INTRODUCTION
Convective drying is an essential operation in many processes that lead to
development of basic materials to finished products. The objective of drying
plant aromatic and medicinal plants is to reduce the water activity of these
products and to ensure their conservation value. This phenomenon must meet certain
criteria related to product quality (Ahmed et al.,
2002; Kouhila et al., 2002; Dvin
et al., 2011).
In order to contribute to studies on the determination of optimal conditions
for plant conservation, this study has been done to study the kinetics of drying
leaves of myrtle (Myrtus communis harvested in northern Tunisia (Bizerte
region).
The Myrtus communis is the only natural species of the family Myrtaceae
in the Mediterranean. In Tunisia, the myrtle grows naturally in the north west
of Tunisia and Cape Bon. This species is highly sought for the operation of
these fruits and the essential oil extracted from these leaves and its distillate,
known as the angel of water.
The determination of sorption isotherms of leaves of Myrtus communis
is an essential step in the study of the drying and preservation, because it
determines the moisture balance (Yogananth et al.,
2011). It provides valuable information on the equilibrium moisture of the
product. The first objective in this study is to determine experimentally the
isotherms of adsorption and desorption.
The second is devoted to obtaining the drying kinetics of Myrtus communis
in conditions aerothermal controlled temperature and humidity of drying air.
MATERIALS AND METHODS
The experimental apparatus consists of eleven glass jars each with insulated
closing this glass jars is filled to quarter depth with saturated salt solution
(KOH, MgC12, K2CO3, NaNO3, KCl and
BaCl2) (Greenspan, 1977), so as to have a
relative humidity which varies from 5 to 97%. For the desorption process, the
sample of fresh Myrtus communis are put in the sample holder. But, for
the adsorption process, Myrtus communis is dehydrated in an oven regulated
at temperature of 50°C until reaching maximum dehydration.
The sample is weighted every four days. When the mass of sample become stationary,
the experiment is stopped and the sample are weighted and placed in an oven
whose the temperature is fixed at 105°C. This operation lasts 24 h.
The objective of this operation is to determine the dry masses of sample Md.
The moisture content Xeq of the product at hygroscopic equilibrium:
The same operation is repeated for both adsorption and desorption processes
at temperature 30, 40 and 50°C.
RESULTS AND DISCUSSION
The hygroscopic equilibrium of Myrtus communis is determinated in 15
days for adsorption and 20 days desorption. The results of these experiments
are presented in Fig. 1-3 at different temperature.
|
| Fig. 1: |
Isotherm of adsorption and desorption of Myrtle for T = 30°C |
|
| Fig. 2: |
Isotherm of adsorption and desorption of Myrtle for T = 40°C |
|
| Fig. 3: |
Isotherm of adsorption and desorption of Myrtle for T = 50°C |
These isotherms have the same profile for many food materials in the literature
(Pfost et al., 1976; Ait Mohamed
et al., 2005; Bag et al., 2009; Barbosa-Canovas
and Vega-Mercad, 1996; Chou and Chua, 2001; Van
den Berg and Burin, 1981).
These Fig. 1-3 show that the equilibrium
moisture content increases with decreasing temperature at constant relative
humidity. And yet, one can notice that the adsorption curve does not overlap
with the desorption curve showing a hysteresis phenomenon. Several studies in
the literature are suggested to explain this phenomenon. They showed that the
hysteresis is due to the fact that the deformation of the body during their
dehydration is not going so elastic (Ait Mohamed et al.,
2005; Al-Muhtaseb et al., 2002; Ghribi
and Chlendi, 2011).
Modeling of the adsorption and desorption isotherms: Many models have
been proposed in the literature for the adsorption and desorption isotherms
(Lahsasni et al., 2002). In this present study,
we use Chung-Pfost, BET, Henderson, Oswin, Smith…to describe the relationship
between the equilibrium moisture content data and the relative humidity. The
mean relative error was used to evaluate the fitting quality. According this
error, the Chung-Pfost model describes well the isotherms.
Drying kinetics of Myrtus communis equipment: The experimental
device for studying the kinetics of drying tunnel is a controlled atmosphere
that we have equipped with the appropriate measuring instruments.
This is a wind tunnel where you can control the temperature, velocity and humidity
(Hii et al., 2011; Chong
et al., 2008).
