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Rheological, physical and chemical characteristics of mulberry pekmez
Food Control 16 (2005) 73–76 www.elsevier.com/locate/foodcont
Rheological, physical and chemical characteristics of mulberry pekmez Memnune Seng€ ul, M. Fatih Ertugay *, Mustafa Seng€ ul Department of Food Engineering, Faculty of Agriculture, Atat€urk University, 25240 Erzurum, Turkey Received 16 February 2003; received in revised form 21 November 2003; accepted 26 November 2003
Abstract The physical, chemical properties and the rheological behaviour of mulberry pekmez were investigated. Viscosity was measured at 30, 40, 50, 60 and 70 C using a rotational viscometer equipped with spindle 5 at the speed of 5, 10, 20, 50 and 100 rpm. An empirical power-law model was used to describe the rheological behaviour of mulberry pekmez with correlation coefficients between 0.991 and 0.999. The mulberry pekmez exhibited a pseudoplastic behaviour. An Arrhenius equation was used to describe the effect of temperature on viscosity and Ea value of the mulberry pekmez was calculated as 17.97 kJ mol1 . 2004 Elsevier Ltd. All rights reserved. Keywords: Mulberry; Pekmez; Concentrated fruit juices; Rheological properties
1. Introduction Pekmez is one of the traditional food products in Turkey and it is commonly produced from grape and mulberry by concentration of juices up to 70–80 soluble dry matter content. Pekmez, consumed generally for breakfast (Alpaslan & Hayta, 2002), can also be produced from fruits containing high amounts of sugar like apple, carnob, plum, watermelon, apricot, sugar beet and fig. Since pekmez contains high amounts of sugar, mineral and organic acid, it is a very important food product in human nutrition (Demir€ oz€ u, S€ okmen, Ucßak, Yılmaz, & G€ ulderen, 2002; Ust€ un & Tosun, 1997). Pekmez easily passes into the blood without digestion because most of its carbohydrate is in the form of monosaccarides like glucose and fructose. This is nutritionally important, especially for babies, children, sportsmen and in situations demanding urgent energy. Pekmez has been produced with different techniques considering species of fruits used in production. For mulberry pekmez production, fresh mulberry is used as raw material. Firstly, mulberry is placed to boiling vessels after cleaning. Then, 8–10 l water is added for 20–30 kg mulberry. After adding water, the mixture is
*
Corresponding author. Tel.: +90-442-2312484; fax: +90-4422360958. E-mail address: [email protected] (M. Fatih Ertugay). 0956-7135/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodcont.2003.11.010
completely stirred, and is boiled by stirring for 1 h. It has various colors from light brown to dark brown depending on the process. After then, the mixture is cooled up to about 40–50 C. It is pressed and then filtered in order to obtain clear mulberry juice. Mulberry juice is concentrated in open vessels to obtain 65–72 Brix. Then, mulberry juice is cooled up to 40 C; this product is called mulberry pekmez. Finally, mulberry pekmez is packaged and stored at room temperature (Aksu & Nas, 1996). In the food industry, rheological characteristics of concentrated fruit juices are a significant property in addition to chemical and physical properties. Moreover, rheological characteristics depend on both the chemical composition of fruits and processing conditions. However, a knowledge of the flow behaviour of concentrated fruit juices will be useful in quality control, calculating energy usage, process control and equipment selection (Kaya & Belibaglı, 2002). Viscosity, a rheological property, is also considered as an important physical characteristic related to the quality of liquid food products (Saravacos, 1970). There is a number of research about rheological (Alpaslan & Hayta, 2002; Kaya & Belibaglı, 2002) and chemical properties of grape pekmez (Batu, 1993; Batu €ßs , & Eren, 1999; & Yurdagel, 1993; Bozkurt, G€ ogu Demir€ oz€ u et al., 2002; Karakaya & Artık, 1990; Nas & Nas, 1987; Ozk€ ok, 1989; Tosun & Ust€ un, 2003; Ust€ un & Tosun, 1997). However, there is little information about
74
M. Seng€ul et al. / Food Control 16 (2005) 73–76
rheological and chemical properties of mulberry pekmez. Therefore, the purpose of this study was to characterize the rheological behaviour of mulberry pekmez and to determine chemical and physical properties.
2. Materials and methods The commercial pekmez was supplied from local supermarket and all chemicals were reagent grade. 2.1. Chemical and physical analyses Total and soluble dry matter, protein, ash, pH and titratable acidity were determined according to standard method AOAC (1984); pH was determined with a ATI ORION 420A model pH meter; titratable acidity, expressed as percentage of citric acid, was determined with 0.1 N NaOH up to pH 8.1; solubles dry matter was determined with an Abbe-Zeis refractometer; protein content was determined by the Kjeldal method (Nx6.25). Total sugar, invert sugar and sucrose contents were analysed by the Lane-Eynon method (Cemero glu, 1992). Hydroxymethylfurfural (HMF) was determined by the IFJJP (1964) with a spectrophotometer (Schimadzu, UV-120-01 Model) at 550 nm. For color analysis, the instrument was calibrated with a white reference tile before measurements. Color of mulberry pekmez was analysed by measuring Hunter L (brightness; 100: white, 0: black), a (+: red; ): green) and b (+: yellow; ): blue) parameters with a colorimeter (Model CR 200, Chromometer, Minolta, Japan).
