Water sorption isotherms and drying characteristics of rupturewort (Herniaria hirsuta) during a convective solar drying for a better conservation

Solar Energy 201 (2020) 916–926

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Water sorption isotherms and drying characteristics of rupturewort (Herniaria hirsuta) during a convective solar drying for a better conservation

T

Younes Bahammoua, Haytem Moussaouia, Hamza Lamsayeha, Zakaria Tagnamasa, ⁎ Mounir Kouhilaa, Rachida Ouaaboub, Abdelkader Lamharrara, , Ali Idlimama a

Laboratory of Solar Energy and Medicinal Plants, Cadi Ayyad University, BP 2400 Marrakesh, Morocco LICVEDDE (Laboratory of Innovation and Sustainable Development & Expertise in Green Chemistry), Faculty of Science Semlalia, Department of Chemistry, Cadi Ayyad University, B.P. 2390, Marrakesh 40000, Morocco

b

A R T I C LE I N FO

A B S T R A C T

Keywords: Herniaria hirsuta Solar drying Drying characteristic curve Modeling Bioactive compounds Conservation

Free water contained in porous structure of food plays a fundamental role in proliferation of microorganisms and bacteria that can damage the product materials. This work presents a thermo-kinetic study carried out in a natural and forced convection solar dryer; the objective was to establish the optimal conditions for drying and storing Herniaria hirsuta by testing the impact of different aerothermal conditions (air temperature, air velocity) on water loss of fresh Herniaria hirsuta. The kinetics of drying is studied for three temperatures (50, 60 and 70 °C) in two-air velocities (0.09 and 0.18 m/s). The optimal water activity which is related to conservation is 0.28. The LESPAM model was found to be the most appropriate for describing the sorption curves. The air drying temperature is a principal factor influencing the drying kinetics; the drying rate decreases in low air drying temperature, the air velocity had a small impact on the drying kinetics of Herniaria hirsuta. The Midilli-Kuck model was found to be the best fitted drying curves in thin layers for Herniaria hirsuta. The effective diffusivity of moisture water values changed from 2.5312 10−9 to 18.0511 10−9 m2·s−1, and increased simultaneously with the increase of temperature. The average activation energy of the diffusion process is obtained to be 2938.46 kJ/ kg; it expresses the temperature effect on the diffusion coefficient. Dried Herniaria hirsuta under the maximum drying parameters showed highest retention of antioxidant activity, while the total phenolics and flavonoids were decreased by 10 and 46%, respectively.

1. Introduction Aromatic and medicinal plants were the focus of several studies (physical, chemical, biological…), this is due to their predominance and utilization in various sectors such as economical, industrial and pharmaceutical fields (Bueno-Sánchez et al., 2009; Sengul et al., 2009). Herniaria hirsuta is one of the plant which is characterized by medicinal proprieties (Fouada et al., 2006; van Dooren et al., 2015). It is a light green colour, with a somewhat thick root 5–20 cm in depth, commonly known as rupturewort in Europe and locally named “Harrast lhjar” (Ouhaddou et al., 2014). It is an astringent, actively diuretic and expectorant (Rovčanin et al., 2015). It has also gained a reputation for treating hernias, nerve inflammation, respiratory disorders, dropsy treatment, catarrh of the bladder, cystitis and kidney stones, removing excess of mucus in the stomach and increasing the flow of urine,

⁎

externally. It has been used as a poultice to speed the healing of ulcers (Atmani et al., 2004; Rovčanin et al., 2015 Dooren et al., 2015). In Moroccan traditional medicine, it is collected when still in flower (May to July), it has widely used in the pharmaceutical and therapeutic sectors to treat and cure several diseases (Atmani et al., 2004; Dooren et al., 2015). Aromatic and medicinal plants including Herniaria hirsuta can be used in different basic forms; essential oils, active ingredient, organic solutions or extracts aqueous and vegetable drugs fresh or dried. The dried form is highly demanded, if compared to the fresh one, because it makes transport and storage easier (Bahammou et al., 2019b; Moussaoui et al., 2018b), as a result, it is a preferable way to conserve aromatic and medicinal plants and to exploit their seasonal abundance. Water activity (aw) and sorption isotherms are the two most powerful concepts available in the literature for understanding and controlling the agri-food’s shelf-life. Water activity is, by definition, the

Corresponding author. E-mail address: [email protected] (A. Lamharrar).

https://doi.org/10.1016/j.solener.2020.03.071 Received 7 October 2019; Received in revised form 28 January 2020; Accepted 19 March 2020 Available online 31 March 2020 0038-092X/ © 2020 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.

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physical measure of ‘active’ water existing in agri-foods. This active water is responsible for the development of microorganisms, chemical reactions and enzymatic activities that can damage the agri-food materials. Although parameters such as temperature, pH and osmotic pressure and several others can influence how long the agri-foods can be stored, water activity is the one which influences the shelf-life of most agri-foods. For example, growth of most bacteria is inhibited below water activity (aw) of 0.91 and most moulds are inhibited at water activity below 0.80. Furthermore, water activity can have a major impact on the colour, taste and aroma of agri-foods. On the other hand, sorption isotherms describe how actively water is bound to a solid and present thermodynamic data which are necessary to design calculations belonging to the drying process (Lahnine et al., 2016a; Moussaoui et al., 2018a; Červenka et al., 2015). Sorption isotherms make it also possible to envisage the paces of drying, indicate the distribution and the intensity of the connections of water and then to classify materials by order of hygroscopicity. Determination of the optimal water activity for conservation by using the sorption isotherms is necessary to know during the drying operation. It allows to approach a state of steady balance and thus to guarantee better conservation (Basu et al., 2006). Drying is considered as an old method to conserve medicinal and aromatic herbs (Krokida et al., 2003). Since times, solar energy has been considered an alternative source and renewable energy for treating and conserving agri-foods. It has been identified as a promising solution to dry aromatic and medicinal plants in developing countries like Morocco. This is due to its minimal operational in terms of energy cost (Sandnes and Rekstad, 2002). Solar drying appears as a non-polluting process. It decreases the conventional rate of energy consumption caused by conventional drying systems and considerably reduces the water and microbiological activity of agri-food products. It minimizes physical and chemical reactions during their storage and improves their high quality (Bahammou et al., 2019a). In this perspective and in order to contribute to the conservation of the aromatic and medicinal plants of Morocco, this work aims to study the influence of different drying parameters on water loss of Herniaria hirsuta by using an indirect forced convection solar dryer. The standard static gravimetric method at three temperatures 30, 40 and 50 °C was used to determine the sorption isotherms of Herniaria hirsuta. This study was mainly concerned with:

Fig.1. Fresh medicinal plant Herniaria hirsuta.

Table 1 Standard values of the water activities of the salts used in the experiments.

30 °C 40 °C 50 °C

KOH

MgCl2, 6H2O

K2CO3

NaNO3

KCl

BaCl2, 2H2O

0.7380 0.6260 0.5720

0.3238 0.3159 0.3054

0.4317 0.4230 0.4091

0.7275 0.7100 0.6904

0.8362 0.8232 0.8120

0.8980 0.8910 0.8823

The solutions were prepared in hermetic jars and were maintained isothermally in a drying oven-controlled in temperature (Baucour and Daudin, 2000; Zungur Bastıoğlu et al., 2017). The mass used in the experiment was 2 ± 0.0001 g and 1 ± 0.0001 g for desorption and adsorption, respectively. The sample for desorption was weighed as a fresh form and placed into glass jars. The sample for adsorption was dried in a drying oven at 70 °C for 48 h after it was weighed as a dried form and placed into glass jars (Fig. 2). The samples are suspended in the jar, above salts (Fig. 2) and remain in a stabilized environment in terms of temperature and relative humidity. The weight recording period was about 4 days; the weight was measured daily for 4 days until the sample weight became constant. Then, the equilibrium moisture content of the samples was determined in a drying oven at 70 °C for 48 h. A dried mass of samples is determined, which allows determining the equilibrium moisture content of each sample for a given water activity. The hygroscopic equilibrium of Herniaria hirsuta was reached in 10 days and 7 days for desorption and adsorption, respectively. Thus, the equilibrium moisture content (Zeq) of the product at hygroscopic equilibrium is calculated by the following expression (Moussaoui et al., 2018a):

• Study the influence of water activity and temperature on the sorption isotherms of Herniaria hirsuta; • Evaluation the effect of several drying parameters on the drying kinetics of Herniaria hirsuta; • Determination of the characteristic drying curve (CDC) of Herniaria hirsuta; • Fitting the sorption isotherms and the drying curves with several numbers of computer modeling programs; • Evaluation the effects of solar convective drying on bioactive compounds of Herniaria hirsuta.

Zeq =

2. Material and methods

m w − md md

(1)

where: m w and md are the mass before and after drying oven at 70 °C for 48 h, respectively.

2.1. Sorption isotherms 2.1.1. Herniaria hirsuta Fresh Herniaria hirsuta (rupturewort), used in the sorption isotherms and the drying experiments, was obtained from a region of the province of Marrakesh, Morocco (Fig. 1). The standard static gravimetric technique was adopted to determine the sorption isotherms of Herniaria hirsuta, it consists of using six saturated salt solutions: KOH, (MgCl2, 6H2O), K2CO3, NaNO3, KCl, and (BaCl2, 2H2O). According to different temperature values, the saturated salt solutions have a range of water activities of 0.3054–0.8980 as shown in Table 1 (Moussaoui et al., 2018a).

2.1.2. Mathematical description of sorption isotherms The experimental isotherms of sorption were studied using some several models existing in the literature as shown in the following table (Basu et al., 2006) (see Table 2): Where: A, B, C, and D are the model coefficient’s and θ (°C) is the temperature of sorption isotherm, To choose the appropriate equation for describing the sorption isotherms and the drying kinetics of Herniaria hirsuta, it is better to rely on the following statistical parameters: the correlation coefficient (r), 917

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(1) Salt solution, (2) sample holder (tripod), (3) beaker for the product, (4) airtight (hermetic) jar, (5) controlled temperature oven. Fig. 2. Experimental apparatus for measuring the sorption isotherms. (1) Salt solution, (2) sample holder (tripod), (3) beaker for the product, (4) airtight (hermetic) jar, (5) controlled temperature oven.

moisture content Zeq of the third degree; consequently, it’s possible to calculate the value of the optimum relative humidity for conservation by the second derivative of Zeq called “inflection point” (Moussaoui et al., 2018a).

Table 2 Moisture sorption isotherm models used to analyze the experimental data for Herniaria hirsuta. Name of the model

Mathematical expression

GAB

Zeq =

LESPAM

ABCaw (1 − Baw )(1 − Baw + BCaw )

Zeq = A exp

Enderby

Zeq =

Smith

(

2.2. Drying experiments

( )+C Baw θ

A 1 − Baw

−

C 1 − Daw

)a

2.2.1. Indirect forced convection solar dryer Herniaria hirsuta drying experiments were carried out in an indirect forced convection solar dryer coupled with a solar collector (Fig. 3). It is a system without storage with total or partial recycling of air. The system’s components are as follows:

w

Zeq = A + B log(1 − aw )

Modified Oswin

Zeq = (A + Bθ)

(

C aw 1 − aw

)

• A simple circulation solar sensor with single glazing, 2.5 m

2

the standard error of estimate (Sr) and the reduced minimum square ( χ 2 ) (Akpinar and Bicer, 2008; Menges and Ertekin, 2006; Midilli and Kucuk, 2003): N

