EFFECTIVE HEAT FLUX MODEL FOR VACUUM PRECOOLING

An effective heat flux model for vacuum cooling is developed in this article. It has great advantages over traditional models in numerical procedure. In this model,λ_eis used to represent the combined effect of thermal conduction and inner vapor evaporation. Therefore, only two differential equations are needed to describe the cooling process. We also provide experiments of vacuum cooling for green grocery and cucumbers under different pressures. The good agreement between simulation and experimental data validates this model, which is also helpful in understanding the mechanism of the precooling process.     

THERMAL CONDUCTIVITY of CONCENTRATED WHOLE MILK

The information of thermal conductivity of milk is required for the design of heat exchanger equipment in the dairy processing. Thermal conductivity of whole milk was experimentally determined by a steady-state, parallel disk, relative method at various concentrations (from 37 to 72.4% total solids) and temperature range between 40 to 90°C. This thermal property of milk increased with rise in temperature and decreased with increase in total solids content and its values varied from 0.278 to 0.491 W/m K. an expression has been proposed for derivation of milk thermal conductivity from temperature and total solids content.     

CALCULATION of INITIAL FREEZING POINT,EFFECTIVE MOLECULAR WEIGHT and UNFREEZABLE WATER of FOOD MATERIALS FROM COMPOSITION and THERMAL CONDUCTIVITY DATA1

A procedure was developed for simultaneous determination of the effective molecular weight, unfreezable water, and initial freezing point of foods based on composition and thermal conductivity data. Freezing properties were determined by trial and error and optimized by minimizing the difference between calculated and reported values of thermal conductivity. the procedure involved determination of the ice fraction at several temperature levels. Corresponding thermal conductivity values at each temperature level were then calculated. Results showed that calculated values of effective molecular weight, unfreezable water, and initial freezing point for various types of meat and fish are comparable to those published. the parallel-perpendicular model was found an excellent predictor of thermal conductivity of frozen meat and fish with muscle fibers oriented toward the direction of the heat flow.     

THERMAL CONDUCTIVITY of INDIAN UNLEAVENED FLAT BREAD (CHAPATI) AT VARIOUS STAGES of BAKING

A steady state technique was used to measure thermal conductivity of whole wheat dough (rolled chapati) and baked chapati (round disc prepared mostly from whole wheat flour dough) at various stages of cooking. Based on the experimental data of chapati at various stages of cooking, a linear equation K = -0.5677 + 0.01396 M + 0.005131 T is proposed for conductivity of wheat flour dough and baked chapati at moisture levels (M): 35 < M < 50 and temperature ranges 35 < T < 60 A separate equation K = 0.3204 + 0.0091 M -0.00805 T predicts thermal conductivity of rolled chapati and baked chapati at T > 60C.     

COMPARISON of TWO OPERATING METHODS of A PLATE HEAT EXCHANGER UNDER CONSTANT HEAT FLUX CONDITION and THEIR EFFECT ON the TEMPERATURE PROFILE DURING MILK FOULING

Two methods of operating a plate heat exchanger under constant heat flux were studied: Method A) control by increasing the flow rate of the heating medium; and Method B) control by increasing the inlet temperature of the heating medium. the cross-sectional temperature variation in the plate used in this study was smaller than that of an intermating-type plate. A change in the axial temperature profile was discovered as a result of the uneven formation of milk deposit along the channel. the temperature changes of both the milk and the solid-milk interface (stainless steel-milk or deposit-milk) during fouling were larger when the effects of fouling were overcome by increased flow rate of heating medium than when the effects of fouling were overcome by increased temperature of the heating medium. A quadratic empirical model underpredicted the temperatures in the lightly fouled region and overpredicted the temperatures in the heavily fouled region.     

