- Email: [email protected]
Effect of sugar on the rheological properties of sunflower oil–water emulsions
Journal of Food Engineering 43 (2000) 173±177
www.elsevier.com/locate/jfoodeng
Eect of sugar on the rheological properties of sun¯ower oil±water emulsions ßs Medeni Maskan *, Fahrettin Go gu Department of Food Engineering, University of Gaziantep, Engineering Faculty, 27310 Gaziantep, Turkey Received 23 April 1999; accepted 16 October 1999
Abstract The rheological properties of sun¯ower oil±water emulsions were studied at constant oil (79%), with varying sugar concentrations (0.0±8.0%) and dierent temperatures (25±65°C). Sugar improved the emulsion stability. The empirical power law ®tted the apparent viscosity-rotational speed data. All emulsions exhibited pseudoplastic behaviour. An average ¯ow behaviour index of 0:49 0:03 was proposed as power law index for all emulsions. The consistency index was in¯uenced by the sugar content and temperature. This dependency was described by an exponential equation. The eect of temperature on consistency of emulsions followed an Arrhenius type equation. Depending on the sugar content, the activation energy values varied from 31077 to 10716 kJ/kg mol. Ó 2000 Elsevier Science Ltd. All rights reserved.
Notation C sugar content (g sugar/100 g emulsion) Ea activation energy (kJ/kg mol) k consistency index (mPa sn ) pre-exponential factor in Arrhenius equation (mPa sn ) k0 n ¯ow behaviour index (dimensionless) r correlation coecient (dimensionless) R universal gas constant (8.314 kJ/kg mol K) rpm revolution per minute T temperature (K) c rotational speed (sÿ1 ) apparent viscosity (mPa s) gapp
1. Introduction Many food formulations are thermodynamically multiphase, unstable colloidal dispersions. The longterm stability of such complex materials is a function of several variables, including the physical and functional properties of the individual components, temperature and concentration. An important goal of research is to predict food stability and its ¯ow behaviour for industrial applications.
*
Corresponding author. E-mail address: [email protected] (M. Maskan).
An emulsion is a thermodynamically unstable system and forms the basis of many food products. It is created when oil and water are rapidly mixed in the presence of a food emulsi®er. Study of the rheological behaviour of food emulsions is of particular interest in quality control, product development, consumer acceptability and sensory evaluation. The ¯ow behaviour of emulsions is also essential for the engineering calculations related to processing and handling, design and evaluation of food processing equipment such as mixing equipment, piping and pumps (Rao & Anantheswaran, 1982; Rao, Cooley & Vitali, 1984). Rheological studies on model oil±water (O/W) emulsions containing dierent vegetable oils, proteins, acids, seasonings, salts and polysaccharides have been reported earlier (Sanderson, 1981; Hennock, Rahalkar & Richmond, 1984; Gladwell, Rahalkar & Richmond, 1985b; Gladwell, Grimson, Rahalkar & Richmond, 1985a; Coia & Stauer, 1987; Yilmazer, Carrillo & Kokini, 1991; Suzuki, Maeda, Matsuoka & Kubota, 1991). They found that the emulsions behaved as pseudoplastic ¯uids. However, published work on the behaviour of oil±water emulsions containing sugar is scarce. The objective of this work was ®rst to investigate the eect of temperature and sugar concentration on ¯ow behaviour and stability of sun¯ower oil±water emulsions and to examine the suitability of commonly used rheological models.
0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 9 9 ) 0 0 1 4 7 - 8
M. Maskan, F. G og ußs / Journal of Food Engineering 43 (2000) 173±177
174
2. Materials and methods 2.1. Materials Deionized water was used for the preparation of emulsions. A sun¯ower oil purchased from a local supermarket with fatty acid composition 14:0, 0.2%; 16:0, 5.8%; 18:0, 4.1%; 20:0, 0.3%; 16:1, 0.2%; 18:1, 22.7%; 18:2, 66.4%; 18:3, 0.3% was used. Emulsi®er (lecithin) and sugar (sucrose) were of commercial grades (Sigma Chemical, USA). 2.2. Preparation of emulsions Emulsions were prepared with sugar concentration ranging from 0 to 8 g sugar/100 g emulsion while keeping the amount of lecithin (1 g/100 g emulsion) and oil (79 g/100 g emulsion) constant. Table 1 shows the composition of emulsions. Lecithin was dissolved in sun¯ower oil and the oil was poured into previously prepared sugar-water solutions stirring gently by hand. Then, the mixture was emulsi®ed in a homogenizer (ArtMiccra D-8 Model, Art Labortechnik, M ullheim, Germany) at 19 000 rpm for 3 min at room temperature (25°C). Homogenizing more than 3 min resulted in a break in emulsions. Air, forced into the emulsions during homogenizing was evacuated at 70 mm Hg until bubble growth stopped (about 10 min). 2.3. Viscosity measurement Rheological measurements were performed at dierent sugar contents (0%, 2%, 4%, 6%, 8%) and controlled temperatures of 25°C, 40°C, 50°C and 65°C using a digital rotational Brook®eld Viscometer (Model RVT, Brook®eld Engineering Laboratories, Stoughton, MA). A thermostatic water bath was used to control the working temperature within the range 25±65°C. Spindle No. 3 rotating at 2.5, 5, 10, 20, 50 and 100 rpm was used to give dial readings between 0 and 100 on the scale. A 600 ml beaker was used for all measurements with the viscometer guard leg on, and enough sample was added to the beaker to just cover the immersion groove on each Table 1 Composition of sun¯ower oil±water emulsionsa Sugar (%)
Water (g)
Oil (g)
0 2 4 6 8
40 36 32 28 24
158 158 158 158 158
a Lecithin was added at a level of 2 g to each emulsion. The total weight of emulsion was 200 g.
