MODELLING of TEMPERATURE HISTORIES DURING VENTING of STILL RETORTS

Experimental tests were carried on a pilot retort to gather information on the temperature evolution inside batch retorts during the venting stage. to fit the coming-up curves the equation for the response of an over-damped second-order system to a step input was proposed. the parameters that characterize this equation were related to steam flow, initial temperature and retort load. Results allow the prediction of coming-up curves for different operating conditions.     

MODELING OF TEMPERATURE PROFILES UNDER CONTINUOUS TUBE-FLOW MICROWAVE AND STEAM HEATING CONDITIONS

Mathematical models were developed based on perfectly mixed flow (PMF), piston flow with heat diffusion (PFHD) and laminar flow (LF) approaches to predict liquid temperature history under continuous tube‐flow microwave and steam heating conditions. Two helical glass coils placed inside domestic microwave ovens (one coil in each of the two 700 W capacity ovens) or in a steam cabinet were used for heating and a spiral condenser at the exit was used for cooling. Transient and steady state mean temperatures of the fluid were experimentally measured at the exit and were compared with predictions from the mathematical models for both systems. The residence time, velocity distribution as well as temperature profiles, along the radius and the length of the tubes, were computed using the models. The PFHD and the LF models better described temperature profiles during the initial transient period, while the PMF model shovsed a better agreement with experimental data during steady staie conditions. The occurrence of secondary turbulence in the helical coil (associated with high Dean numbers) was believed to be responsible for reducing the radial temperature gradients and achieving close to “perfectly mixed piston flow” situation. A relatively larger temperature gradient across the radius was observed under microwave heating conditions than under steam heating conditions. The time‐temperature effects were integrated to predict the lethality at selected temperatures and flow rates for both continuous‐flow thermal processing.     

MODELING the POWER CONSUMPTION DURING SHEETING of DOUGHS FOR TRADITIONAL INDIAN FOODS

The model given by Levine has been extended to account for the effects of moisture, salt and fat through appropriate modifications based on theory and computational experience. Appropriate modifications to consistency index and flow behavior index as a function of moisture, salt and fat are given. the thickness of sheeted dough has been modeled as a function of gap, reduction ratio and the flow behavior index which constitutes an important contribution. the model so modified was fitted to experimental data on power consumption in sheeting dough prepared from two flour types, viz., whole wheat flour and resultant atta, respectively, at five combinations of gap and reduction ratios and three levels each of moisture, salt and fat (conforming to actual use ranges) using Nelder-Mead algorithm for nonlinear optimization. These models provided 12 constants for each flour type which gave fundamental information on the dough properties for each flour type. Three-dimensional graphics have been used to interpret the model. These models appear to be of much practical use.     

TIME-VARIABLE RETORT TEMPERATURE PROFILES FOR CYLINDRICAL CANS: BATCH PROCESS TIME,ENERGY CONSUMPTION,and QUALITY RETENTION MODEL1

The batch retort model developed uses a heat transfer equation for heat conduction in cylindrical cans, first order kinetics for microbial inactivation, first order kinetics for quality losses and a transient energy balance to estimate steam consumption. For a given retort, lethality process and quality retention, the transient energy balance equation in the model allowed the identification of feasible time-temperature profiles reducing energy consumption, total process time or both. In the examples analyzed and depending upon product specifications, time-variable retort temperatures reduced process time by 18–55 min. These examples suggested that a change from constant to time-variable retort temperatures could increase canning capacity by 20–50%.     

A MODEL FOR TEMPERATURE AND MOISTURE DISTRIBUTION DURING CONTINUOUS MICROWAVE DRYING*

A heat and mass transfer model of continuous drying of farmer stock (in‐shell, uncured) peanuts (Arachis hypogaea L.) in a planar microwave applicator was developed and investigated. Transport phenomena equations previously developed for batch‐type microwave drying were successfully adapted to account for the spatial variation of the electric field inside the applicator. The theoretical equations developed, together with experimental methods, were used to determine the effect of microwave power level and dielectric properties on the temperature profiles and reduction in peanuts’ moisture content (mc). The temperature profiles from the solution of these equations matched the experimental ones determined using fiber optic temperature probes inserted into drying peanut pods. An exact theoretical determination of mc reduction during microwave drying was not possible due to the dependence of dielectric properties on mc. The surface temperature distribution of the peanut bed measured using infrared pyrometry was well correlated with internal temperature profiles.     

METHYL BROMIDE, TIME and TEMPERATURE of EXPOSURE ON APPLE QUALITY

ABSTRACT. The use of methyl bromide (MeBr) as a fumigant to control codling moth in 'Delicious'apples resulted in a loss of firmness, internal color and therefore a reduction in the amount of acceptable fruit. Time and temperature of MeBr exposure were directly related to firmness and internal color loss. As the exposure time was increased beyond 2 h and exposure temperature above 6°C firmness and internal color loss were accelerated. an 8-day ambient storage period exacerbated firmness and internal color loss as time and temperature of MeBr exposure were increased. A fumigation regime of 56 g MeBr/m³ at 6°C for 2 h resulted in acceptable fruit during a 60 day refrigerated storage period. Increased exposure times or temperatures beyond 56 g MeBr/m³ at 6°C for 2 h resulted in unacceptable firmness and internal color loss, coupled with a major loss in acceptable fruit.     

