Vaclav Sobolik

Fruits & Vegetables Drying Combining Hot Air, DIC Technology and Microwaves

Dehydration of fruits and vegetables is one of the most ancient and efficient preservation methods. The quality of the product and its cost depend mainly on the final stage of drying. In the present paper we analyze physical mechanisms occurring during drying throughout heat and mass transfer for defining an efficient and economic three stage drying process of hot air drying combined to a texturing stage by DIC (Instant Controlled Pressure Drop) and finally using microwave assisted by ambient temperature air dehydration.

Isolation of Indonesian cananga oil using multi-cycle pressure drop process

New process, instantaneous controlled pressure drop (DIC) was applied on Cananga odorata dry flowers with the aim to isolate essential oil. DIC is based on high temperature, short time heating followed by an abrupt pressure drop into a vacuum. A part of volatile compounds is carried away from flowers in the form of vapor (DIC direct oil) that evolves adiabatically during the pressure drop (proper isolation process) and the other part remains in the DIC-treated flowers (DIC residual oil). In the present paper, the effect of DIC cycle number (1-9) and heating time (4.3-15.7 min) on the availability of oil compounds was investigated at three levels of steam pressure (0.28, 0.4 and 0.6 MPa). The availability was defined as the amount of a compound in direct or residual oil divided by the amount of this compound in the reference oil extracted from non-treated flowers by chloroform during 2h. The total availability and yield of volatiles in the direct oil increased with pressure and cycle number. At a higher pressure, the effect of heating time was insignificant. The amount of oxygenated monoterpenes and other light oxygenated compounds (i.e. predominantly exogenous compounds) in the residual flowers was lower than in the direct oil and this amount decreased with cycle number. On the other hand, the availability of oxygenated sesquiterpenes and other heavy oxygenated compounds (i.e. predominantly endogenous compounds) in residual flowers exhibited a maximum for about five cycles and their quantity at this point was three times as much as in the direct oil. The total availability of each compound at 0.6 MPa was higher than one. The rapid DIC process (0.6 MPa, 8 cycles, 6 min) gave better results than steam distillation (16 h) concerning direct oil yield (2.8%dm versus 2.5%dm) and content of oxygenated compounds (72.5% versus 61.7%).

Viscosity and electrical conductivity of concentrated solutions of soluble coffee

Abstract Viscosity of concentrated aqueous solutions ( ω =0.5–0.8) of soluble coffee was measured in the temperature range 25–95°C. After some time of the shear application in the viscometer and passing a temperature of 95°C, the rheological behaviour was found to be Newtonian. The viscosity was correlated by a five parameter function of coffee mass fraction and temperature. Specific electrical conductivity of coffee solutions in tap water ( ω =0–0.8) was measured in the temperature range 25–72°C. A seven parameter model based on the assumption that the solution is composed of a partially dissociated species and water describes very well the measured data. A modified Casteel–Amis model equation has been identified for comparison. The conductivity dependence on mass fraction exhibits maxima, which are shifted towards higher concentrations the higher is the temperature. Viscosity, refractivity index, density and thermal conductivity of aqueous coffee solutions ( ω =0–0.5) are reviewed.

Wall shear rate in the Taylor–Couette–Poiseuille flow at low axial Reynolds number

Abstract An experimental investigation was done by the use of visual observations and the electrochemical technique in order to study the appearance of hydrodynamic instabilities at low Re axg in the gap between two coaxial cylinders, with radius ratio R r / R s =0.615 and aspect ratio L / e =24. A motor drove the inner cylinder and the outer cylinder was fixed. A Newtonian fluid (Emkarox HV45) and two non-Newtonian fluids (aqueous solutions of guar and CMC) have been used. The analysis of the evolution of the size of the Taylor vortices is carried out for Re axg Re axg Ta g ≈80, and a stretching phase for Ta g >80. For higher Re axg , the contraction phase vanished and the vortices progressively stretched with increasing Ta g . A dimensionless representation was also proposed for wall friction generalized to non-Newtonian fluids following the Ostwald law.

Experimental investigation of the wall shear stress in a circular impinging jet

The influence of the large-scale vortical structures on the wall shear stress in a circular impinging jet is investigated experimentally for a Reynolds number of 1260. Time-resolved particle image velocimetry and polarographic measurements are performed simultaneously. It is found that the instantaneous wall shear stress is strongly dependent on the vortex dynamics, particularly for different parts of the transverse vortex. The influence of the vortex ring, the secondary and tertiary vortices on the ejection/sweep process near the wall is the main mechanism involved in the wall shear stress variation. In the region of the boundary layer separation, the wall shear stress amplitude increases just upstream of the separation and dramatically decreases in the recirculation zone downstream from the separation. The interaction between primary and secondary structures and their pairing process with the tertiary structure affects the sweep/ejection process near the wall and subsequently the wall shear stress variation. A comparison between the Finite Time Lyapunov Exponent (FTLE) method and the phase average technique is performed. It is shown that both methods describe the flow dynamics in the impinging region of the vortex ring. However, the FTLE method is more suitable for describing the unsteady separation of the boundary layer.

