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Volatility profiles of monoterpenes loaded onto cellulosic-based materials
Industrial Crops and Products 51 (2013) 100–106
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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Volatility profiles of monoterpenes loaded onto cellulosic-based materials Igor W.K. Ouédraogo a,b , Julien De Winter b , Pascal Gerbaux b , Yvonne L. Bonzi-Coulibaly a,∗ a
Laboratoire de Chimie Organique: Structure et Réactivité, UFR-SEA, Université de Ouagadougou, 03 BP 7021, Ouagadougou 03, Burkina Faso Groupe de Recherche en Spectrométrie de Masse, Centre Interdisciplinaire de Spectrométrie de Masse, Université de Mons – UMONS, Place du Parc 23, B-7000 Mons, Belgium b
a r t i c l e
i n f o
Article history: Received 8 June 2013 Received in revised form 21 July 2013 Accepted 9 August 2013 Keywords: Monoterpene Cellulose-based matrices Impregnation Volatility Formulation
a b s t r a c t Monoterpene compounds were loaded onto pure cellulose and two cellulose-based matrices by impregnation method. The effects of initial ratio, structures of volatile compounds and polymers, i.e. cellulose, acetate cellulose and rice husk, on the release profiles were investigated. Four monoterpenes, namely ˛-pinene, citronellal, carvone and terpinen-4-ol, were tested as volatile compound models. In the case of carvone loaded onto cellulose, we observed that the release of the volatile molecule increases with increasing initial ratio in the formulation. Using different cellulose-based matrix to study the volatility of monoterpene models, the lowest release rates was obtained with rice husk formulations, with highly retention capacity over 50% for carvone, terpinen-4-ol and citronellal after 20 days at 21 ◦ C. It was also concluded that the impregnation of terpinen-4-ol and carvone into cellulose and cellulose acetate respectively, could effectively help in prolonging the retention of these volatiles. ˛-Pinene, a highly hydrophobic molecule showed no significant retention. These results indicate that cellulosebased matrices could be potentially used as good carriers of active compounds for ecological pesticides formulation for postharvest applications. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Essential oils from aromatic plants are mainly monoterpene derivatives and are involved in a wide range of applications in sanitary, cosmetic, agricultural, pharmaceutical and food industries. Successful uses of essential oils against agricultural pests and foodborne microorganisms are also convincingly (Isman et al., 2011; Nguemtchouina et al., 2010; Sacchetti et al., 2005) reported. One important point to consider in the different applications is the capability of the used formulations to release the active constituent(s) in a controlled manner. This is even more problematic when considering essential oils given the really high volatility of the constituting monoterpene compounds. Given the low vapor pressure (high boiling point) and their molecular structures, monoterpenes are prone to rapid evaporation and easy degradation, respectively (Hoskovec et al., 2005; Lai et al., 2006). Various reactions such as oxidation or Norrish type II photofragmentation (Rochat et al., 2000) are well known to limit their use over time. Different strategies have then been developed to protect the monoterpene constituents from
∗ Corresponding author. Tel.: +226 50 30 70 64; fax: +226 50 30 72 42. E-mail addresses: [email protected] (P. Gerbaux), [email protected] (Y.L. Bonzi-Coulibaly). 0926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2013.08.032
degradation and/or to control their release and diffusion. Most of the procedures involve neighboring-group participation (De Saint Laumer et al., 2003), profragrances formation (Levrand et al., 2006), encapsulations (Cevallos et al., 2010; Luo et al., 2011) or nitrile and oxime-based derivations (Narula, 2004; Dikusar et al., 2008; Ouédraogo et al., 2009). Those methods were demonstrated to prolong the long-lasting effect of volatile compounds and, as an additional benefit, to increase their stability in aggressive media (air oxidation, light, moisture and higher temperatures). Intensive studies have been conducted to consider bio-based materials, i.e. cellulose, as adsorbents of many inorganic or organic compounds in the context of water treatment (Ji et al., 2012; Wojnárovits et al., 2010; Wang and Li, 2013; Takács et al., 2012). Also various biopolymers have been used as matrix for entrapping different active compounds for medical or food purposes (Soottitantawat et al., 2005; Sánchez-González et al., 2011a,b; Tongnuanchan et al., 2012). Incorporation of essential oils into polymer matrices for fruit protection or against food borne pathogens has been showed to represent a successful approach (Marcuzzo et al., 2010; Paula et al., 2011; Bosquez-Molina et al., 2010; Woranuch and Yoksan, 2013). Sánchez-González et al. (2011b) prepared antimicrobial films by incorporating various essential oils from bergamot, lemon and tea tree, into chitosan and hydroxypropylmethylcellulose films. The association of these bio-based
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materials with adsorbed essential oils improves the physical and biological/chemical properties of the film. In addition, when adsorbed on the biopolymer, the volatility of the essential oil constituents is reduced and the activity time of the essential oil is of course prolonged. Cellulose-based materials could be promising materials for natural volatile molecule formulation in crops protection strategies against insects or fungi. In the context of seed protection, feedstocks could be selected and serve as low-cost adsorbents. Indeed, rice husk and derivative products like cellulose and cellulose acetate are interesting polymers given several recognized properties such as no toxicity, availability, biodegradability (Sun et al., 2004; Liu et al., 2007; Asadi et al., 2008). Rice husks are an important by-product of rice milling process. According to the statistical data of Food and Agriculture Organization (FAO), the world annual paddy production is approximately 582 million tons and rice husk represent 22–25% of the rice grain. Rice husk as described in the literature is mainly composed of cellulose (32.24–42.8%), hemicellulose (21.34–32.7%), lignin (11.96–24.5%) and mineral ash (15.05–18.86%) which is in large amount of silica (SiO2 ) (Genieva et al., 2008; Abbas and Ansumali, 2010; Osman et al., 2010; Wan Ngah and Hanafiah., 2008; Adel et al., 2011; Chaudhary and Jollands, 2004; Saha et al., 2005; Saha and Cotta, 2007). Thus, due to the availability, attention has been focused on sorbents production from rice husk. In this context, we decided to probe the adsorption capabilities of some cellulosic-based matrices toward some typical monoterpene compounds. For the present work, cellulose, cellulose acetate and rice husk have been selected as model matrices for the adsorption of four monoterpenes used as volatile compound candidates. The selected molecules are (+)-˛-pinene (unsaturated hydrocarbon), citronellal (aldehyde), carvone (ketone) and terpinen-4-ol (alcohol). The release behaviors of the four compounds loaded onto cellulosicbased materials will be investigated by gas chromatography–mass spectrometry (GC–MS) analyses. 2. Materials and methods 2.1. Materials Cellulose powder (DS-0), for thin layer chromatography (TLC) was purchased from Sigma–Aldrich. Cellulose acetate (average M.W. 100.000) was obtained from ACROS Organics. Monoterpene chemical used were (+)-˛-pinene (purity >98%, Alfa Aesar), carvone (purity >99%, ACROS Organics), citronellal (purity >93%, ACROS Organics) and terpinen-4-ol (purity >95%, Aldrich). Rice husk was obtained from local rice collected in Burkina Faso. All other reagents used throughout the study were analytical grade. 2.2. Rice husk preparation and characterization The dried material of rice husk (size fractions of 0.063–0.125 mm) was successively washed with deionized water, ethanol and acetone several times to remove dust and fines. Afterwards, the collected material was dried in an oven at 60 ◦ C for 24 h. The material was characterized by Fourier transforminfrared (FT-IR) analysis on a Perkin Elmer BX II spectrophotometer employing the KBr pellet method. The specific surface area of rice husk was determined in comparison with cellulose and cellulose acetate by N2 adsorption at 77 K using an ASAP 2020 Micromeritics instrument and the Brunauer–Emmett–Teller (BET) method (Sing et al., 1985). FT-IR spectra: 3600–3200 cm−1 O H stretching, 1732 cm−1 C O stretching (hemicellulose), 1606 and 1515 cm−1 aromatic rings of
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lignin, 799 cm−1 Si O Si bond, 465–485 cm−1 O Si O bending vibration. Specific surface area: 7.1 m2 /g. 2.3. Volatile compound formulations and volatility studies Formulation of monoterpenes loaded onto cellulose-based matrices was carried out by impregnation: 1 mg of each monoterpene was first diluted with 200 L of acetone and then added to 100 mg of cellulose into glass vials (inner diameter × height: 20 mm × 40 mm). The volatility study was performed by using different amounts, expressed as weight of volatile molecule per 100 g of cellulose-based matrix. For the study of the influence of the adsorbent nature, 0.25 mg of each monoterpene was mixed to get 1 mg of a mixture of monoterpenes. This mixture was diluted with 200 L of acetone (for cellulose or rice husk formulations) or hexane (for cellulose acetate formulation) and then added to 100 mg of cellulose-based matrix into glass vials (inner diameter × height: 20 mm × 40 mm). The resulting mixtures were left uncovered for solvent and monoterpene evaporation. Monitoring was carried out under room temperature (21 ± 2 ◦ C). Each experiment was replicated 3 times and the mean (average) values are reported. Monoterpenes (without cellulose-based matrix) were also diluted with 200 L of acetone and exposed for monitoring the volatilization. 2.4. Extraction method At set time intervals, the amount of the residual volatile molecules was determined. The remaining volatile molecules were extracted from the cellulose matrices by 5 mL of dry ethanol. The suspension was vigorously shaken with vortex apparatus (Fisher Scientific) for 15 s and sonicated for 15 min. The solid phase was then separated from the ethanolic mixture by centrifugation (Eppendorf centrifuge, 5417R) at 7000 rpm for 10 min at 21 ◦ C. The supernatant containing the volatile molecules was collected and the monoterpene compound contents were quantified by gas chromatography–mass spectrometry (GC–MS). The extraction procedure was demonstrated to be quantitative by measuring the extracted amounts of the four monoterpene compounds, directly after the adsorption onto the three different matrices. The amount of remaining monoterpenes was expressed as the relative retention (%) and was determined using the following equation: Relative retention (%) =
Mt × 100 M0
where Mt and M0 are respectively the amount (mg) of monoterpene in sample at time t and initial time t0 . For each formulation, the relative retention at t0 was measured to be equal to the unity, i.e. the extraction procedure must be quantitative. 2.5. Gas chromatography–mass spectrometry analysis (GC–MS) Each extraction sample was diluted in 1 mL of chloroform and 1 L was directly injected into the GC–MS apparatus. The GC–MS analyses were performed using a Waters GCT Premier (from Waters Corporation company, Manchester, England) instrument based on a time-of-flight analyzer. The gas chromatograph was equipped with a Restek Rtx-5Sil MS column (30 m length, 0.25 mm inner diameter and 0.25 m film thickness). Typical GC conditions were: injector temperature, 250 ◦ C; splitless mode; Helium carrier gas flow rate, 1 mL/min; interface temperature: 250 ◦ C. The temperature program was as follow: initial temperature, 55 ◦ C; 1 ◦ C/min ramp; final temperature, 150 ◦ C;
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Table 1 Structures, physico-chemical parameters and GC–MS data of selected monoterpene molecules.
