 Revised: February 23, 2019 Accepted: February 23, 2019

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DOI: 10.1021/acs.jafc.8b07100 J. Agric. Food Chem. XXXX, XXX, XXX−XXX

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Journal of Agricultural and Food Chemistry Table 1. MRM Quantification Transitions and Collision Energies Used in the Stable-Isotope-Dilution Assays odorant 2,3-butanedione (1) 2,3-pentanedione (2) 4-hydroxy-2,5-dimethyl-3(2H)-furanone (3) maltol (4) 2-acetyl-1-pyrroline (5) 2-propionyl-1-pyrroline (6) 2-acetyl-2-thiazoline (7) 2-acetylthiazole (8)

MRM transition (m/z) 86 → 43 100 → 57 128 → 43 126 111 125 129 127

→ → → → →

71 83 69 60 99

internal standard [13C4]-2,3-butanedione [2H5]-2,3-pentanedione [13C2]-4-hydroxy-2,5-dimethyl-3(2H)furanone [2H3]-maltol [13C2]-2-acetyl-1-pyrroline [13C2]-2-propionyl-1-pyrroline [13C2]-2-acetyl-1-thiazoline, [13C2]-2-acetyl-1-thiazole

type 550 SN (Groupe MinoteriesMoulins de Granges SA, GrangesMarnand, Switzerland), palm olein from fractionated palm oil (Loders Croklaan B.V., Wormerveer, Netherlands), and sodium bicarbonate (Univar, Zürich, Switzerland). Aqueous Reaction Mixtures. Equimolar amounts (100 μmol) of sugar and amino acid (glycine, L-proline, or L-cysteine) were dissolved in 1 mL phosphate buffer (pH 7, 0.1 M) and placed into a 20 mL headspace vial. The vial was closed with a screw cap and heated at 120 °C for 20 min in the heating block of an SPME module of a PAL 1 system (CTC Analytics AG, Zwingen, Switzerland) under agitation of 500 rpm. Then the vial was cooled down in a water bath. The reaction mixture was spiked with a solution of labeled standards in methanol (20 μL), homogenized by means of a vortex agitator, and subjected to aroma analysis by head-space solid-phase microextraction in combination with gas chromatography and tandem mass spectrometry (HS-SPME-GC-MS/MS). The heating experiment and following aroma analysis of each amino acid−sugar mixture was performed in five replicates for all sugars except isomaltotriose and panose, whose analyses were performed in two replicates. Wafer Baking (Food Model). Sugar (1.8 mmol), glycine (0.6 mmol), L-proline (0.6 mmol), L-cysteine (0.6 mmol), and sodium bicarbonate (180 mg) were dissolved in water (75 g) using an overhead stirrer (Heidolph RZR2020, Schwabach, Germany); this was followed by the addition of palm olein (3 g). Refined wheat flour (70.7−71.3 g depending on sugar type) was then gradually added under vigorous stirring to obtain 150 g of batter. After the addition of the whole amount of the flour, the batter was stirred for another 2 min. Wafers (between 8.5 and 9.5 g each) were prepared by baking at 160 °C for 110 s using laboratory equipment for production of wafer sheets (Hebenstreit, Mörfelden-Walldorf, Germany). Three wafers from each recipe were prepared. In addition, reference wafers were prepared without the addition of sugar and amino acids. Preparation of Wafer Samples for Aroma Analysis. Each wafer was ground using a coffee grinder (Tristar, Tilburg, Netherlands). Ground wafer (1000 ± 2 mg) was exactly weighed into a 20 mL headspace vial. Ultrapure water (10 mL), a magnetic stir bar and a solution of labeled standards in methanol (20 μL) were added. The vial was closed with a screw cap, and the mixture was homogenized by means of a vortex agitator for 5 s and then stirred for 15 min using a magnetic stirrer. Then, the mixture was centrifuged at 4000 rpm for 3 min. An aliquot of supernatant (2 mL) was transferred into a new 20 mL headspace vial containing 2 g of sodium chloride. The vial was closed with a screw cap, and the mixture was vortexed for 5 s by means of a vortex agitator and subjected to aroma analysis by HSSPME-GC-MS/MS. The analysis of each wafer was performed in two replicates. Aroma Analysis by HS-SPME-GC-MS/MS. Head-space solidphase microextraction in combination with gas chromatography and tandem mass spectrometry (HS-SPME-GC-MS/MS) was applied for aroma analysis of both the aqueous reaction mixture and the aqueous wafer extract. Aroma compounds were extracted from the headspace over 15 min at 80 °C under agitation (500 rpm) using a divinylbenzene−carboxen−polydimethylsiloxane fiber (StableFlex DVB/CAR/PDMS; 2 cm; film thickness, 50/30 μm; Supelco, Buchs, Switzerland).

