Solution Viscoelastic Properties of OATRIMalO and Cooked Oat Bran

Cereal Chern. 75(3):354-359 The solution rheological behaviors of OATRIM-1O and cooked oat bran were investigated. The rheological properties of the materials were investigated using both thixotropic loop and small-amplitude oscillatory shear experiments. The cooked oat bran exhibited shear-thinning behavior during a thixotropic loop experiment over a shear rate range of a250/sec. The shear-thinning behavior was reproduced during the measurement of a second thixotropic loop. In contrast, OATRIM-1O exhibited an unexpected region of shear-thickening behavior at 2a-SO/sec. The shear-thickening and subsequent shear-thinning regions for OATRIM10 could be described by a transient network model indicating that the shear-thickening behavior is caused by a shear-induced entangled network Oat bran and other oat materials are used in numerous food applications where high-fiber content is desired in food products. The soluble fiber of oats is (l-3),(l-4)-13-D-glucan, which is highly regarded for contributing health benefits to food products (Inglett 1997), Numerous studies have shown that soluble fibers obtained from sources such as oat flours benefit health by lowering blood cholesterol (AHA 1980, Topping 1991, Malkki et al 1992, Uusitupa et al 1992, FDA 1997a,b). Consequently, the utilization of oat (l-3),(1-4)-13-D-glucan in the food industry has been an area of much active research recently, A characterization of the rheological behavior of the systems is required to develop and produce materials efficiently, Rheological properties are of import in goveming many industrial processes such as extrusion, injection molding, and baking. In addition, rheological measurements can be used to distinguish the effects of different processing conditions on the flow properties of materials, Application of rheological theories can aid in understanding of the conformational dynamics of the macromolecules and can be used to predict the flow behavior of the materials in processing, Validated rheological models can aid in the timely and cost-efficient development of new processes as well as in the development of new die and screw designs. Despite the importance of rheological properties in many industrial processes, a limited number of studies on the viscoelastic properties of oat-based biopolymers has been reported in literature (Wood 1984; Autio et al 1987, 1992; Autio 1988, Doublier and Wood 1995). Semidilute solutions of oat gums have displayed shear-thinning behavior at shear rate ranges of 0-1,500/sec. The limited data on the oscillatory shear viscoelastic properties of oat (1-3),(l-4)-13-D-glucan appear to display classic non-Newtonian flow behavior, In this study, initial investigations of the solution viscoelastic properties of OATRIM-lO and cooked oat bran are reported. The materials were subjected to both thixotropic loop and small-amI Biomaterials Processing Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, USDA, 1815 N. University Street, Peoria, IL 61604. Names are necessary to report factually on available data; however, the USDA neither guarantees nor warrants the standard of the product, and the use of the name by the USDA implies no approval of the product to the exclusion of others that may also be suitable. 2 Corresponding author. E-mail: [email protected] Phone: 309/6816240. Fax: 309/681-6685 3 Biopolymer Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, USDA, Peoria, IL. Publication no. C-1998-0407-07R. This article is in the public domain and not copyrightable. It may be freely reprinted with customary crediting of the source. American Association of Cereal Chemists, Inc., 1998, 354 CEREAL CHEMISTRY that is partially disentangled at higher shear rates. Subsequent thixotropic loop experiments displayed the shear-thickening region for OATRIM-JO, indicating that the network structure can be reformed during the imposition of a shear field. Small-amplitude oscillatory shear data for cooked oat bran can be described reasonably well using a generalized linear viscoelastic (GLV) model. The oscillatory shear data obtained for OATRIM-lO could not be described by the GLV model. OATRIM-1O exhibited a distinctive plateau centered at IO/sec, and the low frequency response of storage modulus G' decreased with a much larger slope in frequency than was predicted by the GLV model. plitude oscillatory shear experiments. Cooked oat bran exhibited shear-thinning behavior over the shear rate range investigated. In contrast, OATRIM-lO exhibited an unexpected region of shearthickening behavior. The data obtained from the thixotropic loop experiments for both of the materials was interpreted using a power law model. The oscillatory shear rheological properties of the cooked oat bran and OATRIM-lO were compared to the predictions developed from a generalized linear viscoelastic (GLV) model. MATERIALS AND METHODS Materials and Sample Preparation The oat bran material used in this study was obtained from Grain Millers (Saint Ansgar, IA). The oat bran was designated as lot number IN-046-97-408 and was used as received. The cooked oat bran sample was prepared by creating a 5% (w/w) aqueous slurry of the oat bran in water. The slurry was brought to a boil in a microwave oven and