3. Results and discussion 3.1. Chemical and physical characteristics The chemical and physical characteristics of mulberry pekmez are given in Table 1. As shown in Table 1, mulberry pekmez contained high amounts of total sugar, which is composed of approximately 100% invert sugar as glucose and fructose. This is very important in human nutrition because of its easy digestibility and these sugars provide a readily available energy source since they easily pass to blood (Kavas, 1990). In addition, glucose, energy source of the brain, enhances the transport of tryptophan through the blood–brain barrier and it is useful in seratonin synthesis that has a function in brain working (Birch & Parker, 1979). However, this pekmez contains very small contents of sucrose and protein. HMF, an indicator of quality deterioration, occurs as a result of excessive heating in foods containing carbohydrates (Cemeroglu, 1982). Therefore, high amounts of HMF are not desired in processed fruit juices. The Turkish Standard Institute recommends a maximum HMF concentration of 75 mg l1 for first quality mulberry pekmez (TS, 1996). In this study, a low HMF content of mulberry pekmez was found here (viz. 6.34 mg l1 ). As shown in Table 1, L, a and b values were measured 19.57, +15.91 and )0.14, respectively. A high redness (a) value is not desired because it occurs as a result of excessive caramelization of sugars. Therefore, a low redness (a) and a high brightness (L) values indicate a good quality pekmez (Aksu & Nas, 1996).
2.2. Rheological behaviour
3.2. Rheological behaviour
The viscosity (Pa s) of mulberry pekmez was measured at 30, 40, 50, 60 and 70 C using a rotational viscometer (Model Poulten RV-8, Selfe Elee Ltd., Wickford Essex SSII 8BJ, England) equipped with spindle 5 at the speed of 5, 10, 20, 50 and 100 rpm. Three readings were taken per sample at 30 s intervals. A 300 ml beaker was used for viscosity measurements and a thermostatic water bath was used to control the processing temperature within the (30–70 ± 1) C range.
Several rheological models have been employed to fit data on fruit juices. The rheological behaviour of mulberry pekmez was described by the power law model (Chhinnan, McWaters, & Rao, 1985; Heikal & Chhinnan, 1990):
2.3. Statistical analysis In this study, five different speed levels (5, 10, 20, 50 and 100 rpm) and temperatures (30, 40, 50, 60 and 70 C) were selected as experimental factors. The experiment was set up as 5 · 5 the factorial experimental design, and the analysis was carried out according to completely randomized blocks design with three replications (G€ urb€ uz, 1993).
Table 1 The chemical and physical characteristics of mulberry pekmez Parameters
Values
Total dry matter Soluble dry matter Total sugar Invert sugar Sucrose Ash Protein Hydroxymethylfurfural (HMF) pH Titratable acidity (citric acid) Color
74.33% 72% 60.22 g/100 g 59.56 g/100 g 0.66 g/100 g 2.02% 0.36% 6.34 mg/l 5.15 0.52% L: 19.27 a: +15.91 b: )0.14
M. Seng€ul et al. / Food Control 16 (2005) 73–76
ga ¼ kcðn1Þ
1600
ð1Þ
Table 2 The parameters of power law model for the mulberry pekmez at different temperatures n
2
Temperature (C)
n
k (Pa s )
r
30 40 50 60 70
0.816 0.796 0.78 0.709 0.684
1.9665 1.5102 1.0814 0.9105 0.9042
0.999 0.998 0.991 0.988 0.995
30°C 40°C 50°C 60°C 70°C
1400 Viscosity (mPa s)
where ga is the viscosity (Pa s), k the consistency index (Pa sn ), c the rotational speed (s1 ), n the flow behaviour index (dimensionless). In order to obtain the flow behaviour index (n) and consistency index (k) values, Eq. (1) was analysed by linear regression, and n, k and correlation coefficients (r2 ) at different temperatures are given in Table 2. As shown in Table 2, it was found that the power law model properly explained the flow behaviour of mulberry pekmez with high correlation coefficients. n and k values ranged from 0.684 to 0.816, 0.9042 to 1.9665, respectively. Mulberry pekmez exhibited a pseudoplastic behaviour because the values of flow behaviour index (n), a measure of the departure from Newtonian flow (Chhinnan et al., 1985), were less than 1 (GrigelmoMiguel, Ibarz-Ribas, & Martin-Belleso, 1999; Khan, Stehlı, Weı, Steinberg, & Yamashita, 1990; Sopade & Filibus, 1995). However, the consistency index (k) decreased as temperature increased. Similar results were also obtained in other studies (Alpaslan & Hayta, 2002; Grigelmo-Miguel et al., 1999; Kaya & Beliba glı, 2002; Sopade & Filibus, 1995). The effect of the interaction between speed and temperature on viscosity of mulberry pekmez was found to be statistically significant at P < 0:01, and the plot of viscosity versus speed for mulberry pekmez at different temperatures is shown in Fig. 1. The viscosity of liquids generally decreases as temperature increases. As shown in Fig. 1, as temperature and speed increased, the viscosity of mulberry pekmez decreased. Similar observations had been reported by several researchers (Finley, 1973; Heikal & Chhinnan, 1990; Khan et al., 1990; Sopade & Kassum, 1992). An activation energy is necessary for moving of a molecule, and as the temperature increases, the liquid flows more easily due to higher an activation energy in high temperatures (G€ urses & Bayrakcßeken, 1996). Besides, Rha (1975) noted that the decrease in viscosity with increasing speed is related to the increasing alignment of constituent molecules. The consistency index, an indication of the viscous nature of food, can be used to describe the variation in viscosity with temperature using the Arrhenius equation
