Hydrogen production by anaerobic digestion from Agave lechuguilla hydrolysates

Abstract

Hydrogen production from enzymatic hydrolysates of Agave lechuguilla pretreated by autohydrolysis was assessed in this work. The pretreatment was carried out in a high-pressure reactor using a solid/liquid ratio of 1:6 (w/v) at 190 °C for 30 min at 200 rpm. The pretreated solids were enzymatically hydrolyzed and then were digested with a treated mixed consortium under specified conditions with a Taguchi (L₉(3⁴)) experimental array. The results showed that the xylan was 65.2% solubilized during pretreatment, and the glucan preserved was 77.5% hydrolyzed, obtaining a hydrolysate with 55 g/L of glucose. The production of hydrogen after anaerobic digestion of hydrolysates was significantly influenced mainly by the temperature (80.6%) and glucose concentration (15.1%). The best conditions were 40 ºC, glucose 20 g/L, inoculum 5% (v/v), and initial pH 7. Under optimal conditions, the hydrogen yield achieved was 3.48 mol H₂/mol glucose consumed at 120 h.

Full Article

Hydrogen Production by Anaerobic Digestion from Agave lechuguilla Hydrolysates

Leopoldo J. Rios-González,* Thelma K. Morales-Martínez, Gabriela G. Hernández-Enríquez, José A. Rodríguez-De la Garza, and Mayela Moreno-Dávila

Hydrogen production from enzymatic hydrolysates of Agave lechuguilla pretreated by autohydrolysis was assessed in this work. The pretreatment was carried out in a high-pressure reactor using a solid/liquid ratio of 1:6 (w/v) at 190 °C for 30 min at 200 rpm. The pretreated solids were enzymatically hydrolyzed and then were digested with a treated mixed consortium under specified conditions with a Taguchi (L₉(3⁴)) experimental array. The results showed that the xylan was 65.2% solubilized during pretreatment, and the glucan preserved was 77.5% hydrolyzed, obtaining a hydrolysate with 55 g/L of glucose. The production of hydrogen after anaerobic digestion of hydrolysates was significantly influenced mainly by the temperature (80.6%) and glucose concentration (15.1%). The best conditions were 40 ºC, glucose 20 g/L, inoculum 5% (v/v), and initial pH 7. Under optimal conditions, the hydrogen yield achieved was 3.48 mol H₂/mol glucose consumed at 120 h.

Keywords: Agave lechuguilla; Autohydrolysis pretreatment; Anaerobic digestion; Hydrogen

Contact information: Departamento de Biotecnología, Facultad de Ciencias Químicas, Universidad Autónoma de Coahuila, Saltillo, Coahuila México; *Corresponding author: leopoldo.rios@uadec.edu.mx

INTRODUCTION

Energy plays a major role in world economic and social development; however, today’s energy is produced mainly from non-renewable sources that are considered pollutants. Therefore, continuous diversification of energy sources is of crucial importance to every nation, and Mexico is no exception (Arreola-Vargas et al. 2015). Hydrogen is one of the most promising energy carriers due to its high energy-yield efficiency and low generation of pollutants (Han et al. 2016). Natural gas reforming is a well-established technology used in many refineries and chemical industries in Mexico for large-scale H₂ production (Ortiz et al. 2016). However, the production of hydrogen via this process generates large quantities of carbon dioxide (CO₂), one of the main causes of global warming (Arriaga et al. 2011).

The biological process of anaerobic digestion is an environmental friendly process and can utilize a wide range of substrates (Sattar et al. 2016), including different lignocellulosic feedstocks such as forest and agricultural residues or crops not used for food or feed (Liu et al. 2014). However, due to the complex plant cell wall structures, lignocellulosic materials are not capable of undergoing fermentation without previous pretreatment and hydrolysis (Zhao et al. 2013). In second-generation (2G) biofuel production process from lignocellulosic biomass, the pretreatment stage is of major importance; therefore, the selection of an adequate pretreatment method that can improve the hydrolysis of structural carbohydrates (cellulose and hemicellulose) and can also be ecofriendly and low cost is crucial. Autohydrolysis pretreatment is a method that does not require the use or addition of chemicals and only uses water, making this method an attractive option to obtain a pretreated material with a high digestibility (Rios-González et al. 2017). To our knowledge, previous reports by Rios-González et al. (2017) and Ortíz-Méndez et al. (2017) are the only ones to mention the use of autohydrolysis to pretreat agave biomass (Agave tequilana and Agave lechuguilla respectively) to produce 2G ethanol.

