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1 Department of Dairy Technology, Mustafa Kemal University, Antakya, Turkey
2 Department of Food Engineering, Onsekiz Mart University, Canakkale, Turkey
3 Department of Food Science, Southeast Dairy Foods Research Center, North Carolina State University, Raleigh
4 Department of Food Science and Human Nutrition, University of Illinois, Urbana
Corresponding author: M. A. Drake; e-mail: mdrake{at}unity.ncsu.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Key Words: Cheddar cheese • cheese flavor • nutty flavor • Strecker aldehyde
Abbreviation key: DHS-GC-MS = dynamic headspace analysis-gas chromatography-mass spectrometry, DSE = direct solvent extraction, FID = flame ionization detector, GC-MS = gas chromatography-mass spectrometry, GCO = gas chromatography-olfactometry, GCO-DHS = gas chromatography-olfactometry dynamic headspace analysis, HVD = high vacuum distillation, N = nutty, NN = not nutty
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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The majority of studies on nutty flavor in cheese have been carried out on Swiss type cheese due to its distinct sweet and nutty notes. A range of compounds, such as ketones, lactones, esters, alcohols, aldehydes, pyrazines, sulfurous compounds, carbonyl compounds, free fatty acids, free amino acids, and salts have been reported to contribute to nutty flavor (Biede and Hammond, 1979a, 1979b; Liardon et al., 1982; Vangtal and Hammon, 1986; Warmke et al., 1996; Preininger et al., 1996; Rychlik and Bosset, 2001a). Specifically, acetic and propionic acids, the major products of propionic acid bacteria, were claimed to play an important role in nutty flavors of this particular cheese type. It is important to note that descriptive sensory analysis using a defined sensory language was not conducted in these studies nor were model studies conducted to confirm or pinpoint the exact cause of nutty flavors.
Numerous studies have likewise been conducted to reveal the overall chemical profile of Cheddar cheese flavor and identify the most potent compounds (Milo and Reineccius, 1997; Suriyaphan et al., 2001; O’Riordan and Delahunty, 2001; Zehentbaur and Reineccius, 2002). The presence of pyrazines in Cheddar cheese has been reported in a few studies (Suriyaphan et al., 2001). In addition, 2-acetylthiazoline (Milo and Reineccius, 1997), 2-acetyl-1-pyroline, and 2-acetyl-2-thiazoline (Zehentbauer and Reineccius, 2002) were identified as compounds in Cheddar cheese that exhibited nutty aromas. However, no sensory analysis revealing the relationship between those compounds and the nutty flavor of Cheddar cheese were reported. In their research, Fernandez-Espla and Fox (1998) produced Cheddar cheese with nutty flavor using propionic acid bacteria as an adjunct culture. The researchers stated that this flavor was associated with increased levels of free amino acids and that the cheese flavor resembled that of Swiss cheese. Descriptive sensory analyses were not conducted.
Identifying specific chemical compounds associated with particular flavors requires extensive and specific instrumental and sensory analysis. First, the sensory perceived flavor of a food should be identified by descriptive sensory analysis. Descriptive sensory analysis qualitatively and quantitatively identifies all of the sensory-perceived flavor and tastes present in the food and provides definitions and references for the perceived flavors and tastes (Drake and Civille, 2003). Instrumental analysis can then be conducted to identify volatile compounds that contribute to flavor. The presence of a particular compound, identified by gas chromatography-mass spectrometry, does not necessarily mean the compound plays a role in flavor because many volatile compounds can be present in a food, but concentrations may be below actual sensory thresholds (McGorrin, 2002). Gas chromatography-olfactometry (GCO) can assist in identification of compounds that are actually present in the sensory threshold range, and it is often used as a way of further screening volatile compounds that play key roles in flavor (Friedrich and Acree, 1998). However, the aroma of an individual compound identified by instrumental analysis is not necessarily indicative of its role on flavor in a food due to interactions with the matrix and other compounds (Drake and Civille, 2003). Model systems, similar to the actual food, should then be constructed to evaluate the role of specific compound(s) on sensory-perceived flavor. To our knowledge, compounds responsible for nutty flavor in Cheddar cheese have not been reported and specifically linked to sensory perception of nutty flavor in Cheddar cheese. The purpose of this study was, therefore, to identify and quantify volatile compound(s) responsible for nutty flavor in Cheddar cheese.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Cheese Selection
Forty 5-kg blocks of Cheddar cheese were purchased on the retail market and screened for nutty flavor by 3 sensory experts, each with more than 150 h of experience in the sensory evaluation of cheese flavor. Cheeses selected (15 total) were 1 to 3 yr old. Cheeses with intense nutty (N) flavors (designated as N1, N2, N3, ..., N8) and without nutty (NN) flavor (designated as NN1, NN2, NN3, ..., N7) were selected for volatile aroma analyses and descriptive sensory analysis.
