Patent Application
20140342424
The present invention relates to a novel yeast and a method for producing ethanol using the same.
In recent years, large-scale production of bioethanol has been conducted throughout the world as a countermeasure against global warming. A main raw material for bioethanol is edible biomass such as corn biomass or sugarcane biomass. Such edible biomass is problematic in terms of a competition between the use as a raw material for bioethanol and the use as a food material.
In order to avoid such a problem, it has been desired to develop a technique of producing ethanol from a cellulosic biomass of wood, non-edible herb and the like, which are not used as food materials (particularly, cellulosic biomass obtained from agricultural residues, forestry residues, etc.).
In such a technique, cellulose or hemicellulose which is comprised in cellulosic biomass, or a polysaccharide as a partially decomposed product of such cellulose or hemicellulose, is hydrolyzed to obtain a saccharified solution comprising, as main ingredients, hexose (glucose, mannose and galactose) and pentose (xylose). Subsequently, sugar(s) comprised in the saccharified solution are fermented by microorganisms, so as to obtain ethanol.
Saccharomyces cerevisiae is known as a yeast capable of efficiently producing ethanol from glucose and mannose. However, there are only several types of microorganisms, which are capable of efficiently producing ethanol from xylose or galactose.
For instance, Pichia stipitis, Candida shehatae and Pachysolen tannophilus (Non Patent Literature 1), and Candida intermedia (Non Patent Literature 2), are known as a few examples of microorganisms capable of producing ethanol not only from glucose but also from xylose.
However, the yeast described in Non Patent Literature 1 has been problematic in that, when ethanol fermentation is carried out in the presence of both glucose and xylose comprised in a raw material liquid derived from cellulosic biomass, almost no xylose is consumed until glucose has been almost completely consumed as a result of catabolite repression caused by glucose, and thus that a long period of time is required for fermentation, and ethanol productivity is thereby lowered.
The present inventor conducted studies regarding the yeast described in Non Patent Literature 2. As a result, it was found that catabolite repression is hardly caused by glucose, depending on production conditions, and that although there may be a case in which ethanol fermentation can be carried out from both glucose and xylose when it is carried out in the presence of the glucose and the xylose, the yeast described in Non Patent Literature 2 is problematic in that xylose consumption efficiency is poor, and in that the yeast described in Non Patent Literature 2 does not have a sufficient ability to produce ethanol from xylose.
Therefore, the present invention relates to provision of a novel yeast having an ability to efficiently produce ethanol from glucose and xylose in a short time in the coexistence of the glucose and the xylose and a method for producing ethanol using the novel yeast.
Hence, as a result of intensive studies, the present inventor found that a specific yeast has an ability to efficiently produce ethanol from glucose and xylose in a short time in the coexistence of the glucose and the xylose, thereby completing the present invention.
Specifically, the present invention provides a yeast, which was designated as Candida intermedia 4-6-4T2 and was deposited as FERM BP-11509.
In addition, the present invention also provides a method for producing ethanol, which comprises a step of fermenting a raw material liquid comprising one or more monosaccharides selected from glucose and xylose using the aforementioned yeast.
The yeast of the present invention has an ability to efficiently produce ethanol from glucose and xylose in a short time in the coexistence of the glucose and the xylose. Moreover, using such a yeast, ethanol can be efficiently produced in a short time even in the case of using a raw material derived from cellulosic biomass comprising glucose or xylose.
Therefore, according to the method for producing ethanol of the present invention, ethanol can be efficiently produced in a short time from a cellulosic biomass-derived raw material.
The yeast of the present invention is a yeast, which was designated as Candida intermedia 4-6-4T2 and was deposited at the International Patent Organism Depositary (IPOD), National Institute of Technology and Evaluation (NITE), Incorporated Administrative Agency (address: Tsukuba Central 6, Higashi 1-1-1, Tsukuba, Ibaraki, Japan, postal code: 305-8566) under accession No. FERM BP-11509 on Sep. 20, 2012. This yeast was obtained by subjecting Candida intermedia “NBRC10601” used as a parent strain to spontaneous mutation according to an ordinary method and selecting a strain having a higher ethanol production ability than the parent strain. It is to be noted that the aforementioned “NBRC10601” is a yeast available from the International Patent Organism Depositary (IPOD), National Institute of Technology and Evaluation (NITE), Incorporated Administrative Agency.
