Evaluation of sensory properties and their correlation coefficients with physico-chemical indices in Turkish set-type yoghurts

doi:10.4236/ojas.2011.11002

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Vol.1, No.1, 10-15 (2011) Open Journal of Animal Sciences

doi:10.4236/ojas.2011.11002

Evaluation of sensory properties and their correlation

coefficients with physico-chemical indices in Turkish

set–type yoghurts

Correlations between sensory and chemical parameters of Turkish yoghurts

Zehra Güler1, Young W. Park2*

1Department of Food Engineering, Mustafa Kemal University, 31034-Antakya-Hatay, TURKEY

2Georgia Small Ruminant Research and Extension Center, Fort Valley State University, Fort Valley, GA 31030,USA; parky@fvsu.edu

Received March 2,2011;revised April 2,2011;accepted April 9,2011

ABSTRAC T

Sensory properties and physico-chemical para-

meters of 10 most popular brands of commer-

cial set-type Turkish yoghurts were evaluated

and correlation coefficients between the two

indices were investigated. The results indicated

that increases in volatile compounds (ace-

taldehyde, 2-butanone, 2-nanonane, ethyl ace-

tate), titratable acidity, ash and fat contents

inversely correlated with the overall accepta-

bility score of the yoghurt. However, diacetyl, C4

to C12 free fatty acids, pH, whiteness index and

texture positively correlated with overall accep-

tability of the yoghurt products. It was con-

cluded that the acceptability of the Turkish

set-type yoghurts is mainly governed by the

fifteen volatile compounds as well as the phy-

sico-chemical properties determined. Thus, the

overall acceptability of the yoghurts was not

influenced by a single characteristic, but rather

by complex in nature.

Keywords: Turkish Set-Type Yoghurt; Sensory

Properties; Physico-Chemical Parameters;

Correlation Coefficient

1. INTRODUCTION

The sensory quality characteristics of cultured dairy

products are not as clearly defined as for other dairy

products. Specific geographical differences may exist in

consumer preferences for flavor intensity, body and tex-

ture characteristics, and /or color and appearance fea-

tures of many cultured dairy products (Bodyfelt et al.

1988). Consumers are very heterogeneous in their

likings and not all consumers prefer sweeter yoghurt.

For example, the apparent preference of Turkish consu-

mers is more acid, delicious and fatty flavor of yoghurt.

Pohjanheimo and Sandell (2009) have indicated that

food choice motives are connected to the liking. Subjects

who are considered natural content, ethical concern, and

health as important food choice motives perceived sourer,

thicker, and more genuine yoghurt flavour as more

pleasant, compared to subjects who are considered

convenience, price, mood, and familiarity more impor-

tant, considered sweeter and smoother yoghurt as more

pleasant. In addition, brand information is significantly

increased the liking for domestic yoghurts but did not

alter the main connections between food choice motives

and liking.

The sensory properties of Turkish set-type yoghurt as

well as gross-chemical composition are stated in Turkish

yoghurt standard (TSI 2006). Unfortunately, systematic

or routine sensory evaluation of cultured milk products

has received less attention than most other traditional

dairy products as cheeses. Yoghurt sensory character-

ristics may be influenced by different factors such as the

chemical composition of milk base, type of milk, pro-

cessing conditions, the ratio, activity and strains of

starter culture during the incubation period (Beshkova et

al. 1998; Kneifel et al. 1992; Tamime and Robinson

2001; Ulbert and Kneifel 1992).

Since sensory attributes play a key role in determining

consumer preference, elucidation of sensory causing

components of Turkish yoghurts is of paramount im-

portance to yoghurt producers. In Turkey, yoghurt is one

of the greatest volume of dairy products, which is

1,010,000 tons/per year (FAO 2006). However, no data

on the correlations the between sensory and physico-

chemical properties are available in literature. Thus, the

Z. Güler et al. / Open Journal of Animal Sciences 1 (2011) 10-15

aim of this study was to determine the correlations

between some physicochemical characteristics and sen-

sory properties of the commercially marketed Turkish

set-type yoghurts.

