The rheological behavior of selected jams was analyzed at different temperatures, from 20 to 40°C in a rotational viscosimeter (HAAKE VT550). The rheograms were fitted with Power-Law, Carreau, Herschel-Bulkley, and Cross models. It was observed that the jams presented a pseudoplastic behavior, and the suspended solids influenced the consistency index.
INTRODUCTION
The study of manufactured products of fruits, such as juices, nectars, ice creams, and jellies, whose basic raw material is the fruit pulp, which is used in the unit operations, such as pumps, agitation, heat exchangers, and separations, have a great interest. For such industrial processes to be technically and economically feasible, it is important to have the knowledge of the physicochemical properties. The rheological properties of foods, such as fruit purees, play an important role in the processing of these materials. These rheological properties are also important as quality control parameters in the final products.[Citation1] The most common rheological property used to characterize fruit purees is the shear-stress against the shear–rate relationship. For fruit purees with high solid contents, the yield stress is another important rheological parameter used by engineers to describe the flow behavior of these materials. The yield stress is just as important as the shear-stress against shear–rate relationship in the design of equipment for processing purees, and in the assessment of quality of the final products.[Citation2]
Most foods are subjected to variations in their temperature during production, transport, storage, preparation, and consumption, e.g., pasteurization, sterilization, evaporation, cooking, freezing, chilling, etc. Temperature changes cause alterations in the physical and chemical properties of food components, which influence the overall properties of the final product, e.g., taste, appearance, texture, and stability. Chemical reactions such as hydrolysis, oxidation, or reduction may be promoted, or physical changes, such as evaporation, melting, crystallization, aggregation, or gelation may occur. A better understanding of the influence of temperature on the properties of foods enables food manufacturers to optimize processing conditions and improve product quality.[Citation3]
Most fluid foods do not have the simple Newtonian rheological model; in other words, their viscosities are independent of shear rate or shear stress and not constant with temperature. Therefore, it is necessary to develop more complex models to describe their behavior[Citation4]. Several researchers used the Power Law model to describe rheological behaviour of fruit pulps, juices, and puree.[Citation5,Citation6,Citation7] However, their experimental data are not sometimes adjusted to the Power Law model with great precision for some foods; it is because to these foods have the yield stress that is also an important characteristic.[Citation8,Citation9] For that, other authors proposed other models with three or four parameters as are the Herschel-Bulkley (which is the power law with a yield stress), the Cross, and the Carreau model. The rheological model most often used to describe the flow behavior of pseudoplastic foods is the power law with or without a yield stress
where η is the viscosity in Pa·s, τ0 is the yield stress expressed in Pa, γ is the shear rate in s−1, n is the flow behavior index, and K is the consistence index in Pa·sn. The power law model without the yield (Eq. 1) is also known as the Ostwald Waele model, while that with the yield stress (Eq. 2) is known as the Herschel-Bulkley model. Cross [Citation10] proposed the following expression:
The Carreau model is described in two forms below as
where η is the viscosity in Pa·s, η0 is the initial viscosity expressed in Pa·s, η∝ is the viscosity to infinite time expressed in Pa·s and its constant value is 0, γ is the shear rate in s−1, n is the flow behaviour index, γb and λ are time parameters in s−1 and s, respectively, and a has a constant value of 2. This article reports rheological properties of different jams at different temperatures to analyze their consistency when the temperature changes. Five rheological models were used as tools for calculation of the relationship between the shear stress and shear rate.
MATERIALS AND METHODS
Preparation of Jams
The fruits pulps were pasteurised in order to produce jams. The jams were employed in their commercial form. The pH values were adjusted to 2.5–3.2, with citric acid and controlled with a KRISSON pH meter. The ingredients of the prune jam are prune pulp, sugar, and glucose syrup, thickener (E-440, E-410), acidity corrector (E-330), antirust (E‐300), coloring (E-141), conservator (E-202). The ingredients of the peach jam are peach, sugar, and fruit pectin. The ingredients of the apricot jam are apricots, sugar, glucose-fructose syrup, acidifier (citric acid), and gelling agent (pectin). The ingredients of the strawberry jam are fructose syrup, strawberries, pectin, and citric acid. The ingredients of the raspberries jam are raspberries, glucose syrup, pectin of fruits, flour of seeds, acidifier (citric acid), and conservator. The ingredients of the fruits jam are strawberry, raspberry, currant, sugar, citric acid, and pectin. The nutritional information, composition and pH of all fruits are shown in .