MRM transition (m/z)

collision energy (V)

90 → 45 105 → 59 130 → 45

5 0 15

129 → 72 113 → 84 127 → 69 131 → 60 129 → 100

15 5 10 10 5

The extracted compounds were thermally desorbed for 1 min in a split−splitless injector maintained at 250 °C and operated in split mode (ratio 1:2); then, the fiber was baked for 4 min at 270 °C. Automated sample handling and extraction (HS-SPME) was achieved with a PAL 1 system (CTC Analytics AG, Zwingen, Switzerland). Separation was carried out on a 60 m × 0.25 mm × 0.25 μm DB624-MS-UI column (Agilent, Basel, Switzerland) using an Agilent 7890A gas chromatograph (Agilent, Basel, Switzerland). Helium was used as the carrier gas with a constant flow of 1.0 mL/min. The GC oven was temperature-programmed from 100 to 150 °C at 15 °C/min and then to 240 °C at 30 °C/min, and finally the temperature was maintained at 240 °C for 10 min. Mass spectrometry was performed on an Agilent 7010 triple-quadrupole mass spectrometer (Agilent, Basel, Switzerland) operating in multiple-reaction-monitoring (MRM) mode and equipped with a high-sensitivity electronionization source (HS-EI). The collision gas (nitrogen) and quench gas (helium) were held at constant flow rates of 2.25 and 1.5 mL/min, respectively. Instrument control and data acquisition were achieved with Agilent MassHunter GCMS Data Aquisition software version B.07.04.2704 (Agilent, Basel, Switzerland), and data treatment was performed using Agilent Mass Hunter Workstation Quantitative Analysis software version B.08.00/Build 8.0.598.0. Quantification was accomplished with a stable-isotope-dilution assay (SIDA). The chosen MRM quantification transitions are depicted in Table 1. Measurement of Absorbance. The absorbance at 420 nm was measured in aqueous reaction mixtures after HS-SPME extraction using a Shimadzu UV-2600 spectrophotometer (Kyoto, Japan). Reaction mixtures from the replicates were mixed together, and 1 mL of the mixture was transferred into a plastic cuvette. The absorbance of a blank sample containing the buffer and the mixture of the labeled standards was measured and subtracted from the absorbances determined for the reaction mixtures. Two independent measurements were performed for each sample, showing good repeatability with a coefficient of variation below 1%. Statistical Analysis. The least significant differences (LSDs) were computed for individual compounds and each block of experiments (sugar−glycine, sugar−proline, and sugar−cysteine models and wafer baking) after having pooled together the SD values that were derived from each pair of mean and CV results; they are reported in Tables 3−5. All LSD values correspond to confidence levels of 95%. In each block, two mean values are considered significantly different at a 95% confidence level if their absolute difference is greater than the corresponding LSD value.

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RESULTS AND DISCUSSION Browning in Aqueous Reaction Mixtures. The absorbance at 420 nm was measured in the reaction mixtures (Table 2) in order to evaluate the impact of Maillard precursors on browning. The highest absorbance was obtained in the reaction mixtures with glycine (1.096 to 3.486) followed by those with proline (1.057 to 3.144) and cysteine (0.146 to 1.125). Except of panose in the presence of cystein, brown B


