| DOI: 10.3933/APPLRHEOL-24-24766 | WWW.APPLIEDRHEOLOGY.ORG Comparison of Viscoelastic Properties of Chestnut and Acorn Starch by Means of Mechanical Models with an In-built Springpot Magdalena Orczykowska, Marek Dziubi ski* ń 1Faculty of Process and Environmental Engineering, ódz University of Technology, ul. Wólczanska 213, 90-924 ódz,Ł Poland Ł *Corresponding author: [email protected] Fax: x48.42.6365663 Received: 25.9.2013, Final version: 6.12.2013 Abstract: The effect of concentration on viscoelastic properties of chestnut and acorn starch is discussed in the paper. The starch struc- ture was assessed using a rheological fractional standard linear solid model FSLSM in contrary to very simple power-law mod- el usually used in many published papers concerning determination of rheological properties of starch. Rheological parame- ters of this model were determined and their changes for different concentrations of the two tested types of starch were discussed. The values of the rheological parameter of FSLSM model give a useful of information concerning the elastic prop- erties of materials such as total elasticity of networks, network oscillations, gel stiffness, structure of cross-linking and relax- ation time of the materials. The proposed method for the interpretation of rheological measurements of the two types of starch allows for a comprehensive estimation of the analyzed biomaterial structure. The fractional rheological models can be very useful to control the biomaterial structure the needs of the final to meet envisaged product which is particularly signif- icant from the point of view of materials engineering. Key words: fractional standard linear solid model, chestnut and acorn starch, viscoelastic behavior 1 INTRODUCTION pea (Pisum sativum) [4, 5], but also acorn (Quercus ilex L.) [6–11] and sweet chestnut (Castanea sativa Mill.), Polysaccharides, including starch, are the biopolymers etc. can be found in the literature [12–20]. which due to their physicochemical properties are com- An interesting source of starch is sweet chestnut monly used in many industrial applications. However, (Castanea sativa) which should not be confused with primarily, they find the widest applications in the food the common horse chestnut (Aesculus hippocastanus). processing industry. It is estimated that annually in the Sweet chestnuts contain up to 70% starch, 17% sugar world about 60 million tons of starch is extracted from (saccharose), 6% protein and 2% fat. A high content of various types of cereals, tubers and root crops, at which starch in the chestnut makes it attractive for use in food. about 60% is used in food related applications (e.g. Similarly, another interesting source of starch are bread, sauces, soups, syrups, ice cream, snacks, meat acorns, i.e. apparent fruits of oak trees (Quercus) com- products, baby food, drinks, and fat substitutes), while posed of a nut and a cupule. Oak seeds, i.e. acorns con- the remaining 40% is applied in the production of med- tain starch and other carbohydrates amounting to 37% icines and in the manufacturing of packaging materi- and 7%, respectively. They also contain 8.1% protein als [1]. In food industry starch of various botanical ori- and 31.4% fat as well as about 7% tannins. gins has been used for years, e.g. starch obtained from Due to the growing interest in both chestnut and potatoes, corn, wheat, oat, rye, rice, or tapioca, which acorns starch, the authors of the present paper have differs in the shape and size of grains or the content of undertaken to analyze the mechanical state of structure amylose and amylopectin. The extremely wide applic- of pure paste obtained from chestnut and acorn starch, ability is a reason for searching for the new sources of using the results obtained by Kim and Yoo and Moreira starch, hence studies on starch obtained from kiwifruit [9, 18] and to explain the effect of the concentration of (Actinidia deliciosa) [2], banana (Musa paradisiaca) [3], these two types of starch on the rheological properties © Appl. Rheol. 24 (2014) 24766 | DOI: 10.3933/ApplRheol-24-24766 | 1 | measurements [24]. Various rheological models are used to describe the rheological behavior of pastes. One of them is the standard linear solid model (SLSM). The standard linear solid model consists of a spring and a Figure 1: Fractional standard linear solid model – FSLSM. system of elements of the Maxwell model, i.e. the spring connected in series with a dashpot. Such a of biomaterials produced from them by applying a rhe- mechanical model describing the behavior of media ological fractional standard linear solid model FSLSM. representing properties of solid bodies, in which elas- The proposed method of discussion of the results of rhe- tic properties dominate over viscous ones (G’ > G”). In ological measurements makes it possible to present a this model, Newton’s viscous element, i.e., the dashpot, comprehensive evaluation of the analyzed medium can be replaced by the