Title:

Fermentation of biomass sugars to ethanol using native

industrial yeast strains. 

Abstract (1):

In this paper, the feasibility of a technology for fermenting sugar mixtures representative of cellulosic biomass hydrolyzates with native industrial yeast strains is demonstrated.

This paper explores the isomerization of xylose to xylulose using a bi-layered enzyme pellet system capable of sustaining a micro-environmental pH gradient.

Abstract (2):

This ability allows for considerable flexibility in conducting the isomerization and fermentation steps.

With this method, the isomerization and fermentation could be conducted sequentially, in fed-batch, or simultaneously to maximize utilization of both C5 and C6 sugars and ethanol yield.

Abstract (3):

Commercially-provenSaccharomyces cerevisiae strains from the corn–ethanol industry were used and shown to be very effective in implementation of the technology for ethanol production.

 

Abstract (4):

Saccharomyces cerevisiae is well-adapted to industrial use due to its near theoretical ethanol yields and tolerance to a wide spectrum of inhibitors and elevated osmotic pressure. S. cerevisiae contains all necessary enzymes for

the conversion of xylose to ethanol, except xylose isomerase (XI). Hence, addition of exogenous xylose isomerase

for isomerization of xylose to xylulose and fermentation of xylulose to ethanol using native yeast strains was proposed.

Organisms and chemicals

Baker’s yeast (BY) (S. cerevisiae, Type II) was purchased in dried pellet form from Sigma–Aldrich (St. Louis, MO). The commercially-proven yeasts from the corn–ethanol industry, namely ethanol red (ER) (Fermentis, Lesaffre, Marcq-en-Baroeul, France)

Keyword

hydrolysate ; a substance or mixture produced by hydrolysis

 UREASE; An enzyme that catalyzes the hydrolysis of urea into carbon dioxide and ammonia; is present in intestinal bacteria.

AZIDE;  a chemical compound containing the azido group combined with an element or radical

Azide is the anion with the formula N3−. It is the 

conjugate base of hydrazoic acid. N3− is a linear

anion that is isoelectronic with CO2 and N2O

Method for sustaining two ph

Enhancing conversion of xylose to xylulose in co-immobilized enzyme pellets

Simultanaeous isomerization and fermentation

Sequential and SIF of mixed sugars

Result and Discussion (1):

For analysis of metabolites from fermentation, a 200–300 μl sample was collected at each time

point and centrifuged to separate yeast.

The supernatant was collected and diluted 1:3 with deionized water and filtered through a

0.2 μm filter.

Calibration standards for glucose, xylose, xylulose, glycerol, acetic acid, xylitol, urea and

ethanol were prepared in a similar manner.

Result and Discussion (2):

All standards and samples were analyzed by HPLC using a 30 μl injection volume with a 100 μl

injection loop.

For sugar, glycerol, acetic acid and ethanol detection and analysis, Bio-Rad Aminex HPX-87 H (300 × 7.8 mm) and Phenomenex Rezex RFQ fast acid (150 × 7.8 mm) ion exchange columns were

used in series.

Result and Discussion (3):

The column temperature was maintained at 50 °C with a mobile phase of 0.4 ml/min of 5 mM H2SO4.

At these operating conditions, when xylulose concentrations are low relative to xylose, the peaks are not well-resolved and the calculated xylulose concentrations may be subject to higher uncertainty.

For xylitol and urea analysis, a Bio-Rad Aminex HPX-87 C (300 × 7.8 mm) column was used at 80 °C with a 0.6 ml/min mobile phase of ultra-pure water.

Result and Discussion (4):

The stoichiometric fermentations of glucose and xylulose to ethanol are given by:

Result and Discussion (5):Xylulose fermentations with BY and ER

As noted, the additives used for isomerization of xylose had no adverse effect on the glucose fermentation using either BY and ER. The experiments in this section were conducted

to assess the relative effectiveness of BY and ER in fermenting xylulose to ethanol. Xylose (60 g/l) was partially isomerized and

fermented using BY or ER. The xylose was pre-isomerized to xylulose with

0.05 M borax and 0.01 M urea for a 55% conversion (∼33 g xylulose/l).

Result and Discussion (6):

By allowing a significant proportion of xylose to remain in the media, the effectiveness of azide in suppressing xylitol formation on the two yeast strains can be assessed.