Monoterpene molecules (+)-˛-Pinene Formula Boiling point (◦ C) Molecular weight (g/mol) Density Retention time (min)a a
C10 H16 154–156 136.23 0.858 5.68
Citronellal C10 H18 O 201–207 154.25 0.855 8.89
Terpinen-4-ol
Carvone
C10 H18 O 212 154.25 0.933 9.36
C10 H14 O 230–231 150.22 0.958 10.28
Gas chromatography–mass spectrometry (GC–MS).
5 ◦ C/min ramp; final temperature, 250 ◦ C (hold 5 min). Electron ionization (EI) source conditions were: source temperature, 200 ◦ C; electron energy, 70 eV; trap current, 200 A; emission current, 400 A. All ions were transmitted into the pusher region of the time-of-flight analyzer where they were mass-analyzed with a 1 s integration time. Data were acquired in continuum mode. The GCT Premier instrument is a high-sensitivity instrument and, for instance, in EI positive ionization mode, 1 pg of hexachlorobenzene gives signal-to-noise ratio (S/N) >10/1 whilst acquiring full spectra over a mass range up to m/z 800. For the quantitative approach, a calibration curve was constructed using various concentrations. Quantitative control was carried out using the calibration curve of the standards. The relative concentration (%) of the individual component was calculated based on the GC peak areas. 3. Results and discussion 3.1. Material characterization Cellulosic-based materials used were cellulose, cellulose acetate and rice husk. These materials have been extensively characterized by FT-IR spectroscopy (Sun et al., 2004; Liu et al., 2007; Ouédraogo and Bonzi-Coulibaly, 2009; Chockalingam and Subramanian, 2006; Asadi et al., 2008). Adsorption isotherms employed to determine the BET surface areas showed that surface areas were 3.4, 13.4 and 7.1 m2 /g respectively, for cellulose, cellulose acetate and rice husk. Table 1 also gives a full overview of the physico-chemical properties of the four selected monoterpenes. GC–MS parameters, i.e. retention times, are also provided in Table 1. In order to define the volatility propensity of the four selected molecules, the volatility profiles of each of the free monoterpenes is presented in Fig. 1 and were obtained when 1 mg of unloaded mixture of monoterpenes is put into glass vials at 21 ◦ C. The volatility of the four monoterpenes from the mixture of monoterpenes can be, as expected, correlated to the different boiling points (Tanzi et al.,
Fig. 1. Release profiles of unloaded mixture of monoterpenes during storage at 21 ± 2 ◦ C. Initial relative retention of each monoterpene = 100%.
2012; Sansukcharearnpon et al., 2010; Hoskovec et al., 2005). Our results indicate that the investigated molecules are characterized by different vaporization rates under our experimental conditions. As expected, ˛-pinene (Bp = 155 ◦ C) is quickly and completely volatilized after 6 h (1/4 day). In the same conditions and, after 6 h, the amounts of evaporated citronellal (Bp = 203 ◦ C), terpinen-4-ol (Bp = 212 ◦ C) and carvone (Bp = 230 ◦ C) are estimated to respectively 85%, 75% and 60%, in close agreement with the relative boiling points (Table 1). This experiment is also important since the good correlation between the evaporation relative order from the mixture of monoterpenes and the respective boiling points could also indicate that the evaporation processes are not too influenced by heterogenic intermolecular interactions between the four different molecules constituting the mixture of monoterpenes. In other words, the relative volatility of the selected monoterpenes is mainly dependent on their intrinsic volatility (Bp) and is not modified by additional intermolecular interactions. This was important to demonstrate for the following part of the work to avoid misunderstanding on the exact role of the cellulose-based matrices. 3.2. Evaporation of carvone loaded on cellulose-based matrices: concentration effect Based on lower volatility, carvone was selected as model compound to carry out the influence of the concentration of the monoterpene on the volatility profile. This was investigated using various concentrations of carvone loaded on cellulose and stored at 21 ◦ C. Retention capacity of carvone impregnated onto cellulose was monitored via the determination of the remaining concentration of carvone in the solid matrix over time. Carvone release profiles when loaded onto cellulose are reported in Fig. 2. Three weight ratios, i.e. 1:100, 2:100 and 5:100 (w/w) were tested and the remaining amount of carvone was determined after a selected 4-days storage time. For all concentrations, there is a significant reduction of the concentration of carvone. In other words, this already shows that the evaporation of carvone is not totally
Fig. 2. Release profile of carvone loaded onto cellulose during storage at 21 ± 2 ◦ C. Initial relative retention of carvone = 100%.
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prevented when impregnated onto cellulose. Nevertheless, when compared with the evaporation rate determined for the unloaded material (Fig. 1), the evaporation of carvone is significantly reduced when loaded onto cellulose. For instance, 6 h are required to evaporate 60% of unloaded carvone, whereas, for the lowest tested concentration (1:100), after 4 days, more than 50% of the starting amount of carvone is still adsorbed onto cellulose. A closer analysis of the release profiles reveals that the evaporation of monoterpene is especially marked during the first day, whatever the starting concentration. This phenomenon is mainly observed for the highest concentrations of carvone in the solid matrix. Indeed, for the 2:100 and 5:100 concentrations, the evaporation yield of carvone amounts to almost 40% and 45%, respectively. On the other hand, for the concentration 1:100, nearly 80% of the starting quantity of carvone is still present in the solid matrix after one day. Saturation effects can explain this result, when a higher concentration is used; it is likely that all the available sites for adsorption will be solicited leaving more non adsorbed carvone molecules at the surface of the solid matrix that are ready to escape by evaporation. Based on this observation, we can already propose that, using cellulose as sorbent for monoterpene conditioning, can allow reducing the quantity of active product. Nevertheless, in the specific case of essential oils, due to the intrinsic high volatility of the monoterpenes, more compounds than required are usually needed to achieve the active local concentrations and keep it constant over time. Comparing the carvone content after 4 days with different ratios indicates that, for the preparation of an efficient formulation, saturation of cellulose with essential oil is quickly reached at concentration 1:100. Many works describes the encapsulation of essential oil onto polysaccharides using similar concentrations. For instance, different chitosan-based formulations were obtained with loading values between 0.05 and 0.15% (w/w) for both lime and thyme essential oils (Bosquez-Molina et al., 2010; Ramos-García et al., 2012), and 0.5–3% (w/w) for bergamot essential oil (Sánchez-González et al., 2010). Beads based on chitosan and cashew gum were also
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loaded with Lippia sidoides essential oil (Paula et al., 2011), with a relative concentrations ranged from 2.4% to 4.4% (w/w). More recently, Abreu et al. (2012) also improved this essential oil loading from 5 to 11.8% by using a new matrix composed of chitosan and cashew gum. Finally, supercritical impregnation of lavender oil in n-octenyl succinate (OSA) modified starch has also been proposed with loading concentration ranging from 2.5 to 15% (Varona et al., 2011). 3.3. Evaporation of monoterpenes loaded on cellulose-based matrices: nature of the monoterpene The releases of ˛-pinene, carvone, citronellal and terpinen-4-ol impregnated onto cellulose were individually investigated with a ratio of 1:100 (w/w). The release profiles of volatile molecules individually loaded onto cellulose are shown in Fig. 3 in comparison with unloaded monoterpenes. In general, monoterpenes of essential oils are considered as non-polar molecules which have low interaction with the hydrophilic surface of cellulose. Nevertheless, all the data presented in Fig. 3 demonstrates that the evaporation rates are significantly reduced upon adsorption. Indeed, total evaporation of unloaded volatile molecules was achieved after 6 h for ˛-pinene and after 24 h for carvone, citronellal and terpinen-4-ol. Fig. 3 clearly shows that the impregnation of a volatile molecule onto cellulose slows the rate of the evaporation process when compared to non-impregnated volatiles. After storage at 21 ◦ C, the evaporation of ˛-pinene is notable during the first day, since more than 70% of the starting material has escaped from the solid matrix after 6 h. This is readily explained since ˛-pinene, being the most hydrophobic compound, weakly interacts with the polar cellulose matrix. In the case of carvone and citronellal, both being quite polar molecules, the relative retentions amount are about 60% and 40%, respectively, during the first two days. Finally, the evaporation of terpinen-4-ol is strongly limited when adsorbed on cellulose since, even after 10 days, 60% of the starting amount is still present in the solid matrix. Though, after 10 days, ˛-pinene and citronellal
Fig. 3. Release profiles of monoterpenes unloaded and loaded onto cellulose matrix during storage at 21 ± 2 ◦ C. Initial relative retention of monoterpene = 100%.
