Effect of convective solar drying on colour, total phenols and radical scavenging activity of olive leaves (Olea europaea L.)

Original article

Effect of convective solar drying on colour, total phenols and

radical scavenging activity of olive leaves (Olea europaea L.)

Neila Bahloul,1* Nourhene Boudhrioua,1 Mohammed Kouhila2 & Nabil Kechaou1

1 Groupe de Genie des Procedes Agro-alimentaires, Unite de Recherche en Mecanique des Fluides Appliquee et Modelisation, Ecole Nationale

d’Ingenieurs de Sfax, BP 1173, 3038, Sfax, Tunisia

2 Laboratoire d’Energie Solaire et des Plantes Aromatiques et Medicinales, Ecole Normale Superieure, BP 2400, Marrakech, Morocco

(Received 25 June 2009; Accepted in revised form 1 September 2009)

Summary In this study, olive leaves, which are known for their therapeutic and antioxidant properties, were used to

assess the effect of solar drying conditions (temperature and flow rate) on the drying time and quality

parameters of olive leaves. Samples were dried at three drying temperatures (40, 50 and 60 �C) and at two

drying air flow rates (1.62 and 3.3 m3 min)1) in a convective laboratory solar dryer. From the experimental

results, it was noted that the drying time required to reduce the moisture content to 0.10 kg kg)1 d.b.

depends mainly on the drying temperature. The quality attributes of the dehydrated olive leaf samples were

investigated in term of colour, total phenols and radical scavenging activity. The effect of solar drying on L*,

a* b* parameters was significant (P < 0.05) for all the studied olive leaves. Besides, the total phenols of olive

leaves were significantly (P < 0.05) influenced by drying air conditions and tended to decrease with

increased drying time. The DPPH radical scavenging activity was higher in fresh [EC50 39.40 (ZR)-39.95

(CH)] than in dried leaves. However, the radical scavenging activity was also high in leaves dried at 60 �C,3.3 m3 min)1 [EC50 54.21 (ZR)–68.79 (CL)].

Keywords Colour, convective solar drying, olive leaves, radical scavenging activity, total phenols.

Introduction

Olive leaves (Olea europaea L.) have been used formedicinal purposes for centuries. Several reports haveshown that olive leaf extract has the capacity to treatdiabetic hyperglycemia (Gonzalez et al., 1992), to lowerblood pressure (Singh et al., 2008) and to preventintestinal muscle spasms (Tessier, 1994). Many phenoliccompounds such as oleuropein and flavonoids have beenidentified in olive leaves.Recently, there is an increasing trend among food

scientists to replace the synthetic antioxidants such asbutylated hydroxyanisole (BHA), butylated hydroxytol-uene (BHT) with natural ones which, in general, aresupposed to be safer. The phenolic compounds arecurrently the object of much attention in the foodpreservation industries due to their antioxidant andradical scavenging properties. In fact, it was found thatthe phenolic compounds from olive leaves have a strongprotective effect against oil oxidation. The enrichmentwith 1 kg of olive leaves extract is sufficient to fortify

50–320 L of refined olive oil to a similar stability as avirgin olive oil (Paiva-Martins et al., 2007). Similarly,the phenolic extracts obtained from the olive plant(fruit, leaves and pomace) showed remarkable anti-oxidant activity in retarding sunflower oil oxidativerancidity (Farag et al., 2003).The olive leaves are often prepared as an infusion or

decoction extraction. Nowadays, many homoeopathicremedies are sold as capsules containing the powder orthe extract of dried olive leaves. The leaves are oftendried before extraction to reduce their moisture contentand to avoid the interference of water on the process. InTunisia, olive culture is one of the most importantagricultural activities. Olive plantations count about 66million trees; however most homoeopathic remediescontaining olive leaves are imported. This could beattributed to the lack of adequate process of preserva-tion and storage of olive leaves. The immediate dryingof these products is the most important operation inpost-harvest processing to avoid quality losses and toprevent possible degradation due to microbiological orbiochemical reactions (Soysal & Oztekin, 2001).Sun drying is still the most common method used to

preserve agricultural products in the world especially*Correspondent: Fax: +216 74275595;

e-mail: [email protected]