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Journal of Agricultural and Food Chemistry

observed much higher browning for isomaltose and melibiose than for disaccharides with (1→4) linkages, such as maltose, cellobiose, and lactose. In the presence of glycine, galactose induced browning far more strongly than glucose and fructose, leading to absorbance comparable to those in IOS mixtures. Interestingly, in the presence of proline, fructose revealed a higher browning potential than those of galactose and glucose. The impact of the structure of the reducing sugar on browning from the Maillard reaction was explained by the chemical stabilities of the respective Amadori compounds against further degradation resulting in browning.17,18 Lower browning in the mixtures with cysteine can be explained by formation of 2-hydroxyalkyl-substituted thiazolidine-4-carboxylic acid derivatives and their conversion to the corresponding anions, which are described as inhibiting browning because of their high stability.19 Generation of Maillard-Derived Odorants in Aqueous Reaction Mixtures. Selected Maillard-derived odorants, such as α-diketones (buttery), cyclic enolones (caramel), and

Table 2. Absorbance at 420 nm Measured in Aqueous Reaction Mixtures glucose galactose fructose maltose lactose panose isomaltose melibiose isomaltotriose

glycine

proline

cysteine

1.881 3.387 1.096 1.944 1.802 2.212 3.004 3.485 3.486

1.057 1.410 1.831 1.373 1.264 1.098 2.049 2.312 3.144

0.213 0.223 0.146 0.152 0.256 1.125 0.394 0.298 0.937

colorization was far more induced in the mixtures with IOSs than in mixtures with other oligosaccharides or even a majority of monosaccharides. These observations are well in line with the results published by Kato et al.,14 who in the presence of ovalbumin (50 °C, 65% relative humidity for 20 days)

Table 3. Formation of α-Diketones and Cyclic Enolones in Aqueous Reaction Systems 2,3-butanedione (1) b

2,3-pentanedione (2)

model system

mean (μmol/mol)

CV (%)

mean (μmol/mol)

glucose−glycine galactose−glycine fructose−glycine maltose−glycine lactose−glycine panose−glycine isomaltulose−glycine isomaltose−glycine melibiose−glycine isomaltotriose−glycine LSDd

790.9 1111.8 621.0 298.6 251.1 311.7 1137.1 1561.0 1529.8 2099.1 36.0

4.8 1.5 5.8 1.1 1.3 6.4 2.0 2.2 1.1 0.8

23.5 18.4 21.4 1.7 nd nd 25.5 74.4 60.1 51.4 3.6

glucose−proline galactose−proline fructose−proline maltose−proline lactose−proline panose−proline isomaltulose−proline isomaltose−proline melibiose−proline isomaltotriose−proline LSDd

168.0 229.6 267.0 119.0 117.1 138.6 493.5 441.5 445.5 707.2 16.0

5.7 9.4 1.7 2.3 3.0 2.9 1.7 2.5 3.0 1.1

38.0 39.4 54.1 14.5 18.1 nd 51.9 59.4 52.5 20.9 3.4

glucose−cysteine galactose−cysteine fructose−cysteine maltose−cysteine lactose−cysteine panose−cysteine isomaltulose−cysteine isomaltose−cysteine melibiose−cysteine isomaltotriose−cysteine LSDd

30.7 40.7 49.1 56.9 57.7 120.9 48.8 27.8 26.7 55.5 14.0

20.8 21.0 21.7 21.4 26.5 14.9 8.4 10.9 22.0 9.6

b

HDMFa (3)

maltol (4)

mean (μmol/mol)

CVb (%)

mean (μmol/mol)

CVb (%)

10.4 9.8 13.1 14.6   4.9 6.0 5.8 1.3

3.0 9.3 1.1 0.3 1.6 0.3 9.7 16.4 29.7 2.9 4.0

20.5 29.3 27.0 29.1 13.7 20.2 25.7 19.6 21.5 8.4

ndc nd nd 21.1 128.9 14.7 nd nd nd nd 4.9

   26.9 6.0 29.7    

2.4 6.5 3.5 2.6 2.5  6.0 2.7 9.2 5.9

2.8 4.7 3.6 0.4 1.3 0.7 18.4 19.5 22.5 10.4 4.3

28.6 27.5 26.1 18.2 5.8 29.8 24.2 28.1 19.7 23.3

1.3 1.4 1.8 20.9 79.1 21.3 2.9 2.1 2.1 1.7 2.3

5.7 12.2 16.3 13.2 4.6 6.7 25.4 11.7 13.6 6.8

1.2 3.4 0.9 0.2 0.1 0.5 3.2 2.5 2.2 2.1 0.7

13.3 28.5 13.4 14.9 27.9 28.9 25.5 17.6 13.5 20.3

nd nd nd 16.4 13.0 8.4 nd nd nd nd 1.0

   7.9 10.8 12.5    

CV (%)

