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Article

Enhanced Enzymatic Saccharification of Tomato Stalk by Combination Pretreatment with NaOH and ChCl:Urea-Thioure in One-Pot Manner

1
School of Pharmacy, Changzhou University, Changzhou 213164, China
2
State Key Laboratory of Biocatalysis and Enzyme Engineering, Hubei University, Wuhan 430062, China
*
Authors to whom correspondence should be addressed.
These authors contributed equally to this work.
Processes 2022, 10(10), 1905; https://doi.org/10.3390/pr10101905
Submission received: 9 August 2022 / Revised: 11 September 2022 / Accepted: 14 September 2022 / Published: 20 September 2022
(This article belongs to the Section Sustainable Processes)

Abstract

:
In this study, the mixture of NaOH and deep eutectic solvent (DES) ChCl:UA-TA was firstly used to pretreat waste tomato stalk (TS). The effects of pretreatment time, pretreatment temperature, NaOH dosage, and DES dose were investigated, and the synergistic effects of dilute NaOH and DES combination pretreatment were tested on the influence of enzymatic saccharification. It was found that the relationship between delignification and saccharification rate had a significant linear correction. When TS was pretreated with NaOH (7 wt%)–ChCl:UA-TA (8 wt%) in a solid-to-liquid ratio of 1:10 (wt:wt) at 75 °C for 60 min, the delignification reached 82.1%. The highest yield of reducing sugars from NaOH–ChCl:UA-TA-treated TS could reach 62.5% in an acetate buffer (50 mM, pH 4.8) system containing cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS) at 50 °C. In summary, effective enzymatic saccharification of TS was developed by a combination pretreatment with dilute NaOH and ChCl:UA-TA, which has potential application in the future.

1. Introduction

In recent years, the valorization of renewable and sustainable biomass into useful biobased chemicals and biofuels has attracted rapidly increasing attention due to the gradual depletion of petrochemical resources [1,2,3]. Lignocellulosic biomass (LCB), which is composed of carbohydrates (hemicellulose and cellulose) and lignin [4], is regarded as one of the most promising alternative energy resources owing to its abundance, low-cost, non-toxicity, and renewability [5]. However, these three main polymers (e.g., hemicellulose, cellulose, and lignin) in the cell wall are tightly complexed by hydrogen and covalent bonds [6]. To enhance the enzymatic saccharification of lignocellulosic biomass, it is necessary to utilize an efficient pretreatment strategy for the disruption of recalcitrant structure in lignocellulosic biomass [7,8].
Pretreatment is known as one of the most important units in the valorization of lignocellulosic materials [9], which currently presents the most critical challenge and determines the capital cost of the biomass-to-product process [10]. Presently, various pretreatments (e.g., physical, physicochemical, biological methods or their combinations) have been developed and utilized to disrupt a series of lignocellulosic biomasses for subsequent conversion [4,11,12,13,14,15,16]. Alkaline treatment can be used to modify the fibers based on the removal of lignin, pectin, wax, and other components, thereby enhancing the enzymatic hydrolysis of lignocellulosic biomass [17]. NaOH solution has been utilized for swelling and dissolving the cellulose components, which can destroy the neighboring crystalline regions [18]. Aqueous urea/NaOH and thiourea/NaOH solutions may dissolve cotton linter [19,20,21]. Urea/NaOH can be utilized to soak biomass at low temperatures, achieving a high saccharification yield (80.2%) [20]. Probably, NaOH, urea (UA), and thiourea (TA) might capture free water molecules to confine the intermolecular interactions between hydrogen bonds and cellulose molecules, resulting in the enhancement of enzymatic saccharification of lignocellulose [22,23]. In addition, deep eutectic solvents (DESs) have attracted much attention as benign solvents to pretreat lignocellulosic biomass due to their biodegradability, low corrosion, and high stability [24,25,26]. Considering the merit of NaOH and DES pretreatment, the combination by mixing NaOH and DES ChCl:UA-TA in a one-pot manner can be attempted to pretreat biomass.
According to the statistics, the annual production of tomato (Solanum Lycopersicum L.) is over 60 million tons (http://www.fao.org/), which accounts for ~30% of the production of total greenhouse vegetables. As a byproduct, waste tomato stalk (TS) is nearly 8.5 million tons of dry weight per year [27]. However, there is limited information about the efficient enzymatic saccharification of TS. In this work, dilute NaOH and DES ChCl:UA-TA were mixed as a pretreatment solvent to pretreat TS. The pretreatment parameters, including pretreatment time, pretreatment temperature, NaOH dosage, and DES dose, were investigated to influence the enzymatic saccharification of TS, and the synergistic effects of dilute NaOH and DES in a one-pot manner were evaluated. The relationship between delignification and saccharification rate was explored. To effectively utilize TS, significant enzymatic saccharification of TS was developed by a combination pretreatment with dilute NaOH and ChCl:UA-TA in a one-pot manner.