The yeast of the present invention has an ability to efficiently produce ethanol from glucose and xylose in a short time in the coexistence of the glucose and the xylose.
Herein, the phrase “in the coexistence of glucose and xylose” is used to mean that the yeast of the present invention coexists in a raw material liquid (fermentation liquor) comprising at least glucose and xylose. As mentioned above, the conventional yeast has not had sufficient xylose consumption efficiency, or it has had an ability to produce ethanol from either one of glucose and xylose. In the case of the conventional yeast, when both glucose and xylose were present, almost no xylose was consumed, until glucose had been completely consumed by catabolite repression. In contrast, the yeast of the present invention has an ability to efficiently produce ethanol from both glucose and xylose in a short time, even when both the glucose and the xylose are present.
Moreover, while the yeast of the present invention efficiently produces ethanol from a raw material liquid comprising glucose and xylose in a short time, it produces almost no xylitol as a by-product. Furthermore, the yeast of the present invention has the same properties as those of its parent strain, other than such an ability to produce ethanol from sugar(s).
The method for producing ethanol of the present invention is characterized in that it comprises a step of fermenting a raw material liquid comprising one or more monosaccharides selected from glucose and xylose, using the aforementioned yeast (hereinafter also referred to as a “fermentation step”). According to this production method, ethanol can be efficiently produced in a short time.
The amount of the yeast used is not particularly limited. The yeast is used at a ratio of generally approximately from 0.01 to 100 mass parts, and preferably from 0.1 to 10 mass parts, based on 1 mass part of the above-mentioned monosaccharide(s).
The above-mentioned raw material liquid (for example, a saccharified solution) preferably comprises glucose and xylose. Moreover, in addition to these monosaccharides, the raw material liquid may also comprise other monosaccharides (hexoses) such as mannose or galactose. The production method of the present invention is hardly affected by catabolite repression. Hence, according to the present production method, even if the raw material liquid comprises the aforementioned monosaccharides (xylose, mannose, or galactose) as well as glucose, ethanol can be efficiently produced.
When the above-mentioned raw material liquid comprises glucose and xylose, with regard to the content ratios of glucose and xylose, xylose is used at a ratio of preferably 0.1 to 5 mass parts, and more preferably 0.5 to 3 mass parts, based on 1 mass part of glucose. Using the yeast of the present invention, even if the content ratios of glucose and xylose are within the above described ranges, ethanol can be produced not only from glucose but also from xylose. Moreover, each of the contents of glucose and xylose in the raw material liquid is preferably 1 to 100 g/L, and more preferably 5 to 50 g/L. Using the yeast of the present invention, even if each of the contents of glucose and xylose is within the above-described ranges, ethanol can be produced. Furthermore, the content ratio of glucose with respect to sugar(s) comprised in the raw material liquid is not particularly limited. Glucose is contained at a mass percentage of preferably 5% to 99%, and more preferably 10% to 70%, based on the total mass of the sugar(s) comprised in the raw material liquid.
The total content of monosaccharides comprised in the above described raw material liquid is preferably 1 to 100 g/L.
The raw material liquid used in ethanol production may be any one of a raw material liquid derived from animals, a raw material liquid derived from plants, and a raw material liquid derived from industrial wastes. Among these, a raw material liquid derived from plants is preferable. From the viewpoint of preventing a competition between the use as a raw material for bioethanol and the use as a food material, a cellulosic biomass hydrolysate is more preferable. The term “cellulosic biomass” is used herein to mean biomass comprising cellulose and hemicellulose. Glucose is obtained by hydrolyzing cellulose comprised in such biomass, whereas glucose, xylose, mannose, and galactose are obtained by hydrolyzing hemicellulose comprised therein.
From the viewpoint of economic advantage in ethanol production, such cellulosic biomass is preferably obtained from agricultural residues (rice straw, wheat straw, etc.), forestry residues (lumbers, etc.), and the like.
The pH (30° C.) of the above described raw material liquid is preferably 3.5 to 6.5, more preferably 4 to 6, further preferably 4.5 to 5.5, and particularly preferably 4.5 to 5. According to the yeast of the present invention, ethanol production is possible even in such a low pH range. Thus, even in the case of using a saccharified solution comprising acetic acid or the like (an acid hydrolysate of biomass cellulose), ethanol can be produced.