Glycine

Proline

Cysteine          

nd nd nd nd nd nd nd nd nd nd

4-Hydroxy-2,5-dimethyl-3(2H)-furanone. bCoefficients of variation of five replicates for all sugars except isomaltotriose and panose, for which the coefficients of variation correspond to two replicates. cNot detected. dLeast significant difference corresponding to a confidence level of 95%. a

C


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Journal of Agricultural and Food Chemistry Table 4. Formation of Heterocyclic Compounds in Aqueous Reaction Systems 2-acetyl-1-pyrroline (5)

2-propionyl-1-pyrroline (6)

model system

mean (μmol/mol)

CVa (%)

mean (μmol/mol)

glucose−proline galactose−proline fructose−proline maltose−proline lactose−proline panose−proline isomaltulose−proline isomaltose−proline melibiose−proline isomaltotriose−proline LSDc

112.3 89.1 115.3 32.2 38.4 18.3 105.0 148.8 148.3 135.5 5.6

5.6 5.9 3.2 2.8 2.5 1.2 2.9 1.3 3.6 0.6

1.5 1.6 1.7 0.4 0.6 0.2 1.1 2.3 2.4 2.4 0.3

glucose−cysteine galactose−cysteine fructose−cysteine maltose−cysteine lactose−cysteine panose−cysteine isomaltulose−cysteine isomaltose−cysteine melibiose−cysteine isomaltotriose−cysteine LSDc


CVa (%)

2-acetyl-2-thiazoline (7) mean (μmol/mol)

2-acetylthiazole (8)

CVa (%)

mean (μmol/mol)

CVa (%)

Proline 16 14 8 32 15 6 24 8 7 18

ndb nd nd nd nd nd nd nd nd nd

         


         

Cysteine          

         


8.5§ 6.4 16.8 5.9 4.2 1.9 23.8 7.9 4.7 2.3 3.9

28.7 28.7 26.7 13.9 11.1 21.1 22.6 18.0 21.0 24.5

0.04 0.07 0.39 0.02 0.01 0.05 2.04 0.11 0.08 0.14 0.08

15 9.1 31.8 27.4 28 16.1 5.6 15.5 15.7 3.3 15

a Coefficients of variation of five replicates for all sugars except isomaltotriose and panose, for which the coefficients of variation correspond to two replicates. bNot detected. cLeast significant difference corresponding to a confidence level of 95%.

pentanedione (2). For example, isomaltose yielded 74.4 μmol/ mol 2,3-pentanedione (2), whereas maltose yielded only 1.7 μmol/mol (a factor of 44) in the presence of glycine. In the mixtures with cysteine, no clear trend was observed, apart from the fact that panose showed prominent potential to generate 2,3-butanedione (121 μmol/mol). 2,3-Pentanedione (2) was not detected in any of the mixtures containing cysteine. Several labeling studies have indicated that both diketones are formed either by condensation of carbonyl fragments or from intact sugar skeletons.20−23 Our previous study applying the carbon-module-labeling technique (CAMOLA) revealed that wet heating (135 °C for 20 min, pH 7) of glucose with proline or glycine generates both diketones entirely (proline) or mainly (glycine) by recombination of carbonyl fragments.23 Combinations of C3 and C1 fragments (Figure 1A) and of C3 and C2 fragments (Figure 1B) were found as the major formation pathways of 2,3-butanedione (1) and 2,3-pentanedione (2), respectively. The higher potentials of IOSs as compared with those of disaccharides with (1→4) linkages can be best explained by increased generation of 1-hydroxy-2propanone (acetol), which is proposed to be the C3 fragment involved in the formation of both diketones. The retro-aldol cleavage of isomerized 1-deoxyosone providing acetol with the intact C1−C2−C3 carbon chain of the sugar (Figure 2) was reported as major pathway of acetol formation.24 It is likely that retro-aldol cleavage of C3−C4 of the sugar at the reducing end is significantly suppressed in (1→4) disaccharides because of the sugar moiety bound at the C4−OH position. On the other hand, IOS undergoes this cleavage more easily because of the sugar moiety bound at the C6−OH position. The αdiketones are then formed after the condensation of acetol