There is abundant research on hydrogen production from lignocellulosic hydrolysates (Arriaga et al. 2011; Zhao et al. 2014; Arreola-Vargas et al. 2015; Baêta et al. 2016; Ding et al. 2016; Gonzales et al. 2016; Han et al. 2016; Rorke and Kana 2016; Sangyoka et al. 2016; Sattar et al. 2016; Kumar et al. 2017). However, only a few studies of hydrogen production from agave biomass are available in the current literature. These studies were carried out using Agave tequilana bagasse without pretreatment and only applying hydrolysis with diluted acid at high temperatures (Arreola-Vargas et al. 2015, 2016) and by enzymatic hydrolysis (Contreras et al. 2017).

Agave lechuguilla (lechuguilla) is a common plant of northern Mexico, occupying the largest range of all agaves with almost 20 million hectares of the arid and semiarid lands of Mexico (Castillo et al. 2011). A. lechuguilla traditionally has been exploited for extracting fibers used in the manufacture of metal polishing brushes, furniture and car seat fillings, carpets and cleaning brushes, as a construction material in combination with thermoplastic resins, and as a concrete reinforcement (Pando-Moreno et al. 2008). A. lechuguilla cogollos (the heart or pulpy central stem with attached leaf bases) can be harvested many times without sacrificing the whole plant. The annual productivity, with 427 mm of average rainfall, is 4 tons per hectare (Escamilla-Treviño 2012), making it an attractive energy crop. Moreover, A. lechuguilla has potential as a feedstock for ethanol production (Morales-Martínez et al. 2017; Ortíz-Mendez et al. 2017; Díaz-Blanco et al. 2018).

During anaerobic digestion, the sludge contains different microorganism groups. Hydrogen consumers such as methanogens are present and are strongly subjected to deactivation viadifferent methods (high temperature, UV radiation, extremely low or high pH) to obtain an enriched mixed consortium of Clostridium-like microorganisms that are known to produce hydrogen (Sattar et al. 2016). Several factors affect hydrogen production in addition to the nature and treatment of the mixed consortia, such as temperature, pH, mineral medium formulation, type of inoculum, the profile of organic acids produced, and the type and concentration of substrate (Sangyoka et al. 2016).

The present work assesses the effects of temperature, substrate concentration, initial pH, and inoculum on hydrogen production from A. lechuguilla hydrolysates in batch reactors to optimize the process via a Taguchi (L₉(3⁴)) experimental array.

EXPERIMENTAL

Materials and Methods

Feedstock

All cogollos (A. lechuguilla) were harvested from Ramos Arizpe, Coahuila, Mexico (latitude 25° 55’ 47” North, longitude 101° 55’ 47” West). For storage purposes, the cladodes of the cogollos were completely separated and dried for 24 to 30 h at 45 °C in a tray dehydrator (model KL10, Queretaro, Mexico). The dried material was milled to obtain a particle size of 2 mm in a cutting mill (Retsch SM100, Retsch, Haan, Germany). After milling, the material was stored in plastic containers at room temperature until further use.

Determination of feedstock composition and pretreatment

Before chemical composition determination, the moisture content was determined with a moisture analyzer (Moisture Analyzer OHAUS; Parsippany, NJ). Cellulose (glucan), hemicellulose (xylan), and lignin were quantified according to Rios et al. (2017) using A. lechuguilla extractives free, which was removed and determined using the analytical method NREL/TP-510-42619 (Sluiter et al. 2005). Finally, ashes and proteins were determined using the analytical method NREL/TP-510-42622 (Sluiter et al. 2008) and Kjeldahl method, respectively.

