G. Djelveh

Static Mixers in the Process Industries—A Review

This paper summarizes the field of static mixers including recent improvements and applications to industrial processes. The most commonly used static mixers are described and compared. Their respective advantages and limitations are emphasized. Efficiencies of static mixers are compared based both on theory and experimental results from the literature. The operations, which can benefit from the use of static mixers, are explored, namely, mixing of miscible fluids, liquid–liquid and gas–liquid interface generation, liquid–solid dispersion and heat transfer. Design parameters governing the performance of the various mixers in these applications are reported. The key parameters needed for the selection of a suitable mixer are highlighted.

Extraction, fractionation and functional properties of proteins from the microalgae Chlorella vulgaris

This paper deals with the extraction and emulsifying properties of proteins from Chlorella vulgaris. Solubilisation of proteins has been achieved using high pressure cell disrupter under pH=7 or pH=12. The higher solubilisation yield (52±3%w/w) was obtained using a combination of alkaline conditions and mechanical treatments (2.7kbar). After solubilisation, proteins were recovered by two procedures: precipitation in acid media and concentration/fractionation by tangential ultrafiltration. Proteins were analysed for their molecular weights, isoelectric points and amino acids compositions and their emulsifying properties were quantified and compared to those of commercial ingredients. In spite of lower yield, better emulsifying capacity was obtained when protein solubilisation takes place at pH=7 and when using proteins from permeate of tangential ultrafiltration. In all cases, emulsifying capacity (1780±20 and 3090±50mLoil/g protein) and stability (72±1% and 79±1%) of microalgae proteins remained comparable or higher than the commercial ingredients such as sodium caseinate.

Influence of operating conditions and impeller design on the continuous manufacturing of food foams

The influence of impeller design and operating conditions on the continuous manufacture of food foams was studied in a narrow gap mechanically stirred unit using a model food. Quality and texture of foams were characterised on the basis of density, bubble size and rheological measurements. Mixing mechanisms were investigated using residence time distribution (RTD) experiments. The results show that mean bubble size in foams depends only on the intensity of viscous forces generated by the impeller. For a similar intensity of shear forces, impellers with a small internal volume are to be preferred for foaming because they improve bubble dispersion and mixing of surface-active agents. These allow the manufacture of foams with a lower density, which has been shown to improve their texture and increase their mechanical strength. RTD analysis is shown to be a simple diagnosis tool for estimating the mixing efficiency of new impellers which may be helpful in improving choice of adequate impeller design and operating conditions to produce foams with desired properties.

Separation and fractionation of exopolysaccharides from Porphyridium cruentum

In this work the extraction of EPSs from culture media of Porphyridium cruentum, by dialysis, solvent-precipitation with 3 polar alcohols (methanol, ethanol and isopropanol) and membrane separation techniques has been studied. Diafiltration (DF) using a membrane with a 300 kDa molecular weight cut off was the most efficient technique compared to solvent-extraction and dialysis methods. After extraction, EPS fraction was characterized in terms of rheological properties and biochemical content. The product exhibited shear thinning behavior and a critical overlap concentration equal to 0.6 g/L. The monosaccharide composition was investigated after acidic hydrolysis. Xylose, galactose, glucose and glucuronic acid were identified as the main constitutive monomers.

Rheological behaviour of blood plasmas concentrated by ultrafiltration and by evaporation in relation to liquid–gel transition temperature

Abstract Dehydrated blood plasma finds use as a protein ingredient for its gelation properties especially in meat derivative products. Generally, blood is concentrated by either ultrafiltration or evaporation under reduced pressure. The resulting pH and ash concentration (mineral salts and sodium citrate) differ, so modifying the gelation properties of the products. We studied the influence of pH, NaCl concentration and time–temperature pattern on the liquid-to-gel transition temperature; T g of the blood plasma in these two processes. We showed that at all the NaCl concentrations, increasing the pH from 4 to 7.8 increased the blood plasma T g from 35 to 54 °C. NaCl concentrations in the range 0.2–1.65 (mass%) had no significant effect on this temperature, and particular time–temperature heating procedure suppressed the T g . This last result is of particular interest for pasteurisation.