several roasty- and nutty-smelling heterocycles, were detected in the headspaces of the reaction mixtures. In order to be able to analyze all odorants in the same run, including the cyclic enolones, higher-temperature SPME experiments were privileged (80 °C). To minimize the possible formation of artifacts upon SPME, no equilibration was used, to shorten the exposure of the sample to this higher temperature. As the quantification was done by isotope-dilution assays, and the temperature of the extraction was far below the reaction temperature used for the experiments (120 °C), no impact on the yields was observed as compared with analysis at 60 °C with equilibration time (15 min of equilibration with 15 min of extraction, data not shown). The generation of monitored odorants showed good repeatability between individual heating experiments, with coefficients of variation below 30%. In general, yields of odorants (Tables 3 and 4) were higher in the mixtures with glycine and proline than in the mixtures with cysteine, which can be explained by the formation of the anionic form of thiazolidine carboxylic acid, which is very stable against further degradation.19 Generation of α-Diketones. In the majority of the reaction mixtures with glycine or proline, IOSs revealed superior potential to generate 2,3-butanedione (1) and 2,3pentanedione (2, Table 3). For example, isomaltose and isomaltotriose in the presence of proline generated, respectively, 2.6- and 4.2-fold more 2,3-butanedione (1) than glucose. In the presence of glycine or proline, the generation of diketones was dependent on the linkage in the disaccharide. Yields of 2,3-butanedione (1) were 4- to 6-fold higher in mixtures with (1→6) linked disaccharides (isomaltose and melibiose) than in mixtures with (1→4) linked disaccharides (maltose and lactose). Similar trends were observed for 2,3D


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Figure 1. Schematic formation of selected odorants. R1, sugar residue.

with formaldehyde and acetaldehyde22,23,25 (Figure 1A and 1B). The predominant potential of panose to form 2,3butanedione (1) in the presence of cysteine was surprising, and further research is needed to explain this phenomena. Generation of Cyclic Enolones. The yields of the cyclic enolones 4-hydroxy-2,5-dimethyl-3(2H)-furanone (HDMF, 3) and 2-methyl-3-hydroxy-4H-pyran-4-one (maltol, 4) were strictly dependent on the sugar structure (Table 3). The oligosaccharides with (1→6) linkages at their reducing ends (isomaltulose, isomaltose, melibiose, and isomaltotriose) revealed high potential to generate HDMF (3), whereas maltol (4) was generated predominantly from oligosaccharides with (1→4) linkages at the reducing ends (maltose, lactose, and panose). Although the latter was expected and is welldocumented in the literature, the high potential of IOSs to generate HDMF (3) was truly surprising.

Disaccharides with (1→6) linkages generated far more HDMF (3) than disaccharides with (1→4) linkages (by a factor of 17 to 51). The highest potential was detected for melibiose in the presence of glycine and proline, which provided, respectively, 11- and 8-fold higher HDMF (3) yields than glucose. The higher potential of melibiose as compared to other IOSs could be related to presence of galactose in this disaccharide. In the presence of glycine, the system with galactose provided significantly more (9.3 μmol/mol) HDMF (3) than the systems with glucose (3.0 μmol/mol) and fructose (1.1 μmol/mol). In the presence of proline, galactose also provided a higher yield (4.7 μmol/mol) of HDMF (3) as compared with other monosaccharides (2.8 and 3.6 μmol/ mol); however, this difference was not significant. In the presence of cysteine, galactose generated levels (3.4 μmol/ mol) of HDMF (3) comparable to those of isomaltulose (3.2 μmol/mol). Other IOSs generated lower amounts (2.1−2.5 μmol/mol) of HDMF (3); however, those levels were still E