spindle shaft (Sopade & Filibus, 1995). The measurements were repeated three times and the average values were used to calculate the apparent viscosity. Exact shear stresses and shear rates could not be obtained because of instrument limitations. The dial readings of the viscometer were converted to apparent viscosity values by multiplying with the conversion factors, supplied by the manufacturer for spindle no. 3 connected to RVT model. Chhinnan, McWatters and Rao (1985) have reported good correlations between rheological data calculated from viscosity-speed data obtained from similar viscometers and that obtained from exact shear stressshear rate data using capillary extrusion and Brook®eld Viscometers. Therefore, they reported that fundamental rheological information could still be estimated from viscosity-speed data instead of shear stress-shear rate data. 2.4. Oiling o Measured amounts of emulsions were poured into measuring cylinders (100 ml capacity) of identical size and stored at the temperatures studied. The height of the clear liquid (oil) at the top was measured at the end of 24 h. 2.5. Statistical analysis An analysis of variance (two-way ANOVA) was used to statistically analyse the data using Statgraphics (1991) package at a con®dence level of 95%. Regression analysis was carried out using the method of least squares. 3. Results and discussion The rotational speed range studied includes the shear rates at which the oral ¯ow index and apparent viscosity quality control index of O/W emulsions are measured. Coia and Stauer (1987) made a calibration between the Brook®eld (RVT) viscometer rotational speeds and shear rates (sÿ1 ). They also found the oral stimulus to be the shear stress approximately at a constant shear rate of 10 sÿ1 , which relates to the Brook®eld RVT at 50 rpm and the apparent viscosity quality control index is at Brook®eld 20 rpm for food emulsions. Therefore, Brook®eld viscometer may be used in determining the ¯ow properties and quality control index of O/W emulsions. The relationship between viscosity and rotational speed for the emulsions at 50°C is shown in Fig. 1. A systematic increase of viscosity with sugar concentration was observed. The increase of the emulsion viscosity when the quantity of sugar increases is related to the molecular movements, interfacial ®lms formation and
M. Maskan, F. G og ußs / Journal of Food Engineering 43 (2000) 173±177
Fig. 1. The relationship between apparent viscosity and rotational speed at 50°C for varying sugar content of emulsions.
setting up physical barriers with ingredients. The emulsions are non-Newtonian samples exhibiting shear thinning behaviour at all sugar concentrations which are true for the remaining temperatures also. Increase in speed decreased the apparent viscosity and shows two zones depending on the magnitude of the speed. At low speeds, apparent viscosity decreased sharply whereas at higher speeds the changes in viscosity were rather low. The change indicates a continuous breakdown of structure or emulsion aggregates at high speeds resulting in less resistance to ¯ow (Rha, 1978). This is consistent with observations on O/W emulsions by Hennock et al. (1984), semi-solid foods by Dervisßo glu and Kokini (1986), Bengal gram ¯our suspensions by Bhattacharya, Bhat and Raghuveer (1992) and concentrated food emulsions by Campanella, Dorward and Singh (1995). Since the viscosity of emulsions decreases as speed increases, pumping eciency increases as pump ¯ow rate increases (Race, 1991). Fig. 2 shows the eect of temperature on the viscosity of emulsions at 50 rpm. An increase in temperature decreased the viscosity at all sugar concentrations and rotational speeds. Fig. 1 also shows that an increase in sugar concentration increased the viscosity. These results agree with the published studies (Suzuki et al., 1991; Bhattacharya et al., 1992; Maskan, 1999) that an increase in solid content increases the viscosity of a food system, but appear to be in opposition to the ®ndings of Sopade and Filibus (1995) and Rezzoug, Bouvier, Allaf and Patras (1998). The latter researchers observed that addition of sugar caused a fall in the viscosity because sugar softens the dough or dilutes the starch due to its physico-chemical role in relation to water and starch. But in our system, one single phase can be considered because of emulsi®cation, and presence of more sugar with no other solute in the emulsion results in an increase in viscosity.
