https://doi.org/10.15255/KUI.2005.002
Published: Kem. Ind. 54 (7-8) (2005) 341–346
Paper reference number: KUI-02/2005
Paper type: Original scientific paper
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Influence of Dry Solid Fraction on the Rheological Properties of Apple Puree at Low Temperatures Before and During the Freezing

A. Pozderović, T. Moslavac and A. Pichler

Abstract

The knowledge of rheological properties is very important either for achieving specific food properties, or for designing production procedure. The consistency is one of the significant factors, which specifies the quality of foodstuffs, including those, which are kept by freezing in order to preserve their stability for a longer period. The aim of this work was to research the influence of soluble and insoluble matter before and during freezing. Investigation was carried out with apple puree of Idared sort by different insoluble dry solid fraction. The apple puree was made by mixing serum and pulp at different rates that had been previously separated in the process of centrifuge. The puree with varied soluble dry solid fraction was prepared by adding the sucrose. Measures were carried out using the rotary viscosimeter Rheotest 3 (VEB MLW, Germany) with application of concentric cylinder systems. Thermostat “Ultra-Kryostat MK70" with temperature regulation from 60 °C to –30 °C was used for cooling the sample to the freezing temperature. The rheological properties of purees were measured and shown by graphic representation showing relation between shear stress and shear rate at the temperature of 10 °C and by calculating the consistency coefficient and the flow behaviour index. Results of these measurements (Table 1) showed that all of analysed apple purees were pseudoplastic, Non-Newtonian fluids. The consistency of examined samples (given with consistency coefficient) was lower in apple purees with bigger fraction of soluble matter. By increasing fraction of insoluble matter, examined samples consistency was increased. Shear stress, apparent viscosity temperature, dependence of apple puree with the different dry solid fraction, was investigated at the constant shear rate (Fig. 1–3). The apple purees were continuously cooling from 10 °C to the complet puree freezing. It was measured the temperature at which shear stress increases and the temperature at which (Tk) falls. The lowest temperature of cooling at which the shear (Tm) is still present was also measured. During cooling of the apple puree, the shear stress and apparent viscosity is suddenly increased after initial puree freezing at temperature Tk because of ice crystals growth. The examinations showed that during continuous cooling at low temperatures, the rheological properties of apple puree as well as temperatures Tk and Tm depended on soluble and insoluble dry solid fraction and shear rate (Table 2). Temperatures Tk and Tm were reduced by an increase of insoluble matter fraction. At the same effect of insoluble matter fraction, apparent viscosity as well as times tk and tm were increased. Temperatures Tk and Tm were decreased with soluble matter fraction increase in apple puree. At the same effect of soluble matter fraction, apparent viscosity at temperature Tm was decreased and times tk and tm were increased. Decrease of temperatures Tk and Tm was greater with soluble matter fraction increasing than with an increase of insoluble matter fraction. It was also clear that the shear rate and cooling of certain puree sorts influenced the puree rheological properties during the freezing process in different ways. If the apple purees were cooled at a freezing temperature, their temperature (Tm) depended on the way of cooling, shear rate, and dry solid fraction. There were some significant changes of rheological properties of apple puree while being cooled due to ice crystals formation and growth. They were formed as solid particles and observed at temperatures ranging from –5.55 to –10.50 °C, depending on dry solid matter and shear rate.

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Keywords

apple puree, rheological properties, dry solid, shear rate, low temperature