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formulations still contained 20% and 25%, respectively. Carvone is also quite efficiently associated to the cellulose matrix since, after 10 days, only 50% of the starting amount was evaporated. It is of course expected that, after being adsorbed on a solid matrix, the molecular structures in terms of size and functional groups present on the volatile molecules represent the key-parameter to account for the increase over time stability. Indeed, several authors previously commented in their studies that release characteristics are directly related (i) to the diffusion of the molecules through the polymer matrix and (ii) to the different chemical interactions realized by different fragrance molecules with the matrix (Sansukcharearnpon et al., 2010; Sánchez-González et al., 2011c; Madene et al., 2006; Ponce Cevallos et al., 2010). Cellulose is a polysaccharide polymer which has two free secondary and one free primary alcohol functions per repeating unit. This makes cellulose a quite polar macromolecule able to create strong H-bonds with hydroxylic groups from alcohol molecules. The hydrophilic cellulose has then more potential interactions with alcohols as terpinen-4-ol than with apolar compounds as pinene. For that reason, the terpinen-4-ol formulation shows a lower release rate than the associations of cellulose with carvone, citronellal and ˛-pinene. These interactions definitively reduce the volatility of the alcohol. The ketone functional group of carvone presents a similar character that the aldehyde functional group of citronellal. Nevertheless, citronellal and carvone have significantly different behaviors with carvone being less efficiently evaporated from the solid matrix. Carvone and citronellal being respectively a cyclic and a linear molecule, the diffusion rates of those molecules are also expected to be different throughout cellulosic matrix. An additional effect can also come from the highest intrinsic polarity of carvone, as exemplified by its highest Bp. 3.4. Evaporation of monoterpenes loaded on cellulose-based matrices: effect of the nature of the adsorbent This part of the investigation has been performed using the synthetic mixture of monoterpenes. Three different cellulosic-based matrices have been selected and correspond to cellulose, cellulose acetate and rice husk. The previously determined ratio of 1:100 (w/w) has also been selected and the prepared formulations were stored over 20 days. Fig. 4 describes the release efficiencies of this mixture impregnated onto cellulose, cellulose acetate and rice husk over time. First of all, it is important to probe the putative influence of the intermolecular interactions between the monoterpenes when analyzing the mixture of monoterpenes when compared to the individual formulations. This part of the work was only performed in the case of cellulose as a model system. When comparing Figs. 4 and 3, we can easily conclude that, with the concentrations used for this study, the evaporation rates of the four selected compounds individually or jointly loaded on cellulose are really similar. Basically, after 20 days at 21 ◦ C, the residual quantities of terpinen-4-ol, carvone, citronellal and ˛-pinene into cellulose are reproducibly measured around 65%, 50%, 20% and 10%, respectively. As far as the cellulose acetate formulations (Fig. 4), it is interesting to already remind that the polarity of cellulose acetate polymer is reduced upon (partial) acetylation when compared with cellulose. Accordingly, terpinen-4-ol is no longer the most retained molecule given that strong and numerous H-bonds are no longer feasible. Actually, when loaded on cellulose acetate, citronellal and terpinen-4-ol show medium but similar releases with about 35% and 21% remaining after 10 and 20 days, respectively, for both compounds. However, carvone is efficiently retained on cellulose acetate since the retention was recorded at 80% and 70% after 10 and 20 days, respectively. All together, the reported retention order follows carvone > terpinen-4-ol ∼ citronellal > ˛-pinene.
Fig. 4. Release profiles of mixture of monoterpenes loaded cellulosic-based matrices during storage at 21 ± 2 ◦ C. Initial relative retention of each monoterpene = 100%.
Finally, the retention profiles for the mixture loaded onto rice husk reveal a high release of ˛-pinene during the first day of storage with retention of 45% of the initial amount in the formulation. Basically, the retention efficiencies ranking is really similar to the cellulose situation with terpinen-4-ol > carvone > citronellal > ˛pinene. However, the evaporation of the polar compounds is significantly limited when loaded onto rice husk with retention of 90%, 85% and 53% for terpinen-4-ol, carvone and citronellal, respectively, after 20 days. The efficiency of rice husk to prevent polar compounds from evaporating could be explained on the basis of the physical and chemical properties of rice husk. Rice husk is a highly polar solid matrix that contains carboxylic acid functions (COOH) and hydroxyl groups such as silanol (SiOH) (Osman et al., 2010; Chockalingam and Subramanian, 2006; Asadi et al., 2008). Those functional groups are obviously likely to be involved in forming strong H-bonds. Therefore, the retention of selected compounds on rice husk is highly governed by such chemical interactions. On the other hand, rice husk possesses a larger surface area than cellulose, i.e. 3.4 and 7.1 m2 /g for, respectively, cellulose and rice husk. As a direct consequence, the rice husk particles have a large amount of H-bond sites available for interaction with the guest molecules. On the other hand, it seems that the larger specific surface area determined for cellulose acetate at 13.4 m2 /g does not contribute greatly to the retention processes. This of course confirms that the key-parameter that induces retention and prevents evaporation of
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the monoterpenes loaded onto cellulose-like matrices is the adequacy between chemical functionalities present on the molecule and the solid phase. But, for a given monoterpene/matrix pair, the role of the specific surface area becomes important since more strong interactions are then expected to occur. 4. Conclusion In the present study, the incorporation of volatile compounds onto three cellulose-based polymers is demonstrated to induce a significant reduction of the volatility of several terpenes from essential oils. The retention tests confirm that the overtime increased stability of the volatile molecules is highly dependent (i) on the initial concentration of the volatile molecules loaded onto the solid material, (ii) on the volatile molecule intrinsic volatility (Bp) and (iii) on the chemical structure of the volatile molecules. As far as the selected system is concerned, we found that rice husk present excellent retention (≥50%) capacity up to 20 days for terpinen-4-ol, carvone and citronellal. The impregnation of terpinen-4-ol and carvone onto the cellulose and cellulose acetate effectively prolongs the retention of the volatiles over time. These results can be considered as the starting point for the development of formulations dedicated to essential oils with lowcost biosorbents for crops protection against pests. This kind of formulation could represent a great potential for large scale postharvest application taking in account the low concentration of active essential oil required in the biopolymer matrix. Acknowledgements The authors are grateful to CUD (Commission Universitaire pour le Développement) of Belgium for financial support for this work. MS laboratory at UMons acknowledges the “Fonds de la Recherche Scientifique (FRS-FNRS)” for its contribution to the acquisition of the GC–MS Spectrometer. P.G. is FNRS Senior Research Associate. References Abbas, A., Ansumali, S., 2010. Global potential of rice husk as a renewable feedstock for ethanol biofuel production. Bioenergy Res. 3, 328–334. Abreu, F.O.M.S., Oliveira, E.F., Paula, H.C.B., De Paula, R.C.M., 2012. Chitosan/cashew gum nanogels for essential oil encapsulation. Carbohydr. Polym. 89, 1277–1282. Adel, A.M., Abd El-Wahab, Z.H., Ibrahim, A.A., Al-Shemy, M.T., 2011. Characterization of microcrystalline cellulose prepared from lignocellulosic materials. Part II: physicochemical properties. Carbohydr. Polym. 83, 676–687. Asadi, F., Shariatmadari, H., Mirghaffari, N., 2008. Modification of rice hull and sawdust sorptive characteristics for remove heavy metals from synthetic solutions and wastewater. J. Hazard. Mater. 154, 451–458. Bosquez-Molina, E., Ronquillo-de Jesús, E., Bautista-Banos, S., Verde-Calvo, J.R., Morales-López, J., 2010. Inhibitory effect of essential oils against Colletotrichum gloeosporioides and Rhizopus stolonifer in stored papaya fruit and their possible application in coatings. Postharvest Biol. Technol. 57, 132–137. Cevallos, P.A.P., Buera, M.P., Elizalde, B.E., 2010. Encapsulation of cinnamon and thyme essential oils components (cinnamaldehyde and thymol) in cyclodextrin: effect of interactions with water on complex stability. J. Food Eng. 99, 70–75. Chaudhary, D.S., Jollands, M.C., 2004. Characterization of rice hull ash. J. Appl. Polym. Sci. 93, 1–8. Chockalingam, E., Subramanian, S., 2006. Studies on removal of metal ions and sulphate reduction using rice husk and Desulfotomaculum nigrificans with reference to remediation of acid mine drainage. Chemosphere 62, 699–708. De Saint Laumer, J.-Y., Frérot, E., Herrmann, A., 2003. Controlled release of perfumery alcohols by Neighboring-Group participation. Comparison of the rate constants for the alkaline hydrolysis of 2-acyl-, 2-(hydroxymethyl)-, and 2carbamoylbenzoates. HeIv. Chim. Acta 86, 2871–2899. Dikusar, E.A., Zhukovskaya, N.A., Moiseichuk, K.L., Zalesskaya, E.G., Vyglazov, O.G., Kurman, P.V., 2008. Synthesis and structure: aroma correlation of citral oxime esters. Chem. Nat. Compd. 44, 81–83. Genieva, S.D., Turmanova, S.Ch., Dimitrova, A.S., Vlaev, L.T., 2008. Characterization of rice husks and the products of its thermal degradation in air or nitrogen atmosphere. J. Therm. Anal. Calorim. 93, 387–396. Hoskovec, M., Grygarová, D., Cvaˇcka, J., Streinz, L., Zima, J., Verevkin, S.P., Koutek, B., 2005. Determining the vapour pressures of plant volatiles from gas chromatographic retention data. J. Chromatogr. A 1083, 161–172.
