Aseptic Processing System Research Articles

Residence time distribution (RTD) characteristics of meat and carrot cubes in starch solutions in a vertical scraped surface heat exchanger (SSHE)

The residence time distribution (RTD) of meat and carrot cubes was evaluated in the vertical scraped surface heat exchanger of a pilot scale aseptic processing system using a full factorial design of experiments employing flow rate (10, 15, 20 and 25 kg/min), particle size (meat cubes of size 10, 15 and 20 mm; and carrot cubes of size 6 and 13 mm) and concentration of the carrier fluid (4, 5 and 6% w w starch) as factors. All factors influenced ( P < 0.05) the fastest particle residence time (FPRT), the mean particle residence time (MPRT), standard deviation of the residence time distribution (SRTD) and their normalized versions. A generalized logistic model showed a reasonable fit for the experimental RTD under all test conditions ( r 2 > 0.95). Analysis of variance revealed that all test factors were responsive ( P < 0.05) to parameters associated with the model: the half concentration internal age ( HRT & HNT), the particle accumulation rate ( B) and the upper limit particle concentration ( U).

Residence time distribution (RTD) characteristics of meat and carrot cubes in starch solutions in a vertical scraped surface heat exchanger (SSHE)

The residence time distribution (RTD) of meat and carrot cubes was evaluated in the vertical scraped surface heat exchanger of a pilot scale aseptic processing system using a full factorial design of experiments employing flow rate (10, 15, 20 and 25 kg/min), particle size (meat cubes of size 10, 15 and 20 mm; and carrot cubes of size 6 and 13 mm) and concentration of the carrier fluid (4, 5 and 6% w/w starch) as factors. All factors influenced ( P ≤ 0·05) the fastest particle residence time (FPRT), the mean particle residence time (MPRT), standard deviation of the residence time distribution (SRTD) and their normalized versions. A generalized logistic model showed a reasonable fit for the experimental RTD under all test conditions ( r 2 > 0·95). Analysis of variance revealed that all test factors were responsive ( P ≤ 0·05) to parameters associated with the model: the half concentration internal age ( HRT & HNT), the particle accumulation rate ( B) and the upper limit particle concentration ( U).

Convective Heat Transfer at Particle‐Liquid Interface in Continuous Tube Flow at Elevated Fluid Temperatures

ABSTRACTLiquid‐to‐particle convective heat transfer coefficients are useful in developing aseptic food processing systems. They were determined for continuous flow through a holding tube at 115.5°C using liquid crystal and relative velocity methods with sodium carboxymethylcellulose solution to simulate non‐Newtonian fluid characteristics. An on‐line tube viscometer was used for in situ estimation of rheological characteristics. Minimum and maximum values of hfp determined from the liquid crystal method ranged from 986 W/m20K to 2270 W/m20K, (Nusselt numbers from 26.4 to 54.6). Values from the relative velocity method ranged from 1143 to 2270 W/m20K (Nusselt numbers from 33.2 to 63.1) when using the Ranz and Marshall relation, and from 598 to 1456 W/m20C (Nusselt numbers from 13.6 to 24.1) with a flat‐plate correlation. Heat transfer coefficients increased significantly with decreasing carrier medium viscosity and decreasing particle‐to‐tube diameter ratio and increased with flow rate.

Mathematical Characterization of Residence Time Distribution Curves of Carrot Cubes in a Pilot Scale Aseptic Processing System

Residence time distribution (RTD) of food particles and carrier fluid was evaluated in a commercial pilot scale aseptic processing system using o full factorial design of experiments employing flow rate (15 and 20 kg/min), temperature (80 and 100°C), holding tube length (1.5, 17.5 and 26.7m) particle (carrot cubes) size (6 and 13 mm) and starch concentration (30 and 50 g/kg) of the carrier fluid as factors. RTD curves of carrot cubes were characterized in this study using a special case of the logistic model (autocatalytic or inverse exponential model). Three model parameters (an accumulation rate factor, B; a concentration limit factor, U and a half-time factor, M) fully described the RTD curve: F = C/[1 + e -B(n-M)]. The model was used to describe the influence of various process parameters and to obtain E-type RTD curves. All test factors were significant (P < 0.05) in influencing the parameters associated with the logistic model. Using multiple regression, the logistic parameters were related to the experimental factors (R 2 > 0. 78).