International Journal of Food Science and Technology 2009, 44, 2561–2567 2561

doi:10.1111/j.1365-2621.2009.02084.x

� 2009 The Authors. Journal compilation � 2009 Institute of Food Science and Technology

herbs and spices (Prasad, 2009). However, it has someproblems related to the over drying and contaminationwith dust, soil, micro-organisms and insects, and forbeing weather dependent (Doymaz, 2006). The qualityof the dried products may also be lowered significantly.Therefore, the drying process should be undertaken inclosed and controlled equipment to improve the qualityof the final product. Compared to traditional dryingmethods, the use of the solar dryer reduces drying timesignificantly and prevents mass losses. In fact, Moham-mad et al. (2009) reported that solar tunnel drying ofapricots resulted in hygienic product and reduced thedrying time to 3–4 days compared with 14–16 days inopen air drying. Furthermore, product quality can beimproved essentially (Thanaraj et al., 2006; Sacilik,2007). Solar energy is an important alternative source ofenergy especially in locations with good sunshine and itis preferred to other energy sources because it isabundant, inexhaustible and non-polluting. It is alsorenewable, cheap and environmental friendly (Basunia& Abe, 2001).Many researches cited in the literature have addressed

the influence of drying conditions on the qualitycharacteristics of dehydrated agricultural products suchas bay leaves (Demir et al., 2004), apricot (Karabulutet al., 2007) and spinach (Ozkan et al., 2007). In general,drying processes provide a lot of advantageous such asextended shelf-life; however, some undesirable chemicalor physical changes like colour losses or browning mayoccur simultaneously. Browning is caused primarily bythe oxidation of some phenolic substrates to corre-sponding quinones by enzymatic activity. High temper-atures or long drying times in conventional air drying,may cause serious damage to the quality attributes ofthe dried product such as flavour, colour, nutrients andstability of active substances (Vadivambal & Jayas,2007; Krokida et al., 2008). Colour is an importantquality attribute in the agricultural products. Thereby,undesirable changes in the colour may lead to a decreasein the quality and marketing value of these products.Therefore, the selection of proper drying conditions is ofprime importance for decreasing thermal stress andmaintaining the quality of the dried product.The main objective of this study was to investigate the

effect of solar drying conditions on the drying time andon some quality parameters of olive leaves particularlythe colour, total phenols and the radical scavengingactivity.

Materials and methods

Materials

Olive leaves used for the solar drying experiments wereobtained from the ‘Olive Tree Institute’ farm in Sfaxregion (Tunisia). Four varieties were selected for this

study: Chemlali (CL), Chemchali (CH), Chetoui (CT)and Zarrazi (ZR). The caffeic acid, the 2,2-diphenyl-1-picrylhydrazyl (DPPH) and the Folin–Ciocalteu re-agents were purchased from Fluka (Buchs, Switzerland).

Moisture content

The initial moisture content of each sample was deter-mined by drying the samples in a drying oven at 105 �Cfor 24 h. The sample weight was measured by ananalytical balance (METTLER-TOLEDO) with a pre-cision of ±0.0001 g. The difference of weight before(mw) and after (md) drying in the oven gives the initialmoisture content of the product (X0) as indicated in theequation given below:

X0 ¼mw �md

m dð1Þ

Colour measurements

The colour of the fresh and dried leaves was determinedon the green face of the olive leaves by using a handheldtristimulus colorimeter (Minolta chroma Meter CR-300,CIE, 1976) and a CIE standard illuminant C todetermine CIE colour space coordinates, L*, a* andb* values.Lightness value, L* (varying from 0: black to 100:

white) indicates how dark ⁄ light the sample is; a*parameter (varying from )60 to +60) is a measure ofgreenness (negative values) ⁄ redness (positive values).The parameter b* (varying from )60 to +60) is thegrade of blueness (negative values) ⁄yellowness (positivevalues). The colorimeter was calibrated against a stan-dard white plate before each colour measurement.Values of different parameters were expressed as themean ± SD of repeatability.

