Seddik Khalloufi

Interactions between flaxseed gums and WPI-stabilized emulsion droplets assessed in situ using diffusing wave spectroscopy

Diffusing wave spectroscopy (DWS) was used to investigate the behavior of flaxseed gums when added to WPI-stabilized emulsions. The effect of different concentrations (0-0.33% (w/v)) of flaxseed gum, derived from two seed varieties, namely Emerson and McDuff, was studied at acidic and neutral pH. At pH 7.0 and low gum concentrations the dynamic and spatial characteristics of the system remained unchanged. While at gum concentrations from 0.075% to 0.20% a rapid phase separation was observed, at higher concentrations phase separation was retarded because of the increased background viscosity slowing down the mobility of the emulsion droplets. At pH 3.5, the difference in overall electrical charge between the gum (negative) and the protein on the emulsion surface (positive) led to electrostatic interactions. While at low concentration of flaxseed gum the general characteristics of the emulsions were not significantly different, at intermediate concentrations, bridging flocculation occurred. When sufficient flaxseed gum was present, the emulsion droplets mobility was arrested in a gel-like state. In spite of the compositional differences in the ratio of acidic and neutral fraction between the two polysaccharides extracted from different seed varieties, at both values of pH the behavior of the emulsions after addition of the polysaccharide was comparable.

The role of drying methods on enzymatic activity and phenolics content of impregnated dried apple

ABSTRACT Infusion of antioxidants into vegetables is a new food strategy managed by matrix processing. Raw and blanched apple were air- or freeze-dried. In the case of freeze-dried samples, different freezing methods were previously applied: conventional (−28°C), blast freezing (−30°C), and liquid N2 (−196°C). Afterwards, air- and freeze-dried samples at different conditions were impregnated with a concentrated (40°Brix) tea extract and finally, air-dried for their stabilization. Total phenolic content (TPC), antioxidant capacity (AC), enzymatic activity, and microstructure were analyzed. Regardless of pretreatments, the impregnation and the further drying improved the antioxidant potential. Samples with the most porous microstructure free of degradative enzymes provided high AC (78.5 ± 0.9 mg Trolox/g dried matter) and TPC (16.7 ± 0.2 mg GAE/g dried matter).

Mathematical investigation of the case hardening phenomenon explained by shrinkage and collapse mechanisms occurring during drying processes

Although drying is one of the oldest approaches that have been used by humans to preserve foods, this technology is still an active scientific topic for researchers in both industry and academia. Indeed, during drying processes, food products undergo several physical, chemical, and structural changes which have a direct impact on the quality of the final products and therefore on the consumer perception and the acceptance of dried foods. To translate the consumer needs, scientists use some measurable attributes, such as bulk density which affects the visual aspects. In addition, density plays a major role in heat/mass transfers, which are crucial for process optimization and hence the mathematical modeling. In the literature, several models are available to describe density during drying. The majority of these models are empirical and very few are theoretical. Recently, our group (Khalloufi et al. 2010) has developed a new fundamental approach for predicting bulk density as a function of moisture content. This approach included, for the first time, simultaneous variations of both initial and instantaneous porosities during drying processes. The aim of this contribution is to assess the ability of this new theoretical model to investigate the temperature effect on bulk density and therefore to support the theoretical background behind the case hardening phenomenon. Experimental data obtained by an independent group for apple dried at three different temperatures (50oC, 80oC or 105oC) were used. The model was implemented and solved in Matlab using the fmincon, and then applied to simulate the bulk density behaviors for the three temperatures. For all three temperatures, the average deviation between the present model and the experimental data was <10%. The investigation of the temperature effect on bulk density was performed by using the two physical mechanisms involved in this model, namely the collapse and shrinkage phenomena. The results of this assessment showed that both mechanisms have the same profiles for the three temperatures, and their values at the end of the drying process were found to be temperature dependant. Indeed, the increase in drying temperatures leads to: (i) more preservation of the initial air existing at the beginning of the process and (ii) more replacement by air of the water removed during drying. Therefore, drying at high temperatures (e.g. 105oC) results in low bulk density. In the literature, this behavior is hypothetically ascribed to the case hardening phenomenon. This phenomenon consists in an instantaneous drying of the external layer of food products at high temperature, resulting in crust formation (sort of protective shell) which in turns leads to less shrinkage and low density. The results obtained by the present mathematical model support the concept of case hardening via a theoretical explanation based on the shrinkage and collapse mechanisms.

