Drying of Gelidium sesquipedale ( Rhodophyta ) Under Microwave Irradiation : Study of Thermal and Kinetic Aspects of the Drying Process

Microwave technology is currently very interesting because it offers, in many industrial processes, a new alternative energy for drying and treatment of various materials. In this context, the drying of the Gelidium sesquipedale (Rhodophyta), which is a red algae, is carried out under microwave irradiation in the aim to optimize the energy consumption in the drying process and the quality of the extracted products. The drying process of Gelidium sesquipedale under microwave irradiation is carried out by studying the thermal and kinetic aspects of drying under different irradiation conditions. The experiments were performed on samples of different initial masses ranging from 2 to 30 g. The samples are irradiated in an experimental device combining a gravimetric balance and microwave assembly that may impose different power microwave ranging from 50 to 200 W. The analysis of the experimental curves of the drying process shows that the initial mass of the sample has little influence and the applied microwave power has a significant effect on the drying kinetics. The comparative study of the drying of Gelidium sesquipedale by conventional heating and under electromagnetic field shows that the controlling kinetic mechanism depends on the way in which the sample is dried. The action of the electromagnetic field allows faster heat transfer leading to the rapid evacuation of water molecules from the seaweed.


Introduction
The drying process becomes a basic step but necessary in the food industry where the interest resides primarily in its ability to ensure a long-term storage for the food if the package is suitable.Indeed, many degradation reactions can be significantly slowed and many others can be virtually stopped (Labuza, 1975;Morris, 2011;Saravacos & Maroulis, 2011) due to a decrease in the moisture content in the food product to values less than 25%.
Over the years, the drying is performed by various methods: by convection of preheated air (Karathanos & Belessiotis, 1997;Toǧrul & Pehlivan, 2004) direct solar drying (Yaldiz & Ertekin, 2001;Doymaz, 2005;Vijaya Venkata Raman et al., 2012), drying by irradiation (Rodríguez et al., 2005;Wang, 2006;Zhang et al., 2006;Bilbao-Sàinz et al., 2006;Giri & Prasad, 2007), by freeze-drying (Graciela et al., 1993;Burgschweiger & Tsotsas, 2002;Kettner et al., 2006), under fluidized bed (Soponronnarit et al., 1997;Kannan & Subramanian, 1998;Syahrul et al., 2002), by osmotic dehydration (Raoult-Wack, 1994;Simal et al., 1997;Della Rosa & Giroux, 2001;Ozen et al., 2002;Mišljenović et al., 2012), etc.Currently, through continuous improvement, the thermal drying is one of the industrial operations that consume the most energy and the dehydration process is significant in the worldwide energy industry.In Morocco, an example of developing country, the energy used in the drying process is estimated at about 15% of the industrial energy consumption.This bill is about 25% in industrialized countries.This explains why our country has established a policy that encourages the promotion of   The analysis of the experimental curves obtained shows that all the curves obtained converge to the same final mass corresponding to the total desorption of the sample.For the initial mass of 10 g of Gelidium sesquipedale, the amount of desorbed water reaches a value close to 7.8 g (78%).The drying time decreases when the incident microwave power increase.

Kinetic
The desorption curves have a nearly sigmoid shape and start after a short latent period whose duration becomes shorter when the incident microwave power increases.Figure 3 illustrates the variation of the rate of progress of drying as a function of time for initial masses ranging from 10 to 25 g.The rate progress α is defined as the ratio of the mass of water desorbed at time t and that desorbed at the end of drying (Hnini et al., 2014): The analysis of these results shows that all the experimental curves have the same shape for all initial masses used.The drying kinetics depends slightly on the mass m 0 used.
As shown in Figure 4, the desorption of water from the seaweed starts upon application of the microwave irradiation.The instantaneous rate of desorption, which is initially very low, increases gradually to reach a maximum, then decreases continuously towards zero.It is strongly influenced by the incident microwave power.
The maximum speed of drying increases with the increase of the incident microwave power.In the range of the initial mass of the sample studied, the speed curves present the same evolution and show that m 0 has very low effect on the kinetics of drying, Figure 4. Drying of Gelidium sesquipedale under microwave irradiation.Variation of the instantaneous speed of drying against the time (Initial sample mass m 0 = 10 g)

