Procedimiento

An in-line measurement is performed in a process line; an online measurement is per-formed in a bypass loop from the main process line, and the food may be returned to the main process line after the measurement is performed. A near-line measurement is performed on a sample taken from a process line, which is often discarded after measure-ment. Because foods are complex materials (e.g., suspensions, emulsions, gels), structural changes may take place during sampling (e.g., flow through a valve) for online and near-line measurements (Roberts, 2003). Nevertheless, in principle, the previously described capillary flow, concentric cylinder, plate–cone, and mixer viscometers may be used for in-line, onin-line, and near-line measurements. The empirical measurement methods described previously are used primarily in near-line measurements.

Roberts (2003) listed several requirements that both in-line and online measuring sys-tems for foods should satisfy, including:

Free of hygiene risk. The system must be constructed with a food-grade material, per-mit standard clean-in-place procedures, and be free of dead flow zones.

nondestructive. The system should not alter the quality of the product or perturb the production schedule and process.

Real-time operation. To minimize down time and waste or rework of a product, the response time should be short, typically seconds.

Physically robust and stable. In general, the system must require little maintenance and withstand the process operating conditions (e.g., temperature and pressure).

The sensor signal must be unaffected by the typical environment in a processing

RHeologiCAl PRoPeRties oF Fluid Foods

plant (e.g., mechanical vibration, electrical interference) and amenable to control operations.

easy operation. It would be desirable that the sensor and system be easy to operate and provide an acceptable signal for process control, and that the results not be depen-dent on operator skills. However, determination of non-Newtonian rheological behavior also requires knowledge of the flow characteristics of the fluid food and its structure, as well as potential changes that can occur due to the shear rate and tem-perature prevalent in the measurement system. Thus, in addition to a skilled opera-tor, it would be desirable to have supervisors with a thorough knowledge of the rheological and physicochemical behavior of the food product being manufactured.

5.4.3.2.1 Tomographic Techniques

Some, if not all, of the requirements of in-line measurement techniques are satisfied by tomographic techniques that provide spatially and temporally resolved data. The tech-niques utilize the inherent properties of the food material and include those based on magnetic, acoustic, optical, and electrical signals (Choi et al., 2002). These techniques have also been used in measurement of velocity profiles in tubes and rheology of stationary materials. Here, the emphasis is on determination of in-line measurement of rheological behavior of fluid foods using tube flow.

Magnetic resonance imaging (MRI) is a spectroscopic technique based on the interac-tion between nuclear magnetic moments and applied external magnetic fields. A sample is placed in a magnetic field within a radio frequency probe and the response of the test material in terms of attenuation, frequency, and phase to energy added in that frequency range is recorded. Two notable constraints of MRI are the need to include a nonmetallic and nonmagnetic test section in the flow system and the high cost of setting up MRI sys-tems in processing plants.

Ultrasonic refers to sound waves with frequencies of 20,000 Hz or greater, which are beyond the range of human hearing. The sound waves are transmitted through the wall of a pipe, and the reflections are analyzed. In principle, there are two different kinds of ultra-sonic flow meters: transit time and Doppler flow meters. Both kinds measure primarily velocity. The principal advantages of ultrasonic Doppler velocity (UDV) meters over other types, such as turbine and conductivity meters, are as follows:

• No moving parts involved

• Nonintrusive

• Low maintenance

• Hard to block

• Can be used with nonconductive media

The UDV and MRI methods offer similar potential for rheological measurements under fully developed, steady, pressure-driven tube flow. In addition, the data-processing techniques for MRI and UDV are somewhat similar.

Doppler meters measure the frequency shifts caused by liquid flow. The frequency shift is proportional to the liquid’s velocity. Time-of-flight meters use the speed of the signal traveling between two transducers, which increases or decreases with the direction

of transmission and the velocity of the liquid being measured. Important parameters to consider when specifying ultrasonic flow meters include flow rate range, operating pres-sure, fluid temperature, and accuracy. One-beam Doppler flow meters are widely used, but multibeam profiling Doppler flow meters have been reported.

A schematic diagram of an UDV system is shown in Figure 5.10. The relationships among fluid velocity, v, and UDV data are given by

v cf f

= d

2 0cosθ (5.77)

where v is the velocity component along the axis of the ultrasound transducer, f_d is the Doppler shift frequency, f₀ is the frequency of the transmitted pulse, c is the speed of sound, and θ is the angle between the transducer and the flow direction, typically 45°

(Dogan et al., 2003).

