Effects of heat treatment and fat reduction on the rheological and functional properties of Gaziantep cheese

ARTICLE IN PRESS

International Dairy Journal 13 (2003) 867–875

Effects of heat treatment and fat reduction on the rheological and functional properties of Gaziantep cheese Talip Kahyaoglu, Sevim Kaya* Food Engineering Deparment, University of Gaziantep, Gaziantep 27310, Turkey Received 17 October 2002; accepted 14 May 2003

Abstract Gaziantep cheeses with different fat contents (50.4%, 33.4%, and 13.5% w/w) were made and the curds of these were dipped into hot whey at three different temperatures (75
C, 85
C and 95
C) in accordance with the traditional techniques. Cheeses, the curds of which were not dipped into hot whey, were prepared as controls. Cheese samples were analyzed with respect to viscoelastic parameters, meltability and hardness. Reduction in the fat content resulted in significant (Po0:05) increases in protein and moisture contents and decreases in the amount of moisture in non-fat substance (MNFS). Viscoelastic parameters of cheeses were studied using oscillatory dynamic experiments (frequency sweep and temperature sweep tests). A decrease in the fat content and the curd dipping process increased the elastic shear modulus (G0 ). Temperature sweep test on heating the cheese from 10
C to 70
C showed that reduction of fat content results in a decrease in the magnitude of the phase angle, d: However, when the curd dipping temperature was increased, the magnitude of the phase angle (d) increased, at temperatures >35
C. The temperature at which G0 ¼ G 00 ; i.e. tan d equals 1.0, is called as crossover temperature. The crossover temperature was accepted as melting temperature (Tm ) and there was a good correlation between Tm and meltability measured with Arnott test for control cheeses. Reduction of the fat content increased the Tm values and decreased meltability. In contrast, increasing the curd dipping temperatures resulted in lowering of both meltability and Tm due to possible structural changes upon heating. r 2003 Elsevier Ltd. All rights reserved. Keywords: Gaziantep cheese; Fat reduction; Hardness; Melting; Viscoelasticity

1. Introduction Gaziantep cheese is an unripened, semi-hard cheese, which is produced traditionally in the southeast part of Turkey (Kaya, 2002). It is consumed extensively for breakfast and used as ingredient in many prepared local foods. High-fat intake is a risk factor in coronary heart disease. The reduction of fat intake in the diet is an effective means of decreasing the risk of the coronary heart disease and related health problems (Woteki & Thomas, 1993). Cheese is naturally a high-fat food and its consumption through the world is high. Consequently, much effort is being made in the cheese industry to produce good-quality raw reduced-fat cheese (RFC) (Anonymous, 1995). Unfortunately, reduction of fat has some negative effects on the flavor, body and texture of *Corresponding author. Tel.: +90-342-3601105; fax: +90-3423601100. E-mail address: [email protected] (S. Kaya). 0958-6946/03/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0958-6946(03)00113-4

cheese (Drake & Swanson, 1995; Bryant, Ustunol, & Steffe, 1995; Rodriguez, 1998). Much research has been undertaken to investigate effect of fat reduction on the rheology, microstructure, texture and functional properties of cheeses (Tunick, Mackey, Smith, & Holsinger, 1991; Fife, McMahon, & Oberg, 1996; Ma, Drake, Barbosa-Canovas, & Swanson, 1996; Guinee, Auty, Mullins, Corcoran, & Mulholland, 2000a). During the production of Gaziantep cheese, some of the producers frequently dip their curd into hot whey or water for a few minutes after pressing, but some do not apply this technique (Kaya, Kaya, & Oner, 1999). Cheeses that are dipped will be referred to as heat treated (HT) in the current study; whereas those are not will be controls and denoted non-heat treated (NHT). The authors are unaware of any studies on the effect of temperatures during curd dipping on the rheological characteristics of cheeses. Meltability is one of the properties of cheese at elevated temperatures and it may be defined as the ease

