Page 73 - Advances in bioenergy (2016)

P. stipitis has been mainly used in applications where not only cellulose but also hemicellulose
is to be fermented, as it is considered one of the most promising naturally occurring C5
fermenting organisms. P. stipitis has been used as an ethanologen to ferment various types of
pretreated biomass such as red oaks, wheat straw, sugarcane bagasse, rice straw, corn cob,
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corn stover, aspen wood, pine wood, and poplar wood (Table 2.3). Lately, K. marxianus has
received much attention because of the potential cost savings, which could be obtained by

continuous evaporation of ethanol from the broth under reduced pressure. Promising ethanol
yields from various lignocellulosic woody and herbaceous materials, during SSF in the
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presence of commercial cellulases, have been achieved by Ballesteros et al., while two
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years ago Yanase et al. have reported ethanol production by K. marxianus displaying
cellulolytic enzymes.

Saccharomyces cerevisiae

Although S. cerevisiae is used in industrial fermentation applications for many years, the
commercialization of cellulosic ethanol using this microorganism has been proved to be a

strong challenge. The efficient cofermentation of C6 and C5 sugars (simultaneous fermentation
of both pentoses and hexoses) by S. cerevisiae has always been (and still is) considered as a
major concern. Apart from this, the inhibitory effects of (toxic) lignocellulose derivatives on
the fermentative performance of S. cerevisiae—the stressful conditions due to high ethanol
concentration or due to high sugars concentrations during the fermentation, the high cost of
separation of ethanol from the fermentation broth—are major restraints which the advance of
biofuels technology is trying to overcome. The majority of the scientific reports published so
far could be divided in two distinguished, though supplementary categories; the process
optimization strategies and the strain development strategies.

High Cell Density Processes

S. cerevisiae immobilization has been applied in industrial alcohol fermentation aiming mainly
at the increment in fermentation rates. High cell density, greater volumetric productivity,
increased tolerance to high concentrations of inhibitors, relative easiness of downstream
processing are some of the properties that have been attributed to these processes.

Flocculating S. cerevisiae strains is another way to increase cell density during fermentation
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process. Flocculation is the asexual, reversible, and Ca -dependent aggregation of thousands
of vegetative cells into flocs. Detailed biochemical and structural studies of the responsible
adhesins (flocculins) have been performed, enlightening the biological aspects of this
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mechanism based on a lectin–carbohydrate interaction. Recently, Ruiz et al. reported the
successful production of ethanol from hydrothermally treated wheat straw during SSF by a
thermotolerant flocculating yeast strain. As shown, the flocculating strain performed very well
at 45°C. In other words, it was possible to perform the SSF at a temperature close to the
optimal for the enzyme action overcoming a major restraint of SSF processes.

A novel type of cell–cell flocculation induced by trehalose-6-phosphate synthase 1 (TPS1)
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promoter was reported recently by Li et al. From the experimental results, it was shown that