2. Materials and Methods

2.1. Materials

Tomato stalk (TS) was collected from a suburb in Changzhou, China. The dried TS powder (40 mesh) was stored at room temperature for future use. Cellulase (C805042, 10,000 U/g) and xylanase (X832362, 100,000 U/g) were obtained from Macklin (Shanghai, China). Choline chloride, urea, thiourea, tetracycline hydrochloride, sodium acetate, acetic acid and other chemicals were bought from Sinopharm Group Chemical Reagent Co., (Shanghai, China).

2.2. Synthesis of DES ChCl:UA-TA

Choline chloride (ChCl), urea (UA), and thiourea (TA) were mixed in the molar ratio of 1:1:1. This mixture was stirred continuously (200 rpm) in a stoppered three-neck flask Nanjing Kangluoda Experiment Technology Co.; Nanjing, China) (at 80 °C until a clear solvent was formed. Then, the generated DES ChCl:UA-TA was stored under low-temperature drying conditions.

2.3. Pretreatment of TS with Dilute NaOH and ChCl: UA-TA

Dry TS (solid-liquid ratio of 1:10, wt:wt) was added to aqueous solutions containing NaOH (0.5−9 wt%) or/and ChCl:UA-TA (0−16 wt%) in a 250 mL round bottom flask at −25−125 °C by using a magnetic stirring device. After being pretreated for 10−120 min, an equal volume of deionized water (DI water, as an antisolvent) was added to the pretreatment system under vigorous agitation. After 5 min, the regenerated TS solid was washed repeatedly with DI water to remove NaOH, ChCl:UA-TA, or other residuals. Subsequently, the solid TS was oven-dried at 60 °C and further used for enzymatic saccharification.

2.4. Enzymatic Hydrolysis of TS

Enzymatic hydrolysis of untreated or pretreated TS (50 g/L) was conducted in a 25 mL of acetate buffer (50 mM, pH 4.8) system containing cellulase (10.0 FPU/g TS), xylanase (30.0 CBU/g TS) and 40 μL of tetracycline hydrochloride (10.0 g/L).

2.5. Analytical Methods

The compositions of pretreated and unpretreated samples were determined according to National Renewable Energy Laboratory (NREL) standard analytical procedures [28]. Reduced sugars were determined by the DNS method [29]. Glucose and xylose were assayed with HPLC (Waters 600, Milford, MA, USA) [30]. The yield of reducing sugars was calculated as previously reported [31]. The total delignification and solid recovery of TS were calculated as previously reported [32,33].

