Flow Behavior of Wheat Flour - Water Dough Using a Capillary Rheometer . I . Effect of Capillary Geometry

Cereal Chem. 70(l):59-63 A capillary extrusion rheometer was employed for detailed investigation coefficient of 2,395 Pa-sec 03 4 . The flow curves, corrected for end effects of the flow behavior of wheat flour-water dough. Dough was extruded and for effect of die diameter on shear rates, were independent of capillary at ambient conditions through capillaries of different lengths and diamdimensions. The capillary rheometer technique was found to be a reliable eters. In the shear rate range of 9-5,000 sec-1, the dough exhibited shear and repeatable method for determining flow parameters of viscous mathinning with an average flow behavior index of 0.34 and consistency terials such as dough. As with most foods, rheological properties are important in breadmaking. The final quality of the bread can be related to the rheological properties of its flour dough, which explains the amount of research being conducted in this area. Researchers have addressed different aspects of dough rheology and have described the methods used to evaluate the rheological properties (Bloksma 1975; Hibberd and Parker 1975; Matsumoto et al 1975; Matsuo and Irvine 1975; Rasper 1975; Bushuk 1985; Dick 1985; Faubion et al 1985; Hoseney 1985; Nagao 1985; Dreese et al 1988a,b; Fitzgerald et al 1988; Refai et al 1988). Their work concentrated on the qualitative evaluation of flours. Instruments such as the farinograph or mixograph were used to obtain optimum 'Published as journal series 9881, Agricultural Research Division, University of Nebraska. 2 Use of a company or product name by the U.S. Department of Agriculture does not imply approval or recommendation of the product to the exclusion of others that may also be suitable. 3 Packaging engineer, Westreco, Inc., St. Joseph, MO; professor, Biological Systems Engineering, University of Nebraska, Lincoln; research leader, USDA-ARS, Beltsville, MD, respectively. @1993 American Association of Cereal Chemists, Inc. mix time and absorption data. These instruments provide useful information about dough, but they cannot generate numerical data to characterize flow behavior. In automated bakeries, knowledge of dough viscosity plays an important role in production control and equipment design. Dough viscosity may relate to the quality of the baked product and may control that quality in some instances. On-line methods to sense dough consistency require knowledge of dough flow behavior over a wide range of flow conditions. Dough can be pumped from the point of mixing to the fermentation chamber and then to the oven, but suitable pumps cannot be designed or specified unless the dough's rheological parameters and flow behavior properties are known. None of the studies referenced above give the basic information on flow behavior needed for engineering design. Basic information on flow behavior of wheat flour-water dough based on a capillary extrusion rheometer was reported by Sharma et al (1990). They found it was possible to describe the flow behavior of dough using the capillary rheometer results. Because only one size capillary (3.21 mm diameter) with different lengths was used, the reliability of the technique needs verification at other capillary diameters. Therefore, this study was undertaken to investigate the effects of capillary geometry on the flow behavior of wheat flour-water dough. Vol. 70, No. 1, 1993 59 MATERIALS AND METHODS Rheometer The capillary extrusion rheometer consisted of a cylinder-piston arrangement (Sharma and Hanna 1992). The stainless steel cylinder had an inside diameter of 37 mm and a wall thickness of 17 mm. A close-fitting plunger was constructed from a solid brass rod. The working length of the plunger was the same as the 100-mm height of the cylinder. One end of the plunger had a threaded portion that could be screwed into a coupler, which, in turn, was screwed to the cross-head of a Universal Testing Machine (UTM) (model 1123, Instron Corp., Canton, MA). The capillaries were hollow brass tubes. One end of each tube was soldered to a threaded stainless steel section that could be screwed into the threaded hole at the bottom of the cylinder's supporting collar to be flush with the inside surface. The inside diameter of the stainless steel supporting collar was slightly bigger than the outside diameter of the cylinder. The height of the supporting collar was 56 mm to avoid buckling due to excessive pressure exerted on the cylinder side walls. Three set screws were used to secure the cylinder in the supporting collar. The supporting collar was held vertically on a flat plate and the entire assembly was supported by a heavy steel stand. The complete unit was positioned under the cross-head of the UTM so that the brass plunger would smoothly slide down inside the cylinder containing the dough and force-extrude the dough through the various capillary sizes. Dough Preparation Bakery flour was obtained from ConAgra (Omaha, NE). The moisture content of flour was determined to be 12.6% on a wet basis (method 44-15A, AACC 1983) of drying 3 g of flour at 130'C for 1 hr in a convection oven. Bakery flour was tested in a 10-g mixograph for optimum mix time and absorption. These were found to be 3 min and 64.5 g of H20 per 100 g of flour (on a 14% absorption basis), respectively. Subsequently, larger quantities of dough were made by mixing 100 g of flour with 64.5 g of distilled water for 3 min in a pin mixer (National Manufacturing Co., Lincoln, NE). Rheometer Operation The inside wall of the cylinder was lightly lubricated with corn oil. Small pieces of dough were placed in the cylinder and pushed down using a wooden rod. When the cylinder was completely filled, a lightly lubricated plastic sheet was placed on top of the sample and hand-pressed to squeeze out the excess dough and to level the dough surface. A capillary tube of known dimensions was then screwed in place. Different lengths of capillary tubes with inside diameters of 3.2, 4.0, and 5.6 mm were used to achieve various combinations of length-to-diameter ratios (L/D ratio). These ratios are given in Table I. Each capillary was first used at its maximum length. After ensuring that sufficient data were collected using one length, the capillary tubes were cut to obtain shorter lengths. After the cylinder was placed inside the supporting collar and secured, the whole assembly was placed under the cross-head of the Instron, to which the brass plunger had already been attached. The plunger was moved down at preselected speeds of 5, 10, 20, 50, 100, and 500 mm/min to extrude the dough through the capillary tube. The extruded dough was collected TABLE I Length-Diameter Ratios for Various Capillaries