75
1200 1000 800 600
v
400 200 0 5
10
20 Speed (rpm)
50
100
Fig. 1. The flow behaviour of mulberry pekmez at different temperatures.
Table 3 The parameters of Arrhenius equation Soluble dry matter (Brix)
Kt (Pa sn )
Ea (kJ mol1 )
r2
72
1.486
17.97
0.933
(Costaldo, Plamer, Lo Voi, & Costabile, 1990; Morgan, Steffe, & Ofoli, 1989; Vergnes & Villemarie, 1987): ln K ¼ ln Kt þ Ea =Rg Ta
ð2Þ n
where K is the consistency index (Pa s ), Kt the consistency index at a reference temperature (Pa sn ), Ea the activation energy (J mol1 ), Rg the universal gas constant (J mol1 ), Ta the absolute temperature (K). Applying the linear regression analysis in Eq. (2), values of Ea and Kt were calculated; the Arrhenius equation parameters with correlation coefficients (r2 ) are given in Table 3. The value of activation energy (17.97 kJ mol1 ) obtained at 72 Brix for mulberry pekmez in our study was found lower than those reported by other authors for Gaziantep pekmez (concentrated grape juice) at 72.9 Brix (43.85 kJ mol1 ). Consequently, these values indicate that mulberry pekmez has more homogeneous texture and uniform particle distribution than grape pekmez.
4. Conclusions Mulberry pekmez contained high amounts of total sugar, which is composed of approximately 100% invert sugar as glucose and fructose. This is especially important in human nutrition. In addition, the rheological behaviour of mulberry pekmez was found to be most adequately described by the power law model and exhibited a pseudoplastic behaviour. Viscosity decreased with increasing temperature as was expected. The effect
76
M. Seng€ul et al. / Food Control 16 (2005) 73–76
of temperature on viscosity was described by the Arrhenius equation, with an activation energy for mulberry pekmez at 72 Brix lower than those of grape pekmez. References € retim teknigi ve ßcesßitli Aksu, I., & Nas, S. (1996). Dut pekmezi u €zellikleri. Gıda, 21, 83–88. fiziksel-kimyasal o Alpaslan, M., & Hayta, M. (2002). Rheological and sensory properties of pekmez (grape molasses/tahin) (sesame paste blends). Journal of Food Engineering, 54, 89–93. AOAC, 1984. AOAC, Official methods of analysis. Association of Official Analytical Chemists. 14th ed., Arlington, VA. €z€ Batu, A. (1993). Kuru u um ve pekmezin insan saglıgı ve beslenmesi € nemi. Gıda, 18, 303–307. acßısından o Batu, A., & Yurdagel, U. (1993). Degisßik katkıların kullanımı ile beyaz €z€ €zerine bir arasßtırma. Gıda, 18, 157–163. katı u um pekmezi eldesi u Birch, G. G., & Parker, K. J. (1979). Sugar: science and technology. London: Applied Science Publisher Ltd. €ßs , F., & Eren, S. (1999). Nonenzymic browning Bozkurt, H., G€ o gu reaction in boiled grape juice and its models during storage. Food Chemistry, 64, 89–93. € retim teknolojisi. Teknik Basm Cemero glu, B. (1982). Meyve suyu u Sanayii Matbaası (297 pp), Ankara. Cemero glu, B. (1992). Meyve ve sebze isßleme end€ ustrisinde temel analiz metotları. Biltav Yayınları, Ankara. Chhinnan, M. S., McWaters, K. H., & Rao, V. N. M. (1985). Rheological characterization of grain legume pastes and effect of hydration time and water level on apparent viscosity. Journal of Food Science, 50, 1167–1171. Costaldo, D., Plamer, L., Lo Voi, A., & Costabile, P. (1990). Rheological characterisation of purees and concentrates of babaco. In W. E. L. Spiess & H. Schubert (Eds.), Engineering and Food: Vol. 1. Physical properties of process control (pp. 143–150). London: Elsevier Science Publishers. Demir€ oz€ u, B., S€ okmen, M., Ucßak, A., Yılmaz, H., & G€ ulderen, S. (2002). Variation of copper, iron, and zinc levels in pekmez products. Bulletin Environmental Contamination and Toxicology, 69, 330–334. Finley, J. R. E. E. (1973). Elementary concepts of rheology relevant to food texture studies. In A. Kramer & A. S. Szczesniak (Eds.), Texture measurements of foods (pp. 33–51). Dordrecht, The Netherlands: Reidel. G€ urb€ uz, F. (1993). Minitab Inc. 3081 Enterprise Drive State Collage, PA 16801-3008 814-238-3280, Copyright [C], Minitab Inc. G€ urses, A., & Bayrakcßeken, S. (1996). Deneysel Fizikokimya. Atat€ urk € Universitesi Yayn No: 807. K.K. Egitim Fak. Yayın No: 62. Ders Kitapları Serisi No: 48. Erzurum, Turkiye.
