Colloidal stability and rheological properties of sesame paste
Introduction
The particle size analysis and rheological characterization of food products are important in their formulation, processing, transportation and storage, especially for emulsions and suspensions. Sesame paste is a kind of colloidal suspension mainly composed of hydrophilic solids suspended in sesame oil (Abu-Jdayil, 2004, Altay and Ak, 2005). During the production of sesame paste, the selected sesame seeds are mechanically dehulled, roasted and ground into a paste (Kahyaoglu and Kaya, 2006). Sesame has a high nutritive value with respect to high amount of proteins, lipids and important minerals and vitamins (Abu-Jdayil et al., 2002). Antioxidant activity of sesamol and sesaminol in sesame also makes sesame paste shelf stable (Nagata et al., 1987).
Although sesame paste is shelf stable with respect to chemical deteriorative reactions, colloidal instability is the main problem during the storage of sesame paste (Isa, 2001). Sedimentation of solid particles causes negative effect on consumer acceptability on the market and also it is very difficult to redisperse the sediment cake for further processing. Due to the good health beneficial effects of sesame paste, there has been great interest for using it as an ingredient for creamy desserts and sauces. Thus, in order to use the sesame paste properly as an ingredient in foods or develop new creamy products, it is important to improve its colloidal stability and to understand rheological properties and their correlations with particle size.
Recently, much study has been reported about the rheological properties of sesame paste, and steady shear behavior of sesame paste was reported as pseudoplastic and thixotropic (Alpaslan and Hayta, 2002, Abu-Jdayil et al., 2002, Altay and Ak, 2005). None of these reports took into account the instrumentally measured particle size distribution that may play an important role in rheological properties and colloidal stability of sesame paste. Since the instrument/process based on the laser diffraction principle has become a standard tool for particle size analysis in food and allied industries (Singh et al., 2006), the exact data are required for particle size distribution of sesame paste. Dynamic shear rheological tests have been used to characterize the viscoelastic properties of macromolecular dispersions (Rao, 1999). Dynamic rheological properties of sesame paste do not appear to have been studied.
The objectives of our study, therefore, were to perform particle size analysis with a laser-scattering instrument and to determine the effect of particle size distribution on the colloidal stability, on the steady and the viscoelastic behavior, and on the color of sesame paste.
Section snippets
Sample preparation
The mechanically dehulled sesame seeds were roasted (150 °C for 100 min) and then milled using a commercial miller (Gürmaksan Co., Turkey) to produce sesame paste in a laboratory. In this device, the sesame paste passes into a narrow gap between a high speed rotating grooved disc and the stationary casing of the mill. Intense shearing action results in breakage of the feed. Seven sesame paste samples having different particle sizes were produced by manually controlling the gap between the discs.
Particle size distribution measurement
Composition
Chemical composition of sesame paste was 52.2% oil, 27.2% protein, 0.3% moisture, and 1.0% ash. The oil and protein contents of sesame paste used were found within the range (50.0–65.0% and 17.0–27.0%, respectively) given in the literature (Sawaya et al., 1985, Damir, 1994, Özcan and Akgül, 1994). The density values of sesame paste and oil phase separated measured at room temperature were 1094 and 944 kg m−3, respectively. The density of solid particles in sesame paste calculated using Eq. (1)
Conclusion
Sesame paste was a colloidal suspension and particle size distribution plays an important role in its stability. Particle sizes smaller than 5 μm increased the colloidal stability. The result of colloidal stability was in a good agreement with rheological data. The pseudoplastic and thixotropic characteristics were observed for all sesame paste samples. The magnitudes of thixotropic area decreased exponentially with the decreasing particle size of sesame paste samples. Decreasing particle size
Acknowledgements
The support of this work by the Turkish Scientific and Technical Research Council (TUBITAK) with the Project TOVAG 105 O 374 is gratefully acknowledged. The authors also thank the Scientific Research Projects Executive Council of University of Gaziantep for financial supports.
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