According to a previous report by Ortíz-Méndez et al. (2017), the optimum conditions to pretreat A. lechuguilla biomass by autohydrolysis were at 190 °C for 30 min, corresponding to a severity factor (SF) of 4.127. Under these conditions, a greater quantity of xylan is hydrolyzed and glucan is largely preserved. Additionally, the enzymatic digestibility increases. Therefore, the autohydrolysis pretreatment of A. lechuguilla was carried out in a 5-gallon high-pressure stainless-steel reactor equipped with a Rushton impeller agitation system (Parr Instruments Company, Moline, IL, USA). The dried and milled material (2.192 kg of A. lechuguilla) was suspended in 13.15 L of distilled water (resulting in a 1:6 w/v solid/liquid ratio) at 190 °C and 200 rpm for 30 min, corresponding to a SF of 4.127. The heating up (190 °C) and cooling down (50 °C) time was 56 min and 45 min respectively. However, to calculate the SF according to Fan and Ragauskas (2012) this time is not taken in account,

where t is the reaction time in minutes, T_H is the reaction temperature in °C, and T_R is the reference temperature (100 °C). The value of 14.75 is an empirical parameter related to the activation energy and temperature.

The reactor was opened when the temperature had decreased to 50 °C. Then the pretreated material was washed with water, and the glucan, xylan, and lignin content were determined as described previously (without extractives removal). The enzymatic loading calculations were established according to the glucan content present in the pretreated material.

Enzymatic hydrolysis of the pretreated solids

The pretreated solid fraction was hydrolyzed using Cellic^® CTec3 (kindly provided by Novozymes A/S, Kalundborg, Denmark), with a cellulase activity of 217 FPU/mL. FPU is the activity unit of cellulase when filter paper is used as the enzymatic hydrolysis substrate, according to Ghose (1987). According to previous studies carried out by Morales-Martínez et al. (2017), the enzymatic hydrolysis was carried out at a solids loading of 25% (w/w) in the presence of 0.05 M sodium citrate buffer at a pH of 4.8, using an enzyme loading of 15 FPU per gram of glucan in a 15 L glass reactor with mechanical stirring and controlled with an eZ Applikon^® module (Schiedam, Netherlands). The reactor was maintained at 50 °C and 200 rpm for 72 h. The slurry obtained was withdrawn from the reactor and centrifuged at 10000 × g for 15 min. The liquid fraction (hydrolysate) was filtered and analyzed by HPLC to determine the glucose concentration.

The enzymatic hydrolysis yield was expressed as the relationship between the amount of glucose released during saccharification and the initial amount of glucan present in the pretreated material. The enzymatic hydrolysis yield was calculated as described by Fang et al. (2010).

Inoculum and treatment

The anaerobic granular sludge used in the present work was obtained from a large-scale up-flow anaerobic sludge blanket (UASB) reactor provided by the Modelo S. De R. L. De C. V. brewery located in Torreon, Coahuila, Mexico. To eliminate hydrogen-consuming microorganisms (methanogens) and to favor hydrogen spore-formers mainly Clostridium species, the sludge was heated in a boiling water bath at 105 °C for 30 min, then cooled down and followed by an acid treatment that involved decreasing the pH of the sludge to 3.0 using 0.1 N HCl solution for 24 h. After this period, the pH was adjusted to 7.0 with a 0.1 N NaOH solution (Hu and Chen 2007). The sludge obtained was separated by filtration and used as an inoculum in the hydrogen production assays.

Optimization of hydrogen production from enzymatic hydrolysate

An orthogonal experimental array (L₉(3⁴)) was applied to optimize the hydrogen production from enzymatic hydrolysates of A. lechuguilla biomass. Nine experiments were carried out at the temperatures 30, 40, and 50 °C; glucose concentrations 20, 37.5, and 55 g/L; inoculum of 5, 10, and 15% (v/v); and initial pH values 5, 6, and 7. The hydrogen fermentation conditions and the orthogonal experiment array are shown in the Tables 1 and 2, respectively.

Table 1. Factors and Levels in the Orthogonal Experiments

Table 2. Orthogonal Experiment Array

Each experiment was carried out in a 25 mL glass bottle that contained 10 mL of hydrolysate. The hydrolysate was previously supplemented with a mineral medium described by Contreras-Dávila et al. (2017), in g/L: NH₄H₂PO₄, 4.5; Na₂HPO₄, 0.635; K₂HPO₄, 0.125; MgCl₂·6H₂O, 0.1; ZnCl₂, 0.075; FeSO₄·7H₂O, 0.025; MnSO₄·H₂O, 0.009; and CuSO₄·5H₂O, 0.005. The glucose concentration and pH of the hydrolysates were adjusted as described in the experimental design. The amount of anaerobic treated sludge (inoculum) added for each experiment is described in Table 2.