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Figure 2. Formation of reactive intermediates from iso-oligosaccharides. R1, sugar residue; R2, amino acid residue; RA(3,4), retro-aldol cleavage at C3−C4.

significantly higher as compared with those from the disaccharides with (1→4) linkages (0.1−0.5 μmol/mol) and those from glucose (1.2 μmol/mol) and fructose (0.9 μmol/ mol). Previous studies revealed that HDMF (3) is formed by thermal degradation of hexoses, proceeding either through incorporation of an intact sugar skeleton or recombination of C3-fragments (2-oxopropanal and dihydroxyacetone).26 Acetylformoin, a reactive degradation product of hexoses, was established as a key intermediate in the formation of HDMF (3). The prominent potential of IOSs can be best explained by enolization of 1-deoxyosone followed by the loss of the intact sugar moiety, which yields acetylformoin directly (Figure 2). HDMF (3) is then formed upon a redox reaction of acetylformoin with reductones, such as acetylformoin itself or other reductones available in the reaction system, or via a Strecker-type reaction with amino acids (Figure 1H).26 Analogous HDMF (3) formation was described for sugar 6phosphates, particularly for glucose-6-phosphate, fructose-6phosphate, and fructose-1,6-diphosphate in wheat bread crust and bakers yeast.27 It was suggested that a phosphate group at the C6−OH position is a good leaving group, for which elimination followed by enolization results in acetylformoin. Our previous studies (unpublished data) revealed that corresponding sugar 6-phosphates, upon reaction with glycine under the same conditions as applied in the present study, yield significantly more HDMF (3) than the original sugars (factors of 6 and 21 for glucose and fructose, respectively). These data thus indicate that both a sugar moiety and a phosphate group bound at C6−OH position are better leaving groups than a hydrogen atom and hence facilitate the formation of acetylformoin, the key HDMF (3) intermediate.

This mechanism is in fact similar to the mechanism of maltol (4) formation from (1→4) linked disaccharides, which is welldocumented in the literature. It was suggested that maltol (4) is generated from the β-pyranone that is formed after the elimination of water from the cyclic form of 1-deoxyosone.28,29 Enolization and loss of the intact sugar moiety yields maltol (4). In the reaction systems with glycine or cysteine, maltol (4) was detected exclusively in mixtures containing maltose, lactose, or panose. In the systems with proline, maltol (4) was detected in all mixtures, with prominent yields in the mixtures containing maltose, lactose, or panose. Lactose in the presence of proline generated almost 4 times more maltol (4, 79.1 μmol/mol) than maltose and panose (20.9 and 21.3 μmol/mol, respectively) and 62 times more maltol than glucose (1.3 μmol/mol). These results confirmed the literature data, showing that maltol (4) is formed in a Maillard reaction from reducing disaccharides but hardly from monosaccharides30 and reporting that maltose and lactose are effective maltol (4) precursors for the reaction in aqueous systems.31 Generation of Heterocyclic Compounds. The generation of four roasty- and nutty-smelling heterocyclic compounds, namely, 2-acetyl-1-pyrroline (2-AP, 5), 2-propionyl-1pyrroline (2-PP, 6), 2-acetyl-2-thiazoline (2-ATZN, 7), and 2acetylthiazole (2-AT, 8), was monitored in the systems with proline and cysteine. IOSs generated substantially higher amounts of 2-AP (5) and 2-PP (6) than (1→4) linked disaccharides. In several of the systems, the potentials of IOSs were even higher than the potentials of the monosaccharides (Table 4). The formation of 2-AP (5) is most probably linked to effective generation of acetylformoin (Figure 2), which was suggested as an intermediate of 2-AP (5).32 The formation of 2-AP (5) was F


Article

G

a

4-Hydroxy-2,5-dimethyl-3(2H)-furanone. bCoefficient of variation of three replicate wafer trials with duplicate analysis of each wafer. cLeast significant difference corresponding to a confidence level of 95%.