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with which cheese flows or spreads upon heating (Muthukumarappan, Wang, & Gunasekaran, 1999a). For most types of the cheeses, meltability is one of the most important physical properties and in dairy industry there are many empirical methods to determine it (Ustunol, Kawachi, & Steffe, 1994). The most popular methods for measuring meltability are the Schreiber and Arnott test (Campanella, Popplewell, Rosenau, & Peleg 1987). Measuring meltability using one of the empirical methods has not been so successful yet, since there has not been a correlation between the meltabilty results obtained using different methods (Park, Rosenau, & Peleg, 1984). For research purposes, the lack of objectivity of empirical tests is a distinct disadvantage. To solve these problems scientists have tried to correlate and find the relationship between meltability and dynamic rheological parameters of cheeses. Ruegg, Eberhard, Popplewell, and Peleg (1991) stated that loss tangent could be potentially used as a predictor for cheese meltability. Kuo, Wang, and Gunasekaran, (2000) have proposed that the instantaneous slope of the creep curve can be used to distinguish the meltability of Cheddar cheese of different ages and fat levels. They used transient tests for estimating softening point of cheese. Mounsey and O’Riordan (1999) used the maximum tan d as a meltability index for imitation cheese and found good correlation between maximum tan d and meltability as measured using the Olson and Price method. The objectives of our study were to investigate the effects of fat reduction and different curd dipping temperatures on the composition and rheology of Gaziantep cheese. The melting characteristics of the cheese samples were determined using a dynamic rheological method and Arnott test.

2. Materials and methods 2.1. Cheese manufacture Gaziantep cheeses used for this study were manufactured in duplicate in a local plant during the spring season. Cows’ milk was obtained from a local farm. Raw milk was filtered via cheesecloth. One-third of the milk was used to prepare full-fat cheese (FFC). The rest of the milk was separated at 45
C to lower the fat content of milk to 1% (w/w). Half of the resultant lowfat milk was used to produce low-fat cheese (LFC). For RFC cream was added to the skim milk to give reducedfat milk with desired fat level. The milk batches were pasteurized at 72
C  15 s. After pasteurization, they were cooled to 37
C and commercial rennin (Ren-Na, 15,000 MCU mL1, Mayasan Inc., Istanbul, Turkey) was added at a level of 0.55 mL L1. The milks were incubated at 37
C for 1 h. Then, the curd was cut,

stirred and poured into specially prepared cloth bags and left for 30 min to drain its whey. The curds were molded in spherical shape (the diameter is around B10 cm). After whey separation, they were pressed for 3 h using 40 kg m2. For different curd treatments, the whey was first heated to the required temperature (75
C, 85
C, or 95
C) and the cheese curd (B250 g) was then dipped into the heated whey (the volume was 2 L) for a period of 100 s. The dipped cheese curds were then left at room temperature for 30 min, packaged in plastic films to prevent water loss and immediately brought to laboratory for analyses. In the laboratory, cheese samples were allowed to equilibrate at room temperature (2072
C) for 1 h prior to analysis. The chemical analyses, hardness and rheological analyses of cheese were carried out on the same day. The different cheeses are coded as FFCNHT, RFCNHT, LFCNHT, which correspond to full-, reduced- and low-fat Gaziantep cheeses, respectively, the curds of which were not heat treated; FFC75HT, RFC75HT, LFC75HT, correspond to full-, reduced- and low-fat Gaziantep cheeses, respectively, the curds of which were heat treated at 75
C; FFC85HT, RFC85HT, LFC85HT correspond to full-, reduced- and low-fat Gaziantep cheeses, respectively, the curds of which were heat treated at 85
C; FFC95HT, RFC95HT, LFC95HT correspond to full-, reduced- and low-fat Gaziantep cheeses, respectively, the curds of which were heat treated at 95
C. 2.2. Chemical analysis The fat content of milks and cheese samples were analyzed using the Gerber method (Elmer, 1978). The moisture contents of cheese were determined using the oven method (16 h at 105
C) (Elmer, 1978). The protein contents of cheese and milk samples were determined by Kjeldahl method (Egan, Kirk, & Sawyer, 1981). The pH of milk and cheese was measured using a Nel model pHmeter (Nel Electronic Inc., Ankara, Turkey); cheese was dispersed in distilled water (5 g cheese 5 mL1 water) prior to measuring pH (Kaya & Oner, 1996). 2.3. Dynamic mechanical analysis Gaziantep cheese samples were cut into disk-shaped slices (3.070.3 mm thick and 3571 mm diameter), by using a device with two parallel stainless-steel wires, and covered with stretch film to prevent dehydration. Samples were equilibrated to room temperature for at least 1 h prior to testing. Viscoelastic measurements were carried out on a HAAKE RheoStress RS rheometer coupled with a Peltier/Plate TCP/P temperature control unit (HAAKE GmbH, Karlshure, Germany) using a cone and plate system (diameter, d; 35 mm; cone angle, a; 2
). A circulator DC10 unit was used to control the