HEAT TREATMENT EFFECT ON TEXTURE CHANGES and THERMAL DENATURATION of PROTEINS IN BEEF MUSCLE

Meat tenderness is one of the most important quality criteria when evaluating results of cooking conditions. Changes in tenderness and weight losses of heat-treated meat (semitendinosus muscle) for different time-temperature combinations were analyzed; the relationship between protein denaturation and textural changes was studied.
Heat treatments of meat samples (1.5 cm in diameter, 2 cm long) were performed in a thermostatic bath in the 60–90C range. Maximum heating times were 180 min. Meat hardness was determined by Warner-Bratzler measurements using an Instron testing machine. Protein denaturation was followed by Differential Scanning Calorimetry (DSC) analyzing peaks for myosin (I and II), sarcoplasmatic proteins and collagen (II) and actin (III).
Between 60 and 64C, hardness decreased with cooking time until reaching the lowest asymptotic values. This was related to protein denaturation of peak I and II. Between 66 and 68C, hardness decreased at first but increased later due to actin denaturation; at the temperatures 81 and 90C no modifications were observed and hardness remained at its higher values.
The kinetic model proposed fit the experimental results satisfactorily. Activation energies of tenderizing and toughening processes are similar to those of protein denaturation of peak II and III. Weight losses due to cooking were also modelled increasing through the entire temperature range.     

THERMAL DEATH PARAMETERS of ORANGE JUICE and EFFECT of MINIMAL HEAT TREATMENT and CARBON DIOXIDE ON SHELF-LIFE

Freshly squeezed, refrigerated orange juice has a relatively short shelf-life, which could be extended by minimal processing. D and z values of orange juice microflora were obtained using the capillary method, as well as a plate heat exchanger. the effect of low levels of CO₂ on the shelf-life of the juice was also evaluated. the D₆₀ value of typical orange juice flora was about 5 s and the z was 4–5C. A combination of minimal heat treatment (15 s at 60C) and 6 mM CO₂ extended the storage life of orange juice to 35 days at 4C.
Carbon dioxide flushed into a 10% headspace of 350 ml jars resulted in 6 mM dissolved CO₂ in the juice at 4C. This level of CO₂ extended the shelf-life of unpasteurized juice to 25 days at 4C and 10 days at 10C, as compared to 17 and 5 days without CO₂, respectively. No significant difference in organoleptic evaluations was found between minimally heat treated juices with or without CO₂ and fresh untreated juices without CO₂ during the first week of storage.     

HEAT TRANSFER DURING EVAPORATION of MILK to HIGH SOLIDS IN THIN FILM SCRAPED SURFACE HEAT EXCHANGER

Thermal performance of thin film scraped surface heat exchanger was evaluated for concentration of milk to high solids with process variables such as mass flow rate, steam condensing, temperature, etc. Appropriate dimensionless groups were formulated and fitted in Cobb-Douglas model to obtain a correlation. This relationship which is in the form of a Nusselt equation will be useful in predicting the scraped film coefficient during milk concentration to high solids. the effect of process variables on scraped film coefficient were discussed.     

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.     

HEAT TRANSFER to and TRANSPORT PROPERTIES of WHEAT GLUTEN IN A TUBULAR REACTOR

The design and modeling of a tubular reactor, similar to an extrusion rheometer, which accounts for reacting flow is described. A numerical study (finite differences) of heat transfer to a flowing and reacting biopolymer melt (wheat gluten), similar to extrusion processing, is described and compared to experimental data. the cylindrical wall is nonisothermal in the axial direction, but is adequately characterized by a third-order polynomial determined experimentally. Convective and viscous dissipation terms are included in the energy equation. the rheological behavior of the melt is described by a power law model, . The consistency coefficient pre-exponential, m₀, is 530,000–2,100,000 Pa-secⁿ and the power law exponent or flow index, n, is 0.38–0.65 for a 25–30% moisture content range and a 110–209 C temperature range. the flow index decreases with increasing conversion due to polymerization reactions occurring at elevated temperatures. Flow curves, bulk temperatures, and temperature profiles are presented for a variety of cylinder wall temperatures and biopolymer moisture contents.     