0.3 3.2 10.0 2.5 1.2 10.6 5.8 2.0 7.6 3.5 2.3 8.0 167.3 127.0 404.3 70.4 63.5 84.8 843.7 225.7 218.9 225.3 22.0 24.8 9.6 18.3 11.9 9.5 11.9 7.2 9.6 2.6 4.2 0.1 1.3 29.9 20.8 72.5 10.7 9.4 11.8 155.0 40.4 35.9 38.5 12.0 11.4 21.5 30.4 28.3 30.1 23.9 1.3 15.3 18.0 10.4 0.3 0.1 8.0 6.3 13.6 2.1 2.1 1.9 14.1 7.9 6.5 6.1 3.7 24.8 28.5 6.9 12.0 21.5 19.7 1.9 22.9 19.2 20.4 11.5 2.7 20.4 33.2 26.1 7.8 7.1 7.0 21.9 19.5 18.1 13.1 7.2 28.2 11.0 1.5 19.8 25.1 28.6 4.4 19.2 24.5 16.2 23.0 503.8 501.5 1076.1 1062.1 1190.8 1075.9 610.8 1001.8 654.1 752.5 389.7 398.0 13.7 16.8 28.6 29.1 28.1 23.6 10.6 21.8 1.0 6.7 2.7 146.2 268.4 524.8 196.9 55.7 65.9 55.0 462.5 590.5 665.8 561.0 139.0 20.8 7.0 17.5 17.4 18.5 15.8 19.6 4.7 13.6 18.7 7.9 38.7 201.4 235.9 115.9 52.8 53.2 48.8 153.8 185.9 189.1 169.0 47.0 22.5 17.2 24.6 22.4 18.9 23.1 2.0 17.5 8.6 7.6 4.3 267.0 1061.9 1204.0 1065.4 859.4 735.3 754.0 1500.5 1886.6 1600.1 1581.5 406.0 reference glucose galactose fructose maltose lactose panose isomaltulose isomaltose melibiose isomaltotriose LSDc

CVb (%) mean (μg/kg) CVb (%) mean (μg/kg) CVb (%) mean (μg/kg) CVb (%) mean (μg/kg) CVb (%) mean (μg/kg) CVb (%) mean (μg/kg) mean (μg/kg) CVb (%) mean (μg/kg)

CVb (%)

2-acetyl-2-thiazoline(7) 2-propionyl-1-pyrroline (6) 2-acetyl-1-pyrroline (5) maltol (4) HDMFa (3) 2,3-pentane-dione (2) 2,3-butanedione (1)

Table 5. Concentrations of Odorants in the Reference Wafer and in the Wafers with Added Amino Acids (Glycine, Proline, and Cysteine) and Individual Sugars

