Mathematical modeling of drying behavior of single emulsion droplets containing functional oil

doi:10.1016/j.fbp.2016.10.012

Food and Bioproducts Processing

Volume 101, January 2017, Pages 100-109

https://doi.org/10.1016/j.fbp.2016.10.012 Get rights and content

Highlights

•
Developing a mathematical model for drying of single emulsion droplets.
•
Drying modeling using an ALE frame by temperature- and moisture-dependent properties.
•
Fairly good agreement between simulation results and experimental data.
•
Potential of the model for predicting drying behavior of emulsion droplets.

Abstract

In this study, a mathematical model was developed to study drying behavior of single emulsion droplets containing functional oil. For this aim, drying process was divided into two successive steps by considering a receding emulsion/air interface in the first step and a wet core/crust interface in the second step. A system of differential and algebraic equations was solved numerically using an arbitrary Lagrangian–Eulerian (ALE) framework by considering temperature-dependent thermophysical properties for the droplet/particle constituents. The developed model was then validated using experimental drying data obtained for emulsion droplets containing skim milk powder and walnut oil as wall and core materials, respectively. After validation of the developed model, the effects of drying air temperature (80–140 °C), air velocity (0.02–3 m/s), and droplet initial diameter (0.01–0.75 mm) on the droplet/particle drying behavior were systematically investigated. The simulation results agreed fairly well with the experimental data indicating the suitability of the developed model for predicting drying characteristics of liquid droplets containing functional oils. According to the results, drying behavior of emulsion droplets/particles was significantly affected by the variables considered. This work further demonstrated the merits of phenomenological models for tracking and controlling the physicochemical attributes of droplets/particles containing functional ingredients. Accordingly, the developed model could facilitate ongoing attempts to improve the encapsulation quality and to minimize the lipid oxidation in large-scale spray drying systems being employed for microencapsulation of functional oils.

Introduction

Nowadays, application of functional food ingredients such as long chain n-3 poly unsaturated fatty acids has found an increasing attention in the food and pharmaceutical industries due to their beneficial effects on human health (Anwar et al., 2010, Aghbashlo et al., 2013, Wang et al., 2014). However, successful incorporation of these invaluable constituents into processed foods is not straight forward because of their susceptibility to auto-oxidation leading to the production of off-flavors and toxic compounds. This problem becomes even more serious because of low solubility of functional oils in most food systems (Gharsallaoui et al., 2007, Aghbashlo et al., 2012). It is fortunate that microencapsulation technology could solve these problems to a large extent by isolating functional oils from the deteriorating effects of air, mitigating the evaporation rate of volatile cores, masking the taste or odor of core materials, and isolating reactive core materials from chemical attacks (Gharsallaoui et al., 2007, Jafari et al., 2008b, Borrmann et al., 2011, Borrmann et al., 2013).

Microencapsulation can be carried out using various techniques such as spray drying, spray cooling/chilling, extrusion, liposome entrapment, coacervation, and fluidized bed coating (Goula and Adamopoulos, 2012b). Among various technologies developed, spray drying is one of the most promising methods to encapsulate functional oils because of its low expenditure and the availability of equipment (Jafari et al., 2008a, Anwar and Kunz, 2011, Aghbashlo et al., 2013). However, improving the encapsulation quality and reducing the lipid oxidation are still important challenges of spray drying technique that must be addressed. On the other hand, characterization of drying behavior of droplets/particles containing functional oils using phenomenological models can provide invaluable information on the underlying transport mechanisms and the structure of produced powders. These insights can then be used to track the encapsulation efficiency and lipid oxidation during drying of droplets/particles embodying functional oils.

Drying of a single emulsion droplet often occurs in two successive stages. In the first drying stage, the droplet contains an excessive amount of solvent and drying occurs at a constant rate along with droplet diameter shrinkage. The second drying stage commences when the droplet moisture content decreases to the critical value. At this moment, termed “locking point”, the wet core is covered by a porous-structured dry solid crust (Bück et al., 2012, Tsotsas, 2012, Tsotsas, 2015). Accordingly, the rate of internal solvent evaporation dramatically decreases towards the end of drying process due to an increase in the thickness of formed crust. During the second drying stage, the outer diameter of particle remains constant, whereas the interface of the crust/wet core retreats progressively by solvent evaporation (Castilla and Munz, 2007b, Mezhericher et al., 2015). Generally, modeling drying behavior of single emulsion droplets containing functional oils using the above-mentioned approach can lead to invaluable information to monitor and control the physiochemical attributes of dried particles.

A number of studies have been reported on the microencapsulation of various functional oils using spray drying technology with their focus on technical feasibility and particle characterization (Klaypradit and Huang, 2008, Tonon et al., 2011, Goula and Adamopoulos, 2012a, Frascareli et al., 2012, Taksima et al., 2015). However, to the best of our knowledge, there is no information to date on the phenomenological modeling of drying behavior of single emulsion droplets containing functional oil. Understanding drying behavior of single emulsion droplets has utmost importance for predicting the physicochemical properties or functionality of finished microparticles in industrial-scale operations. Therefore, the main objective of this study was to develop a mathematical model for drying of single emulsion droplets containing functional oil and to verify the presented model using experimental drying data. More specifically, drying behavior of single emulsion droplets was studied at various drying air temperatures, velocities, and droplet initial diameters. The outcomes of this study would be envisaged to be of paramount interest to researchers and engineers involved with design, optimization, and retrofitting of spray drying systems being employed for encapsulation of food ingredients.

Section snippets

Model development

In this study, the proposed two-stage drying approach by Mezhericher et al. (2007) was considered and adapted for modeling drying behavior of single droplets containing functional oil. Moreover, the following assumptions were made in order to develop mathematical model:

1)
The geometry of droplet/particle was postulated to be perfectly symmetric.
2)
In the first drying stage, the solvent evaporation occurred only at the droplet surface.
3)
The radiative heat transfer was ignored.
4)
Heat and mass transfer were

Model assessment

The developed model was validated by comparing the numerical results with experimental data obtained in our previous study (Shamaei et al., 2016) for single emulsion droplets containing walnut oil and skim milk powder at drying air temperatures 80, 110, and 140 °C and air velocity of 0.02 m/s. Fig. 2 compares the experimental data and simulation results for drying of a single emulsion droplet/particle containing walnut oil. Obviously, an increase in drying air temperature decreased the required

Conclusions

In this study, a two-stage mathematical model was developed to predict drying behavior of single liquid droplets containing functional oil. The model was then validated using experimental data obtained from the drying process of emulsion droplets containing skim milk powder and walnut oil as wall and core materials, respectively. The developed model predicted the droplet/particle moisture content and temperature as well as diameter with an acceptable accuracy. Moreover, the effects of the

Acknowledgments

The authors acknowledge the support provided by Tabriz University.

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Mathematical modeling of drying behavior of single emulsion droplets containing functional oil

Highlights

Abstract

Introduction

Section snippets

Model development

Model assessment

Conclusions

Acknowledgments

Food Res. Int.

J. Food Eng.

J. Aerosol Sci.

Food Bioprod. Process.

Int. J. Heat Mass Transf.

J. Food Eng.

Int. J. Refrig.

Chem. Eng. Sci.

Food Bioprod. Process.

Food Res. Int.

Food Bioprod. Process.

Food Res. Int.

LWT Food Sci. Technol.

J. Food Eng.

Chem. Eng. J.

Chem. Eng. Sci.

Adv. Powder Technol.

LWT Food Sci. Technol.

Food Res. Int.

Chem. Eng. Sci.