The flow of liquid foods in an agitated vessel using PEPT: Implications for the use of TTI to assess thermal treatment

The motion of Time Temperature Integrators (TTI) has been assessed within a scale model vessel designed for thermal processing of viscous complex food fluids using Positron Emission Particle Tracking (PEPT). The vessel, based on the ‘Vesuvio’ vessel made by Giusti Ltd, was filled with water and starch solutions of varying rheology and the effect of rotational speed and fill height was assessed. PEPT was used to measure both the motion of the fluid and the TTI separately, since for the TTI to give a reliable measure of the thermal processing it should be isokinetic and follow the fluid streamlines. For low viscosity fluids, significant settling of the particle relative to the fluid was observed. This did not occur for the more viscous starch solutions where the TTI and fluid behaved similarly regardless of the mixing quality, which was significantly affected by changing fill height and rotational speed. Sub-circulatory regions observed close to the impeller could lead to overprocessing of the food fluids based on TTI measurements in these regions, since they would be expected to be in the coldest part of the vessel. This study shows that TTI can be applied to the processing of viscous fluids within agitated thermal processes, provided that requisite knowledge of the fluid motion and likelihood of settling of the TTI is assessed in tandem.

Industrial relevance

Time Temperature Integrators (TTI) are potentially very valuable tools for the measurement and validation of thermal processes. In this paper, conditions under which TTI would be expected to give valid thermal measurements in agitated batch vessels are assessed by measurement of their relative flow to the fluid using a novel visualisation technique. As well as ensuring the correct implementation of TTI, this work could be used to improve vessel design.     

APPLICATION of LASER DOPPLER ANEMOMETRY to MEASURE VELOCITY DISTRIBUTION INSIDE the SCREW CHANNEL of A TWIN-SCREW EXTRUDER

Velocity measurements inside the screw channels of a co-rotating, self-wiping twin-screw extruder (ZSK-30) have been carried out using the noninvasive technique of Laser Doppler Anemometry. A two-dimensional Argon-ion laser Doppler system was used to measure tangential and axial velocity components in one of the screws, away from the intermeshing zone. Heavy corn syrup with naturally occurring particles was used in the extrusion experiments. the measured tangential velocity distribution agreed with the expected distribution for a Newtonian fluid. Measurements also indicated that near the screw root and away from the flights, shear rates were substantially lower as compared to the shear rates near the barrel. the results indicated that this technique is suitable for making velocity measurements in a twin-screw extruder using model fluids.     

Velocity and temperature field characteristics of water and air during natural convection heating in cans

Presence of headspace during canning is required since an adequate amount allows forming vacuum during the process. Sealing technology may not totally eliminate all entrapped gases, and headspace might affect heat transfer. Not much attention has been given to solve this problem in computational studies, and cans, for example, were mostly assumed to be fully filled with product. Therefore, the objective of this study was to determine velocity and temperature evolution of water and air in cans during heating to evaluate the relevance of headspace in the transport mechanism. For this purpose, canned water samples with a certain headspace were used, and required governing continuity, energy, and momentum equations were solved using a finite volume approach coupled with a volume of fluid element model. Simulation results correlated well with experimental results validating faster heating effects of headspace rather than insulation effects as reported in the literature. The organized velocity motions along the air-water interface were also shown. Practical Application: Canning is a universal and economic method for processing of food products, and presence of adequate headspace is required to form vacuum during sealing of the cans. Since sealing technology may not totally eliminate the entrapped gases, mainly air, headspace might affect heating rates in cans. This study demonstrated the increased heating rates in the presence of headspace in contrast with some studies in the literature. By applying the effect of headspace, required processing time for thermally processed foods can be reduced leading to more rapid processes and lower energy consumptions.     

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

Velocity 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.     