RELATION of BACTERIAL DESTRUCTION to CHEMICAL MARKER FORMATION DURING PROCESSING BY THERMAL PULSES1

This paper presents a formulation in dimensionless terms of the equations pertaining to first-order processes that follow Arrhenius kinetics, and applies them to problems in food thermoprocessing. the need for it arose from attempts to understand the relation between destruction of pathogenic microorganisms and formation of chemical markers in the food. During processing food is subjected to a thermal pulse that affects the markers and microorganisms. the dimensionless formulation makes it convenient to obtain equations expressing the relation between the decadic reduction in microbial population and the rise in marker concentration for temperature-time profiles of forms hitherto not conveniently calculable. Measurements of chemical markers can be used with these formulas to predict destruction of microorganisms. Examples are given of calculations demonstrating the feasibility of this approach when a single marker is monitored.     

TEXTURAL CHANGES IN VEGETABLES DURING THERMAL PROCESSING. I. A DESCRIPTIVE METHOD to SEGREGATE EFFECTS of PROCESS TREATMENTS

Criteria for assessing the effect of processing treatments on the texture of processed foods were developed. Five parameters were defined, relating the hardening or softening of a food material and allowing for the comparison between different processing procedures. In order to determine the usefulness of this method, the parameters were applied to experimental data reported in literature for thermal processing of fruits and vegetables. Three parameters were defined for identifying the effect of a given preprocessing treatment on the raw material texture, the effect of the preprocessing treatment on the sensitivity of the vegetable tissue to the thermal processing and the overall effect. the two remaining parameters were used to describe additional information regarding the process kinetics, that is, to assess the effect of time and temperature on the rate of softening/hardening.     

LIQUID-TO-PARTICLE HEAT TRANSFER DURING CONTINUOUS TUBE FLOW: INFLUENCE of FLOW RATE and PARTICLE to TUBE DIAMETER RATIO1

Liquid-to-particle convective heat transfer coefficients were measured during continuous flow through tubes, using an experimental technique in which a thermocouple was moved at the same speed as the particle. Water was used as the carrier fluid and transducer particles were made hollow to approximate densities of real food particles. Results from over 250 experimental runs over a fluid Reynolds number range from 7300 to 43600, showed that the convective coefficient was increased significantly with increasing fluid flow rate and particle to tube diameter ratio. Convective coefficient values ranged from 688 to 3005 w/m²⁰C depending on the experimental conditions. Dimensionless correlations obtained between the Nusselt number, particle Reynolds number, particle to tube diameter ratio, and the particle Froude number yielded R² values ranging from 0.82 to 0.92 depending on the complexity of the relation.     

NONLINEAR CONSTRAINED OPTIMIZATION of THERMAL PROCESSING II. VARIABLE PROCESS TEMPERATURE PROFILES to REDUCE PROCESS TIME and to IMPROVE NUTRIENT RETENTION IN SPHERICAL and FINITE CYLINDRICAL GEOMETRIES

Conventional methods for thermal processing of foods use constant processing temperature profiles (CPTPs) for a prescribed processing time, which is based on achieving a required microbial lethality to comply with public health standards. This also results in degradation of nutrients and quality factors. the variable process temperature profiles (VPTPs) obtained by using optimization methods can reduce quality losses and/or processing time compared to CPTPs. the objective of this research was to evaluate VPTPs using the Complex Method to reduce the processing time and/or improve quality retention for a specified level of lethality in thermal processing of conduction heated foods. the VPTPs were obtained for volume average retention of thiamine considering different sizes of spheres (small and large) and finite cylinders (small and large), and the thiamine retention and processing time results were compared with a conventional method (processing at 121.1C) for a specified lethality level. the use of VPTPs resulted in a 37 and 10% decrease in processing times in spherical and 40 % and 6 % for finite cylindrical shapes, for the same objective function value and specified lethality compared to the CPTP process. For the same processing time, the improvements in thiamine destruction were 3.7 and 2 % for spheres, and 3.9 and 2.2% for finite cylinders.