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Isman, M.B., Miresmailli, S., Machial, C., 2011. Commercial opportunities for pesticides based on plant essential oils in agriculture, industry and consumer products. Phytochem. Rev. 10, 197–204. Ji, F., Li, C., Tang, B., Xu, J., Lu, G., Liu, P., 2012. Preparation of cellulose acetate/zeolite composite fiber and its adsorption behavior for heavy metal ions in aqueous solution. Chem. Eng. J. 209, 325–333. Lai, F., Wissing, S.A., Müller, R.H., Fadda, A.M., 2006. Artemisia arborescens L essential oil-loaded solid lipid nanoparticles for potential agricultural application: preparation and characterization. AAPS PharmSciTech 7, E1–E9. Levrand, B., Ruff, Y., Lehn, J.-M., Herrmann, A., 2006. Controlled release of volatile aldehydes and ketones by reversible hydrazone formation – “classical” profragrances are getting dynamic. Chem. Commun., 2965–2967. Liu, Z.-T., Fan, X., Wu, J., Zhang, L., Song, L., Gao, Z., Dong, W., Xiong, H., Peng, Y., Tang, S., 2007. A green route to prepare cellulose acetate particle from ramie fiber. React. Funct. Polym. 67, 104–112. Luo, Y., Zhang, B., Whent, M., (Lucy) Yu, L., Wang, Q., 2011. Preparation and characterization of zein/chitosan complex for encapsulation of ␣-tocopherol, and it’s in vitro controlled release study. Colloids Surf. B: Biointerfaces 85, 145–152. Madene, A., Jacquot, M., Scher, J., Desobry, S., 2006. Flavour encapsulation and controlled release – a review. Int. J. Food Sci. Technol. 41, 1–21. Marcuzzo, E., Sensidoni, A., Debeaufort, F., Voilley, A., 2010. Encapsulation of aroma compounds in biopolymeric emulsion based edible films to control flavour release. Carbohydr. Polym. 80, 984–988. Narula, A.P.S., 2004. The search for new fragrance ingredients for functional perfumery. Chem. Biodivers. 1, 1992–2000. Nguemtchouina, M.M.G., Ngassoum, M.B., Ngamo, L.S.T., Gaudu, X., Cretin, M., 2010. Insecticidal formulation based on Xylopia aethiopica essential oil and kaolinite clay for maize protection. Crop Prot. 29, 985–991. Ouédraogo, W.I., Bonzi-Coulibaly, Y.L., 2009. Extraction et analyse de la cellulose issue de biomasses végétales de rejet disponibles au Burkina Faso. Sci. Tech. 3, 73–81. Ouédraogo, W.I., Boulvin, M., Flammang, R., Gerbaux, P., Bonzi-Coulibaly, Y.L., 2009. Conversion of natural aldehydes from Eucalyptus citriodora, Cymbopogon citratus, and Lippia multiflora into oxime: GC–MS and FT–IR analysis. Molecules 14, 3275–3285. Osman, H.E., Badwy, R.K., Ahmad, H.F., 2010. Usage of some agricultural by-product in the removal of some heavy metals from industrial wastewater. J. Phytol. 2, 51–62. Paula, H.C.B., Sombra, F.M., De Freitas Cavalcante, R., Abreu, F.O.M.S., De Paula, R.C.M., 2011. Preparation and characterization of chitosan/cashew gum beads loaded with Lippia sidoides essential oil. Mater. Sci. Eng. C 31, 173–178. Ponce Cevallos, P.A., Buera, M.P., Elizalde, B.E., 2010. Encapsulation of cinnamon and thyme essential oils components (cinnamaldehyde and thymol) in cyclodextrin: effect of interactions with water on complex stability. J. Food Eng. 99, 70–75. Ramos-García, M., Bosquez-Molina, E., Hernández-Romano, J., Zavala-Padilla, G., Terrés-Rojas, E., Alia-Tejacal, I., Barrera-Necha, L., Hernández-López, M., ˜ S., 2012. Use of chitosan-based edible coatings in combination Bautista-Banos, with other natural compounds, to control Rhizopus stolonifer and Escherichia coli DH5␣ in fresh tomatoes. Crop Protect. 38, 1–6. Rochat, S., Minardi, C., De Saint Laumer, J.-Y., Herrmann, A., 2000. Controlled release of perfumery aldehydes and ketones by Norrish type-II photofragmentation of ␣-keto esters in undegassed solution. Helv. Chim. Acta 83, 1645–1671. Sacchetti, G., Maietti, S., Muzzoli, M., Scaglianti, M., Manfredini, S., Radice, M., Bruni, R., 2005. Comparative evaluation of 11 essential oils of different origin as functional antioxidants, antiradicals and antimicrobials in foods. Food Chem. 91, 621–632. Saha, B.C., Cotta, M.A., 2007. Enzymatic saccharification and fermentation of alkaline peroxide pretreated rice hulls to ethanol. Enzyme Microb. Technol. 41, 528–532. Saha, B.C., Iten, L.B., Cotta, M.A., Victor Wu, Y., 2005. Dilute acid pretreatment, enzymatic saccharification, and fermentation of rice hulls to ethanol. Biotechnol. Prog. 21, 816–822. Sánchez-González, L., Cháfer, M., Chiralt, A., González-Martínez, C., 2010. Physical properties of edible chitosan films containing bergamot essential oil and their inhibitory action on Penicillium italicum. Carbohydr. Polym. 82, 277–283. Sánchez-González, L., Pastor, C., Vargas, M., Chiralt, A., González-Martínez, C., Cháfer, M., 2011a. Effect of hydroxypropylmethylcellulose and chitosan coatings with and without bergamot essential oil on quality and safety of cold-stored grapes. Postharvest Biol. Technol. 60, 57–63. Sánchez-González, L., Cháfer, M., Hernández, M., Chiralt, A., González-Martínez, C., 2011b. Antimicrobial activity of polysaccharide films containing essential oils. Food Control 22, 1302–1310. Sánchez-González, L., Cháfer, M., González-Martínez, C., Chiralt, A., Desobry, S., 2011c. Study of the release of limonene present in chitosan films enriched with bergamot oil in food simulants. J. Food Eng. 105, 138–143. Sansukcharearnpon, A., Wanichwecharungruang, S., Leepipatpaiboon, N., Kerdcharoend, T., Arayachukeat, S., 2010. High loading fragrance encapsulation based on a polymer-blend: preparation and release behavior. Int. J. Pharm. 391, 267–273. Sing, K.S.W., Everett, D.H., Haul, R.A.W., Moscou, L., Pierotti, R.A., Rouquerol, J., Siemieniewska, T., 1985. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl. Chem. 57, 603–619.
106
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Soottitantawat, A., Bigeard, F., Yoshii, H., Furuta, T., Ohkawara, M., Linko, P., 2005. Influence of emulsion and powder size on the stability of encapsulated dlimonene by spray drying. Innov. Food Sci. Emerg. Technol. 6, 107–114. Sun, J.X., Sun, X.F., Zhao, H., Sun, R.C., 2004. Isolation and characterization of cellulose from sugarcane bagasse. Polym. Degrad. Stab. 84, 331–339. Takács, E., Wojnárovits, L., Horváth, É.K., Fekete, T., Borsa, J., 2012. Improvement of pesticide adsorption capacity of cellulose fibre by high-energy irradiation-initiated grafting of glycidyl methacrylate. Radiat. Phys. Chem. 81, 1389–1392. Tanzi, C.D., Vian, M.A., Ginies, C., Elmaataoui, M., Chemat, F., 2012. Terpenes as green solvents for extraction of oil from microalgae. Molecules 17, 8196–8205. Tongnuanchan, P., Benjakul, S., Prodpran, T., 2012. Properties and antioxidant activity of fish skin gelatin film incorporated with citrus essential oils. Food Chem. 134, 1571–1579.
Varona, S., Rodríguez-Rojo, S., Martín, Á., Cocero, M.J., Duarte, C.M.M., 2011. Supercritical impregnation of lavandin (Lavandula hybrida) essential oil in modified starch. J. Supercrit. Fluids 58, 313–319. Wan Ngah, W.S., Hanafiah, M.A.K.M., 2008. Removal of heavy metal ions from wastewater by chemically modified plant wastes as adsorbents: a review. Bioresour. Technol. 99, 3935–3948. Wang, L., Li, J., 2013. Adsorption of C.I. Reactive Red 228 dye from aqueous solution by modified cellulose from flax shive: kinetics, equilibrium, and thermodynamics. Ind. Crops Prod. 42, 153–158. Wojnárovits, L., Földváry, Cs.M., Takács, E., 2010. Radiation-induced grafting of cellulose for adsorption of hazardous water pollutants: a review. Radiat. Phys. Chem. 79, 848–862. Woranuch, S., Yoksan, R., 2013. Eugenol-loaded chitosan nanoparticles: II. Application in bio-based plastics for active packaging. Carbohydr. Polym. 96, 586–592.