MODELING THE BEHAVIOR OF LISTERIA MONOCYTOGENES IN pH-MODIFIED CHICKEN SALAD DURING COLD STORAGE AND TEMPERATURE ABUSE CONDITIONS

Listeria monocytogenes grows at refrigeration temperatures (5C or below) and tolerates various environmental stressors. The Food and Drug Administration specifies a zero tolerance for this pathogen in certain ready‐to‐eat processed foods. Modeling its dynamic behavior to fluctuation in temperature at various pH levels is critical to the safety of food. This study presents linear and nonlinear models to predict the behavior of L. monocytogenes in pH‐modified chicken salad at various cold storage and temperature abuse conditions. A linear model of the kinetics accounting for simple and interactive effects of storage time, temperature and pH was developed. Predictions of the linear model were inconsistent with laboratory observations. The limitations of the linear model were reflected in the poor correlation of model predictions to the observed values (r² = 0.58). A proposed nonlinear model was therefore used to model the observed data. The four model parameters (N(0), C_c(0), k_max and N_res ) were optimized for each of the nine treatments. Correlation coefficient (r²) values ranged from 0.70 (pH 5.2, 7.2C) to 0.99 (pH 4.0, 21.1C), indicating an improved accuracy. Developing a functional and validated microbial predictive model for chicken salad requires further analyses and collection of data at additional pH and temperature values to determine a single set of parameter values that would represent the microbial behavior at the full range of pH and temperatures observed under storage conditions. Future experiments should address the adaptive nature of L. monocytogenes, as the response to environmental stressors affects the survival of the organism in food systems.     

SCREW CONFIGURATION EFFECTS ON RESIDENCE TIME DISTRIBUTION AND MIXING IN TWIN-SCREW EXTRUDERS DURING EXTRUSION OF RICE FLOUR

Screw configuration effects on residence time distribution and mixing were investigated during twin-screw extrusion of rice flour. The type, length, and position of mixing elements, and spacing between two elements significantly affected residence time distribution and mixing. Mean residence time (t_mean) and extent of mixing were lower for screw profiles with KB than those with RSE. A systematic increase in t_mean was observed as the mixing elements were moved farther away from the die, with longer elements, and with increased spacing between two elements.     

RHEOLOGICAL MODELING of POTATO FLOUR DURING EXTRUSION COOKING

A generalized model for predicting the effects of shear rate, temperature, moisture content, time-temperature history and strain history on viscosity has been evaluated for extruded potato flour doughs. an Instron Capillary Rheometer and a 50 mm Baker Perkins co-rotating twin screw extruder were used to evaluate all effects incorporated in the model, except strain history. the power law model was used to describe shear rate effects in the range 10–10000 s^?1. the generalized model fit observed data for temperatures of 25–95°C and moisture contents ranging from 22 to 50%, wet basis. Since potato flour by its manufacturing process is pregelatinized, it was unnecessary to evaluate the effects of time-temperature history. Strain history was found to have an insignificant influence on the viscosity.     

EFFECT OF GERMINATION TIME,TEMPERATURE AND MOISTURE ON MALTING OF SORGHUM

The effect of germination conditions on sorghum malt quality and malting loss was studied by germinating sorghum for different periods of time up to 6 days over a range of temperatures (24 to approximately 36°C) and moisture conditions. The moisture conditions varied from that sufficient to maintain green malt weight to that where surface moisture remained on the malt throughout germination. Germination time, temperature, moisture and the three possible pairwise interactions all had a highly significant effect on malt diastatic power, free α-amino nitrogen and extract. Malting loss was highly significantly affected by germination time and moisture and their pair-wise interaction. However, over the range examined, germination temperature had no significant effect on malting loss. In general diastatic power, free α-amino nitrogen, extract, and malting loss all increased with germination time. Germination temperatures of 24° and 28°C were both equally good for the development of diastatic power, free α-amino nitrogen and extract but higher temperatures were progressively worse. Distatic power, free α-amino nitrogen, extract and malting loss were, in general, all increased by high moisture during germination. However, high moisture and a negative effect on diastatic power towards the end of the germination period.     

MATHEMATICAL MODELLING of MOISTURE, VOLUME and TEMPERATURE CHANGES DURING DRYING of PRETREATED APRICOTS

Simultaneous heat and mass transport in dehydration of apricots is presented. the moisture variation of apricots, pretreated with various combinations of sodium-meta-bisulphite and ethyl oleate solutions, was analyzed by Exponential and Page equations. Use of ethyl oleate in addition to sodium-meta—bisulphite as a pretreatment solution was observed to decrease drying times of apricots. the system of differential equations determining the unsteady-state temperature distribution was solved. Thermophysical properties were taken to be moisture dependent. the variation of volume upon drying (shrinkage) was introduced into the solution by taking space intervals to be variable in the apricot side while they were taken to be constants in the stone side. the dominant mass transfer mechanism was diffusion while diffusivity of water changed depending upon the pretreatment solution. Comparison between predicted and experimental data shows good agreement for both moisture and temperature variation of the apricots throughout the drying process.