Fermentation of the 33/27 g/l of xylulose/xylose mixtures was initiated by adding 100 g/l yeast to the media. Ethanol production with the two strains is shown in Figure 1.

Result and Discussion (7):

The initial fermentation rates are higher for BY but the overall ethanol yields are similar for both strains, as was the case for glucose. Both strains were able to convert the xylulose to

near theoretical yield for ethanol.

Total acetic acid and glycerol amounts of 0.06–0.08 g/g total sugar were observed in both cases, which are similar to literature reports for BY.

Result and Discussion (8):Fig. 1. Fermentation of ∼33 g/l xylulose with both ER and BY. Xylulose was produced by the partial isomerization of

60 g/l xylose with the bi-layered enzyme pellets. Both strains fermented xylulose to ethanol with high yield, but with significantly slower kinetics than for glucose. Both

experiments contained 4.6 mM azide to block respiration.

Result and Discussion (9):

Xylitol has been found to be a measurable by-product in xylose/xylulose fermentations when a significant quantity of xylose is present in the medium.Xylitol production tends to increase as the

temperature decreases (less than 35 °C) or the pH increases (pH > 5).For these experiments at 34 °C and pH 4.5 with

azide, xylitol formation is not likely.

Result and Discussion (10): Fermentation of mixed sugars

The ultimate goal of the bi-layered enzyme technology is its implementation with biomass hydrolyzates.Hydrolyzates may contain various fermentation

inhibitors depending on the method of biomass pretreatment employed.Accordingly, detoxification/conditioning may be

necessary prior to the isomerization/fermentation. Removal of fermentation inhibitors prior to isomerization will also eliminate the risk of inhibition of the XI and urease activities.

Result and Discussion (11):

A recently developed ionic liquid pretreatment method was shown to produce hydrolyzate free of the fermentation inhibitors traditionally found with dilute acid pretreatment.Thus, as new pretreatment methods are

developed, the need for hydrolyzate preconditioning may be eliminated.Even in the absence of fermentation inhibitors,

when S. cerevisiae are presented with mixtures of glucose and xylulose, the overall ethanol yields can be much different than that achieved with either sugar separately.Accordingly, the experiments in this section

focus on simulated glucose/xylulose mixtures.

Result and Discussion (12): Effect of yeast loading on fermentation of mixed

sugars

Mixed sugar fermentations were conducted with 90 g/l glucose and 30 g/l xylose both with and without the addition of azide.

WHAT IS AZIDE? Azide is the anion with the formula N3

−. It is the conjugate base of hydrazoic acid.N3

− is a linear anion that is isoelectronic with CO2 and N2O.

Per valence bond theory, azide can be described by several resonance structures, an important one being N−=N+=N−.Azide is also a functional group in organic

chemistry, RN3.

Result and Discussion (13):

To ensure immediate fermentable sugar availability, the xylose was first pre-isomerized for 24 h to obtain ∼86% xylulose.

Fermentations were performed with three yeast loadings (50, 100, and 200 g/l).

Results of these fermentations in the absence of azide are shown in Fig. 2a–c.

Result and Discussion (14):Fig. 2. Mixed sugar fermentation of 90 g/l glucose and 30 g/l xylose

(pre-isomerized to xylulose) without azide (a–c) and with 4.6 mM azide (d–f). Three yeast loadings were tested: 50 g/l (a,d); 100 g/l

(b,e); and 200 g/l (c,f).

Result and Discussion (15):

In all cases, glucose was consumed within the first 5 h. As yeast loading increases from panels a to c, the overall ethanol yield increases and the by-product formation decreases by about 20%.With 50 g/l yeast, more than 6 g/l xylulose

remain after 28 h of fermentation.At this low yeast loading, uptake of xylulose may

limit its utilization.At the higher yeast loadings shown in Figure2b

and 2c, xylulose utilization is much improved as is the overall ethanol production.

Result and Discussion (16):

To assess the impact of azide in mixed sugar fermentations, the fermentations shown in Fig. 2a–c were repeated with 4.6 mM azide (see Fig. 2d–f).