Coating quality as affected by core particle segregation in fluidized bed processing

Fluidized bed coating is an important technique in the food powder industry, where often particles of a wide size distribution are dealt with. In this paper, glass beads of different particle size distribution were coated with sodium caseinate in a top-spray fluid bed unit. Positron Emission Particle Tracking (PEPT) was used to visualize and quantify the particle motion in the fluidized bed. Confocal Laser Scanning Microscopy combined with image analysis were used to investigate the effect of core particle size and its distribution on the thickness and quality of the coating. Particle size significantly affected the thickness and quality of the coating, due to differences in the corresponding fluidization patterns, as corroborated by PEPT observations. As the particle size distribution becomes narrower, segregation is less likely to occur. This results in a thicker coating which is, however, less uniform compared to when cores of a wider particle size distribution are spray coated.     

MEASUREMENT OF VELOCITY DISTRIBUTION IN THE BRABENDER FARINOGRAPH AS A MODEL MIXER, USING LASER-DOPPLER ANEMOMETRY

The nonintrusive technique of Laser-Doppler Anemometry was used to characterize flow velocity distribution in the Brabender Farinograph, which is often used to mix rheologically diverse food materials. A two-dimensional Argonion laser Doppler system was used to obtain local velocity data at selected locations in this complex mixer. Three components of velocity were obtained as a function of time and spatial location for model fluids with characteristic rheological properties. The results revealed that the velocity distribution in the Farinograph changes with the time-relative motion of the blades and with fluid rheology. Velocity values at locations close to the blades were much higher for the non-Newtonian fluids than for the Newtonian fluid studied. The prevalence of static zones at regions away from the blades was noticeable, particularly in the non-Newtonian fluids. These findings indicate that Laser Doppler Anemo metry is a valuable tool in the design and analysis of complex mixing processes.     

Influence of large solid spherical particles on the residence time distribution of the fluid in two-phase tubular flow

The effect of large solid particles on the residence time distribution of the fluid in two-phase flow was studied in a pilot scale continuous tubular system. The residence time distribution was characterized with four parameters: the normalized minimum and mean residence time, the normalized standard deviation and the skewness. The effect of particle concentration (1 and 3%) and particle diameter (diameters approximately 0.3 and 0.4 of the tube diameter) were analysed for fluids with different viscosities and at different flow rates. It was observed that the presence of particles, even at very low concentration, has a significant effect, increasing the mean residence time and decreasing the standardized dispersion. This effect was attributed to particles breaking the velocity profile of the fluid and causing some localized mixing due to their rotational velocity. Although the presence of particles affected the results, their concentration (or number) was not relevant, in the range tested, which is very low for particulate foods.     

DETERMINATION of RESIDENCE TIME DISTRIBUTION of NONSETTLING FOOD PARTICLES IN VISCOUS FOOD CARRIER FLUIDS USING HALL EFFECT SENSORS

Hall effect sensors were used to determine residence time distributions for diced carrot particles in 4% Colflo 67 carrier fluid. Four sensors at either end of a 3.2 m long 2 in. IDF tube viscometer allowed residence times to be measured for carrot particles incorporating a ceramic magnet. Mean particle residence times were greater than mean bulk residence time for 8 mm diced carrots, whereas 15 mm carrots showed trials in which particles travelled faster than the bulk fluid. Increasing concentration of 15 mm diced carrots from 3.25, 6.30, 9.16 to 11.85% w/w resulted in decreasing mean particle residence times from 17.6, 17.0, 15.9 to 14.3 s, with minimum residence times of 16.4, 16.2, 14.8 and 13.4 s, respectively. This sensing technique operates through stainless steel, providing applications for UHT foods containing particles. In addition, the technique was not affected by high particle concentration, and will operate for any distribution of particle size, shape or type.     

