Aseptic processing of a food fluid containing particles is much more sophisticated than that of a homogeneous food fluid. From public health standpoint, the system design and Fvalue calculation should be based on the time–temperature history of the slowest heating particle in the heating section and the residence time of the fastest moving particle in the holding section.
In the aseptic processing of a homogeneous food fluid, heat transfer occurs between the heating or cooling medium and the food liquid and within the food liquid as well. When processing a food fluid that contains particles, the heat trans- fer in the heat exchanger, usually in scraped-surface type but sometimes a tubular heat exchanger may be used, becomes more complicated. In addition to the heat transfer between the medium and the liquid part of the food and within the liq- uid, there is heat transfer between the liquid and the particles and inside the particles.
The particles are heated in the heat exchanger by surround- ing liquid. The center of a particle is heated slowest in the whole body. Therefore, the time–temperature history at the center of the particle is indicative to the effectiveness of the heating section. The settings for the operation of the heating section are supposed to ensure that the center in every particle is heated to a temperature no lower than the predetermined holding temperature before discharging into the holding sec- tion. Meanwhile, the settings for the holding section are to ensure that the fastest moving particle resides in this section
no shorter than the expected holding time for achieving the desiredFvalue.
The time–temperature history and RTD in the aseptic pro- cessing of food fluids containing particles are affected by fac- tors related to the particles themselves (shape, size, density, structure, concentration, and thermal properties), the liquid part or “carrier fluid” as it is often called (density, flow rate, concentration, thermal and rheological properties), and the aseptic system (configurations, operation temperature, mu- tator speed of the scraped-surface heat exchanger, and the characteristics of pumps).
Numerous studies using various mathematical models, simulated systems, and indicators have been performed to predict the time–temperature history and RTD. Simu- lated food particles, such as polystyrene spheres, acrylic beads, and rubber cubes, are usually used in the sim- ulated system (Sandeep and Zuritz 1995, Palazoglu and Sandeep 2002, 2004). Some methods have been proposed for time–temperature measurements of food particles in simulated processes, such as ultrasonic tomography (Beller 1993), magnetic resonance imaging (Kantt et al. 1998), and microthermometry (Reiffel 2001, Higgins 2004). Palazoglu et al. (2006) reported a more advanced method that monitors the time–temperature history in real food particles during the processing of a food fluid containing particles in the heating section and holding tube in a tubular heat exchanger by small magnetic particles embedded in these particles.
The reported methods for evaluating the RDT of a food fluid containing particles include visual observation and man- ual timing, the use of photoelectric sensor, laser beam meth- ods, magnetic tracer, Hall-effect sensor, electrical conduc- tivity measurement, etc. For example, several workers have used a video camera and subsequent frame-by-frame anal- ysis, often with mirrors surrounding the tube to enable the tracing of particles in the holding tube (Lewis and Heppell 2000).
It is difficult to identify the fastest-moving particle in a commercial system operating under actual processing condi- tions. As an alternative, residence times of numerous parti- cles may be recorded in a tedious task. The residence time of the fastest-moving particle can then be predicted statis- tically. Fortunately, for most foods, it appears to be conser- vative and safe just to assume that the residence time for the fastest-moving particle is half of the bulk average resi- dence time for the food fluid containing particles. Any devi- ation from this assumption in establishing a process would require detailed studies on the RTD and the MRT of parti- cles. Tests should be conducted using the actual food product flowing at the production flow rate (Berry 1989, Singh and Morgan 2010).
An example of aseptic products containing particles is stirred yogurt with diced fruit. In its processing, fruit particles are pumped through a scraped-surface heat exchanger to be heated up to at least 90◦C, held for 3 minutes, cooled to 27–33◦C, mixed with pasteurized yogurt fluid, and then filled
into plastic cups and sealed aseptically (Haque et al. 2001, O’Rell and Chandan 2006, Hui 2007).
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