Grigelmo-Miguel, N., Ibarz-Ribas, A., & Martin-Belleso, O. (1999). Rheology of peach dietary fibre suspensions. Journal of Food Engineering, 39, 91–99. Heikal, Y. A., & Chhinnan, M. S. (1990). Rheological characterization of tomato puree at different temperatures using to types of viscometers. In W. E. L. Spiess & H. Schubert (Eds.), Engineering and food: Vol. I. Physical properties and process control (pp. 151– 158). London: Elsevier Science Publishers. IFJJP (1964). IFJJP, International federation of fruit juice producers methods. Analyses; Fruit-Union Suise Assoc., Zug, Switzerland. € retim teknigi ve Karakaya, M., & Artık, N. (1990). Zile pekmezi u bilesßim unsurlarının belirlenmesi. Gıda, 15, 151–154. € um ve incirin beslenmedeki yeri ve o €nemi. Saglıklı Kavas, A. (1990). Uz€ € nemi €z€ beslenmede kuru ve incir ve ßcekirdeksiz kuru u um€ un o semineri (pp. 53–65). Tebligi No: 4. Kaya, A., & Belibaglı, K. B. (2002). Rheology of solid Gaziantep pekmez. Journal of Food Engineering, 54, 221–226. Khan, R., Stehlı, D., Weı, L. S., Steinberg, M. P., & Yamashita, N. (1990). Activity and mobility of water in sweetened concentrated desulged soy beverages and their rheological properties. Journal of Food Science, 55, 537–542. Morgan, R. H., Steffe, J. F., & Ofoli, R. Y. (1989). A generalised viscosity model for extrusion of protein doughs. Journal of Food Process Engineering, 11, 55–78. Nas, S., & Nas, M. (1987). Pekmez ve pestilin yapılısßı, bilesßimi ve €nemi. Gıda, 6, 347–352. o €retilen pekmezlerin u €retim Ozk€ ok, Z. (1989). Izmir ili ve ßcevresinde u €zerinde arasßtırmalar. Tarım teknikleri ve analitik karakterleri u Orman ve K€ oy Isßleri Bakanlıgi Koruma ve Kontrol Genel € , yayın No: 30. M€ ud€ url€ ugu Rha, C. (1975). Theories and principles of viscosity. In C. Rha (Ed.), Theory: determination and control of physical properties of food materials (pp. 7–249). Dordrecht, The Netherlands: Reidel. Saravacos, G. D. (1970). Effect of temperature on viscosity of fruit juices and purees. Journal of Food Science, 35, 122–125. Sopade, P. A., & Filibus, T. E. (1995). The influence of solid and sugar contents on rheological characteristics of Akamu, a semi-liquid maize food. Journal of Food Engineering, 24, 197–211. Sopade, P. A., & Kassum, A. L. (1992). Rheological characterisation of Akamu, a semi-liquid food from maize, millet and sorghum. Journal of Cereal Science, 15, 193–202. Tosun, I., & Ust€ un, N. S. (2003). Nonenzymic browning during storage of white hard grape pekmez (Zile pekmezi). Food Chemistry, 80, 441–443. TS (1996). Mulberry pekmez. Turkish Standard Institute, 12001. Necatibey Caddesi, 112, Bakanlıklar, Ankara. Ust€ un, N. S., & Tosun, I. (1997). Pekmezlerin bileimi. Gıda, 22, 417– 423. Vergnes, B., & Villemarie, J. P. (1987). Rheological behaviour of low moisture molten maize starch. Rheologica Acta, 26, 570–576.
