Dynamic viscoelastic properties of sweet potato studied by dynamic mechanical analyzer

Abstract

The relaxation, creep, temperature-dependence and frequency-dependence characteristics of sweet potato roots were evaluated using a dynamic mechanical analyzer (DMA). The sweet potato was cut into rectangular to meet the testing requirements and wrapped with sealing film or aluminum foil to prevent water loss. The temperature scanning tests were carried out at 2 °C/min and 10 °C/min in the temperature range of 30–100 °C, and the frequency sweep tests were conducted in a range of 50–0.1 Hz. The regression results suggested that 5-element Maxwell model described relaxation behavior better for consisting of two relaxation times; the creep behavior matched the Burgers model well, and changes in creep parameters were observed after each creep cycle. The temperature scanning tests revealed that starch gelatinization was only obtained when the temperature increased at 2 °C/min. A resonance frequency was detected both in 3-point bending and compression deforming clamps.

Introduction

Sweet potato (Ipomoea batatas L. (Lam.)) is widely grown in many countries, especially in China and in Southeast Asian countries (An, Frankow-Lindberg, & Lindberg, 2003). The sweet potato is comprised of 60–80% of water and 10–30% of starch (Zhang, Christopher, & Harold, 2002), while it also contains a variety of nutrients, such as dietary fiber, carotenoid, vitamins, lysine and mineral elements etc., thus sweet potato is extensively used in food industry, light chemical industry, and feed industry. Recent studies show that sweet potato has brilliant anti-cancer and antioxidant effect as well (Teow et al., 2007, Tian and Wang, 2008).

Food can be regarded as some kind of complex polymer. Different food matrix shows different mechanical properties in different viscoelastic regions: the glass-like region, where the material shows a rigid and brittle character and the modulus is relatively high; the glass-transition region, where the storage modulus of the material decreases remarkably; the rubber-like region, where the material shows a high-elastic property; and the terminal region, where the material flows like liquid (Le Meste, Champion, Roudaut, Blond, & Simatos, 2002). The dynamic mechanical analyzer, DMA, has been widely used to determine the structural and mechanical changes in different kinds of matrix, such as starch film (Zhou et al., 2009), dental composite resins (Mesquita, Axmann, & Geis-Gerstorfer, 2006), cheese (Del Nobile et al., 2007a), chocolate (Tremeac, Hayert, & Le-Bail, 2008), frozen dough (Laaksonen & Roost, 2000), meat (Del Nobile, Chillo, Mentana, & Baiano, 2007b), rice kernels (Chen et al., 2007).

In order to determine the viscoelastic behavior of a food matrix, the transient and dynamic tests can be performed by DMA. The most typical transient tests are represented by creep-recovery and stress relaxation experiments (Del Nobile et al., 2007b). The relaxation and creep-recovery tests, which are quite common in the life cycle of materials, are both especially useful for studying materials under very low shear rates or frequencies, under long test times, or under real use conditions (Menard, 1999a).

The Kelvin model, Maxwell model and Burgers model are the most commonly used models to describe viscoelastic behaviors of the matrix (Del Nobile et al., 2007b, Mohsenin and Mittal, 1977) under static load. The Kelvin model, consisting a spring and a dashpot in parallel, represents the start point for the development of mechanical analogs describing the creep behavior (Del Nobile et al., 2007b). The Maxwell model, consisting of a Hookean spring and a Newtonian dashpot in series, is suitable for understanding stress relaxation data, but not able to express the equilibrium of stress; while the generalized Maxwell model, consisting of several Maxwell elements in parallel with a spring, can describe stress-equilibrium behavior better (Steffe, 1992). The Burgers model, consisting of a spring, a dashpot and a Kelvin component in series, can show the instantaneous elastic deformation, delayed elastic deformation and the character of viscous flow at the same time when the material is under the external force. Burgers model is one of the most used rheological models to describe the creep behavior.

DMA also gives the information of various transitions in a polymer as temperature changing. Dynamic mechanical properties of polymers can represent their molecular motion, which has a close relationship with the condensed, chain structures of polymers (Zhou et al., 2009). The thermal transitions in polymers can be explained by theory of free volume (Champion et al., 2000, Chen et al., 2007, Menard, 1999b), according to which, there is free volume (defined as the space a molecule has for internal movements) existing between molecules. When temperature rises, the free volume of the chain segment of a polymer increases, and its ability to move in various directions also increases. This increased mobility makes the food matrix softened and thus results in a greater compliance (or lower modulus).

Apart from the free volume for molecular movements, the water molecules in the food matrix also have a close relation to the viscoelastic properties of the materials (Chen et al., 2007, Lievonen and Roos, 2002). Water acts as one of the most effective plasticizers for biopolymers. Furthermore, the viscoelastic properties of a food matrix vary with the different amounts of water content in it. Thus, it is of great necessity to keep the water content constant by means of wrapping or coating to materials (Zhou et al., 2009).

The frequency scanning test by DMA gives information of materials’ melting property and this test was usually done to the liquid samples. Many thermal analysts and material experts use DMA to study the temperature, time or strain rate (Mulliken & Boyce, 2006; Yi, Boyce, Lee, & Balizer, 2006) dependence of the matrix’s mechanical property and neglect the frequency dependence of it. For a dynamic test conducted by DMA, the loading frequency played a vital role to the materials’ mechanical response. The low frequency range is where viscous or liquid behavior predominates; when the frequency is relatively high, the material will act in an elastic way and behave stiff. The change caused by increasing in frequency is similar to that by decreasing in temperature (Menard, 1999c). Thus frequency dependence of the materials’ viscoelastic property may also be explained by the molecular or chain movements in the free volume theory.

In this study, the relaxation, creep-recovery, temperature and frequency scanning behaviors of sweet potato were tested by DMA. The aim of this work was to model the relaxation and creep-recovery behaviors of the sweet potato sample, and characterize the temperature and frequency dependences of its viscoelastic properties.