Experimental protocol: After fixation of different experimental conditions,
the products to be dried are placed on a support grid and traversed by hot air.
When the mass of plant becomes constant, we stopped the operation. Knowing the
wet mass, the dry mass is obtained by putting the plant at the end of every
experiment in an oven regulated at 105°C until dewatering completely the
product.
Influence of temperature: Figure 4 show that the moisture
content Xeq of the product of myrtle decreases when the temperature
of the drying air increases. From these results, we note the absence of phase
0 and phase 1, there is only the presence of the falling rate period (phase
2). This result is compatible with the drying literature (Van
den Berg and Burin, 1981; Al-Muhtaseb et al.,
2002).
|
| Fig. 4: |
Influence of drying air temperature on the evolution of water
content as a function of time |
This decrease is explained by the fact that at the beginning of drying, evaporation
of water from the surface of the product does not require a lot of energy, against
the diffusion of water from the interior part of product to the surface requires
much time.
Influence of humidity of air: The initial water content of the product
is equal to 0.23 g g-1 of dry mass sample. In this study, we present
the results obtained at T = 70°C for the other temperatures show no special
behavior to that obtained at 70°C.
Figure 5 shows the evolution of water content during drying
to the relative humidities used. This Fig. 5 demonstrates
the presence of the declining phase during the drying process. These results
are consistent with other work (Chou and Chua, 2001).
Determination of extraction efficiency and quality of essential oil:
The quality of essential oils was followed by determining their chemical composition
using gas chromatography coupled with mass spectrometry (GC/MS).
Extraction efficiency of essential oil: The Myrtus communis was
submitted to hydrodistillation with a Clevenger-type apparatus according to
the European Pharmacopoeia and extracted with water for three hours until no
more essential oil was obtained (Aliyu et al., 2011;
May and Perre, 2002; Everette and
Islam, 2012).
Vapor condensation obtained by hydrodistillation led to two phases:
| • |
An aqueous phase also called aromatic water |
| • |
An organic phase which is added sodium sulfate Na2SO4
to remove traces of water. This phase is called essential oil |
The extraction yields of essential oils are defined as follows:
|
| Fig. 5: |
Influence of drying air humidity on the evolution of water
content as a function of time |
|
| Fig. 6: |
Influence of drying temperature on extraction efficiency
of essential oils of myrtle |
| % ηorg |
: |
Efficiency of essential oils organic phase |
| % ηaq |
: |
Efficiency of essential oils aqueous phase |
| % ηTot |
: |
Total efficiency of essential oils |
The results obtained show that the extraction efficiency varies considerably
with temperature drying in the tunnel; it seems that the best efficiency was
obtained at a temperature of 80°C (Fig. 6). These results
can be explained by the fact that for too high temperature, it can cause evaporation
of some volatile compounds of essential oils from Myrtle so reducing the extraction
efficiency. The drying tunnel is a very effective method for drying products:
we noticed the speed and very good extraction efficiency for an optimum temperature
equal to 80°C.
Quality of essential oils: The essential oil is generally in the form
of a complex mixture of many compounds in varying proportions. In present study,
the qualitative analysis of essential oil was made by method of gas chromatography
coupled with mass (GC-MS) (Ismail et al., 2011;
Rashid et al., 2012; Lopez
et al., 2000).
This analysis has shown that the major components of the essential oil are
α-pinene, 1,8 cineole, limonene, para-cymene. But increasing the temperature
of drying wind tunnel caused the decrease of the compounds detected. It further
notes that the levels of compounds such as: α-pinene (major compound) and
1.8 cineole have experienced a decrease, respectively, 15.30 and 12.33%. However,
the content of other compounds such as limonene disappeared.
So we can conclude that the optimum temperature for best quality of essential
oils is 40°C.
CONCLUSION
Experimentally, we have studied for leaves of myrtle isotherms of adsorption
and desorption by the static gravimetric method for different temperatures (30,
40 and 50°C). These curves can be prescient about the behavior of the product
during (conservation and storage) to avoid changes in chemical and biological
characteristics of essential oils.
The experimental study of the kinetics of drying of this product showed the
influence of different parameters on the air drying process.
ACKNOWLEDGMENT
The authors gratefully acknowledge the Applied Thermodynamics and Environment
Unit Research ENIG for providing support of a project.