2. MATERIAL AND METHODS

2.1. Preparation of experimental yoghurts

Ten commercially produced cow milk set-type

yoghurts samples from different manufacturers, pac-

kaged in PS (Polystrene) plastic cap of about 1 kg, were

purchased from local markets in Hatay, Turkey. Taking

into consideration packing information, all of the

yoghurts were corresponded with regulations stated in

Turkish Yoghurt Standart (TSI 2006). Four yoghurt

samples were obtained from each brand during two

different periods, such as March and December, 2007.

All samples were analyzed at the last 14th day before

their shelf life expire.

2.2. Sensory Analysis

Sensory evaluation was performed by 14 experienced

panelists (ten males and four female) who have been

trained with yoghurt sensory scores characteristics. The

panel consisted of academic staff and students from

Food Engineering Department of Mustafa Kemal

University, Hatay, Turkey. Yoghurts were removed from

refrigerator (4˚C) 1 h prior to sensory evaluation, kept at

room temperature (22 ± 2˚C). Appearance (unnatural

color to natural color), acid taste and atypical yoghurt

flavor scores by hedonic scales (none to extremely

strong) rated immediately after opening yoghurt caps.

Whey drainage was examined by visually observing the

gel surface of the products and after inserting a spoon

into gel. The evaluation of the texture (weak to very firm)

was also based on visual observation after stirring the

product with spoon. Each sensory attribute was clearly

defined to the panelists as described by Bodyfelt et al.

(1988). The intensity of sensory attributes was measured

on a 4-point hedonic scale where 3 corresponded to

„much too strong‟ and 0 corresponded to „none‟. By

using a 9-point hedonic sale (1 = dislike extremely, 5 =

neither like nor dislike, 9 = like extremely), consumers

rated overall acceptability. Yoghurt was evaluated in

duplicate by the panel members.

2.3. Chemical Analyses

2.3.1. Analyses of basic nutrients and

physicochemical indices

Total solids, fat, protein, ash contents and titratable

acidity value of yoghurts were determined according to

the Association of Official Analytical Chemist (AOAC,

2003) methods. pH was measured using a pH meter

(Orion, Thermo, USA). Lactose content was estimated

as the difference between total solids and the sum of fat,

protein and ash contents.

Color characterisitcs were measured by using a

Minolta Chromameter (model CR-400 Tokyo, Japan)

calibrated with a manufacturer-supplied white cali-

bration plate. The L (dark = 0 and light = 100), a (red=

+a and green= –a) and b (yellow= +b and blue= –b)

values were measured. The L*, a* and b* reading was

carried out in triplicate for each sample. Results were

expressed as Chroma (C* = [(a*)² + (b*)²]0.5), hue angle

(hab = tan–1[(a*)(b*)–1]), and whiteness index (WI = 100

– [(100 – L)2 + a2 + b2]0.5 ). Analyses were carried out in

duplicate obtaining Two yoghurt samples from each the

brand were collected at two experimental period (March

and December, 2007), and each sample was analyzed in

duplicates.

2.3.2. Analyses of free fatty acids (FFA) and

benzoic acid

Extraction and quantification of FFA and benzoic acid

were carried out according to the method of Deeth et al.

(1983) with slight modifications as reported by Güler

(2008). Heptanoic acid was added to all experimental

yoghurt samples at the time of extraction. FFAs were

analyzed by a GC-MS (Agilent 6890 gas chromatograph

and 5973 N mass selective detector; Agilent, Palo Alto,

CA, USA). Column used for FFA and benzoic acid

separation was a DB-FFAP-column (30 m × 0.25 mm id

× 0.25 µm film thickness). Analyses were carried out in

triplicate.

For GC operating conditions of FFA and benzoic acid

analysis, helium was used as a carrier gas with a

constant flow rate of 1 mL min–1. The GC oven tempe-

rature was set to 50˚C for 5 min then raised to 230˚C at a

rate of 5˚C min–1 and held at 230˚C for 20 min. The

injector temperature was 250˚C, and the run time was 58

min. The GC column was connected without splitting to

the ion source of the Agilent 5973N model quadrupole

mass selective detector which was operating in the scan

mode within a mass range 33 to 330 m z–1 at 1 scan s–1.

