kinnow waste and banana peels for ethanol production was
performed by Sharma et al. (2007). Effects of different fermentation parameters were studied to find out the ethanol
yield (0.426 g/g) and fermentation efficiency (83.52%)
under the optimized conditions (Sharma et al. 2007).
4.4 Ethanologens for Ethanol Production
from Pectin-rich Substrates
S. cerevisiae: S. cerevisiae is the most widely used
microorganism when it comes to ethanol production. It is
considered a very robust microorganism due to its high
threshold toward both substrate and ethanol concentrations
as well as its tolerance toward comparatively lower levels of
oxygen and pH (Gujjari et al. 2009). The metabolic pathway
of S. cerevisiae is such that the product channeling using
sugar as a substrate is mainly toward ethanol, thus lesser
by-products are formed. This yeast strain can also break
down sucrose without any hindrance during the degradation
process to form ethanol. However, S. cerevisiae cannot
effectively metabolize pentose sugars and galacturonic acid,
the prominent constituents of pectin-rich biomass (Edwards
and Doran-Peterson 2012). Out of the four pectic polysaccharides present in pectin, rhamnogalacturonan I (RG I)
contributes to 20–35% of the pectin structure and contains
arabinose and arabinan linkages (Mohnen 2008). On that
account, researchers have developed xylose and arabinose
fermenting strains of S. cerevisiae. Detailed research has also
gone into the development of galacturonic-acid fermenting
strains of S. cerevisiae (Sedlak and Ho 2001; Van Maris
et al. 2006).
E. coli: E. coli, in contrast to S. cerevisiae, has very
limited tolerance toward both ethanol as well as S. cerevisiae. It has, however, a very low pH requirement (Gujjari
et al. 2009). This remains the primary reason for the inefficient performance of E. coli during fermentations involving
break down of biomass structure in acidic medium. Nevertheless, the metabolic pathway of E. coli is such that it is
competent enough to utilize different carbohydrate sources,
including arabinose and galacturonic acid. Galacturonic-acid
utilization by E. coli has been widely studied by different
research groups (Richard and Hilditch 2009). The varied
product distributions for sugar fermentation by E. coli
interferes with its fermentation efficiency. The products
formed, other than ethanol, during sugar catabolism include
acids, acetates, formates, succinates, and lactates (Jarboe
et al. 2007). The production of these bio-based chemicals
can lessen the dependency on petroleum-based products.
Different research groups have adopted bioengineering of
E. coli in order to modify its metabolic pathway so as to limit
the product distribution more toward ethanol. The major
issues regarding fermentation of pectin-rich biomasses are
acetate formation along with ethanol and the presence of
inhibitors, for example, limonene in citrus wastes (Shaw
1979).
The chemistry involving the fermentation reaction for
ethanol production suggests that ratio wise, 1 mol of
galacturonic acid forms 1 mol of ethanol and acetate each.
The reason behind this lies in the existence of galacturonic
acid in higher oxidation states which is why consecutively
the requirement for NAD(P)H molecules increases. The need
for NAD(P)H molecules is met through pyruvate formate
lyase pathway, which gives both ethanol and acetate
(Grohmann et al. 1994).
As mentioned earlier, the presence of inhibitory compounds in the biomass is yet another problem. For example,
the industrial processing of citrus fruits results in a residue
rich in limonene, which constitutes 86–95% of essential oils
of this biomass (Shaw 1979). Limonene comprises
monoterpenes, an aromatic compound, which are said to
disintegrate membrane integrity of cells resulting in movement of cellular components in and out of the cells along
with the loss of driving forces which include proton movement and K
+ gradient (Koroch et al. 2007). Limonene offers
inhibition toward fermentation even at concentrations of
1.4% (v/v).
Food waste has a varied range of carbohydrate sources.
Their structural degradation gives different groups of
monomeric units, namely pentoses and hexoses. The
ethanologenic organism, therefore, must be able to maximally ferment the mixture of the substrates. This enhances
the viability of using the biochemical route with respect to
the economic competitiveness of the whole process (Kumar
et al. 2009). In this regard, many approaches have been
adopted for engineering the fermenting organism in order to
expand its metabolizing capability for different carbohydrate
sources.
5 Physico-Chemical Characteristics of Food
Wastes
The food waste composed of biodegradable constituents can
be categorized into solid, liquid, or semi-solid waste. The
uniformity in the product composition ensures lesser variations in the composition of the corresponding food waste
generated (Zhang et al. 2014). A few examples of solid and
liquid food wastes are given below.
Solid food waste: Wastes like tomato waste, apple
pomace, grape pomace produced during wine production are
mainly characterized by starch, cellulose, hemicellulose, and
lignin.
Liquid food waste: Residues from cheese-and yogurtprocessing industries, brewery industries, characterized by
nitrogen content, fats and oils, and other suspended solids.