3. Results and Discussion

3.1. Optimization of NaOH–ChCl:UA-TA Pretreatment

ChCl:UA-TA was confirmed by 1H NMR (Figure S1) and 13C NMR (Figure S2). 13C NMR (101 MHz, DMSO): δ 184.32 (TA), 160.23 (UA), 67.4 (ChCl, -CH2-N), 55.54 (ChCl, -CH2-O), 53.62 (ChCl, CH3-N). 1H NMR (400 MHz, DMSO): δ 7.22 (4H, TA), 5.52 (4H, UA), 3.83 (2H, ChCl, -CH2-N), 3.44 (2H, ChCl, -CH2-O), 3.41 (1H, ChCl, -OH), 3.15 (9H, ChCl, -CH3). ChCl:UA-TA was also assayed with FT-IR (Figure S3); the peaks near 3287 and 3170 cm−1 are ascribed to the stretching mode of -OH and -NH2. The peaks around 1661, 1604, and 1392 cm−1 are associated with TA and UA. The peak near 1079 cm−1 is related to the stretching mode of C-O. The peak around 951 cm−1 is ascribed to the C-N group of ChCl.
To efficiently saccharify biomass, it is of great interest to optimize the pretreatment conditions [34,35]. In this work, the effects of pretreatment temperature, pretreatment time, NaOH dosage, DES (ChCl:UA-TA) dose, and TS concentration were examined during the pretreatment.
Efficient removal of lignin in lignocellulosic biomass with NaOH can facilitate enzymatic saccharification [36]. Since NaOH might be used to remove lignin that can adsorb or inhibit cellulase [37], thereby decreasing the concentration of enzyme required. Figure 1 depicts that different NaOH dosages could significantly affect enzymatic saccharification. In the presence of 7 wt%, the highest reducing sugar yield of 62.5% was obtained. The highest yield of reducing sugars reached 62.5% in a solid-to-liquid ratio of 1:10 (wt:wt). The concentration of DES might have a profound influence on the enzymatic saccharification of TS [38,39]. Various ChCl:UA-TA dosages (0−16 wt%) were examined on the enzymatic saccharification of TS. As illustrated in Figure 2, the yield of reducing sugars increased from 51.4% to 62.5% when the DES dosage was raised from 0 wt% to 8 wt%. Further increasing the DES dosage from 8 to 16 wt%, the yield of reducing sugar dropped from 62.5% to 54.5%. High concentration of DES ChCl:UA-TA (over 12 wt%) might promote hemicellulose removal, which led to a reduced reducing sugar yield. Hence, the optimum dose of DES was 8 wt%.
Pretreatment temperature is a key parameter for enhancing the enzymatic saccharification of biomass [39,40]. To promote enzymatic hydrolysis of TS, the effects of pretreatment temperature (−25 °C, 0 °C, 25 °C, 50 °C, 75 °C, 100 °C, and 125 °C) were investigated after 1 h of pretreatment. As revealed in Figure 3, the yield of reducing sugars was raised from 31.0% to 62.5% by increasing the pretreatment temperature from −25 °C to 75 °C. Upon increasing the temperature from 75 °C to 125 °C, the saccharification yield decreased significantly. Obviously, the appropriate pretreatment temperature was 75 °C. High pretreatment temperature (over 75 °C) could disrupt and remove cellulose and hemicellulose, which would reduce the reducing sugar yield. Several efficient alkaline pretreatments had been carried out at high-performance temperature (100 °C) and/or concentrated alkali [29], which required a higher performance cost and complicated control system. Our work provided an effective alkaline pretreatment, which could be conducted at 75 °C and give high enzymatic saccharification (62.5%).
Recently, various alkaline pretreatments have been utilized to improve the enzymatic hydrolysis of lignocellulosic biomass [30,40]. Long pretreatment time was required at low-performance temperature [41]. In addition to the performance temperature [42], the pretreatment time is also a very important factor influencing biomass pretreatment and enzymatic saccharification [43]. As revealed in Figure 4, by increasing the pretreatment time from 10 to 60 min, the yield of reducing sugar was raised from 45.9% to 62.5%. As the pretreatment time rose from 1 to 2 h, the yield of reducing sugar only increased by ~2%. A long pretreatment duration was not economical for biomass pretreatment. Hence, the optimal pretreatment time was 60 min.
Overall, combination pretreatment of TS was conducted in dilute NaOH (7 wt%)–ChCl:UA-TA (8 wt%) at 75 °C for 60 min in the solid-to-liquid of 1:10 (wt:wt). This established pretreatment was easy to operate under the relatively mild performance condition. The obtained DNC-TS could be enzymatically saccharified into reducing sugars in a yield of 62.5% after 48 h of enzymatic hydrolysis.