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temperature within the range 10–70
C. Frequency sweep tests were performed at 2071
C; samples were loaded with a gap width of 2.5 mm and kept undisturbed for 5 min to allow sample relaxation. The elastic shear modulus (G 0 ) and loss modulus (G 00 ) were measured during a frequency sweep varying from 0.01 to 15 Hz at a constant stress of 200 Pa, which was within the linear viscoelastic region of the Gaziantep cheeses. The viscoelasticty changes during heating from 10
C to 70
C were measured using a temperature sweep test. Cheese samples were equilibrated at 10
C for 5 min and subjected to a harmonic low-amplitude shear stress at 200 Pa and a frequency of 1 Hz. The temperature was increased to 70
C at a rate of 3
C min1. Data were analyzed using a RheoWin Data Manager (RheoWin Pro V.2.64, HAAKE GmbH, Karlshure, Germany). 2.4. Hardness Cheeses were cut into cylinders (1070.5 mm height and 2571 mm diameter), which were wrapped in plastic, and held at room temperature (B20
C) for 1 h. The hardness of the samples was measured using an aluminum cylinder probe (25 mm diameter) on a TA.XT2 Texture Analyzer (Texture Technologies Corp., Scarsdale, NY/Stable Microsystems, Godalming, UK). Samples were compressed to 25% of original height. Hardness is defined as the maximum force during the first compression cycle (Konstance & Holsinger, 1992). 2.5. Meltability Meltability of cheeses was determined using the Arnott test (Park et al., 1984). Cheese samples were cut into cylinders (2571 mm in diameter and 1070.5 mm in height) with a knife and a device with two parallel stainless-steel wires. Each specimen was placed in the center of a glass dish. The dishes were heated in an oven at 100
C for 15 min. The center height of each sample was measured by a micrometer. The percent decrease in height was measured after cooling to room temperature for 30 min, which was reported as the Arnott meltability on a scale of 0–100. 2.6. Experimental design and statistical analysis The design of the experiment was a 3  4 factorial in a complete randomized design with two replications of cheese making. The three levels of fat content (FFC, RFC, and LFC) and four different types of heat treatment (NHT, non-heat treated; 75HT, heat treated at 75
C; 85HT, heat treated at 85
C; 95HT, heat treated at 95
C) were the main effects. Analysis of variance (ANOVA) was carried out using a general linear model (GLM) procedure of SPSS for Windows Release 8.0

869

(SPSS Inc., Chicago, USA). The least significant difference (LSD) test was used for multiple comparisons at 5% significance level.

3. Results and discussion 3.1. Composition of cheesemilk The chemical compositions of the milks used for cheese manufacture are given in Table 1. The fat content of the standardized milks was ranged from 3.76% to 0.84% w/w to achieve the desired reductions in the fat content of the cheeses. The protein contents of the milks increased significantly (Po0:05) as the fat content decreased. As expected, the moisture contents of milks also increased significantly (Po0:05) with decreasing fat content. There was no statistically significant difference in the calculated SNF contents and pH values of milks (P > 0:05). 3.2. Composition of cheese Cheese production was performed in a dairy plant in order to provide commercial conditions during production. The chemical composition of the cheese samples produced from the standardized milks with different fat contents is shown in Table 2. In agreement with the literature (Tunick et al., 1993; Guinee, Auty, & Fenelon, 2000b), the moisture and protein contents increased significantly (Po0:05) when the fat content of samples was reduced. Water in cheese is found as either free or bound to the protein since fat, the other major component, is hydrophobic (Subramanian & Gunasekaran, 1997a). Hence, the high protein contents of RFC probably caused retention of more water in the cheese samples. Decreasing the fat level also resulted in significant decreases (Po0:05) in the level of moisture in non-fat substance (MNFS). The reductions in MNFS and other compositional results are consistent with