Total phenols

The total phenols of olive leaves were determinedaccording to the Folin–Ciocalteu method (Bonnely etal., 2003; Skerget et al., 2005). Each sample of fresh anddried leaves was separately crushed in a laboratoryblade cutter and extracted with methanol (1:25 w ⁄v).The total phenols in extracts were measured by UVspectrophotometry (CECIL CIE, 2021) at 760 nm,based on a colorimetric oxidation ⁄ reduction reaction.The oxidising agent used was Folin–Ciocalteu reagent.Briefly, to 0.5 mL of diluted extract, 2.5 mL of Folin–Ciocalteu reagent (diluted ten times with water) wasadded and, after that (within time interval from 0.5–8 min), 2 mL of Na2CO3 (75 g L)1) was added. Thesample was incubated for 5 min at 50 �C and thencooled. For a control sample, 0.5 mL of distilled waterwas used. The results were expressed in gram of caffeic

Effect of convective solar drying on olive leaves N. Bahloul et al.2562

International Journal of Food Science and Technology 2009 � 2009 The Authors. Journal compilation � 2009 Institute of Food Science and Technology

acid per 100 g of dry leaves (g caffeic acid 100 g)1 drymatter).

DPPH radical scavenging activity

The DPPH radical is a stable organic free radical withan absorption maximum band around 515–528 nm andthus, it is a useful reagent for evaluation of antioxidantactivity of compounds. In the DPPH test, the antiox-idants reduce the DPPH radical to a yellow-colouredcompound, diphenylpicrylhydrazine, and the extent ofthe reaction will depend on the hydrogen donatingability of the antioxidants (Ardestani & Yazdanparast,2007).DPPH scavenging potential of fresh and dried olive

leaves was measured, based on the scavenging ability ofstable 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals byolive leaves antioxidants. Briefly, 2 mL of each meth-anolic extract solution of olive leaves (prepared atvariable concentrations 25–700 lg mL)1 in methanol)was added to 0.1 mL of 0.4 mm methanolic solution ofDPPH. The mixture was then vortexed vigorously andleft for 30 min at room temperature in the dark. Theabsorbance was measured at 517 nm and the activitywas expressed as percentage DPPH-scavenging activityrelative to the control, using the following equation:

%Radical scavenging activity ¼ AControl�ASample

AControl�100

ð2Þ

where AControl is the absorbance of the control andAsample is the absorbance of the sample.The percentage of DPPH radical scavenging activity

was plotted against the plant extract concentration (lgmL)1) to determine the amount of extract necessary todecrease the DPPH radical concentration by 50%(called EC50).

Convective solar drying equipment

The experimental apparatus consists of an indirectforced convection laboratory solar dryer with a solarair collector (dimensions of 1 m · 2.5 m), an auxiliaryheater, a circulation fan and a drying cabinet [dimen-sions, 1.40 m (length), 0.5 m (width), and 0.90 m(depth)] (Ait Mohamed et al.).

Drying experiments

Three drying temperatures 40 �C (T1), 50 �C (T2) and60 �C (T3) and two drying air flow rates of1.62 m3 min)1 (Q1) and 3.3 m3 min)1 (Q2) were selectedto perform the drying curves of olive leaves.The leaves were uniformly spread in a thin layer on a

drying tray that was then placed on the first shelf of the

drying cabinet. The sample mass was kept constant at20 g by tray for each run.A digital weighing apparatus (±0.001 g) allows the

measurement of the mass loss of the product duringthe drying process. During each drying experiment, theweight of the product on the tray was measured byremoving it from the drying cabinet for approximately15–20 s. Drying was continued until the moisturecontent of sample reached about 0.10 kg kg)1 d.b.

Statistical analysis

The analysis of variance (anova) was performed inorder to estimate the effect of solar drying conditionson the colorimetric measurements (L*, a* and b*), thetotal phenols and the radical scavenging activity ofolive leaves. Statistical analyses were performed byusing spss 13.0 for Windows and the Tukey test wasapplied. The P < 0.05 was considered significant. Allthe experimental treatments were conducted in threereplicates.

Results and discussion

Drying kinetics

The initial moisture content of the Chemlali (CL),Chemchali (CH), Chetoui (CT) and Zarrazi (ZR) oliveleaves varieties are 0.941, 0.912, 0.918 and 0.837 kg kg)1

d.b., respectively. The initial moisture content of theleaves was reduced to the final moisture content of0.10 kg kg)1 d.b. Figure 1 presents the variations ofmoisture content of olive leaf (CH variety) with dryingtime under different drying conditions. (Data is notshown for the other varieties.) It is evident from thesecurves that the moisture content decreases continuouslywith the drying time.