Rheological, physical and chemical characteristics of mulberry pekmez Memnune Seng€ ul, M. Fatih Ertugay *, Mustafa Seng€ ul Department of Food Engineering, Faculty of Agriculture, Atat€urk University, 25240 Erzurum, Turkey Received 16 February 2003; received in revised form 21 November 2003; accepted 26 November 2003
Abstract The physical, chemical properties and the rheological behaviour of mulberry pekmez were investigated. Viscosity was measured at 30, 40, 50, 60 and 70 C using a rotational viscometer equipped with spindle 5 at the speed of 5, 10, 20, 50 and 100 rpm. An empirical power-law model was used to describe the rheological behaviour of mulberry pekmez with correlation coefficients between 0.991 and 0.999. The mulberry pekmez exhibited a pseudoplastic behaviour. An Arrhenius equation was used to describe the effect of temperature on viscosity and Ea value of the mulberry pekmez was calculated as 17.97 kJ mol1 . 2004 Elsevier Ltd. All rights reserved. Keywords: Mulberry; Pekmez; Concentrated fruit juices; Rheological properties
1. Introduction Pekmez is one of the traditional food products in Turkey and it is commonly produced from grape and mulberry by concentration of juices up to 70–80 soluble dry matter content. Pekmez, consumed generally for breakfast (Alpaslan & Hayta, 2002), can also be produced from fruits containing high amounts of sugar like apple, carnob, plum, watermelon, apricot, sugar beet and fig. Since pekmez contains high amounts of sugar, mineral and organic acid, it is a very important food product in human nutrition (Demir€ oz€ u, S€ okmen, Ucßak, Yılmaz, & G€ ulderen, 2002; Ust€ un & Tosun, 1997). Pekmez easily passes into the blood without digestion because most of its carbohydrate is in the form of monosaccarides like glucose and fructose. This is nutritionally important, especially for babies, children, sportsmen and in situations demanding urgent energy. Pekmez has been produced with different techniques considering species of fruits used in production. For mulberry pekmez production, fresh mulberry is used as raw material. Firstly, mulberry is placed to boiling vessels after cleaning. Then, 8–10 l water is added for 20–30 kg mulberry. After adding water, the mixture is
*
Corresponding author. Tel.: +90-442-2312484; fax: +90-4422360958. E-mail address: [email protected] (M. Fatih Ertugay). 0956-7135/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodcont.2003.11.010
completely stirred, and is boiled by stirring for 1 h. It has various colors from light brown to dark brown depending on the process. After then, the mixture is cooled up to about 40–50 C. It is pressed and then filtered in order to obtain clear mulberry juice. Mulberry juice is concentrated in open vessels to obtain 65–72 Brix. Then, mulberry juice is cooled up to 40 C; this product is called mulberry pekmez. Finally, mulberry pekmez is packaged and stored at room temperature (Aksu & Nas, 1996). In the food industry, rheological characteristics of concentrated fruit juices are a significant property in addition to chemical and physical properties. Moreover, rheological characteristics depend on both the chemical composition of fruits and processing conditions. However, a knowledge of the flow behaviour of concentrated fruit juices will be useful in quality control, calculating energy usage, process control and equipment selection (Kaya & Belibaglı, 2002). Viscosity, a rheological property, is also considered as an important physical characteristic related to the quality of liquid food products (Saravacos, 1970). There is a number of research about rheological (Alpaslan & Hayta, 2002; Kaya & Belibaglı, 2002) and chemical properties of grape pekmez (Batu, 1993; Batu €ßs , & Eren, 1999; & Yurdagel, 1993; Bozkurt, G€ ogu Demir€ oz€ u et al., 2002; Karakaya & Artık, 1990; Nas & Nas, 1987; Ozk€ ok, 1989; Tosun & Ust€ un, 2003; Ust€ un & Tosun, 1997). However, there is little information about
74
M. Seng€ul et al. / Food Control 16 (2005) 73–76
rheological and chemical properties of mulberry pekmez. Therefore, the purpose of this study was to characterize the rheological behaviour of mulberry pekmez and to determine chemical and physical properties.
2. Materials and methods The commercial pekmez was supplied from local supermarket and all chemicals were reagent grade. 2.1. Chemical and physical analyses Total and soluble dry matter, protein, ash, pH and titratable acidity were determined according to standard method AOAC (1984); pH was determined with a ATI ORION 420A model pH meter; titratable acidity, expressed as percentage of citric acid, was determined with 0.1 N NaOH up to pH 8.1; solubles dry matter was determined with an Abbe-Zeis refractometer; protein content was determined by the Kjeldal method (Nx6.25). Total sugar, invert sugar and sucrose contents were analysed by the Lane-Eynon method (Cemero glu, 1992). Hydroxymethylfurfural (HMF) was determined by the IFJJP (1964) with a spectrophotometer (Schimadzu, UV-120-01 Model) at 550 nm. For color analysis, the instrument was calibrated with a white reference tile before measurements. Color of mulberry pekmez was analysed by measuring Hunter L (brightness; 100: white, 0: black), a (+: red; ): green) and b (+: yellow; ): blue) parameters with a colorimeter (Model CR 200, Chromometer, Minolta, Japan).
