Abstract
In this study, the volatile aroma profiles of a variety of economically important cheeses for the Turkish dairy sector were characterized. A total of 75 samples belonging to 11 Turkish cheese varieties, including Civil, Canak, Dil, Divle Tulum, Ezine, Hellim, Malatya, Mihalic, Orgu, Urfa, and Van Otlu, were comparatively studied for their volatile profiles. One hundred and twelve volatile compounds were identified in the cheeses by solid-phase microextraction combined with gas chromatography-mass spectrometry and the results are discussed based on their chemical classes (31 esters, 7 acids, 18 ketones, 3 aldehydes, 24 alcohols, 10 terpenes, and 19 miscellaneous compounds). Esters, ketones, and alcohols were the most abundant classes identified and were highly dependent on the variety of cheese. Principal component analysis was applied to aid the interpretation of the gas chromatography-mass spectrometry data and to distinguish the cheeses. Divle Tulum cheese had high levels of aldehydes, ketones, and alcohols and separated from all the other cheeses, and the cheeses including Dil, Hellim, Malatya, Orgu, and Urfa grouped together. The last group of cheeses had low levels of volatiles stemming probably from the restricting effect of scalding or cooking that are employed in the manufacture of these cheeses and on the biochemical and/or microbial activity. Civil, Ezine, and Mihalic cheeses had somewhat different aroma profiles, but they were closely located near the cheeses including Dil, Urfa, Orgu, etc. The results suggest that each variety of cheese had different volatiles profile and that the manufacturing technique as well as ripening conditions of the cheeses played a major role on the individual volatile profiles.
INTRODUCTION
The development of cheese flavor is due to several complex microbial and biochemical activities that occur during manufacture and ripening. Cheese flavor is composed of both volatile and non-volatile fractions, which originate from milk fat, protein, and carbohydrate. The final flavor in cheese is a key parameter in consumer choice, cheese quality, and variety.[Citation1] Investigation of cheese volatile compounds have involved the use of several extraction techniques, including steam distillation, simultaneous distillation extraction, high vacuum distillation, dynamic head space, and solid phase microextraction.[Citation2, Citation3] Recently, many scientific studies have been performed to determine the flavor profile of numerous cheeses over the world.[Citation4] A number of volatile components belonging to several chemical groups, i.e., acids, esters, aldehydes, ketones, alcohols, sulfur compounds and other aromatic hydrocarbons, have been identified in different cheeses.[Citation5 − Citation8] The formation of these compounds in cheese results from proteolysis, lipolysis, and the metabolism of lactose and lactate.[Citation6, Citation9] These reactions depend on the cheese variety, microflora, and ripening conditions. Proteolysis is the most important biochemical event that affects the flavor profile of the cheese. During proteolysis, caseins are hydrolyzed into various lower molecular weight peptides and/or free amino acids; some of them are converted into amines, aldehydes, phenols, indole, alcohols, or ammonia by catabolism of the free amino acids.[Citation9] Free fatty acids, esters, lactones, and ketones contribute directly to the cheese flavor even at low threshold levels and are formed by the metabolism of lipids in cheese during ripening.[Citation1, Citation6, Citation10] Lactate metabolism can occur through several pathways and the resultant products can also contribute to the flavor of the cheeses.[Citation9]
There are more than 50 varieties of cheese in Turkey, including brine ripened cheeses (e.g., white cheese), high scald temperature cheeses (>90°C) (e.g., Halloumi), pasta-filata type cheeses (e.g., Kashar or Kashkaval), and herby or spicy cheeses (e.g., Van Otlu cheese). Canak cheese is ripened in an earthenware pot for at least 6 months under ground during which strong flavors, noted by consumers, are produced. Also, Civil cheese is ripened by the spontaneous growth of molds on the surface and is characterized by a rancid taste although usually containing a low level of fat. Some varieties of cheeses (Dil, Hellim, Malatya, Orgu, and Urfa) are scalded by dipping the cheese blocks into hot water or whey (at 90°C or more) and then ripened under brine. Limited lipolysis is expected for these types of cheeses; however, there is lack of information on their individual free fatty acid (FFA) or aroma profiles. Another cheese variety is Divle Tulum, which is ripened in natural cave conditions (4–5°C and 80 ± 5% relative humidity) for long periods. One of the most interesting cheeses in Turkey is a variety called Otlu cheese, which uses some special herbs, including Allium, Thymus, Silene, and Ferula species, individually or in appropriate mixtures in the manufacture.[Citation11 − Citation13] Some details on the manufacturing and ripening characteristics of the varieties of cheese used in this study are summarized in .