Drying time (min)

Moi

stur

e co

nten

t (k

g kg

–1 d

.b.)

0

0.2

0.4

0.6

0.8

1

0 100 200 400 500 600300

CH T1, Q1 T1, Q2 T2, Q1 T2, Q2 T3, Q1 T3, Q2

Figure 1 Variations of moisture content with drying time of olive

leaves (CH variety) for drying temperatures of 40 �C (T1), 50 �C (T2)

and 60 �C (T3) and air flow rates of 1.62 m3 min)1 (Q1) and

3.3 m3 min)1 (Q2).

Effect of convective solar drying on olive leaves N. Bahloul et al. 2563

� 2009 The Authors. Journal compilation � 2009 Institute of Food Science and Technology International Journal of Food Science and Technology 2009

Experimental results showed that drying temperatureis an effective parameter for the drying of olive leaves.The drying time was reduced by increasing dryingtemperature. For CH variety for instance, the timerequired for the lowering of moisture content of oliveleaves to the final moisture content of 0.10 kg kg)1 d.b.decreases from 490 min at 40 �C to 280 min at 50 �Cand to 180 min at 60 �C with drying air flow rate of1.62 m3 min)1. A reduction in drying time occurred alsowith increase of air flow rate to 3.3 m3 min)1. In thiscase, the time taken to reduce the moisture content ofthe leaves to 0.10 kg kg)1 d.b. was 460, 250 and 140 minat the temperatures of 40, 50 and 60 �C, respectively.However, the influence of the drying air flow rate on thedrying time is less important than that of the dryingtemperature. As the drying temperature and flow rateincreased, moisture removal increases thus resulted intosubstantial decrease in drying time. The same trend wasobserved for the other olive leaves varieties. Cui et al.(2004) reported that the drying kinetics is often used todescribe the combined macroscopic and microscopicmechanisms of heat and mass transfer, and it may beaffected by the drying conditions, types of dryer and thecharacteristics of the materials to be dried.The drying time required to decrease the moisture

content of CL and CH variety to 0.10 kg kg)1 d.b. wereless (from 100 to 490 min) than that of CT and ZRvariety (from 180 to 500 min). This means that CT andZR varieties required more drying time than CL and CHvariety to reach the final moisture content. This factcould be attributed to the difference on the tissue ofolive leaves and on the size and the distribution of their

porous morphometric features (trichome layers andstomata). These structures are known for playing animportant role on the transpiration and undoubtedly onthe dehydration process (Bahloul et al., 2008).

Colour analysis

The colour parameters L*, a* and b* have been widelyused to describe colour changes during thermal process-ing of agricultural products. These colour variables havebeen related to the types and quantities of somecomponents present in these products.The L*, a* and b* values of fresh and solar dried

olive leaves are presented in Fig. 2. For the examinedfresh olive leaves, the parameter a* is negativeindicating the green colour of the leaves and it variesfrom )9.143 (CH) to )5.013 (ZR). The L* parametervaries from 29.793 (ZR) to 35.027 (CT) and the b*parameter is positive and it ranges from 6.243 (ZR) to11.370 (CH).The values of L* parameter of the solar dried olive

leaves increase if compared with those of fresh leaves.The luminance of the leaves is so improved by solardrying. However, the greenness of the leaves is reduced.In fact, the values of a* increased and became positive(red chroma) for all the olive leaf varieties under all thedrying conditions (except for CH variety dried at 40 �Cand 1.62 m3 min)1 air flow rate). Regarding the param-eter b* values, the olive leaves CL, CH, CT and ZRdried at 40 �C (at air flow rate of 1.62 or 3.3 m3 min)1)were comparable to the fresh leaves and were signifi-cantly different (P < 0.05 at 95%) to the leaves dried at