Table 1 The chemical compositions of milks for Gaziantep cheeses of different fat contentsa Component

Moisture content (% w/w) Fat content (% w/w) Protein content (% w/w) Solid non-fat content (% w/w) pH

Milk typeb

SEMc

FFM

RFM

LFM

87.12a 3.76a 3.12a 9.12a 6.54a

88.47b 2.42b 3.13a 9.11a 6.55a

89.98c 0.84c 3.18b 9.08a 6.52a

0.5 0.19 0.01 0.007 0.01

a Values within the same row not sharing a common letter differ significantly, Po0:05: b FFM: full-fat milk; RFM: reduced-fat milk; LFM: low-fat milk. c Standard error of mean.

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Table 2 The effects of curd dipping temperature and fat content on the composition of Gaziantep cheesea Protein (% w/w)

Moisture (% w/w)

FDMc (% w/w)

MNFSd (%)

FFCNHT FFC75HT FFC85HT FFC95HT

22.40z 26.01v 26.54u 27.02t

51.24p 47.93s 47.95s 46.78t

50.45y 46.57z 46.23z 45.13p

67.95y 63.27v 63.14v 61.57u

RFCNHT RFC75HT RFC85HT RFC95HT

29.04s 29.47r 29.58r 30.23p

53.17z 50.50u 50.52u 49.48r

33.42v 31.55r 30.96r 31.42r

63.03v 59.85z 59.82z 58.76r

LFCNHT LFC75HT LFC85HT LFC95HT

32.90u 32.97u 33.20z 33.41y

57.42y 53.83z 53.55z 49.63r

13.57s 12.51t 12.25t 10.14u

60.94u 57.13s 56.79s 52.53t

0.18

0.26

0.82

0.91

25

G' or G" (kPa)

Cheese typeb

30

20

15

10

5

0 0

SEMe a

Values within a column not sharing a common letter (p, r, s, t, u, v, etc.) differ significantly, Po0:05; presented values are the means of two replicate treatments. b FFCNHT, RFCNHT, LFCNHT: full-, reduced- and low-fat Gaziantep cheeses, respectively, the curds of which were not heated following pressing. FFC75HT, RFC75HT, LFC75HT: full-, reducedand low-fat Gaziantep cheeses, respectively, the curds of which were heat treated at 75
C following pressing. FFC85HT, RFC85HT, LFC85HT: full-, reduced- and low-fat Gaziantep cheeses, respectively, the curds of which were heat treated at 85
C following pressing. FFC95HT, RFC95HT, LFC95HT: full-, reduced- and low-fat Gaziantep cheeses, respectively, the curds of which were heat treated at 95
C following pressing. c FDM: fat in dry matter. d MNFS: moisture in the non-fat substance. e SEM: standard error of mean.

other studies (Gilles & Lawrence, 1985; Tunick & Shieh, 1995; Subramanian & Gunasekaran, 1997b; Rudan, Barbano, Guo, & Kindstedt, 1998; Fenelon & Guinee, 2000). Heat treatment of the curds (dipping) decreased the contents of moisture and MNFS but increased the protein content significantly (Po0:05). Increasing the curd dipping temperature to which the curd was heated increased the water loss significantly from the cheese samples (Po0:05). Tunick et al. (1993) also reported similar results for cooking temperatures of Mozzarella cheese. 3.3. Dynamic rheological measurements 3.3.1. Frequency sweep Frequency sweep tests were used to determine if the fat reduction and temperature of curd heat-treatment influence the characteristics of the cheese. The dynamic viscoelastic properties of Gaziantep cheeses were measured within the linear viscoelastic range. G 0 is a measure

2

4

6

8

10

12

14

Frequency (Hz)

