The nutrients distribution in the food waste is based on
their respective sizes and are broadly classified as a
macronutrients (carbohydrates, proteins, and lipids) and
micronutrients (sodium Na
+ , potassium K
+
, calcium Ca
2+ ,
magnesium Mg
2+ , to name a few). Some of the characteristics are as follows (Hegde et al. 2018):
• pH: The pH value of the biomass is suggestive of the
period over which it has been stored. The lesser the value,
the fresher is the biomass.
• Chemical oxygen demand (COD): It gives a quantitative
estimation of the organic compounds present in the biomass. Biomass COD values are proportional to the
nutritional values of the same.
• Total solids: The quantifiable constant weight solid
fraction produced by the heat treatment (at 100 °C) of the
waste.
• Ash content: The measurement of this parameter tells
about the inorganic fraction present in the biomass.
• Metal ions: They are the cofactors of enzymes involved in
alcohol synthesis. A balanced metal-ion requirement is
required for optimum cell growth (van Vliet et al. 2001).
• Carbohydrate content: Higher the carbohydrate content of
the substrate (food waste), higher will be the probability
toward conversion to alcohol.
• Lipid content: Lipid molecules with a high content of free
fatty acids (FFA) enhance tolerance limits of the fermenting organism (yeast) toward ethanol produced in the
fermentation broth (van Vliet et al. 2001).
• Protein content: Proteins are a source of nitrogen content
and need to be present in an optimum carbon to nitrogen
ratio. Protein limitation or excessive overloading can
affect the hydrolysis process and ultimately hinder substrate utilization (Hegde et al. 2018; Zhao 2008).
6 Comparison of Conversion Strategies
for Ethanol Production
Feedstock utilization for 2G bioethanol production is carried
out worldwide using collateral conversion routes, namely
thermochemical and biochemical (as mentioned in Sects. 3
and 4) routes. A detailed comparative study of both these
routes has been reported by Mu et al. (2010).
Thermochemical process: The entire process can be
broken down into 6–7 stages. It starts with feedstock production followed by an assemblage of the same and further
transportation to the thermochemical-conversion facility. At
the facility, the substrate undergoes initial drying using flue
gas produced from the gasification of the substrate in the
presence of steam, olivine, magnesium oxide, and air.
Gasification results in the synthesis of carbon monoxide,
hydrogen, and some sulfur content. The gaseous mixture is
then subjected to oxidation, with the addition of some oxidizing agent and catalyst, along with the simultaneous
removal of the sulfur produced. The alcohol synthesis takes
place in the presence of a catalyst immediately after syngas
conditioning. The later stages of this process include alcohol
separation into ethanol and other higher alcohols produced.
Biochemical process: The first stage of this process
remains the same as that of the previous one which is biomass cultivation and further carrying it to the required
biorefinery facility. Since this process involves a biochemical route for ethanol production, the biomass undergoes
pretreatment which can be physical, biological, and chemical
depending on the structural complexity of the biomass. The
pretreatment is done to break down the complex structure of
the substrate into an easily hydrolyzable one consisting of
monomeric sugars. Pretreatment of the substrate is followed
by hydrolysis using calcium oxide, or acid, water, and steam
to form a fermentable mixture (selection of the fermentation
medium components may vary depending on the substrate)
which then undergoes fermentation and/or co-fermentation
using the chosen yeast or bacterial strain to then form a
fermentation broth containing ethanol and other by-products.
Ethanol is then usually recovered by steam distillation. The
lignin in the lignocellulosic biomass may, however, not
disintegrate effectively. It is therefore subjected to combustion resulting in heat and electricity production which in
turn are used during the conversion process.
7 Pretreatment Technologies Involved
in Biochemical Conversion Route
Food waste is one class of lignocellulosic biomass. The
structural composition of the lignocellulosic biomass is
mainly by cellulose, hemicellulose, lignin, and some amount
of pectin. These four components may be present in varied
fractions in different biomasses (Pérez et al. 2002). An
overview of the lignocellulosic structure of this class of
biomass (i.e. food waste) depicts that of all the components
present, the accessibility for hydrolysis by the cellulose
pretreatment techniques is hindered by the presence of
hemicellulose and lignin (Mcmillan 1994). Lignin separation
ensures the removal of non-productive adsorption sites. The
pretreatment techniques are intended for disruption of the
structural configuration of the components and thus removal
of any steric hindrance toward the accessibility of the cellulosic fibers (Fig. 2). Most commonly, acid (dilute acid at
high temperature and pressure or concentrated acid treatment
at low temperature and atmospheric pressure) and enzymatic
pretreatments (using an enzyme called cellulase) are adopted
(Kumar et al. 2009; Iranmahboob et al. 2002). The structural
breakdown of cellulose gives its glucose (hexose)
Bioconversion of Food Waste into Ethanol: A Review
51
their respective sizes and are broadly classified as a
macronutrients (carbohydrates, proteins, and lipids) and
micronutrients (sodium Na
+ , potassium K
+
, calcium Ca
2+ ,
magnesium Mg
2+ , to name a few). Some of the characteristics are as follows (Hegde et al. 2018):
• pH: The pH value of the biomass is suggestive of the
period over which it has been stored. The lesser the value,
the fresher is the biomass.
