favorable due to the very high distillation costs involved in
ethanol recovery from the fermentation broths (Guimarães
et al. 2010).
3.2 Coffee Pulp and Husk
The by-products generated during coffee-beans production
include pulp, wastewater, and husk, all of which are rich
carbon sources for ethanol fermentation. These are, otherwise, dumped into water sources polluting them extensively,
and making water highly acidic.
Processing yields one-tonne pulp per two tonnes of coffee
beans (Roussos et al. 1995). The compositional analysis (dry
weight basis) of coffee pulps suggests that these are mainly
made up of 23–27% fermentable sugars and 8.25% protein.
The pretreatment of coffee pulp using dilute sulfuric acid
results in a hydrolysate of composition: xylose (up to
3.23 g/L), arabinose (up to 11.26 g/L), fructose (up to
3 g/L), glucose (up to 6.31 g/L), sucrose (up to 96 g/L), and
maltose (up to 3.50 g/L) (Urbaneja et al. 1996).
Apart from sulfuric-acid treatment, even distilled water is
used to pretreat the coffee pulp prior to fermentation. This
treatment had been adopted and carried out for 4 h by Kefale
(2011) prior to fermentation by S. cerevisiae, which was
performed for 24 h at 30 °C to yield ethanol of 7.4 g/L.
In addition, many other by-products of coffee-bean
industries, for example, coffee husk, can be used for ethanol production. For every 1 kg of coffee beans produced, an
equivalent amount of 1 kg of coffee husks is produced.
Coffee husks provide other uses, for example, as a fuel, as
an animal feed, and many others (Franca and Oliveira 2009).
Ethanol concentration up to a titer value of 13.6 g/L was
achieved by Gouvea et al. (2009) by fermenting whole and
ground coffee husks by S. cerevisiae.
3.3 Potato-processing Waste
Potatoes are considered a significant food crop that is made
up of starch molecules. Starch, a polymer of glucose molecules, serves as the main source of energy in plants. It is the
main carbohydrate source from which energy is derived for
all the metabolic functions of the plant (Fonseca et al. 2008).
A large fraction of potatoes is wasted during its production
as well as in the potato-chips industries (Fadel 2000). Glucose recovered from the wastewater, produced during the
manufacturing of the potato chips, can be used as the carbon
source for bioethanol production. Optimization studies on
hydrolysis had been conducted by Izmirlioglu and Dermicri for the potato-processing wastes to be fermented by S.
cerevisiae for bioethanol production (Izmirlioglu and
Demirci 2012).
3.4 Rice Husk
Rice is an important food crop in most of the Asian countries. Rice processing produces rice husks, which contribute
to 22% of the processing waste, whereas the rest 78%
comprises rice, broken rice, and bran (Nagrale et al. 2012).
Rice husk could be used as a fermentable carbon source for
ethanol fermentation, instead of dumping it as such creating
environmental pollution.
Many fermentation techniques have been adopted for
ethanol production from rice husks, which include separate
hydrolysis prior to fermentation, simultaneous saccharification and fermentation, and many others. Rice husk is composed of 50% cellulose, 25–30% lignin, and 15–20% silica
(Akbas and Stark 2016). The presence of lignin in rice husk
makes the availability of cellular enzymes to cellulose very
cumbersome and hence provides resistance during hydrolysis (Rostagno et al. 2015). It is, therefore, preferred that the
addition of the rice husk to the fermentation medium takes
place after pretreating the rice husk to form fermentable
sugars, else the overall efficiency of the process goes down.
Saha and Cotta had done lime treatment followed by
enzymatic treatment using enzymes like cellulase, hemicellulase, and b-galactosidase to produce a fermentable
hydrolysate of monomeric sugars which yielded 9.8 g/L
ethanol after 19 h of fermentation at 35 °C by Escherichia
coli (E. coli) FBR5 (Saha and Cotta 2008). An ethanol titer
value of 11 g/L was produced by simultaneous saccharification and fermentation carried out for 53 h at 35 °C.
Other fermentation techniques have been investigated
too. Acid pretreatment of the rice husk prior to fermentation
by Moon et al. (2012) gave an ethanol yield which is 4%
lesser than the theoretical maximum. The cell growth is
slightly compromised because of the presence of inhibitors
produced during the acid pretreatment.
4 Pectin-Rich Biomass
Pectin is a polysaccharide which is in plenty in cell walls of
plants and is an important source for the overall growth of
the plant (e.g. morphogenesis, defense, wall porosity, seed
hydration, and fruit development). The uses of pectin
include a gelling and stabilizing agent, production of adhesives, plasticizers, and in drug delivery systems. The study
on the structure of pectin suggests that it is covalently
bonded by galacturonic acids (70%) (Mohnen 2008). Many
other constituents are also expected in pectin, like rhamnose,
xylose, arabinose, and galactose (Edwards and
Doran-Peterson 2012). The four pectic polysaccharides
comprising pectin are homogalacturonan (HG), xylogalacturonan (XGA), rhamnogalacturonan I (RG-I), and
rhamnogalacturonan II (RG-II) (Scheller et al. 2007).
