Rheological properties of Lepidium sativum seed extract as a function of concentration, temperature and time Hojjat Karazhiyan a , Seyed M.A. Razavi a , Glyn O. Phillips b , Yapeng Fang b, * , Saphwan Al-Assaf b , Katsuyoshi Nishinari b , Reza Farhoosh a a Department of Food Science and Technology, Ferdowsi University of Mashhad, POBox: 91775-1163 Mashhad, Iran b Glyn O. Phillips Hydrocolloid Research Center, Glyndwr University, Plas Coch, Mold Road, Wrexham LL112AW, United Kingdom article info Article history: Received 26 March 2009 Accepted 27 March 2009 Keywords: Lepidium sativum Semi-rigid chain Rheology Shear thinning Thixotropy abstract The seeds of Lepidium sativum (Garden Cress) were selected as a new source of hydrocolloid and its chemical composition and molecular parameters were determined. The macromolecular component of the extract had a molecular weight of 540 kDa, and was nearly as rigid as xanthan with regard to chain conformation. The main rheological features were investigated as a function of shear rate, concentration and temperature. The extract exhibited strong shear-thinning behaviour, which was even more pronounced than for xanthan. An increase in concentration or temperature led to an increase in pseu- doplasticity. The Arrhenius model was applied to the temperature dependence of viscosity, and the activation energy (E a ) was found to decrease with increasing concentration. The extract solutions showed thixotropic behaviour at all the concentrations and temperatures studied, and the first-order stress decay model with a non-zero equilibrium stress fairly described the time-dependent behaviour. The rheological characteristics found indicated a potential application of the extract as a novel thickener. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Hydrocolloids are broadly used in food systems for various purposes, for example as thickeners, gelling agents, texture modi- fiers, and stabilizers (Williams & Phillips, 2000). There has been an increase in the demand for hydrocolloids in the last decade. The volume share of hydrocolloids depends on the security of their supply, quality and price. Hydrocolloids from plants have the advantage over those from animals because of their friendly image to consumers. Starch and derivatives, pectin, galactomannans, carrageenans, alginates, agars, gum arabic and cellulose and its derivatives are the main plant hydrocolloids used in food systems. Studies on these hydrocolloids are currently concentrated on the structure–functionality relationships in order to maximize effec- tiveness and produce novel textures. Consequentially there are opportunities in the hydrocolloid market for new sources of plant hydrocolloids to meet the demand for ingredients with more specific functionalities. A number of studies have been carried out to quantify the rheological characteristics of food hydrocolloids individually or in food formulations (Abdelrahim, Ramaswamy, & van de Voort, 1995; Clasen & Kulicke, 2001; Da Silva, Pedro, Oliveira, & Rao, 1997; Da Silva & Rao, 1995; Dickie & Kokini, 1983; Krumel & Sarkar, 1975; Lapasin & Pricl, 1999; Ma & Barbosa-Canovas, 1996; Sanderson, 1981; Stanley, 1990; White, Davidson, & Otten, 1993; Williams & Phillips, 2000). The viscosity of solutions of hydrocolloids can be significantly affected by variables such as shear rate, concentration, temperature, ionic strength, pH etc. Several models have been used to characterize the shear rate dependence of flow behaviour of gum solutions and among these the power-law model is the most used (Barnes, Hutton, & Walters, 1989). The effect of concentration on the apparent viscosity of hydrocolloids is generally described by either an exponential or a power relationship (Rao & Kenny, 1975; Speers & Tung, 1986). Temperature also has an important influence on the flow behaviour of hydrocolloid solutions. The effect of temperature on viscosity at a specified shear rate is generally expressed by an Arrhenius-type model (Rao & Anantheswaran, 1982; Rao & Kenny,1975; Speers & Tung, 1986). Hydrocolloid solutions may also exhibit time-dependent prop- erties, mainly thixotropy. When a material is sheared at a constant shear rate, the viscosity of a thixotropic material will decrease over a period of time, implying a progressive breakdown of the structure (Abu-Jdayil & Mohameed, 2004). Modeling of the thixotropic behaviour of food products has been based on equations, such as the Weltmann model (Weltmann, 1943), first-order stress decay * Corresponding author. E-mail address: y.fang@glyndwr.ac.uk (Y. Fang). Contents lists available at ScienceDirect Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd 0268-005X/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2009.03.019 Food Hydrocolloids 23 (2009) 2062–2068