W.L.P. Bredie and M.A. Petersen (Editors)
Flavour Science: Recent Advances and Trends
© 2006 Elsevier B.V. All rights reserved.
Formation of 4-hydroxy-5-methyl-3(2i/)-furanone
(norfuraneol) in structured fluids
Imre Blank, Tomas Davidek, Stephanie Devaud, Laurent
Sagalowicz, Martin E. Leser and Martin Michel
Nestle Research Center, Vers-chez-les-Blanc, 1000 Lausanne 26,
Switzerland
ABSTRACT
This study investigated the influence of structured fluids on Maillard-type reactions.
The decomposition of xylose and the formation of volatile compounds were affected by
the type of structured fluid used as reaction medium. In model systems based on xylose
and glycine or leucine, xylose was preferably degraded in the hexagonal phase,
compared to other mesophases and the aqueous sample. In parallel, norfuraneol was
accumulated in the hexagonal phase. The data obtained indicate that molecular
organisation of the reaction medium and flavour precursors can play an important role
in food systems containing ingredients that tend to form self-assembly structures.
1. INTRODUCTION
The use of structured fluids as micro-reactors for food applications has recently been
highlighted in enzymatic and Maillard-type reactions [1]. Micro-emulsions are capable
of solubilising considerable amounts of water-soluble and oil-soluble guest molecules.
This represents an attractive approach to make better use of the potential of flavour
precursors with respect to increasing the yields, guiding reaction pathways, stabilising
flavour compounds generated, and controlling their delivery in the proper moment of
use. Water-in-oil microemulsions have recently been reported as a means to generate
and deliver novel flavours by thermal degradation of suitable intermediates during
frying [2]. Reaction of pentose sugars in the presence of cysteine resulted in higher
amounts of the odorants 2-methyl-3-furanthiol and 2-furfurylthiol in cubic phases based
on water/monoglyceride (20/80), as compared to aqueous systems [3]. This paper deals
with flavour formation using various structured fluids as reaction media. We focus on
the formation of the well-known flavour intermediate norfuraneol from xylose in the
presence of glycine or L-leucine.
2. MATERIALS AND METHODS
Materials. The following chemicals were commercially available: monosodium
dihydrogenphosphate monohydrate, anhydrous sodium sulfate, sodium chloride (NaCl),
aqueous hydrochloride (HC1) solution, diethylether (Merck, Darmstadt, Germany); Dxylose (Xyl), glycine (Gly), L-leucine (Leu), 2-hydroxy-3-methyl-2-cyclopenten-l-one
(cyclotene, Fluka, Buchs, Switzerland); monoglycerides Dimodan U and Dimodan HR
(Danisco, Copenhagen, Denmark).
Reactions performed in phosphate buffer: A solution of xylose (0.330 g, 2.2 mmol) and
glycine (0.165 g, 2.2 mmol) or leucine (0.288 g, 2.2 mmol) in phosphate buffer (10 ml,
0.2 mol/1; pH 6.0) was heated in a Pyrex tube (30 ml, 70 °C, 7 h). Aliquots of the
reaction mixture were taken at regular time intervals for analysis. Each reaction mixture
(2 g) was diluted with water (8 g), cyclotene (372 jig) was added as internal standard,
and the mixture saturated with NaCl (3 g). The pH was adjusted to 4 (aqueous HC1, 2
mol/1) and the volatiles were extracted with diethylether (2 x 15 ml, 45 min). The
organic phase was separated, dried over sodium sulfate, and concentrated to 0.5 ml
using a Vigreux column (50 cm x 1 cm) and a micro-distillation device [5]. All
experiments were performed in duplicate.
Reactions performed in mesophases. The monoglyceride (8 g) in a Pyrex tube (30 ml)
was heated in a silicone bath (116 °C). After 23 min, a solution of Gly (0.033 g, 0.44
mmol) or Leu (0.057 g, 0.44 mmol) in phosphate buffer (1 ml, 0.2 mol/1, pH 6.0)
preheated (3 min, 116 °C), was added. After vigorous stirring (Vortex) the tube was
further heated (116 °C) and stirred until a homogonous mixture was obtained (22 min).
