Simultaneous Saccharification and Fermentation

In late 1970s, Gulf Oil Company developed a process for simultaneous saccharification of cellulose and fermentation of the hydrolysis product to ethanol (Katzen and Monceaux 1995). Researchers at the University of Arkansas later improved upon the SSF process, summarized in Figure 1. Conceptually, we simply replace the acid hydrolysis step with an enzymatic one and carry out the combined operations of hydrolysis and fermentation in a single reactor to convert biomass into ethanol.

Most reports on the SSF process cite combining two separate reactors for saccharification and fermentation into one reactor as an advantage; however, this claim is misleading. Perhaps because the researchers may have simply obtained the cellulase enzyme off the shelf for their studies, they ignore the fact that the enzyme has to be produced separately in an economical fashion (Shin et al. 2000). In practice, there is an additional cellulase production step, which is nontrivial from the operational and economic viewpoint. Fermenting biomass with the fungus T. reesei to produce the cellulase needed in SSF is perhaps just as complex and expensive an operation as the cellulose-to-ethanol conversion portion.

3.2.1 Process Description

We give the description of a typical SSF process that aims to recycle solid waste into ethanol, which functions as an alternative fuel. The solid utilized in this process is approximately two-thirds urban waste and one-third pulp mill waste. This solid mixture contains approximately 57% in cellulose. The waste is pretreated, sterilized, and then forwarded to three reactors of 2500 gal each. A culture of mutant fungus T. reesei is inoculated into each reactor. The fungus continuously produces a full complement of cellulases that degrade cellulose. The total residence time for each cellulase production strain is 48 h. Ninety percent of the cellulose introduced into the reactors is degraded into sugars, such as pentose, xylose, arabinose, and glucose. Subsequently, the degraded cellulose is cooled in a heat exchanger and sent into 12 reactors for fermentation into ethanol. In this system, one can shut down four reactors while continuing operating the remaining reactors in full swing. The four reactors have an effective residence time of 48 h and are maintained at 40°C. The initial feed to the reactors contains 8% of intact cellulose, and the remaining 92% is degraded cellulose, which is hydrolyzed to glucose. The yeast simultaneously ferments the glucose generated to produce 2-3.6 w/v% ethanol beer slurry. Any unconverted material is recycled back to the process.

In Szczodrak's study (1989), the filamentous fungi T. reesei produced the cellulase, and a thermotolerant yeast strain Kluyveromyces fragilis FT 23 carried out the conversion of sugars to ethanol. Table 5 gives a summary of the percentage of ethanol produced.

Figure 1 The SSF process flow chart. From So and Brown (1999).

Table 5 The SSF of chemically modified straw by T. reesei cellulase preparations derived from the parent and bGDase mutant strain and K. fragilis cells

Activity (U/g straw)

Cellulase source Cellulase ßGdase Time (h) Ethanol percent (w/v) Glucose hydrolyzate (mg) Final pH

Table 5 The SSF of chemically modified straw by T. reesei cellulase preparations derived from the parent and bGDase mutant strain and K. fragilis cells

Activity (U/g straw)

Cellulase source Cellulase ßGdase Time (h) Ethanol percent (w/v) Glucose hydrolyzate (mg) Final pH

T. reesei F-522 (parent)

40

14.1

24

1.9

0

5.0

48

2.5

0

4.9

T. reesei F-522-V-7 (mutant)

40

64.2

24

3.4

4.7

48

3.4

0

4.7

Source: Szczodrak (1989). One unit of enzyme activity (U) is defined as the amount of the enzyme that liberates one moles of reducing sugars (calculated as glucose) per minute under the assay conditions. (bGdase stands for endo p-1,4-D-glucanase.

Source: Szczodrak (1989). One unit of enzyme activity (U) is defined as the amount of the enzyme that liberates one moles of reducing sugars (calculated as glucose) per minute under the assay conditions. (bGdase stands for endo p-1,4-D-glucanase.

3.2.2 Mixed Cultures and Fermentation

One disadvantage of the SSF process is the number of steps because of the separate steps needed for cellulase generation. In the original SSF process, cellulase, yeast, cellulose, and other nutrient supplements are all thrown into one bioreactor. To reduce the number of steps and cost, a modified process termed mixed cultures and fermentation has been developed. In this process, more than one organism coexist in the same reactor. While one organism degrades cellulose into sugars, another ferments sugars into ethanol, acetic acid, and lactic acid. This process is more efficient because it eliminates the steps associated with the separation of the enzyme from the products. In the process shown in Figure 2, C. thermocellum saccharifies cellulose into glucose and cellobiose; the same organism also hydrolyzes hemicellulose to xylose and xylobiose. Furthermore, it ferments glucose and cellobiose to ethanol, acetic acid, and lactic acid. The second organism C. thermosaccharolyticum cannot degrade cellulose, but it ferments glucose, cellobiose, xylose, and xylobiose to ethanol, acetic acid, and lactic acid (Wilke et al. 1983). Mixed culture fermentation is performed under anaerobic conditions, at a temperature of 60°C and pH 7. In summary, C. thermocellum efficiently degrades cellulose and hemi-cellulose, and C. thermosaccharolyticum then metabolizes xylose to generate ethanol and useful byproducts.

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