MATHEMATICAL MODELING of PARTICULATE TWO-PHASE FLOW IN A HELICAL PIPE

The design of helical tubes for continuous sterilization of low-acid foods (pH > 4.6) containing large discrete particles depends on the knowledge of the flow behavior of the suspended particles. the flow behavior of a carrier fluid with suspended solid spherical particles in a helical tube was numerically simulated through an iterative solution of the Navier-Stokes equations and the particle dynamic equations in three dimensions. the Lagrangian approach was used to predict the individual particle velocities and trajectories under solid-liquid two-phase flow situation. the results indicate that the residence time distribution of the particles in helical tube flow was narrower than in a conventional holding tube (consisting of two straight tubes connected with a 180° bend). Also, the average velocity of the particles was closer to the average velocity of the carrier fluid in the helical tubes. the secondary flow induced by the tube curvature greatly reduced the axial dispersion of the particles. There was a substantial particle influence on the fluid flow field. the pressure drop in helical tube was 18% higher for single-phase flow and 51% higher for two-phase flow than in conventional holding tube of equal length.     

THE LETHALITY-FOURIER NUMBER METHOD. HEATING RATE VARIATIONS AND LETHALITY CONFIDENCE INTERVALS FOR FORCED-CONVECTION HEATED FOODS IN CONTAINERS

Heat transfer and lethality in foods heated by force-convection in agitated, reel-type retorts (Steritort) were investigated. Equations describing the temperature profile at any position in the food were developed and experimentally verified for fluids and fluids with particles. For both types of food, the temperature profiles are a function of the heat transfer coefficient (h) at the inside surface of the container (the fluid container interface). For foods with particulate pieces, the temperature profiles, in both the fluid and the particle, are also a function of the heat transfer coefficient (h_p) at the fluid particle interface. Empirical models are presented correlating both heat transfer coefficients (h or h_p) with system parameters. Equations describing the temperature profile at the coldest point in the food (i.e. the point receiving least lethal treatment) were used in a modified form of the lethality-Fourier number equation (Lenz and Lund 1977a) to calculate lethality. This method of calculating lethality agreed with lethalities calculated by the Improved General Method. Confidence intervals for lethality for forced-convection heated products were determined using the Monte Carlo procedure described in Lenz and Lund (1977b). The 95% confidence intervals for the lethality ranged from 20–60% of the median value of the lethality depending on process conditions. The fluid/particle interface heat transfer coefficient (h_p) had a significant effect on the lethality distributions.     

Estimating Heat Transfer Coefficients in Liquid/Particulate Canned Foods using Only Liquid Temperature Data

A mathematical procedure was developed to estimate heat transfer coefficients for heated rotating cans of food comprised of a liquid and spherical particles. The method uses experimental data for only the fluid temperature, and heat transfer coefficients are obtained by comparing predicted with experimental fluid temperature data in the Laplace transform plane. Results obtained using the procedure were compared with experimentally obtained heat transfer coefficients from the literature for three cases: potato spheres in deionized water, Teflon spheres in deionized water, and aluminum spheres in silicone fluid. Agreement between predicted and experimental results depended upon the agreement of the experimental conditions and the assumptions associated with the model.     

粉体抽气除尘的流场分析与模型仿真

粉体下料过程中产生的粉尘会对环境、员工造成影响。为此在给袋式粉体充填设备上设置了粉体除尘罩。一方面对除尘罩内部的粉尘颗粒进行受力分析,得到了负压场会以气流曳力形式作用在颗粒上;通过分析流体雷诺数,确定气流速度是影响粉尘收集的本质因素。另一方面通过Fluent模拟颗粒轨迹和探究合理的抽气速度。发现气流风速过小时,不足以带动部分粒径的颗粒上升,反而增加了罩口粉尘的悬浮量;过大的风速则使大颗粒粉尘不易沉降。而当抽气速度为26-39m/s时,除尘效果最佳。     