175
Fig. 2. Eect of temperature on the apparent viscosity of emulsions at 50 rpm for varying sugar content.
Emulsion stability was observed with respect to oiling o. During storage the continuous oil droplets coalesce to larger oil droplets and tend to migrate towards the top because of the density dierence. Finally, an oil layer is produced at the top of an emulsion. The oil layers (measured at 65°C) after 24 h were 0.3, 0.5, 1.5, 2.1 and 4.6 cm for emulsions containing 8%, 6%, 4%, 2% and 0% sugar, respectively. These results demonstrated that sugar increases the stability of emulsions. Although, Sanderson (1981) reported that polysaccharides can be used as a thickener, stabiliser and emulsi®er, our results revealed that sucrose can also be used as a stabiliser. Similar results were obtained for the other temperatures, and decrease in temperature increased the oiling o of the emulsions. This may be attributed to the mobility of molecules at elevated temperatures generating more friction and hence, delaying the settling of the sugar particles. The analysis of variance (two-way ANOVA) showed that there is a signi®cant eect of temperature and sugar content on the viscosity of emulsions (P < 0.05), and the multiple range analysis denoted no statistically signi®cant dierence between the viscosity values of 6% and 8% sugar content emulsions. For viscometers that do not give exact shear rates, an empirical mathematical equation (Eq. (1)) between apparent viscosity and rotational speed has been described which can be related to the power law model (Sopade & Filibus, 1995). gapp kc nÿ1 :
1
Table 2 contains the values for power law model (Eq. (1)) showing the empirical consistency and ¯ow behaviour indices. This model suitably explains the experimental data where r2 values ranged from 0.985 to 0.999. Values of the ¯ow behaviour index were consistently
M. Maskan, F. G og ußs / Journal of Food Engineering 43 (2000) 173±177
176
Table 2 Rheological parameters estimated from Eq. (1) Temperature (°C)
Sugar (%)
k
n
r2
25
0 2 4 6 8
2607 39 2666 49 3187 47 3507 75 3590 321
0:51 8:5 10ÿ3 0:52 1:0 10ÿ2 0:51 8:5 10ÿ3 0:50 1:3 10ÿ2 0:53 5:6 10ÿ2
0.999 0.999 0.999 0.998 0.966
40
0 2 4 6 8
1716 138 2218 107 2318 31 3220 69 3270 71
0:48 3:9 10ÿ2 0:45 2:2 10ÿ2 0:48 5:7 10ÿ3 0:47 1:3 10ÿ2 0:46 1:2 10ÿ2
0.986 0.996 0.999 0.999 0.999
50
0 2 4 6 8
1300 83 1939 67 2114 53 2616 40 2754 42
0:44 2:9 10ÿ2 0:43 2:1 10ÿ2 0:50 1:4 10ÿ2 0:49 8:9 10ÿ3 0:49 8:5 10ÿ3
0.995 0.997 0.998 0.999 0.999
65
0 2 4 6 8
569 33 1110 82 1618 63 2022 55 2164 57
0:57 3:2 10ÿ2 0:51 8:2 10ÿ2 0:50 2:3 10ÿ2 0:49 1:6 10ÿ2 0:48 1:6 10ÿ2
0.985 0.996 0.995 0.998 0.998
below unity indicating a pseudoplastic behaviour of the emulsions at all concentrations and temperatures tested. No consistent trend of the ¯ow behaviour index as affected by sugar concentration and temperature was noticed. In contrast, the consistency index decreased with increasing temperature and decreasing sugar concentration. Similar observations have been reported by various workers (Vitali & Rao, 1984; Chhinnan et al., 1985; Sopade & Filibus, 1995). Since the empirical consistency index is an indication of the viscous nature of the foods, it can be used to investigate the in¯uence of temperature on viscosity using Arrhenius-type relationship. The parameters of this equation are shown in Table 3. The Ea values were from 31 077 to 10 716 kJ/kg mol as sugar content varied 0±8%, respectively indicating that Ea decreases with increasing sugar content. The empirical equations for Ea and k0 are shown in Eqs. (2) and (3), respectively with r values being 0.999 and 0.992: 4
4
Ea 1:069 10 2:033 10 exp ÿ0:5105C;
k0 ÿ14:6 12:69 exp 0:2045C:
3
Thus, the variation of consistency index with temperature and sugar concentration was ®tted to Eq. (4) by multiple regression with a correlation coecient of 0.922: k 2:32 exp 2046=T 0:095C:
4
Applying the t-test, the mean values of ¯ow behaviour index were not signi®cantly dierent (P > 0.05) from each other. Combining the 20 mean values obtained for emulsions, a mean n value of 0:49 0:03 was proposed to de®ne the empirical power law index of emulsions. Finally, substituting Eq. (4) into Eq. (1), an empirical equation Eq. (5), was obtained for prediction of viscosity: gapp 2:32 exp 2046=T 0:095Ccÿ0:51 :
5
2
4. Conclusion
Table 3 Estimated parameters of the Arrhenius type equation Sugar (%)
k0 (mPa sn )
Ea (kJ/kg mol)
r2
0 2 4 6 8
0.01 2.32 11.82 32.14 49.40
31 077 17 683 13 817 11 755 10 716
0.946 0.887 0.988 0.929 0.939
Sun¯ower oil±water emulsions behaved as pseudoplastic ¯uids. The empirical power law model ®tted the experimental data well. The inclusion of sugar improved the stability of emulsions. An empirical equation was developed for the prediction of apparent viscosity of these emulsions as a function of sugar concentration, temperature and rotational speed of the spindle.
