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Chapter 11

Biogeneration of Roasted Notes Based on 2-Acetyl-2-thiazoline and the Precursor

2-( 1 -Hydroxyethyl)-4,5-dihydrothiazole R. Bel Rhlid, Y. Fleury, S. Devaud, L. B. Fay, I. Blank, and M. A. Juillerat

Nestec Ltd, Nestlé Research Center, Vers-Chez-les-blanc, P.O. Box 44, 1000-Lausanne 26, Switzerland

Roasted notes contribute to the flavor of thermally processed foods such as meat and bread. 2-Acetyl-2-thiazoline (2-AT) is one of the key volatile compounds responsible for the roasted and popcom-like aroma character. We report on the biogeneration of flavoring preparations with intense roasted notes, which are characterized by a high content of 2-AT. Moreover, we found that the addition of 2 4 1-hydroxyethy1)- 4,5-dihydrothiazole (HDT) as aroma precursor added to pizza dough resulted in an enhancement of the roasted note.

Introduction

Several types of compounds are known to elicit a roasted note, mainly heterocyclic components formed in the course of the Maillard reaction, such as pyrazines, pyrrolines, pyridines, thiazolines and thiazines. These heterocyclic compounds are important constituents in many foods, such as cooked and roasted meat, bread, chocolate, coffee, and beer ( I ) . Among these aroma volatiles, thiazolines and thiazoline derivatives play a key role in roasted flavors, particularly in meat products (2), and have received increasing research attention

One of the most important thiazolines, which exhibits an intense roasted aroma character, is 2-acetyl-2-thiazoline (2-AT), (Figure 1). It was reported for the fxst time as a volatile constituent of beef broth (4) and was later

(3).

HETEROATOMIC AROMA COMPOUNDS (ACS Symposium Series 826) Edited by Gary A. Reineccius and Terry A. Reineccius Washington : American Chemical Society, 2002 ISBN 0-8412-3777-8

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identified as a sensory relevant constituent of roasted beef (5). Several methods to generate 2-AT by organic synthesis (6) and the Maillard reaction (7) have been published. Moreover, Hofmann and Schieberle (7) proposed a reaction pathway involving cysteamine and methylglyoxal as substrates to produce 2-( 1 - hydroxyethyl)-4,5-dihydrothiazole (HDT), 2 (Figure 1), which was then transformed to 2-AT by heat treatment in a model reaction. However, the impact of HDT as a potential precursor of 2-AT in a food model has never been demonstrated.

OH 2-acetyl-2-thiazoline 2 4 1 -hydroxyethyl)-4,5 -dihydrothiazole

2 - 1 -

Figure 1. Important thiazolines having intense roasted aroma characters.

This paper reports on the generation of roasted notes based on aerobic incubation of cysteamine, ethyl-L-lactate and D-glucose with baker's yeast. Sensory-directed chemical analysis was applied to characterize the key flavor compounds and the reaction intermediates. In addition, we report here on the potential of HDT as aroma precursor to improve the roasted note of pizza dough after baking.

Experimental

Bioconversion

A flavoring preparation was obtained by fermentation of cysteamine and ethyl-L-lactate with baker's yeast (Figure 2). Commercial yeast cream solution (150 ml) was placed in a 500 ml flask equipped with an electrode and a magnetic stirrer (500 rpm). The flask was kept at 35OC using an oil bath and the pH adjusted to 9.8 with 2M sodium hydroxide. The pH was automatically maintained throughout the reaction using a Metrohm pH-stat device (Impulsomat 614). Cysteamine (385 mg, 5 m o l ) and ethyl-L-lactate (590 mg, 5 mmol) were then added. Ten and 5 g of D-glucose were added after 4h and 24h of incubation, respectively. A kinetic study was performed and after 48h of reaction, the mixture was centrifuged and the supernatant was further treated.

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cysteamine e thy l-L- lacate D-glucose I

Centrifugation

Supernatant

1 Extraction Heat treatment Heat treatment

pH 9.8 pH 6.5 pH 10

extraction extraction 1 i 4

Figure 2. General procedure for preparation offlavor samples.

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Preparation of Flavoring Samples

Thrty ml of the liquid phase (supematant), obtainec as described above (Figure 2), were saturated with sodium chloride and extracted with diethyl ether to give sample A. Another 30 ml of the liquid phase were acidified to pH 6.5 with 15% hydrochloric acid and refluxed for 30 min in a 50 ml flask equipped with a reflux condenser and magnetic stirrer to obtain sample B. A third part of the supernatant (30 mi) was refluxed in the same manner as above but without acidification (pH 10) to obtain sample C. After cooling to room temperature, the aqueous solutions were evaluated sensorially, then saturated with sodium chloride and extracted with diethyl ether overnight using a rotary perforator (liquid-liquid extraction). The organic phases were dried over anhydrous sodium sulfate and purified by high vacuum distillation at 3 x mbar. The contents of the traps were combined, dried and concentrated to about 1 ml for chromatographic analyses.