Mechanisms of the Drying Process Under Microwave Field
The evolution of temperature versus time at constant incident microwave power is shown in Figure 5.The plotted curves (t) show that for every incident power:  the maximum temperature  m is as greater as the incident power is high,  the heating is accelerated with the increasing of the incident microwave power and stopped after a time t p (t p = 15 min for 50 W and t p = 8 min for 200 W).
 beyond this period, the drying continues at a constant temperature and the system is considered isothermal.
Taking into account the kinetic curves obtained experimentally and their evolution as a function of the incident microwave power, we note that the drying of Gelidium sesquipedale under microwave radiation seems to be realized in two different thermal phases.A first non-isothermal phase, which takes place during the transitional period t p , is due to the heating rate which increases with the temperature imposed by the microwave field.It is followed by a second phase which is carried out under isothermal conditions.As described in our previous work (Hnini et al., 2014), the variation of rate of progress α with time follows a differential law of the form: which leads by integration, under isothermal conditions to the function F(), given by: For a non-isothermal process, Equation (2) leads to the Achar equation (Achar et al., 1966), which is often used to describe the experimental curves obtained under non-isothermal conditions (Coats & Redfern, 1963, 1964;Jarez et al., 1987): where  = (dT/dt) is the heating rate, A is the pre-exponential factor and E a the apparent activation energy.
To determine the kinetic regime that controls the drying process under microwave irradiation, we use kinetic functions which are widely used in heterogeneous kinetic processes for the determination of isothermal and non-isothermal reaction mechanisms (Hnini et al., 2014).Indeed, two approaches were discussed to describe the shapes of the experimental curves.The first approach is to check the linearity of the first part of the curves, where the drying is performed with a non-isothermal heating rate which depends on the incident microwave power.The use of Equation ( 4) provides a better linearization if we choose which reflects a drying process controlled by a monodimensional diffusion regime comparable to that of the evaporation of weakly bounded water molecules.For each incident microwave power, the apparent activation energy E a can be determined from the plotted / ( ) versus 1/T (Figure 6).The values of the apparent activation energy are reported in Table 1.
versus 1/T according the Achar equation (Equation ( 4)) Table 1.Drying of Gelidium sesquipedale under microwave irradiation.Values of the activation energy E a (given by the Achar's method) The second approach allows to the linearization of the second part of the curves (t > t p ) using the isothermal kinetic models characterized by the kinetic equations, depending in particular on the nature of the mechanism of the phenomenon (decomposition, dehydration, drying).The experimental curves are better transformed (Figure 7) according to the Jander's equation reported by Sharp et al. (1966): which shows that beyond of t p , the kinetic regime that regulates the drying process is the three-dimensional diffusion.
The slopes k'(T) are attributed to the rate constants and related to the temperature imposed by the microwave field.According to the Arrhenius law, the plotted of Lnk'(T) versus 1/T (Figure 8) is a straight line that allows the determination of the apparent activation energy E a .The value obtained is 10.3 kJ•mol -1 .

