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RHEOLOGICAL PROPERTIES OF FROZEN CONCENTRATED ORANGE JUICE (FCOJ) AT SUBZERO TEMPERATURES Tavares, D. T.1, Alcantara, M. R. 2, Tadini, C. C. 1*,Telis-Romero, J.3 1

Escola Politécnica – São Paulo University, Brazil Instituto de Química – São Paulo University, Brazil 3 Food Eng. and Tech. Dept. – Universidade Estadual Paulista, Brazil 2

Abstract. This study was aimed to obtain the data on flow properties of Frozen Concentrated Orange Juice (FCOJ) produced from oranges cv. Pera-Rio (65.04 ºBrix, 8.8 % w/w pulp content, 0.133 % w/w pectin, 3.84% citric acid, 1.2934 g/cm3) from – 18ºC to 0ºC and fit appropriate predictive models. This temperature interval was chosen because of the flow transport of FCOJ is done at temperatures within this range. Rheological measurements were carried out of using a rheometer (model MCR 300, Physica) with plate/plate geometry (49.96 mm) and a Peltier temperature control from – 40ºC to + 150ºC. A steady shear rate sweep was performed to determine the dependence of apparent viscosity and shear stress with rate. First the sweep was performed by increasing shear rate from 0 to 100 s-1 and then the shear rate was decreased back to initial value using the same sample at temperature constant. The experiments were repeated twice, at least, always using new FCOJ sample. The rheograms were obtained for FCOJ between 46.56 ºBrix to 65.04 ºBrix at temperature range from – 18ºC to 0 ºC. In order to obtain with different concentrations, the FCOJ was diluted with distilled water. In general, the power law fitted well the experimental data. In higher concentrations the tixotropy was observed. Arrhenius model was applied to describe the temperature effect, for each concentration. Among the samples, higher concentrations showed more dependency temperature.

Keywords: Frozen concentrate orange juice, Rheological properties and Tixotropy.

1. Introduction Concentrated orange juice is one of the most important commodities over the world and Brazil is the major producer. Rheological behavior knowledge of frozen concentrated orange juice (FCOJ) is quite important for flow transport. The temperature and soluble solids content influence the rheological behavior of FCOJ and are responsible for non-Newtonian flow behavior. Some authors reported that fruit concentrates and purees revealed thixotropic behavior, related to pectin content, fibers and also pulp content. Ibarz and Ramos (1998) reported that orange juice with pulp and pectin presented thixotropic behavior at soluble solids concentrations of 55 and 60 ºBrix at temperature range from 0 to 20 ºC and shear rate interval from 7.2 to 57.6 s-1. The thixotropic behavior was described by a kinetic model. This study was aimed to obtain the data on flow properties of Frozen Concentrated Orange Juice (FCOJ) produced from oranges cv. Pera-Rio (65.04 ºBrix, 8.8 % w/w pulp content, 0.133 % w/w pectin, 3.84% citric

*

To whom all correspondence should be addressed. Address: Food Eng. Lab. Chemical Eng. Dept., Escola Politécnica, São Paulo University, P. O. Box: 61548, 05424970 – Brazil E-mail: [email protected]

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acid, 1.2934 g/cm3) from – 18ºC to 0ºC and fit appropriate predictive models. This temperature interval was chosen because of the flow transport of FCOJ is done at temperatures within this range.

2. Material and Methods

2.1. Sample preparation All the experimental measurements were conducted with samples from the same batch of frozen concentrated orange juice (FCOJ) produced from oranges cv. Pera-Rio (65.04 ºBrix, 8.8 % w/w pulp content, 0.133 % w/w pectin, 3.84% citric acid, 1.2934 g/cm3) in a 7 – stage TASTE® evaporator and stored at – 30ºC. In order to obtain different concentrations, the FCOJ was diluted with distilled water. 2.2. Analytical measurements Citric acid was determined, in triplicate, according to the Kimball (1991) conducted in the pH-STAT (RADIOMETER, mod. PHM290, France). Soluble solids were determined, in triplicate, by a refractometer (CARLZEISS JENA, mod. 711849, Germany) and corrected for acidity and temperature values according to Kimball (1991). 2.3. Rheological measurements Rheological measurements were carried out of using a rheometer (model MCR 300, Physica) with plate/plate geometry (49.96 mm) and a Peltier temperature control from – 40ºC to + 150ºC. A steady shear rate sweep was performed to determine the dependence of apparent viscosity and shear stress with rate. First the sweep was performed by increasing shear rate from 0 to 100 s-1 (2 s-1 step) and then the shear rate was decreased back to initial value using the same sample at temperature constant. The experiments were repeated twice, at least, always using new FCOJ sample. Shear stress – shear rate data were gathered as rheograms. The rheograms were obtained for FCOJ between 46.56 ºBrix to 65.04 ºBrix at temperature range from – 18ºC to 0 ºC. For thixotropic characterization, working temperatures of –18 ºC, - 10ºC and 0 ºC and shear rates of 1

10 s-

, 50 s-1 and 70 s-1 were chosen. Curves were obtained using the same rheometer and same Peltier temperature

control. The variation of shear stress with time at each constant shear rate was acquired automatically, during 10 min.

2.4. Data analysis Fitted models were obtained by using nonlinear estimation procedure from the statistical program Statgraphics® v. 4.0 (Manugistics). The suitability of the fitted models was evaluated by the determination coefficient (r2), the significance level (p