Sensory Evaluation

The aroma of each sample was evaluated by sniffmg the headspace of the freshly prepared samples (A, By C). Ten assessors were asked to describe the aroma quality and intensity. For descriptive analysis, a limited number of aroma descriptors were provided to the panel in order to reduce the number of attributes and simpli@ the aroma characterization. The intensity range of the aroma was scored from 1 (weak) to 3 (intense).

Analyses by Gas Chromatography and Mass Spectrometry

GC and GC-O analyses were performed using a Carlo Erba gas chromatograph (Mega 2, GC 8000) equipped with automatic cold on-column injector, FID, and sniffing port. Fused silica capillary columns (OV-1701 and DB-FFAP) were used, both 30 m x 0.32 mm and film thickness 0.25 pm. The temperature program for the OV-1701 was: 35OC (2 min), 40°C/min to 5OoC ( 1 min), 6OC/min to 24OOC (10 min), and for the FFAP was: 5OoC (2 min), 6"C/min to 18OoC, 10°C/min to 24OOC (10 min). GC-MS analyses were carried out using a Finnigan MAT-8430 mass spectrometer using the same GC conditions described above. The MS-EI spectra were generated at 70 eV and MS-CI at 150 eV with ammonia as the reagent gas.

Applications of HDT in Pizza

HDT is commercially unavailable and was prepared by microbial reduction of the carbonyl group of 2-AT using baker's yeast as biocatalyst. This approach

183

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2-AT 1 -

baker's yeast

ÒH HDT

2 -

Figure 3. Biogeneration of 2-(I -hydroxyethyl)-4,5-dihydrothiazole.

was identified as the most appropriate and easiest way to produce HDT in one step and under mild conditions (Figure 3).

The application of HDT was performed in two ways using pizza dough as a food model. In the first application, a solution of HDT (1,6 mg/ml) was mixed with other ingredients of the dough. Three different concentrations of HDT were tested, ranging from 0.01 to 1.0 g of HDT solution per 50 g of raw dough. In the second application, HDT solution was applied as a surface coating at five different levels ranging fiom 0.04 to 1.0 g per 50 g of raw dough. The highest amount corresponded to the maximum limit before appearance of white spots during baking.

For the frozen pizza, samples were pre-baked for 8 min at 220 OC, wrapped in plastic bags without modified atmosphere and kept frozen for 2 weeks. For the refrigerated pizza, samples were wrapped in plastic bags with modified atmosphere (50% N2/ 50% 02) and kept refrigerated for maximum of 1 week. Frozen and refrigerated pizza samples were baked for 8 and 15 min respectively at 2OOOC in a rotary convection oven.

Sensory evaluation by triangle test procedures was performed on the baked pizza samples. Samples spiked with HDT were compared to the corresponding reference samples. Tasting sessions were performed under red light to avoid visual identification of the different product.

Panelists (12) were asked to identi@ which pizza sample out of the three was different. The samples were presented in the following two schemes: 1 reference, 2 HDT spiked samples or 2 references and 1 HDT spiked sample. The assessors were asked to provide comments about the samples on why a sample was selected as different.

Results and Discussion

Biogeneration of Flavoring Preparations

A flavoring preparation of commercial interest was obtained by fermentation of cysteamine and ethyl-L-lactate with baker's yeast. Ten assessors

184

described the aroma quality of the freshly concentrated extract as roasted, dried sausage and sausage skin-like and of high odor intensity.

As shown in Table I, sixteen odor-active volatile compounds were detected by GC-O using two capillary columns of different polarity. Thirteen odorants were identified by matching retention indices, odor qualities and mass spectra with those of reference compounds, if available, or with literature data. 2- Methylthiazolidine, isovaleric acid and 2-acetyl-2-thiazoline the most intensely smelled compounds in the sample. Several other odorants were also identified in this flavoring preparation, i.e., 2-methyl-3-furanthiol (MFT), 3-mercapto-2- pentanone, 2-ethyl-3,5-dimethyl-pyrazine, and 2-AT. These odorants have been cited as characteristic constituents of boiled and roasted meat ( 4 3 . As shown in this study, these aroma impact compounds can also be generated by fermentation using suitable precursors and without applying any heat treatment.

Several, mainly cyclic, sulfur-containing compounds were also identified such as thiazolines and thiazolidines. Three thiazolines were identified as 2- acetyl-2-thiazoline 1, 2-( l-hydroxyethyl)-4,5-dihydrothiazole (HDT) 2 and 2- methyl-2-thiazoline 3 (Figure 4). 2-AT was the most intensely smelling odorant and is describesd as having a pleasant roasted, popcorn-like odor. This compound was identified by GC-O, GC-MS, and the sensory and chromatographic properties of this compound were identical to those of the reference compound.