Table 1 Short notes on the characteristics of cheeses analyzed
Although Turkey has many traditional or local cheeses, three varieties, namely, Beyaz peynir, Kashar, and Tulum, have enjoyed a wide consumer acceptance. However, other traditional cheeses have also been gaining in popularity. Consequently, consumer demand has resulted in the market share of these cheeses to be increased in the larger urban cities. However, no studies have been reported in the literature, which characterized the volatile composition of the most widely consumed cheeses in Turkey, including Civil, Canak, Divle Tulum, Dil, Hellim, Mihalic, Van Otlu, etc. Comparative studies on the volatile profiles of cheeses have previously been performed in the EU[Citation14, Citation15] and in Italy,[Citation16 − Citation19] Portugal,[Citation20] Spain,[Citation21] Austria,[Citation22] and Greece.[Citation23 − Citation25] The volatile profiles of some Turkish cheese varieties have been previously characterized;[Citation26 − Citation29] however, further studies need to be performed especially for the native cheeses. A comparative study on the volatile profiles of several cheeses would result in new knowledge for both cheese scientists and the cheese industry, which could be used in, for example, the classification of cheeses based on volatile profiles, standardizing of some manufacturing parameters, obtaining data for consumers, and encouraging the attempts for “protection of their geographical origin and manufacturing methodology.” The aim of the present study was to evaluate and characterize the volatile compounds in 11 different Turkish cheese varieties, including Civil, Canak, Dil, Divle Tulum, Ezine, Hellim, Malatya, Mihalic, Orgu, Urfa, and Van Otlu, which have a high market share in Turkey.
MATERIALS AND METHODS
Materials
Seventy-five samples from 11 different types of cheese were collected from different regions of Turkey in 2009. The names of the cheeses were Civil (moldy/mildewy), Canak, Dil, Divle Tulum, Ezine, Hellim, Malatya, Mihalic, Orgu, Urfa, and Van Otlu. The ages of the cheeses (5 to 8 samples per variety) were between 2–8 months old. As far as possible, fully ripened and/or ready for sale cheese samples were collected. Production dates on labels were noted when possible; however, some samples are still produced by artisanal methods without packaging and labeling. So, to assess the production dates for these, the manufacturer's declarations were taken into account. Cheese samples were stored in sterile plastic bags and transported to the laboratory at the Food Engineering Department, Inonu University, Turkey. Cheese samples were vacuum packed and stored at -20°C until analysis. Analysis was performed within 1 month after sample collection for each variety.