–10

0

10

20

30

40

50

60

Fresh T1, Q2 T2, Q1 T2, Q2 T3, Q1 T3, Q2

Drying air conditions

L*a

*b*

valu

es

L*a

*b*

valu

es

L* a* b* L* a* b*

L* a* b* L* a* b*

–20

–10

0

10

20

30

40

50


Drying air conditions –10

0

10

20

30

40

T1, Q1 Fresh T1, Q2 T2, Q1 T2, Q2 T3, Q1 T3, Q2 T1, Q1


L*a

*b*

valu

es

L*a

*b*

valu

es

–20

–10

0

10

20

30

40

50

Fresh T1, Q2 T2, Q1 T2, Q2 T3, Q1 T3, Q2 T1, Q1 Fresh T1, Q2 T2, Q1 T2, Q2 T3, Q1 T3, Q2 T1, Q1

CL CH

CT ZR

Figure 2 L*a*b* values of olive leaves

obtained under different drying conditions:

drying temperatures of 40 �C (T1), 50 �C (T2)

and 60 �C (T3) and air flow rates of

1.62 m3 min)1 (Q1) and 3.3 m3 min)1 (Q2).



50 �C and at 60 �C (at air flow rate of 1.62 or3.3 m3 min)1).The anova analysis results show that the effect of

solar drying effects on L*, a* and b* parameters forolive leaves is significant for all the examined olive leavesvarieties (P < 0.05 at 95%). Regarding the L* param-eter, the effect of solar drying is very significant(P < 0.001 at 95%) for CL, CT and ZR variety and itis highly significant (P < 0.01 at 95%) for CH variety.The effect is very significant (P < 0.001 at 95%) on a*parameter for all the varieties. However, the effect ofsolar drying on b* parameter seems to depend on theolive leaves variety. It is very significant (P < 0.001 at95%) for CL and CT variety and significant for CH andZR (P < 0.05 at 95%).The increase of a* value denotes a more red chroma,

which is indicative of the browning reaction (Vadivam-bal & Jayas, 2007). The increase in b* could be causedby the development of some brown pigments related tonon-enzymatic reactions activated during drying pro-cess. It has been reported that many reactions can affectcolour during thermal processing of agricultural prod-ucts; among them, the most common are pigmentdegradation, especially carotenoids and chlorophylls(Nyambaka & Ryley, 2004).

Total phenols

The effect of solar drying conditions on total phenols ofthe CL, CH, CT and ZR olive leaves is shown inTable 1. The total phenols in fresh leaves, range from2.203 (CH) to 2.606 (ZR) g caffeic acid 100 g)1 d.m.These values are higher than those obtained for thedried leaves [0.972 (CL)–2.408 (CT) g caffeic acid100 g)1 d.m.].The total phenols of olive leaves were significantly

(P < 0.05) influenced by drying air conditions andtended to decrease with increased drying time. Indeed,dried leaves at 40 �C, 1.62 m3 min)1, which varied from0.972 (CL) to 1.349 (CH) g caffeic acid 100 g)1 d.m.contain the lowest total phenols in comparison withfresh leaves. The considerable decrease of total phenols

observed at 40 �C could be attributed to the long dryingperiod required to reduce the moisture content of theleaves to 0.1 kg kg)1 d.b. Further, the long exposure toair and light may lead to the oxidation of some phenolicsubstrates to corresponding quinones by enzymaticactivity. Park et al. (2006) studied the effect of sundrying and dehydration at 60 �C in a cabinet dryer ontotal phenols of persimmons. The authors reportedsignificant differences (P < 0.05) between the fresh andthe dried fruits in term of total phenols. The fresh fruitswere found to contain the highest values of totalphenols. However, dried leaves at 60 �C (air flow ratesof 1.62 and 3.3 m3 min)1) contain relatively highamounts of total phenols in comparison to the otherdried olive leaves. Dehydrated olive leaves at 60 �C,3.3 m3 min)1 have total phenols close to those obtainedfor the properly fresh leaves.