Chemicals
Diethyl ether (anhydrous, 99.8%), sodium chloride (99%), sodium sulfate (99%), and 2-methyl-3-heptanone (internal standard for neutral/basic fractions) were purchased from Aldrich Chemical Company (St. Louis, MO) and 2-methylpentanoic acid (internal standard for acidic fractions) was obtained from Lancaster (Windham, NH). Aroma compounds listed in Tables 2
, 3, 4, and 7 below were provided from the following commercial sources: numbers 1 to 4, 6, 8, 9, 10 to 12, 14 to 20, 22 to 26, 28, 29, 30, 33, 35, 37 to 39, 42 to 46, 50, 52 to 54, 56, 59, 61 to 66 (Aldrich Chemical Co.); numbers 47 and 48 (Sigma, St. Louis, MO), number 13 (Lancaster), and number 59 (Firmenich Inc., Plainsboro, NJ). Sodium bicarbonate (99.7%), hydrochloric acid (36.5%), and acetic acid (number 42) were obtained from Fisher Scientific (Pittsburgh, PA).
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. Panelists each received 75 h training on descriptive analysis of cheese flavor using the Spectrum descriptive analysis method (Meilgaard et al., 1999). Panelists evaluated cheeses individually in booths with free access to bottled water and unsalted crackers for palate cleansing. Panelists expectorated cheeses following analysis. Cheeses were presented in 2- x 2-cm cubes with 3-digit codes at 10°C. Cheeses were evaluated in triplicate by each panelist.
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High vacuum distillation (HVD).
Separation of volatile compounds from cheese extract was achieved by means of high vacuum distillation as described by Karagul-Yuceer and co-workers (2001). The assembly used was similar to that described by Sen and co-workers (1991). For HVD, the cheese solvent extract was transferred to a 1-L, round-bottom flask and immersed in a Dewar flask containing liquid nitrogen until it was frozen. The flask was then connected to a unit equipped with a rough pump/diffusion pump, as a vacuum source; a receiving tube; and a waste tube. The receiving tube and waste tube were held in separate Dewar flasks containing liquid nitrogen at all times. Distillation was carried out for 4 h under vacuum (approx. 10–5 Torr). For the first 2 h, the sample flask was kept at room temperature; for the second 2 h, the sample was kept in a water bath at 50°C. After distillation, the contents of the receiving flask were concentrated to 20 mL under a gentle stream of nitrogen gas. Concentrated distillate was then washed 2x with 15 mL of sodium bicarbonate (0.5 M) and 3x with saturated sodium chloride solution. The upper layer (ether) containing the neutral/basic fraction was collected, dried over anhydrous sodium sulfate, and concentrated to 0.5 mL under a gentle stream of nitrogen gas. To recover acidic volatiles, the bottom layer (aqueous phase) was acidified with hydrochloric acid (18%) to 2 to 2.5 pH, extracted 3x with diethyl ether, and dried over anhydrous sodium sulfate before concentrating to 0.5 mL under a nitrogen gas stream.
Dynamic headspace analysis-gas chromatography-mass spectrometry (DHS-GC-MS).