The interface line to MS was set at 280˚C. The MS was

operated in an electron impact mode at electron energy

of 70eV and was calibrated by auto-tuning. Identif-

ication of the compounds was performed by a computer-

matching of their mass spectral data with those of known

compounds from the Mass Spectral Database (Wiley7n.1

/Nist02.L.). To compensate the amount of loss during the

extraction and clean-up, heptanoic acid (C7) was used as

internal standard. Pre-analyses of the milk and yoghurt

had ensured that heptanoic acid was absent.

Z. Güler et al. / Open Journal of Animal Sciences 1 (2011) 10-15

2.3.3. Analysis of volatile compounds (VC)

VCs were determined by static head space technique

according to Güler (2007). VCs were analyzed using a

Agilent model 6890 gas chromatography (GC) and 5973

N mass selective detector (MS) (Agilent, Palo Alto, CA,

USA). Columns used for FFA separation HP-INNOWAX

capillary column (30 m × 0.32 mm id × 0.25 µm film

thickness). The volatile compounds were separated un-

der the following conditions: injector temperature 200˚C;

carrier gas helium at a flow rate of 1.4 mL



min–1; oven

temperature program initially held at 50˚C for 6 min and

then programmed from 50˚C to 180˚C at 8 ˚C



min–1

held at 180˚C for 5 min. The interface line to MS was set

at 250˚C. Identification of the compounds was also con-

ducted by a computer-matching of their mass spectral

data with those of known compounds from the Nist 02.L.

Mass Spectral Database. Based on the peak resolution,

their areas were estimated from the integrations per-

formed on selected ions. The resulting peak areas were

expressed in the arbitrary area units. Quantification of

constituents was calculated by external standard tech-

nique.

2.4. Statistical Analysis

Statistical analysis was performed using the SPSS

version 9.05 for Macintosh (SPSS Inc./Chicago, III.,

U.S.A.). Data were expressed as means ± standard

deviation. The coefficient of variation (CV) between the

samples was expressed as relative standard deviation

(%). Sensory properties were submitted to one-way

analysis of variance (ANOVA). Duncan multiple mean

comparison test (P < 0.05) was used to state the

differences among the yoghurts. Pearson‟s correlation

coefficient (r) was also performed measure of the

strength of the association between the variables. Linear

Discriminat Analysis was applied to detect the presence

of classes within the yoghurt samples. The variables

were selected by forward stepwise analysis, and Wilk‟s

lambda and F-value were used to determine the signifi-

cance of the changes in lambda when a new variable is

tested. Validation of these results was performed by

leave-one-out cross validation.

3. RESULT AND DISCUSION

Results of sensory evaluation on the yoghurt samples

are presented in Figure 1. None of the yoghurts received

the maximum overall acceptability score of 9 (excellent)

described by the sensory evaluation form. Linear

Discriminant Analysis (LDA) was applied to distinguish

among yoghurt samples. Using the yoghurt samples as a

classification variable, the selected variables were the

scores of sensory properties. LDA achieved a high

recognition percentage for the classification of yoghurt

samples according to the sensory properties, reaching a

percentage of 91.2% for yoghurt samples (Figure 2).

According to the sensory scores, yoghurt samples fell

Figure 1. The sensory evaluation of yoghurt samples obtained

from 10 brands.

Figure2. Scattered plot of the samples projected in the plane

defined by the discriminant functions according to sensory

properties.

into four distinct groupings, where each group consisted

of yoghurts samples with similar characteristics. As

shown in Table 1, yoghurts are predominantly grouped

according to the magnitude of overall acceptability and

texture scores. This indicates that panelists were able to

distinguish differences among samples, as well as to

make similar assessments for duplicates within a sample.