DPPH radical scavenging activity

The antioxidant activity of plant extracts containingphenolic compounds is due to their capacity to bedonors of hydrogen atoms or electrons and to capturethe free radicals. DPPH analysis is one of the tests usedto prove the ability of the components of plants extractsto act as donors of hydrogen atoms (Ardestani &Yazdanparast, 2007). The effect of solar drying condi-tions on DPPH radical scavenging activity of oliveleaves varieties is shown in Fig. 3. As can be seen, thefree radical scavenging activity of the methanolicextracts of olive leaves (CL, CH, CT and ZR variety)increased with the increased concentration. In fact, thehighest free radical scavenging activities were recordedat a concentration of 700 lg mL)1. The lowest valueswere obtained at a concentration of 25 lg mL)1. Fur-ther, the fresh olive leaves extracts showed excellentscavenging effect on DPPH radicals, reaching up to 86%at a concentration of 700 lg mL)1. The level was higherthan that obtained for the dried leaves.The amounts of extract required to scavenge 50% of

DPPH radicals present in the reaction mixture (EC50)were calculated from data presented in Fig. 3. The EC50

Table 1 Effect of solar drying conditions on

total phenols (g caffeic acid 100 g)1 d.m.)

of the olive leavesDrying conditions

Olive leaf varieties

CL CH CT ZR

Fresh 2.209a ± 0.091 2.203a ± 0.047 2.583a ± 0.111 2.606a ± 0.034

40 �C, 1.62 m3 min)1 0.972f ± 0.009 1.349f ± 0.013 1.085e ± 0.004 1.253f ± 0.019

40 �C, 3.3 m3 min)1 1.276e ± 0.004 1.481e ± 0.018 1.576d ± 0.013 1.331e ± 0.004

50 �C, 1.62 m3 min)1 1.354d,e ± 0.009 1.585d ± 0.004 1.622d ± 0.004 1.354e,d ± 0.009

50 �C, 3.3 m3 min)1 1.449c,d ± 0.004 1.867c ± 0.004 2.004c ± 0.041 1.413c,d ± 0.023

60 �C, 1.62 m3 min)1 1.522c ± 0.013 2.065b ± 0.002 1.988c ± 0.007 1.481c ± 0.009

60 �C, 3.3 m3 min)1 1.686b ± 0.023 2.172a ± 0.009 2.408b ± 0.045 2.281b ± 0.045

a–fColumn mean values with different letters are significantly different at the 95% level.



values for olive leaves are shown in Table 2. In fact, alower value of EC50 indicates a higher DPPH radicalscavenging activity. Therefore, extracts obtained fromfresh leaves registered the highest DPPH radical scav-enging activity [EC50 39.40 (ZR)–39.95 (CH)]. However,the EC50 values of fresh olive leaves varieties werestatistically similar (P > 0.05).The DPPH radical scavenging activity of dried olive

leaves was significantly influenced by drying air condi-tions (P < 0.05). In fact, the EC50 values of fresh leaveswere significantly (P < 0.05) higher than those of thedried ones. Besides, the olive leaves dried at 60 �C,3.3 m3 min)1 gave higher DPPH radical scavenging

activity [EC50 54.21 (ZR)–68.79 (CL)] than thatobtained at 40 �C, 3.3 m3 min)1 [EC50 120.39 (CL)–164.24 (CH)].The olive leaves dehydrated at 40 �C, 1.62 m3 min)1

exhibited the lowest DPPH radical scavenging activities[EC50 152.72 (CL)–179.40 (CT)]. The decrease of theDPPH radical scavenging activity observed at oliveleaves dried at 40 �C could be attributed to the decreaseon their total phenols. Indeed, it is noted that the highertotal phenols measured for each olive leaf variety yieldedthe higher DPPH radical scavenging activity; this isprobably due to the combined effect of the phenoliccompounds and their high hydrogen atom donating

0

20

40

60

80

100

0 100 200 300 400 500 600 700 800

Concentration (µg mL–1) Concentration (µg mL–1)

Concentration (µg mL–1) Concentration (µg mL–1)

0

20

40

60

80

100

Fresh

T1, Q1

T1, Q2

T2, Q1

T2, Q2

T3, Q1

T3, Q2

Fresh

T1, Q1

T1, Q2

T2, Q1

T2, Q2

T3, Q1

T3, Q2

0 100 200 300 400 500 600 700 800

0

20

40

60

80

100

0

20

40

60

80

100

200 300 4000 100 500 600 700 800 200 300 4000 100 500 600 700 800

CL CH

ZRCT

Fresh

T1, Q1

T1, Q2

T2, Q1

T2, Q2

T3, Q1

T3, Q2

Fresh

T1, Q1

T1, Q2

T2, Q1

T2, Q2

T3, Q1

T3, Q2

Figure 3 DPPH radical scavenging activity of

the methanolic extracts of fresh and dried

olive leaves for different drying conditions:

drying temperatures of 40 �C (T1), 50 �C (T2)

and 60 �C (T3) and air flow rates of

1.62 m3 min)1 (Q1) and 3.3 m3 min)1 (Q2).