M. Maskan, F. G og ußs / Journal of Food Engineering 43 (2000) 173±177
References Bhattacharya, S., Bhat, K. K., & Raghuver, K. G. (1992). Rheology of Bengal gram (cicer arietinum) ¯our suspensions. Journal of Food Engineering, 17 (2), 83±96. Campanella, O. H., Dorward, N. M., & Singh, H. (1995). A study of the rheological properties of concentrated food emulsions. Journal of Food Engineering, 25 (3), 427±440. Chhinnan, M. S., McWatters, K. H., & Rao, V. N. M. (1985). Rheological characterization of grain legume pastes and eect of hydration time and water level on apparent viscosity. Journal of Food Science, 50 (4), 1167±1171. Coia, K. A., & Stauer, K. R. (1987). Shelf life study of oil/water emulsions using various commercial hydrocolloids. Journal of Food Science, 52 (1), 167±172. Dervisßoglu, M., & Kokini, J. L. (1986). Steady shear rheology and ¯uid mechanics of four semi-solid foods. Journal of Food Science, 51 (3), 541±546,625. Gladwell, N., Grimson, M. J., Rahalkar, R. R., & Richmond, P. (1985a). Rheological behavior of soya oil±water emulsions: Dependence upon oil concentration. Journal of Food Science, 50 (2), 441±443. Gladwell, N., Rahalkar, R. R., & Richmond, P. (1985b). Creep/ recovery behavior of oil±water emulsions: in¯uence of disperse phase concentration. Journal of Food Science, 50 (5), 1477±1481. Hennock, M., Rahalkar, R. R., & Richmond, P. (1984). Eect of xantan gum upon the rheology and stability of oil±water emulsions. Journal of Food Science, 49 (5), 1271±1274. Maskan, M. (1999). Rheological behaviour of liquorice (Glycyrrhiza glabra) extract. Journal of Food Engineering, 39 (4), 389±393.
177
Race, S. W. (1991). Improved product quality through viscosity measurement. Food Technology, 45 (7), 86±88. Rao, M. A., & Anantheswaran, R. C. (1982). Rheology of ¯uids in food processing. Food Technology, 36 (2), 116±126. Rao, M. A., Cooley, H. J., & Vitali, A. A. (1984). Flow properties of concentrated juices at low temperatures. Food Technology, 38 (3), 113±119. Rezzoug, M. Z., Bouvier, J. M., Allaf, K., & Patras, C. (1998). Eect of principal ingredients on the rheological behaviour of biscuit and on quality of biscuits. Journal of Food Engineering, 35 (1), 23±42. Rha, C. (1978). Rheology of ¯uid foods. Food Technology, 32 (7), 77±81. Sanderson, G. R. (1981). Polysaccharides in foods. Food Technology, 35(7), 50±56, 83. Sopade, P. A., & Filibus, T. E. (1995). The in¯uence of solid and sugar contents on rheological characteristics of akamu, a semi-liquid maize food. Journal of Food Engineering, 24 (2), 197±211. Statgraphics (1991). Statistical graphics system, reference manual. Rockville, STSC Inc., Maryland, USA. Suzuki, K., Maeda, T., Matsuoka, K., & Kubota, K. (1991). Eects of constituent concentration on rheological properties of corn oil-in-water emulsions. Journal of Food Science, 56 (3), 796±798, 854. Vitali, A. A., & Rao, M. A. (1984). Flow properties of low-pulp concentrated orange juices: eect of temperature and concentration. Journal of Food Science, 49 (3), 882±888. Yilmazer, G., Carrillo, A. R., & Kokini, J. L. (1991). Eect of propylene glycol alginate and xanthan gum on stability of o/w emulsions. Journal of Food Science, 56 (2), 513±517.
