Consistometers

Chapter HI relates to measurement of flow properties of foods that are primarily fluid in nature, unithi.i surveys the nature of viscosity and its relationship to foods. An overview of the various flow behaviors found in different fluid foods is presented. The concept of non-Newtonian foods is developed, along with methods for measurement of the complete flow curve. The quantitative or fundamental measurement of apparent shear viscosity of fluid foods with rotational viscometers or rheometers is described, unithi.2 describes two protocols for the measurement of non-Newtonian fluids. The first is for time-independent fluids, and the second is for time-dependent fluids. Both protocols use rotational rheometers, unit hi.3 describes a protocol for simple Newtonian fluids, which include aqueous solutions or oils. As rotational rheometers are new and expensive, many evaluations of fluid foods have been made with empirical methods. Such methods yield data that are not fundamental but are useful in comparing variations in consistency or texture of a food product, unit hi.4 describes a popular empirical method, the Bostwick Consistometer, which has been used to measure the consistency of tomato paste. It is a well-known method in the food industry and has also been used to evaluate other fruit pastes and juices as well. 

Close the gate of a Bostwick consistometer and adjust leveling screws until the leveling bubble indicates that the consistometer is level. 

Fill the consistometer reservoir to the point of overflow (e.g., -300 g of 12 °Brix paste) and remove excess from top of reservoir using a spatula. 

McCarthy, K.L. and Seymour, J.D. 1994. Gravity current analysis of the Bostwick consistometer for power law foods. J. Texture Stud. 25 207-220. 

DeVaney and Shelton (1940) developed a consistometer (viscosimeter) which is easily made in the laboratory and has proved fairly accurate for a wide variety of suspensions, both large and fine. Because of its simplicity and its range of usefulness a description is presented here, y., The device is shown in Figure 92 and con-... 

Figure 93. Comparison of Observed and Calculated Rates of Flow for Sucrose Solutions in Consistometer.

Effect of Specific Gravity—The effect of specific gravity on consistency was studied by DeVaney and Shelton (1940) using the consistometer described above. Suspensions were made of fine substances ranging from a density of 2.65 for quartz to 11.3 for metallic lead. Each of the materials was screened so as to pass a 200-mesh sieve and be retained on a 325-mesh. The materials were carefully washed so no slime... 

Davey, K.R., Jones, P.N. 1985. Evaluation of a sliding pin consistometer for measurement of the hardness and spreadability of butter and margarine. J. Text. Stud. 16, 75-84. 

Pretesting is done by using a consistometer (Fig. 15.2), in which the cement mixing slurry cup is placed in a bath of oil. The oil temperature and pressure can be raised or... 

In a typical consistometer, such as the one shown in Fig. 15.2, the temperamre and pressures can be varied from ambient conditions to 400 °F (215°C) and 21,000 psi (147 MPa), but consistometers with higher temperatures and pressures are available. Freezing temperatures are simulated by connecting a chiller to the consistometer. 

For pumping ease, the initial Be should be very low, preferably < 30 Be, which can be measured by the consistometer. The rheological behavior can be measured by means of a rotational viscometer, which consists of an outer sleeve and an inner drum, both rotating at different speeds. The outer sleeve is rotated at a constant speed, which causes a torque on the inner dmm that can be measured on a dial. Starting from 600 rpm, the rotor speed is lowered successively at 20 s intervals, and the measurements of the torque are taken at the end of each period. Typically the speed is lowered in steps of 6 rpm, and the results are represented graphically. 

The water in the slurry should be intimately mixed, and the solid particles in the slurry should not be segregated. For this reason, free water is measured immediately after the slurry is mixed in the blender and before it is placed in the slurry cup of the consistometer. It is poured in a graduated glass cylinder, and the cylinder is kept at an angle of 45° for 2 h. The free water standing above the slurry is then decanted and measured. 

The compressive strength of the set sealant is measured with ASTM standard specimens of a (2 X 2 X 2 in. ) cube [6]. The slurry from the consistometer is poured in molds and is allowed to set in a curing chamber. It is then taken out after a desired period, and the strength is measured by applying a load on it in a uniaxial press. Knowing the total load and the area of the face on which the load is applied, one can calculate the compressive strength. 

Needle penetration Hoppler consistometer, Needle track tester A—>C 34... 

Concentrated compositions that increase in viscosity when diluted are described in U.S. Patent 6,150,320. A Bostwick consistometer is used for all viscosity measurements and equivalence is offered to Brookfield measurements using Spindle no. 1, at 60 r/min, and the Zahn viscometer, no. 1 cup. A low-viscosity hard surface cleaning emulsion, approximately 12 mPa s, is described in U.S. Patent 5,934,375 that increases in viscosity upon dilution with water to 800 to 1200 mPas. 

Consistometer n. An instrument for measuring the flow characteristics of a viscous or plastic material. 

The earliest consistometer was that used by R. D. Cooke (7. Amer. Ceram. Soc., 7, 651,1924) this was of the capillary type and was developed into an instrument suitable for works control by W. N. Harrison (ibid., 10, 970,1927). Other instruments for controlling the properties of vitreous-enamel slips include the GARDNER MOBILOMETER (q.V.). 

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