Fig. 1. Elastic shear modulus (G 0 (—)) and loss modulus (G 00 (- - -)) of full-fat (FFC) (54. 45% w/w fat in dry matter ()), reduced-fat (RFC) (33.42% w/w fat in dry matter (J)) and low-fat (LFC) (13.57% w/w fat in dry matter (.)) Gaziantep cheeses, the curds of which were not heated following pressing. See text for details of curd manufacture and heating.

of the energy stored and subsequently released per cycle of deformation and is the property that relates to the molecular events of elastic nature. G 00 is a measure of the energy dissipated as heat per cycle of deformation per unit volume (Gunesakaran & Ak, 2000). The results of frequency sweep tests (frequency=0.02–15 Hz) on the FFCNHT, RFCNHT and LFCNHT cheese samples are presented in Fig. 1. Both G 0 and G00 of the cheeses were dependent on frequency indicating the viscoelastic nature of the materials (Drake & Gerard, 1999). G 0 was greater than G00 at any given point for all NHT cheese samples, which indicates a dominant contribution of the elastic component to the viscoelasticity (Subramanian et al., 1997a). This behavior is typical for a viscoelastic solid (Rao & Steffe, 1992). The NHT cheese samples, shown in Fig. 1, were solid-like gels with rheological spectra resembling that of weak gel (Morris, 1984; Ross-Murphy, 1988; Richardson, Morris, Ross-Murphy, Taylor, & Dea, 1989). Typical weak gel characteristics were observed, i.e., G 0 was greater than G 00 throughout the frequency range, and the moduli showed a slight dependence on frequency. Such behavior has been reported in processed cheese spreads (Lee & Klostermeyer, 2001) and some biopolymer gels (Richardson et al., 1989; Ross-Murphy, 1995). The values of G0 and G 00 of the LFCNHT were greater than that of RFCNHT and FFCNHT cheeses. This could be a consequence of the reduction in fat content and the increase in the protein content; this would probably increase the elastic (or solid-like) character of cheese. Thus, the LFCNHT could be considered to have higher solid-like viscoelastic structure than the FFCNHT. This kind of response has also been reported

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Elastic shear modulus G' (kPa)

60

50

40

30

20

10

0 0

2

4

6

8

10

12

14

Frequency (Hz)

Fig. 2. Effect of temperature to which curd is heated after pressing on the elastic shear modulus (G0 ) of low-fat (13.57% w/w fat in dry matter) Gaziantep cheeses: non-heat treated control curd (), and curds heat treated at 75
C (J), 85
C (.), or 95
C (,).

by other researchers (Drake, Truong, & Daubert, 1999; Guinee et al., 2000b). However, our results were not similar to those obtained by Ma et al. (1996) who reported that the full-fat Cheddar cheeses had higher solid-like structure than the fat-reduced cheese at 20
C. The HT cheese samples showed trends similar to those of the NHT cheese samples. G 0 was higher than G 00 in the whole frequency range. It was found that the G 0 and G 00 of cheese samples increased when the curd was heat treated (dipped). The solid-like characteristics of the HT cheeses were greater than those of NHT cheeses. This may be due to the lower moisture content of the former cheeses (which could result in less freedom of movement for protein molecules), larger amounts of intact casein and a firmer casein matrix (Tunick et al., 1993; Attaie, Richter, & Risch, 1996). The effect of curd heattreatment temperatures on the G 0 of LFC samples is shown in Fig. 2. The magnitude of G0 increased as the temperature of curd was increased. Increasing the curd heat-treatment temperatures also resulted in a reduction in moisture level (Table 2). Water acts as a lubricant or a plasticizer between different proteins (Subramanian et al., 1997a); therefore, lower moisture content increased the magnitude of G 0 : The combined effects of fat content and moisture level on the textural characteristics of cheese are very significant. Rudan et al. (1998) used the filled gel composite model to help understand the changes in the rheological characteristics of Mozzarella cheese. According to this model, the fat and moisture represent the filler within casein network. They are both responsible for the viscous properties of cheese. If the fat has no interaction with the matrix, then, as its volume fraction is decreased, there is more matrix to deform per unit volume, and consequently, the cheese becomes more elastic. Subramanian et al. (1997a) reported that the fat content had a larger effect than moisture content