Impact of Swell-Drying Process on Water Activity and Drying Kinetics of Moroccan Pepper (Capsicum annum)

The impacts of total hot air drying (THD) and the swell-drying process (SD), which is hot air drying coupled with an instant controlled pressure drop DIC process, on the drying kinetics and adsorption isotherm behavior, were determined in the case of Moroccan peppers. Drying kinetics were studied through a model that takes into account the starting accessibility and the internal diffusion model with Ficks law. Adsorption experimental data were obtained by using the gravimetric method at 25, 40, and 50°C. Data were fitted to the GAB, Halsey, and Oswin models. Studied responses were: effective diffusivity (Deff), starting accessibility (δWs), monolayer moisture content (Xm), specific surface area (σ), internal porosity (ϵ), and microstructure. Mean relative percentage deviation (E) and root mean square (RMS) were used to evaluate the fitting of models. Results showed that: pepper adsorption isotherms followed the type II isotherm curve;the equilibrium moisture content (Xeq) depended on the temperature and the drying method; the GAB and Halsey models fitted well the experimental data; at selected conditions of the DIC treatment (0.35 MPa and 5 s), the monolayer moisture content (Xm), the specific surface area (σ), and the internal porosity values were higher than those of THD samples; the DIC process intensified the drying operation, by increasing the effective diffusivity and the starting accessibility. Although the DIC process improves the drying kinetics, this behavior could be ignored related to a higher aw. This study shows that, thanks to the new expanded structure obtained by the DIC process (higher porosity), the adsorption capacities of products are improved (aw is reduced). Thus, the SD process improves both the drying operation and the stability of dried pepper products.

Isolation of Indonesian Cananga Oil by Instantaneous Controlled Pressure Drop

Abstract The isolation of cananga oil by a new process, Instantaneous Controlled Pressure Drop (DIC), was investigated. This process consists in heating the dry cananga (Cananga odorata Hook. fil. et Thomson, forma macrophylla) flowers for a short time period by steam, followed by an abrupt pressure drop into a vacuum (about 5 kPa). This pressure drop provokes auto-vaporization of the volatile compounds, puffing of flowers, breaking of cell walls and cooling. The effect of the process parameters, namely number of DIC cycles (1–9), saturated steam pressure (0.2–0.6 MPa), and heating time (0.5–20 min) on the oil yield and oil composition was examined. The results indicated a significant increase of oil yield with increasing processing pressure and number of DIC cycles, however the total heating time was not a significant parameter. The DIC oil was compared with the oil obtained using steam distillation (SD). DIC exhibited better results than SD concerning rapidity (4 min versus 24 h), oil yield (2.74% versus 2.60%) and also oil quality.

Calibration of a segmented friction probe flush mounted in a sphere

A calibration theory for segmented probes flush mounted in a sphere exposed to uniform fluid flow was developed. The theory was checked experimentally by towing the sphere of diameter 2R = 22.2 mm with a three-segment circular electrodiffusion probe of diameter 2RE = 0.526 mm through a pool of aqueous solution of potassium ferro/ferricyanide. Oriented wall shear rates were detected and analysed at Reynolds number 660. Hydrodynamic models of the wall shear rate distribution along the sphere surface in a uniform stream are reviewed. The principles of the direction specific flow measurements using three-segment electrodiffusion friction probes are explained.

Yield and Composition of Indonesian Cananga Oil Obtained by Steam Distillation and Organic Solvent Extraction

Abstract Cananga oil has high economical value because of its utilisation as fragrance and food flavour. In the present work, the oil was isolated by steam distillation and organic solvent extraction from dry mature flowers of Cananga odorata Hook. fil. et Thomson, forma macrophylla. Chloroform and hexane were used as solvents with different polarities. The yield of volatile oil and its grouped compounds (LOC, sesquiterpenes hydrocarbons and HOC) was investigated as a function of time. LOC consists of oxygenated monoterpenes and other light oxygenated compounds whereas HOC comprises oxygenated sesquiterpenes and other heavy oxygenated compounds. The oil yields of 5.1, 3.8 and 2.5 % dry matter were obtained with chloroform (8 h), hexane (8 h) and steam distillation (16 h), respectively. The isolation rate was described by a first-order kinetics equation. The rate was lowest with steam distillation, intermediate with hexane extraction and best with chloroform extraction. The oil composition was analyzed by gas chromatography coupled to mass spectrometry. The steam-distilled oil and chloroform oil had a similar composition, with a high content of oxygenated compounds (61.7% and 58.3%), while hexane oil was rich in sesquiterpenes hydrocarbons (66.5%).

Wall shear rates and stagnation mass transfer on a plate in axisymmetric and cross impinging jets

This article presents a study on the wall shear rate and mass transfer of impinging jets on a flat plate in the vicinity of stagnation point. The performance of a cross-shaped orifice nozzle was compared with a reference convergent circular nozzle having similar equivalent diameter. An array of electrodiffusion micro probes inserted into the plate was used for wall shear rates measurements. Mass transfer in the impinging region was calculated from the measured wall shear rates for a Reynolds number around 5500 and over a range of streamwise distances between the nozzle and the impinging plane within 1 to 5 nozzle equivalent diameters. The most important observation in the present investigation is that the wall shear rates and the mass transfer in the impingement region of the cross-shaped orifice nozzle are up to 175% and 40%, respectively, higher than that of the convergent nozzle. The performance of the cross-shaped orifice jet is probably related to its particular vortex dynamics characteristic of the near exit region. All the results confirm that the jet passive control enhance the mass transfer.