Table 2 DPPH EC50 values of fresh and dried

olive leaves

Drying conditions

Olive leaf varieties

CL CH CT ZR

Fresh 39.62 ± 0.20 39.95 ± 0.34 39.45 ± 0.07 39.40 ± 0.09

40 �C, 1.62 m3 min)1 152.72 ± 0.67 178.50 ± 1.06 179.40 ± 2.12 162.13 ± 1.54

40 �C, 3.3 m3 min)1 120.39 ± 0.31 164.24 ± 1.21 138.98 ± 2.26 149.23 ± 0.97

50 �C, 1.62 m3 min)1 93.80 ± 0.93 80.24 ± 0.26 113.22 ± 0.35 129.07 ± 1.87

50 �C, 3.3 m3 min)1 90.88 ± 0.24 78.87 ± 2.76 98.15 ± 1.70 100.12 ± 2.50

60 �C, 1.62 m3 min)1 82.06 ± 0.13 62.51 ± 2.25 79.21 ± 0.32 60.24 ± 0.53

60 �C, 3.3 m3 min)1 68.79 ± 0.23 55.52 ± 0.85 60.31 ± 0.24 54.21 ± 0.06



abilities. Similarly, a correlation between DPPH radicalscavenging activity and total phenols has been observedin different vegetables and fruits (Jimenez-Escrig et al.,2001; Park et al., 2006).

Conclusions

In this work, the effect of solar drying conditions ondrying kinetics, colour, total phenols and radical scav-enging activity was investigated. The results verified thatdrying temperature is the main factor in controlling thedrying time. Dehydrated leaves were compared to theproperly fresh ones. The effect of solar drying on L*, a*,b* parameters was significant for all the studied oliveleaves. The total phenols and the radical scavengingactivity of olive leaves were significantly (P < 0.05)influenced by drying conditions and tended to decreasewith increased drying time.

Acknowledgment

The authors would like to thank Mrs Naziha KAM-MOUN, researcher in the Olive Tree Institute of Sfax(Tunisia), who guaranteed us the supply of olive leaves.

References

Ait Mohamed, L., Kane, C.S.E., Kouhila, M., Jamali, A., Mahrouz,M. & Kechaou, N. (2008). Thin layer modelling of Gelidiumsesquipedale solar drying process. Energy Conversion and Manage-ment, 49, 940–946.

Ardestani, A. & Yazdanparast, R. (2007). Antioxidant and free radicalscavenging potential of Achillea santolina extracts. Food Chemistry,104, 21–29.

Bahloul, N., Boudhrioua, N. & Kechaou, N. (2008). Moisturedesorption-adsorption isotherms and isosteric heats of sorption ofTunisian olive leaves (Olea europaea L.). Journal of Industrial Cropsand Products, 28, 162–176.

Basunia, M.A & Abe, T. (2001). Thin layer solar drying characteristicsof rough rice under natural convection. Journal of Food Engineering,47, 295–301.

Bonnely, S., Davis, A.L., Lewis, J.R. & Astill, C. (2003). A modeloxidation system to study oxidised compounds present in black tea.Food Chemistry, 83, 485–492.

Cui, Z.W, Xu, S.Y. & Sun, D.W. (2004). Microwave–vacuum dryingkinetics of carrot slices. Journal of Food Engineering, 65, 157–164.

Demir, V., Gunhan, T., Yagcioglu, A.K. & Degirmencioglu, A. (2004).Mathematical modelling and the determination of some qualityparameters of air-dried bay leaves. Biosystems Engineering, PH-Postharvest Technology, 88, 325–335.

Doymaz, I. (2006). Thin layer drying behaviour of mint leaves. Journalof Food Engineering, 74, 370–375.