871

on the rheological properties of cheese. Guinee et al. (2000b) concluded that fat had a major effect on the microstructure, texture and functionality of Cheddar cheese. From the results of our study, it was found that when fat content decreased, the contribution of moisture content to the rheological character of the cheese increased. Hence moisture level had a more pronounced effect in LFC than in RFC and FFC. The magnitudes of G 0 ; G 00 ; and of all cheeses from the frequency sweep experiments were similar to the values for Cheddar, as obtained by Drake et al. (1999), and lower than for Mozzeralla as obtained by Yun, Hsieh, Barbano, and Rohn (1994). When cheese is subject to a sinusoidal shear stress, the shear strain is out of phase with the stress by an angle d: The tangent of this loss angle [tan d ¼ G 00 =G 0 ] denotes the relative effects of viscous and elastic components on viscoelastic behavior (Gunesakaran et al., 2000). The values of tan d of all Gaziantep cheeses (at frequencies ranging from 0.01 to 15 Hz) were between 0.31 and 0.43. This range is similar to the values reported for Mozzarella cheeses by Yun et al. (1994). The power-law model was used to interpret the frequency sweep data; G 0 and G 00 were transformed using this model, i.e. P ¼ aob ; where P (Pa) is G0 or G 00 ; while ‘‘a’’ (Pa s) and ‘‘b’’ (dimensionless) are constants and o (rad s1) is the angular frequency. The values of the constants ‘‘a’’ and ‘‘b’’ were shown in Table 3 for all cheese samples. The mean values of the constant ‘‘b’’ for G 0 and G 00 of FFC ranged from 0.18 to 0.20, which are similar to the values reported by Yun et al. (1994) for

Table 3 Power-law modela of elastic shear modulus (G0 ) and loss modulus (G00 ) of Gaziantep cheese samples with varying fat contents (tested at 200 Pa and 20
C, r2 ¼ 0:97  0:99) Cheese typeb

Viscoelastic parametersc Elastic shear modulus G0 (kPa)

Loss modulus G00 (kPa)

FFCNHT FFC75HT FFC85HT FFC95HT

3:02ð70:02Þo0:18ð70:001Þ 6:66ð70:03Þo0:18ð70:001Þ 8:61ð70:03Þo0:18ð70:001Þ 9:76ð70:02Þo0:19ð70:002Þ

1:00ð70:02Þo0:2ð70:003Þ 2:22ð70:03Þo0:2ð70:003Þ 2:83ð70:03Þo0:2ð70:004Þ 3:39ð70:04Þo0:19ð70:005Þ

RFCNHT RFC75HT RFC85HT RFC95HT

7:60ð70:03Þo0:2ð70:004Þ 11:00ð70:03Þo0:21ð70:004Þ 14:62ð70:03Þo0:22ð70:004Þ 9:66ð70:03Þo0:21ð70:005Þ

2:68ð70:05Þo0:2ð70:006Þ 3:78ð70:04Þo0:21ð70:007Þ 5:13ð70:05Þo0:22ð70:006Þ 3:22ð70:05Þo0:22ð70:006Þ

LFCNHT LFC75HT LFC85HT LFC95HT

14:66ð70:02Þo0:19ð70:005Þ 19:12ð70:02Þo0:21ð70:005Þ 25:53ð70:03Þo0:21ð70:005Þ 40:33ð70:03Þo0:21ð70:006Þ

4:81ð70:03Þo0:21ð70:005Þ 6:55ð70:03Þo:0:20ð70:005Þ 8:51ð70:03Þo0:21ð70:005Þ 1:35ð70:03Þo0:20ð70:005Þ

a

P ¼ aob ; where P can be G0 or G00 ; while a and b are constants; o is the angular frequency (rad s1) b See footnote b of Table 2. c Presented values are the means of two replicate trials (mean7standard deviation).