48
N. Dey and A. N. Bhaskarwar
ethanol recovery from the fermentation broths (Guimarães
et al. 2010).
3.2 Coffee Pulp and Husk
The by-products generated during coffee-beans production
include pulp, wastewater, and husk, all of which are rich
carbon sources for ethanol fermentation. These are, otherwise, dumped into water sources polluting them extensively,
and making water highly acidic.
Processing yields one-tonne pulp per two tonnes of coffee
beans (Roussos et al. 1995). The compositional analysis (dry
weight basis) of coffee pulps suggests that these are mainly
made up of 23–27% fermentable sugars and 8.25% protein.
The pretreatment of coffee pulp using dilute sulfuric acid
results in a hydrolysate of composition: xylose (up to
3.23 g/L), arabinose (up to 11.26 g/L), fructose (up to
3 g/L), glucose (up to 6.31 g/L), sucrose (up to 96 g/L), and
maltose (up to 3.50 g/L) (Urbaneja et al. 1996).
Apart from sulfuric-acid treatment, even distilled water is
used to pretreat the coffee pulp prior to fermentation. This
treatment had been adopted and carried out for 4 h by Kefale
(2011) prior to fermentation by S. cerevisiae, which was
performed for 24 h at 30 °C to yield ethanol of 7.4 g/L.
In addition, many other by-products of coffee-bean
industries, for example, coffee husk, can be used for ethanol production. For every 1 kg of coffee beans produced, an
equivalent amount of 1 kg of coffee husks is produced.
Coffee husks provide other uses, for example, as a fuel, as
an animal feed, and many others (Franca and Oliveira 2009).
Ethanol concentration up to a titer value of 13.6 g/L was
achieved by Gouvea et al. (2009) by fermenting whole and
ground coffee husks by S. cerevisiae.
3.3 Potato-processing Waste
Potatoes are considered a significant food crop that is made
up of starch molecules. Starch, a polymer of glucose molecules, serves as the main source of energy in plants. It is the
main carbohydrate source from which energy is derived for
all the metabolic functions of the plant (Fonseca et al. 2008).
A large fraction of potatoes is wasted during its production
as well as in the potato-chips industries (Fadel 2000). Glucose recovered from the wastewater, produced during the
manufacturing of the potato chips, can be used as the carbon
source for bioethanol production. Optimization studies on
hydrolysis had been conducted by Izmirlioglu and Dermicri for the potato-processing wastes to be fermented by S.
cerevisiae for bioethanol production (Izmirlioglu and
Demirci 2012).
3.4 Rice Husk
Rice is an important food crop in most of the Asian countries. Rice processing produces rice husks, which contribute
to 22% of the processing waste, whereas the rest 78%
comprises rice, broken rice, and bran (Nagrale et al. 2012).
Rice husk could be used as a fermentable carbon source for
ethanol fermentation, instead of dumping it as such creating
environmental pollution.
Many fermentation techniques have been adopted for
ethanol production from rice husks, which include separate
hydrolysis prior to fermentation, simultaneous saccharification and fermentation, and many others. Rice husk is composed of 50% cellulose, 25–30% lignin, and 15–20% silica
(Akbas and Stark 2016). The presence of lignin in rice husk
makes the availability of cellular enzymes to cellulose very
cumbersome and hence provides resistance during hydrolysis (Rostagno et al. 2015). It is, therefore, preferred that the
addition of the rice husk to the fermentation medium takes
place after pretreating the rice husk to form fermentable
sugars, else the overall efficiency of the process goes down.
Saha and Cotta had done lime treatment followed by
enzymatic treatment using enzymes like cellulase, hemicellulase, and b-galactosidase to produce a fermentable
hydrolysate of monomeric sugars which yielded 9.8 g/L
ethanol after 19 h of fermentation at 35 °C by Escherichia
coli (E. coli) FBR5 (Saha and Cotta 2008). An ethanol titer
value of 11 g/L was produced by simultaneous saccharification and fermentation carried out for 53 h at 35 °C.
Other fermentation techniques have been investigated
too. Acid pretreatment of the rice husk prior to fermentation
by Moon et al. (2012) gave an ethanol yield which is 4%
lesser than the theoretical maximum. The cell growth is
slightly compromised because of the presence of inhibitors
produced during the acid pretreatment.
4 Pectin-Rich Biomass
Pectin is a polysaccharide which is in plenty in cell walls of
plants and is an important source for the overall growth of
the plant (e.g. morphogenesis, defense, wall porosity, seed
hydration, and fruit development). The uses of pectin
include a gelling and stabilizing agent, production of adhesives, plasticizers, and in drug delivery systems. The study
on the structure of pectin suggests that it is covalently
bonded by galacturonic acids (70%) (Mohnen 2008). Many
other constituents are also expected in pectin, like rhamnose,
xylose, arabinose, and galactose (Edwards and
Doran-Peterson 2012). The four pectic polysaccharides
comprising pectin are homogalacturonan (HG), xylogalacturonan (XGA), rhamnogalacturonan I (RG-I), and
rhamnogalacturonan II (RG-II) (Scheller et al. 2007).
48
N. Dey and A. N. Bhaskarwar