Analysis of Volatile Compounds in Cheese Using Solid-Phase Microextraction
Vacuum packed cheese samples were frozen-sliced into small granules of 1–2 mm in size and immediately placed in glass vials (15-mL; Supelco, Bellefonte, PA, USA) in a freezer at −20°C. On the day of analysis, 81 ppm of the internal standard (2-methyl-3-heptanone in methanol; Sigma-Aldrich Co., St. Louis, MO, USA) was added to 3.00 g of the sample and allowed to equilibrate at 40°C for 30 min. Extraction of volatiles was carried out using a solventless extraction technique.[Citation30] The method is suitable for the isolation of volatile compounds from the sample matrix and has been used in various configurations for the characterization of cheese samples. Essentially, extraction is achieved by introducing a 75-μm carboxen-polydimethylsiloxane (CAR/PDMS) fiber (Supelco) through the septum into the vial and exposing it to the headspace for 30 min at 40°C. The fiber was positioned at a depth of 3.0 scale units for each run. Desorption of the extracted volatiles was performed using a Shimadzu GC-2010 gas chromatography-QP-2010 mass spectrometry system (Shimadzu Corporation, Kyoto, Japan) and run in split (ratio was 1:20) mode. During desorption, the SPME fiber remained in the injector for 2 min at a temperature of 250°C, with helium as the carrier gas at a flow rate of 1.0 mL/min. The volatile compounds were separated on a DB-Wax column (60 m × 0.25 mm × 0.25 μm; J&W Scientific, Folsom, CA, USA). The oven was held at 40°C for 2 min (desorption period), then increased at 5°C per min to 240°C, where it was held for 5 min to give a total run time of 47 min. The mass spectrometer was set to scan from 33 to 450 amu (threshold 1000) at a sampling rate of 1.11 scans/s. The identification of the volatile compounds was performed by calculation of retention indices (RI) of each compound by using n-alkane series (C10 to C26) under the same conditions. The tentative identifications were based on comparing mass spectra of unknown compounds with those in Wiley 7 (7th edition) and NIST/EPA/NIH 02 mass spectral library. Identifications were also confirmed by comparing retention times with reference standards when available. A total of 33 authentic standard compounds (Sigma Chemical Co., St. Louis, MO, USA) were used to confirm the identities of volatile compounds in the cheese samples. The RI values were also compared with those described in the literature as determined under the same conditions for matching the compounds. The concentrations were calculated by the comparison of the internal standard's and unknown compounds’ peak area. Each compound was expressed as μg/100 g of cheese.
Statistical Analysis
Volatile compound data from GC–MS analysis were investigated using multivariate statistical techniques to simplify interpretation of the results. Principal component analysis (PCA) was performed using a covariance matrix and varimax rotation between the cheeses. Hierarchical cluster analysis (HCA) was performed using Euclidean distance and average linkage. The concentrations of volatile compounds (GC–MS) were used as variables. PCA and HCA were carried out using SPSS statistical package program version 9.0 for Windows (SPSS Inc., Chicago, IL, USA).
RESULTS AND DISCUSSION
A total of 112 volatile compounds were identified from 11 different varieties of cheeses and were grouped into chemical classes including 31 esters, 7 acids, 18 ketones, 3 aldehydes, 24 alcohols, 10 terpenes, and 19 miscellaneous compounds. Duplicate analyses for all samples were performed and the average values with SD are listed in –. The volatile profiles of most cheeses have not previously been characterized and the results are discussed by comparison to data reported by other researchers for different cheeses.
Esters
Esters were the most abundant chemical group identified in the volatile fractions of the cheeses. Esters, which are responsible for the fruity flavor in cheese, are formed through two enzymatic reactions, esterification, and alcoholysis. Esterification is the formation of esters from alcohols and carboxylic acids, whereas alcoholysis is the production of esters from alcohols and acylglycerols or from alcohols or acyl-coenzyme A.[Citation31] Thirty-one esters were found in 11 different varieties of cheese and the most frequently identified sub-group was ethyl (9), propyl (5), and methyl (5) esters (). The highest (934.7 μg/100 g) and lowest (19.1 μg/100 g) levels of esters were identified in Canak and Hellim cheeses, respectively. Ethyl hexanoate, ethyl butanoate, methyl butanoate, and ethyl-acetate were found at the highest levels in all cheeses. Twenty esters were found in Canak cheeses and ethyl esters including ethyl hexanoate, ethyl decanoate, ethyl butanoate, and ethyl octanoate were the most abundant ones. The ethyl esters were also present at large quantities in Tulum,[Citation32] Grana Padano,[Citation5] Roncal,[Citation33] and other cheeses in the EU countries.