RESIDENCE TIME DISTRIBUTION (RTD) AND GOODNESS OF MIXING (GM) DURING CO2-INJECTION IN TWIN-SCREW EXTRUSION PART II: GM STUDIES

Experimental residence-time distribution (RTD) characteristics of an extrusion process may be used to estimate the extent of mixing experienced in the extruder. Using earlier theories of laminar mixing and striation thickness reduction, a new approach to estimate efficacy of mixing of two phases in the mixing zone of the extruder is proposed. Predicting the time required to achieve complete mixing and comparing it with the minimum time (plug flow time) that the extrudate spends in the mixing zone gives a ratio that can be used to evaluate the adequacy of extruder operating conditions for good mixing of gas/fluid injection and extrudate. Twin-screw extrusion of cornmeal with carbon dioxide injection were used to investigate the effect of CO₂ injection pressure and extruder screw speeds on mixing. Lower CO₂ injection pressures (25 bars) and lower screw speeds (150 rpm) were theoretically shown to enhance mixing. Scanning electron micrographs (SEMs) of experimental samples taken showed smaller and more uniform cell structures for those conditions which were predicted to favor better mixing.     

Thermal Conductivity of Liquid Foods by the Thermal Comparator Method

The thermal comparator is shown to be a rapid, accurate method for determining the thermal conductivity of liquid foods. Increasing solids content decreased thermal conductivity for apple juice concentrate, orange juice concentrate, and milk samples. Equations for predicting the thermal conductivity of apple juice, orange juice, and milk are shown to be specific for each commodity. A comparison of apple juice and orange juice suggests that qualitative attributes of the solids fraction as well as the quantity present are important in determining thermal conductivity.     

RHEOLOGICAL BEHAVIOR OF PROCESSED MUSTARD. I: EFFECT OF MILLING TREATMENT

The influence of milling level on the rheological behavior of processed mustard was investigated using three different particle sizes (slightly-coarse, standard, and fine) prepared according to a standard formulation and pilot-plant replication of the commercial methodology. All samples showed unimodal population-based particle size distributions which were described by the population-mode (d_pop). On a volume basis, all milling levels had well-defined bimodal distributions consisting of a smaller- and a larger-size group of particles described by their modes (d and D). The most important particle size index in relating milling level to rheological behavior was not the population mode (d_pop) but the larger-size volume mode (D) from the bimodal distribution. The coarser the milling, the higher the larger-size volume mode (D), and higher the values of: a) Apparent viscosity of nonmixed samples (η_p), b) Bingham yield stress (o⁻_o) and plastic apparent viscosity (σ_p), c) Shear stress constant (A) and coefficient of thixotropic breakdown (B) from the Weltman stress decay model, d) Yield stress (o⁻_o) and consistency index (K) from the Herschel-Bulkley model, and e) Storage (G¹) and loss (G^") moduli. As indicators of structure-texture relationship, the above rheological parameters can be profitably utilized to study the effect of manufacturing practices on long term storage stability of the product.     

OPTIMIZATION OF PARAMETERS FOR PACKAGING OF FRESH PEACHES IN POLYMERTIC FILMS1

Fresh produce packaged in polymeric films represents a dynamic respiration-permeation model. Control of equilibrium concentrations of respiratory gases is an important variable in extending the shelf life if critical vectors of quality deterioration and transport properties of packaging materials are known. Fresh peaches were stored at 5 × 1° C under various modified atmospheres for a period of up to thirty days. Samples were removed at ten day intervals for chemical and physical analyses of vulnerable quality factors. Parameters evaluated include soluble solids, titratable acidity, and texture and profile parameters such as hardness, elasticity, and cohesiveness. An internal atmosphere of 10 to 15% O₂ and 15 to 25% CO₂ inside the package provided better retention of quality attributes such as hardness and cohesiveness than any other treatments. Springiness did not provide any significant index of quality. Soluble solids decreased over time as did the amount of acid present in the fruit. Anaerobic fermentation did not occur under any storage conditions. Subsequent to establishing the optimal gaseous concentrations, respiration rates were determined as a function of time and gas composition. Permeability of polymeric films were determined using ASTM method. Analytical equations were employed to optimize packaging parameters or to estimate transient and equilibrium gas concentrations in polymeric packages. Equations were set up to estimate transient and steady state time values for oxygen and carbon dioxide concentrations inside a package. This approach is applicable in extending the storage quality of other fresh produce.     

RHEOLOGICAL CHARACTERIZATION OF BUTTER USING LUBRICATED SQUEEZING FLOW

Rheological characteristics of butters made from anhydrous milk fat (AMF) and supercritically fractionated high melting triglyceride (HMT) fraction were compared. An Instron Universal Testing Machine was used to compress the samples between two lubricated plates at constant deformation rate (0.5 mm/min). Stress growth coefficients, computed from the ratios of stresses and biaxial strain rates, were plotted as function of time. HMT butter had higher stress growth coefficients than AMF butter, indicating its greater degree of firmness. Stress growth coefficients were negatively correlated to temperature due to lower crystallinity of fat at higher temperatures and increased on storage due to formation of weak van der Waal forces of attraction between crystalline fat chains.     

A Simple Nondestructive Method for Determination of Air Content in Polymeric Packages

A new nondestructive method of estimating the residual air volume in food packages was developed and compared with the standard destructive technique. The nondestructive method involved enclosing the test package in a nonflexing, plexiglass container and placing this container into a vacuum chamber. Monitoring changes in volume at two vacuum levels and using Boyle's Law, the air content of more than fifty polymeric packages was computed. The difference between the destructive and nondestructive volumes was found to be statistically insignificant.     

Determination of Cross-link Density in Egg White Gels from Stress Relaxation Data

The concentration of permanent cross-links in egg white gels was determined using stress relaxation experiments and equations derived from the theory of rubber elasticity. Cross-link densities of 1.2–1.3 covalent cross-links/molecule were observed. The concentration of covalent cross-links, most likely disulfide bonds, was only slightly higher for gels formed in water as compared to those formed in 0.6M NaCl-0.2M phosphate buffer at pH 6.0. These results differ from the 3—4 cross-links/molecule previously reported for ovalbumin gels in 0.6M urea.     

Water Activity Calculation by Direct Measurement of Vapor Pressure

An improved method for calculation of water activity from vapor pressure manometric data, which takes into account the volume expansion of nonadsorbing gases, the initial pressure in the desiccant flask and the pressure exerted by the desiccant, was developed. The water activity of pure water was measured over a range of nonadsorbing gas pressures and initial pressures in the desiccant flask. Standard deviations in a_w of pure water calculated using no corrections, corrections for volume expansion only, and full corrections were 0.221, 0.011, and 0.005, respectively. Correction only for volume expansion may under some conditions result in larger errors than when no corrections are made at all. Based on the improved equation, an experimental modification which reduces loss of water from samples during measurement was proposed.     

Anhydrous Milk Fat Fractionation with Continuous Countercurrent Supercritical Carbon Dioxide

A continuous pilot-scale supercritical carbon dioxide system was designed and built for separation and fractionation up to 400 g/hr of anhydrous milk fat. This fat was separated into five fractions (S1-S5) in the pressure range 24.1–3.4 MPa at 40–75°C. The solvent/feed ratio was 62 with extraction yield 78 wt%. The short-chain (C4-C8) and medium-chain (C10-C12) fatty acids increased from S1S5. The long-chain and unsaturated fatty acids (C14-C18) decreased gradually from S1-S5. The triglycerides followed the same trend as fatty acids. Cholestrol content decreased by 51% and β-carotene concentration increased by 145% in the raffinate (S1).