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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Volatility profiles of monoterpenes loaded onto cellulosic-based materials Igor W.K. Ouédraogo a,b , Julien De Winter b , Pascal Gerbaux b , Yvonne L. Bonzi-Coulibaly a,∗ a
Laboratoire de Chimie Organique: Structure et Réactivité, UFR-SEA, Université de Ouagadougou, 03 BP 7021, Ouagadougou 03, Burkina Faso Groupe de Recherche en Spectrométrie de Masse, Centre Interdisciplinaire de Spectrométrie de Masse, Université de Mons – UMONS, Place du Parc 23, B-7000 Mons, Belgium b
a r t i c l e
i n f o
Article history: Received 8 June 2013 Received in revised form 21 July 2013 Accepted 9 August 2013 Keywords: Monoterpene Cellulose-based matrices Impregnation Volatility Formulation
a b s t r a c t Monoterpene compounds were loaded onto pure cellulose and two cellulose-based matrices by impregnation method. The effects of initial ratio, structures of volatile compounds and polymers, i.e. cellulose, acetate cellulose and rice husk, on the release profiles were investigated. Four monoterpenes, namely ˛-pinene, citronellal, carvone and terpinen-4-ol, were tested as volatile compound models. In the case of carvone loaded onto cellulose, we observed that the release of the volatile molecule increases with increasing initial ratio in the formulation. Using different cellulose-based matrix to study the volatility of monoterpene models, the lowest release rates was obtained with rice husk formulations, with highly retention capacity over 50% for carvone, terpinen-4-ol and citronellal after 20 days at 21 ◦ C. It was also concluded that the impregnation of terpinen-4-ol and carvone into cellulose and cellulose acetate respectively, could effectively help in prolonging the retention of these volatiles. ˛-Pinene, a highly hydrophobic molecule showed no significant retention. These results indicate that cellulosebased matrices could be potentially used as good carriers of active compounds for ecological pesticides formulation for postharvest applications. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Essential oils from aromatic plants are mainly monoterpene derivatives and are involved in a wide range of applications in sanitary, cosmetic, agricultural, pharmaceutical and food industries. Successful uses of essential oils against agricultural pests and foodborne microorganisms are also convincingly (Isman et al., 2011; Nguemtchouina et al., 2010; Sacchetti et al., 2005) reported. One important point to consider in the different applications is the capability of the used formulations to release the active constituent(s) in a controlled manner. This is even more problematic when considering essential oils given the really high volatility of the constituting monoterpene compounds. Given the low vapor pressure (high boiling point) and their molecular structures, monoterpenes are prone to rapid evaporation and easy degradation, respectively (Hoskovec et al., 2005; Lai et al., 2006). Various reactions such as oxidation or Norrish type II photofragmentation (Rochat et al., 2000) are well known to limit their use over time. Different strategies have then been developed to protect the monoterpene constituents from
∗ Corresponding author. Tel.: +226 50 30 70 64; fax: +226 50 30 72 42. E-mail addresses: [email protected] (P. Gerbaux), [email protected] (Y.L. Bonzi-Coulibaly). 0926-6690/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.indcrop.2013.08.032
degradation and/or to control their release and diffusion. Most of the procedures involve neighboring-group participation (De Saint Laumer et al., 2003), profragrances formation (Levrand et al., 2006), encapsulations (Cevallos et al., 2010; Luo et al., 2011) or nitrile and oxime-based derivations (Narula, 2004; Dikusar et al., 2008; Ouédraogo et al., 2009). Those methods were demonstrated to prolong the long-lasting effect of volatile compounds and, as an additional benefit, to increase their stability in aggressive media (air oxidation, light, moisture and higher temperatures). Intensive studies have been conducted to consider bio-based materials, i.e. cellulose, as adsorbents of many inorganic or organic compounds in the context of water treatment (Ji et al., 2012; Wojnárovits et al., 2010; Wang and Li, 2013; Takács et al., 2012). Also various biopolymers have been used as matrix for entrapping different active compounds for medical or food purposes (Soottitantawat et al., 2005; Sánchez-González et al., 2011a,b; Tongnuanchan et al., 2012). Incorporation of essential oils into polymer matrices for fruit protection or against food borne pathogens has been showed to represent a successful approach (Marcuzzo et al., 2010; Paula et al., 2011; Bosquez-Molina et al., 2010; Woranuch and Yoksan, 2013). Sánchez-González et al. (2011b) prepared antimicrobial films by incorporating various essential oils from bergamot, lemon and tea tree, into chitosan and hydroxypropylmethylcellulose films. The association of these bio-based
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materials with adsorbed essential oils improves the physical and biological/chemical properties of the film. In addition, when adsorbed on the biopolymer, the volatility of the essential oil constituents is reduced and the activity time of the essential oil is of course prolonged. Cellulose-based materials could be promising materials for natural volatile molecule formulation in crops protection strategies against insects or fungi. In the context of seed protection, feedstocks could be selected and serve as low-cost adsorbents. Indeed, rice husk and derivative products like cellulose and cellulose acetate are interesting polymers given several recognized properties such as no toxicity, availability, biodegradability (Sun et al., 2004; Liu et al., 2007; Asadi et al., 2008). Rice husks are an important by-product of rice milling process. According to the statistical data of Food and Agriculture Organization (FAO), the world annual paddy production is approximately 582 million tons and rice husk represent 22–25% of the rice grain. Rice husk as described in the literature is mainly composed of cellulose (32.24–42.8%), hemicellulose (21.34–32.7%), lignin (11.96–24.5%) and mineral ash (15.05–18.86%) which is in large amount of silica (SiO2 ) (Genieva et al., 2008; Abbas and Ansumali, 2010; Osman et al., 2010; Wan Ngah and Hanafiah., 2008; Adel et al., 2011; Chaudhary and Jollands, 2004; Saha et al., 2005; Saha and Cotta, 2007). Thus, due to the availability, attention has been focused on sorbents production from rice husk. In this context, we decided to probe the adsorption capabilities of some cellulosic-based matrices toward some typical monoterpene compounds. For the present work, cellulose, cellulose acetate and rice husk have been selected as model matrices for the adsorption of four monoterpenes used as volatile compound candidates. The selected molecules are (+)-˛-pinene (unsaturated hydrocarbon), citronellal (aldehyde), carvone (ketone) and terpinen-4-ol (alcohol). The release behaviors of the four compounds loaded onto cellulosicbased materials will be investigated by gas chromatography–mass spectrometry (GC–MS) analyses. 2. Materials and methods 2.1. Materials Cellulose powder (DS-0), for thin layer chromatography (TLC) was purchased from Sigma–Aldrich. Cellulose acetate (average M.W. 100.000) was obtained from ACROS Organics. Monoterpene chemical used were (+)-˛-pinene (purity >98%, Alfa Aesar), carvone (purity >99%, ACROS Organics), citronellal (purity >93%, ACROS Organics) and terpinen-4-ol (purity >95%, Aldrich). Rice husk was obtained from local rice collected in Burkina Faso. All other reagents used throughout the study were analytical grade. 2.2. Rice husk preparation and characterization The dried material of rice husk (size fractions of 0.063–0.125 mm) was successively washed with deionized water, ethanol and acetone several times to remove dust and fines. Afterwards, the collected material was dried in an oven at 60 ◦ C for 24 h. The material was characterized by Fourier transforminfrared (FT-IR) analysis on a Perkin Elmer BX II spectrophotometer employing the KBr pellet method. The specific surface area of rice husk was determined in comparison with cellulose and cellulose acetate by N2 adsorption at 77 K using an ASAP 2020 Micromeritics instrument and the Brunauer–Emmett–Teller (BET) method (Sing et al., 1985). FT-IR spectra: 3600–3200 cm−1 O H stretching, 1732 cm−1 C O stretching (hemicellulose), 1606 and 1515 cm−1 aromatic rings of
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lignin, 799 cm−1 Si O Si bond, 465–485 cm−1 O Si O bending vibration. Specific surface area: 7.1 m2 /g. 2.3. Volatile compound formulations and volatility studies Formulation of monoterpenes loaded onto cellulose-based matrices was carried out by impregnation: 1 mg of each monoterpene was first diluted with 200 L of acetone and then added to 100 mg of cellulose into glass vials (inner diameter × height: 20 mm × 40 mm). The volatility study was performed by using different amounts, expressed as weight of volatile molecule per 100 g of cellulose-based matrix. For the study of the influence of the adsorbent nature, 0.25 mg of each monoterpene was mixed to get 1 mg of a mixture of monoterpenes. This mixture was diluted with 200 L of acetone (for cellulose or rice husk formulations) or hexane (for cellulose acetate formulation) and then added to 100 mg of cellulose-based matrix into glass vials (inner diameter × height: 20 mm × 40 mm). The resulting mixtures were left uncovered for solvent and monoterpene evaporation. Monitoring was carried out under room temperature (21 ± 2 ◦ C). Each experiment was replicated 3 times and the mean (average) values are reported. Monoterpenes (without cellulose-based matrix) were also diluted with 200 L of acetone and exposed for monitoring the volatilization. 2.4. Extraction method At set time intervals, the amount of the residual volatile molecules was determined. The remaining volatile molecules were extracted from the cellulose matrices by 5 mL of dry ethanol. The suspension was vigorously shaken with vortex apparatus (Fisher Scientific) for 15 s and sonicated for 15 min. The solid phase was then separated from the ethanolic mixture by centrifugation (Eppendorf centrifuge, 5417R) at 7000 rpm for 10 min at 21 ◦ C. The supernatant containing the volatile molecules was collected and the monoterpene compound contents were quantified by gas chromatography–mass spectrometry (GC–MS). The extraction procedure was demonstrated to be quantitative by measuring the extracted amounts of the four monoterpene compounds, directly after the adsorption onto the three different matrices. The amount of remaining monoterpenes was expressed as the relative retention (%) and was determined using the following equation: Relative retention (%) =
Mt × 100 M0
where Mt and M0 are respectively the amount (mg) of monoterpene in sample at time t and initial time t0 . For each formulation, the relative retention at t0 was measured to be equal to the unity, i.e. the extraction procedure must be quantitative. 2.5. Gas chromatography–mass spectrometry analysis (GC–MS) Each extraction sample was diluted in 1 mL of chloroform and 1 L was directly injected into the GC–MS apparatus. The GC–MS analyses were performed using a Waters GCT Premier (from Waters Corporation company, Manchester, England) instrument based on a time-of-flight analyzer. The gas chromatograph was equipped with a Restek Rtx-5Sil MS column (30 m length, 0.25 mm inner diameter and 0.25 m film thickness). Typical GC conditions were: injector temperature, 250 ◦ C; splitless mode; Helium carrier gas flow rate, 1 mL/min; interface temperature: 250 ◦ C. The temperature program was as follow: initial temperature, 55 ◦ C; 1 ◦ C/min ramp; final temperature, 150 ◦ C;
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Table 1 Structures, physico-chemical parameters and GC–MS data of selected monoterpene molecules.