∗ − Z¯ ∗eqi,exp)2 ∑ (Zeq i, pred

r=

i=1 N

∗ − Z¯ ∗eqi,exp)2 ∑ (Zeq i,exp

i=1 N

Sr =

∑

•

(2)

(Z ∗eqi,exp − Z ∗eqi, pred )2 f

i=1

(3)

• •

N

∗ ∗ 2 ∑ (Zpre , i − Zexp, i )

χ2 =

i=1

N−n

(4)

where: ∗ ème Moisture ratio predicted by the model Zpre ,i i ∗ ème Experimental moisture ratio Zexp i ,i n Number of variables in each model N Number of experimental data f Degree of freedom of the regression model

• •

2.1.3. Determination of the optimal relative equilibrium moisture for drying and storing The study of the adsorption-desorption isotherms allows us to know the optimal water activity for the conservation of this medicinal plant and the water content of equilibrium to arrive at the end of the drying. In addition, it provides researchers with accurate information on how to control a product during storage and conservation (Lahnine et al., 2016a). For this purpose, the optimal water activity for conservation a wop was determined. It models the whole of the experimental points of the sorption isotherm by a polynomial equation of the equilibrium

surface, inclined 31° to the horizontal plane and facing south. The cover is in ordinary glass. The sensor has an area of 2.5 m2 (2.1 m long and 1 m wide). The absorber of the solar collector is made of a black galvanized iron sheet of 0.5 mm thickness with a non-selective surface. The insulating absorber distance is 0.025 m and the glass absorber distance is 0.02 m. The rear thermal insulation is 0.05 m thick polyurethane foam sandwiched between two steel sheets. A ventilation duct consisting of a section tunnel cuboid. A double T (consisting of two nested T) allows total or partial recirculation of the air leaving the drying chamber after crossing all racks. The double T has a throttle valve to control the air flow. A drying chamber (dimensions 1.40 m high, 0.90 m deep and 0.50 m wide). It consists of ten racks. A centrifugal fan (0.0889 m3.s−1; 80mmCE; 220 V. 0.1Kw), allows a theoretical air velocity of 1.7 m·s−1, with an upstream throttling that allows to vary the air flow A thermoregulator (range 0–100 °C and precision 0.1 °C) connected to a platinum probe PT100 acting on the electric auxiliary heating alows to fix the set temperature at the entrance of the drying chamber. Electrical power heaters with 4 kW acting (auxiliary source of energy).

Detail of different instruments used in the drying experiments is given in the following table (Table 3): 2.2.2. Experimental procedure The fresh plant Herniaria hirsuta used in the drying experiments was grown in Marrakech, Morocco. The fresh mass used in drying experiments was (20 ± 0.1) g by tray (127.80 ± 0.1 g presents the mass of sample with tray) (Fig. 4). However, the samples were uniformly spread evenly on a drying tray that was then placed on the first shelf of 918

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(1) control box, (2) air drying flow channel, (3) electrical back-up, (4) fan, (5) ventilation duct, (6) air flap, (7) air outlet , (8) humidity sensor, (9) thermocouple , (10) floors, (11) sample holder, (12) drying cabinet, (13) air inlet, (14) solar collector, (15) Sun rays. Fig. 3. Indirect solar dryer used in the experiment. (1) Control box, (2) air drying flow channel, (3) electrical back-up, (4) fan, (5) ventilation duct, (6) air flap, (7) air outlet, (8) humidity sensor, (9) thermocouple, (10) floors, (11) sample holder, (12) drying cabinet, (13) air inlet, (14) solar collector, (15) Sun rays.

becomes constant (Bahammou et al., 2019a; Ouaabou et al., 2018).

Table 3 Different instruments used in all drying experiments. Instrument

Characteristic

Thermo-hygrometer HI 9564

°C/°F temperature readout ABS body RH probe with built-in microchip, 250-hour battery life with a battery level indicator Data hold function to hold measurement values High accuracy and rapid response Sunlight measurement up to 1999 W/m2 or 634BTU/ (ft2*h). Unit and sign display for easy reading Manual scale selection Direct reading with no adjustments needed Measuring unit selection among W/m2 and BTU/ (ft2*h) Maximum and minimum values and low battery indication RS232 Interface Accuracy 0.001 g Glass wind-shield

Solar power meter

Digital weighing device

2.2.3. Moisture content and characteristic drying curves The final water content Zeq is an essential feature to be determined for each product (which is deduced from the sorption isotherms), it allows to know the optimal value for which the product does not deteriorate and keeps its organoleptic and nutritional qualities during the drying process (texture, color, form and essential oils) (Karathanos, 1999). For all the experiments conducted in the outdoor conditions, drying Herniaria hirsuta is carried out in a solar dryer by setting the temperature and following the evolution of the mass loss m(t) of the product over time by successive weightings until m(t) becomes stationary (when he final water content is reached). This is followed by total dehydration of the final mass (mf ) in an oven at 70 °C for 48 h for determining the dry mass (mf) of Herniaria hirsuta. Dry based moisture content at time (t) is defined by:

Z=

m (t ) − mf mf

(5)

The dimensionless moisture content is calculated as follows:

the drying cabinet. The heated air enters the drying cabinet below the trays and flows upwards through the samples (Fig. 4). During each drying experiment, the weight of Herniaria hirsuta on the tray was measured by removing it from the drying cabinet for about 15–20 s, these measurements were conducted each 10 min at the beginning of the experiment and 60 min at the end until the weight of the mass

Z → Z∗ =

Z − Zeq Z0 − Zeq

0 ⩽ Z∗ ⩽ 1

(6)

where Z0 presents the initial water content and Zeq presents the final water content

Fig. 4. Drying chamber and digital weighing device used in the drying experiments of Herniaria hirsuta. 919

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gas constant (J / mol. k ) , and T is the air drying temperature (K).