A computer simulation program for evaluation of the continuous heat treatment of particulate food products. Part 2: Utilisation

A computer program for calculation of temperatures in a liquid containing particles during continuous heat treatment has been developed at the SIK. The program facilitates the study of the influence of processing on product quality in an aseptic processing system. The calculated F value is related to the safety, and the cook value (C value) is related to the quality of the product. In this paper, results of some calculations using the program are presented. They concentrate on the influence of the processing conditions on the process design. These results indicate that factors such as particle heat transfer coefficient, both in the holding tube and in the scraped surface heat exchangers (SSHEs), flow conditions, particle diameter and sterilisation temperature have consequences for the holding tube length, i.e. the processing time, for a given lethality. For example, for small particle heat transfer coefficients (< 100 W m 2 K), a small change will have a large effect on the required processing time (i.e. the holding tube length). However, at particle heat transfer coefficients > 150–200 W m 2 K a small change does not greatly affect the processing time (i.e. holding tube length). The program can thus be used to examine whether the surface heat transfer coefficient is the rate-limiting factor, or whether the heat conduction inside the particles is more important in the design of the process.

A computer simulation program for evaluation of the continuous heat treatment of particulate food products. Part 1: Design

Computer simulation programs used by the food industry today are not very well developed, partly because few food processes ar easily described by mathematical equations. The heat treatment of food, however, is one group of processes for which mathematical modelling exists. Programs used for prediction of the time-temperature relationship for various food products during heat treatment have been developed at SIK. A specially designed program calculates the temperature in viscous liquid foods with discrete solid particles. This program facilitates study of the influence of processing conditions on the product quality in an aseptic processing system. The F value is a measure of the safety and a so-called C value (cook value) is a measure of the quality of the product. The simulation program has been further developed to take into account product flow rate, residence time distribution, particle size and geometry, equipment specifications, particle load and the thermal properties of the product. The simulation program makes it possible to analyse the entire sterilisation system, including cooling, and determine the required holding time to reach a desired lethality value in the centre of the fastest-moving particle. The design of the program is described in this paper, and some applications are treated in a following paper.

Residence time distribution of carrot cubes in starch solutions in a pilot scale aseptic processing system

Residence time distribution (RTD) of carrot cubes flowing in starch solutions was evaluated in a commercial pilot-scale aseptic processing system using a full factorial design of experiments employing flow rate (15 and 20 kg/min), temperature (80 and 100°C), holding tube length (0, 60 and 100%), particle size (6 and 13 mm) and starch concentration (3 and 5% w/w) as factors. All factors were found to be significant ( p<0·05) in influencing the fastest particle residence time (FPRT), mean particle residence time (MPRT), particle residence time variance (PRTV) and their normalized versions. While particle size, holding tube length and starch concentration increased the FPRT, fluid flow rate and temperature had the opposite effect. The FPRT was lower than the average fluid retention time under all testing conditions.

Lethality Distribution within Particles in the Holding Section of an Aseptic Processing System

ABSTRACTResidence time distribution (RTD) affected lethal effects of heat on 1.27 cm diameter particles in the holding section of an aseptic processing system. The variations in particle center and particle surface F0 were determined as a function of particle density, flow rate, and particle to fluid heat transfer coefficient, hfp. Particles with density ratios of 1.00‐1.04 relative to the carrier fluid showed diverse RTD characteristics in the hold tube. Particles with density ratio of 1.01 had the least residence time. The distribution of the particle center F0 value increased with increase in the hfp.