[Citation2] Ethyl esters contribute to the cheese flavor as fruity or floral notes by smoothing the rancidity or sharpness from the acids and ketones.[Citation22] Relatively low levels of esters (below 60 μg/100 g) were identified in the scalded varieties, such as Dil, Hellim, Malatya, Orgu, and Urfa cheeses, indicating the scalding process inversely influenced the concentrations of esters in these cheeses. Ethyl acetate (Hellim, Malatya), ethyl hexanoate (Dil), ethyl decanoate and ethyl butanoate (Urfa), and propyl acetate (Orgu) were principal esters in five cheese varieties as shown in . Ethyl acetate was also the main ester in Malatya
Table 2 Ester compounds (μg/100 g) in eleven Turkish cheese varieties (n = 75)
Table 3 Acids (μg/100 g) in eleven Turkish cheese varieties (n = 75)
Table 4 Ketones (μg/100 g) in eleven Turkish cheese varieties (n = 75)
Table 5 Aldehydes (μg/100 g) in eleven Turkish cheese varieties (n = 75)
Table 6 Alcohols (μg/100 g) in eleven Turkish cheese varieties (n = 75)
Table 7 Terpenes (μg/100 g) eleven Turkish cheese varieties (n = 75)
Table 8 Miscellaneous compounds (μg/100 g) in eleven Turkish cheese varieties (n = 75)
cheese made from raw or pasteurized milk.[Citation26] Civil (16 esters), Ezine (11 esters), and Van Otlu (13 esters) cheeses were rich in esters and their concentrations ranged from 105.4 to 170.0 μg/100 g. Methyl esters were dominant in Civil cheese, which is a mold-ripened variety; however, Ezine and Van Otlu cheeses were rich in ethyl esters including ethyl butanoate, ethyl acetate, and ethyl hexanoate (). Ethyl butanoate were also identified by Yuceer et al.[Citation29] in Ezine cheese and surprisingly 1-methylpropyl acetate was only identified in Ezine cheese, probably due to use of ewe's and goat's milk in production and different botanical species. Ester profile of Van Otlu cheese was substantially different from the others, being rich in ethyl esters, including ethyl hexanoate, ethyl butanoate, and ethyl acetate.
Acids
Most of the acids present in cheese originate from the lipolysis of the milk fat.[Citation10] Ethanoic (acetic), propanoic, butanoic, pentanoic, or octanoic acids are produced by lipolysis or by the fermentation of lactose or lactic acid; however, 2-methyl propanoic and 3-methyl butanoic acids are produced by the metabolism of the AA Val and Ile, respectively.[Citation2, Citation6, Citation10] The principal acid was butanoic acid followed by acetic and hexanoic acids in the majority of the cheeses (). High concentrations of these acids were detected in Ezine[Citation29] and Malatya[Citation26] cheeses. No acids were detected in any samples of Hellim cheese. Six different acids were identified in Canak, Mihalic, and Van Otlu cheeses and their concentrations were 1306.8, 486.7, and 553.0 μg/100 g, respectively. Similarly, six acids were identified in Civil cheese samples but at very low concentrations. Acids are considered to be important components of the volatile fraction of cheese due to their low perception thresholds as well as the fact that they are precursors for the formation of methyl ketones, alcohols, lactones, and esters.[Citation5, Citation6, Citation20] Only acetic acid was identified in Malatya (30.4 μg/100 g) and Orgu (13.7 μg/100 g) cheeses, while considerably higher levels of butanoic acid (101.6 μg/100 g) was detected in Dil cheeses. High levels of 3-methyl butanoic (Civil, Canak, Mihalic, and Van Otlu) and 2-methyl propanoic acids (Civil, Mihalic, and Van Otlu) were detected in the cheeses, indicating high activity of proteolytic enzymes in these cheeses.[Citation2]
Ketones
Methyl ketones are the principal flavor compounds in Blue cheese and are formed by enzymatic oxidation of FFA to β-keto acids and their consequent decarboxylation to methyl ketones,[Citation9, Citation34] which contribute to the pungent aroma of Blue cheese.[Citation2] Ketones identified in the cheese samples are shown in . The highest (1935.8 μg/100 g) and lowest (65.3 μg/100 g) concentration of ketones were identified in Divle Tulum and Hellim cheeses, respectively. About 15 ketones were identified in Civil cheeses and high concentrations of 2-heptanone and 2-nonanone were detected in the Civil samples. In addition, some ketones, including 3-octanone, 2-octanone, 6-methyl-5-hepten-2-one, 3-methyl-2-pentanone, and 2-methyl-3-octanone, were rarely identified in other cheeses; however, substantial levels were detected in Civil cheeses. Similar profiles of ketones were identified in Canak, Dil, Hellim, Malatya, Orgu, and Urfa cheeses and their concentrations ranged from 65.3 to 196 μg/100 g. 2-Butanone, 2-pentanone, and 3-hydroxy-2-butanone were the main ketones in the last group of cheeses as previously described in Hayaloglu and Brechany[Citation26] for Malatya cheeses made from raw or pasteurized milk. Ezine, Mihalic, and Van Otlu cheeses were differentiated from the other cheeses due to their similar ketone concentrations with 296.3, 246.2, and 177.9 μg/100 g, respectively. 