It has been reported that a saccharified solution with a low pH value obtained by hydrolyzing cellulosic biomass using an acid comprises an inhibitory substances that inhibit ethanol production by microorganisms, such as acetic acid (Nigam J N. Journal of Applied Microbiology, Vol. 90, 208-215 (2001)). It has also been reported that, in particular, ethanol fermentation by microorganisms capable of producing ethanol from xylose is inhibited in a low pH range such as pH 5 or less (Palmqvist E., Hahn-Hagardal B. Bioresource Technology Vol. 74, 25-33 (2000)).
The above described production method is carried out at a temperature of preferably 20° C. to 35° C., and more preferably 25° C. to 30° C.
Moreover, in the above described production method, while the above described yeast may be allowed to grow under its growing conditions, ethanol may be produced. Alternatively, the yeast may be left in the state of resting cells, namely, the yeast may be left under conditions in which a nitrogen source is reduced to an amount insufficient for the growth of the yeast and a carbon source used as a raw material for ethanol is abundant, so that ethanol can be produced under conditions in which the growth of the yeast is suspended. Among these conditions, it is preferable to produce ethanol by the yeast which is in the state of resting cells, since ethanol production is hardly affected by ethanol production inhibitory substances, such as acetic acid or sulfurous acid, under such conditions. When ethanol production is carried out by the yeast which is in the state of resting cells, the concentration of the yeast is preferably 5 to 100 g/L. Moreover, when ethanol is produced by the yeast which is in the state of resting cells, it is not necessary to add a nitrogen source, a yeast extract, etc. to the raw material liquid. It is preferable that the raw material liquid be adjusted to have the aforementioned pH value by addition of a phosphate buffer, sodium hydroxide or the like. The recovery of ethanol may be carried out according to ordinary means such as distillation.
Furthermore, in the above described production method, from the viewpoint of the amount of cells, it is preferable that a pre-culture be carried out before the aforementioned fermentation step. As a medium used in such a pre-culture, a medium comprising glucose and one or more sugars selected from mannose, galactose and xylose is preferable. As such a medium, a medium comprising the aforementioned cellulosic biomass hydrolysate may be used.
The concentration of the aforementioned sugars in total is preferably 1 to 100 g/L, and more preferably 10 to 50 g/L. The content ratio of one or more sugars selected from mannose, galactose and xylose to glucose is not particularly limited.
When a cellulosic biomass hydrolysate is used as a carbon source in the above described pre-culture, the hydrolysate is used at a volume percentage of generally 20% or less, and preferably 10% or less, based on the volume of the medium. Other ingredients comprised in the medium are not particularly limited. Examples of such other ingredients include: nitrogen sources suitable for growth, such as amino acid, urea, polypeptone, and an amino acid-free nitrogen base; and a yeast extract. The temperature applied to the pre-culture is preferably 10° C. to 37° C., and more preferably 25° C. to 30° C. The pH applied to the pre-culture is preferably 4 to 7, and more preferably 4.5 to 6.5. In addition, the pre-culture is preferably carried out under aerobic conditions.
Hereinafter, the present invention will be described in detail in the following examples. However, these examples are not intended to limit the scope of the present invention.
In accordance with the following procedures, the yeast Candida intermedia “NBRC10601,” which was deposited at the International Patent Organism Depositary (IPOD), National Institute of Technology and Evaluation (NITE), Incorporated Administrative Agency, was used as a parent strain, and the yeast Candida intermedia “NBRC10601” was subjected to acclimation and spontaneous mutation, thereby obtaining a yeast strain 4-6-4T2.
First, the pH of an acetic acid aqueous solution comprising glucose and xylose (each 1 mass %) was adjusted to be 5.0 with magnesium hydroxide, and 20% of this solution was then mixed with 80% of a liquid medium (1% yeast extract, 2% amino acid-free nitrogen base). Thereafter, 1% xylose was added to 10 mL of the obtained mixed solution, and one platinum loop of the yeast Candida intermedia “NBRC10601” was then inoculated into the mixed solution. The thus obtained mixture was cultured at 30° C. for 3 days to obtain a culture solution.
