Rheological behavior of cupuaçu and taperebá juice with added inulin

Pedro Henrique Campelo pedrocampelo@ufam.edu.br Faculdade de Ciências Agrárias, Universidade Federal do Amazonas, Manaus, Brasil. Eyde Cristianne Saraiva Bonatto bonatto@gmail.com Faculdade de Ciências Agrárias, Universidade Federal do Amazonas, Manaus, Brasil. Samara Claudia Picanço Batista samarabatista@gmail.com Departamento de Engenharia Agrícola e Solos, Universidade Federal do Amazonas, Manaus, Amazonas. Romuald Euloge Yomkil romualdyk@hotmail.fr Faculdade de Ciências Agrárias, Universidade Federal do Amazonas, Manaus, Brasil. Rheological behavior of liquid foods is an important physicochemical characteristic for processing in the food industry. This work has the objective to evaluate the rheological behavior of cupuaçu and taperebá juice with added inulin. Analyzes of centesimal composition, soluble solids, pH and titratable acidity of pulp and juice were performed plus rheology of inulin-added juices at temperatures of 5, 25 and 45 °C. The rheological data were adjusted for the power law model. The centesimal composition confirmed the physicochemical characteristics of fruit pulps, with acceptable quality parameters and recommended by current legislation. It was observed that temperature and solids addition (inulin) influenced the rheological behavior of cupuaçu and taperebá juice, with viscosity reduction with increasing temperature; and increase in viscosity with the addition of inulin. The power law adjusted the flow curve data, evidencing the pseudo-plastic behavior of fruit juices. The rheological behavior of juices is an important tool in the food industry as it helps in the dimensioning of transmission lines and unit operations equipment, such as pasteurizers.


INTRODUCTION
Among the native tropical fruits of the Amazon, cupuaçu (Theobroma grandiflorum) is one that meets the best conditions for industrial use. It is a fruit originating in the South and Southeast of the Amazon and is appreciated for its acidic and intensely aromatic pulp (BUENO et al., 2002). The taperebá (Spondias mombin L.) is a fruitful species originating in tropical America, common in the Amazon region, where it occurs in the wild state. Fruit pulp presents high economic potential and growing demand in the North and Northeast regions of Brazil (SOUZA, 2000).
Inulin is a prebiotic formed from fructose units with a degree of polymerization (DP) ranging from 10 to 60 extracted from chicory roots. Inulin is widely used in functional foods around the world because of its properties related to improved health and technological possibilities. Prebiotics, such as inulin, are at the forefront of the emerging trend toward functional foods (CAMPELO-FELIX et al., 2017;SOUZA;CUNHA, 2009). Prebiotics are nondigestible nutritional ingredients that beneficially affect the host by selectively stimulating the growth and activity of one or more species of beneficial intestinal bacteria, improving the health of its host (SOUZA; MENEZES; CUNHA, 2009).
The consumption habits of industrialized fruit juices have been increasing all over the world, due to several aspects such as the search for health, concern with the consumption of healthy foods, practicality offered by the products, substitution of carbonated beverages for the consumption and nutritive value (LIMA et al., 2008). The behavior of industrial materials became of interest in physics, mechanics, mathematics and even colloid chemistry. Rheology aims to study the deformation and flow of materials. It is the study of the way in which materials respond to tension or deformation. During processing, the fruit juices show variations in their concentrations and temperatures. These are subjected to unitary operations such as pumping, evaporation, drying and spraying. For a correct dimensioning, operation and control of the process, the knowledge of the rheological parameters is of fundamental importance (CABRAL; QUEIROZ; FIGUEIREDO, 2002).
From the above, the objective is to evaluate the rheological behavior of cupuaçu and tapereba juice with added inulin. Centesimal composition, soluble solids, pH and titratable acidity of pulps and juices were performed; rheology of added inulin juices at temperatures of 5, 25 and 45°C.

PHYSICAL-CHEMICAL CHARACTERIZATION OF THE RAW MATERIAL: CENTESIMAL COMPOSITION
Cupuaçu and taperebá pulps were characterized in terms of moisture, ash, protein, lipids, carbohydrate and fiber (AOAC, 2016).

SOLUBLE SOLIDS
The fruit pulps were homogenized and 1 to 2 drops transferred to the refractometer prism, neglecting any large particles. When the temperature of the sample and the temperature of the apparatus were equal and the reading was performed on the °Brix scale.
pH Fruit pulp pH was measured using a peagameter. The equipment was calibrated with buffer solutions, and pH values were measured by inserting the electrode directly into the sample at 25°C.

TITRATABLE ACIDITY
To determine the titratable acidity, 20 mL of the homogenized sample was pipetted into an Erlenmeyer flask, diluted with approximately 100 mL of water, and received 0.3 mL of phenolphthalein solution. Flask contents were titrated with 0.1 M sodium hydroxide solution under constant stirring until pink staining persisted for 30 seconds (ADOLFO LUTZ, 2008).