Photo‐sensor Methodology for Determining Residence Time Distributions of Particles in Continuous Flow Thermal Processing Systems

ABSTRACTA method applying photo‐sensors was developed to determine particle residence time distributions in the holding tube of aseptic processing systems. Multiple sensor pairs installed around a transparent glass tube (50.8 mm, i.d.) created a two‐dimensional “optical‐grid” for detecting particle movement. Detection of spherical particles (polystyrene, 19.1 mm diameter) could be achieved at particle velocities below 2.67 m/s to comply with sensor response time. A normal distribution of particle residence times was obtained from 75 replicated runs. The laminar flow assumption for the fastest fluid element was found to be conservative for public safety concerns in solid‐liquid two‐phase systems using the single particle basis for sizing holding sections.

Measurement of Residence Time Distribution of Fluid and Particles in Turbulent Flow

ABSTRACTResidence time distribution (RTD) of fluid and particles was measured in the holding tube of an aseptic processing system. A unique injector device instantaneously injected a large number of particles at one time without interruption of flow. A fraction collector device sampled exiting fluid at 1‐sec intervals. Exiting particles at specific lapsed times from injection, were counted from those collected. RTD of fluid was determined by dye injection. RTD of fluid and particles under turbulent flow fitted a normal distribution function. Mean residence time of fluid was 0.975 times that calculated from volumetric flow rate with standard deviation ± 4.16%. Mean residence time of particles was 1.032 times that of the fluid, standard deviation was ± 4.62%.

Velocity Distributions of Food Particle Suspensions In Holding Tube Flow: Distribution Characteristics and Fastest‐Particle Velocities

ABSTRACTVelocity distributions of model food particles were investigated by videotaping particles suspended in sodium carboxymethylcellulose (CMC) solutions during passage through a transparent holding tube similar in dimension to that of commercial aseptic processing systems. The distributions could be well described by log‐normal models. Fastest particle velocities were below theoretical centerline velocities for Newtonian fluids but were above theoretical values for the fluids used. Results indicated that for particle concentration levels used in these studies, channeling and particle‐fluid interaction effects may be significant.

Velocity Distributions of Food Particle Suspensions in Holding Tube Flow: Experimental and Modeling Studies on Average Particle Velocities

ABSTRACTAverage particle velocities of model food particles were investigated by videotaping particles suspended in sodium carboxymethylcellulose solutions during passage through a transparent holding tube similar in dimension to that of commercial aseptic processing systems. Results indicated the mean normalized velocity of particles was significantly affected by particle Froude number and a dimensionless viscosity. Within the range of this study, particle concentration effects were significant only with respect to standard deviations of distributions. Generalized Reynolds number poorly correlated with mean normalized particle velocity. A mathematical model for particle action was in qualitative agreement with experimental results.

PARTICLE/FLUID INTERFACE HEAT TRANSFER UNDER UHT CONDITIONS AT LOW PARTICLE/FLUID RELATIVE VELOCITIES1

ABSTRACTMathematical models are often used for determining commercial thermal process schedules for heterogeneous foods processed in an aseptic processing system. the particle/fluid interface convective heat transfer coefficient is a critical input parameter for these models and must be determined experimentally. Experiments were conducted at 129.4°C in a specially designed apparatus to determine the particle/fluid interface convective heat transfer coefficient for model food particles heated by Newtonian and non‐Newtonian fluids. Values of the heat transfer coefficient ranged between 55.63 and 89.5 W/m10C for particles heated in a non‐Newtonian fluid and between 65.67 and 107.11 W/m20C for particles heated in a Newtonian fluid.

THE DESIGN OF COMPLETE ASEPTIC PROCESSING SYSTEMS

The paper shows the design development during the last decade of UHT plants in the United Kingdom. With the increase of automation in the dairy industry the progress in the design of aseptic processing systems leading to the simplification of the basic design is described in detail. Indirect and direct UHT plant are considered and the advantages and disadvantages of design evaluated. The operation of aseptic processing lines is discussed and the need for a high standard of experience in operators stressed. Variations in design of UHT plant and aseptic tank systems are examined with further development in the use of aseptic fillers.