Monoterpene molecules (+)-˛-Pinene Formula Boiling point (◦ C) Molecular weight (g/mol) Density Retention time (min)a a
C10 H16 154–156 136.23 0.858 5.68
Citronellal C10 H18 O 201–207 154.25 0.855 8.89
Terpinen-4-ol
Carvone
C10 H18 O 212 154.25 0.933 9.36
C10 H14 O 230–231 150.22 0.958 10.28
Gas chromatography–mass spectrometry (GC–MS).
5 ◦ C/min ramp; final temperature, 250 ◦ C (hold 5 min). Electron ionization (EI) source conditions were: source temperature, 200 ◦ C; electron energy, 70 eV; trap current, 200 A; emission current, 400 A. All ions were transmitted into the pusher region of the time-of-flight analyzer where they were mass-analyzed with a 1 s integration time. Data were acquired in continuum mode. The GCT Premier instrument is a high-sensitivity instrument and, for instance, in EI positive ionization mode, 1 pg of hexachlorobenzene gives signal-to-noise ratio (S/N) >10/1 whilst acquiring full spectra over a mass range up to m/z 800. For the quantitative approach, a calibration curve was constructed using various concentrations. Quantitative control was carried out using the calibration curve of the standards. The relative concentration (%) of the individual component was calculated based on the GC peak areas. 3. Results and discussion 3.1. Material characterization Cellulosic-based materials used were cellulose, cellulose acetate and rice husk. These materials have been extensively characterized by FT-IR spectroscopy (Sun et al., 2004; Liu et al., 2007; Ouédraogo and Bonzi-Coulibaly, 2009; Chockalingam and Subramanian, 2006; Asadi et al., 2008). Adsorption isotherms employed to determine the BET surface areas showed that surface areas were 3.4, 13.4 and 7.1 m2 /g respectively, for cellulose, cellulose acetate and rice husk. Table 1 also gives a full overview of the physico-chemical properties of the four selected monoterpenes. GC–MS parameters, i.e. retention times, are also provided in Table 1. In order to define the volatility propensity of the four selected molecules, the volatility profiles of each of the free monoterpenes is presented in Fig. 1 and were obtained when 1 mg of unloaded mixture of monoterpenes is put into glass vials at 21 ◦ C. The volatility of the four monoterpenes from the mixture of monoterpenes can be, as expected, correlated to the different boiling points (Tanzi et al.,
Fig. 1. Release profiles of unloaded mixture of monoterpenes during storage at 21 ± 2 ◦ C. Initial relative retention of each monoterpene = 100%.
2012; Sansukcharearnpon et al., 2010; Hoskovec et al., 2005). Our results indicate that the investigated molecules are characterized by different vaporization rates under our experimental conditions. As expected, ˛-pinene (Bp = 155 ◦ C) is quickly and completely volatilized after 6 h (1/4 day). In the same conditions and, after 6 h, the amounts of evaporated citronellal (Bp = 203 ◦ C), terpinen-4-ol (Bp = 212 ◦ C) and carvone (Bp = 230 ◦ C) are estimated to respectively 85%, 75% and 60%, in close agreement with the relative boiling points (Table 1). This experiment is also important since the good correlation between the evaporation relative order from the mixture of monoterpenes and the respective boiling points could also indicate that the evaporation processes are not too influenced by heterogenic intermolecular interactions between the four different molecules constituting the mixture of monoterpenes. In other words, the relative volatility of the selected monoterpenes is mainly dependent on their intrinsic volatility (Bp) and is not modified by additional intermolecular interactions. This was important to demonstrate for the following part of the work to avoid misunderstanding on the exact role of the cellulose-based matrices. 3.2. Evaporation of carvone loaded on cellulose-based matrices: concentration effect Based on lower volatility, carvone was selected as model compound to carry out the influence of the concentration of the monoterpene on the volatility profile. This was investigated using various concentrations of carvone loaded on cellulose and stored at 21 ◦ C. Retention capacity of carvone impregnated onto cellulose was monitored via the determination of the remaining concentration of carvone in the solid matrix over time. Carvone release profiles when loaded onto cellulose are reported in Fig. 2. Three weight ratios, i.e. 1:100, 2:100 and 5:100 (w/w) were tested and the remaining amount of carvone was determined after a selected 4-days storage time. For all concentrations, there is a significant reduction of the concentration of carvone. In other words, this already shows that the evaporation of carvone is not totally
Fig. 2. Release profile of carvone loaded onto cellulose during storage at 21 ± 2 ◦ C. Initial relative retention of carvone = 100%.
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prevented when impregnated onto cellulose. Nevertheless, when compared with the evaporation rate determined for the unloaded material (Fig. 1), the evaporation of carvone is significantly reduced when loaded onto cellulose. For instance, 6 h are required to evaporate 60% of unloaded carvone, whereas, for the lowest tested concentration (1:100), after 4 days, more than 50% of the starting amount of carvone is still adsorbed onto cellulose. A closer analysis of the release profiles reveals that the evaporation of monoterpene is especially marked during the first day, whatever the starting concentration. This phenomenon is mainly observed for the highest concentrations of carvone in the solid matrix. Indeed, for the 2:100 and 5:100 concentrations, the evaporation yield of carvone amounts to almost 40% and 45%, respectively. On the other hand, for the concentration 1:100, nearly 80% of the starting quantity of carvone is still present in the solid matrix after one day. Saturation effects can explain this result, when a higher concentration is used; it is likely that all the available sites for adsorption will be solicited leaving more non adsorbed carvone molecules at the surface of the solid matrix that are ready to escape by evaporation. Based on this observation, we can already propose that, using cellulose as sorbent for monoterpene conditioning, can allow reducing the quantity of active product. Nevertheless, in the specific case of essential oils, due to the intrinsic high volatility of the monoterpenes, more compounds than required are usually needed to achieve the active local concentrations and keep it constant over time. Comparing the carvone content after 4 days with different ratios indicates that, for the preparation of an efficient formulation, saturation of cellulose with essential oil is quickly reached at concentration 1:100. Many works describes the encapsulation of essential oil onto polysaccharides using similar concentrations. For instance, different chitosan-based formulations were obtained with loading values between 0.05 and 0.15% (w/w) for both lime and thyme essential oils (Bosquez-Molina et al., 2010; Ramos-García et al., 2012), and 0.5–3% (w/w) for bergamot essential oil (Sánchez-González et al., 2010). Beads based on chitosan and cashew gum were also
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loaded with Lippia sidoides essential oil (Paula et al., 2011), with a relative concentrations ranged from 2.4% to 4.4% (w/w). More recently, Abreu et al. (2012) also improved this essential oil loading from 5 to 11.8% by using a new matrix composed of chitosan and cashew gum. Finally, supercritical impregnation of lavender oil in n-octenyl succinate (OSA) modified starch has also been proposed with loading concentration ranging from 2.5 to 15% (Varona et al., 2011). 3.3. Evaporation of monoterpenes loaded on cellulose-based matrices: nature of the monoterpene The releases of ˛-pinene, carvone, citronellal and terpinen-4-ol impregnated onto cellulose were individually investigated with a ratio of 1:100 (w/w). The release profiles of volatile molecules individually loaded onto cellulose are shown in Fig. 3 in comparison with unloaded monoterpenes. In general, monoterpenes of essential oils are considered as non-polar molecules which have low interaction with the hydrophilic surface of cellulose. Nevertheless, all the data presented in Fig. 3 demonstrates that the evaporation rates are significantly reduced upon adsorption. Indeed, total evaporation of unloaded volatile molecules was achieved after 6 h for ˛-pinene and after 24 h for carvone, citronellal and terpinen-4-ol. Fig. 3 clearly shows that the impregnation of a volatile molecule onto cellulose slows the rate of the evaporation process when compared to non-impregnated volatiles. After storage at 21 ◦ C, the evaporation of ˛-pinene is notable during the first day, since more than 70% of the starting material has escaped from the solid matrix after 6 h. This is readily explained since ˛-pinene, being the most hydrophobic compound, weakly interacts with the polar cellulose matrix. In the case of carvone and citronellal, both being quite polar molecules, the relative retentions amount are about 60% and 40%, respectively, during the first two days. Finally, the evaporation of terpinen-4-ol is strongly limited when adsorbed on cellulose since, even after 10 days, 60% of the starting amount is still present in the solid matrix. Though, after 10 days, ˛-pinene and citronellal
Fig. 3. Release profiles of monoterpenes unloaded and loaded onto cellulose matrix during storage at 21 ± 2 ◦ C. Initial relative retention of monoterpene = 100%.