2-Heptanone, 2-pentanone, and 2-nonanone were detected in Divle Tulum cheese at the highest levels. On the other hand, 3-methyl-2-pentanone was identified only in this cheese variety. Seven different types of ketones were identified in Ezine cheeses with the concentration of 2-butanone (172 μg/100 g) and pentanone (42 μg/100 g) being the greatest (). Mihalic cheese was also rich in ketones (8 types) with 2-butanone (168.8 μg/100 g) and 3-hydroxy-2-butanone (56.7 μg/100 g) being most predominant, while the others were present at considerably lower levels (1.8–7.2 μg/100 g). Similar types of ketones were also detected at high levels in Van Otlu cheeses. Overall, the profile of ketones was similar in Van Otlu, Ezine, and Mihalic cheeses.
Aldehydes
Aldehydes are produced by the catabolism of fatty acids or amino acids via decarboxylation or deamination.[Citation9, Citation35] Only three aldehydes were identified in the cheeses (). The predominant aldehyde, 3-methyl butanal is produced by the Strecker degradation or transamination of Leucine. Hexanal and 3-methyl butanal were identified in Ezine cheese. Hexanal, 2-methyl butanal, and 2-pentanal were also isolated from this variety by Yuceer et al.[Citation29] Two aldehydes, including pentanal and 3-methylbutanal, were isolated in Orgu cheese and their total concentration was 6.7 μg/100 g. Overall, low levels of aldehydes were qualitatively and quantitatively identified in the 11 different varieties of Turkish cheeses, probably due to their catabolism into alcohols during ripening. Aldehydes are transitory compounds and do not accumulate significantly in cheese as they are rapidly transformed into alcohols or to corresponding acids.[Citation6]
Alcohols
Twenty-four different alcohols consisting of primary, secondary, and branched-chain alcohols were present in the cheese samples. These compounds are synthesized via many metabolic pathways, i.e., metabolism of lactose and AA, degradation of methyl ketones, and linoleic and linolenic acids.[Citation6] Alcohols are considered to be important volatile compounds, which may contribute to the flavor of the cheeses and impart an alcoholic, winey, sweet, fruity, and harsh note in cheeses.[Citation36] Alcohols were quantitatively the most abundant chemical class in Divle Tulum and Ezine cheeses () with 15 different alcohols being identified in each of these cheeses. The concentration of alcohols in the other cheeses may have been higher than the reported levels as it has been reported that the level of alcohols can fluctuate during ripening (e.g., Torta del Casar,[Citation37] Tulum,[Citation32] Malatya,[Citation26] and Hispanico cheese[Citation4]). Hence, at the time of analysis, the cheeses of this study may not have been sampled at a time where the level of the various alcohols was at their optimum. Higher concentrations of secondary alcohols were detected in all cheeses in comparison to the other alcohols, with the principal alcohols, ethanol, 2-butanol, and 3-methyl-1-butanol, detected in all samples. Fourteen alcohols were identified in Civil cheese with the main ones being 2-nonanol, 2-heptanol, and 3-methyl-1-butanol. 3-Methyl-1-butanol has a fruity and alcoholic note and is associated with an unclean and harsh flavor in Cheddar cheese.[Citation6, Citation35] Civil cheese did not contain high levels of alcohols, however, the profile of alcohols in Civil cheese is different to the other cheeses probably due to the action of mold growth on the surface. Similar concentrations of total alcohols were isolated in Civil (185.6 μg/100 g), Canak (218.9 μg/100 g), Mihalic (209.2 μg/100 g), and Van Otlu (205.7 μg/100 g) cheeses, although the predominant alcohols differed between these cheeses. For example, ethanol was at high concentrations in all samples, except for Civil cheese. On the other hand, 2-butanol was found at the highest level in all samples; however, it was not isolated from Civil cheese. On the contrary, 7-octen-2-ol, 4 penten-2-ol, and 3-cyclohexen-1-methanol were isolated only in Civil cheese. 2-Butanol and ethanol were the main alcohols in the samples of Van Otlu cheese; however, 1-propanol was specific for the cheese. Similarly, terpinen-4-ol was only identified in Ezine cheese and this volatile compound may be characteristic for Ezine due to plant flora of the corresponding geographic area. Low concentrations of alcohols were detected (<100 μg/100 g) in Dil, Hellim, Malatya, Orgu, and Urfa cheeses due probably to the scalding process.