Dynamic headspace analysis was conducted using a CDS 6000 headspace analyzer (CDS Analytical, Inc., Oxford, PA). A cheese slurry mixture (5 g) was prepared by mixing grated cheese with deodorized water (1:2 ratio, wt/wt), to which 2 g of NaCl and 50 ppm of 2-methyl-2-butenal (internal standard) were added. The sample was placed in a dark area for equilibration at room temperature for 60 min. The sample was then placed into the analyzer where helium gas (30 mL/min) was purged into the mixture at 30°C for 30 min. Once the purge process was complete, the trap was dry-purged for 10 min to eliminate moisture prior to desorption and transfer. The transfer line was set at 180°C. The injector temperature was 250°C. Volatiles were thermally desorbed and injected in split mode (1:1 ratio, vol/vol). The chromatographic separation system consisted of an HP GC 5890 and MSD 5972 (Hewlett-Packard, Palo Alto, CA) equipped with a DB5 column (30-m x 0.25-mm i.d. x 0.25-µm film thickness, J&W Scientific, Folsom, CA). The oven temperature was programmed from –20 to 220°C at 8°C/min from –20 to 60°C for 0 min, and 6°C/min from 60 to 220°C for 5 min. Initial and final hold times were 6.5 and 5 min, respectively.
Gas chromatography-olfactometry (GCO) of solvent extracts.
Two methods of sniffing were used for solvent extract fractions: postpeak intensity and aroma extract dilution analysis (Friedrich and Acree, 1998; Van Ruth, 2001). For postpeak intensity, an HP5890 series II gas chromatograph (Hewlett-Packard, Palo Alto, CA) equipped with a flame-ionization detector (FID), a sniffing port, and a splitless injector was used. From each fraction of every extract, 2 µL was injected into a polar capillary column (DB-WAX 30-m length x 0.25-mm i.d. x 0.25-µm film thickness (df); J&W Scientific) and a nonpolar column (DB-5ms 30-m length x 0.25-mm i.d. x 0.25-µm df; J&W Scientific). Column effluent was split 1:1 between FID and sniffing port using deactivated fused silica capillaries (1 m length x 0.25 mm i.d.). Gas chromatographic oven temperature was programmed from 40 to 200°C at a rate of 10°C/min, with initial and final hold times of 5 and 15 min, respectively. The FID and sniffing port were maintained at a temperature of 250°C. The sniffing port was supplied with humidified air at 30 mL/min. Three experienced sniffers evaluated the neutral/basic and acid fractions of cheese extract 2x on the 2 different columns, described the odor, and scored the intensity using a 10-point numerical intensity scale. Sniffers each had more than 60 h of experience with GCO and GCO techniques, including scaling and aroma description.
Aroma extract dilution analysis of solvent extract fractions was conducted using an HP6890 series GC (Agilent Technologies Inc., Palo Alto, CA) equipped with a DB-FFAP (15-m x 0.32-mm i.d. x 0.25-µm film thickness; J&W Scientific), a cool on-column injector, an FID, and a sniff port (DATU Technology Transfer, Geneva, NY). Oven temperature was programmed from 35 to 225°C at 10°C/min, with an initial and final hold times of 5 and 15 min, respectively. Helium was used as a carrier gas at 10 mL/min (velocity of 70 cm/s). Solvent extracts were diluted stepwise with diethyl ether at a ratio of 1/3. The dilution procedure was followed until no odorants were detected by sniffers. The highest dilution was reported as the flavor dilution factor (Grosch, 1993).
Gas chromatography-olfactometry of dynamic headspace analysis (GCO-DHS).