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Mozzarella cheese which ranged from 0.16 to 0.18. The values of constant ‘‘b’’ for LFC and RFC were 0.20– 0.22, slightly higher than ‘‘b’’ values for the FFC. 3.3.2. Temperature sweep test The use of cheese as an ingredient in many prepared, ready-to-eat foods is increasing. Hence the characterization of the cheese behavior at elevated temperatures is important. The reduction of fat content resulted in a decrease in the magnitude of the phase angle, d: The changes in G 0 and d during heating FFC Gaziantep cheese samples from 10
C to 70
C are shown in Fig. 3. It was observed that when the temperature was increased, G0 decreased and phase angle, d; increased. At 10
C the value of d was E15–18
which indicates that cheese is more elastic than viscous. However, at temperatures >40
C, the cheese was more viscous (d ¼ 35280
) than elastic. It was interestingly found that when the curd heat-treatment temperature was increased, the magnitude of the phase angle (d)

Elastic shear modulus G', kPa

50

40

30

20

increased, at temperatures >40
C. This showed that at higher temperatures, HT samples showed more viscous character than NHT samples. The changes in d indicate a phase transition from an unheated cheese to a melted cheese (Guinee et al., 2000a). G 0 decreased rapidly as the temperature was raised from 10
C to 35–50
C. The decrease in G0 indicated a softening of the cheese. This may be due to the liquefaction of the fat phase, which is fully liquid at 35–40
C (Guniee et al., 2000a). Since G 0 also decreased in the LFC samples, other factors in addition to fat liquefaction may contribute to softening, e.g. an increase in para-casein salvation as affected by change in casein conformation or pH reduction (Guinee et al., 2000a). These results are in agreement with a previous study on Cheddar cheese (Guinee et al., 2000a). The temperature at which G 0 =G 00 ; i.e. tan d equals 1.0, is called crossover temperature. At higher temperatures, the viscous character is more dominant in the viscoelasticity (Ustunol et al., 1994). Therefore, the crossover temperature might be accepted as the beginning of the melting. The crossover temperature (Tm ) for the various cheeses is shown in Fig. 4. As the fat content decreased, Tm of the cheese samples increased. Similarly Muthukumarappan, Wang, and Gunasekaran (1999b) reported that the softening point of cheese increased with reduction in fat content. Although HT samples had lower fat contents than NHT samples (Table 2), the Tm values of HT samples were lower than those of the NHT samples (Po0:05). Increasing heat-treatment temperature caused a reduction in the Tm values. The difference between the Tm values of FFC95HT and FFCNHT

10

70

0 (Α)

20

30

40

50

60

70 Crossover temperature (Tm) °C

10

90 80

Phase angle δ,°

70 60 50 40

60

NHT 75HT 85HT 95HT

50

40

30 30

20

FFC

10 0 10 (Β)

RFC

LFC

Fat Level, %,w/w.

20

30

40

50

60

70

Temperature, °C

Fig. 3. Elastic shear modulus G0 (A) and phase angle (B) as a function of temperature for full-fat (FFC) Gaziantep cheeses: non-heat treated control curd (), and curds heat treated at 75
C (J), 85
C (.), or 95
C (,).

Fig. 4. Crossover temperatures Tm (at this temperature tan d ¼ 1:0) of full-fat (54.45% w/w fat in dry matter; FFC), reduced-fat (33.42% w/w fat in dry matter; RFC), and low-fat (13.57% w/w fat in dry matter; LFC) Gaziantep cheeses. During manufacture, the pressed curds were either not heated (control, NHT) or heat treated at 75
C (75HT), 85
C (85HT), or 95
C (95HT). See text for details of curd manufacture and heating, and determination of Tm : The values presented are the means of two replicate trials.

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Meltabilty (height decrease (%))

60

NHT 75HT 85HT 95HT

50

40

30

20

10

873

compared to the NHT samples may be due to the presence of more aggregated proteins in the HT cheeses. The HT samples softened at lower temperatures but did not melt as much as NHT samples. The determination of melting temperature by dynamic rheological data might be useful also for estimating the softening temperature of cheeses; in many applications cheeses that soften but do not melt are needed (Kuo, Wang, Gunasakaran, & Olson, 2001). 3.4. Hardness

0

FFC

RFC

LFC

Fat Level, % w/w.