www.elsevier.com/locate/jfoodeng
Eect of sugar on the rheological properties of sun¯ower oil±water emulsions ßs Medeni Maskan *, Fahrettin Go gu Department of Food Engineering, University of Gaziantep, Engineering Faculty, 27310 Gaziantep, Turkey Received 23 April 1999; accepted 16 October 1999
Abstract The rheological properties of sun¯ower oil±water emulsions were studied at constant oil (79%), with varying sugar concentrations (0.0±8.0%) and dierent temperatures (25±65°C). Sugar improved the emulsion stability. The empirical power law ®tted the apparent viscosity-rotational speed data. All emulsions exhibited pseudoplastic behaviour. An average ¯ow behaviour index of 0:49 0:03 was proposed as power law index for all emulsions. The consistency index was in¯uenced by the sugar content and temperature. This dependency was described by an exponential equation. The eect of temperature on consistency of emulsions followed an Arrhenius type equation. Depending on the sugar content, the activation energy values varied from 31077 to 10716 kJ/kg mol. Ó 2000 Elsevier Science Ltd. All rights reserved.
Notation C sugar content (g sugar/100 g emulsion) Ea activation energy (kJ/kg mol) k consistency index (mPa sn ) pre-exponential factor in Arrhenius equation (mPa sn ) k0 n ¯ow behaviour index (dimensionless) r correlation coecient (dimensionless) R universal gas constant (8.314 kJ/kg mol K) rpm revolution per minute T temperature (K) c rotational speed (sÿ1 ) apparent viscosity (mPa s) gapp
1. Introduction Many food formulations are thermodynamically multiphase, unstable colloidal dispersions. The longterm stability of such complex materials is a function of several variables, including the physical and functional properties of the individual components, temperature and concentration. An important goal of research is to predict food stability and its ¯ow behaviour for industrial applications.
*
Corresponding author. E-mail address: [email protected] (M. Maskan).
An emulsion is a thermodynamically unstable system and forms the basis of many food products. It is created when oil and water are rapidly mixed in the presence of a food emulsi®er. Study of the rheological behaviour of food emulsions is of particular interest in quality control, product development, consumer acceptability and sensory evaluation. The ¯ow behaviour of emulsions is also essential for the engineering calculations related to processing and handling, design and evaluation of food processing equipment such as mixing equipment, piping and pumps (Rao & Anantheswaran, 1982; Rao, Cooley & Vitali, 1984). Rheological studies on model oil±water (O/W) emulsions containing dierent vegetable oils, proteins, acids, seasonings, salts and polysaccharides have been reported earlier (Sanderson, 1981; Hennock, Rahalkar & Richmond, 1984; Gladwell, Rahalkar & Richmond, 1985b; Gladwell, Grimson, Rahalkar & Richmond, 1985a; Coia & Stauer, 1987; Yilmazer, Carrillo & Kokini, 1991; Suzuki, Maeda, Matsuoka & Kubota, 1991). They found that the emulsions behaved as pseudoplastic ¯uids. However, published work on the behaviour of oil±water emulsions containing sugar is scarce. The objective of this work was ®rst to investigate the eect of temperature and sugar concentration on ¯ow behaviour and stability of sun¯ower oil±water emulsions and to examine the suitability of commonly used rheological models.
0260-8774/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 0 - 8 7 7 4 ( 9 9 ) 0 0 1 4 7 - 8
M. Maskan, F. G og ußs / Journal of Food Engineering 43 (2000) 173±177
174
2. Materials and methods 2.1. Materials Deionized water was used for the preparation of emulsions. A sun¯ower oil purchased from a local supermarket with fatty acid composition 14:0, 0.2%; 16:0, 5.8%; 18:0, 4.1%; 20:0, 0.3%; 16:1, 0.2%; 18:1, 22.7%; 18:2, 66.4%; 18:3, 0.3% was used. Emulsi®er (lecithin) and sugar (sucrose) were of commercial grades (Sigma Chemical, USA). 2.2. Preparation of emulsions Emulsions were prepared with sugar concentration ranging from 0 to 8 g sugar/100 g emulsion while keeping the amount of lecithin (1 g/100 g emulsion) and oil (79 g/100 g emulsion) constant. Table 1 shows the composition of emulsions. Lecithin was dissolved in sun¯ower oil and the oil was poured into previously prepared sugar-water solutions stirring gently by hand. Then, the mixture was emulsi®ed in a homogenizer (ArtMiccra D-8 Model, Art Labortechnik, M ullheim, Germany) at 19 000 rpm for 3 min at room temperature (25°C). Homogenizing more than 3 min resulted in a break in emulsions. Air, forced into the emulsions during homogenizing was evacuated at 70 mm Hg until bubble growth stopped (about 10 min). 2.3. Viscosity measurement Rheological measurements were performed at dierent sugar contents (0%, 2%, 4%, 6%, 8%) and controlled temperatures of 25°C, 40°C, 50°C and 65°C using a digital rotational Brook®eld Viscometer (Model RVT, Brook®eld Engineering Laboratories, Stoughton, MA). A thermostatic water bath was used to control the working temperature within the range 25±65°C. Spindle No. 3 rotating at 2.5, 5, 10, 20, 50 and 100 rpm was used to give dial readings between 0 and 100 on the scale. A 600 ml beaker was used for all measurements with the viscometer guard leg on, and enough sample was added to the beaker to just cover the immersion groove on each Table 1 Composition of sun¯ower oil±water emulsionsa Sugar (%)
Water (g)
Oil (g)
0 2 4 6 8
40 36 32 28 24
158 158 158 158 158
a Lecithin was added at a level of 2 g to each emulsion. The total weight of emulsion was 200 g.
