Detoxification of Eucheuma spinosum Hydrolysates with Activated Carbon for Ethanol Production by the Salt-Tolerant Yeast Candida tropicalis

Abstract

The objective of this study was to optimize the slurry contents and salt concentrations for ethanol production from hydrolysates of the seaweed Eucheuma spinosum. A monosaccharide concentration of 44.2 g/l as 49.6% conversion of total carbohydrate of 89.1 g/l was obtained from 120 g dw/l seaweed slurry. Monosaccharides from E. spinosum slurry were obtained by thermal acid hydrolysis and enzymatic hydrolysis. Addition of activated carbon at 2.5% (w/v) and the adsorption time of 2 min were used in subsequent adsorption treatments to prevent the inhibitory effect of HMF. The adsorption surface area of the activated carbon powder was 1,400-1,600 m²/g and showed selectivity to 5-hydroxymethyl furfural (HMF) from monosaccharides. Candida tropicalis KCTC 7212 was cultured in yeast extract, peptone, glucose, and high-salt medium, and exposed to 80, 90, 100, and 110 practical salinity unit (psu) salt concentrations in the lysates. The 100 psu salt concentration showed maximum cell growth and ethanol production. The ethanol fermentations with activated carbon treatment and use of C. tropicalis acclimated to a high salt concentration of 100 psu produced 17.9 g/l of ethanol with a yield (Y_EtOH) of 0.40 from E. spinosum seaweed.

Introduction

Seaweed is considered a third-generation biomass source for bioethanol production. Seaweed has high productivity per unit area per year, and there is no competition with food crops [4]. Currently, Eucheuma spinosum is cultivated commercially in the Philippines, China, Indonesia, Malaysia (Sabah), Tanzania, and Kiribati [17]. The polysaccharides in Eucheuma species are largely in the form of carrageenan, as cell wall components. Carrageenans are the major polysaccharide present in many red macroalgae (seaweeds). These gel-forming polysaccharides have a linear backbone of D-galactose residues, linked with alternating α-(1,3) and β-(1,4) linkages, which are substituted by one (κ-carrageenan), two (ι-carrageenan), or three (λ-carrageenan) ester-sulfonic groups per di-galactose repeating unit [3]. The monosaccharide contents of carbohydrates in Eucheuma spp. are 56.2% D-galactose and 43.8% 3,6-anhydrogalactose [8].

Adsorption by activated carbon has been extensively used in lignocellulosic hydrolyzates. Activated carbon treatment is effective for the removal of furfural, 5-hydroxymethylfurfural (HMF), and phenolic compounds in various hydrolyzates [10]. Carrageenan is a polysaccharide contained in E. spinosum and can be hydrolyzed readily to monosaccharides using thermal acidic and enzymatic hydrolyses. Thermal acidic hydrolysis is economically more feasible than enzymatic hydrolysis. However, the main drawback of the thermal acid process, versus the enzymatic process, is the formation of byproduct compounds, such as furfural and HMF, which severely inhibit cell growth and ethanol production from the hydrolysate [14]. Thus, additional treatments are required to address the problems of high concentrations of inhibitory compounds to facilitate the ethanol fermentation process. Activated carbon adsorption was introduced as a physical treatment to reduce the concentration of HMF.

Another problem encountered in seaweed hydrolysates, such as of E. spinosum, is the high concentration of NaCl due to its sea water origin. Salt stress has two effects, ion toxicity and osmotic stress, to yeasts [5]. To overcome the problems caused by the high salt concentrations of seaweed hydrolysates, techniques for the acclimation of yeasts to high salt concentrations have been developed [15,18]. Differences of this study from previous work [2] are that ethanol fermentation was carried out using the salt-tolerant yeast Candida tropicalis KCTC 7212. Using inocula without and with pre-acclimation to high salt concentrations was determined for the red seaweed E. spinosum.

The objective of this study was to optimize the slurry content and salt concentration for the hydrolysis of E. spinosum. Adsorption conditions were optimized by the types of activated carbon, activated carbon dosage, and adsorption time. Ethanol fermentation was carried out using the hydrolysate and C. tropicalis KCTC 7212 acclimated to high salt concentrations.

 

Materials and Methods

Yeast Strains and Seaweed Preparation

C. tropicalis KCTC 7212, a salt-tolerant yeast, was obtained from the Korean Collection for Type Cultures (KCTC) Biological Resource Center (Daejeon, Korea). E. spinosum, a product of Indonesia, was obtained from the Biolsystems Co., Ltd., Goheung, Jeonnam, Korea.

The seaweed was dried in hot air and ground in a hammer mill. The E. spinosum powder was sieved with a 200-mesh sieve prior to pretreatment. The composition of E. spinosum was determined by the Feed and Foods Nutrition Research Center at Pukyong National University, Korea, according to the AOAC method [16].

Pretreatment Processes of E. spinosum for Monosaccharide Conversion

Thermal acid hydrolysis was carried out on 8-20% (w/v) seaweed slurry with 281 mM H2SO4 at 121℃ for 60 min using 100 ml of seaweed slurry in a 250 ml Erlenmeyer flask.

E. spinosum hydrolysates were neutralized to pH 5.0 using 5 N NaOH. For the hydrolysis of fiber, 1.2 U/ml of Viscozyme L (82 FBG/ml, beta-glucanase; Novozymes, Bagsvaerd, Denmark) and 8.4 U/ml of Celluclast 1.5L (573 EGU/ml, cellulase; Novozymes) were used per 100 g/l of seaweed slurry after a thermal acid hydrolysis at pH 5.0, 40℃ for 48 h. The activities of cellulase and β-glucosidase were determined according to the procedures described in Kubicek [7] and Mandels et al. [11].

The efficiency of pretreatment and saccharification (EPS, %) was calculated as the increase in galactose concentration (g/l) during thermal acid hydrolysis and as the increase in glucose concentration (g/l) during enzymatic saccharification after pretreatment from the carbohydrate content (g/l) of E. spinosum.

Removal of HMF from E. spinosum hydrolysates

Removal of HMF from E. spinosum hydrolysates was carried out using various activated carbon contents and adsorption times. Two types of activated carbon, granular and powder (Duksan Pure Chemical Co., Ltd., Ansan, Korea), were evaluated. The removal of HMF was carried out from 100 ml of hydrolysate in a flask containing 0, 1.0, 2.5, 5.0, or 10.0 g of activated carbon. The flasks were placed in a shaking water bath at 100 rpm and 50℃ for adsorption times of 0, 2, 4, 6, 8, 10, and 12 min. The adsorption surface areas of the activated carbon used were 1,000-1,100 m2/g for the granular and 1,400-1,600 m2/g for the powder. After treatment, solids were removed by centrifugation (1,390 ×g, 10 min). The supernatant was recovered and used for monosaccharide and HMF measurements.

Acclimation of Yeast to High Salt Concentrations and Ethanol Fermentation

The growth rates of C. tropicalis KCTC 7212 in 10 g /l yeast extract, 20 g/l peptone, 20 g/l glucose, and high-salt (YPGHS) medium with initial salt concentrations of 80, 90, 100, and 110 practical salinity unit (psu), were evaluated for ethanol fermentation. C. tropicalis KCTC 7212 was acclimated in YPGHS medium with increasing salt concentrations. The culture profiles of non-acclimated and acclimated yeasts in YPGHS were evaluated.

Ethanol fermentation was performed with 100 ml of E. spinosum hydrolysate in a 250 ml Erlenmeyer flask under semi-anaerobic conditions. Yeast was precultured for 48 h until the OD600 reached 4.0 and then 7.58 g dcw/l of yeast was inoculated into 100 ml of E. spinosum hydrolysate. The seaweed hydrolysates were fermented at 30℃ and 150 rpm with detoxification treatment for HMF and/or yeast acclimated to high salt concentrations. Samples were taken periodically and stored at -20℃ prior to measurements of ethanol, residual sugars, and HMF concentrations. The ethanol yield (YEtOH, g/g) was defined as the maximum ethanol concentration (g/l) relative to the total initial fermentable sugar (galactose + glucose) concentration at the onset of fermentation (g/l).

Analytical Methods

Cell growth was measured at OD600 and converted to dry cell weight using a standard curve determined by dry cell weight vs. OD600. For the determination of dry cell weight, 1 ml of the sample was centrifuged at 10,000 ×g for 10 min. The cell pellet was washed with phosphate buffer saline (PBS), pH 7.0. The cell pellet was dried at 60℃ for 48 h. The glucose, galactose, HMF, and ethanol concentrations were determined by HPLC (Agilent 1100 Series; Agilent. Inc., Santa Clara, CA, USA) equipped with a refractive index detector. The Bio-Rad Aminex HPX-87H column (300 × 7.8 mm) was maintained at 65℃ and samples were eluted with 5 mM H2SO4 at 0.6 ml/min. Salinity was measured using a salinometer (Salinity Refractometer, ATAGO Inc., Japan). The values are reported as the mean of triplicate experiments.

 

Results and Discussion

Pretreatments of E. spinosum

Monosaccharide (glucose and galactose) concentrations increased with increasing slurry content following thermal acid hydrolysis and enzymatic saccharification (Fig. 1). The maximum monosaccharide concentration was 60.3 g/l with an EPS of 43.4% with 20% (w/v) slurry content. However, increasing the slurry content from 12% to 20% during thermal acid hydrolysis resulted in a decrease in EPS, from 49.6 to 43.4%. The EPS after treatment with 14% to 20% (w/v) slurry content was not greater than that with 12% (w/v) slurry content at 281 mM H2SO4 and 121℃ for 60 min. The acetic acid was produced from the hydrolysis of the acetyl groups bound to the hemicellulosic monomers ]undefined[1]. The composition of E. spinosum contains very low amounts of crude fiber. Thus, acetic acid concentrations of increasing slurry contents showed a slight increase with the increase of slurry contents.

Fig. 1.Pretreatment of various seaweed slurries by thermal acid hydrolysis with 281 mM H2SO4 at 121℃ for 60 min, followed by enzymatic saccharification by Viscozyme L and Celluclast 1.5 L at pH 5.0, 45℃ for 48 h.

High slurry content increased the psu of the pretreated slurry, from 64 to 140 psu (Fig. 1). Omori et al. [13] reported a high salt concentrations—above 120 psu—showed rapid inhibition of cell growth and ethanol fermentation. Use of seaweed slurry above 8% (w/v) increased the lag phase before ethanol production owing to the presence of HMF. Increased HMF concentration inhibited both galactose or glucose uptake and ethanol production by the yeast [9,14]. However, the presence of <5 g/l HMF in the fermentation broth resulted in rapid glucose and galactose uptake and ethanol production. Thus, ethanol fermentation could be carried out without damage to the yeast by decreasing the HMF concentration. Therefore, 12% (w/v) seaweed content with 49.6% EPS, 95 psu salt concentration, and 9.6 g/l HMF were selected for the ethanol production. The HMF concentration could be reduced by adsorption with activated carbon, thus a high concentration of HMF did not induce the inhibition problem. However, a salinity above 100 psu resulted in growth inhibition (see Fig. 3D). Thus, HMF was removed by activated carbon adsorption, and ethanol production was carried out using yeasts acclimated to high salt concentrations.

Effects of Activated Carbon Type, Dosage, and Adsorption Time

To evaluate the adsorption of HMF produced from the E. spinosum hydrolysates, granular and powder activated carbon types were assessed in a shaking water bath at 100 rpm and 50℃. Fig. 2A shows a comparison of granular and powder activated carbon, in the presence of galactose, glucose, acetic acid, and HMF from 12% (w/v) E. spinosum hydrolysate. Galactose, glucose, and acetic acid concentrations decreased slightly with the addition of 2.5% (w/v) activated carbon and a 10 min adsorption time. The HMF concentration decreased with both the granular and powder forms of activated carbon, and the HMF removal efficiency was higher with the powder. Zhang et al. [19] reported that the powder form of activated carbon shows rapid mass transfer property and equilibrium due to its small powder size. Thus, activated carbon powder was used as the optimal adsorbent.

Fig. 2.Evaluation of (A) activated carbon types, (B) dosage, and (C) adsorption time on the adsorption of HMF. Conditions: 100 rpm and 50℃. Adsorption time was evaluated using 2.5% activated carbon.

The effects of optimization of activated carbon dosage (Fig. 2B) and adsorption time (Fig. 2C) on HMF adsorption were evaluated. Lee et al. [10] reported that HMF and furfural were preferentially adsorbed, compared with monosaccharides, by activated carbon at a given adsorption time. Thus, a strategy to improve the efficiency of the adsorption process was developed to maintain high monosaccharide concentrations and HMF <5 g/l in the medium. The data in Figs. 2B and 2C indicate that 2.5% activated carbon and a 2 min adsorption time showed optimum HMF removal. This result indicates that activated carbon can act as an excellent adsorbent for HMF without significant loss of monosaccharides. Thus, an adsorption treatment with 2.5% activated carbon and an adsorption time of 2 min was selected as the optimal treatment conditions.

Effects of High Salt Acclimation on Cell Growth

As shown in Fig. 3, the effects of salt acclimation on yeast cell growth were assessed using various salt concentrations and non-acclimated and acclimated yeasts in YPGHS medium. C. tropicalis KCTC 7212 was grown in the YPGHS medium with salt concentrations of 80 to 110 psu, as shown in Figs. 3A, 3B, 3C, and 3D, respectively. At 80 (Fig. 3A) and 90 psu (Fig. 3B), the non-acclimated control and yeast acclimated to salt showed similar trends in cell growth. However, yeast acclimated to a salt concentration of 100 psu (Fig. 3C) showed only a 6 h lag period in cell growth, whereas the lag period of non-acclimated yeast was up to 24 h. Under exposure to 110 psu salt (Fig. 3D), non-acclimated control and yeast acclimated to the salt concentration showed no substantial cell growth. Thus, a salt concentration of 100 psu was determined to be the maximum concentration for cell growth and ethanol production.

Fig. 3Effect of salt stress on growth using non-acclimated control or C. tropicalis KCTC 7212 acclimated to the following salt concentrations: (A) 80 psu, (B) 90 psu, (C) 100 psu, and (D) 110 psu.

Ethanol Fermentation by Salt-Tolerant Yeast Under Severe Conditions

A slurry content of 12% (w/v), optimal for thermal acid hydrolysis and enzymatic hydrolysis, was then used for ethanol fermentation. Separate hydrolysis and fermentation (SHF) was carried out by non-acclimated or acclimated C. tropicalis KCTC 7212 to the high salt concentration.

As shown in Fig. 4, the impact of pretreatment conditions on ethanol production was assessed using no activated carbon treatment of non-acclimated yeast (Fig. 4A), no activated carbon treatment of acclimated yeast (Fig. 4B), activated carbon treatment of non-acclimated yeast (Fig. 4C), and activated carbon treatment of acclimated yeast (Fig. 4D). No activated carbon treatment of non-acclimated yeast showed no cell growth and ethanol production during 140 h (Fig. 4A). Ethanol production and monosaccharide uptake by the yeast were near zero owing to the synergistic inhibitory effects of HMF and the high salt concentration. These results indicated that increased HMF and salt concentration inhibited both galactose and/or glucose uptake and ethanol production by the yeast. However, Fig. 4B shows that the non-activated carbon treatment of hydrolysates with acclimated yeast showed a 24 h lag period for ethanol production. When the HMF concentration was decreased to <5 g/l from 40 to 72 h, the acclimated C. tropicalis KCTC 7212 produced an ethanol concentration of 10.8 g/l and a yield (YEtOH) of 0.24 after 140 h of fermentation. To further improve the efficiency of ethanol fermentation, the optimum HMF removal condition was used, which involved maintenance of high monosaccharide concentrations and HMF <5 g/l in the medium.

Fig. 4Ethanol production by SHF with 12% (w/v) E. spinosum hydrolysates at 30℃, 150 rpm for 140 h using non-activated carbon treatment with either (A) non-acclimated yeast or (B) acclimated yeast, and activated carbon treatment with either (C) non-acclimated yeast or (D) yeast acclimated to a high salt concentration.

As shown in Fig. 4C, activated carbon-treated hydrolysate with non-acclimated yeast showed a reduction in lag time, from 24 to 6 h, compared with that of Fig. 4B. The ethanol concentration was 12.7 g/l with a YEtOH of 0.29 after 140 h of fermentation. These results indicated that the efficiency of ethanol fermentation with activated carbon addition was higher than that of yeast acclimated to a high salt concentration. The effects of both activated carbon treatment to decrease the HMF concentration and the use of acclimated yeast were evaluated, as shown in Fig. 4D. Activated carbon treatment and the use of yeast acclimated to the high salt concentration showed a synergistic effect, and exhibited the maximum ethanol production from E. spinosum hydrolysates. Thus, activated carbon-treated hydrolysate with acclimated yeast showed high monosaccharide uptake and an ethanol concentration of 17.9 g/l and yield (YEtOH) of 0.40 after 140 h of fermentation (Fig. 4D). Ethanol yields (YEtOH) of 0.40 using C. tropicalis KCTC 7212 in this study showed slightly higher than YEtOH of 0.38 from ethanol fermentation using the yeast S. cerevisiae using Yeast extract, Peptone, and Galactose (YPD) medium [6].

Fermentation inhibitors generated from the acidic hydrolysis of biomass inhibit cell growth and ethanol fermentation using E. spinosum hydrolysates. Following optimization of pretreatment by thermal acid hydrolysis and enzymatic saccharification, a 12% (w/v) seaweed content and 49.6% EPS, 95 psu salt concentration, and 9.6 g/l HMF were selected for ethanol production. Owing to the inhibitory effect of HMF >5 g/l in the medium, subsequent adsorption treatment was performed using 2.5% activated carbon with an adsorption time of 2 min. The activated carbon treatment and use of yeast acclimated to high salt concentrations showed synergistic effects and produced an ethanol concentration of 17.9 g/l with a yield (YEtOH) of 0.40 after 140 h of fermentation. Therefore, a detoxification step can increase the sugar utilization from seaweed hydrolysates, resulting in higher ethanol yields in the fermentation broth.