PRODUCTION OF JUICES
The juices were elaborated using the following ingredients: citric acid, commercial sucrose, water and inulin. Four formulations were developed: two standards (without the addition of inulin), and two with prebiotic concentrations of 1.5%, which are presented in Table 1. The nectars were mixed in an Ultraturrax homogenizer (T25, IKA, USA) for 5 min, speed 7000 rpm.

RHEOLOGICAL MEASUREMENTS
The rheological measurements were performed on a rotational viscometer (LV model, Brookfield, USA) at temperatures of 4°C, 25°C and 45°C, connected to a computerized Rheocalc data acquisition system to record the apparent viscosity (ηap), shear stress (σ) and shear rate (γ) were obtained using WinGatherо software. We used: a SC4-18 Spindle, which allows deformation rate reading (γ) in the range of 0 to 10 s (-1), and the accessory for small sample quantity (20 ml), DIN adapter including temperature and approximate refrigeration, storage and pasteurization respectively. With the experimental data of shear stress, viscosity and deformation rate obtained in the viscometer, adjustments were made with the Ostwald-De-Waele rheological models (Power Law). The analysis of the model adjustments to the experimental results was done through the statistical parameters coefficient of determination (R²) and sum of squares of residues (SSR). Therefore, the model that best fits is that with high R² values and low SSR values.
PHYSICOCHEMICAL PROPERTIES OF JUICE: SOLUBLE SOLIDS, pH AND TITRATABLE ACIDITY Soluble solids, pH and titratable acidify analyzes were performed according to the methodologies previously described.

VITAMIN C
Approximately 10 mL of each juice sample were transferred to a 300 mL Erlenmeyer flask. 1 mL of the potassium iodide solution (10% w / w) and 1 mL of the starch solution (1% w / w) were added. The solutions were titrated with potassium iodide solution (0.02M) to blue coloration (ADOLFO LUTZ, 2008).

CENTESIMAL COMPOSITION OF FRUITS PULP
Climatic factors, soil type, genetic variability and season of the year can interfere the centesimal composition of fruits (MATOS, 2007). The moisture content corroborated with other studies (CANUTO et al., 2010), with values of 89.2% (cupuaçu) and 93.8% (taperebá). In studies with taperebá values of 0.62, 1.06 and 13.9% (TIBURSKI et al., 2011) for lipids, proteins and total carbohydrates, respectively, were obtained corroborating those of this present work. Canuto et al., (2010) observed lipid values of 0.2 and 0.3% for taperebá and cupuaçu, respectively. The content of this compound can vary between different regions of the country, depending on factors such as temperature, light intensity and moisture content (CANUTO et al., 2010). pH and total titratable acidity (TTA) are two methods used to measure fruit acidity, which reveal, respectively, the percentage of organic acid and the concentration of hydrogen ion. Generally, to indicate the acid taste parameter, total titratable acidity is the most viable method, measured in predominant organic acids (CHITARRA; CHITARRA, 1990). The pH values (Table 2) found are in agreement with other studies (MORZELLE et al., 2009;CANUTO et al., 2010;CARVALHO et al., 2011) which reported a pH less than 4.0 for industrialized fruit nectar. pH values can directly interfere with the color of the product and consequently its acceptance by the consumer. In addition, pH regulates microbial growth, which may be related to the useful life of the juice.
Other studies obtained TTA values for cupuaçu of 1.0% (BUENO et al., 2002) and 2.27% (COSTA et al., 2003); and for taperebá of 1.86% (MATTIETTO; LOPES; MENEZES, 2010) and 1.20 -1.93% (CARVALHO et al., 2011). Fruits with high acidity generally have low sugar content. High levels of acidity may present low acceptance for in natura consumption but it is a good characteristic for industry Página | 38 because it avoids the use of organic acids to produce by-products such as jellies and sweets (MATOS, 2007). Total soluble solids content correlates with sugars and organic acids, a characteristic of interest for products sold in natura, since the consumer market prefers sweet fruits (CANUTO et al., 2010). The total soluble solids content of 10º Brix for both fruits was presented for other studies with cupuaçu (FERREIRA; GUIMARÃES; MAIA, 2008) and taperebá (MATTIETTO; LOPES; MENEZES, 2010). Others authors (CANUTO et al., 2010) observed lower values than those of the present study. The content of total soluble solids tends to increase with the degree of maturation of the fruit, because with maturation the starch is hydrolyzed and the complex sugars are transformed into simple sugars and as a consequence acidity reduces. In fruit production, climatic conditions, mainly precipitation and high temperatures, are factors that influence the formation of soluble solids (MATOS, 2007;FERREIRA;MAIA, 2008). Figure 1 shows the rheological behavior from the flow curves of cupuaçu and tapereba juice with added inulin. A gradual increase is observed in the shear stress in line with the increase of the deformation rate, an effect that towards stability under high deformation rates. This non-linear flow curve behavior is typical of pseudoplastic fluids (PELEGRINE; VIDAL; GASPARETTO, 2000). Fruit juices tend to present non-linear flow curve behavior due to the presence of pectin, facilitating the formation of gels and increasing the viscosity of the medium (TIZIANI; VODOVOTZ, 2005).

