Holding Tube Research Articles

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.

Flow Characteristics and Holding Time Calculations of Ice Cream Mixes in HTST Holding Tubes

In order to determine the potential for development of laminar flow and consequential underholding in the holding tubes of HTST pasteurizers, a study on the relationship between ice cream mix viscosity and shear rate at 80°C has been conducted. Typical shear rates at the wall were calculated for HTST holding tubes of standard industry sizes and flow rates. Shear rates in the holding tube were found to vary from 50 to 180 s−1, depending on the conditions. Viscosity of ice cream mixes as a function of shear rate, stabilizer type, and stabilizer concentration were measured. Ice cream mix was found to be non-Newtonian and pseudoplastic. Viscosities ranged from 8.7 cP in an unstabilized mix at high shear rate (relative to the inside of the holding tube) to 103 cP for 0.25% carboxymethyl cellulose at low shear rate. Generalized Reynolds numbers inside the holding tubes varied from 100 to 1700, indicating a strong potential for the development of laminar flow. The apparent viscosities required to result in a minimum generalized Reynolds number of 2100 are very near to or less than the actual viscosities of stabilized ice cream mixes, and thus the potential for a laminar flow pattern within the holding tube needs to be addressed in determining holding tube lengths for a required holding time.

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.

MATHEMATICAL ANALYSIS of CRITICAL PARAMETERS IN ASEPTIC PARTICULATE PROCESSING SYSTEMS

ABSTRACTA sensitivity analysis of a one‐dimensional heat transfer model for a heat‐hold‐cool aseptic system of food products containing particulates was completed for eighteen different design, product, and process control parameters. All of the parameters, except the direct effect of the fluid specific heat, were determined to be statistically significant in influencing the accumulated lethality of the processes. the most influential parameters were surface heat transfer coefficient in the hold tube at the fluid‐particle interface, exit temperature of the fluid from the heater, initial temperature, holding tube size, and holding tube‐U value. the process/product‐related parameters that are significant in the establishement of a process are: particle diameter, surface heat transfer coefficient in the hold tube at the fluid‐particle interface, thermal properties of the fluid and particulates, maximum heater velocity ratio, maximum hold tube velocity ratio, surface heat transfer coefficient in the heater at the fluid‐particle interface, product flow rate, and particle to fluid ratio.

Heat Transfer and Simulated Sterilization of Particulate Solids in a Continuously Flowing System

ABSTRACTThe heat transfer coefficients (h) between a cube and water were 239 and 303 W/m2K at 0 and 0.86 cm/s relative velocity. In 35% sucrose and 0 cm/sec, h was 146 W/m2K. A model for heat lethality in cubes during continuous sterilization which used finite difference equations for heat conduction and above values of h, revealed that hold times for 6D inactivation of PA 3679 (F0= 7.2 min) in an isothermal fluid at 132.2°C were 1.78 and 5.2 min for 1 and 2 cm cubes when h was 303 W/m2K and 1.23 and 4.13 min when h was 2500 or higher. When h was 239 W/m2K, hold times were 2.36 and 6.02 min for 1 and 2 cm cubes. When fluid entered the hold tube at 132.2°C and particle to fluid ratio was 0.56, fluid‐particle heat exchange increased hold time to 12.2 and 119 min for 1 and 2 cm cubes.

CLEANING KINETICS MODELING OF HOLDING TUBES FOULED DURING MILK PASTEURIZATION1

ABSTRACTIn the present study, 25 cm tubes fouled during pilot plant scale pasteurization of raw whole milk were cleaned with sodium hydroxide. The pasteurization conditions resembled those of the dairy industry. An optical sensor sensitive to the turbidity of suspended deposits removed by the cleaning solution was placed successively at the outlet of each fouled tube. Use of this sensor combined with a nonrecycled cleaning solution process enabled us to measure the cleaning rate on line. In these conditions it was observed that, at zero time, the cleaning rate was null. Based on the above observations a kinetic model of cleaning was established. It consists of two steps in series. The adhesive deposit passes through an intermediate state before being washed away by the cleaning solution. Each step is a first order reaction related to the mass of the adhesive deposit and the mass of the intermediate compound, respectively.Among the many physico‐chemical parameters affecting the cleaning process, three major ones were studied; temperature, concentration and circulation rate of the soda solution. An empirical model of the rate constant was set up which takes into account those three parameters and the initial weight of deposit to be cleaned. A 1% increase in the amount of NaOH caused the same increase in the constant rate as 8°C or 0.5 m/s.

Temperature Histories in a UHT Indirect Heat Exchanger

An indirect UHT processing system was modeled by a computer program to predict the temperature profile of a mass element of fluid as it progressed through heating, hold and cooling units. The computer program consisted of successive calculations of the temperature at the exit of numerous increments in each unit based on the overall heat transfer coefficient determined for the total length of the unit. The validity of the computer program was determined by correlation of computer results with experimental determinations of sucrose inversion. The percentage of total inversion of sucrose occurring in the heater was 36%, indicating the error associated with the assumption of chemical reactions based exclusively on hold tube temperature, residence time and laminar flow pattern.

Temperature Variations in the Holding Tube During Ultra High-Temperature Pasteurization by Steam Injection

Temperature variations after steam injection were monitored at 10 positions along a 61-cm holding tube with highspeed thermocouples (thermometric lag of approximately 40 milliseconds). Three steam injectors were tested with water (tap and deaerated) and milk at from 880 to 1,330 liters per hour, temperature increases by steam injection from 22 to 33C, holding temperatures from 88 to 110C, and holding pressures from 1.2 to 3.0atm (abs.). Skimmilk (9% and 42% solids), chocolate milk, cream, and ice cream mix were also studied under selected processing conditions. Temperatures varied greatly (±15C) with water, milk, and skimmilk (42% solids) under some conditions; however, temperature variations at the injector exits (5cm downstream) were eliminated when the following were imposed: a) all injector ports were orificed to isolate the product and steam flows from pressure fluctuations in the injection chamber, b) the process was as free as possible of noncondensable gases, and c) the holding pressure was at least 0.2atm above the product saturation vapor pressure. Accordingly, temperature variations in the holding tube can be eliminated by adhering to these operating conditions.

Residence Time of Milk Products in Holding Tubes of High-Temperature Short-Time Pasteurizers

Holding tubes are sized by performing the salt conductivity test on water flowing at pasteurization temperature. Because dairy products have a higher viscosity than water, the fastest product particles could be held for shorter times than predicted by the salt test. A study was done to determine the error attributable to this practice. A high-temperature short-time pasteurizer unit was operated at 1,130 liters/hour at pasteurization temperature, and residence times were determined by measuring the time required for an injection of radio-active iodine to traverse the holding tube. Residence times of milk products were 21.9 and 23.5sec, respectively, for 16.5 and 10% milk-fat ice cream mix, 22.7sec for cream (40% milk fat), and 17.2sec for condensed skimmilk (40% solids) flowing in a tube sized for 26sec by salt injection; and 15.8 and 15.5sec, respectively, for milk and chocolate milk flowing in a 16-sec tube. Sizing holding tubes by the salt test is satisfactory for milk and chocolate milk; however, to ensure proper holding times for ice cream mix, cream, and condensed skimmilk, tubes should be sized by calculating fastest particle velocity of product in laminar flow.