3. Results and discussion 3.1. Chemical and physical characteristics The chemical and physical characteristics of mulberry pekmez are given in Table 1. As shown in Table 1, mulberry pekmez contained high amounts of total sugar, which is composed of approximately 100% invert sugar as glucose and fructose. This is very important in human nutrition because of its easy digestibility and these sugars provide a readily available energy source since they easily pass to blood (Kavas, 1990). In addition, glucose, energy source of the brain, enhances the transport of tryptophan through the blood–brain barrier and it is useful in seratonin synthesis that has a function in brain working (Birch & Parker, 1979). However, this pekmez contains very small contents of sucrose and protein. HMF, an indicator of quality deterioration, occurs as a result of excessive heating in foods containing carbohydrates (Cemeroglu, 1982). Therefore, high amounts of HMF are not desired in processed fruit juices. The Turkish Standard Institute recommends a maximum HMF concentration of 75 mg l1 for first quality mulberry pekmez (TS, 1996). In this study, a low HMF content of mulberry pekmez was found here (viz. 6.34 mg l1 ). As shown in Table 1, L, a and b values were measured 19.57, +15.91 and )0.14, respectively. A high redness (a) value is not desired because it occurs as a result of excessive caramelization of sugars. Therefore, a low redness (a) and a high brightness (L) values indicate a good quality pekmez (Aksu & Nas, 1996).
2.2. Rheological behaviour
3.2. Rheological behaviour
The viscosity (Pa s) of mulberry pekmez was measured at 30, 40, 50, 60 and 70 C using a rotational viscometer (Model Poulten RV-8, Selfe Elee Ltd., Wickford Essex SSII 8BJ, England) equipped with spindle 5 at the speed of 5, 10, 20, 50 and 100 rpm. Three readings were taken per sample at 30 s intervals. A 300 ml beaker was used for viscosity measurements and a thermostatic water bath was used to control the processing temperature within the (30–70 ± 1) C range.
Several rheological models have been employed to fit data on fruit juices. The rheological behaviour of mulberry pekmez was described by the power law model (Chhinnan, McWaters, & Rao, 1985; Heikal & Chhinnan, 1990):
2.3. Statistical analysis In this study, five different speed levels (5, 10, 20, 50 and 100 rpm) and temperatures (30, 40, 50, 60 and 70 C) were selected as experimental factors. The experiment was set up as 5 · 5 the factorial experimental design, and the analysis was carried out according to completely randomized blocks design with three replications (G€ urb€ uz, 1993).
Table 1 The chemical and physical characteristics of mulberry pekmez Parameters
Values
Total dry matter Soluble dry matter Total sugar Invert sugar Sucrose Ash Protein Hydroxymethylfurfural (HMF) pH Titratable acidity (citric acid) Color
74.33% 72% 60.22 g/100 g 59.56 g/100 g 0.66 g/100 g 2.02% 0.36% 6.34 mg/l 5.15 0.52% L: 19.27 a: +15.91 b: )0.14
M. Seng€ul et al. / Food Control 16 (2005) 73–76
ga ¼ kcðn1Þ
1600
ð1Þ
Table 2 The parameters of power law model for the mulberry pekmez at different temperatures n
2
Temperature (C)
n
k (Pa s )
r
30 40 50 60 70
0.816 0.796 0.78 0.709 0.684
1.9665 1.5102 1.0814 0.9105 0.9042
0.999 0.998 0.991 0.988 0.995
30°C 40°C 50°C 60°C 70°C
1400 Viscosity (mPa s)
where ga is the viscosity (Pa s), k the consistency index (Pa sn ), c the rotational speed (s1 ), n the flow behaviour index (dimensionless). In order to obtain the flow behaviour index (n) and consistency index (k) values, Eq. (1) was analysed by linear regression, and n, k and correlation coefficients (r2 ) at different temperatures are given in Table 2. As shown in Table 2, it was found that the power law model properly explained the flow behaviour of mulberry pekmez with high correlation coefficients. n and k values ranged from 0.684 to 0.816, 0.9042 to 1.9665, respectively. Mulberry pekmez exhibited a pseudoplastic behaviour because the values of flow behaviour index (n), a measure of the departure from Newtonian flow (Chhinnan et al., 1985), were less than 1 (GrigelmoMiguel, Ibarz-Ribas, & Martin-Belleso, 1999; Khan, Stehlı, Weı, Steinberg, & Yamashita, 1990; Sopade & Filibus, 1995). However, the consistency index (k) decreased as temperature increased. Similar results were also obtained in other studies (Alpaslan & Hayta, 2002; Grigelmo-Miguel et al., 1999; Kaya & Beliba glı, 2002; Sopade & Filibus, 1995). The effect of the interaction between speed and temperature on viscosity of mulberry pekmez was found to be statistically significant at P < 0:01, and the plot of viscosity versus speed for mulberry pekmez at different temperatures is shown in Fig. 1. The viscosity of liquids generally decreases as temperature increases. As shown in Fig. 1, as temperature and speed increased, the viscosity of mulberry pekmez decreased. Similar observations had been reported by several researchers (Finley, 1973; Heikal & Chhinnan, 1990; Khan et al., 1990; Sopade & Kassum, 1992). An activation energy is necessary for moving of a molecule, and as the temperature increases, the liquid flows more easily due to higher an activation energy in high temperatures (G€ urses & Bayrakcßeken, 1996). Besides, Rha (1975) noted that the decrease in viscosity with increasing speed is related to the increasing alignment of constituent molecules. The consistency index, an indication of the viscous nature of food, can be used to describe the variation in viscosity with temperature using the Arrhenius equation