After adding the inoculum, the reactors were sealed with butyl rubber stoppers and aluminum caps to avoid gas leakage and flushed with N₂ (100%) gas for 15 min to promote an anaerobic environment. Hydrogen and methane production were determined by gas chromatography and measured at 20, 44, 68, and 92 h. After every measurement, the reactors were flushed with N₂ (100%) as described above. The initial and final glucose concentration were determined by High Performance Liquid Chromatography (HPLC). The hydrogen production yield (mol H₂/mol of consumed glucose) was considered the dependent variable. The experimental data was analyzed statistically by the ANOVA method using Qualitek-4^® software (Nutek, Inc., Bloomfield Hills, MI, USA). To validate the results, a set of experiments were further performed using the obtained optimized conditions with a 50.5 mL of hydrolysates.

Analytical Methods

The hydrogen and methane produced were measured by gas chromatography (Varian 3400, Palo Alto, CA, USA) equipped with a TCD detector at 200 °C and a Molecular Sieve 5A packed column at 30 °C, using argon as the carrier gas with a flow rate of 6 mL/min. The sugars (glucose, xylose, cellobiose, and arabinose) were determined by HPLC (Agilent 1260 Infinity, CA, USA) equipped with a refractive index detector at 45 °C, using an Agilent Hi-Plex H column at 35 °C (7.7 x 300 mm, CA, USA) and 5 mM H₂SO₄ as the mobile phase at a flow rate of 0.5 mL/min. All experiments were carried out in triplicate, and the average values are reported.

RESULTS AND DISCUSSION

Composition of A. lechuguilla and Autohydrolysis Pretreatment

The extractives were the main component in A. lechuguilla cogollos in dry base w/w (29.8%). The glucan, xylan, and lignin contents were 18.2%, 7.7%, and 21.7%, respectively. The ashes and protein contents were 8% and 5.5%, respectively and a 9.07% corresponded to non-detected components (Table 3).

The solids composition of A. lechuguilla pretreated by autohydrolysis is summarized in Table 3. The solids recovered after pretreatment were 55.8% from the raw material, mainly due to the solubilization of extractives and xylan during the process. The glucan content was increased after pretreatment compared with untreated biomass (increasing from 18.2% to 28.2%). From the initial glucan content present in the untreated material, 86.4% remained in the solid phase. However, the glucan content present in the pretreated material (28.19%) does not coincide with previous results reported by Ortíz-Méndez et al. (2017) and Morales-Martínez et al. (2017) in which the same SF factor was applied (4.127). These differences can be attributed to the different location at which the raw material was collected. The hydrolysis of xylan is one of the main effects of the autohydrolysis process, and its degradation products are dissolved in the liquid phase during pretreatment. As expected, autohydrolysis mainly affected the hemicellulosic components, and under these conditions 65.2% of the original xylan content was solubilized.

The glucan and most of the insoluble lignin were retained completely in the solid phase (Amiri and Karimi 2015; Zhuang et al. 2016). The lignin was not solubilized during the pretreatment process, recovering 99.2%. These results are similar to previous reports (Ortíz-Méndez et al. 2017; Morales-Martínez et al. 2017; Rios-González et al. 2017) and with other materials pretreated with the same method (Moniz et al. 2013; Buruiana et al. 2014).

Delignification is not the only factor to decrease lignocellulose recalcitrance; in a previous study on ethanol production from Agave tequilana bagasse pretreated by autohydrolysis (Rios-González et al. 2017), the hemicellulose (xylan) removal improved the enzymatic hydrolysis (obtaining an 81.5% hydrolysis yield).

Table 3. A. lechuguilla Composition after Autohydrolysis Pretreatment

Enzymatic Hydrolysis of the Pretreated Solids

Table 4 shows the enzymatic hydrolysis of pretreated A. lechuguilla, reaching a glucose concentration of 55 g/L at 72 h, corresponding to a hydrolysis yield of 77.5%. However, no significant increase in the glucose concentration was observed after 24 h of enzymatic hydrolysis, and it is likely that in this period, the cellulases started to present inhibition. This result can be attributed to an increase in the diffusional limitation of cellulases by a high solid loading, or to lignin’s effect on the action of enzymes via the blocking of access to the cellulose (López-Linares et al. 2014). Comparing the results obtained in the present study with the results reported previously by Ortíz-Méndez et al. (2017), the hydrolysis yield was similar (60.8%) at a solid and enzyme loadings of 20% (w/w) and 25 FPU per gram of glucan, respectively, with the difference that the enzyme complex used in this previous report was Acellerase 1500 (Genencor®, USA). Meanwhile, the glucose released and the hydrolysis yield were lower than those reported (previous results) by Morales-Martínez et al. (2017) using the same enzyme complex (Cellic^® CTec3, Novozymes) used in the present study.