For the dynamic headspace sampling, a grated cheese sample (10 g), in duplicate, was first equilibrated in a 3-neck glass purge vessel (280-mL volume, jacketed; Custom Glass Shop) to 45°C for 20 min, followed by purging of headspace volatiles on to adsorbent Tenax TA 60/80 (200 mg/trap) using nitrogen (flow rate 50 mL/min, ultra high purity). Flavor dilution analysis was performed by varying headspace purge times (25, 5, and 1 min), as described by Cadwallader and Baek (1998). Volatiles adsorbed on the Tenax were thermally desorbed (splitless-mode; Thermal Desorption system TDS2, Gerstel GmbH & Co. KG, Germany) and cryo-focused before injection (–150°C, solvent venting mode; cooled injection system CIS4, Gerstel GmbH & Co.) for GCO analysis. The GCO system consisted of an HP6890 series GC (Agilent Technologies Inc.) equipped with a DB-FFAP (15-m x 0.53-mm i.d. x 1-µ film thickness; J&W Scientific), an FID, and a sniffing port. Oven temperature was programmed from 30 to 225°C at 10°C/min, with initial and final hold times of 2 and 10 min, respectively. Helium was used as a carrier gas at 10 mL/min (velocity of 70 cm/s). Two experienced sniffers evaluated each sample/purge time combination. A flavor dilution factor was calculated for each odorant by dividing the highest purge time tested (25 min) by the purge time at which it was last detected by GCO-DHS (e.g., either 25, 5, or 1 min).
For the quantification of selected headspace volatiles, 5 µL of internal standard solution (1.03 µL/µL of 2-methyl-3-heptanone in methanol) was added to grated cheese (25 g), in duplicate. The mixture was quickly kneaded (10 to 15 s) and reformed into a block, wrapped in aluminum foil, and kept at 4°C for 180 min for equilibration. At the end of the equilibration period, the cheese block was grated, and 10-g cheese aliquot, in duplicate, was first equilibrated in a 3-neck glass purge vessel (280-mL volume) to 45°C for 20 min followed by purging of headspace volatiles on to adsorbent Tenax TA 60/80 (200 mg/trap) using nitrogen (flow rate 50 mL/min, ultra high purity). Volatiles adsorbed on the Tenax were thermally desorbed (splitless-mode; Thermal Desorption system TDS2) and cryo-focused before injection (–150°C, solvent venting mode; cooled injection system CIS4) for GC-MS analysis. The GC-MS system consisted of an HP6890 series GC/5973 mass selective detector (Agilent Technologies Inc.), and separation of desorbed volatiles was performed on a DB-FFAP (30-m x 0.25-mm i.d. x 0.25-µm film thickness; J&W Scientific). Oven temperature was programmed from 20 to 225°C at 4°C/min, with initial and final hold times of 5 and 15 min, respectively. Helium was used as a carrier gas at 1.2 mL/min. The MSD conditions were as follows: capillary direct interface temperature, 280°C; ionization energy, 70 eV; mass range, 35 to 300 amu; scan rate, 5.27 scans/s.
Gas chromatography-mass spectrometry.
For GC-MS analysis of solvent extracts, a HP5890 Series II GC/HP 5972 mass selective detector (MSD, Hewlett-Packard) was used. Separations were performed on fused silica capillary column (DB-WAX 30-m length x 0.25-mm i.d. x 0.25-µm df, J&W Scientific). Helium gas was used as a carrier at a constant flow of 1 mL/min. Oven temperature was programmed from 40 to 200°C at a rate of 5°C/min, with initial and final hold times of 5 and 45 min, respectively. The MSD conditions were as follows: capillary direct interface temperature, 280°C; ionization energy, 70 eV; mass range, 33 to 330 amu; EM voltage (Atune+200 V); scan rate, 5 scans/s. Each extract (2 µL) was injected in the splitless mode. Duplicate analyses were performed on each sample. Based on MS results, relative concentrations of the compounds identified positively were calculated.
Identification of Odorants
For positive identifications, retention indices, mass spectra, and odor properties of unknowns were compared with those of authentic standard compounds analyzed under identical conditions. Tentative identifications were based on comparing mass spectra of unknown compounds with those in the National Institute of Standards and Technology (NIST, 1992) mass spectral database or on matching the retention indices values and odor properties of unknowns against those of authentic standards. For the calculation of retention indices, an n-alkane series was used (Van den Dool and Kratz, 1963).