75
1200 1000 800 600
v
400 200 0 5
10
20 Speed (rpm)
50
100
Fig. 1. The flow behaviour of mulberry pekmez at different temperatures.
Table 3 The parameters of Arrhenius equation Soluble dry matter (Brix)
Kt (Pa sn )
Ea (kJ mol1 )
r2
72
1.486
17.97
0.933
(Costaldo, Plamer, Lo Voi, & Costabile, 1990; Morgan, Steffe, & Ofoli, 1989; Vergnes & Villemarie, 1987): ln K ¼ ln Kt þ Ea =Rg Ta
ð2Þ n
where K is the consistency index (Pa s ), Kt the consistency index at a reference temperature (Pa sn ), Ea the activation energy (J mol1 ), Rg the universal gas constant (J mol1 ), Ta the absolute temperature (K). Applying the linear regression analysis in Eq. (2), values of Ea and Kt were calculated; the Arrhenius equation parameters with correlation coefficients (r2 ) are given in Table 3. The value of activation energy (17.97 kJ mol1 ) obtained at 72 Brix for mulberry pekmez in our study was found lower than those reported by other authors for Gaziantep pekmez (concentrated grape juice) at 72.9 Brix (43.85 kJ mol1 ). Consequently, these values indicate that mulberry pekmez has more homogeneous texture and uniform particle distribution than grape pekmez.
4. Conclusions Mulberry pekmez contained high amounts of total sugar, which is composed of approximately 100% invert sugar as glucose and fructose. This is especially important in human nutrition. In addition, the rheological behaviour of mulberry pekmez was found to be most adequately described by the power law model and exhibited a pseudoplastic behaviour. Viscosity decreased with increasing temperature as was expected. The effect
76
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of temperature on viscosity was described by the Arrhenius equation, with an activation energy for mulberry pekmez at 72 Brix lower than those of grape pekmez. References € retim teknigi ve ßcesßitli Aksu, I., & Nas, S. (1996). Dut pekmezi u €zellikleri. Gıda, 21, 83–88. fiziksel-kimyasal o Alpaslan, M., & Hayta, M. (2002). Rheological and sensory properties of pekmez (grape molasses/tahin) (sesame paste blends). Journal of Food Engineering, 54, 89–93. AOAC, 1984. AOAC, Official methods of analysis. Association of Official Analytical Chemists. 14th ed., Arlington, VA. €z€ Batu, A. (1993). Kuru u um ve pekmezin insan saglıgı ve beslenmesi € nemi. Gıda, 18, 303–307. acßısından o Batu, A., & Yurdagel, U. (1993). Degisßik katkıların kullanımı ile beyaz €z€ €zerine bir arasßtırma. Gıda, 18, 157–163. katı u um pekmezi eldesi u Birch, G. G., & Parker, K. J. (1979). Sugar: science and technology. London: Applied Science Publisher Ltd. €ßs , F., & Eren, S. (1999). Nonenzymic browning Bozkurt, H., G€ o gu reaction in boiled grape juice and its models during storage. Food Chemistry, 64, 89–93. € retim teknolojisi. Teknik Basm Cemero glu, B. (1982). Meyve suyu u Sanayii Matbaası (297 pp), Ankara. Cemero glu, B. (1992). Meyve ve sebze isßleme end€ ustrisinde temel analiz metotları. Biltav Yayınları, Ankara. Chhinnan, M. S., McWaters, K. H., & Rao, V. N. M. (1985). Rheological characterization of grain legume pastes and effect of hydration time and water level on apparent viscosity. Journal of Food Science, 50, 1167–1171. Costaldo, D., Plamer, L., Lo Voi, A., & Costabile, P. (1990). Rheological characterisation of purees and concentrates of babaco. In W. E. L. Spiess & H. Schubert (Eds.), Engineering and Food: Vol. 1. Physical properties of process control (pp. 143–150). London: Elsevier Science Publishers. Demir€ oz€ u, B., S€ okmen, M., Ucßak, A., Yılmaz, H., & G€ ulderen, S. (2002). Variation of copper, iron, and zinc levels in pekmez products. Bulletin Environmental Contamination and Toxicology, 69, 330–334. Finley, J. R. E. E. (1973). Elementary concepts of rheology relevant to food texture studies. In A. Kramer & A. S. Szczesniak (Eds.), Texture measurements of foods (pp. 33–51). Dordrecht, The Netherlands: Reidel. G€ urb€ uz, F. (1993). Minitab Inc. 3081 Enterprise Drive State Collage, PA 16801-3008 814-238-3280, Copyright [C], Minitab Inc. G€ urses, A., & Bayrakcßeken, S. (1996). Deneysel Fizikokimya. Atat€ urk € Universitesi Yayn No: 807. K.K. Egitim Fak. Yayın No: 62. Ders Kitapları Serisi No: 48. Erzurum, Turkiye.