Then, a solution of Xyl (0.066 g, 0.44 mmol) in phosphate buffer (1 ml, 0.2 mol/1 pH
6.0) preheated (3 min, 116 °C), was added. The mixture was vigorously stirred, heated
(5 min, 116 °C), and finally cooled down to room temperature. The tube containing the
reaction mixture was heated in a silicone bath (70 °C, up to 7 h). Dimodan U and
Dimodan HR led to the hexagonal and lamellar phase, respectively. To obtain the cubic
phase, a mixture of Dimodan HR and Dimodan U (1:1, by weight) was used.
Each reaction mixture (10 g) was transferred into a Pyrex bottle (250 ml). Diethyl ether
(50 ml) and the internal standard (372 \±g of cyclotene) were added and the mixture was
shaken (30 min). Samples containing Dimodan HR were passed through a paper filter to
separate undissolved monoglycerides. Samples containing Dimodan U were completely
dissolved and thus not filtered. The resulting solution was extracted with phosphate
buffer (2 x 30 ml, 0.1 mol/1, pH 8.0) and the combined extracts were centrifuged (2500
rpm, 5 min). The aqueous phase was saturated with NaCl (18 g) followed by the cleanup described above. All experiments were performed in duplicate.
Analysis of xylose. This was carried out by high performance anion exchange
chromatography (HPAEC) using an electrochemical detector (ECD), as recently
described [4]. An aliquot of the reaction mixture in phosphate buffer was diluted 250times with deionised water and filtered through a PVDF filter (polyvinylidene fluoride,
0.22 |Lim/25 mm). The mesophase-based reaction mixture (10 g) was transferred into a
Pyrex bottle (100 ml). Diethyl ether (60 ml) was added and the mixture was shaken (30
min) to break the mesophase. Water (18 ml) was added and the mixture was shaken (35
min). Samples containing Dimodan HR were centrifuged (3500 rpm, 15 min) to
separate the aqueous and organic phase. Samples containing Dimodan U were
completely separated without centrifugation. The water phase was diluted 25-times with
deionised water, filtered through a PVDF filter, and analysed by HPAEC-ECD.
Analysis ofnorfuraneol. This was performed by gas chromatography-mass spectrometry
(GC-MS) using a GC 6890A coupled to an MSD 5973N (Agilent, Palo Alto, USA) and
equipped with a DB-Wax capillary column (J&W Scientific, Folsom, USA): 60 m x
0.25 mm, film thickness 0.25 |im. Carrier gas: helium (1.5 ml/min, constant flow).
Sample injection: 1 jil, splitless, 250 °C. Oven temperature program: 2 min at 35 °C, 6
°C/min to 240 °C with a hold for 25 min. Ion source temperature: 280 °C. The electron
impact (El) MS spectra were generated at 70 eV.
3. RESULTS AND DISCUSSION
Model systems based on xylose and glycine. Various Maillard model systems were
compared by heating precursors under mild reaction conditions (70 °C) in aqueous
buffer solution (pH 6) and several monoglyceride-based mesophases, such as lamellar,
cubic, and hexagonal phases. As shown in Figure 1, xylose was preferably decomposed
in the hexagonal phase. In agreement with that, norfuraneol was accumulated in the
hexagonal phase, as compared to other mesophases and the aqueous buffered sample.
Norfuraneol (ug/mmol xyl)
Residual xylose (%)
Buffer
Lamellar
Cubic
Hexagonal
Buffer
Lamellar
Cubic
Hexagonal
Figure 1. Decomposition of xylose in the presence of glycine at 70 °C after 7 h and concomitant
formation of its degradation product norfuraneol.
Model systems based on xylose and leucine. Maillard model systems were studied based
on xylose and L-leucine in various mesophases and aqueous buffer solution (pH 6)
under the same mild reaction conditions (70 °C). Xylose was again preferably
decomposed in the hexagonal phase (Figure 2). On the other hand, norfuraneol was
readily generated in the hexagonal phase, as compared to the aqueous buffered sample.
350
Xylose/Glycine
i: •
Buffer
Xylose/Leucine
1y
53
47
Hexagonal
Buffer
Hexagonal
Hexagonal
Figure 2. Decomposition of xylose (a) and formation of norfuraneol (b) in Xyl/Gly (black) and
Xyl/Leu (grey).
4. CONCLUSION
The data obtained in this study indicate that molecular organisation of the reaction
medium and flavour precursors can play an important role in food systems containing
ingredients that tend to form self-assembly structures. Both amino acids behave in the
same way with regard to the decomposition of xylose and formation of norfuraneol. The
accumulation of norfuraneol may be due the increased protection of the mesophase.
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