The sugar (glucose) concentrations used (55 g/L, 37.5 g/L, and 20 g/L) are similar to other reports of H₂ production using hydrolysates obtained from A. tequilana bagasse. Arreola-Vargas et al. (2015) assessed the H₂ production using hydrolysates, with total sugar concentrations of 27.9 and 18.9 g/L from cooked and uncooked bagasse, respectively, obtained by acid hydrolysis with HCl. Using the same raw material, Arreola-Vargas et al. (2016) assessed H₂ production using acid and enzymatic hydrolysates, with total sugar concentrations of 17.3 g/L and 8.9 g/L, respectively. Contreras-Dávila et al. (2017) assessed this same process (H₂ production) using two enzymatic hydrolysates with a total sugar concentration that ranged from 11 to 12.5 g/L.

Table 4. Performance of Enzymatic Hydrolysis of Pretreated Solids

Optimization of Hydrogen Production from Enzymatic Hydrolysate

Table 5 shows that maximum H₂ production was obtained at 40 °C (3.58 to 4.09 × 10^-3 mol), with a significant decrease at 50 °C (0.733 to 0.936 × 10^-3 mol). The optimal temperature for H₂ production varies widely based on the nature of the biocatalyst and the type of substrate used. For mixed consortia, diverse optimum temperatures have been reported (Bharathiraja et al. 2016). Glucose consumption was observed in all cases; however, higher hydrogen yields were observed in the cases where the initial glucose concentration was lower (20 g/L), while the maximum hydrogen yield (3.3 mol H₂/mol glucose consumed) was observed in the case of an experiment carried out at 30 °C, 10% (v/v) of inoculum and initial pH of 7.0. The decrease in the hydrogen yield can be attributed to an inhibition caused by high initial substrate concentration or to the presence of a higher concentration of inhibitory by-products present in the hydrolysate, such as furfural and HMF (Gonzales et al. 2016). In addition, a high substrate concentration can cause a buildup of cell concentration and volatile fatty acids (VFAs) in the system, leading to a decline of pH in the reactor that could inhibit hydrogen production (Fan et al. 2004).

Table 5. Hydrogen Production and Glucose Consumption from A. lechuguilla Hydrolysate at 92 h of Fermentation

As mentioned above, an inhibitory effect of high substrate concentration generally occurs in anaerobic digestion processes, depending on the type of substrates and microorganisms. The initial substrate concentration plays an important role on the yield and production rate of hydrogen (Fabiano and Perego 2002). Sangyoka et al. (2016) observed a decrease in H₂production at a sugar concentration of 30 g/L present in the hydrolysate of sugar cane bagasse that was previously subjected to acidic hydrolysis. Chen et al. (2005) reported that H₂production from sucrose by Clostridium butyricum CGS5 was higher at an initial sucrose concentration of 20 g-COD (Chemical Oxygen Demand)/L, while the fermentation process was inhibited at an initial sucrose concentration of 30 g-COD/L. Similar behavior was mentioned by Oh et al. (2003), who reported the H₂ production by Citrobacter sp. Y19 with glucose as a carbon source and mentioned that hydrogen yield gradually decreased with the increase of glucose concentration at levels higher than 20 g/L.

Table 5 shows that pH values were similar for all cases (4.8 to 5.2). The pH, as mentioned before, is an important factor in anaerobic biological processes, due to its effects on Fe-hydrogenase activity, metabolic pathways, and the duration of the lag phase (Lay 2000). Carbohydrate-rich substrates have a greater potential to acidify the media during anaerobic fermentation, and as a result, in some situations system stability is hard to maintain. In hydrogen fermentation, reactors tend to acidify readily, and a reduction in the pH may take place. Low initial pH values below 5.0 inhibit hydrogen production. On the other hand, high initial pH values such as 9.0 decrease the lag phase time but tend to produce less hydrogen (Sangyoka et al. 2016).