spindle shaft (Sopade & Filibus, 1995). The measurements were repeated three times and the average values were used to calculate the apparent viscosity. Exact shear stresses and shear rates could not be obtained because of instrument limitations. The dial readings of the viscometer were converted to apparent viscosity values by multiplying with the conversion factors, supplied by the manufacturer for spindle no. 3 connected to RVT model. Chhinnan, McWatters and Rao (1985) have reported good correlations between rheological data calculated from viscosity-speed data obtained from similar viscometers and that obtained from exact shear stressshear rate data using capillary extrusion and Brook®eld Viscometers. Therefore, they reported that fundamental rheological information could still be estimated from viscosity-speed data instead of shear stress-shear rate data. 2.4. Oiling o Measured amounts of emulsions were poured into measuring cylinders (100 ml capacity) of identical size and stored at the temperatures studied. The height of the clear liquid (oil) at the top was measured at the end of 24 h. 2.5. Statistical analysis An analysis of variance (two-way ANOVA) was used to statistically analyse the data using Statgraphics (1991) package at a con®dence level of 95%. Regression analysis was carried out using the method of least squares. 3. Results and discussion The rotational speed range studied includes the shear rates at which the oral ¯ow index and apparent viscosity quality control index of O/W emulsions are measured. Coia and Stauer (1987) made a calibration between the Brook®eld (RVT) viscometer rotational speeds and shear rates (sÿ1 ). They also found the oral stimulus to be the shear stress approximately at a constant shear rate of 10 sÿ1 , which relates to the Brook®eld RVT at 50 rpm and the apparent viscosity quality control index is at Brook®eld 20 rpm for food emulsions. Therefore, Brook®eld viscometer may be used in determining the ¯ow properties and quality control index of O/W emulsions. The relationship between viscosity and rotational speed for the emulsions at 50°C is shown in Fig. 1. A systematic increase of viscosity with sugar concentration was observed. The increase of the emulsion viscosity when the quantity of sugar increases is related to the molecular movements, interfacial ®lms formation and
M. Maskan, F. G og ußs / Journal of Food Engineering 43 (2000) 173±177
Fig. 1. The relationship between apparent viscosity and rotational speed at 50°C for varying sugar content of emulsions.
setting up physical barriers with ingredients. The emulsions are non-Newtonian samples exhibiting shear thinning behaviour at all sugar concentrations which are true for the remaining temperatures also. Increase in speed decreased the apparent viscosity and shows two zones depending on the magnitude of the speed. At low speeds, apparent viscosity decreased sharply whereas at higher speeds the changes in viscosity were rather low. The change indicates a continuous breakdown of structure or emulsion aggregates at high speeds resulting in less resistance to ¯ow (Rha, 1978). This is consistent with observations on O/W emulsions by Hennock et al. (1984), semi-solid foods by Dervisßo glu and Kokini (1986), Bengal gram ¯our suspensions by Bhattacharya, Bhat and Raghuveer (1992) and concentrated food emulsions by Campanella, Dorward and Singh (1995). Since the viscosity of emulsions decreases as speed increases, pumping eciency increases as pump ¯ow rate increases (Race, 1991). Fig. 2 shows the eect of temperature on the viscosity of emulsions at 50 rpm. An increase in temperature decreased the viscosity at all sugar concentrations and rotational speeds. Fig. 1 also shows that an increase in sugar concentration increased the viscosity. These results agree with the published studies (Suzuki et al., 1991; Bhattacharya et al., 1992; Maskan, 1999) that an increase in solid content increases the viscosity of a food system, but appear to be in opposition to the ®ndings of Sopade and Filibus (1995) and Rezzoug, Bouvier, Allaf and Patras (1998). The latter researchers observed that addition of sugar caused a fall in the viscosity because sugar softens the dough or dilutes the starch due to its physico-chemical role in relation to water and starch. But in our system, one single phase can be considered because of emulsi®cation, and presence of more sugar with no other solute in the emulsion results in an increase in viscosity.