Subsequently, in the same manner as described above, 50% of an acetic acid aqueous solution comprising glucose and xylose (each 1 mass %) that had been adjusted to have pH 5.0 was mixed with 50% of a liquid medium. Thereafter, to 10 mL of this mixed solution, 100 μL of the culture solution obtained as a result of the aforementioned culture for 3 days was added, and the thus obtained mixture was further cultured for 7 days. Thereafter, in the same manner as described above, 80% of an acetic acid aqueous solution comprising glucose and xylose (each 1 mass %) that had been adjusted to have pH 5.0 was mixed with 20% of a medium. To 10 mL of this mixed solution, 100 μL of the culture solution obtained as a result of the aforementioned culture for 7 days was added, and the obtained mixture was further cultured for 30 days, so as to prepare an acclimated strain solution.
The prepared acclimated strain solution was diluted by 1000 times, and the thus diluted solution was applied onto a YNB agar medium (5% glucose, 1% yeast extract, 2% amino acid-free nitrogen base, 2% agar), and the resultant was then cultured at 25° C. for 4 days. Thereafter, a strain that formed a colony was obtained.
The obtained strain was applied onto a YNB agar medium (2% trehalose, 1% yeast extract, 2% amino acid-free nitrogen base, 2% agar), and the resultant was cultured 25° C. for 3 days. Thereafter, formation of a colony was confirmed, and the culture was then stored at 4° C. Colonies, which had grown at 4° C., were selected, and thereafter, using the selected colonies, an ethanol production test was carried out in a phosphate buffer (2.5% xylose, 0.1 M KH2PO4, pH=5.0, 0.006 M MgSO4.7H2O). A strain having an ability to produce ethanol, which was higher than the parent strain, was selected.
Thus, a yeast of interest was selected, and was designated as Candida intermedia 4-6-4T2. The present yeast strain was deposited at the International Patent Organism Depositary (IPOD), National Institute of Technology and Evaluation (NITE), Incorporated Administrative Agency. The accession number is FERM BP-11509.
The parent strain (NBRC10601) and the strain 4-6-4T2 were each inoculated into a YNB medium (0.5% glucose, 2% xylose, 1% yeast extract, 2% amino acid-free nitrogen base), and the obtained mixtures were each subjected to a shaking culture at 30° C. at 120 rpm (24 to 48 hours, OD660=18 to 25). The weight of a dry yeast strain in 1 mL of the culture solution was calculated according to the following relational expression (1), which was obtained empirically. (According to the expression (1), the weight of a dry yeast strain could be calculated to be 0.0062 g in the case of OD660=20, for example.)
Weight of dry yeast strain (g/mL)=0.00032×OD660−0.00017 (1)
A necessary amount of culture solution was recovered by centrifugation (3000 rpm, 2 minutes) such that the amount of the yeast strain became 2% in terms of dry weight. Thereafter, a fermentation solution prepared by mixing the sugars shown in Tables 1 to 4 below (raw materials for ethanol) into a phosphate buffer (KH2PO4/K2HPO4: 0.01M, pH=5.0) was added to each yeast strain, followed by performing fermentation. A change over time in the amount of ethanol produced was confirmed. The results are shown in Tables 1 to 4 and
As shown in Table 1 and
In contrast, as shown in Table 2 and
In addition, as shown in Table 3 and
In contrast, as shown in Table 4 and
The reason why there is a difference between the strain 4-6-4T2 and the parent strain in terms of ability to produce ethanol from xylose has not been necessarily clarified. From the above
That is to say, from the comparison between
Thus, it is assumed that the reason that ethanol can be efficiently produced using the yeast of the present invention would be that the present yeast is hardly affected by catabolite repression caused by glucose, and also that production of xylitol is suppressed and thereby ethanol productivity is hardly inhibited by such xylitol.
The yeasts shown in Table 5 (approximately 2% dry weight) were each added to a fermentation solution (0.01 M phosphate buffer, pH 5.0) each comprising the sugars shown in the same table. Twelve hours later, the amount of ethanol produced (% (v/v)) was determined. The results are shown in Table 5.
C. intermedia
S. cerevisiae
P. tannophilus
C. shehatae
P. stipits
From Table 5, it is found that the strain 4-6-4T2 exhibits an ability to produce ethanol, which is higher than that of the parent strain or known yeast strains, regardless of the types of sugars used.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-240158 | Nov 2011 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/077428 | 10/24/2012 | WO | 00 | 3/14/2014 |