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formulations still contained 20% and 25%, respectively. Carvone is also quite efficiently associated to the cellulose matrix since, after 10 days, only 50% of the starting amount was evaporated. It is of course expected that, after being adsorbed on a solid matrix, the molecular structures in terms of size and functional groups present on the volatile molecules represent the key-parameter to account for the increase over time stability. Indeed, several authors previously commented in their studies that release characteristics are directly related (i) to the diffusion of the molecules through the polymer matrix and (ii) to the different chemical interactions realized by different fragrance molecules with the matrix (Sansukcharearnpon et al., 2010; Sánchez-González et al., 2011c; Madene et al., 2006; Ponce Cevallos et al., 2010). Cellulose is a polysaccharide polymer which has two free secondary and one free primary alcohol functions per repeating unit. This makes cellulose a quite polar macromolecule able to create strong H-bonds with hydroxylic groups from alcohol molecules. The hydrophilic cellulose has then more potential interactions with alcohols as terpinen-4-ol than with apolar compounds as pinene. For that reason, the terpinen-4-ol formulation shows a lower release rate than the associations of cellulose with carvone, citronellal and ˛-pinene. These interactions definitively reduce the volatility of the alcohol. The ketone functional group of carvone presents a similar character that the aldehyde functional group of citronellal. Nevertheless, citronellal and carvone have significantly different behaviors with carvone being less efficiently evaporated from the solid matrix. Carvone and citronellal being respectively a cyclic and a linear molecule, the diffusion rates of those molecules are also expected to be different throughout cellulosic matrix. An additional effect can also come from the highest intrinsic polarity of carvone, as exemplified by its highest Bp. 3.4. Evaporation of monoterpenes loaded on cellulose-based matrices: effect of the nature of the adsorbent This part of the investigation has been performed using the synthetic mixture of monoterpenes. Three different cellulosic-based matrices have been selected and correspond to cellulose, cellulose acetate and rice husk. The previously determined ratio of 1:100 (w/w) has also been selected and the prepared formulations were stored over 20 days. Fig. 4 describes the release efficiencies of this mixture impregnated onto cellulose, cellulose acetate and rice husk over time. First of all, it is important to probe the putative influence of the intermolecular interactions between the monoterpenes when analyzing the mixture of monoterpenes when compared to the individual formulations. This part of the work was only performed in the case of cellulose as a model system. When comparing Figs. 4 and 3, we can easily conclude that, with the concentrations used for this study, the evaporation rates of the four selected compounds individually or jointly loaded on cellulose are really similar. Basically, after 20 days at 21 ◦ C, the residual quantities of terpinen-4-ol, carvone, citronellal and ˛-pinene into cellulose are reproducibly measured around 65%, 50%, 20% and 10%, respectively. As far as the cellulose acetate formulations (Fig. 4), it is interesting to already remind that the polarity of cellulose acetate polymer is reduced upon (partial) acetylation when compared with cellulose. Accordingly, terpinen-4-ol is no longer the most retained molecule given that strong and numerous H-bonds are no longer feasible. Actually, when loaded on cellulose acetate, citronellal and terpinen-4-ol show medium but similar releases with about 35% and 21% remaining after 10 and 20 days, respectively, for both compounds. However, carvone is efficiently retained on cellulose acetate since the retention was recorded at 80% and 70% after 10 and 20 days, respectively. All together, the reported retention order follows carvone > terpinen-4-ol ∼ citronellal > ˛-pinene.
Fig. 4. Release profiles of mixture of monoterpenes loaded cellulosic-based matrices during storage at 21 ± 2 ◦ C. Initial relative retention of each monoterpene = 100%.
Finally, the retention profiles for the mixture loaded onto rice husk reveal a high release of ˛-pinene during the first day of storage with retention of 45% of the initial amount in the formulation. Basically, the retention efficiencies ranking is really similar to the cellulose situation with terpinen-4-ol > carvone > citronellal > ˛pinene. However, the evaporation of the polar compounds is significantly limited when loaded onto rice husk with retention of 90%, 85% and 53% for terpinen-4-ol, carvone and citronellal, respectively, after 20 days. The efficiency of rice husk to prevent polar compounds from evaporating could be explained on the basis of the physical and chemical properties of rice husk. Rice husk is a highly polar solid matrix that contains carboxylic acid functions (COOH) and hydroxyl groups such as silanol (SiOH) (Osman et al., 2010; Chockalingam and Subramanian, 2006; Asadi et al., 2008). Those functional groups are obviously likely to be involved in forming strong H-bonds. Therefore, the retention of selected compounds on rice husk is highly governed by such chemical interactions. On the other hand, rice husk possesses a larger surface area than cellulose, i.e. 3.4 and 7.1 m2 /g for, respectively, cellulose and rice husk. As a direct consequence, the rice husk particles have a large amount of H-bond sites available for interaction with the guest molecules. On the other hand, it seems that the larger specific surface area determined for cellulose acetate at 13.4 m2 /g does not contribute greatly to the retention processes. This of course confirms that the key-parameter that induces retention and prevents evaporation of
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the monoterpenes loaded onto cellulose-like matrices is the adequacy between chemical functionalities present on the molecule and the solid phase. But, for a given monoterpene/matrix pair, the role of the specific surface area becomes important since more strong interactions are then expected to occur. 4. Conclusion In the present study, the incorporation of volatile compounds onto three cellulose-based polymers is demonstrated to induce a significant reduction of the volatility of several terpenes from essential oils. The retention tests confirm that the overtime increased stability of the volatile molecules is highly dependent (i) on the initial concentration of the volatile molecules loaded onto the solid material, (ii) on the volatile molecule intrinsic volatility (Bp) and (iii) on the chemical structure of the volatile molecules. As far as the selected system is concerned, we found that rice husk present excellent retention (≥50%) capacity up to 20 days for terpinen-4-ol, carvone and citronellal. The impregnation of terpinen-4-ol and carvone onto the cellulose and cellulose acetate effectively prolongs the retention of the volatiles over time. These results can be considered as the starting point for the development of formulations dedicated to essential oils with lowcost biosorbents for crops protection against pests. This kind of formulation could represent a great potential for large scale postharvest application taking in account the low concentration of active essential oil required in the biopolymer matrix. Acknowledgements The authors are grateful to CUD (Commission Universitaire pour le Développement) of Belgium for financial support for this work. MS laboratory at UMons acknowledges the “Fonds de la Recherche Scientifique (FRS-FNRS)” for its contribution to the acquisition of the GC–MS Spectrometer. P.G. is FNRS Senior Research Associate. References Abbas, A., Ansumali, S., 2010. Global potential of rice husk as a renewable feedstock for ethanol biofuel production. Bioenergy Res. 3, 328–334. Abreu, F.O.M.S., Oliveira, E.F., Paula, H.C.B., De Paula, R.C.M., 2012. Chitosan/cashew gum nanogels for essential oil encapsulation. Carbohydr. Polym. 89, 1277–1282. Adel, A.M., Abd El-Wahab, Z.H., Ibrahim, A.A., Al-Shemy, M.T., 2011. Characterization of microcrystalline cellulose prepared from lignocellulosic materials. Part II: physicochemical properties. Carbohydr. Polym. 83, 676–687. Asadi, F., Shariatmadari, H., Mirghaffari, N., 2008. Modification of rice hull and sawdust sorptive characteristics for remove heavy metals from synthetic solutions and wastewater. J. Hazard. Mater. 154, 451–458. Bosquez-Molina, E., Ronquillo-de Jesús, E., Bautista-Banos, S., Verde-Calvo, J.R., Morales-López, J., 2010. Inhibitory effect of essential oils against Colletotrichum gloeosporioides and Rhizopus stolonifer in stored papaya fruit and their possible application in coatings. Postharvest Biol. Technol. 57, 132–137. Cevallos, P.A.P., Buera, M.P., Elizalde, B.E., 2010. Encapsulation of cinnamon and thyme essential oils components (cinnamaldehyde and thymol) in cyclodextrin: effect of interactions with water on complex stability. J. Food Eng. 99, 70–75. Chaudhary, D.S., Jollands, M.C., 2004. Characterization of rice hull ash. J. Appl. Polym. Sci. 93, 1–8. Chockalingam, E., Subramanian, S., 2006. Studies on removal of metal ions and sulphate reduction using rice husk and Desulfotomaculum nigrificans with reference to remediation of acid mine drainage. Chemosphere 62, 699–708. De Saint Laumer, J.-Y., Frérot, E., Herrmann, A., 2003. Controlled release of perfumery alcohols by Neighboring-Group participation. Comparison of the rate constants for the alkaline hydrolysis of 2-acyl-, 2-(hydroxymethyl)-, and 2carbamoylbenzoates. HeIv. Chim. Acta 86, 2871–2899. Dikusar, E.A., Zhukovskaya, N.A., Moiseichuk, K.L., Zalesskaya, E.G., Vyglazov, O.G., Kurman, P.V., 2008. Synthesis and structure: aroma correlation of citral oxime esters. Chem. Nat. Compd. 44, 81–83. Genieva, S.D., Turmanova, S.Ch., Dimitrova, A.S., Vlaev, L.T., 2008. Characterization of rice husks and the products of its thermal degradation in air or nitrogen atmosphere. J. Therm. Anal. Calorim. 93, 387–396. Hoskovec, M., Grygarová, D., Cvaˇcka, J., Streinz, L., Zima, J., Verevkin, S.P., Koutek, B., 2005. Determining the vapour pressures of plant volatiles from gas chromatographic retention data. J. Chromatogr. A 1083, 161–172.