postulated to proceed via aldol addition of 1-pyrroline (a Strecker-degradation product of proline) to acetylformoin (Figure 1G), followed by retro-aldol cleavage yielding 2acetylpyrrolidine, which oxidizes to 2-AP (5). An alternative pathway for 2-AP (5) formation is a reaction of 1-pyrroline with a hydrate of 2-oxopropanal (methylglyoxal, Figure 1E)33 or with 1-deoxyosone (Figure 1F).34 Formation of 2-oxopropanal (Figure 2) is likely to be more effective from oligosaccharides with (1→6) linkages at their reducing ends. The formation of 2-oxopropanal was postulated via retro-aldol cleavage of 1-deoxyosone or 3-deoxyosone (Figure 2).24,35 Similarly to the formation of acetol discussed above, it appears that the formation of 2-oxopropanal is also significantly suppressed in (1→4) linked disaccharides because of the sugar moiety at the C4−OH position, which hinders the retroaldol cleavage of C3−C4. In addition, the oxidation of acetol may also be considered to contribute to the generation of methylglyoxal. The formation of 2-PP (6) was proposed to proceed analogously via the reaction of 1-pyrroline and the hydrate of 2-oxobutanal (ethylglyoxal).36 Several studies showed that methylglyoxal is also a key intermediate in the formation of 2-ATZN (7) and 2-AT (8).37,38 The formation was proposed to proceed via reaction with cysteamine (Figure 1D), a decarboxylation product of cysteine, yielding 2-acetylthiazolidine, which then oxidizes to 2-ATZN (7) and further to 2-AT (8). Interestingly, isomaltulose generated significantly higher levels of 2-ATZN (7, 23.8 μmol/mol) and 2-AT (8, 2.04 μmol/mol) than other IOSs tested (2.3 to 7.9 μmol/mol 2-ATZN and 0.08−0.14 μmol/mol 2-AT). Similarly fructose generated significantly higher levels of both compounds as compared with glucose or galactose. The higher levels generated from fructose and isomaltulose could be explained by the formation of 2-ATZN (7) and 2-AT (8) via alternative reaction pathways starting from 4-deoxyosone (derived from fructose by the elimination of water) and cysteamine, as proposed by Engel and Schieberle (Figure 1C).39 Isomaltulose generated more 2-ATZN (7, a factor of 1.4) and 2-AT (8, a factor of 5) than fructose, thus indicating the positive impact of the bound sugar moiety at the C6−OH position on the formation of both compounds. Generation of Selected Maillard-Derived Odorants in the Wafer Model. In order to validate the findings from the simple model systems in more complex reaction environments and under different temperature and moisture conditions, the generation of selected odorants from IOSs was studied upon wafer baking and compared with that from other oligosaccharides and monosaccharides. To increase the complexity compared with the binary model system, the reactions of individual sugars were studied in the presence of all three amino acids (glycine, proline, and cysteine). The reference wafer was prepared analogously without added sugar and amino acids. Table 5 depicts the concentrations of odorants in the reference wafer and in the wafers with added amino acids and individual sugars. The generation of monitored odorants during three replicate wafer trials showed good repeatability, with coefficients of variation below 30%. The coefficient of variation of duplicate wafer analysis was below 10%. As expected, the majority of monitored odorants increased after spiking of the wafers with Maillard precursors. Similarly to the model systems, the position of the glycosidic linkage had an important impact on the formation of the monitored

2-acetyl-thiazole (8)



Article


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odorants from the disaccharides. With the exception of maltol (4), the disaccharides with (1→6) linkages generated significantly higher levels of the monitored odorants than the disaccharides with (1→4) linkages and similar or sometimes even higher levels than the corresponding monosaccharides. For example, the highest yields of 2-ATZN (7) and 2-AT (8) were detected in the wafers containing isomaltulose (155 and 843.7 μg/kg, respectively) followed by those containing fructose (72.5 and 404.3 μg/kg, respectively). This is well in line with the findings in aqueous models and indicates the potential role of 4-deoxyosone in the formation of both compounds. Similarly, the results from the wafers confirmed the high potential of galactose (524.8 μg/kg) and the galactose-containing IOS melibiose (665.8 μg/kg) to generate HDMF (3). Despite the different process conditions, the findings from the food model confirmed the results from the simple Maillard system. This newly obtained data brought, for the first time, evidence about the unexpected potential of IOSs in Maillard reactions that result in the formation of several potent food odorants. This group of carbohydrates may thus bring, in addition to health benefits, improvement of the organoleptic properties of thermally processed food.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +41244427342. E-mail: [email protected]. com. ORCID

Tomas Davidek: 0000-0002-4555-8492 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Enrico Alberto Chavez from Nestlé Research and Development Orbe for statistical treatment of the data.

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ABBREVIATIONS USED IOS, iso-oligosaccharide; IMO, isomalto-oligosaccharide; GOS, galacto-oligosaccharide; HS, headspace; SPME, solidphase microextraction; GC, gas chromatography; MS/MS, tandem mass spectrometry; DVB/CAR/PDMS, divinylbenzene−carboxen−polydimethylsiloxane; MRM, multiple-reaction monitoring; HS-EI, high-sensitivity electron-ionization source; CAMOLA, carbon module labeling; HDMF, 4hydroxy-2,5-dimethyl-3(2H)-furanone; 2-AP, 2-acetyl-1-pyrroline; 2-PP, 2-propionyl-1-pyrroline; 2-AT, 2-acetylthiazole; 2ATZN, 2-acetyl-2-thiazoline

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REFERENCES

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Unexpected Potential of Iso-oligosaccharides in the Generation of

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