Grigelmo-Miguel, N., Ibarz-Ribas, A., & Martin-Belleso, O. (1999). Rheology of peach dietary fibre suspensions. Journal of Food Engineering, 39, 91–99. Heikal, Y. A., & Chhinnan, M. S. (1990). Rheological characterization of tomato puree at different temperatures using to types of viscometers. In W. E. L. Spiess & H. Schubert (Eds.), Engineering and food: Vol. I. Physical properties and process control (pp. 151– 158). London: Elsevier Science Publishers. IFJJP (1964). IFJJP, International federation of fruit juice producers methods. Analyses; Fruit-Union Suise Assoc., Zug, Switzerland. € retim teknigi ve Karakaya, M., & Artık, N. (1990). Zile pekmezi u bilesßim unsurlarının belirlenmesi. Gıda, 15, 151–154. € um ve incirin beslenmedeki yeri ve o €nemi. Saglıklı Kavas, A. (1990). Uz€ € nemi €z€ beslenmede kuru ve incir ve ßcekirdeksiz kuru u um€ un o semineri (pp. 53–65). Tebligi No: 4. Kaya, A., & Belibaglı, K. B. (2002). Rheology of solid Gaziantep pekmez. Journal of Food Engineering, 54, 221–226. Khan, R., Stehlı, D., Weı, L. S., Steinberg, M. P., & Yamashita, N. (1990). Activity and mobility of water in sweetened concentrated desulged soy beverages and their rheological properties. Journal of Food Science, 55, 537–542. Morgan, R. H., Steffe, J. F., & Ofoli, R. Y. (1989). A generalised viscosity model for extrusion of protein doughs. Journal of Food Process Engineering, 11, 55–78. Nas, S., & Nas, M. (1987). Pekmez ve pestilin yapılısßı, bilesßimi ve €nemi. Gıda, 6, 347–352. o €retilen pekmezlerin u €retim Ozk€ ok, Z. (1989). Izmir ili ve ßcevresinde u €zerinde arasßtırmalar. Tarım teknikleri ve analitik karakterleri u Orman ve K€ oy Isßleri Bakanlıgi Koruma ve Kontrol Genel € , yayın No: 30. M€ ud€ url€ ugu Rha, C. (1975). Theories and principles of viscosity. In C. Rha (Ed.), Theory: determination and control of physical properties of food materials (pp. 7–249). Dordrecht, The Netherlands: Reidel. Saravacos, G. D. (1970). Effect of temperature on viscosity of fruit juices and purees. Journal of Food Science, 35, 122–125. Sopade, P. A., & Filibus, T. E. (1995). The influence of solid and sugar contents on rheological characteristics of Akamu, a semi-liquid maize food. Journal of Food Engineering, 24, 197–211. Sopade, P. A., & Kassum, A. L. (1992). Rheological characterisation of Akamu, a semi-liquid food from maize, millet and sorghum. Journal of Cereal Science, 15, 193–202. Tosun, I., & Ust€ un, N. S. (2003). Nonenzymic browning during storage of white hard grape pekmez (Zile pekmezi). Food Chemistry, 80, 441–443. TS (1996). Mulberry pekmez. Turkish Standard Institute, 12001. Necatibey Caddesi, 112, Bakanlıklar, Ankara. Ust€ un, N. S., & Tosun, I. (1997). Pekmezlerin bileimi. Gıda, 22, 417– 423. Vergnes, B., & Villemarie, J. P. (1987). Rheological behaviour of low moisture molten maize starch. Rheologica Acta, 26, 570–576.