Syed S. H. Rizvi

A review of effects of carbon dioxide on microbial growth and food quality

Carbon dioxide is effective for extending the shelf-life of perishable foods by retarding bacterial growth. The overall effect of carbon dioxide is to increase both the lag phase and the generation time of spoilage microorganisms; however, the specific mechanism for the bacteriostatic effect is not known. Displacement of oxygen and intracellular acidification were possible mechanisms that were proposed, then discounted, by early researchers. Rapid cellular penetration and alteration of cell permeability characteristics have also been reported, but their relation to the overall mechanism is not clear. Several researchers have proposed that carbon dioxide may first be solubilized into the liquid phase of the treated tissue to form carbonic acid (H2CO3), and investigations by the authors tend to confirm this step, as well as to indicate the possible direct use of carbonic acid for retarding bacterial spoilage. Most recently, a metabolic mechanism has been studied by a number of researchers whereby carbon dioxide in the cell has negative effects on various enzymatic and biochemical pathways. The combined effect of these metabolic interferences are thought to constitute a stress on the system, and result in a slowing of the growth rate. The degree to which carbon dioxide is effective generally increases with concentration, but high levels raise the possibility of establishing conditions where pathogenic organisms such as Clostridium botulinum may survive. It is thought that such risks can be minimized with proper sanitation and temperature control, and that the commercial development of food packaging systems employing carbon dioxide will increase in the coming years.

Solubilities of fatty acids, fatty acid esters, triglycerides, and fats and oils in supercritical carbon dioxide

Using supercritical carbon dioxide (SC-CO2) as the solvent, the solubility of methyl esters, ethyl esters, fatty acids, triglycerides, and fats and oils were studied over a range of temperature and pressures. Equilibrium data from the literature were correlated using the Peng-Robinson equation of state. With the van der Waals (VDW) and Panagiotopoulos and Reid (PR) mixing rules. The data were also correlated using statistical methods and the solubilities predicted by various models were compared with experimental solubilities at the temperature and pressure conditions studied. The equation of state method provided good agreement of theory with experiment for the solubilities of fatty acids and their esters, but less so for the triglycerides, fats and oils. The statistical methods on the other hand, not only predicted the solubilities of fatty acids and fatty acid esters well, but also gave good agreement with the solubilities of triglycerides, fats and oils. Chain length, degree of unsaturation, and functional groups in compounds are known to affect the vapor pressures of these compounds which in turn influence their solubilities in SCCO2. As expected solubilities decreased in going from fatty acid esters, to fatty acids, to triglycerides.

An Overview of Starch-Based Plastic Blends from Reactive Extrusion

The North American market for biodegradable plastics in 2005 was estimated to be around 60 million pounds (27 kt). Starch-based polymer blends were expected to account for 30 million pounds (14 kt) with significantly lower growth rates than other biodegradable polymers such as polyesters. The main hurdle in the growth of starch-based products is the thermodynamic immiscibility and non-wetting of starch with other polymers which leads to serious deterioration of mechanical properties at >25–30 wt% starch. Higher amounts of starch in the blends entail adding suitable functional groups on starch and other polymers in the blend to make them more compatible. The primary challenge is to develop fast reaction chemistries that can be transformed into viable processes and integrated into existing process lines with economically viable formulations. This article briefly reviews some of the most promising chemistries available for the reactive extrusion of starch-based polymer blends (biodegradable/non-biodegradable).

Structural properties of protein-stabilized starch-based supercritical fluid extrudates

Abstract Supercritical fluid extrusion, a low temperature and low shear process, was used to produce pre-gelatinized corn and potato starch-based extrudates, containing 4–10% thermosetting egg white (EW) or whey protein concentrate (WPC-34), and dried at 22–100°C. Addition of proteins reduced shrinkage of high-moisture extrudates, as indicated by increases in expansion ratio by up to 140 and 341% when the drying temperatures were 22 and 100°C, respectively. Products containing 7% EW or WPC-34 and dried at 85°C expanded best while maintaining an intact structure, with expansion ratio (∼12) and bulk density (∼0.10 g/cm3) comparable to steam extrudates. The products had a unique composite and uniform microcellular structure, with average cell sizes in the range of 50-250 μm and cell density of the order of 106 cells/cm3. The classical nucleation theory and a qualitative model for cell growth and shrinkage based on glass transition temperature were used to explain the microcellular structure.