Fig. 5. Meltability of full-fat (54.45% w/w fat in dry matter; FFC), reduced-fat (33.42% w/w fat in dry matter; RFC), and low-fat (13.57% w/w fat in dry matter; LFC) Gaziantep cheeses. During manufacture, the pressed curds were either not heated (control, NHT) or heat treated at 75
C (75HT), 85
C (85HT), or 95
C (95HT). See text for details of curd manufacture and heating, and melt test. The values presented are the means of two replicate trials.

samples was B20
C, while that between the LFC95HT and LFCNHT was only 3
C. This result showed that heat treatment had a larger effect on the FFC samples than on the RFC or LFC cheeses. This may be due to the low volume fraction of fat phase in the RFC and LFC cheeses (Guinee et al., 2000a). The meltability of the Gaziantep cheese with different fat levels after heat treatment (dipping) at different temperatures is shown in Fig. 5. As the fat content of the cheese decreased, the meltability decreased. Decreasing fat content resulted in a lower MNFS content, causing the protein matrix to become firmer and more likely to support its own weight when heated (Tunick et al., 1993). Rudan, Barbano, Yun, and Kindstedt (1999) also found that Mozzarella cheese meltability decreased as the fat content decreased. But Fife et al. (1996) reported that fat content had no influence on the meltability of Mozzarella cheese. When Tm and meltability were compared for NHT samples a good correlation (r2 ¼ 0:81; degrees of freedom=5) was found. Fat reduction resulted in a decrease in meltability and an increase in Tm : The increase in the heat-treatment temperature caused a decrease of both Tm values and meltability. When cheese is heated, hydrophobic interactions are strengthened and unfolding of proteins occurs at temperatures in the range of 60–80
C (Kuo, Wang, Gunesakaran, & Olson, 2001). Aggregation of the protein molecules may therefore take place at 60– 80
C due to an increase in hydrophobic interactions. During the heat treatment of the curd in hot whey, the temperature of the cheese curds probably reached to 60
C and protein aggregation might have resulted. Hence, the lower meltability of the HT samples

The hardness values of FFC, RFC and LFC cheeses with and without heat treatments are given in Fig. 6. Reducing the fat content significantly increased the hardness (Po0:05). Although LFC and RFC samples had higher moisture contents than the FFC cheeses, the LFC and RFC cheeses were harder. This result showed that fat imparts more softness to the cheese than water. It was obvious that the denser protein matrix causes the increasing of the hardness of RFC. This effect was more pronounced at higher heat-treatment temperatures than at the lower heat-treatment temperatures. When heattreatment temperatures were increased the differences in the hardness between the different cheeses increased, because the HT samples contained less MNFS than their NHT counterparts; a lower MNFS content would lead to less protein hydration, less freedom of movement for

Fig. 6. Hardness of full-fat (54.45% w/w fat in dry matter; FFC), reduced-fat (33.42% w/w fat in dry matter; RFC), and low-fat (13.57%w/w fat in dry matter; LFC) Gaziantep cheeses. During manufacture, the pressed curds were either not heated (control, NHT) or heat treated at 75
C (75HT), 85
C (85HT), or 95
C (95HT). See text for details of curd manufacture and heating. The values presented are the means of two replicate trials.

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the protein molecules, a larger level of intact caseins, and a firmer casein matrix (Olson, Gunesakaran, & Bogenrief, 1996). G 0 measures elasticity, and as expected, hardness was highly correlated with G0 (r2 X0:83; degrees of freedom=23). Drake et al. (1999) also found good correlation between G 0 and hardness for Cheddar, Feta and Parmesan cheeses.

4. Conclusions The study indicated that reduction of fat content and curd heat-treatment temperature had major effects on the rheological properties of Gaziantep cheese. Reduction in the fat content and increasing curd heattreatment temperatures decreased the meltability. The hardness and elasticity of cheeses increased when the fat level was reduced and the curd heat-treatment temperature was increased from 75
C to 90
C. There was a good correlation between G 0 values and hardness of cheeses. Melting temperature (at which tan d equals 1.0) could be used to predict meltability of fresh (NHT) Gaziantep cheeses.

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