In pure xylulose fermentations, close to theoretical yields of ethanol were achieved under similar conditions.

The incomplete utilization of xylulose in mixed sugar fermentations suggests that ethanol produced from glucose fermentation may inhibit xylulose uptake and utilization.

Result and Discussion (17):Effect of mode of addition and strain of yeast on

mixed sugar fermentation

The fermentation of two mixed sugar compositions: (1) 90 g/l glucose and 30 g/l xylose and (2) 130 g/l glucose and 50 g/l xylose, were conducted with fed-batch addition of 50 g/l yeast at time 0, 4, 7 and 24 h to a final concentration of 200 g/l.

FED-BATCHA fed-batch is a biotechnological batch process

which is based on feeding of a growth limiting nutrient substrate to a culture.The fed-batch strategy is typically used in bio-

industrial processes to reach a high cell density in the bioreactor.Mostly the fed solution is highly concentrated to

avoid dilution of the bioreactor.

Result and Discussion (18):

The fermentations were conducted with either BY or ER in the presence of 4.6 mM azide. Xylose was pre-isomerized to 86% conversion prior to fermentation for all cases.

In an attempt to more-closely produce anaerobic conditions, nitrogen was bubbled into the shake flasks for 60 s to purge oxygen after each yeast addition.

Ethanol production for 120 g/l total sugars fermented with 200 g/l yeast added either initially or in fed-batch mode are shown in Fig. 3a.

Result and Discussion (19):Fig. 3. Mixed sugar fermentations with BY and ER in fed-batch yeast addition mode. Xylose was pre-isomerized to 86% conversion for all experiments prior to fermentation; 4.6 mM sodium azide was used in all experiments. The fed-batch yeast addition was implemented by adding 50 g/l of yeast at 0, 4, 7

and 24 h to a total of 200 g/l. Nitrogen was purged into the shake flask after each yeast addition and sample collection to maintain anaerobic conditions.

Ethanol production is shown for (a) for 120 g/l sugar (90 g/l glucose and 30 g/l xylose) fermentation and (b) 180 g/l total sugar (130 g/l glucose and

50 g/l xylose) fermentation.

Result and Discussion (20):

The overall ethanol yield increased from 0.38 to 0.42 g/g total sugar when yeast was switched from initial to fed-batch addition.

Under these conditions, ER proved to be a marginally better fermentor of mixed sugars (ethanol yield of 0.46 g/g total sugar) than BY with fed-batch addition.

Result and Discussion (21):

Ethanol yields from the fermentation of 130 g/l glucose and 50 g/l xylose with fed-batch yeast addition are shown in Fig. 4b.

Ethanol yields at the end of 72 h were 0.35 and 0.41 g ethanol/g total sugar from BY and ER, respectively.

For BY, some reassimilation of ethanol occurred after the first 24 h.

Result and Discussion (22):

Fed-batch yeast addition resulted in better sugar utilization and about 25% higher total product formation, which is reflected as higher ethanol as well as by-product production.ER clearly shows less ethanol inhibition at these

high sugar loadings than BY.This result is consistent with the hypothesis that

an ethanol-tolerant yeast strain may be less susceptible to the synergistic inhibition effects of azide and ethanol. This inhibitory effect may be more clearly seen

in Fig. 4 where xylulose uptake rates are compared for BY with and without azide on pure pentose and mixed sugars.

Result and Discussion (23):Fig. 4. Xylulose uptake in pure and mixed sugar

fermentations. All experiments were conducted with an initial yeast loading of 100 g/l and with 30 g/l xylose pre-

isomerized to yield approximately 25.8 g/l xylulose; Experiments A and B contained 4.6 mM azide.

 

Result and Discussion (24):Xylulose uptake in mixed sugar fermentations for

BYTransient xylulose concentrations for three

different fermentations with 100 g/l BY loading and the same initial xylulose concentration are compared in Fig. 4.

Curve C shows that the most rapid uptake of xylulose occurred in the mixed sugar fermentation without azide (see also Fig. 2b).

In contrast, when azide is added, xylulose uptake rate is reduced (Fig. 4, curve B, see also Fig. 2e).