3.2. Comparison of Different Pretreatments on the Enzymatic Saccharification of TS

The recalcitrance of biomass can hinder the enzymatic hydrolysis of lignocellulose [31]. Prior to enzymatic saccharification, it is necessary to establish an appropriate pretreatment system for disrupting the tight structure of biomass [32,33]. In this work, dilute NaOH (7 wt%), ChCl:UA-TA (8 wt%), and their combination [dilute NaOH (7 wt%)–ChCl:UA-TA (8 wt%)] were separately used as pretreatment media to treat TS. U-TS (untreated TS), DN-TS (dilute NaOH-pretreated TS), C-TS (ChCl:UA-TA-pretreated TS), and DNC-TS (dilute NaOH–ChCl:UA-TA pretreated TS) were individually utilized as substrates for enzymatic hydrolysis. As presented in Figure 5a, pretreatment with dilute NaOH (7 wt%), ChCl:UA-TA, and their combination (dilute NaOH–ChCl:UA-TA) could give the delignification of 71.3%, 41.2%, and 82.1%, respectively. DNC-TS had the lowest solid recovery (30.2%) than those of DN-TS (32.9%) and C-TS (57.0%). It was observed that the relationship about delignification and saccharification rate (48 h) had a significant linear correction [Y(saccharification rate) = 0.692 × X(Delignification) + 4.13, R2 = 0.98] after NaOH–ChCl:UA-TA pretreatment. With the increase in delignification, the saccharification rate had an increasing trend. The yield of reducing sugars from DNC-TS reached 62.5% after 48 h, which was highest than those of DN-TS (51.4%), C-TS (33.2%), and U-TS (14.9%). U-TS gave the lowest saccharification, which might be attributed to its highly tight structure. In addition, the lignin and hemicellulose in U-TS may have an inhibitory effect on enzymatic hydrolysis. The linear relationships of solid sorghum straw recovery-xylan removal (R2 = 0.94) and solid sorghum straw recovery-reducing sugar yield (R2 = 0.81) were observed in the NaOH–ChCl:LAC pretreatment [32]. Tandem pretreatment with NaOH (0.75 wt%) at 121 °C for 1 h and ChCl:LAC at 140 °C for 40 min could remove lignin (78.4%) and xylan (67.6%). Time courses for the enzymatic hydrolysis of U-TS, C-TS, DN-TS, and DNC-TS (50 g/L) were monitored at 50 °C and pH 4.8 (Figure 5b). These pretreated TS samples had higher saccharification rates than U-TS. Saccharification for 30 min, the reducing sugar yield of U-TS, C-TS, DN-TS, and DNC-TS could reach 2.8%, 7.7%, 13.6%, and 19.5%, respectively. By increasing the saccharification time from 30 min to 48 h, the reducing sugar yield increased gradually. DNC-TS had the highest yield of reducing sugar (62.5%) after 48 h of enzymatic saccharification, which was 4.2 times higher than that of U-TS. Therefore, the combination pretreatment (NaOH–ChCl:UA-TA) had the highest pretreatment efficiency to pretreat TS.
To visualize the change in cellulose structure, the SEM micrographs were used to characterize U-TS and DNC-TS (Figure 6). It showed that there were significant surface changes in the morphology of TS before and after pretreatment. U-TS was relatively smooth and continuous and had few pores for the hydrolysis with cellulases. However, the NaOH–ChCl:UA-TA-treated TS (DNC-TS) had looser and rougher surfaces and presented more porosity than U-TS. After U-TS was pretreated with NaOH–ChCl:UA-TA, the rigidity of the cellulose in TS decreased due to the lignin removal and disruption of surface structure.

3.3. Reuse of Pretreatment Medium NaOH–ChCl:UA-TA

Industrially, efficient recovery and reuse of pretreatment medium have gained great interest to reduce the pretreatment costs and avoid environmental pollution [44,45]. After each use, the solids were separated from the pretreatment slurry by filtration, and the pretreatment liquor was reused for the next batch of pretreatment. After each batch, the solid recovery, delignification and enzymatic saccharification were determined. As presented in Figure 7, the saccharification rate and delignification decreased gradually with the increase in the recycling number. The reducing sugar yield dropped from 62.5% to 40.5% from the first to sixth batch, maintaining a high yield of reducing sugars (>40%). The good reusability of pretreatment medium NaOH–ChCl:UA-TA would potentially reduce the pretreatment cost. Although NaOH–ChCl:UA-TA could be used for efficient pretreatment of TS, potential pretreatment mechanisms involving the action of ChCl:UA-TA with alkali and the stability of ChCl:UA-TA in an alkaline medium can be explored in the future.