There were significant differences in acid (P < 0.001)

and atypical (P < 0.05) taste scores in yoghurt samples

(Table 1). Pearsons correlation coefficeints of acid taste

Z. Güler et al. / Open Journal of Animal Sciences 1 (2011) 10-15

scores were significant with titratable acidity (0.61, P<

0.001), appearence (–0.59, P < 0.001) and overall

acceptability (–0.57, P < 0.001). Similar results were

obtained by Chamnas et al. (2006) and Harper et al.

Table 1. Sensory properties of yoghurts grouped according to Linear Discriminant Analysis (LDA).

Yoghurt samples

1Properties

Y1-Y2-Y9

Y5-Y6

Y3-Y7

Y4-Y8-Y10

Meana

Acid taste (0-3)

2.38 ± 0.34

1.57 ± 0.09

2.63 ± 0.18

1.88 ± 0.22

2.12 ± 0.46

***

21.54

Atypical(0-3)

2.21 ± 0.07

1.76 ± 0.18

2.01 ± 0.18

1.75 ± 0.13

1.94 ± 0.24

12.19

Whey separation(0-3)

1.67 ± 0.14

1.43 ± 0.11

1.53 ± 0.04

1.54 ± 0.17

1.55 ± 0.14

9.20

Appearence (0-3)

1.59 ± 0.19

2.07 ± 0.81

1.59 ± 0.23

1.63 ± 0.13

1.70 ± 0.36

***

21.09

Texture(0-3)

1.83 ± 0.14

2.25 ± 0.19

2.32 ± 0.09

2.38 ± 0.25

2.18 ± 0.29

13.15

2Overall acceptability (1-9)

3.75 ± 0.01

4.50 ± 0.00

4.88 ± 0.18

5.04 ± 0.08

4.51 ± 0.57

***

12.57

aMeans ±standard deviations of 40 yoghurt samples; P: significant level; NS: non significant; *P < 0.05; ***P < 0.001; 1Zero-3 points intensity scale; ²One-9

points scale. (1 = dislike extremely, 5 = neither like nor dislike, 9 = like extremely); CV; Coefficients of variation (standard deviation/mean × 100).

(1991). No significant differences in whey separation

scores of yoghurt samples were observed (Table 1).

Whey separation in the yoghurts was correlated with pH

(0.380, P < 0.01). Appearence (color) score of yoghurts

was significantly different from each other (P < 0.05).

Appearance was also significantly correlated with

whiteness index (0.278, P < 0.05), hab (–0.291, P < 0.05),

titratable acidity (–0.309, P < 0.05) values and overall

acceptability (0.56, P < 0.01). One of the most important

sensory attributes for yogurt is texture (Sodini et al.

2004). There were significant differences in texture

scores between yoghurt samples (P < 0.05) (Table 1).

Texture scores has significant correlation coefficients

with protein (0.293, P < 0.05), ash (–0,308, P < 0.05),

whiteness index (0.444, P < 0.01) and pH (–0.280, P <

0.05). The more acidic yoghurt samples revealed the

more firm texture, which is in agreement with the report

by Chammes et al. (2006). According to Modler et al.

(1983), increasing amounts of proteins in milk

formulation increased gel firmness of yogurt. Moreover,

yogurt viscosity was improved as a result of the

increasing of dry matter (Skriver et al. 1999). Texture of

the Turkish yoghurts in this study was positively

correlated with overall acceptability (0.479, P < 0.01),

which was similar to the data reported by Harper et al.

(1991).

Significant correlation coefficients were found be-

tween scores of the sensory attributes and physicco-

chemical properties of the commercial Turkish yoghurts

as shown in Table 2. The correlation coefficient of acet-

aldehyde (sharp, green and white glue) (Lindisay et al.