Prediction of Supercritical Carbon Dioxide Drying of Food Products in Packed Beds

Drying assisted by supercritical carbon dioxide is foreseen to become a promising technology for sensitive food products. In this contribution, a mathematical model is derived to describe the changes in water concentration in both a solid food matrix and a fluid carrier during drying. Finite different element method is used to solve the set of mass balance equations. A remarkable agreement between simulated and experimental data was obtained. Moreover, the simulated changes in water concentration in the solid and fluid carrier gave a coherent description of the process. This model can be used as a tool for optimizing the operating conditions and process scale-up in supercritical carbon dioxide assisted drying.

Mathematical Model for Simulating the Springback Effect of Gel Matrixes During Drying Processes and Its Experimental Validation

Volume change is one a fundamental aspect in characterization of drying processes. Several attempts to simulate the volume change during drying have been reported in the open literature. However, so far no theoretical approach has been used to support these simulations, especially when it comes to dealing with the springback effect. In this contribution, a theoretical model was built to predict the volume change including the springback phenomenon. The theory behind the present model is based on three physical mechanisms, which are represented by the shrinkage, collapse, and swelling functions. The resulting set of equations was implemented and solved in MATLAB(Mathworks, Inc., Natick, MA) by formulating the model as a constrained optimization problem. Data for three gels reported by an independent group and characterized by different profiles in terms of the springback effect were used to validate the model. This validation showed excellent agreement between the predictions obtained by this model and the experimental data. The average error lies somewhere between 1.6 and 4.4% depending on the gel. The information extracted from the parameters included in this theoretical model should assist in understanding the mechanisms that occur during processes involving moisture/solvent changes. Hence, the present model can be used as a reliable tool to predict volume changes and to understand the dynamic mechanisms involved in pore formation/disappearance during drying processes.

Mathematical description of mass transfer in supercritical-carbon-dioxide-drying processes

For thermo-sensitive food products, supercritical-carbon-dioxide (SC-CO2) drying process could be a promising technology. The process takes place in three steps: (i) removal of water from the food matrixes, (ii) adsorption of the water removed in the adsorber bed, and (iii) regeneration of the adsorber with hot air. In this investigation, a mathematical model is derived to describe the changes of water concentration in SCCO2, in the solid food matrix and in the adsober bed during the entire drying processes. The mass balance equations of the model involve several parameters such as the geometry of the autoclave and the adsorber bed, mass transfer coefficients, diffusion coefficients, equilibrium constants between the solids and the fluids, the specific interfacial area of the solid matrixes, the porosities of the packed beds, the SC-CO2 flowrate and the particle size. Preliminary results obtained with the model suggest that each parameter may contribute differently to the drying kinetics. This finding allows the identification of the bottlenecks encountered in drying processes and offer leads and strategies to overcome them. The present model could eventually be used as a tool for optimizing the operating conditions and process scale-up in SC-CO2 drying.

Sensitivity of shrinkage and collapse functions involved in pore formation during drying

Abstract The pore formation during drying is controlled by two mechanisms which are represented by two functions. These functions are expected to be universal, thus recurrent and applicable to relevant physical properties of the products during drying. This contribution aims at studying the sensitivity of shrinkage and collapse functions in predicting the porosity as a function of moisture content. A set of experimental data from an independent research group using air-drying of carrot were used to evaluate the sensitivity of these two functions. The results of this analysis showed that (i) at high moisture content, the porosity is not sensitive to the shrinkage function whatever the value of the collapse function is, (ii) in the case of air drying and at low moisture content, the porosity could be strongly affected by the shrinkage function, and (iii) the collapse function has a strong effect on porosity in products with a high volume of initial air. These findings are reported here for the first time and the approach used in this contribution could be very relevant to assess other parameters involved in drying processes such as bulk density and shrinkage coefficient.