CHARACTERIZATION of SINGLE PARTICLE TUBE-FLOW BEHAVIOR AT ELEVATED TEMPERATURES

The single particle flow behavior of food (carrot, parsnip and potato cubes) and Nylon spheres was investigated in simulated holding tubes at different temperatures under pressurized (110C) and nonpressurized (80 and 100C) flow conditions using different carrier fluids (water and 2, 3 and 4% starch suspensions). It was found that the particle to fluid velocity ratio (u_p/u) was a function of carrier fluid effective viscosity, temperature and flow rate, and particle relative size, density and shape. the maximum value of u_p/u ranged from 0.4–1.9 which increased almost linearly with temperature showing no specific deviations in the flow behavior of particles as conditions changed from the nonpressurized (>100C) to pressurized (110C) flow. In dimensionless form, the particle to fluid velocity ratio (mean and maximum) as well as particle Reynolds number were found be functions of generalized fluid Reynolds number, particle Archimedes number, particle to tube diameter ratio and particle sphericity.     

Statistical Variability of Thermal Process Lethality in Conduction Heating Food—Computerized Simulation

A computerized procedure was developed for estimating the variability of process lethality when there were variations in all independent, thermal process parameters of conduction heating food packages. This was accomplished through the combined application of a Monte Carlo procedure and a reliable mathematical method for thermal process evaluation. By examining published information, statistical variations in physical and reaction kinetic parameters were represented by gamma distributions and others by normal distributions. The coefficients of variations in sterilizing values were estimated from temperature data collected by processing 211 × 300 and 307 × 409 cans of spaghetti in tomato sauce (60 cans each). They agreed well with those computed by the developed computerized procedure.     

RESIDENCE TIME DISTRIBUTION OF FOOD AND SIMULATED PARTICLES IN A MODEL HORIZONTAL SWEPT SURFACE HEAT EXCHANGER1

The residence time distribution (RTD) of food and inert particles flowing in a swept surface heat exchanger at room temperature was investigated. The effects of selected experimental conditions on RTD include particle shape (cube, cylinder, and sphere), particle concentration (10, 20, and 30% w/v), particle type (potato, carrot, turkey, green peas, and polystyrene), fluid viscosity (water, 0.5% CMC, and 1.0% CMC), bulk flow rate (5.8 × 10^?4, 7.28 × 10^?4, and 8.71 × 10^?4 m3/s) and shaft rotational speed (30, 60, and 90 rpm). Although turkey cubes were denser than other food particles, they moved faster than potato and carrot particles. Mean particle normalized residence time (MNPRT) of food particles (green peas) were found to be significantly higher than that of simulated particles (polystyrene). Cylindrical particles stayed longer than cubic particles in water, but not significantly in CMC solution. MNPRT values were found to be dependent on viscosity. Increasing particle concentration (up to 30%) tended to decrease MNPRT. Both bulk flow rate and shaft rotational speed tend to decrease MNPRT, thus reducing the lethality achieved. The distribution of residence time curves were narrowed by decreasing particle concentration, bulk flow rate, and shaft rotational speed, and by using fluids with high viscosity. In general, particle velocity was found to be higher than the average bulk velocity. The fastest particle velocity was found to be 3.23 times the average bulk velocity while the ratio was 0.382 for the slowest.     

The effect of sulphur-containing fungicides on the corrosion of plain cans of fruit

The addition of 2–5 ppm of TMTDS (tetramethylthiuramdisulphide) or ziram markedly decreased the shelf life of canned pears, but 1 ppm of these materials had little effect. The addition of trace levels of sulphur or captan to canned pears appeared to increase the shelf life; addition of sodium sulphide appeared to have a negligible effect on shelf life. The shelf life of canned peaches, apricots, pineapple, apples, and citrate solution was also decreased by the addition of 5 ppm TMTDS or ziram. TMTDS and ziram caused pitting of the base steel of the tinplate with little attack on the tin coating. Pitting was not seen in cans containing added sulphur, sulphide, or captan and the rate of corrosion of the tin coating was less than in cans containing no added chemicals. Formation of passive films and reduction of the hydrogen overpotential were considered to be important factors influencing corrosion of cans containing the added agricultural chemicals.     