Table 4 Selected mathematical models applied to the drying curves. Model name

Model equation

2.3. Quality analysis

Newton Page Henderson and Pabis Wang and Singh

Z ∗ = exp(−kt ) Z ∗ = exp (−kt n) Z ∗ = a exp (−kt )

2.3.1. Extraction procedure 2 ± 0.0001 g of Herniaria hirsuta was powdered and introduced into a flask then a 20 mL of methanol was added. After 30 min of stirring, the mixture was centrifuged and filtered through a whatman filter paper no.1 (Whatman International. Ltd.)

Z ∗ = 1 + at + bt 2 Z ∗ = a exp (− kt ) + (1 − a) exp (− kbt ) Z ∗ = a exp (−kt n) + bt

Approximation of diffusion Midilli-Kucuk

2.3.2. Determination of total phenolics content To determine the total phenolic content, the method of FolinCiocalteu was used (Arabshahi-Delouee and Urooj, 2007); 0.25 mL of extract was mixed with 2 mL of distilled water, 0.25 mL of Folin-Ciocalteu reagent, and 0.25 mL of sodium carbonate (20%). The whole was incubated for 30 min and the reading was taken against a white using a spectrophotometer at 760 nm. The results were expressed as mg gallic acid equivalent in g dry weight (mg GAE/g DW).

And the dimensionless drying rate f (−) is calculated as follows:

( ) ( )

− ⎛− dZ ⎞ → f = dt ⎝ ⎠ −

dZ dt

dZ dt 0

0⩽f⩽1 (7)

2.2.4. Modeling drying curves Modeling drying curves consists in developing a function that verifies the equation of experimental data as a function of drying time. Various models existing in the literature have been tested to describe the drying data of Herniaria hirsuta (Krokida et al., 2003). The following table represents the most appropriate equations selected in the literature to fit the drying data of Herniaria hirsuta (Table 4):

2.3.3. Determination of flavonoid content The determination of total flavonoids was performed according to the colorimetric assay of Quettier-Deleu et al. (2000). A quantity of 1 mL of the extract was mixed with 1 mL of aluminum chloride. The absorbance was measured at 420 nm after incubation for 15 min. The results were expressed in milligram quercetin equivalent in g dry weight (QE mg/g DW).

2.2.5. Effective diffusivities According to the experimental drying curves of Herniaria hirsuta, only a falling drying rate period and liquid flow controls process were observed. Fick’s second law can be used to describe the drying curves. The solution of Fick’s second law in slab geometry was found from using the following equation cited by Driscollb and Buckleb (1996) (Driscollb and Buckleb, 1996).

Z∗ =

8 π2

∞

∑ n=0

(2n + 1)2π 2Deff t ⎞ 1 exp ⎛⎜− ⎟ 2 (2n + 1) 4L2 ⎝ ⎠

2.3.4. Determination of antioxidant activity The free radical-scavenging activity of extracts was evaluated by 1,1-diphenyl-2-picrylhydrazyl radical (DPPH) according to the method reported by Şahin et al. (2004). 50 μL of the extract was mixed with 2 mL of DPPH methanolic solution; then the mixture was agitated vigorously and incubated for 60 min. The absorbance of the samples was measured at 517 nm. The inhibition activity was calculated as follows:

(8)

For long drying periods Eq. (8) can be expressed in logarithmic from: 2 ⎛⎜ π Deff 4L2

8 Ln (Z ∗) = Ln ⎛ 2 ⎞ − ⎝π ⎠ ⎝

% of radical scavenging activity =

t⎞ (9)

where Z ∗ is the moisture ratio, Deff is the effective diffusion coefficient of Herniaria hirsuta and L is the half-thickness of Herniaria hirsuta. A plot of Ln (Z ∗) versus drying time for each fixed air temperature allows determining the effective diffusivity from slope of Eq. (9), the slope was obtained as:

Slope =

(12)

3.1. Sorption isotherms of Herniaria hirsuta (10)

3.1.1. Adsorption and desorption isotherms Fig. 5 represents the result of the sorption isotherms and the hysteresis phenomenon of Herniaria hirsuta at different sorption temperatures. The hygroscopic equilibrium of Herniaria hirsuta was reached in nine days for desorption and eighth days for adsorption. The experimental data of sorption isotherms are illustrated in Fig. 5-a and 5-b. The isotherms have a sigmoid shape (type II), which is common for many hygroscopic products. For any constant water activity, an increase in temperature significantly decreases the equilibrium moisture content (EMC). This effect is due to the activation of the water molecules by temperature that split them from water binding sites, which lowers the equilibrium moisture content (Bensebia and Allia, 2016; Červenka et al., 2015; Lahnine et al., 2016a). Indeed, there exists a phenomenon of hysteresis (Fig. 5-c). For the reason that the bound fraction is always larger on desorption than

2.2.6. Activation energy The activation energy in a drying process Ea is the equivalent of a potential barrier that opposes the progress of the reaction which must be overcome for the drying process (Doymaz, 2007). The origin of the self-diffusion of water content is the thermal agitation. The diffusion of water content is thermally activated, and the diffusion coefficient of water content follows an Arrhenius law. The correlation between the determined values of the effective diffusivity and the drying conditions is expressed by an Arrhenius equation (Hii and Ogugo, 2014) as follows:

E Deff = D0 exp ⎛− a ⎞ ⎝ RT ⎠

× 100

3. Results and discussion

π 2Deff 4L2

Abssample

where: Abscontrol : Absorbance of the control Abssample : Absorbance of the sample. The results obtained were reported as DPPH % per g of dry weight (DPPH % /g DW).