Table 1 Nutritional information per 100 g of fruit, composition, and pH
Rheological Measurements
Different rheological measurements for all jams were made using a rotational concentric cylinder viscosimeter HAAKE VT550,[Citation11] equipped with SV1 rotor adequated for viscosity measurements of high viscosity liquids.[Citation12] The viscosimeter was interfaced to a computer for control and data acquisition. The sample compartment was kept at a constant temperature using a circulator water batch. This apparatus measures the shear rate and the apparent viscosity of a fluid at a certain temperature. Five measures from 20 to 40°C were carried out to study the effect of the temperature. For each test, the filled sample cup and spindle was equilibrated temperature for about 20 minutes and sheared a programmed continuous sequence in which the shear rate was increased linearly from 17.8 to 445 s−1 in the next 10 minutes. Shear stress and shear rate data were collected continuously at 30 s intervals throughout the text. In all cases, the temperature of the samples was controlled during measurement with a precision of ±0.1°C. In order to investigate the reproducibility of the results, three experimental runs were accomplished for each sample, and the resulting shear stress was average of the experimental values ±5%. Most fluid foods do not have the simple Newtonian rheological model; in other words, their viscosities are independent of shear rate or shear stress and not constant with temperature. Therefore, it is necessary to develop more complex models to describe their behavior.[Citation4] The flow curves (rheograms) were evaluated by using the Ostwald Waele, Herschel-Bulkley, Cross, Carreau models.[Citation13] For each adjusted, the determination coefficient (R2) or the chi‐square (χ2) were analyzed.
RESULTS AND DISCUSSION
Rheograms
Shear stress (τ) against shear rate (γ) was represented for one jam (see ) at different temperatures and for six jams at one temperature (see ). From the typical curves on dependence of the shear stress (τ) on shear rate (γ), it could be seen that all of these substances had a non-newtonian and pseudoplastic behaviour. A marked change in the rheological behaviour of all jams with increasing the temperature was observed (see and ). The apparent viscosity decreases with the temperature in all cases. In industrial operations a product is submitted to a range of shear rates and it is important to know how the viscosity will change with temperature at these shear rates to adequately design the equipment for these operations.
Figure 1. Shear stress vs. shear rate for fruits jam at all temperatures: 20°C (▾), 25°C (•), 30°C (♦), 35°C (▪) and 40°C (▴).
Figure 2. Shear stress vs. shear rate for fruits jams at 30°C: prune (▴), apricot (▵), strawberry (•), fruits (▪), peach (□) and raspberry (○).
Figure 3. Viscosity vs. shear rate for two jams at two temperatures: prune 25°C (○) and 35°C (•), fruits 25°C (▿) and 35°C (▾).
Figure 4. Viscosity vs. shear rate for two jams at two temperatures: apricot 25°C (▵) and 35°C (▴), peach 25°C (□) and 35°C (▪).
Rheological Parameters
Once their behavior was analyzed, five rheological models (Eqs. 1–5) were used to determine the rheological parameters. , , , , present the values obtained from Ostwald Waele, Herschel-Bulkley, Cross, and Carreau models fitting, respectively. From and , it can be observed the value of n is smaller than 1 in all cases, concluding that all jams present pseudoplastic characteristics, and the pseudoplasticity is larger for prune jam, as this parameter indicates. The experimental data are in agreement with the five models; it can be noted that the models present low χ2 values (, , , , ). Models were proposed to correlate the dependency of the rheological parameters with the temperature. All parameters change lineally with the temperature (Eq. 6). The fitting parameters are shown in , , , , .