175
Fig. 2. Eect of temperature on the apparent viscosity of emulsions at 50 rpm for varying sugar content.
Emulsion stability was observed with respect to oiling o. During storage the continuous oil droplets coalesce to larger oil droplets and tend to migrate towards the top because of the density dierence. Finally, an oil layer is produced at the top of an emulsion. The oil layers (measured at 65°C) after 24 h were 0.3, 0.5, 1.5, 2.1 and 4.6 cm for emulsions containing 8%, 6%, 4%, 2% and 0% sugar, respectively. These results demonstrated that sugar increases the stability of emulsions. Although, Sanderson (1981) reported that polysaccharides can be used as a thickener, stabiliser and emulsi®er, our results revealed that sucrose can also be used as a stabiliser. Similar results were obtained for the other temperatures, and decrease in temperature increased the oiling o of the emulsions. This may be attributed to the mobility of molecules at elevated temperatures generating more friction and hence, delaying the settling of the sugar particles. The analysis of variance (two-way ANOVA) showed that there is a signi®cant eect of temperature and sugar content on the viscosity of emulsions (P < 0.05), and the multiple range analysis denoted no statistically signi®cant dierence between the viscosity values of 6% and 8% sugar content emulsions. For viscometers that do not give exact shear rates, an empirical mathematical equation (Eq. (1)) between apparent viscosity and rotational speed has been described which can be related to the power law model (Sopade & Filibus, 1995). gapp kc nÿ1 :
1
Table 2 contains the values for power law model (Eq. (1)) showing the empirical consistency and ¯ow behaviour indices. This model suitably explains the experimental data where r2 values ranged from 0.985 to 0.999. Values of the ¯ow behaviour index were consistently
M. Maskan, F. G og ußs / Journal of Food Engineering 43 (2000) 173±177
176
Table 2 Rheological parameters estimated from Eq. (1) Temperature (°C)
Sugar (%)
k
n
r2
25
0 2 4 6 8
2607 39 2666 49 3187 47 3507 75 3590 321
0:51 8:5 10ÿ3 0:52 1:0 10ÿ2 0:51 8:5 10ÿ3 0:50 1:3 10ÿ2 0:53 5:6 10ÿ2
0.999 0.999 0.999 0.998 0.966
40
0 2 4 6 8
1716 138 2218 107 2318 31 3220 69 3270 71
0:48 3:9 10ÿ2 0:45 2:2 10ÿ2 0:48 5:7 10ÿ3 0:47 1:3 10ÿ2 0:46 1:2 10ÿ2
0.986 0.996 0.999 0.999 0.999
50
0 2 4 6 8
1300 83 1939 67 2114 53 2616 40 2754 42
0:44 2:9 10ÿ2 0:43 2:1 10ÿ2 0:50 1:4 10ÿ2 0:49 8:9 10ÿ3 0:49 8:5 10ÿ3
0.995 0.997 0.998 0.999 0.999
65
0 2 4 6 8
569 33 1110 82 1618 63 2022 55 2164 57
0:57 3:2 10ÿ2 0:51 8:2 10ÿ2 0:50 2:3 10ÿ2 0:49 1:6 10ÿ2 0:48 1:6 10ÿ2
0.985 0.996 0.995 0.998 0.998
below unity indicating a pseudoplastic behaviour of the emulsions at all concentrations and temperatures tested. No consistent trend of the ¯ow behaviour index as affected by sugar concentration and temperature was noticed. In contrast, the consistency index decreased with increasing temperature and decreasing sugar concentration. Similar observations have been reported by various workers (Vitali & Rao, 1984; Chhinnan et al., 1985; Sopade & Filibus, 1995). Since the empirical consistency index is an indication of the viscous nature of the foods, it can be used to investigate the in¯uence of temperature on viscosity using Arrhenius-type relationship. The parameters of this equation are shown in Table 3. The Ea values were from 31 077 to 10 716 kJ/kg mol as sugar content varied 0±8%, respectively indicating that Ea decreases with increasing sugar content. The empirical equations for Ea and k0 are shown in Eqs. (2) and (3), respectively with r values being 0.999 and 0.992: 4
4
Ea 1:069 10 2:033 10 exp ÿ0:5105C;
k0 ÿ14:6 12:69 exp 0:2045C:
3
Thus, the variation of consistency index with temperature and sugar concentration was ®tted to Eq. (4) by multiple regression with a correlation coecient of 0.922: k 2:32 exp 2046=T 0:095C:
4
Applying the t-test, the mean values of ¯ow behaviour index were not signi®cantly dierent (P > 0.05) from each other. Combining the 20 mean values obtained for emulsions, a mean n value of 0:49 0:03 was proposed to de®ne the empirical power law index of emulsions. Finally, substituting Eq. (4) into Eq. (1), an empirical equation Eq. (5), was obtained for prediction of viscosity: gapp 2:32 exp 2046=T 0:095Ccÿ0:51 :
5
2
4. Conclusion
Table 3 Estimated parameters of the Arrhenius type equation Sugar (%)
k0 (mPa sn )
Ea (kJ/kg mol)
r2
0 2 4 6 8
0.01 2.32 11.82 32.14 49.40
31 077 17 683 13 817 11 755 10 716
0.946 0.887 0.988 0.929 0.939
Sun¯ower oil±water emulsions behaved as pseudoplastic ¯uids. The empirical power law model ®tted the experimental data well. The inclusion of sugar improved the stability of emulsions. An empirical equation was developed for the prediction of apparent viscosity of these emulsions as a function of sugar concentration, temperature and rotational speed of the spindle.