⎟

⎠

Abscontrol − Abssample

(11)

where Ea is the activation energy of the moisture diffusion D0 is the preexponential factor of the Arrhenius equation (m2 / s ), R is the universal 920

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Y. Bahammou, et al. 60

50

60

Desorption at 50 °C 40

(a)

30

20

10

Adsorption at 40 °C

40

Equilibruim moiture content Zeq (%d.b)

Equilibrium moisture content Zeq(%d.b)

Equilibrium moisture content Zeq (%d.b)

Desorption at 40 °C

50

Desorption at 30 °C

Adsorption at 30 °C

Desorption at 30 °C

Adsorption at 50 °C

30

(b) 20

10

0

0 0.0

0.2

0.4

0.6

0.8

1.0

Adsorption at 30 °C

50

40

(c)

30

20

10

0

0.0

0.2

Water activity a w

0.4

0.6

0.8

1.0

0.0

Water activity a w

0.2

0.4

0.6

0.8

1.0

Water activity a w

Fig. 5. Desorption (a), adsorption (b) isotherms and hysteresis phenomenon (c) of Herniaria hirsuta at sorption temperature conditions. Table 5 Statistical parameters for each model according to the sorption temperature. Model

Parameters

Desorption

GAB

θ (°C)

30

40

50

30

40

50

A B C r Sr A B C r Sr A B C r Sr A B C D r Sr A B r Sr

3,5879 0,873 3,5836 0,9854 3,6890 0,1501 187,2726 6,5245 0,9983 1,2693 −8,4894 0,6837 0,6289 0,9954 2,0720 6,7703 0,9557 1490,7398 −300,0115 0,9995 0,8558 −16,1580 −278,6953 0,9998 0,4213

3,4503 0,836 3,4503 0,9904 2,8632 0,2430 228,596 4,7532 0,9978 1,3613 −0,1405 0,6237 0,6701 0,9947 2,1368 7,5423 0,9401 1,62.108 −5,00.107 0,9974 1,8197 −15,4763 −271,09 0.9981 1,1383

3,2683 0,905 3,2683 0,9873 3,2073 0,2204 292,654 4,0832 0,9950 2,0241 −19,531 0,5892 0,7245 0,9888 3,0196 4,8341 0,9051 4,8341 0,9051 0,9873 3,9281 −17,8052 −28,3909 0,9909 2,6345

3,4278 0,874 3,4262 0,9890 2,9599 0,1899 177,0436 5,2539 0,9975 1,4131 −9,0886 0,6637 0,6389 0,9963 1,7126 6,7160 0,9448 520,736 −131,2457 0,9987 1,2514 −14,7778 −25,5653 0,9998 0,3266

3,1143 0,912 3,1143 0,9898 2,8193 0,1429 250,3819 4,4147 0,9968 1,5968 −14,7904 0,6052 0,7138 0,9931 2,3293 1,891.107 −6,77.106 6,5706 0,95637 0,9958 2,2106 −18,2783 −27,3756 0,9975 1,3400

3,0941 0,912 3,0940 0,9885 2,8530 0,1785 300,52 3,8487 0,9953 1,8243 −19,967 0,5806 0,7337 0,9905 2,5924 6,7064 0,9543 4,3565.108 −1,8356.108 0,9933 2,6632 −17,3803 −26,8074 0,9942 1,9709

LESPAM

Oswin modified

Enderby

Smith

Adsorption

Table 6 Experimental data obtained during the solar drying of Herniaria hirsuta. Experiment

Air drying velocity (m/s)

Air drying temperature (°C)

Ambient air temperature (°C)

Ambient air relative humidity (%)

Zi

Zf

t (min)

1 2 3 4 5 6

0.18 0.18 0.18 0.09 0.09 0.09

50 60 70 50 60 70

26.09 26 30.36 21 27 28.4

25 18 12 38 35 21

2.8321 2.7593 2.6363 2.6527 2.7358 2.7950

0.0278 0.0151 0.0363 0.0909 0.0773 0.0075

140 80 50 190 95 55

on adsorption, hysteresis in the sorption isotherm is a consequence of variation in the fraction of bound water present in the adsorption and desorption processes (Bensebia and Allia, 2016; Červenka et al., 2015; de Burgh and Foster, 2017; Durakova and Menkov, 2005; Lahnine et al., 2016a; Vega-Gálvez et al., 2008).

the basis of standard error Sr and the correlation coefficient r (Table 6). It can be observed that the minimum values for Sr and the maximum r for Herniaria hirsuta were for the LESPAM and Smith models, (Table 5). The LESPAM model was selected to be the most suitable for describing the sorption curves of Herniaria hirsuta.

3.1.2. Modeling of sorption experimental data The experimental data of the adsorption and desorption curves of Herniaria hirsuta were analyzed through the use of different models on

3.1.3. Determination of the optimal water activity for drying and storage Fig. 6 shows the variation of water activity of Herniaria hirsuta as a function of equilibrium moisture content under the experimental 921

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equation of the third degree (Eq. (13)). This calculation method allows to calculate the value of the optimal water activity of conservation. It consists of decomposing polynomial of equilibrium moisture content (EMC), for all the experimental results for each product based on water activity. It can be seen that the experimental results of sorption of Herniaria hirsuta at different temperatures present important information about the conditions to keep it in reserve (Fig. 6). The value of the optimal water activity for the conservation of Herniaria hirsuta is found as 0.28, this value is in agreement with the range of the optimal water activities found in the literature (0.2–0.4) (Bahammou et al., 2019a; Moussaoui et al., 2018a).

Experimental data Polynomial Fit of Data

50 40 30

Inflection point

20 10 0

3.2. Drying kinetics analysis 0.2

0.4

0.6

0.8

1.0

The initial moisture content for all the drying experiments of the Herniaria hirsuta varied from 2.6363 to 2.8321 %d.b (dry basis) and was reduced to the final moisture content, which varied from 0.0075 to 0.0909 %d.b (Table 6). The experimental drying conditions of Herniaria hirsuta are shown in Table 6. The inclined solar radiation during a day period of Herniaria hirsuta was observed to be higher than the horizontal one as observed in Fig. 7.