50
N. Dey and A. N. Bhaskarwar
performed by Sharma et al. (2007). Effects of different fermentation parameters were studied to find out the ethanol
yield (0.426 g/g) and fermentation efficiency (83.52%)
under the optimized conditions (Sharma et al. 2007).
4.4 Ethanologens for Ethanol Production
from Pectin-rich Substrates
S. cerevisiae: S. cerevisiae is the most widely used
microorganism when it comes to ethanol production. It is
considered a very robust microorganism due to its high
threshold toward both substrate and ethanol concentrations
as well as its tolerance toward comparatively lower levels of
oxygen and pH (Gujjari et al. 2009). The metabolic pathway
of S. cerevisiae is such that the product channeling using
sugar as a substrate is mainly toward ethanol, thus lesser
by-products are formed. This yeast strain can also break
down sucrose without any hindrance during the degradation
process to form ethanol. However, S. cerevisiae cannot
effectively metabolize pentose sugars and galacturonic acid,
the prominent constituents of pectin-rich biomass (Edwards
and Doran-Peterson 2012). Out of the four pectic polysaccharides present in pectin, rhamnogalacturonan I (RG I)
contributes to 20–35% of the pectin structure and contains
arabinose and arabinan linkages (Mohnen 2008). On that
account, researchers have developed xylose and arabinose
fermenting strains of S. cerevisiae. Detailed research has also
gone into the development of galacturonic-acid fermenting
strains of S. cerevisiae (Sedlak and Ho 2001; Van Maris
et al. 2006).
E. coli: E. coli, in contrast to S. cerevisiae, has very
limited tolerance toward both ethanol as well as S. cerevisiae. It has, however, a very low pH requirement (Gujjari
et al. 2009). This remains the primary reason for the inefficient performance of E. coli during fermentations involving
break down of biomass structure in acidic medium. Nevertheless, the metabolic pathway of E. coli is such that it is
competent enough to utilize different carbohydrate sources,
including arabinose and galacturonic acid. Galacturonic-acid
utilization by E. coli has been widely studied by different
research groups (Richard and Hilditch 2009). The varied
product distributions for sugar fermentation by E. coli
interferes with its fermentation efficiency. The products
formed, other than ethanol, during sugar catabolism include
acids, acetates, formates, succinates, and lactates (Jarboe
et al. 2007). The production of these bio-based chemicals
can lessen the dependency on petroleum-based products.
Different research groups have adopted bioengineering of
E. coli in order to modify its metabolic pathway so as to limit
the product distribution more toward ethanol. The major
issues regarding fermentation of pectin-rich biomasses are
acetate formation along with ethanol and the presence of
inhibitors, for example, limonene in citrus wastes (Shaw
1979).
The chemistry involving the fermentation reaction for
ethanol production suggests that ratio wise, 1 mol of
galacturonic acid forms 1 mol of ethanol and acetate each.
The reason behind this lies in the existence of galacturonic
acid in higher oxidation states which is why consecutively
the requirement for NAD(P)H molecules increases. The need
for NAD(P)H molecules is met through pyruvate formate
lyase pathway, which gives both ethanol and acetate
(Grohmann et al. 1994).
As mentioned earlier, the presence of inhibitory compounds in the biomass is yet another problem. For example,
the industrial processing of citrus fruits results in a residue
rich in limonene, which constitutes 86–95% of essential oils
of this biomass (Shaw 1979). Limonene comprises
monoterpenes, an aromatic compound, which are said to
disintegrate membrane integrity of cells resulting in movement of cellular components in and out of the cells along
with the loss of driving forces which include proton movement and K
+ gradient (Koroch et al. 2007). Limonene offers
inhibition toward fermentation even at concentrations of
1.4% (v/v).
Food waste has a varied range of carbohydrate sources.
Their structural degradation gives different groups of
monomeric units, namely pentoses and hexoses. The
ethanologenic organism, therefore, must be able to maximally ferment the mixture of the substrates. This enhances
the viability of using the biochemical route with respect to
the economic competitiveness of the whole process (Kumar
et al. 2009). In this regard, many approaches have been
adopted for engineering the fermenting organism in order to
expand its metabolizing capability for different carbohydrate
sources.
5 Physico-Chemical Characteristics of Food
Wastes
The food waste composed of biodegradable constituents can
be categorized into solid, liquid, or semi-solid waste. The
uniformity in the product composition ensures lesser variations in the composition of the corresponding food waste
generated (Zhang et al. 2014). A few examples of solid and
liquid food wastes are given below.
Solid food waste: Wastes like tomato waste, apple
pomace, grape pomace produced during wine production are
mainly characterized by starch, cellulose, hemicellulose, and
lignin.
Liquid food waste: Residues from cheese-and yogurtprocessing industries, brewery industries, characterized by
nitrogen content, fats and oils, and other suspended solids.
50
N. Dey and A. N. Bhaskarwar