Result and Discussion (25):

Indeed, the xylulose uptake in curve B is even slower than observed with pure xylulose alone (curve A) where ethanol inhibition resulting from glucose fermentation is not possible.

It may be that xylulose utilization kinetics are hampered by the combined effects of azide and accumulated intracellular ethanol.

Result and Discussion (26):

Based on the experiments summarized in Fig. 2, Fig. 3 and Fig. 4, it was concluded that the most efficient implementation of SIF for mixed sugar fermentations with BY is high yeast loadings without addition of azide to the media.Under such conditions, xylulose uptake is

benefited by the availability of sufficient transporters,  and  as well as reduced ethanol inhibition.

Result and Discussion (27): Simultaneous-isomerization-and-fermentation of

mixed sugarsIsomerizations and fermentations were

conducted simultaneously on mixed sugars containing 60 g/l glucose and 30 g/l xylose in media without azide.Three separate experiments were conducted to

assess the effect of the extent of xylose isomerization to xylulose on ethanol yield.In these experiments, the bi-layered pellets were

(a) not added to the fermentation medium (control run); (b) added with borax and yeast; or (c) added first, followed by yeast addition after partial isomerization of the sugars with borax.

Result and Discussion (28):

Fig. 5. Simultaneous isomerization and fermentation (SIF) of 60 g/l glucose and 30 g/l xylose. Fermentation with (a)

no isomerization of xylose; (b) SIF; and (c) pre-isomerization of xylose followed by SIF. Data shown are the average values of duplicate experiments; glucose values have standard deviation within ±8%; all other

values have standard deviation within ±3%. For all experiments shown in Fig. 3, carbon balances closed to within 3% (data not shown). Ethanol produced in (c) is

slightly lower than (b) due to lower initial sugar concentrations. Yield of ethanol per total sugar for the

experiments are (a) 67%, (b) 74%, and (c) 81%.

 

Result and Discussion (29):

Result and Discussion (30):

As shown in Fig. 5, glucose was completely consumed within 1–2 h for all three cases.

In the absence of added bi-layer pellets (Fig. 5a), the ethanol yield based on total sugar (glucose and xylose) was 67%; about 20% of the xylose was consumed.

Byproducts formed included acetic acid, glycerol and xylitol.

Result and Discussion (31):

When the bi-layer pellets were added with 0.05 M borax (Fig. 5b), the overall ethanol yield increased from 67% to 74% (±1%), with about one third of the initial xylose remaining at 10 h.The facilitated transport shuttle set up by borax

allows appreciable conversion of xylose to xylulose, presenting yeast with xylulose to ferment.However, as the SIF proceeds, the rate of xylose

isomerization decreases due to xylitol accumulation within the fermentation broth.

Result and Discussion (32):

Xylitol formation began after glucose was depleted and rose gradually to ∼2.5 g/l.

These experiments, while proving the viability of SIF, point to the need to address issues associated with the slow rate of isomerization in this configuration.

Result and Discussion (33):

In Fig 5c, the xylose was pre-isomerized to 70% xylulose and then yeast were added.These high initial xylulose concentrations led to a

further improvement over Fig. 5b with ethanol yield approaching 81%.Indeed, the partial isomerization to xylulose prior

to fermentation leads to lower xylitol production contributing to the increased ethanol productivity.

Result and Discussion (34):

In proportions found in biomass hydrolyzates (concentration of glucose 3–6 times higher than xylose), isomerization of xylose to xylulose will dominate.

Moreover, since glucose is rapidly fermented by yeast, fructose formation is likely to be very low (fructose was not detected in the experiments described inFig. 5b and c).

Even if small quantities of fructose are formed, S. cerevisiae ferments fructose as well as glucose.

Conclusion:

Recently, a method for isomerizing xylose to xylulose at high yield was demonstrated wherein the xylulose complexing agent, pH, and temperature are compatible with fermentation.Thus, isomerization and fermentation could be

conducted sequentially, in fed-batch, or simultaneously to maximize utilization of both C5 and C6 sugars and ethanol yield.For the first time, it was demonstrated that

industrially proven yeast strains such as ethanol red can convert high concentrations of mixed sugars to ethanol in times of about 30 h with yields comparable to engineered strains.

Q & A SESSION

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