Farag, R.S., El-Baroty, G.S. & Basuny, A.M. (2003). The influence ofphenolic extracts obtained from the olive plant (cvs. Picual and

Kronakii), on the stability of sunflower oil. International Journal ofFood Science and Technology, 38, 81–87.

Gonzalez, M., Zarzuelo, A., Ganez, M.J., Utrilla, M.P., Jimenez, J. &Osuna, I. (1992). Hypoglycemic activity of olive leaf. Planta Medica,58, 513–515.

Jimenez-Escrig, A., Rincon, M., Pulido, R. & Saura-Calixto, F. (2001).Guava fruit (Psidium guajava l.) as a new source of antioxidantdietary fiber. Journal of Agricultural and Food Chemistry, 49, 5489–5493.

Karabulut, I., Topcu, A., Duran, A., Turan, S. & Ozturk, B. (2007).Effect of hot air drying and sun drying on colour values and b-carotene content of apricot (Prunus armenica L.). Lebensmittel-Wissenchaft und Technology, 40, 753–758.

Krokida, M.K., Maroulis, Z.B. and Saravacos, G.D. (2008). The effectof the method of drying on the colour of dehydrated products.International Journal of Food Science and Technology, 36, 53–59.

Mohammad, M.A., Peerzada, H.R., Shafi, F. & Abdul, H.R. (2009).Effect of sulphiting and drying methods on physico-chemical andsensorial quality of dried apricots during ambient storage. Interna-tional Journal of Food Science and Technology, 44, 1157–1166.

Nyambaka, H. & Ryley, J. (2004). Multivariate analysis of the sensorychanges in the dehydrated cowpea leaves. Talanta, 64, 23–29.

Ozkan, I.A., Akbudak, B. & Akbudak, N. (2007). Microwave dryingcharacteristics of spinach. Journal of Food Engineering, 78, 577–583.

Paiva-Martins, F., Correia, R., Felix, S., Ferreira, P. & Gordon, M.(2007). Effects of enrichment of refined olive oil with phenoliccompounds from olive leaves. Journal of Agricultural and FoodChemistry, 55, 4139–4143.

Park, Y., Jung, S., Kang, S., et al. (2006). Drying of persimmons(Diospyros kaki L.) and the following changes in the studiedbioactive compounds and the total radical scavenging activities.Lebensmittel-Wissenchaft und Technology, 39, 748–755.

Prasad, J.A. (2009). Convective heat transfer in herb and spices duringopen sun drying. International Journal of Food Science andTechnology, 44, 657–665.

Sacilik, K. (2007). Effect of drying methods on thin-layer dryingcharacteristics of hull-less seed pumpkin (Cucurbita pepo L.). Journalof Food Engineering, 79, 23–30.

Singh, I., Mok, M., Christensen, A., Turner, A.H. & Hawley, J.A.(2008). The effects of polyphenols in olive leaves on plateletfunction. Nutrition, Metabolism & Cardiovascular Diseases, 18,127–132.

Skerget, M., Kotnik, P., Hadolin, M., Hras, A.R., Simonic, M. &Knez, Z. (2005). Phenols, proanthocyanidins, flavones and flavonolsin some plant materials and their antioxidant activities. FoodChemistry, 89, 191–198.

Soysal, Y & Oztekin, S. (2001). Technical and economic performanceof a tray dryer for medicinal and aromatic plants. Journal ofAgricultural Engineering Research, 79, 73–79.

Thanaraj, T., Dharmasena, D.A.N. & Samarajeewa, U. (2006).Comparison of drying behaviour, quality and yield of copraprocessed in either a solar hybrid dryer on in an improved coprakiln. International Journal of Food Science and Technology, 42, 125–132.

Tessier, A. (1994). Phytotherapie Analytique, Phytochimie et Pharma-cologie. Editions Marc-Aurele, France.

Vadivambal, R. & Jayas, D.S. (2007). Changes in quality of micro-wave-treated agricultural products. Biosystems Engineering, PH –Postharvest Technology, 98, 1–16.



Documents

Effect of convective solar drying on colour, total phenols and radical scavenging activity of olive leaves (Olea europaea L.)