The spatial location, d, of the velocity component in the above equation can be identi-fied by a time-of-flight, Δt, measurement that relates the speed of the reflected wave to the distance traveled:

d= ∆c t

2 (5.78)

In turn, the values of d can be converted to the radial location in the pipe so that the velocity profile in a pipe can be obtained. The velocity profile is used to calculate velocity gradients (shear rates), (dv/dr), at specific locations using an even-order polynomial curve fit to the velocity data.

v r( )= +a br²+cr⁴+dr⁶ +er⁸ (5.79) Resolution of the velocity data and removal of data points near the center of the tube, which are distorted by noise, aid robustness of the curve fit; the polynomial curve fit introduced a systematic error when plug-like flow existed at radial positions smaller than 4 mm in a tube of 22 mm diameter. The curve fit method correctly fit the velocity data of

Transducer

Center line

Fully developed velocity profile of food

Sampling time = 2d/c

r d z

Figure 5.10 Schematic diagram of in-line measurement of flow properties using an ultrasonic Doppler velocity meter.

RHeologiCAl PRoPeRties oF Fluid Foods

Newtonian and shear-thinning behaviors but was unable to produce accurate results for shear-thickening fluids (Arola et al., 1999).

With velocimeter-based or pointwise rheological characterization, in addition to cal-culation of shear rate profile, the corresponding shear stress distribution is obtained by combining pressure drop measurements and the linear relationship between the shear stress and the radial position in a pipe, to be discussed later in this chapter.

UDV and pressure drop (ΔP) measurements were carried out on tomato concentrates with total solids of 8.75%, 12.75%, and 17.10% (Dogan et al., 2003); the rheological parame-ters deduced from these data agreed well with those based on capillary flow data obtained at four different flow rates. Also, pointwise rheological characterization using MRI and UDV of 4.3 °Brix tomato juice and 65.7% corn syrup agreed well with off-line data obtained using a rheometer (Choi et al., 2002). The UDV technique was also used on 1- to 3-mm diced tomatoes suspended in tomato juice, and the yield stress of the suspension was char-acterized in terms of the Herschel–Bulkley model (Equation 5.12) and the apparent wall slip region modeled as a Bingham fluid (Dogan et al., 2002).

For reliable characterization of a specific food by the UDV technique, extensive studies would be necessary to establish the operating parameters for that food and flow system.

A change in the type of food or the composition of a specific food may necessitate a thor-ough evaluation of all operating parameters. Nevertheless, this technique may find a place in in-line characterization of rheological properties in food processing plants.

5.4.3.2.2 Vibrational Viscometers

Vibrational viscometers are robust, easy to install for in-line sensing of viscosity, offer minimal disruption of flow in a process line, operate over a wide range of temperatures (e.g., −40 to 400°C), and provide real-time data. Vibrational viscometers actually measure kinematic viscosity (viscosity/density), and units are available capable of measuring kine-matic viscosities ranging from 0.1 to 10⁶ mPa s/g cm⁻³ (centistokes). Typically, a vibrational viscometer employs a high frequency (e.g., 650 Hz) torsional oscillation of a sphere- or rod-shaped probe that undergoes damping by the fluid whose viscosity is of interest. The amplitude of oscillation is small, of the order of a micrometer, and the power consumed is converted to viscosity. When the viscosity of a fluid changes, the power input to maintain constant oscillation amplitude is varied.

However, vibrational viscometers may not indicate the true bulk viscosity of a suspen-sion that forms a thin layer of the continuous phase (e.g., serum of tomato juice) around the immersed probe or when the probe is covered by a higher viscosity gel due to fouling.

Vibrational viscometers are suitable for measuring viscosities of Newtonian fluids, but not the shear-dependent rheological behavior of a non-Newtonian fluid (e.g., to calculate values of the power law parameters).

Vibrational viscometers may also be suitable for following gelation in near-line or laboratory experiments at a constant temperature. For example, a vibrational viscom-eter was used to dviscom-etermine the coagulation time of renneted milk at fixed temperatures (Sharma et al., 1989, 1992). However, in nonisothermal physical gelation, the elastic modu-lus depends on the temperature dependence of the resonant response, so the precise cor-rection for the influence of temperature must be known.