3.4. Enzymatic Hydrolysis of DNC-TS

An effective pretreatment can facilitate the enzymatic saccharification of lignocellulosic materials [46,47,48]. To enhance the saccharification efficiency, the effects of DNC-TS concentration on enzymatic hydrolysis were examined. As illustrated in Figure 8a, time courses for converting different concentrations of DNC-TS (10–100 g/L) were monitored at 50 °C and pH 4.8. Within 48 h, the saccharification rates increased by raising the saccharification time. Over 48 h, the reducing sugar yield had no significant change. Saccharification for 48 h, 10 g/L, 20 g/L, 40 g/L, 50 g/L, 60 g/L, 80 g/L, and 100 g/L of DNC-TS could be saccharified to reducing sugars in the yield of 56.2%, 58.8%, 60.2%, 62.5%, 54.7%, 43.3%, and 29.1% (Figure 8b), respectively. The highest concentration of reducing sugars was also obtained by using 50 g/L of DNC-TS as feedstock. Significantly, the appropriate DNC-TS concentration for enzymatic hydrolysis was 50 g/L. This work provided an effective pretreatment strategy for enhancing the enzymatic saccharification of waste TS.
Lignocellulosic biomass consists of highly crystalline cellulose and hemicellulose polymers whereas lignin interacts with hemicellulose and cellulose to form a complex matrix [49,50,51,52,53,54,55]. NaOH (1 wt%) could remove 60.4% of lignin in corn stover at 70 °C for 1 h [55]. A combined ethylene glycol-HClO4-H2O (88.8:1.2:10, wt/wt/wt) soaking at 130 °C for 0.5 h with NaOH (1 wt%) extraction at 90 °C for 1 h was utilized to treat chestnut shell, achieving a 77.5% of delignification and 77.8% of enzymatic saccharification yield [7]. DES can be used to pretreat biomass in a high delignification (Table 1). Various ChCl-based DESs (e.g., ChCl:FA, ChCl:Gly, ChCl:LA, ChCl:FA:AA, and ChCl:MEA) have been used to remove lignin in a series of biomasses (e.g., corncob, lettuce leave, peach endocarp, rice straw, and wheat straw) at 70–150 °C for 0.5–16 h [50,51,52,53,54]. The delignification rates could reach 40.0–71.4%. Although ChCl:MEA could be used for pretreating wheat straw in a high delignification (71.4%), a high pretreatment time (9 h) was required. Combination pretreatment could enhance the delignification significantly [32]. Sorghum straw could be pretreated by sequential pretreatment with NaOH (0.75 wt%) at 121 °C for 1 h and ChCl:LAC at 140 °C for 40 min, achieving the highest delignification (78.4%). However, this combination pretreatment required high temperatures (≥121 °C) and cumbersome performances. In our study, it was firstly reported that NaOH (7 wt%)–ChCl:UA-TA (8 wt%) pretreated TS at a relatively mild performance temperature (75 °C) within 1 h, and a good delignification reached 82.1%. In addition, this pretreatment medium could be recovered and reused, potentially reducing the pretreatment cost. The development of a cost-efficient and sustainable pretreatment strategy by using green solvent as a pretreatment medium is still in progress.

4. Conclusions

In this work, a feasible combination pretreatment was developed under a relatively mild performance condition. And a significant enhancement of enzymatic saccharification of TS was achieved by combination pretreatment with a mixture of NaOH (7 wt%) and ChCl:Urea-Thioure (8 wt%) in a one-pot manner at 75 °C for 60 min. The pretreated TS (DNC-TS) had a looser structure, more porosity, and more cracks than raw TS. High delignification was obtained at 82.1%, and the highest yield of reducing sugars (62.5%) was obtained. Overall, the NaOH–ChCl:UA-TA pretreatment has high potential in the valorization of lignocellulose.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr10101905/s1. Figure S1: 1H NMR spectra of DES (ChCl:UA-TA); Figure S2: 13C NMR spectra of DES (ChCl:UA-TA); Figure S3: FT-IR spectra of DES (ChCl:UA-TA).

Author Contributions

Conceptualization, Methodology, and Writing original draft, B.F., J.N. and Q.L.; Data curation, Software, Supervision, Review and revising manuscript, Y.H. and C.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

The authors thank the National-Local Joint Engineering Research Center of Biomass Refining and High-Quality Utilization (Changzhou University) for kindly analysis of biomass with SEM.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

LCBLignocellulosic biomass
TSTomato stalk
DESdeep eutectic solvent
ChClCholine chloride
UAurea
TAthiourea
DI waterdeionized water

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Figure 1. Effects of NaOH concentration on the enzymatic saccharification of tomato stalk (TS). [Pretreatment condition: NaOH (0.5−9 wt%)–ChCl:UA-TA (8 wt%), 75 °C, 60 min; Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 oC, pH 4.8]. [U-TS represents untreated TS].
Figure 1. Effects of NaOH concentration on the enzymatic saccharification of tomato stalk (TS). [Pretreatment condition: NaOH (0.5−9 wt%)–ChCl:UA-TA (8 wt%), 75 °C, 60 min; Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 oC, pH 4.8]. [U-TS represents untreated TS].
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Figure 2. Effects of deep eutectic solvent (DES) concentration on the enzymatic saccharification of TS. [Pretreatment condition: NaOH (7 wt%)–ChCl:UA-TA (1−16 wt%), 75 °C, 60 min; Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 °C, pH 4.8]. [U-TS represents untreated TS].
Figure 2. Effects of deep eutectic solvent (DES) concentration on the enzymatic saccharification of TS. [Pretreatment condition: NaOH (7 wt%)–ChCl:UA-TA (1−16 wt%), 75 °C, 60 min; Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 °C, pH 4.8]. [U-TS represents untreated TS].
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Figure 3. Effects of TS pretreatment temperature on the enzymatic saccharification of TS. [Pretreatment condition: NaOH (7 wt%)–ChCl:UA-TA (8 wt%), −25−75 °C, 60 min; Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 oC, pH 4.8]. [U-TS represents untreated TS].
Figure 3. Effects of TS pretreatment temperature on the enzymatic saccharification of TS. [Pretreatment condition: NaOH (7 wt%)–ChCl:UA-TA (8 wt%), −25−75 °C, 60 min; Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 oC, pH 4.8]. [U-TS represents untreated TS].
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Figure 4. Effects of TS pretreatment duration on the enzymatic saccharification of TS. [Pretreatment condition: NaOH (7 wt%)–ChCl:UA-TA (8 wt%), 75 °C, 10−120 min; Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 °C, pH 4.8]. [U-TS represents untreated TS].
Figure 4. Effects of TS pretreatment duration on the enzymatic saccharification of TS. [Pretreatment condition: NaOH (7 wt%)–ChCl:UA-TA (8 wt%), 75 °C, 10−120 min; Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 °C, pH 4.8]. [U-TS represents untreated TS].
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Figure 5. Effects of dilute NaOH (7 wt%), ChCl:UA-TA and their combination (dilute NaOH–ChCl:UA-TA) on the pretreatment and enzymatic saccharification of TS [Pretreatment condition: NaOH (7 wt%)–ChCl:UA-TA (1–16 wt%), 75 °C, 60 min; Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 °C, pH 4.8] (a); Time courses for the enzymatic hydrolysis of U-TS (untreated TS), DN-TS (dilute NaOH-pretreated TS), C-TS (ChCl:UA-TA-pretreated TS), and DNC-TS (dilute NaOH–ChCl:UA-TA pretreated TS) (50 g/L) with cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS) at 50 °C and pH 4.8 (b).
Figure 5. Effects of dilute NaOH (7 wt%), ChCl:UA-TA and their combination (dilute NaOH–ChCl:UA-TA) on the pretreatment and enzymatic saccharification of TS [Pretreatment condition: NaOH (7 wt%)–ChCl:UA-TA (1–16 wt%), 75 °C, 60 min; Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 °C, pH 4.8] (a); Time courses for the enzymatic hydrolysis of U-TS (untreated TS), DN-TS (dilute NaOH-pretreated TS), C-TS (ChCl:UA-TA-pretreated TS), and DNC-TS (dilute NaOH–ChCl:UA-TA pretreated TS) (50 g/L) with cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS) at 50 °C and pH 4.8 (b).
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Figure 6. SEM micrographs of U-TS (left) and DNC-TS (right). [U-TS represents untreated TS; DNC-TS represents dilute NaOH–ChCl:UA-TA pretreated TS].
Figure 6. SEM micrographs of U-TS (left) and DNC-TS (right). [U-TS represents untreated TS; DNC-TS represents dilute NaOH–ChCl:UA-TA pretreated TS].
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Figure 7. Reuse of pretreatment medium NaOH–ChCl:UA-TA [Pretreatment condition: NaOH (7 wt%)–ChCl:UA-TA (8 wt%), 75 °C, 60 min; Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 °C, pH 4.8].
Figure 7. Reuse of pretreatment medium NaOH–ChCl:UA-TA [Pretreatment condition: NaOH (7 wt%)–ChCl:UA-TA (8 wt%), 75 °C, 60 min; Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 °C, pH 4.8].
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Figure 8. Time courses for the enzymatic hydrolysis of DNC-TS concentration (10–100 g/L) (a); Effects of DNC-TS concentration (10–100 g/L) on the enzymatic saccharification (b). [Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 °C, pH 4.8].
Figure 8. Time courses for the enzymatic hydrolysis of DNC-TS concentration (10–100 g/L) (a); Effects of DNC-TS concentration (10–100 g/L) on the enzymatic saccharification (b). [Enzymatic hydrolysis condition: cellulase (10.0 FPU/g TS) and xylanase (30.0 CBU/g TS), 50 °C, pH 4.8].
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Table 1. Related works about DES pretreatment of biomass.
Table 1. Related works about DES pretreatment of biomass.
DES; Pretreatment ConditionsBiomassDelignificationRef.
EaCl:LAC = 1:1; 150 °C, 30 minRice straw67.0%[50]
ChCl:Gly = 1:2; 150 °C, 16 hLettuce leaf~40%[51]
ChCl:LA = 1:2; 145 °C, 6 hPeach endocarp70.2%[52]
ChCl:FA:AA = 1:1:1; 130 °C, 2 hRice straw43.6%[53]
ChCl:MEA = 1:6; 70 °C, 9 hWheat straw71.4%[54]
NaOH (0.75 wt%) 1 h, 121 °C; ChCl:LAC 140 °C, 40 minSorghum straw78.4%[32]
NaOH (7 wt%)–ChCl:UA-TA (8 wt%); 75 °C, 60 minTomato stalk82.1%This study
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Fan, B.; Ni, J.; Li, Q.; He, Y.; Ma, C. Enhanced Enzymatic Saccharification of Tomato Stalk by Combination Pretreatment with NaOH and ChCl:Urea-Thioure in One-Pot Manner. Processes 2022, 10, 1905. https://doi.org/10.3390/pr10101905

AMA Style

Fan B, Ni J, Li Q, He Y, Ma C. Enhanced Enzymatic Saccharification of Tomato Stalk by Combination Pretreatment with NaOH and ChCl:Urea-Thioure in One-Pot Manner. Processes. 2022; 10(10):1905. https://doi.org/10.3390/pr10101905

Chicago/Turabian Style

Fan, Bo, Jiacheng Ni, Qi Li, Yucai He, and Cuiluan Ma. 2022. "Enhanced Enzymatic Saccharification of Tomato Stalk by Combination Pretreatment with NaOH and ChCl:Urea-Thioure in One-Pot Manner" Processes 10, no. 10: 1905. https://doi.org/10.3390/pr10101905

APA Style

Fan, B., Ni, J., Li, Q., He, Y., & Ma, C. (2022). Enhanced Enzymatic Saccharification of Tomato Stalk by Combination Pretreatment with NaOH and ChCl:Urea-Thioure in One-Pot Manner. Processes, 10(10), 1905. https://doi.org/10.3390/pr10101905

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