so called viscoelastic element structure. In the original works of Kim and Yoo [9] and that combines both elastic properties of Hook’s ele- Moreira [18] experimental data only show gel-like ment, i.e. a spring, and viscous properties of Newton’s behaviour of chestnut and acorn starch. Their major element. This element is called Scott-Blair’s element or focus was to show that very interesting additional infor- a springpot. If the dashpot is replaced by a springpot, mation could be extracted from the same experimental the standard linear solid model becomes a fractional data if by employing the FSLSM model. The values of standard linear solid model (FSLSM). The model con- FSLSM model parameters presented a wide spectrum of taining Scott-Blair’s viscoelastic element is therefore a information concerning the structure of starch pastes, fractional mechanical model describing viscoelastic viscoelastic properties of material, total elasticity of net- behavior of solids. A fractional standard linear solid works, network oscillations, gel stiffness, structure model is shown in Figure 1. cross-linking and relaxation time of the material. A sim- An advantage of the fractional rheological models ilar approach for describing the rheological data by is that they can describe dynamic behavior by means of Bahlouli and Melito [21, 22]. a single equation which contains a number of constant parameters determining the viscoelastic properties of a material being tested. In the case of application of the 2 MATERIALS AND METHODS fractional rheological model, it is very important to iden tify the parameters of this model on the basis of The curves of the storage G’ and loss G” moduli for experimental data. The process of identification is the chestnut starch, obtained during oscillation measure- so called reciprocal problem. This means that in the ments in the oscillation frequency from 2 to 70 rad/s at process of identification experimental results are first- a constant deformation of 2%, were described by Mor- ly approximated by trigonometric functions, and then eira [18] and for acorn starch in the oscillation range rheological parameters of the model are determined. from 0.62 to 62.8 rad/s at a constant deformation of 2% Dinzart and Lipi ski [25] described several frac- were described by Kim and Yoo [9] with the power-law tional rheological modelsń to determine the viscoelastic model [23] in the following form: properties of tested media. Particularly noteworthy is a fractional standard linear solid model, modulii so called fractional Zener model [26, 27]. The values of the (1) storage G’ and loss G” moduli for this model can be described using trigonometric functions leading to the following equations: (2) The values of the power-law model parameters k’ and k” as well as n’ and n” are available in the original papers of Kim and Yoo [9] and Moreira [18] and hence are not repeated here. In view of the role of starch in food production, the (3) most important are its rheological properties, so to evaluate the structure of starch pastes or mixtures of these pastes with different additions, a number of methods are used such as steady-shear flow, oscillato- ry shear, small and large deformation shear, creep, stress relaxation and large deformation extensional (4) © Appl. Rheol. 24 (2014) 24766 | DOI: 10.3933/ApplRheol-24-24766 | 2 | Table 1: Rheological parameters of the Zener model calculated on the basis of Moreira’s experimental data for chestnut starch pastes at 25°C. (5) 0 where Ge is the equilibrium modulus, GN the plateau modulus, τ0 the relaxation time, and α is the fractional exponent. The Zener model (Equations 3 to 5) has five 0 rheological parameters, Ge, GN , τ0, α and k. These para- meters represent the following properties of tested materials [28, 29]: I Equilibrium modulus Ge: Modulus of elasticity in the Table 2: Rheological parameters of the Zener model calculated steady state flow condition represents the total on the basis of Kim and Yoo’s experimental data for acorn elasticity of the network and its reciprocal is sus- starch pastes at 25°C. ceptibility in the state of equilibrium Je. High values of modulus Geindicate an increase of elastic proper- As a result, eight rheological parameters are obtained ties of the material. which are used in a comprehensive analysis of viscoelas- I 0 Plateau modulus GN : The viscoelastic plateau mod- tic properties of the tested materials. The Zener fractional ulus is identified with the structure of cross-linking standard linear solid model holds if the following condi- 0 power. The higher the value of this modulus the tions are satisfied: Ge ≥ 0, GN ≥ 0, τ0 ≥ 0, and 0 ≤α≤1. higher the structure cross-linking. The reciprocal of this modulus is susceptibility of the structure at 0 cross-linking JN . 3 RESULTS AND DISCUSSION I Relaxation time τ0: The characteristic relaxation time defines the time after which stress relaxation Moreira [18] has determined rheological properties of will occur.