Table 2 Rheological parameters of Ostwald Waele model
Table 3 Rheological parameters of Herschel-Bulkley model
Table 4 Rheological parameters of Cross model
Table 5 Rheological parameters of Carreau-model A model
Table 6 Rheological parameters of Carreau-Yasuda model
Table 7 Relationship between Ostwald Waele model parameters and temperature
Table 8 Relationship between Herschel-Bulkley model parameters and temperature
Table 9 Relationship between Cross model parameters and temperature
Table 10 Relationship between Carreau model A parameters and temperature
Table 11 Relationship between Carreau Yasuda model parameters and temperature
where y is the rheological parameter, a and b are fitting parameters, and x is the temperature in °C. The most important parameters are influenced by the temperature. The yield stress decreased with the temperature as expected. The consistence index tends to decrease with the temperature, which is expected since there is a general tendency for the viscosity to decrease with the temperature. The flow behavior index tends to increase with increasing the temperature, showing that the behavior of the jams tends to become closer to that of a Newtonian fluid although its value is not close to 1.
CONCLUSIONS
Results allow concluding that the studied jams have a non-newtonian pseudoplastic behavior, and the five of the most used rheological models were employed to describe their behavior under shear in rotational concentric cylinder system. The five models are capable of simulating the rheological properties of all foods with a very good fit. Moreover, the effect of temperature on the rheological parameters was analyzed.
Notes
3. Osborne, D.R.; Voogt, P. Análisis de los nutrientes de los alimentos, Ed.;. Acribia: Zaragoza;, 1986.
Related Research Data
REFERENCES
- Yeow , Y.L. , Perona , P. and Leong , Y.K. 2002 . A reliable method of extracting the rheological properties of fruit purees from loop data . Journal of Food Science , 67 : 1407 – 1411 . [CSA]
- Steffe , J.F. Rheological methods in foods process engineering , 2nd , 428 East Lansing, Mich : Freeman .
- 3. Osborne, D.R.; Voogt, P. Análisis de los nutrientes de los alimentos, Ed.;. Acribia: Zaragoza;, 1986.
- Holdsworth , S. D. 1971 . Applicability of rheological models to the interpretation of low and processing behaviour of fluid products . Journal of Texture Studies , 2 : 393 – 418 . [CSA]
- Lombraña , J. I. and Días , J.M. 1985 . Rheological and chemical changes in stored carrot juice . Canadian Institute of Food Science and Technology , 18 : 213 – 219 . [CSA]
- Rao , K. L. , Cooley , H. J. , Nogueira , J. N. and McLellan , M. R. 1986 . Rheology of apple sauce: effect of apple cultivar, firmness and processing parameters . Journal of Food Science , 51 : 176 – 179 . [CSA]
- Pelegrine , D. H. , Silva , F.C. and Gasparetto , C. A. 2002 . Rheological behaviour of pineapple and mango pulps . LWT Food Science and Technology , 35 : 645 – 648 . [CSA]
- Ditchfield , C. , Tadini , C. C. , Singh , R. and Toledo , R.T. 2004 . Rheological protperties of banana puree at high temperatures . International Journal of Food Properties , 7 ( 3 ) : 571 – 584 . [CROSSREF] [CSA]
- Abu-Jdayil , B. , Banat , F. , Jumah , R. , Al-Asheh , S. and Hammad , S. 2004 . A comparative study of rheological characteristics of tomato paste and tomato powder solutions . International Journal of Food Properties , 7 ( 3 ) : 483 – 497 . [CROSSREF] [CSA]
- Cross , M. M. 1965 . Rheology of non-newtonian fluids: a new flow equation for pseudoplastic systems . Journal of Colloid Science , 20 : 417 – 437 . [CROSSREF] [CSA]
- Álvarez , E. , Cancela , M.A. and Maceiras , R. 2004a . Comparison of rheological behaviour of sweet and salad sauces . International Journal of Food Properties , 7 ( 3 ) : 511 – 518 . [CROSSREF] [CSA]
- Álvarez , E. , Cancela , M.A. and Maceiras , R. 2005 . Rheological behaviour of powered baby foods . International Journal of Food Properties , 8 ( 1 ) : 79 – 88 . [CSA]
- Abdelrahim , K.A. , Ramaswamy , H. , Doyon , G. and Toupin , C. 1994 . Effects of concentration and temperature on carboxymethylcellulose rheology . International Journal of Food Science and Technology , 29 : 243 – 253 . [CSA]