1965; Harper et al. 1991) was positive with atypical taste

(0.40, P < 0.01), and negative with overall acceptability

(–0.39, P < 0.05). These observations are coincided with

the report by Barnes et al. (1991). Even though acetone

did not have a significant correlation with overall

acceptability, it was significantly and negatively corre-

lated with acid taste. Warsy (1983) indicated that acetone

may have limited importance for yoghurt flavor. The

mean value (6.9 µg



g–1 ) of aceton (data shown in the

previous paper) is markedly low in yoghurts when

compared to the threshold in water (40.9 µg



g–1)

(Molimard and Spinnler 1996). On the other hand,

correlation coefficient of diacetyl, which is responsible

for buttery flavor (Macciola et al. 2008), was positive

and significant with both acid taste (0.37, P < 0.05) and

overall acceptability (0.53, P < 0.01). This could be

attributable to the high mean content (5.1 µg



g–1) of

diacetyl (data shown in the previous paper) when

compared with threshold in water (0.2 µg



g–1)

(Molimard and Spinnler 1996). This finding is in

agreement with the report by Rysstad and Abrahamsen

(1987). The presence of diacetyl is thought to contribute

to the delicate, full flavor and aroma of yoghurt, and

their presence are important if acetaldehyde content is

low (Beshkova et al. 1998).

Regarding to the other aromatic volatiles, 2-butanone

(acetone), 2-nonanone (fruity, musty) and ethyl acetate

(fruity) were believed to be responsible for various taste

and odor (Molimard and Spinnler 1996), but had nega-

tive correlations with overall acceptability in our study

(Table 2). 2-Nanonane and 2-tridecanone (fruity, green)

showed positive correlation coefficients with atypical

flavor, since ketones with a higher carbon number are

responsible for heated milk flavor as described by Bad-

ings et al. (1981).

As far as free fatty acids are concerned, butanoic (ran-

cid, cheesy), hexanoic (pungent, sour), octanoic (waxy,

goaty), decanoic (rancid, fatty) and dodecanoic (fatty)

acids (Sable and Cottenceau 1999) were positively and

significantly correlated with overall acceptability of the

yoghurts (Table 2). This suggests that these free fatty

acids may contribute to the formation of the specific

flavor-aromatic properties of set-type Turkish yoghurts

as reported earlier by other researchers (Warsy 1983;

Beshkova et al. 1998; Stelios et al. 2007). The mean

values of C4 (6.2 µg



g–1), C6 (7.8 µg



g–1) and C8 (2.5

µg



g–1) free fatty acids in the yoghurt samples

Z. Güler et al. / Open Journal of Animal Sciences 1 (2011) 10-15

(Y1,Y2,Y9) had low overall acceptability scores (Table

1), which were lower than odour threshold reported by

Rychlik et al. (2006). On the other hand, the mean con-

centrations of free fatty acids C4 (10.5 µg



g–1) and C6

(14.5 µg



g–1) in yoghurts (Y4, Y8 and Y10) having high

overall acceptability were higher than odour thresholds

Table 2. The significant pearson‟s correlation coefficients be-

tween physicochemical and sensory propertie.

Component

Acid taste

Untypical

Overall

acceptability

Acetaldehyde

0.46**

0.40**

–0.390*

Acetone

–0.47**

–0.22

0.19

Diacetyl

0.37

–0.18

0.53**

2-Butanone

–0.03

0.28*

–0.34*

2-Nanonane

0.20

0.29*

–0.36*

2-Tridecanone

0.16

0.30*

0.00

Ethylacetate

0.202

0.458**

–0.340*

Butanoic acid (C4)

–0.13

–0.22

0.35*

Hexanoic acid ( C6)

–0.15

–0.24

0.34*

Octanoic acid (C8)

–0.17

–0.34*

0.58**

Decanoic acid (C10)

–0.11

–0.34*

0.59**

Dodecanoic acid

(C12)

–0.02

–0.21

0.32*

Fat

0.18

0.37

–0.41**

Titratable acidity

0.40**

0.07

–0.32*

0.00

–0.31*

0.48**

Ash

–0.08

0.272

–0.55**

Whitenes index

–0.20

0.18

0.44**

Texture

–0.118

–0.23

0.48**

*P<0.05 and **P<0.01

of C4 (6.58 µg



g–1) and C6 (13.63 µg



g–1), whereas the

amount of C8 (6.7 µg



g–1) was lower than threshold

(13.23 µg



g–1) in yoghurt. This result may confirm that

if the level of octanoic acid, which is responsible for

goaty and waxy flavor, is high in yoghurts, it can

negatively affect overall acceptability. The concentration

(5.3 µg



g–1) of decanoic acid is responsible for rancid

and fatty flavor in yoghurts with low overall accep-

tability score, which is almost close to thresholds in oil

or butter (5 µg



g–1 ), while it is higher than that in water

(3.5 µg



g–1). In yoghurts with high overall score,

concentration (9.7 µg



g–1) of decanoic was higher than

threshold in oil.