Discussion and Interpretation of Results
The analysis of all results provides information on the kinetics and mechanisms of the thermal drying and confirms the nature of water molecules previously defined (Hnini et al., 2013).The kinetic mechanism that regulates the drying process under microwave field is not unique (Figure 8, domains I and II).Two mechanisms of desorption of water molecules have been identified in this work:  A mono-dimensional diffusion mechanism, similar to that obtained during a rapid evaporation of molecules of free water surface, takes place at the beginning of the drying process, after a time corresponding to the setting temperature of the sample under microwave irradiation (Figure 5, domain I).If the intensity of the microwave field is high, its effect on the weakly bound water molecules is important, and the amount of desorbed water is high.The drying process studied here concerns the desorption of 80% of water weakly bound molecules.It should be noted that the apparent activation energy is close to the heat obtained during the evaporation of free water, which is about 43.9 kJ•mol -1 .The apparent activation energy, determined from the slope of the transformed curves, encompasses the true value of the activation energy of drying process and other values related to the thermodynamics quantities. A mechanism of three-dimensional diffusion of water molecules according to the Jander's law, which takes place beyond the first minutes, once the temperature reaches the thermal level of the applied microwave power (Figure 5, Domain II).This heat level, similar to a thermal equilibrium, can be linked to a transient state caused by the balance of the microwave energy desorbed (heating mode) and the energy needed for desorption of water molecules (endothermic phenomenon).The heat transfer to the heart of the sample is quick and the desorption of water molecules is also very fast.The drying is done quickly with a mass transfer influenced by the effect of the imposed temperature.The global phenomenon is controlled by the three-dimensional diffusion characterized by the linearization of the experimental curves according to the Jander's equation.This diffusive regime is sometimes dominated by mechanical phenomena which start from the first minutes.
The results also indicate that the apparent activation energy depends on the temperature imposed by the microwave power and how the heat is transferred to the algae.The value of the apparent activation energy obtained under these conditions (domain II) is about 10.3 kJ•mol -1 .This difference in the apparent activation energy is due, on the one hand, unlike the mechanisms involved, and secondly, by the mechanical phenomena that begin early in domain I.

Conclusion
In this work, we studied the kinetic and thermal aspects of drying of Gelidium sesquipedale under microwave irradiation for different initial masses.Two main objectives were set: understanding the influences of the incident microwave power and the initial mass of the sample and determining the mechanisms that control the drying and kinetic parameters that can be used to optimize the drying.
The experimental results were obtained by performing drying under atmospheric pressure at different incident microwave powers varying from 50 to 200 W for different initial masses of between 10 and 25 g.The evolution of the instantaneous mass m d (t) of the sample is carried out by gravimetry using a balance.The experience is completed when the total mass of the sample remains unchanged and the reproducibility of the results is ensured by repeating the experiment three times.
The temporal evolution of the temperature within the sample under different microwave powers shows the existence of two areas: a first one is characterized by a linear increase in temperature with a drying speed proportional to the microwave power used, followed by a second of which operates isothermally.During the first area, we have applied the Achar method which is a non-isothermal kinetic model.The linearization of the curves obtained allowed to determine values of the activation energy ranging between 7.5 and 89.2 kJ•mol -1 .In the second area the kinetic of the isothermal regime is a three-dimensional diffusion according to the Jander's equation; the corresponding activation energy is about 10.3 kJ•mol -1 .
The comparison of these results with those obtained during drying by conventional thermally conducted at atmospheric pressure at different temperatures ranging from 30 to 80 °C with different initial sample masses (Hnini et al., 2014) shows that:  The drying time by conventional heating is greater than the drying time under microwave irradiation which corresponds to a gain in energy. During the conventional heating, the initial sample mass has a significant effect on the kinetic as well as on the duration of the drying process.This will result in more energy consumption at industrial level when drying large masses (overcrowding effect) unlike the microwave technology where the mass effect is small because the microwave irradiation reacts inside the material. The dried sample by microwave maintains its color and its original form whereas for the conventional way, rods become distorted and take a darker color particularly to the high temperatures.This can be explained by the degradation of organic matter in the seaweed.A further work is being finalized, focuses on the qualitative and quantitative of the amount of extracted agar-agar and agarose from the sesquipedale Gelidium dried by conventionnel heating and under microwave irradiation.
Figure 2 s g 10 m 0 

Figure
Figure 3. E

Figure 5 .
Figure 5. Thermal aspect under microwave irradiation of a sample of Gelidium sesquipedale.Curves of temperature changes for a sample of initial mass m 0 = 10 g Figure 6.Variation of / ( ) d Ln f dt

Figure 7 .Figure 8 .
Figure 7. Linear transformations of the drying curves under different incident powers