Sensory Evaluation of Cheese Models
Cheese models were prepared from mild Cheddar cheese (
4 mo age) or aged Cheddar cheese (
1 yr), previously confirmed by descriptive sensory analysis to be free of nutty flavor. The cheeses were grated and portioned (50 g). All chemical solutions to be tested were prepared in ethanol at the suitable concentration so that maximum volume to be added did not exceed 100 µL. Chemical solutions were introduced by a clean, disposable micropipette. After addition of the chemicals, cheese models were kneaded for 3 min, then molded to a rectangular shape, and equilibrated overnight at refrigerator temperature (5°C). Cheese models were evaluated by descriptive sensory analysis using the same procedure applied to nutty and not nutty Cheddar cheeses.
Freshly grated young or aged Cheddar cheeses were separately spiked with 2-methylbutanal (0 to 1000 ppb), 3-methylbutanal (0 to 1000 ppb), 2-methylpropanal (0 to 1000 ppb), propionic acid (0 to 250 ppm), and dimethyl sulfide (0 to 1000 ppb) across a concentration range reasonably expected in Cheddar cheese based on results from this study and on previous literature with Cheddar or Swiss cheeses (Dunn and Lindsay, 1985; Rychlik and Bosset, 2001). These compounds were selected based on the GCO-DHS results and, in the case of propionic acid, hypotheses from many previous studies (Preinenger et al., 1996; Rychlik and Bosset, 2001). Both young and aged Cheddar cheeses were used in the model studies. We hypothesized that nutty flavor perception might require the appropriate balance of other minor compounds that are present in aged Cheddar but not present in young Cheddar cheese, because Cheddar cheeses with intense nutty flavors are generally >9 mo of age.
Statistical Analysis
Sensory data and relevant instrumental quantitative results were analyzed using ANOVA (PROC GLM) with means separation to determine differences among treatments (SAS version 8.2, Cary, NC). Significance was established at P < 0.05.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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). No differences were detected in basic tastes of cheeses (P > 0.05) (data not shown). All cheeses selected were >1 yr of age.
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). Of the compounds, 16 were positively identified, 14 were tentatively identified, and 11 compounds remained unknown. Identified compounds included 7 aldehydes, 6 lactones, 3 ketones, 3 esters, 3 sulfurous compounds, and 7 nitrogen containing compounds. Twelve odor-active compounds were detected in acidic fractions (Table 3), including 6 fatty acids, 1 pyrane, 1 furanone, and 2 unknowns. These compounds have been reported in Cheddar cheese previously (Christensen and Reineccius, 1995; Milo and Reineccius, 1997; Suriyaphan et al., 2001), and their formation pathways were reviewed elsewhere (Urbach, 1997; McSweeney and Sousa, 2000).
Postpeak intensity sniffing scores indicated that there were no consistent differences between the nutty and not nutty cheeses (Tables 2 and 3). Aroma extract dilution analyses of the neutral/basic fractions of cheeses are summarized in Table 4. As mentioned earlier, this analysis was conducted with a different column (FFAP) and cool on-column injection. As might be expected, additional highly volatile components, 2/3-methylbutanal, were detected. However, consistent differences between nutty and not nutty cheeses were not identified. Relative abundance of selected neutral/basic compounds (Tables 5 and 6) support GCO results, with no clear consistent differences.