M. Maskan, F. G og ußs / Journal of Food Engineering 43 (2000) 173±177
References Bhattacharya, S., Bhat, K. K., & Raghuver, K. G. (1992). Rheology of Bengal gram (cicer arietinum) ¯our suspensions. Journal of Food Engineering, 17 (2), 83±96. Campanella, O. H., Dorward, N. M., & Singh, H. (1995). A study of the rheological properties of concentrated food emulsions. Journal of Food Engineering, 25 (3), 427±440. Chhinnan, M. S., McWatters, K. H., & Rao, V. N. M. (1985). Rheological characterization of grain legume pastes and eect of hydration time and water level on apparent viscosity. Journal of Food Science, 50 (4), 1167±1171. Coia, K. A., & Stauer, K. R. (1987). Shelf life study of oil/water emulsions using various commercial hydrocolloids. Journal of Food Science, 52 (1), 167±172. Dervisßoglu, M., & Kokini, J. L. (1986). Steady shear rheology and ¯uid mechanics of four semi-solid foods. Journal of Food Science, 51 (3), 541±546,625. Gladwell, N., Grimson, M. J., Rahalkar, R. R., & Richmond, P. (1985a). Rheological behavior of soya oil±water emulsions: Dependence upon oil concentration. Journal of Food Science, 50 (2), 441±443. Gladwell, N., Rahalkar, R. R., & Richmond, P. (1985b). Creep/ recovery behavior of oil±water emulsions: in¯uence of disperse phase concentration. Journal of Food Science, 50 (5), 1477±1481. Hennock, M., Rahalkar, R. R., & Richmond, P. (1984). Eect of xantan gum upon the rheology and stability of oil±water emulsions. Journal of Food Science, 49 (5), 1271±1274. Maskan, M. (1999). Rheological behaviour of liquorice (Glycyrrhiza glabra) extract. Journal of Food Engineering, 39 (4), 389±393.
177
Race, S. W. (1991). Improved product quality through viscosity measurement. Food Technology, 45 (7), 86±88. Rao, M. A., & Anantheswaran, R. C. (1982). Rheology of ¯uids in food processing. Food Technology, 36 (2), 116±126. Rao, M. A., Cooley, H. J., & Vitali, A. A. (1984). Flow properties of concentrated juices at low temperatures. Food Technology, 38 (3), 113±119. Rezzoug, M. Z., Bouvier, J. M., Allaf, K., & Patras, C. (1998). Eect of principal ingredients on the rheological behaviour of biscuit and on quality of biscuits. Journal of Food Engineering, 35 (1), 23±42. Rha, C. (1978). Rheology of ¯uid foods. Food Technology, 32 (7), 77±81. Sanderson, G. R. (1981). Polysaccharides in foods. Food Technology, 35(7), 50±56, 83. Sopade, P. A., & Filibus, T. E. (1995). The in¯uence of solid and sugar contents on rheological characteristics of akamu, a semi-liquid maize food. Journal of Food Engineering, 24 (2), 197±211. Statgraphics (1991). Statistical graphics system, reference manual. Rockville, STSC Inc., Maryland, USA. Suzuki, K., Maeda, T., Matsuoka, K., & Kubota, K. (1991). Eects of constituent concentration on rheological properties of corn oil-in-water emulsions. Journal of Food Science, 56 (3), 796±798, 854. Vitali, A. A., & Rao, M. A. (1984). Flow properties of low-pulp concentrated orange juices: eect of temperature and concentration. Journal of Food Science, 49 (3), 882±888. Yilmazer, G., Carrillo, A. R., & Kokini, J. L. (1991). Eect of propylene glycol alginate and xanthan gum on stability of o/w emulsions. Journal of Food Science, 56 (2), 513±517.