PARTICULATE HEAT TRANSFER to CANNED SNAP BEANS IN A STERITORT

Connective heat transfer coefficient at the fluid-particle interface was determined for cut green beans in 303 × 406 cans processed in an agitated retort (FMC Steritort) by measuring the temperature of an aluminum bean and that of the surrounding Newtonian fluid. Four retort reel velocities, two sizes of aluminum beans, and six particle/fluid ratios were employed. Several correlation forms were tested. A correlation was developed in terms of the j-factor, Reynolds number (Re), and shape factor with an R² of 0.785. the particle's shape was a critical factor in the heat transfer correlations. the fluid-particle heat transfer rate increased with an increase in rotational velocity of the retort.     

Comparison of heat transfer rates during thermal processing under end-over-end and axial modes of rotation

Thermal processing under conditions enabling container agitation are attractive to food processors since they provide higher rates of heat transfer and more uniform temperature due to product mixing. Two common types of agitations are end-over-end and axial agitation. While end-over-end agitation is common in batch retorts, axial agitation is predominant in continuous cookers in which the cans roll in a helical path along the retort shell. In this study, heat transfer rates to canned Newtonian fluids in end-over-end and axial modes of rotation were evaluated in the same retort. A single basket rotary retort was retrofitted to accommodate simultaneously both end-over-end and two types of axial modes of rotation, free axial and fixed axial. In the free axial rotation, the cans were allowed to rotate on their own axis as they rolled along the retort shell during the bottom third of the rotation, while in the fixed axial mode, the cans in the axial direction were held stationary in the rotating basket. The overall heat transfer coefficient U, heating rate index, lag factor and cook values associated with cans filled with Newtonian fluids were evaluated using a central composite rotatable design of experiments with glycerin at five concentration level (no glycerin to all glycerin), heating medium temperature (111.6-128.4 °C) and five basket-rotational speeds (4-20 rpm). Higher U, and lower heating rate index, lag factor and cook values were associated with higher rotational speed and retort temperatures for all modes of agitation. The heat transfer parameters associated with free axial rotation were significantly higher than in the end-over-end mode which was higher than in the fixed axial mode.     

Monitoring liquid displacement of model and industrial fluids in pipes by in-line ultrasonic rheometry

Liquid displacement of model and industrial fluids in pipes was successfully investigated using the in-line rheometer concept based on the combination of Ultrasound Doppler Velocity Profiling (UVP) and the pressure difference (PD) technique. The velocity profile development, related mixing zone and real-time changes in rheology were monitored when one fluid was displaced by another with different rheological properties. Four sets of model systems, all well described by the power law model and relevant for the food industry, were used to adequately cover different classes of fluids. In addition, two sets of industrially produced foods with similar flow characteristics were used to test the industrial applicability and limitations of the UVP–PD methodology developed. The results demonstrate that the in-line UVP–PD rheometer concept is an attractive multi-point method for real-time process monitoring and control of non-Newtonian fluids.     

EFFECT OF PRODUCT AND PROCESS VARIABLES IN THE FLOW OF SPHERICAL PARTICLES IN A CARRIER FLUID THROUGH STRAIGHT TUBES

Residence time of particles and fluid-to-particle heat transfer coefficient, two of the major unknowns in the aseptic processing of particulate fluid foods, are intimately related with the linear and rotational velocities of the solid particles. These two velocities were measured by videotaping the flow of individual spherical particles in transparent straight tubes. The effect of fluid velocity and viscosity, particle diameter and density and tube diameter and upward inclination was statistically evaluated using a replicated full factorial design at two levels. Particle linear velocities between around one third of the average fluid velocity and 15% higher were obtained. Rotational velocities were usually lower, but of the same order of magnitude. Particle density and particle diameter had a significant effect on the two particle velocities analysed. The effect of fluid velocity was