105
Isman, M.B., Miresmailli, S., Machial, C., 2011. Commercial opportunities for pesticides based on plant essential oils in agriculture, industry and consumer products. Phytochem. Rev. 10, 197–204. Ji, F., Li, C., Tang, B., Xu, J., Lu, G., Liu, P., 2012. Preparation of cellulose acetate/zeolite composite fiber and its adsorption behavior for heavy metal ions in aqueous solution. Chem. Eng. J. 209, 325–333. Lai, F., Wissing, S.A., Müller, R.H., Fadda, A.M., 2006. Artemisia arborescens L essential oil-loaded solid lipid nanoparticles for potential agricultural application: preparation and characterization. AAPS PharmSciTech 7, E1–E9. Levrand, B., Ruff, Y., Lehn, J.-M., Herrmann, A., 2006. Controlled release of volatile aldehydes and ketones by reversible hydrazone formation – “classical” profragrances are getting dynamic. Chem. Commun., 2965–2967. Liu, Z.-T., Fan, X., Wu, J., Zhang, L., Song, L., Gao, Z., Dong, W., Xiong, H., Peng, Y., Tang, S., 2007. A green route to prepare cellulose acetate particle from ramie fiber. React. Funct. Polym. 67, 104–112. Luo, Y., Zhang, B., Whent, M., (Lucy) Yu, L., Wang, Q., 2011. Preparation and characterization of zein/chitosan complex for encapsulation of ␣-tocopherol, and it’s in vitro controlled release study. Colloids Surf. B: Biointerfaces 85, 145–152. Madene, A., Jacquot, M., Scher, J., Desobry, S., 2006. Flavour encapsulation and controlled release – a review. Int. J. Food Sci. Technol. 41, 1–21. Marcuzzo, E., Sensidoni, A., Debeaufort, F., Voilley, A., 2010. Encapsulation of aroma compounds in biopolymeric emulsion based edible films to control flavour release. Carbohydr. Polym. 80, 984–988. Narula, A.P.S., 2004. The search for new fragrance ingredients for functional perfumery. Chem. Biodivers. 1, 1992–2000. Nguemtchouina, M.M.G., Ngassoum, M.B., Ngamo, L.S.T., Gaudu, X., Cretin, M., 2010. Insecticidal formulation based on Xylopia aethiopica essential oil and kaolinite clay for maize protection. Crop Prot. 29, 985–991. Ouédraogo, W.I., Bonzi-Coulibaly, Y.L., 2009. Extraction et analyse de la cellulose issue de biomasses végétales de rejet disponibles au Burkina Faso. Sci. Tech. 3, 73–81. Ouédraogo, W.I., Boulvin, M., Flammang, R., Gerbaux, P., Bonzi-Coulibaly, Y.L., 2009. Conversion of natural aldehydes from Eucalyptus citriodora, Cymbopogon citratus, and Lippia multiflora into oxime: GC–MS and FT–IR analysis. Molecules 14, 3275–3285. Osman, H.E., Badwy, R.K., Ahmad, H.F., 2010. Usage of some agricultural by-product in the removal of some heavy metals from industrial wastewater. J. Phytol. 2, 51–62. Paula, H.C.B., Sombra, F.M., De Freitas Cavalcante, R., Abreu, F.O.M.S., De Paula, R.C.M., 2011. Preparation and characterization of chitosan/cashew gum beads loaded with Lippia sidoides essential oil. Mater. Sci. Eng. C 31, 173–178. Ponce Cevallos, P.A., Buera, M.P., Elizalde, B.E., 2010. Encapsulation of cinnamon and thyme essential oils components (cinnamaldehyde and thymol) in cyclodextrin: effect of interactions with water on complex stability. J. Food Eng. 99, 70–75. Ramos-García, M., Bosquez-Molina, E., Hernández-Romano, J., Zavala-Padilla, G., Terrés-Rojas, E., Alia-Tejacal, I., Barrera-Necha, L., Hernández-López, M., ˜ S., 2012. Use of chitosan-based edible coatings in combination Bautista-Banos, with other natural compounds, to control Rhizopus stolonifer and Escherichia coli DH5␣ in fresh tomatoes. Crop Protect. 38, 1–6. Rochat, S., Minardi, C., De Saint Laumer, J.-Y., Herrmann, A., 2000. Controlled release of perfumery aldehydes and ketones by Norrish type-II photofragmentation of ␣-keto esters in undegassed solution. Helv. Chim. Acta 83, 1645–1671. Sacchetti, G., Maietti, S., Muzzoli, M., Scaglianti, M., Manfredini, S., Radice, M., Bruni, R., 2005. Comparative evaluation of 11 essential oils of different origin as functional antioxidants, antiradicals and antimicrobials in foods. Food Chem. 91, 621–632. Saha, B.C., Cotta, M.A., 2007. Enzymatic saccharification and fermentation of alkaline peroxide pretreated rice hulls to ethanol. Enzyme Microb. Technol. 41, 528–532. Saha, B.C., Iten, L.B., Cotta, M.A., Victor Wu, Y., 2005. Dilute acid pretreatment, enzymatic saccharification, and fermentation of rice hulls to ethanol. Biotechnol. Prog. 21, 816–822. Sánchez-González, L., Cháfer, M., Chiralt, A., González-Martínez, C., 2010. Physical properties of edible chitosan films containing bergamot essential oil and their inhibitory action on Penicillium italicum. Carbohydr. Polym. 82, 277–283. Sánchez-González, L., Pastor, C., Vargas, M., Chiralt, A., González-Martínez, C., Cháfer, M., 2011a. Effect of hydroxypropylmethylcellulose and chitosan coatings with and without bergamot essential oil on quality and safety of cold-stored grapes. Postharvest Biol. Technol. 60, 57–63. Sánchez-González, L., Cháfer, M., Hernández, M., Chiralt, A., González-Martínez, C., 2011b. Antimicrobial activity of polysaccharide films containing essential oils. Food Control 22, 1302–1310. Sánchez-González, L., Cháfer, M., González-Martínez, C., Chiralt, A., Desobry, S., 2011c. Study of the release of limonene present in chitosan films enriched with bergamot oil in food simulants. J. Food Eng. 105, 138–143. Sansukcharearnpon, A., Wanichwecharungruang, S., Leepipatpaiboon, N., Kerdcharoend, T., Arayachukeat, S., 2010. High loading fragrance encapsulation based on a polymer-blend: preparation and release behavior. Int. J. Pharm. 391, 267–273. Sing, K.S.W., Everett, D.H., Haul, R.A.W., Moscou, L., Pierotti, R.A., Rouquerol, J., Siemieniewska, T., 1985. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl. Chem. 57, 603–619.
106
I.W.K. Ouédraogo et al. / Industrial Crops and Products 51 (2013) 100–106
Soottitantawat, A., Bigeard, F., Yoshii, H., Furuta, T., Ohkawara, M., Linko, P., 2005. Influence of emulsion and powder size on the stability of encapsulated dlimonene by spray drying. Innov. Food Sci. Emerg. Technol. 6, 107–114. Sun, J.X., Sun, X.F., Zhao, H., Sun, R.C., 2004. Isolation and characterization of cellulose from sugarcane bagasse. Polym. Degrad. Stab. 84, 331–339. Takács, E., Wojnárovits, L., Horváth, É.K., Fekete, T., Borsa, J., 2012. Improvement of pesticide adsorption capacity of cellulose fibre by high-energy irradiation-initiated grafting of glycidyl methacrylate. Radiat. Phys. Chem. 81, 1389–1392. Tanzi, C.D., Vian, M.A., Ginies, C., Elmaataoui, M., Chemat, F., 2012. Terpenes as green solvents for extraction of oil from microalgae. Molecules 17, 8196–8205. Tongnuanchan, P., Benjakul, S., Prodpran, T., 2012. Properties and antioxidant activity of fish skin gelatin film incorporated with citrus essential oils. Food Chem. 134, 1571–1579.
Varona, S., Rodríguez-Rojo, S., Martín, Á., Cocero, M.J., Duarte, C.M.M., 2011. Supercritical impregnation of lavandin (Lavandula hybrida) essential oil in modified starch. J. Supercrit. Fluids 58, 313–319. Wan Ngah, W.S., Hanafiah, M.A.K.M., 2008. Removal of heavy metal ions from wastewater by chemically modified plant wastes as adsorbents: a review. Bioresour. Technol. 99, 3935–3948. Wang, L., Li, J., 2013. Adsorption of C.I. Reactive Red 228 dye from aqueous solution by modified cellulose from flax shive: kinetics, equilibrium, and thermodynamics. Ind. Crops Prod. 42, 153–158. Wojnárovits, L., Földváry, Cs.M., Takács, E., 2010. Radiation-induced grafting of cellulose for adsorption of hazardous water pollutants: a review. Radiat. Phys. Chem. 79, 848–862. Woranuch, S., Yoksan, R., 2013. Eugenol-loaded chitosan nanoparticles: II. Application in bio-based plastics for active packaging. Carbohydr. Polym. 96, 586–592.