Water activity aw Fig. 6. Optimal water activity for conservation of Herniaria hirsuta. 2

Inclined solar radiation(w/m ) 2 Horizontal solar radiation(w/m ) Ambient air temperature (°C) Ambient air relative humidity(%)

1000

35 34

900

33

3.2.1. Determination of the drying curves Fig. 8 represents the variation of the moisture content and drying rate as a function of drying time under different air drying conditions. Modeling drying time against moisture content and drying rate gives valuable indications about the variation of water loss of fresh Herniaria hirsuta under different drying air conditions, as shown in Fig. 8. The shape of the drying curves for Herniaria hirsuta sample is similar to that obtained for other plants indicating a rapid moisture removal from the product at the initial stage, which later decreased with an increase in drying time. Thus the moisture content decreased continually with drying time. This continuous decrease in moisture content indicates that diffusion has governed the internal mass transfer. This is in agreement with the results of the study on mint (Sallam et al., 2013) and olive-waste cake (Vega-Gálvez et al., 2010). In addition, the influence of air drying temperature and air velocity on changes in the drying rate of Herniaria hirsuta is shown in Fig. 8. As shown, under all operating conditions, there was no constant rate for all curves and all the drying operations occur in the falling rate period. A similar trend was observed by (Doymaz, 2012; Sallam et al., 2013). It is also to be noticed that the drying rate at the end of all drying experiments is virtually zero. This reduction is related to the lower evaporation rate in the product (Lamsayeh et al., 2020). At a constant air velocity, the drying rate increases and the drying time of Herniaria hirsuta decreases greatly when the air drying temperature increases. Indeed, air drying temperature is known to be the

32 31

700

30

600

29

500

27

28 26

400

25 24

300

23 22

200

21 100

20 19

0 7

8

9

10

11

12

13

14

15

16

17

18

19

Time (h) Fig. 7. Variation of climatic conditions versus time during a day period of solar drying Herniaria hirsuta.

conditions. A polynomial equation of equilibrium moisture content based on water activity of Herniaria hirsuta is expressed as follows:

Zeq = 371.01a w + 122.100a w2 − 2.1204a w3

r = 0.9999

(13)

The experimental sorption isotherms are described as a polynomial -1

=50°C Av=0.09 m.s

Moisture content Z(% d.b)

3.0

(a)

-1

-1

=50°C Av=0.18 m.s

2.5

=60°C Av=0.09 m.s

2.0

=60°C Av=0.18 m.s

-1 -1 -1

=70°C Av=0.09 m.s

1.5

-1

=70°C Av=0.18 m.s Natural sun drying

1.0 0.5

=70°C AV=0.18 m.s

(b)

0.14 -1

-2

Solar radiation (W.m )

800

Drying rate (% d.b.min )

Equilibrium moisture content Zeq (%d.b)

60

-1

=70°C AV=0.09 m.s

0.12

=60°C AV=0.18 m.s

0.10

=60°C AV=0.09 m.s

0.08

=50°C AV=0.18 m.s

0.06

=50°C AV=0.09 m.s

-1 -1 -1 -1

Natural sun drying

0.04 0.02 0.00

0.0 0

50

100

150

200

250

300

0

Drying time (min)

50

100

Drying time (min)

Fig. 8. Moisture ratio (a) at drying rate (b) as a function of drying time at drying air conditions. 922

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Table 7 Statistical parameters for each model according to the air drying temperature and air velocity. Av = 0.18 m/s Models

Newton

Page

Henderson and Pabis

Wang and Singh

Approximat-ion of diffusion

Midilli-Kucuk

T (°C)

Model coefficients

Av = 0.09 m/s Statistical parameters

Model coefficients

Sr

r

χ2

50 k = 1.88 10−2 60 k = 4.94 10−2 70 k = 8.51 10−2 Natural sun drying 50 K= 1.21 10−2 n = 1.11 60 k = 4.37 10−2 n = 1.04 70 k = 3.17 10−2 n = 1.37 Natural sun drying

0.0269 0.0111 0.0460

0.9962 0.9993 0.9913

0.9925 0.9987 0.9827

0.0209

0.9979

0.9959

0.0098

0.9995

0.9990

0.0080

0.9997

0.9995

50

a = 1.02 k = 1.91 10−2 60 a = 1.01 k = 4.99 10−2 70 a = 1.05 k = 8.86 10−2 Natural sun drying

0.0268

0.9966

0.9932

0.0109

0.9994

0.9988

0.0452

0.9926

0.9853

a = −1.43 10−2 b = 5.34 10−5 60 a = −3.35 10−2 b = 2.77 10−4 70 a = −5.54 10−2 b = 7.33 10−4 Natural sun drying

0.0302

0.9957

0.9917

0.0615

0.9819

0.9641

0.0496

0.9911

0.9823

a = −1.99 101 k = 2.88 10−3 b = 9.78 10−1 60 a = −2.13 k = 3.74 10−2 b = 1.09 70 a = 2.06 101 k = 1.59 10−1 b = 1.04 Natural sun drying