Terpenes
Terpenes in milk originate from plants in the feed mixture or pasture.[Citation2, Citation38] Nine terpenes were identified in the cheese samples; however, the concentrations of terpenes were higher in Civil and Van Otlu cheeses than the other varieties. No terpene was detected in Mihalic and Urfa cheeses and a limited number (only one or two) of terpenes were detected in Canak, Dil, Ezine, Malatya, and Orgu cheeses (). The most abundant terpene in majority of the cheese samples was limonene which is associated with a citrus like note;[Citation38] however, α-thujene was only found in Van Otlu cheese at the highest level (49.7 μg/100 g). In addition, substantial levels of β-phellandrene, β-pinene, 4-carene, and limonene were detected in Van Otlu cheese in comparison to the other cheeses. This highlights the effect that herbs have on the production of volatile terpene compounds in this type of cheese. Seven different terpene compounds were determined in Civil cheeses and α-pinene was the main terpene. α-Pinene has also been identified in Cheddar,[Citation2] Kuflu,[Citation27] and Manchego[Citation21] cheeses. These compounds originate from pasture plants and are then transferred to milk and milk products due to the grazing of the animals.[Citation38] High levels of terpenes were detected in Hellim (19.4 μg/100 g) and Divle Tulum (19.9 μg/100 g) cheeses with limonene being the main one. Hellim cheese samples were collected from the Mersin province (located at the Mediterranean coast of Turkey) and may have originated from Northern Cyprus. The cheese is also produced under the name “Halloumi,” which is recognized internationally. High levels of terpenes were detected by Papademas and Robinson[Citation39] in Halloumi and the authors linked the high levels of terpenes to the plants consumed by the animals. High levels of 4-carene were identified in Divle Tulum cheese, which is probably responsible for the taste and flavor of this variety. Similar to Hellim and Ezine cheeses, Divle Tulum also has substantial levels of limonene.
Miscellaneous Compounds
Twenty miscellaneous compounds, including sulfurs, pyrazines, benzenes, and many aromatic hydrocarbons, were also identified (). Hexane was the most abundant compound, except for Hellim cheese, while diethyl ether was isolated at various levels in Civil, Dil, Ezine, Malatya, Urfa, and Van Otlu cheeses. Chloroform and toluene (except for Ezine) were identified from these varieties as well as other cheeses by the same method.[Citation26, Citation28, Citation32, Citation40] They are probably present in cheeses as a result of contamination during volatile analysis or originating from the degradation of the carotene in milk.[Citation6] Sulfur compounds, including dimethyl disulfide and methanethiol, were identified only in Civil cheese samples. These compounds are mainly produced by degradation of methionine (Met) or Met-containing peptides during cheese ripening; and methanethiol can be converted to dimethyl disulfide and dimethyl trisulfide by oxidative reactions.[Citation34] In addition, pyrazines (2,6-dimethyl pyrazine and trimethyl pyrazine) were identified in Civil cheese. 2,6-Dimethyl pyrazine is associated with a nutty flavor and it has also been identified in Cheddar,[Citation41] Gruyere, Manchego, and Ragusano[Citation3] cheeses. Styrene, which has a strong plastic odor, was found at the highest concentration (16.6 μg/100 g) in Dil cheese, followed by Civil cheese (7.9 μg/100 g). Carbon dioxide was determined in the majority of the cheeses, indicating the fermentation of lactate, citrate, and fatty acids in these cheeses. Carbon dioxide was also determined in Malatya cheese made from raw or pasteurized milk by Hayaloglu and Brechany.[Citation26]
Principal Component Analysis