cooked for 5 min. The solution was then cooled to room temperature. The procedure to produce the OATRIM-lO sample was as discussed previously (Inglett 1993, Inglett and Newman 1994): 4 kg of the oat bran was slurried in 28 L of water containing 50 ppm of calcium. The slurry was adjusted to pH 5-8 and passed through a steam-injection cooker and collected in a 30-gal steam-heated kettle. An a-amylase enzyme preparation was added to the slurry (Inglett 1993). The solution was stirred at 80-90°C for 5 min, and the enzyme was deactivated by passing the slurry through a steam-injection cooker. The slurry was then centrifuged at 15,000 rpm in a Sharples centrifuge to separate the soluble and insoluble components. The soluble materials were isolated and the water was removed by freeze-drying the solution. The resulting material was designated as OAlRIM. Degrees of polymerization and number average molecular weights of the amylodextrins produced by the enzymatic treatment are summarized in Inglett (1993). In this study, the specific OAlRIM used was designated OATRIM-lO and contains (w/w): 81 % amylodextrins, 10% (1-3),(l-4)-13-D-glucan, 4% various proteins, 4% ash minerals, and 1% crude fat. The OATRIM-I0 solution used in this work was prepared in a manner identical to that used to prepare the oat bran samples. OATRIM-lO was dissolved in water at a concentration of 5% (w/w). The solution was brought to a boil in a microwave for 5 min and then cooled to room temperature. Rheological Measurements Rheological properties were measured using a controlled-stress rheometer (CSU 500, CarriMed, Dorking, England) with a coneand-plate fixture. All the rheological studies were conducted using a 6-cm diameter, 4° cone. The temperature of the sample was controlled using a Peltier plate which enabled the chamber of the viscometer to be controlled to within ±0.1°C. For all the data discussed in this report, the temperature was maintained at 25°C. For each solution, a small-amplitude oscillatory shear experiment was first conducted using frequencies of O.OI-IO/sec. The oscillatory experiments were performed using a constant shear strain of 50%. Prior to the oscillatory experiments, a torque sweep was conducted on each sample to ensure that the applied shear strain was within the linear viscoelastic regime of the material. From the oscillatory experiments, the shear storage and loss moduli (G' and Gil) were measured. After the oscillatory experiment, a thixotropic loop experiment was conducted by accelerating the cone uniformly from rest to a shear rate of 250/sec and back to rest over a 2-min period. The thixotropic loop experiment was repeated immediately. Each of the experiments were repeated a minimum of three times. The average relative standard deviation of the measurements was ±10%. thickening behavior was caused by the high molecular weight amylodextrins present in the sample. Studies of the rheological properties of the individual components of OATRIM-lO will be needed to establish the exact cause of this unexpected behavior. The effect of a second thixotropic loop experiment on cooked oat bran and OATRIM-IO is illustrated in Figs. 2 and 3. For cooked oat bran, the data for the second thixotropic loop overlapped the data obtained during the initial cycle. This result indicates the absence of any flow-induced structure in the fluid caused by the imposition of the shear field. During the second thixotropic loop, OATRlM-1O again exhibited a region a shear-thickening behavior over the same shear rate range. This result indicates that the flow-induced structure formed during the shear-thickening regime is transient and is reversible at higher shear rates, but it can be reformed with a second application of the shear field. This is in contrast to recent findings for semidilute solutions of waxy maize starch which exhibited shear-thickening behavior over a similar shear rate range but produced a stable flow-induced structure (Dintzis and Bagley 1994, 1995; Dintzis et a11996) RESULTS AND DISCUSSION Thixotropic Loop Experiments The effect of shear rate for the first cycle of the thixotropic loop experiment on the measured shear viscosity (11) for OATRIM-1O and the cooked oat bran is illustrated in Fig. 1. The data for the first cycle of the thixotropic loop experiment for oat bran exhibited shear-thinning behavior over the entire shear rate range investigated. The rheological behavior of OATRIM-1O differed markedly from that of the cooked oat bran. It exhibited an initial region of shearthinning behavior followed by a region of shear-thickening behavior at =20-80/sec and then by another region of shear-thinning behavior. We believe that this is the first reported evidence for shear-thickening behavior in solutions containing (l-3),(1-4)-~-D­ glucan and amylodextrin biopolymers. The exact cause of the shearthickening behavior for OATRIM-1O has yet to be determined. However, published work on (l-3),(l-4)-~-D-glucanbiopolymers in solution indicates shear-thinning of these materials over the shear rate range examined in this work, so it is possible that the shearModeling Shear-Thinning and Shear-Thickening Behavior The effect of shear rate on the viscosity of the materials was characterized using a power law model (Macosko 1994) which can be expressed as: