3.1. Acid Hydrolysis of DRB. Huang et al. [25] studied the production of microbial oil from T. fermentans grew in sulfuric-acid-treated rice straw hydrolysate (SARSH). The hydrolysate (SARSH) contained a maximum of 35.2 g/L fermentable monosaccharides including glucose, xylose, and arabinose, and the concentration of pentoses was about six times higher than hexose. In this study, the composition of the defatted rice bran hydrolysate (DRBH) obtained by using 1% H2SO4 at different temperature (60 oC–120 oC) for 1 h showed that glucose was the predominant sugar in the hydrolysate. This result suggests the presence of a higher proportion of starch and glucans in DRB along with other minor pentoses such as xylose and arabinose, which are mainly derived from xylans and arabinoxylans, respectively. The maximum fermentable sugars concentration from DRBH under these conditions was 5.96 ± 0.43 g/L out of which glucose, xylose, and arabinose are 3.16 ± 0.22 g/L, 1.36 ± 0.12 g/L, and 0.74 ± 0.01 g/L, respectively. However, the results suggest that hydrolysis of DRB for 1 h with 1% H2SO4 was not efficient to hydrolyze the DRB to produce fermentable sugars. The optimum temperature was 90 oC, and this was used as a basis for the next hydrolysis reactions with different acid concentrations. Table 1 shows the effects of H2SO4 concentrations (2%, 3%, and 4%) and hydrolysis time (2–8 h) on the composition of DRBH at 90 oC. The concentration of total fermentable sugars using 2% H2SO4 was much lower than that of 3% and 4%, which shows that higher acid concentration could break down the polymerized structure of hemicellulose, amylase, and amylopectin more efficiently. The highest total released sugar concentration (50.22 ± 0.34 g/L) was obtained when DRB was hydrolyzed with 3% H2SO4 at 90 oC for 6 h. These optimum conditions resulted in glucose (43.20 ± 0.28 g/L), xylose (4.93 ± 0.03 g/L), arabinose (2.09 ± 0.01 g/L), HMF (0.32 ± 0.05 g/L), and furfural (0.025 ± 0.003 g/L). Compared to the total fermentable sugars from SARSH (35.2 g/L), our results showed that rice bran has higher concentration of fermentable sugars than sulfuric-acid-pretreated rice straw hydrolysate. On the other hand, the highest monosaccharide sugar concentration obtained in this study (43.20 ± 0.28 g/L) is glucose, while that of SARSH (25.5 g/L) is xylose.
This indicates rice bran is a better source of glucose than rice straw for microbial fermentation. The total sugar obtained in this work was also much higher than that of enzymatic hydrolysis of DRB which resulted in 37 g/L total sugars when defatted rice bran was subjected to saccharification with a mixture of amylase and cellulase [26]. Therefore, dilute acid hydrolysis breaks down the lignocelluloses better than enzymes and can be considered as an efficient method to hydrolyze the DRB to obtain sugars for utilization in microbial fermentation.
The concentration of monosaccharides increases as time increases from 2 h to 8 h using 2% acid concentration. However, at an acid concentration of 3% or 4%, glucose concentration increased with time until 6 h and then started to decrease. The same trend was observed for xylose and arabinose. The rapid increase in the concentration of inhibitors with longer hydrolysis time indicates the decomposition of monosaccharides into less desirable compounds under the severe acid hydrolysis condition (4% H2SO4). A
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similar work also reported that sugar concentration increases with longer hydrolysis time and higher acid concentration until the optimum conditions were reached and decreases thereafter [27].
3.2. Effect of Detoxification on the Composition of DRBH. The presence of inhibitory substances in the fermentation medium can restrict both sugar utilization and growth of microorganisms. It has been reported that oleaginous microorganisms could not grow and reproduce well in the rice straw hydrolysate obtained by acid hydrolysis without detoxification. Low lipid content was obtained after culturing T. fermentans for 8 days owing to both sugar utilization and lipid accumulation being inhibited by various inhibitors [25]. Therefore, detoxification of DRBH is necessary to reduce the amount of inhibitors and achieve higher fermentability of sugars. Various methods such as overliming, neutralization, ion exchange, steam stripping, and treatment with activated carbon have been applied to detoxify the lignocellulosic hydrolysate. However, the most economical way to detoxify lignocellulosic hydrolysate was neutralization by calcium hydroxide [28]. Detoxification of DRBH by using Ca(OH)2 was applied since it can easily remove the residual SO42 − by forming CaSO4
precipitate and efficiently reducing the level of inhibitors simultaneously. The decrease in the concentrations of HMF and furfural is due to the reaction between calcium ions and furans, producing complex ions [29].
During the hydrolysis of DRB, the concentrations of both HMF and furfural increased with increasing acid concentrations. Using 2% sulfuric acid, the concentrations of HMF and furfural were 0.28 g/L and 0.029 g/L, respectively, and when the acid concentration increased from 3% to 4%, the concentration of HMF and furfural also increased from 0.32 g/L to 0.60 g/L and from 0.025 g/L to 0.053 g/L, respectively.
The maximum concentration of total released sugars from nondetoxified DRBH at optimum conditions was 50.22 ± 0.34 g/L, out of which the concentration of glucose was 8.76 times higher than that of xylose HMF and furfural were the main inhibitors in the hydrolysate before neutralization. As shown in Table 2, the concentration of monosaccharides before and after detoxification by Ca(OH)2 did not show a significant
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change. However, the concentrations of HMF and furfural decreased by 25.0% and 24.0% (in the 3% DRBH), respectively, which indicates that detoxification by lime can effectively reduce the amount of inhibitors and can provide a potential alternative medium for better microbial fermentation.
3.3. Growth of Y. lipolytica Po1g in Different Types of DRBH Media. DRB is rich in carbohydrates, fiber, and proteins and can be regarded as a potential nutrient source for microbial oil production [21]. Therefore, detoxified DRBH can be a potential nutrient source for growing Y. lipolytica Po1g.
Three different types of detoxified DRBHs were obtained from rice bran hydrolysis with three different acid concentrations (2%, 3%, and 4%) and named 2%, 3%, and 4%, respectively. The DRBHs were then used as the basic media for the growth of Y. lipolytica Po1g. The concentrations of total fermentable sugars in all three DRBH media were made constant (20 g/L) by appropriately diluting the corresponding initial detoxified DRBHs with deionized water. The protein contents of DRBHs obtained by using 2%, 3%, and 4% sulfuric acid were analyzed to be 1.09, 2.85, and 3.05 g/L, respectively. To study the possibility of using the hydrolysates as direct nutrient sources, comparison of the growth profile of Y. lipolytica Po1g in these media was made, and the lipid contents of the maximum biomass concentrations were determined.
As can be seen in Figure 1, yeast growth was poor (maximum biomass concentration = 4.43 g/L, 3rd day) in the 2% DRBH medium. The corresponding cellular lipid content obtained was 21.66%.
Increases in biomass concentration were observed when Y. lipolytica Po1g grew in the DRBH obtained by hydrolysis using 3% H2SO4. The highest biomass concentration (8.76 g/L) was observed on the 4th day.
The cellular lipid content obtainable is 30.13% and is relatively higher than the one from the medium that contained the DRBH from 2% H2SO4. The sugar concentration in the medium prepared by hydrolysis of rice bran with 3% H2SO4 also decreased faster due to an increase in biomass. As the biomass increases, sugar consumption also increases. On the other hand, both biomass and lipid contents decreased slightly to 7.51 g/L and 27.73%, respectively, when Y. lipolytica Po1g was cultivated in the DRBH obtained by hydrolysis with 4% H2SO4.
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According to a study conducted by Economou et al. [15], rice hulls hydrolysate was used as the sole carbon and energy source for oleaginous fungus M. isabellina. In their study, an increase in acid concentration from 0.03 M to 0.09 M increased the extractable oil amount from 36% to 64.3%, and it was observed that in the rice hulls hydrolysate obtained from sulfuric acid concentration higher than 1 M, the fungus failed to grow, probably due to the high concentrations of toxic compounds in the culture medium. The same trend was observed in our study. The highest acid concentration (4%) might have led to higher concentration of inhibitors during hydrolysis. This shows that neutralization of this hydrolysate with Ca(OH)2 alone did not reduce the inhibitors concentration, and these inhibitors affected the growth of the yeast cells.
A review about lipid accumulation properties of Y. lipolytica shows that the study of de novo lipid accumulation as a result of the conversion of carbohydrate substrates, such as glucose, provides insight into the regulation of the entire lipid synthesis pathway. The accumulation of lipids produced from these substrates is triggered by nutrient limitation. Nitrogen limitation is the principal type of limitation governing lipid accumulation [30]. Our results also suggested that limited nitrogen content in the media (3% DRBH) led to better growth and lipid accumulation while the protein content (1.09 g/L) in the medium from 2% H2SO4 was not enough for the growth of the cells, which limited biomass concentration to 4.43 g/L.
3.4. Effect of DRBH Sugar Concentration on Biomass and Microbial Oil Content. To investigate the effect of sugar concentrations on microbial growth and lipid accumulation, three different concentrations of total fermentable sugars (20 g/L, 30 g/L, and 40 g/L) were prepared by appropriate dilution of the detoxified DRBH (type 3%, Table 2) with an initial total sugar concentration of 48.41 g/L. The total protein content in
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each of these media was adjusted to 5 g/L by addition of appropriate amount of yeast extract.
As shown in Table 3, the lowest cellular lipid content (30.13%) was obtained when the Y. lipolytica Po1g was incubated in the medium with the lowest sugar concentration (20 g/L). However, the highest lipid content (48.02%) and lipid yield (5.16 g/L) were observed when the sugar concentration in the medium was 30 g/L. Both biomass concentration and cellular lipid contents decreased when the total sugar concentration was 40 g/L.
Zhu et al. [4] investigated cell growth and lipid accumulation of T. fermentans in molasses with different total sugar concentrations ranging from 5% to 30%. Both biomass and lipid content increased with the total sugar concentration, and the maximum biomass (29.9 g/L) and lipid content (31.8%) were attained at 15% total sugar concentration. Further rise in the total sugar concentration beyond 15% led to the decrease in biomass and lipid content. The same trend was observed when Y. lipolytica Po1g was cultivated in DRBH with different sugar concentrations which increased both biomass and lipid content when sugar concentration was raised from 20 g/L to 30 g/L. But further increase to 40 g/L resulted in lowering of biomass concentration and lower lipid content. This may be due to substrate inhibition in which excess sugar concentration in the DRBH media inhibits the growth of the microorganism during the early stages of growth.
Another study on the
biochemical behavior of wild-type or genetically modified Y. lipolytica strains cultivated on commercial glucose and nitrogen-limited cultures showed that carbon-excess conditions favored the secretion of organic acids into the growth medium [17]. Cultivation of Y. lipolytica Po1g using 40 g/L in this study also resulted in reduction of biomass concentration, which may most probably be due to the secretion of organic acids.
3.5. Effect of Addition of Different Nitrogen Sources on Growth and Microbial Oil Production.
Factors such as carbon and nitrogen sources have significant influences on cell growth and lipid accumulation of oleaginous microorganisms [31]. To study the effect of addition of different nitrogen sources on biomass concentration and microbial oil production, three different DRBH media with the same total fermentable sugar concentration (30 g/L) and the same initial protein content (2.82 g/L) were prepared. No external nitrogen source was added to the first one, but either peptone (5 g/L) or urea (5 g/L) was added to the other two media.
As it can be seen from Figure 2, the medium without additional nitrogen source was the best for yeast cells to accumulate the highest lipid content (48.02%) with a lipid yield of 5.16 g/L. The addition of urea did not significantly affect (P-value = 0.27) biomass production (10.85 g/L) but caused reduction in cellular lipid
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accumulation to 14.19%. Compared to urea, Y. lipolytica Po1g grew better, and the maximum biomass concentration was 13.53 g/L when the nitrogen source was peptone. The cellular lipid content (26.43%) obtained when cells grew in peptone was much higher than the one with urea, indicating that urea is not a good nitrogen source for microbial oil production from Y. lipolytica Po1g grown in detoxified DRBH medium.
Microbial oil production with both nitrogen supplements showed similar tendencies in which the microorganisms did not accumulate more lipids but produced more biomass than the one without external nitrogen source. Nitrogen is essential for the syntheses of proteins and nucleic acids which are required for cellular proliferation. However, this synthesis will be repressed when nitrogen source is limited, and carbon source will be channeled to lipid synthesis causing accumulation of lipid in the cells [32]. Therefore, nitrogen limitation is effective in enhancing the accumulation of lipid in Y. lipolytica Po1g grown in DRBH media.
Similar results by Zhu et al. [4] also showed that thecellular lipid content of T. fermentans grown in a medium with urea as the nitrogen source is 2.4 times lower than that when peptone was the nitrogen source.
The result of this study shows that the yeast tended to channel sugar into cellular lipid production when it was cultivated on the nitrogen-limited detoxified DRBH resulting in the highest cellular lipid content. On the other hand, according to Papanikolaou et al. [17], growth of either wild-type or genetically modified strains of Y.
lipolytica under nitrogen-limited culture conditions, with glucose or similarly metabolized compounds used as substrates, was not accompanied by significant lipid accumulation. Carbon-excess conditions favored the secretion of organic acids into the growth medium.
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In general, various Y. lipolytica strains, despite being incapable of producing lipid via the de novo lipid accumulation mechanism, are able to accumulate huge amounts of lipid when growth is carried out on various hydrophobic carbon sources as substrates, since accumulation of lipid from fat materials is a completely different process from that from sugars [13].
3.6. Cellular Lipid Analysis. Crude microbial oil from Y. lipolytica Po1g cells grown in a medium with detoxified DRBH (3%) was investigated by TLC, and the contents were determined gravimetrically. After dewaxing and degumming, the neutral lipid content and the fatty acid profile were determined by GC 17A.
The crude microbial oil consisted of gum (52.77%), neutral lipids (41.92%), and wax (5.31%). The results of analysis of lipids isolated from the biomass of the yeast C. lipolytica grown in medium with methanol or glucose as the only carbon source also showed that polar lipids make more than half of the total cell lipids for both carbon sources (52.3 and 64.2%, resp.) [32]. This is in agreement with the results of our work. On the other hand, Makri et al. [19] found that neutral lipids are the major components in Y. lipolytica when grown in a glycerol medium. According to their study, lipid composition in Y. lipolytica showed specific trends with time that largely reflect the physiological role of individual lipids. The increase of neutral lipids during lipogenic phase and their subsequent decrease in citric acid production phase may explain the physiological role of neutral lipids, which is energy storage in times of plenty and energy provision in times of shortage. In fact, during lipid turnover phase, neutral lipids are preferentially degraded, while some quantities of polar lipids are synthesized.
The GC chromatograms (Figures 3(a) and 3(b)) show that the major components of neutral lipid are free fatty acids (FFA, 79.56%), monoacyl glycerides (MAG, 8.91%), diacyl glycerides (DAG, 2.15%), and triacyl glycerides (TAG, 8.59%). Media, type of strain, and cultivation time have significant effect on the amount of FFA in lipids from Y. lipolytica. As reported by Papanikolaou et al. [16], growth of Y. lipolytica on medium containing hydrophobic compounds as (co)substrates resulted in the accumulation of storage lipid containing considerable quantities of FFA (30 to 40 wt% of total lipids). They also found that large quantities of FFA did not result in lethality or growth deficit, which is similar to our result. According to Juliano [21], rice bran is composed of about 15–19.7% lipids, which depends on factors such as type and origin of rice. The solvent used or extraction time used in this study might have an effect in which some amount of oil has not been removed from rice bran. The excess oil may remain in the medium and contribute to the increase in the FFA amount that can be extracted from Y. lipolytica Po1g. The reason for the high FFA content needs further investigation.
The fatty acid profile of the neutral lipid consisted of oleic acid (C18 : 1, 59.91%), palmitic acid (C16 : 0, 20.46%), palmitoleic acid (C16 : 1, 11.41%), and stearic acid (C18 : 0, 5.39%), with 2.83% as unidentified fatty acids. The main cellular fatty acid of Y. lipolytica lipid produced during growth on glucose or glycerol media also showed similar results, and oleic acid, palmitic acid, palmitoleic acid, and stearic acid were the major fatty acids [17, 19]. The combined content of saturated and monounsaturated fatty acid is more than 97.17%, making neutral lipids obtained from the culturing of Y. lipolytica Po1g in rice bran hydrolysate ideal feedstock for biodiesel production.
Table 4 gives comparison of maximum biomass concentration, lipid content, and lipid yield of Y.
lipolytica Po1g grown in DRBH media with those from different strains of Y. lipolytica grown in different
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nutrient sources. In terms of biomass concentration, DRBH is a better nutrient source for the Y. lipolytica Po1g compared to commercial glucose used to cultivate Y. lipolytica ACA-YC 5033. This indicates the potential of defatted rice bran as a nutrient source for oleaginous microorganisms.
According to Karatay and D¨onmez [33], C. lipolytica can accumulate lipids (59%, w/w) when grown in 8% molasses medium. However, the maximum lipid concentration of Y. lipolytica Po1g in detoxified DRBH is 48.02%. On the other hand, our work shows better results compared to that of Y. lipolytica cultivated in methanol and industrial fats which resulted in a lipid content of 4.9% and 44%, respectively.
Lipid yield of Y. lipolytica Po1g (5.6 g/L) is more promising when compared to the results of different strains of Y. lipolytica.
In our previous study, Y. lipolytica Po1g cells were cultivated in sugarcane bagasse hydrolysate, and the maximum biomass, lipid content, and lipid yield attainable were 11.42 g/L, 58.5%, and 6.68 g/L, respectively [18]. Therefore, under different media, different strains of Y. lipolytica have different lipid contents and lipid
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yields.