Table 6 shows the average effect of the factors at the assigned levels on the hydrogen yield. Both the lowest (0.4) and the highest (3.3) hydrogen yield were attributed to Factor A (Temperature) at level 2 (40 ºC) and level 3 (50 ºC), respectively. In the present study, three levels for each factor were selected, and the R values (Range extreme difference) were calculated based on the difference between the highest and lowest hydrogen yield. The highest R value was considered the most influential factor. As seen in the Table 5, the influence of these four factors on hydrogen yield was in the order A, B, D, C, based on R values.

Table 6. Main Effects of Selected Factors

Figure 1 shows the influence of each individual factor. The increase in the glucose concentration resulted in a decrease in hydrogen yield. For the temperature, the hydrogen yield was higher in level 2; increasing the temperature over 40 °C led to a reduction in hydrogen yield. The pH and inoculum were the factors with less influence over hydrogen yield, obtaining similar results for all levels. The pH did not present a significant effect, which was attributed to the fact that the factor was only controlled at the beginning of the experiment and a rapid decrease in the initial pH (after 20 h) was observed neutralizing the effect of the factor during the rest of the experiment. Taking into account the optimization process for the four factors at the levels assessed, the optimum conditions were A2 (40 °C); B1 (20 g/L glucose); C1 (5 % v/v inoculum); D3 (initial pH 7).

Table 7 shows the ANOVA for hydrogen yield. According to the Fisher test (F), the temperature is a more significant factor than the hydrogen yield. After the temperature, the glucose concentration was more significant than hydrogen yield, and pH and inoculum were the factors with the least significance.

Fig. 1. Individual factors performance at different levels

Table 7. Analysis of Variance (ANOVA)

The temperature showed the highest impact on hydrogen yield (80.6%), followed by the glucose concentration (15.18%), the initial pH (2.81%), and finally the inoculum (0.11%). Controlling each factor individually or as a whole can lead to a major increase in hydrogen yield. By studying the main effects of each factor, the general trends of the influence of the factors towards the process can be characterized. The characteristics can be controlled such that a lower or a higher value in a particular influencing factor can produce the preferred result. Therefore, the levels of factors to produce the best results can be predicted, so that the higher levels of hydrogen yield can be achieved with optimized conditions obtained: temperature of 40 ºC, glucose of 20 g/L, inoculum of 5% (v/v), and pH of 7. The expected result under optimum conditions was 3.515 mol H₂/mol glucose consumed.

The final experimental stage consisted of applying the optimum conditions obtained to confirm or validate the results of the previous stage. Figure 2 shows the experimental results using optimum conditions predicted by the Taguchi L9 orthogonal array, from which it can be seen that hydrogen yield is greatly improved at the selected levels, resulting in a value of 3.48 mol H₂/mol glucose consumed at 120 h, very similar to the previously mentioned expected value. The hydrogen yield in the present work was greater than that reported by Arreola-Vargas et al. (2014) and Contreras-Dávila et al. (2017) in which they used an enzymatic hydrolysate from Agave tequilana bagasse, obtaining a hydrogen yield of 3.4 mol H₂/mol hexose (in batch mode) and 1.53 mol H₂/mol substrate (in continuous mode), respectively.

Fig. 2. Hydrogen production (circle mark) and glucose consumption (diamond shape mark) from A. lechuguilla hydrolysate during validation of results under optimum conditions

CONCLUSIONS

1. The results demonstrated the potential of hydrogen production from the enzymatic hydrolysates of A. lechuguilla pretreated by autohydrolysis.
2. The hydrogen production was significantly influenced by the operational conditions, mainly by the temperature and the initial glucose concentration. The hydrogen yield achieved (3.48 mol H₂/mol glucose consumed) was greater compared to early reports using hydrolysates of agaves.
3. Future research will be focused on assessing the production of hydrogen during continuous mode operation at different organic loading rates to study the economic feasibility of this process on a large-scale.

ACKNOWLEDGMENTS

The authors are grateful to Innovation Promotion Program (Programa de Estímulos a la Innovación PEI-2016) from the National Council of Science and Technology of Mexico (CONACyT) for financial support.

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Article submitted: May 31, 2018; Peer review completed: August 10, 2018; Revised version received: August 21, 2018; Accepted: August 22, 2018; Published: August 27, 2018.

DOI: 10.15376/biores.13.4.7766-7779