0.0211

0.9981

0.9962

0.0110

0.9994

0.9989

0.0097

0.9997

0.9993

50

0.0125

0.9994

0.9988

0.0107

0.9995

0.9991

0.0093

0.9995

0.9995

50

50

a = 1.00 k = 1.81 10−2 n = 9.76 10−1 b = −6.50 10−4 60 a = 1.00 k = 4.50 10−2 n = 1.03 b = −4.86 10−5 70 a = 9.98 10−1 k = 3.12 10−2 n = 1.37 b = 2.42 10−5 Natural sun drying

k = 1.13 10−2 k = 4.31 0−2 k = 6.59 10−2 k = 1.98 10−2 K = 2.23 10−3 n= 1.37 k = 6.06 10−2 n = 1.11 k = 2.84 10−2 n = 1.28 K = 4.82 10−3 n = 1.35 a = 7.22 10−1 k = 7.41 10−3 a = 1.03 k = 4.44 10−2 a = 1.05 k = 6.89 10−2 a = 1.07 k = 2.12 10−2 a = −8.69 10−3 b = 1.84 10−5 a = −2.94 10−2 b = 2.10 10−4 a = −4.54 10−2 b = 9.09 10−4 a = −1.13 10−2 b = 2.81 10−5 a = −7.69 101 k = 3.38 10−3 b = 1.19 a = 1.03 101 k = 3.50 10−2 b = 9.72 10−1 a = 1.58 101 k = 1.16 10−1 b = 1.05 a = 1.84 101 k = 3.69 10−2 b = 1.04 a = 0.991 k = 2.69 10−3 n = 1.30 b = −3.07 10−4 a = 1.00 k = 2.82 10−2 n = 1.14 b = 2.24 10−4 a = 1.00 k = 3.05 10−2 n = 1.25 b = −2.20 10−4 a = 9.94 10−1 k = 4.46 10−3 n = 1.36 b = 2.36 10−5

Statistical parameters Sr

r

χ2

0.2835 0.0214 0.0402 0.0426 0.2893

0.4472 0.9977 0.9929 0.9912 0.4804

0.1999 0.9954 0.9858 0.9825 0.2308

0.0152

0.9989

0.9978

0.0095

0.9996

0.9992

0.0077

0.9997

0.9994

0.2590

0.6195

0.3838

0.0196

0.9982

0.9964

0.0375

0.9943

0.9888

0.0363

0.9939

0.9879

0.2894

0.4804

0.2308

0.0620

0.9820

0.9644

0.0296

0.9965

0.9930

0.1004

0.9530

0.9083

0.3026

0.4781

0.2286

0.0215

0.9980

0.9960

0.0106

0.9995

0.9991

0.0087

0.9996

0.9993

0.0171

0.9989

0.9978

0.0144

0.9991

0.9983

0.0089

0.9997

0.9994

0.0074

0.9997

0.9995

temperature and the initial temperature of the product which is very small. The results thus obtained are compatible with other drying behavior found in the literature by solar drying of agri-food products (Ameri et al., 2018; Darvishi, 2013).

main factor influencing the drying kinetics of most agricultural products (Idlimam et al., 2016; Lahnine et al., 2016b; Ouhaddou et al., 2014). Moreover, at a constant air drying temperature, it is observed that the drying rate increases with a decrease of drying time. These evolutions characterize various biological products studied in the literature (Doymaz, 2012). It is noted that the moisture content decreases with the temperature contrary to the drying rate. The absence of phase 0 and I and the unique presence of phase II for drying curves of Herniaria hirsuta was also apparent. This may be due to the difference between the wet air

3.2.2. Fitting of the drying curves The moisture content values at different air drying temperatures and air drying velocities of Herniaria hirsuta were converted to moisture ratio and then the curve fitting as a function of drying time carried on the six drying models existing in the literature, on the basis 923

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-1

(a)

=50°C AV=0.09 m.s

(b)

-1

=50°C AV=0.18 m.s

-1

0.14

=60°C AV=0.09 m.s

0.12

=60°C AV=0.18 m.s

1.4

Dimensionless drying rate f (-)

-1

Drying rate (% d.b.min )

-1 -1

=70°C AV=0.09 m.s

0.10

-1

=70°C AV=0.18 m.s

0.08

Natural sun drying

0.06 0.04 0.02 0.00

1.2 1.0 0.8 0.6 0.4

Experimental data Characterictic drying curve

0.2 0.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0

0.2

0.4

0.6

0.8

1.0

*

Moisture content Z(% d.b)

Moisture ratio Z (-)

Fig. 9. Variation of drying rate with moisture content during drying (a) and characteristic drying curve (b) of Herniaria hirsuta.

Drying time (min) 0

0

50

100

150

200

-1

250

1/T(K )

300

0.00390 -17.0

-1

=70°C AV=0.18 m.s

-1

0.00391

0.00392

0.00393

0.00394

0.00395

-1

=70°C AV=0.09 m.s

-17.5

-1

-2

=60°C AV=0.18 m.s =60°C AV=0.09 m.s

-3

-18.0

-1

=50°C AV=0.18 m.s

Ln(Deff)

*

Ln(Z )

-1

-1

-4

=50°C AV=0.09 m.s Natural sun drying

-5

-18.5 -19.0

-6

-19.5

Fig. 10. Plot of Ln (Z ∗) versus drying time for different air drying conditions.

Experimental data Linear Fit of the experimental data

-20.0

of statistical parameters with a lowest standard error (Sr) and a highest coefficient of correlation (r) the best one. As shown in Table 7. After modeling, it is noted that the Midilli–Kucuk model was found to be the most suitable for describing the drying curves of Herniaria hirsuta.

Fig. 11. Influence of drying air temperature on the effective diffusivity.

From all drying experiments (Fig. 9-a), the CDC of Herniaria hirsuta has been modeled as a polynomial function of a second degree using the non-linear optimization method of Marquardt-Levenberg.

3.2.3. Determination of the characteristic drying curve Fig. 9 shows the variation of drying rate with moisture content and Characteristic Drying Curve (CDC) during drying Herniaria hirsuta experiments. The variation of moisture content versus drying rate is shown in Fig. 9-a, at a constant air drying velocity, the drying rate increases with the increase of drying air temperature. Therefore the water content of Herniaria hirsuta decreases. The aero-thermal conditions (air drying temperature and air drying velocity) influence the drying rate of Herniaria hirsuta, it decreases continuously when the moisture content decreases. The characteristic drying curve (CDC) consists of modeling a law of drying based on some experimental data and from this latest, normalizing the kinetics of drying in a theoretical model (Mghazli et al., 2017). Therefore, the practical concern of the CDC is to reduce all the experimental data so as to put them in a single form and hence to make them exploitable by the scientific community (Koukouch et al., 2015).