In sample A, we identified HDT by matching retention indices and mass spectra with those published in the literature (7). This is the first time that these two compounds (HDT and 2-AT) have been generated via fermentation using baker’s yeast as a biocatalyst, and without applying any heat treatment. The predominant volatile compound generated in sample A was identified as N- acetyl cysteamine 4. This compound, which smells burnt, yeasty and musty, is probably the precursor of 2-methyl-2-thiazoline 3. Indeed, this last aroma compound was generated upon storage; most likely via intramolecular cyclisation followed by elimination of water (Figure 4). However, the contribution of this compound to the overall aroma appears to be rather low.

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O

N-acetylcysteamine 4 -

LL H OH T

H-O

CL N

L 2 -methyl-2-thiazoline - 3

Figure 4. Hypothetical formation of 2-methylthiazoline f iom N-acetyl cysteamine.

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A reference sample was obtained using the same incubation conditions as for sample A, but without addition of cysteamine and ethyl-L-lactate. After extraction and concentration as detailed in the methods section, the odor of the sample was described as only yeasty and musty. GC analyses did not show any of the sulfur-containing compounds found in sample A, thus indicating that the sulfur compounds were generated from cysteamine.

The thermal treatment of the supernatant at pH 6.5 and 10 resulted, after extraction, in samples B and C, respectively. The aroma qualities of these samples were described as roasted, popcorn, and bread crust-like, and all were found to have high odor intensities. However, sample B was more intense than Cy thus suggesting the importance of the pH to flavor formation during heat treatment.

The aqueous solutions obtained after heat treatment were extracted with diethyl ether. The extracts were then concentrated and analyzed by GC-O on two capillary columns of different polarity (FFAP and OV-1701). As shown in Table II, 2-acetyl-2-thiazoline was the dominant aroma compound in both samples B and C. This result is in good agreement with the sensory evaluation of the two samples which were clearly described as roasted and popcom-like.

The amount of 2-AT in sample B was higher than in sample C (Figure 5). However, sample C contained more 2-methylthiazolidine, 2-ethyl-3,5- dimethylpyrazine and trimethypyrazine than sample B. This observation explains the difference in aroma intensity and quality between the two samples, and shows the influence of the pH on the formation of some aroma impact components.

Impact of HDT as an Aroma Precursor

2 4 1 -hydroxyethyl)-4,5-dihydrothiazole has been proposed as a potential precursor of 2-AT in a model reaction. A possible formation mechanism of 2- AT was proposed by Hohann and Schieberle (7) as shown in Figure 6 .

However, the impact of HDT as a precursor to improve the roasted notes of baked goods has never been demonstrated in food models. As shown in Table III, HDT improved the roasted note of pizza dough. When this aroma precursor was incorporated into the dough, panelists described samples as roasted, toasted and popcorn-like. However, when HDT was applied as surface coating, these aroma notes were lost by release into the oven during baking; which led to a nice smelling odor in the kitchen, but less aroma remaining in the baked dough.

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comb ined b iohhermal processing.

189

I Disproportionation

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A Figure 6. Hypothetical formation of 2-ATproposed by Hofmann and Schìeberle.

(Reproduced fiom reference 7. Copyright 1995 American Chemical Sociew.)

190

Table III. Sensory Evaluation of Spiked Samples Compared to the Reference

Frozen Pizza Refr igerated Pizza

p-value = O. 14 different HDT mixed into the dough No difference Significantly

p-value = 0.001 HDT as surface coating Significantly different Significantly

p-value = 0.01 different p-value = 0.002

Conclusions

Bioconversion of cysteamine and ethyl-L-lactate with baker's yeast resulted in a natural flavoring which was described as dried sausage-like. When this reaction mixture was heated under acidic or alkaline conditions, the resulting samples I3 and C, respectively, exhibited attractive and intense roasted, popcorn and bread crust-like notes. High amounts of 2-AT were detected in these samples by different chromatographic techniques. Moreover, HDT seems to be a promising precursor to improve roasted note of baked goods.

Acknowledgements

The authors are grateful to M. Bilat, F. Beaud and S. Metairon for their help and expert technical ass is t ance .

References

1 . Hartmann, G.J.; Jin, Q.Z.; Lee, K.N.; Ho, C.T.; Chang, S. J: Agrzc. Food Chem. 1983,31,1030.

2. Sagaguchi, M.; Shibamoto, T. J . Agric. Food Chem. 1978,26, 1179. 3. Maga, J.A. Critical Reviews in FoodSci andNutr. 1975,4, 135. 4. Tonsbeek, C.H.T.; Copier, H.; Plancken, A.J. J: Agric. Food Chem. 1971,

19, 1014. 5. Cemy, C . ; Grosch, W. Z. Lebensm. Unters. Forsch. 1992, 194, 322. 6. Flament, I. U.S. Patent 3,881,025, 1975. 7. Hofmann, T.; Schieberle, P. 3: Agric. Food Chern. 1995,43, 2946.