Concerning dodecanoic acid responsible for fatty

flavor, its mean concentration (11.4 µg



g–1) in yoghurts

with low and high overall acceptability score was higher

and lower than threshold in water (2.2 µg



g–1) and oil

(50 µg



g–1), respectively (Molimard and Spinnler 1995;

Sable and Cottenceau 1999). This result confirmed that

if dodecanoic acid was much higher in yoghurts than

threshold in oil, it might be negatively correlated to

overall acceptability since the correlation coefficient

between fat content and overall acceptability score was

negative. This indicates that the correlation between

volatile free fatty acids and overall acceptability is

probably due to odour threshold of each free fatty and

characteristic flavor.

On the other hand, a synergistic action of the short-

chain acids may be suggested. For example, compounds

with similar odour attributes occurring in sub-threshold

concentration may enhance each other and thus are

detected. Ethanoic (acetic) acid responsible for harsh-

ness flavour described as “vinegary” (Molimard and

Spinnler 1996; Sable and Cottenceau 1999) did not have

a significant correlation with overall acceptability, since

the mean value (15.3 µg



g–1) of ethanoic acid in yo-

ghurts was lower than minimum threshold (22 µg



g–1)

in water. In addition, the perception of acetic acid by

panelists might be masked by the other free fatty acids.

There were significant correlations between titratable

acidity (–0.39, P < 0.05), pH (0.31, P < 0.05) and overall

acceptability (Table 2). High acidity negatively influ-

enced the overall acceptability, as similar results were

observed by various researchers (Harper et al. 1991;

Kneifel et al. 1992; Ott et al. 2000), where they

emphasized the importance of acidity in yogurt flavor. In

contrast, Barnes et al. (1991) suggested that there were

no relationships betweem any sensory and analytical

measurement for predicting the overall liking of plain

yogurt. However, for US consumers, the relatively high

extent of sourness along with the intensity of ace-

taldehyde (the key volatile compound of yogurt) have

resulted in low consumer acceptance (Barnes et al.

1991).

Concerning other parameters, ash (P < 0.01, –0.55)

and fat (P < 0.01, –0.41) had a negative correlations with

overall acceptability, while whitenes index (P < 0.05,

0.44) and texture (P < 0.01, 0.48) had a possitive corre-

lation coefficient (Table 2). In fact, texture of yoghurt

could affect the perception of volatile compounds during

consumption and also the final quality of product, as

suggested by other investigators (Kneifel et al. 1992;

Serra et al. 2009).

4. CONCLUSION

There were positive and significant correlations be-

tween free fatty acids (C4-C12), pH, whiteness index and

texture and overall acceptability score of Turkish

set-type yoghurts. However, increases in the amounts of

some volatile compounds (acetaldehyde, acetone, 2-bu-

tanone, 2-nonanone, etyl acetate,) and titratable acidity

value caused a decrease in overall acceptability of

yoghurt. Thus, understanding and controlling the overall

acceptability of a yoghurt would be still difficult due to

the differences in relative distribution of sensorially

active compounds exerting a main effect. In addition,

overall acceptability scores of yoghurts may be influ-

Z. Güler et al. / Open Journal of Animal Sciences 1 (2011) 10-15

enced by consumers‟ ethical concern, health, sex and age,

etc.

5. ACKNOWLEDGEMENTS

The authors gratefully acknowledge the members of the sensory

panel conducted this study. We also thank Gökhan DİLER and Ersin

GÖK for assisting in labratory analyses.

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