2

f = a0 × Z ∗ + a1 × Z ∗

(14)

With: a0 = 2.6947 and a1 = −1.6980. And a correlation coefficient value of 0.9417 and a minimum standard error of 0.054. 3.2.4. Effective diffusivities Fig. 10, shows the plot of the experimental results of Ln(Z*) versus drying time for different air drying conditions. It is noted that at a constant drying air velocity, the effective diffusivity increases with the increase of air drying temperature and it has the same evolution for the air drying velocity (Table 8). This is in agreement with the comportment of other crop products quoted in the literature (Hii and Ogugo, 2014; Kara and Doymaz, 2014; Driscollb and Buckleb, 1996; Lamharrar et al., 2017):

Table 8 Values of effective diffusivity of Herniaria hirsuta. Temperature (°C)

Air drying velocity (m/s)

50

60

70

Natural sun drying

Deff (m2/s)

0.18 0.09

3.0524 10−9 2.5339 10−9

7.1856 10−9 5.8420 10−9

18.051 10−9 15.320 10−9

3.2525 10−9

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Fig. 12. Influence of drying parameters on fresh Herniaria hirsuta.

studies in the literature, which found a strong reduction on color of fruits and vegetables during a drying process (Julkunen-Tiitto and Sorsa, 2001; Sadilova et al., 2006).

3.2.5. Activation energy The activation energy is calculated by representing the natural logarithm of the values of effective diffusivity versus the inverse of air drying temperature, which represents the influence of the air drying temperature on the effective diffusivity, as presented in Fig. 11.

64806.32 ln(Deff ) = 235.99 − T

3.3.3. Effect of air drying parameters on antioxidant activity of Herniaria hirsuta Antioxidant properties of fresh and dried samples of Herniaria hirsuta were investigated through DPPH assay. The results indicated that the antioxidant activity of the samples increases from fresh (6.55 ± 2.35%) to dried (13.26 ± 4.16%). Air drying parameters resulted in a higher antioxidant activity (twice the initial value). The increase of antioxidant activity observed may be explained by the shorter time of drying and formation of new compounds with antioxidant activity, such as Maillard reaction products, which continue to be formed later during storage time (Kaur and Kapoor, 2001; Nicoli et al., 1999). This observation is in accordance with results obtained by Ouaabou et al., (2019) in dried cherries.

(15)

The average activation energy of Herniaria hirsuta is obtained to be 2938.46 kJ/kg with a correlation coefficient of 0.9927. 3.3. Quality analysis The influence of drying parameters on fresh Herniaria hirsuta is observed in Fig. 12. The evaluation of the effects of solar convective drying on bioactive compounds of Herniaria hirsuta under the maximum drying parameters is studied. Thus, it is possible to determine the impact of solar drying on the medicinal proprieties of Herniaria hirsuta. 3.3.1. Effect of air drying parameters on total phenolic contents of Herniaria hirsuta The total phenolic contents (TPC) of Herniaria hirsuta were determined using the Folin-Ciocalteu's reagent and it was expressed in terms of gallic acid equivalent. The total phenolic contents of the fresh and dried Herniaria hirsuta at high aerothermal conditions (70 °C and 0.18 m·s−1) were 52.68 ± 4.12 and 47.56 ± 3.67 GAE/g DW, respectively. The phenolic content estimated in these results was probably responsible for the free radical-scavenging activity of Herniaria hirsuta samples. It is extremely important to point out that, there is a correlation between antioxidant activity potential and amount of phenolic compounds in all extracts, which is in agreement with the previous investigation (Anwar et al., 2013).

4. Conclusions The drying kinetics of Herniaria hirsuta in an indirect forced convection solar dryer has been determined. The static gravimetric method was used to determine the sorption curves and the optimal water activity for the conservation of Herniaria hirsuta. The optimal water activity obtained for Herniaria hirsuta was found to be 0.35. The LESPAM model was found as the most appropriate model to describe the experimental sorption of Herniaria hirsuta. The obtained kinetic results show that the main factor influencing the drying kinetics was the air drying temperature. These results have been operated to determine the characteristic drying curve, which is important to accurate information about the drying rate of Herniaria hirsuta. The Midilli-Kucuk drying model was found to be the most suitable for describing the solar drying curves of Herniaria hirsuta. The convective solar drying had no impact on the total phenolic contents and antioxidant activity. However, total flavonoids content were affected negatively by air drying parameters, According to these results, the indirect forced convection solar dryer process had no major impact on Herniaria hirsuta quality. Finally, solar drying is still an alternative solution for conserving and valorizing of aromatic and medicinal plants especially in some developing countries like Morocco where solar energy is an abundant source for the industrials sectors to reduce the economic, environmental and ecological cost caused by conventional drying systems (pollution…). This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

3.3.2. Effect of air drying parameters on total flavonoids content of Herniaria hirsuta The total flavonoids content present as colouring pigments in plants also function as protective antioxidants at various levels. Some studies showed that the total flavonoids content could protect membrane lipids from oxidation (Keen et al., 2005; Tarahovsky et al., 2014). According to the obtained results, the total flavonoids content of fresh and dried samples of Herniaria hirsuta were varied from 32.28 ± 1.22 to 17.64 ± 0.17 mg of quercetin per gram of extract. The total flavonoids content decreased by 46% after drying process at 70 °C and 0.18 m·s−1. The total flavonoids content are sensitive to the air drying parameters. The observed results are in agreement with other 925

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Declaration of Competing Interest

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