Estimation of physical properties of foamed foods using energy dissipation in scraped-surface heat exchangers

Abstract A jacketed stirred column simulating a continuous scraped-surface heat exchanger under steady-state conditions was devised and used to produce foamed foods. The physical properties of these were assessed as a function of rotor speed, paddle diameter and gas/liquid ratio. Apparent viscosity of foamed food was calculated using a Newton number-Reynolds number experimental relation for the stirred column and power consumption measurements. It was shown that the mean apparent viscosity was an inverse function of the density of the foamed food and of the rotation speed. Mean bubble diameter was correlated with the mixing conditions by a critical Weber number. For the starting material used, the mean bubble diameter reached a stationary value beyond a certain rotation speed. These results were explained theoretically.

Foaming of commercial grade food products in a continuous stirred column

A Narrow Annular Gap Unit (NAGU) was used to produce food foams under steadystate conditions. Raw materials consisted of three kinds of emulsions, corresponding to commercial grade food products. The composition of these materials was defined to be representative of a fresh cheese, an ice cream mix and a model emulsion based on meat. Interfacial and rheological properties of these emulsions were determined and related to their composition. The respective influence of formulation and operating conditions on foaming operation was analysed and compared for the three products in the light of foam properties, such as texture, bubble size and foam stability with time. An attempt was made to correlate the combined effects of process parameters and raw material properties on mean bubble size in foams using a dimensionless Weber number. Rotation speed of the impeller was shown to affect only bubble size for the meat model, while it affects mainly texture for ice cream and also overrun for fresh cheese.

Haematococcus pluvialis soluble proteins: Extraction, characterization, concentration/fractionation and emulsifying properties.

A water-soluble matrix was extracted from green vegetative Haematococcus pluvialis through high-pressure cell disruption either at native pH (5.7) or with pH shifting to neutral (7). The resulting supernatant is mainly composed of carbohydrates and proteins, with the highest yield of proteins obtained at neutral pH (73±2% of total biomass proteins). The key emulsification properties of the proteins isolated in neutral supernatant (emulsification capacity (EC): 534±41mLoilg(-1) protein, emulsification stability (ES): 94±3% and emulsification activity index (EAI): 80±1m(2)g(-1)) were comparable to the native supernatant values (EC: 589±21mLoilg(-1) protein, ES: 84±3% and EAI: 75±1m(2)g(-1)). Confronted to sodium caseinate (EC: 664±30mLoilg(-1) protein, ES: 63±4%, and EAI: 56±4m(2)g(-1)) these results highlighted the strong potential of proteins isolated from H. pluvialis as emulsifier agent. Moreover, experiments have shown that the stability of emulsions obtained from supernatants is due to the proteins rather than the carbohydrates.

Mechanical aspects of gas dispersion in continuous foaming food processes using scraped surface heat exchangers

Abstract A stirred column simulating a continuous scraped surface heat exchanger under steady-state conditions was devised and used to study the influence of mechanical mixing, gas input rate and a gas sparger on the foaming capacity of a model food. The bubble size distribution was measured simultaneously. Foaming capacity depended on the number of paddles inside the column and on the gas flow rate. Rotation speed improved the foaming capacity up to 1000 rpm. The average bubble diameter defined as the Sauter diameter was independent of operating conditions. Material balances on the gas phase showed that gas within the foamed product is under pressure so that bubbles increase in diameter when the product is maintained at atmospheric pressure.

Foaming process analysis for a stirred column with a narrow annular region

Abstract A jacketed stirred column was used to produce foamed products under steady-state flow conditions. Power dissipation during foaming processes was measured and used to define gas–liquid mixing behaviour. The influence of paddles’ diameter and rotation speed on the density and the mean bubble diameter within the foamed food was studied for two model foods, with different rheological properties. Weber number was used to express bubble diameter as a function of the physical properties of the raw material and process parameters. Simulation suggests that raw material with high viscosity and an apparatus with narrow gap favours foaming in continuous processes.