RHEOLOGICAL BEHAVIOR
When the influence of temperature is analyzed, the results show that the increase in temperature causes a reduction of the viscosity value in all treatments.  When the addition of solids was compared, it was seen that the juices with inulin presented higher viscosity in relation to the juice without prebiotics. This behavior was noted by other authors who studied Cherry (JUSZCZAK; FORTUNA, 2004) and Nori (KUMORO; RETNOWATI; BUDIYATI, 2011) juices with different concentrations of solids. The increase in viscosity with increasing solids is related to the increase of hydrogen bonds and the distortion of the liquid velocity pattern due to the increase of molecules in the solute (KUMORO; RETNOWATI; BUDIYATI, 2011).
Corroborating the flow curves, cupuaçu juices had lower viscosity values ( Figure 2) when compared to tapereba juices. This behavior is similar to non-Newtonian fluids with high viscosity values at low shear rates, with decrease and stabilization at high rates. The increase in temperature reduced the apparent viscosity values of the juices. This behavior was similar to that studied for other studies with cupuaçu juice (CABRAL; QUEIROZ; FIGUEIREDO, 2002).  Table 3 shows the values of the power law model parameters for fruit juices. All treatments presented a good fit to the Power Law model, with R² values close to 1. All treatments presented values of n <1, evidencing the non-Newtonian behavior (SILVA; GUIMARÃES; GASPARETTO, 2005). In all treatments, the consistency index (k) reduced with increasing temperature, corroborating with studies on rheology of mango, acerola (SILVA; GUIMARÃES; GASPARETTO, 2005) and cherry (VANDRESEN et al., 2009). In this case the increase of the temperature promotes the decrease of the viscosity in the liquid phase due to the increase of the mobility of the particles in suspension (VIDAL et al., 2006).
The increase in solids concentration increased K values. The concentration of soluble solids and insoluble solids has a strong non-linear effect on the viscosity of fluid foods (JUSZCZAK; FORTUNA, 2004). The flow index (n) increased with the addition of inulin and may be related to the increase of the solids content in the nectars, increasing the pseudoplastic character (OLIVEIRA; ROSSI; BARROS, 2011). Similar behavior was observed in rheology studies of pear juice (AUGUSTO et al., 2011) with different solids content.
According to colloidal molecular theory, the non-Newtonian behavior is due to the friction between the dispersed particles and the dispersant liquid molecules themselves, and also due to the interparticle electrostatic forces (OLIVEIRA et al., 2009). These factors are responsible for the increase in flow resistance. The pseudoplastic systems, despite the homogeneous appearance, actually have dispersed particles with irregular shapes, such as filaments and planar plates (OLIVEIRA; ROSSI; BARROS, 2011).

PHYSICOCHEMICAL PROPERTIES OF CUPUAÇU AND TAPEREBÁ NECTARS
The pH values (Table 4) found are in accordance with the studies of Morzelle et al. (2009) andCarvalho et al. (2011), which reported pH lower than 4.0 for industrialized fruit nectar. For the industrial processing of nectar the high titratable acidity content is important, since it reduces the need for addition of acidifiers and provides food security, hindering the development of yeasts (LIMA et al., 2002). The parameters titratable acidity and total soluble solids in all studied nectars remained in accordance with the identity and quality standards for tropical juices (BRASIL, 2003), which establish minimum values of 0.20% titratable acidity in citric acid and 11° Brix for soluble solids. Other authors (SANTOS et al., 2010) observed pH values between 3.40 -3.77 and titratable acidity 0.37 -1.37% for cupuaçu juices. The results of ascorbic acid in the nectars analyzed were very expressive for taperebá and cupuaçu without inulin, since the increase of solids content may interfere with the vitamin C content reduction.

CONCLUSIONS
The centesimal composition confirmed the physicochemical characteristics of fruit pulps, with acceptable quality parameters and recommended by current legislation. Temperature and inulin influenced the rheological behavior of cupuaçu and taperebá juice, with a viscosity reduction according to increasing temperature; and an increase in viscosity with the addition of inulin. The power law adjusted the flow curve data, evidencing the pseudo-plastic behavior of fruit juices. The rheological behavior of juices is an important tool in the food industry as it helps in the dimensioning of transmission lines and unit operations equipment, such as pasteurizers. It is suggested that the rheological behavior of different concentrations of inulin be studied, evaluating the influence on the flow characteristics of Amazonian juices.