Principal component (PC) and hierarchical cluster analyses (HCA) was performed using the concentrations of all the volatile compounds (112 volatile compounds in total) as variables to identify how the volatile profiles discriminated between the cheeses. An initial PCA was also performed that used the individual chemical classes separately as variables (plots were not shown); however, only one plot is shown that used all the volatile compounds as variables and is shown in . According to the data analysis, PC1 and PC2 accounted for 70 and 23% of the total variation, respectively. Clustering by HCA (dendrogram not shown) and distribution of the scores on the first two PCs showed that the cheese samples were separated into four distinct groupings. Group one contained Divle Tulum cheese (separated individually), group 2 contained Civil, Ezine, and Mihalic cheeses, and the third group included Dil, Hellim, Malatya, Orgu, and Urfa cheeses. The last group of cheeses included Canak and Van Otlu, and these cheeses were located on the positive side of PC2 opposite to the other cheeses. Van Otlu cheeses were characterized by high levels of esters, acids, and terpenes, while Canak cheeses were associated with esters and acids. Higher levels of terpenes in Van Otlu cheese resulted from the manufacturing practices as the addition of some special herbs, including Allium, Thymus, Silene, and Ferula species individually or as appropriate mixtures in the manufacture of this type of cheese, is common.[Citation12] Canak cheese also differed from the other cheeses by being associated with high levels of esters and acids; but it was closely located on the plot with Van Otlu. The cheese is ripened in earthenware pots underground for a long period (probably 1 year) and it is characterized by a sharp and piquant taste due to high levels of lipolysis. Civil, Ezine, and Mihalic cheeses were rich in aldehydes, ketones, alcohols, and terpenes in similar proportions. The three cheeses were closely located on the PCA plot; however, Ezine differed slightly from the other two cheeses. The cheeses of group 3, Dil, Hellim, Malatya, Orgu, and Urfa cheeses, were associated with low levels of all volatile compounds and were closely grouped together. It could be thought that the cheeses of groups 2 and 3, Civil, Ezine, Mihalic, Dil, Hellim, Malatya, Orgu, and Urfa, formed one big group. Divle Tulum cheese, which is ripened in a natural cave using a goat skin bag for at least 6 months, is characterized by high levels of aldehydes, ketones, and alcohols and, hence, this cheese was grouped individually. Ripening in a goat skin bag in a cave provided this variety with a unique volatile characteristic.
Figure 1 Score plot obtained by principal component analysis of the volatile composition of eleven different varieties of cheeses in Turkey. (Color figure available online.)
CONCLUSIONS
The aim of this study was to characterize the volatile aroma profile of the cheeses that are economically important for the Turkish dairy sector. The volatile aroma compounds, which were composed of 31 esters, 7 acids, 18 ketones, 3 aldehydes, 24 alcohols, 10 terpenes, and 19 miscellaneous compounds were identified from eleven different Turkish cheese varieties. The principal volatile compounds in the majority of the cheeses were ketones and alcohols, while some volatile compounds were characteristic for only one cheese variety such as Van Otlu cheese. The concentration of volatile compounds varied greatly with high standard deviations, probably due to the lack of a standard manufacturing protocols and age-related differences between the samples for each different variety. This study highlights the fact that the manufacturing technique and ripening conditions of the cheeses play a major role on the formation of volatile compounds in the cheeses. The data presented in this article provides new information on the volatile characterization of some Turkish cheeses. Further studies are necessary to understand the role of the microbial and biochemical agents on aroma production and accumulation in each individual cheese.
ACKNOWLEDGMENTS
This study was funded in part by Scientific Research Projects Unit of Inonu University, Turkey (Project No. 2007/31). Also, Professor B. Ozer is acknowledged for critical reading of the article.
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