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). In addition to 7 compounds identified previously, 2 additional highly volatile compounds, 2-methylpropanal and dimethylsulfide, were identified by GCO-DHS (Table 7). Use of purge time as a dilution factor with DHA-DHS revealed that there were distinct differences between the headspace compositions of the not nutty and nutty cheeses, particularly dimethylsulfide and the Strecker aldehydes, 2-methylpropanal and 2/3-methylbutanal. To further confirm these findings, 9 additional Cheddar cheeses (5 nutty Cheddar cheeses, 4 not nutty Cheddar cheeses, all aged 1 yr) were selected, evaluated by descriptive sensory analysis, and then analyzed by DHA-GC-MS for Strecker aldehydes. Descriptive sensory analysis confirmed the presence or absence of nutty flavors in the nutty and not nutty Cheddar cheeses, respectively (data not shown). In agreement with previous findings, 2-methylpropanal was consistently detected in nutty Cheddar cheeses and was not found in 3 of the 4 not nutty cheeses (Table 8). The other Strecker aldehydes, 2/3-methylbutanal, were also detected in some of the nutty and not nutty Cheddar cheeses, but they were present at higher concentrations in nutty Cheddar cheeses.
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The addition of the Strecker aldehydes 2-methylpropanal, 2-methylbutanal, and 3-methylbutanal did increase nutty flavors in aged Cheddar cheese (Figures 2, 3, and 4), but in young Cheddar cheeses, descriptive panelists described the flavor as nutty/malty. These compounds have been previously reported in Cheddar cheese (Dunn and Lindsay, 1985; Christensen and Reineccius, 1995; Zehentbauer and Reinecccius, 2002). Strecker aldehydes have also been reported as prevalent constituents of Swiss cheese (Preininger et al. 1996; Thierry et al. 1999; Rychlik and Bosset, 2002a; Rychlik and Bosset, 2002b) and Parmesan cheese (Barbieri et al., 1994; Qian and Reineccius, 2002; Qian and Reineccius, 2003). Interestingly, these cheeses are characterized by intense nutty flavors.
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Formation of 2-methylpropanal and 2/3-methylbutanal via Strecker degradation in model solutions at high pH (Griffith and Hammand, 1989) or low pH (Pipis-Nicolau et al., 2000) was already reported. In these studies, valine, isoleucine, and leucine amino acids were found to be responsible for the formation of 2-methylpropanal, 2-methylbutanal, and 3-methylbutanal, respectively. These 3 Strecker aldehydes were reported to be the strongest contributors to chocolate flavor (Counet et al., 2002). Strecker aldehydes have also been reported to be the source of malty off-flavors in milk (Miller et al., 1974).
Braun and Olsen (1986) reported that low-fat Cheddar cheeses between 2 and 6 mo of age with increased levels of Strecker aldehydes (500 to 1000 ppb) from added cell-free extracts had a malty flavor. Descriptive sensory analysis with a defined sensory language was not used to evaluate cheeses. However, their results do concur with our findings on the addition of the Strecker aldehydes to young Cheddar cheese models. Sensory panelists reported that these cheeses exhibited a malty/nutty character. When Strecker aldehydes were added to aged Cheddar cheeses (>1 yr), nutty flavors were reported by sensory panelists. Panelists also noted that these flavors were identical to "typical" nutty flavors observed in Cheddar cheeses. Sensory analysis of cheese models revealed that the 3 Strecker aldehydes (2-methylpropanal, 2-methylbutanal, 3-methylbutanal) can contribute to nutty flavors in aged (>9 mo) Cheddar cheeses. However, quantitative data (Table 8) suggested that 2-methylpropanal may be more important because it was more prevalent in nutty cheeses and present at higher concentrations than the other Strecker aldehydes. Cheddar cheese flavor is very complex and is composed of many volatile compounds present in an appropriate balance (Singh et al., 2003). Many complex biochemical reactions occur throughout the aging process to generate a host of volatile aroma-active compounds that contribute to flavor (Singh et al., 2003). Results with cheese models also suggest that an appropriate balance of other compounds at or below sensory threshold are required in addition to Strecker aldehydes for the sensory perception of nutty flavor in Cheddar cheeses. Nutty flavor typically develops slowly in Cheddar cheese. Cheddar cheeses with intense nutty flavors selected for this study were all greater than 1 yr of age.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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ACKNOWLEDGEMENTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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Received for publication October 29, 2003. Accepted for publication December 12, 2003.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSACKNOWLEDGEMENTSREFERENCES |
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