The combined application of supercritical fluid and extrusion technology

The supercritical fluid extrusion (SCFX) process has the potential for producing a range of puffed food products, such as ready-to-eat cereals, pasta and confectionery products, with improved texture, color and taste. The conventional extrusion process has some limitations with regard to the process-related product characteristics. The SCFX process has been successful in overcoming some of these limitations, making extrusion a more versatile and controllable process. Some novel products produced using the SCFX process are presented in this article in the light of the above remarks. The principles of the SCFX process are described with particular focus on the thermodynamic and rheological fundamentals underlying the process improvements.

Phase equilibria of oleic acid, methyl oleate, and anhydrous milk fat in supercritical carbon dioxide

Abstract A static recirculation method was developed and used to measure phase equilibria of binary, ternary, and multicomponent systems in supercritical carbon dioxide (SC-CO 2 ). Binary fluid-liquid equilibria of oleic acid + SCCO 2 were experimentally determined at 313.15 and 333.15 K and at pressures between 3 and 31 MPa. Fluid-liquid equilibria of the binary system methyl oleate + SCCO 2 were also measured at 313.15 and 333.15 K and at pressures between 2 and 14 MPa. Finally, the phase equilibria of anhydrous milk fat (AMF) in SC-CO 2 were determined at 313.15 and 333.15 K and at pressures between 2 and 31 MPa. The data were correlated using the Peng-Robinson equation-of-state with the Panagiotopoulos and Reid mixing rule.

THERMODYNAMIC ANALYSIS OF DRYING FOODS

Thermodynamic functions computed for food systems showing hysteresis indicate considerable differences. There appears to exist no qualitative agreement between the values obtained. Large differences in values between the adsorption an desorp-tion branches of isotherms make the absolute magnitude of the entropy and enthalpy changes uncertain. The desorption iso-steric heat was found to decrease with temperature while the adsorption branch of the isotherm for a few foods were found to exhibit temperature reversal effect giving higher values at low temperatures. Such trend may indicate either transition in the form of binding energy or the effect of temperature on structural alterations. In designing drying equipment, the isosteric heat of desorption will provide “a worst case” analysis as the true isosteric heat will lie somewhere between the adsorption and desorption isosteric heat. Any overcalculation in designing equipment by using the desorption branch of hysteresis data can only be in favor of increase...

Process dynamics of starch-based microcellular foams produced by supercritical fluid extrusion. I: model development

Abstract Supercritical fluid extrusion (SCFX) has been used successfully to produce biopolymeric foams with bubble size in the range of 50–200 microns, and bubble density to the order of 106 per cm3. Final bubble size and expansion ratio of extrudates depend on process and material parameters like CO2 injection rate, nozzle temperature, oven temperature, melt viscosity, melt yield stress, etc. The objective of this study was to describe SCFX process dynamics and post-extrusion drying mechanism by using a mathematical model for bubble growth at the microscopic level in conjunction with a macroscopic model for flow of starch melt through the extruder nozzle, bulk diffusion of CO2 and water to the atmosphere and heat transfer in the extrudate. The model was written in Visual Basic. The model provided the basis for a good understanding of the mechanisms of bubble growth and collapse, post-extrusion drying and open cell formation during SCFX processing, and is the only such one developed so far for extrusion puffed products. Simulation results and comparison with experimental data are presented in Part II of this paper.

Continuous supercritical carbon dioxide processing of palm oil

Crude palm oil was processed by continuous supercritical carbon dioxide. The process reduces the contents of free fatty acids, monoglycerides and diglycerides, certain triglycerides, and some carotenes. The refined palm oil from the process has less than 0.1% free fatty acids, higher carotene content, and low diglycerides. Solubility of palm oil in supercritical carbon dioxide increased with pressure. A co-solvent improves the refining process of palm oil.

Fluid-Liquid phase equilibria of fatty acids and fatty acid methyl esters in supercritical carbon dioxide

Abstract Fluid-liquid equilibrium data of fatty acids and fatty acid methyl esters in supercritical carbon dioxide at temperatures of 40 °C and 60 °C and pressures up to 300 bar were experimentally determined. The effect of temperature and pressure on solubility in four binary systems was studied. The Redlich-Kwong equation-of-state with a non-quadratic mixing rule was used for data correlation with successful results.