• Chemical oxygen demand (COD): It gives a quantitative
estimation of the organic compounds present in the biomass. Biomass COD values are proportional to the
nutritional values of the same.
• Total solids: The quantifiable constant weight solid
fraction produced by the heat treatment (at 100 °C) of the
waste.
• Ash content: The measurement of this parameter tells
about the inorganic fraction present in the biomass.
• Metal ions: They are the cofactors of enzymes involved in
alcohol synthesis. A balanced metal-ion requirement is
required for optimum cell growth (van Vliet et al. 2001).
• Carbohydrate content: Higher the carbohydrate content of
the substrate (food waste), higher will be the probability
toward conversion to alcohol.
• Lipid content: Lipid molecules with a high content of free
fatty acids (FFA) enhance tolerance limits of the fermenting organism (yeast) toward ethanol produced in the
fermentation broth (van Vliet et al. 2001).
• Protein content: Proteins are a source of nitrogen content
and need to be present in an optimum carbon to nitrogen
ratio. Protein limitation or excessive overloading can
affect the hydrolysis process and ultimately hinder substrate utilization (Hegde et al. 2018; Zhao 2008).
6 Comparison of Conversion Strategies
for Ethanol Production
Feedstock utilization for 2G bioethanol production is carried
out worldwide using collateral conversion routes, namely
thermochemical and biochemical (as mentioned in Sects. 3
and 4) routes. A detailed comparative study of both these
routes has been reported by Mu et al. (2010).
Thermochemical process: The entire process can be
broken down into 6–7 stages. It starts with feedstock production followed by an assemblage of the same and further
transportation to the thermochemical-conversion facility. At
the facility, the substrate undergoes initial drying using flue
gas produced from the gasification of the substrate in the
presence of steam, olivine, magnesium oxide, and air.
Gasification results in the synthesis of carbon monoxide,
hydrogen, and some sulfur content. The gaseous mixture is
then subjected to oxidation, with the addition of some oxidizing agent and catalyst, along with the simultaneous
removal of the sulfur produced. The alcohol synthesis takes
place in the presence of a catalyst immediately after syngas
conditioning. The later stages of this process include alcohol
separation into ethanol and other higher alcohols produced.
Biochemical process: The first stage of this process
remains the same as that of the previous one which is biomass cultivation and further carrying it to the required
biorefinery facility. Since this process involves a biochemical route for ethanol production, the biomass undergoes
pretreatment which can be physical, biological, and chemical
depending on the structural complexity of the biomass. The
pretreatment is done to break down the complex structure of
the substrate into an easily hydrolyzable one consisting of
monomeric sugars. Pretreatment of the substrate is followed
by hydrolysis using calcium oxide, or acid, water, and steam
to form a fermentable mixture (selection of the fermentation
medium components may vary depending on the substrate)
which then undergoes fermentation and/or co-fermentation
using the chosen yeast or bacterial strain to then form a
fermentation broth containing ethanol and other by-products.
Ethanol is then usually recovered by steam distillation. The
lignin in the lignocellulosic biomass may, however, not
disintegrate effectively. It is therefore subjected to combustion resulting in heat and electricity production which in
turn are used during the conversion process.
7 Pretreatment Technologies Involved
in Biochemical Conversion Route
Food waste is one class of lignocellulosic biomass. The
structural composition of the lignocellulosic biomass is
mainly by cellulose, hemicellulose, lignin, and some amount
of pectin. These four components may be present in varied
fractions in different biomasses (Pérez et al. 2002). An
overview of the lignocellulosic structure of this class of
biomass (i.e. food waste) depicts that of all the components
present, the accessibility for hydrolysis by the cellulose
pretreatment techniques is hindered by the presence of
hemicellulose and lignin (Mcmillan 1994). Lignin separation
ensures the removal of non-productive adsorption sites. The
pretreatment techniques are intended for disruption of the
structural configuration of the components and thus removal
of any steric hindrance toward the accessibility of the cellulosic fibers (Fig. 2). Most commonly, acid (dilute acid at
high temperature and pressure or concentrated acid treatment
at low temperature and atmospheric pressure) and enzymatic
pretreatments (using an enzyme called cellulase) are adopted
(Kumar et al. 2009; Iranmahboob et al. 2002). The structural
breakdown of cellulose gives its glucose (hexose)
Bioconversion of Food Waste into Ethanol: A Review
51