tion are on the way to make budding yeast the
ideal microbial cell factory for the production
of biobutanol.
C. Lactic Acid
Lactic acid is a versatile organic acid used in
food, cosmetic, pharmaceutical, and plastic
industries, in the latter serving as building
block for polylactic acid (PLA), a biodegradable and biocompatible polymer. The increasing global lactic acid market is projected to
reach 1.6 million tons in 2024 with a revenue
of US$3.7 billion (Global Industry Analysts,
Inc. 2017). Currently, lactic acid bacteria and
different yeast genera are most widely used
for the industrial production of lactic acid,
although, since the advent of modern metabolic
engineering tools, yeast-based production processes became increasingly important (Sauer
et al. 2010). Besides Baker’s yeast, also other
yeast species are currently being developed as
host for lactic acid production. While many
bacteria require media supplementation with
peptides, amino acids, nucleotides, or vitamins,
most yeasts can grow on simple mineral
medium (maybe with the addition of few vitamins). Therefore, the nutrient and downstream
purification costs are significantly lower as
compared to processes using bacteria. Moreover, yeast cells are more tolerant to low pH
reducing the addition of neutralizing agents
during the fermentation process. In addition,
the low pH of the process greatly decreases the
risk of bacterial contaminations or bacteriophage infections. However, yeast cells do not
produce lactic acid naturally; thus, metabolic
engineering is required. Converting pyruvate
to lactic acid can be achieved by introduction
of a lactate dehydrogenase (e.g., from lactic acid
bacteria). To avoid any carbon loss, the competing ethanol fermentation pathway must be
shut down, e.g., by deleting the genomic pyruvate decarboxylase genes. Furthermore, adaptive laboratory evolution, random mutagenesis,
and process engineering are applied in academic and industrial biotechnology to further
optimize lactic acid production in yeast (Miller
et al. 2011; Adachi et al. 1998; Baek et al. 2016;
Rajgarhia et al. 2007; Porro et al. 1999).
D. Citrate
Citric acid is one of the most important organic
acids in biotechnology with over 2 million tons
produced per year. Around 75% is used as
acidity regulator and flavor enhancer in beverages and food products. Furthermore, due to
its broad range of applications, it is widely
applied in the chemical, cosmetic, pharmaceutical, and agricultural industries (IHS Markit
2015). Naturally, citric acid is found in high
concentrations in a variety of fruits and vegetables. Moreover, it is an important metabolic
intermediate in the TCA cycle and thus present
in all aerobic organisms. Industrial production
of citric acid began at the end of the nineteenth
century by extraction from concentrated lemon
juice. In the early twentieth century, the filamentous fungus Aspergillus niger was identified
as natural citric acid producer (Currie 1917;
Cavallo et al. 2017). From then on, fermentation
of A. niger was developed and optimized and is,
to date, the preferred production route of
numerous companies around the world (Ciriminna et al. 2017). To further lower the production costs, research in the last two decades
focused on identifying and engineering other
citric acid producers, such as the yeast Yarrowia lipolytica. This yeast species has many
advantages over filamentous fungi: it can
metabolize a wide range of substrates and it is
resistant to high substrate concentrations and
contaminants such as metal ions (Cavallo et al.
2017). One low-cost substrate that can be utilized by Y. lipolytica is glycerol, an inexpensive
by-product of the prospering biodiesel economy. Recent studies proved that optimizing
strain development, fermentation processes
and the utilization of waste products as substrates can bring environmental and economical benefits in the near future (Hu et al. 2019;
Egermeier et al. 2017; Khanna et al. 2012;
Almeida et al. 2012).
13 Yeast Cell Factories
323
ideal microbial cell factory for the production
of biobutanol.
C. Lactic Acid
Lactic acid is a versatile organic acid used in
food, cosmetic, pharmaceutical, and plastic
industries, in the latter serving as building
block for polylactic acid (PLA), a biodegradable and biocompatible polymer. The increasing global lactic acid market is projected to
reach 1.6 million tons in 2024 with a revenue
of US$3.7 billion (Global Industry Analysts,
Inc. 2017). Currently, lactic acid bacteria and
different yeast genera are most widely used
for the industrial production of lactic acid,
although, since the advent of modern metabolic
engineering tools, yeast-based production processes became increasingly important (Sauer
et al. 2010). Besides Baker’s yeast, also other
yeast species are currently being developed as
host for lactic acid production. While many
bacteria require media supplementation with
peptides, amino acids, nucleotides, or vitamins,
most yeasts can grow on simple mineral
medium (maybe with the addition of few vitamins). Therefore, the nutrient and downstream
purification costs are significantly lower as
compared to processes using bacteria. Moreover, yeast cells are more tolerant to low pH
reducing the addition of neutralizing agents
during the fermentation process. In addition,
the low pH of the process greatly decreases the
risk of bacterial contaminations or bacteriophage infections. However, yeast cells do not
produce lactic acid naturally; thus, metabolic
engineering is required. Converting pyruvate
to lactic acid can be achieved by introduction
of a lactate dehydrogenase (e.g., from lactic acid
bacteria). To avoid any carbon loss, the competing ethanol fermentation pathway must be
shut down, e.g., by deleting the genomic pyruvate decarboxylase genes. Furthermore, adaptive laboratory evolution, random mutagenesis,
and process engineering are applied in academic and industrial biotechnology to further
optimize lactic acid production in yeast (Miller
et al. 2011; Adachi et al. 1998; Baek et al. 2016;
Rajgarhia et al. 2007; Porro et al. 1999).
D. Citrate
Citric acid is one of the most important organic
acids in biotechnology with over 2 million tons
produced per year. Around 75% is used as
acidity regulator and flavor enhancer in beverages and food products. Furthermore, due to
its broad range of applications, it is widely
applied in the chemical, cosmetic, pharmaceutical, and agricultural industries (IHS Markit
2015). Naturally, citric acid is found in high
concentrations in a variety of fruits and vegetables. Moreover, it is an important metabolic
intermediate in the TCA cycle and thus present
in all aerobic organisms. Industrial production
of citric acid began at the end of the nineteenth
century by extraction from concentrated lemon
juice. In the early twentieth century, the filamentous fungus Aspergillus niger was identified
as natural citric acid producer (Currie 1917;
Cavallo et al. 2017). From then on, fermentation
of A. niger was developed and optimized and is,
to date, the preferred production route of
numerous companies around the world (Ciriminna et al. 2017). To further lower the production costs, research in the last two decades
focused on identifying and engineering other
citric acid producers, such as the yeast Yarrowia lipolytica. This yeast species has many
advantages over filamentous fungi: it can
metabolize a wide range of substrates and it is
resistant to high substrate concentrations and
contaminants such as metal ions (Cavallo et al.
2017). One low-cost substrate that can be utilized by Y. lipolytica is glycerol, an inexpensive
by-product of the prospering biodiesel economy. Recent studies proved that optimizing
strain development, fermentation processes
and the utilization of waste products as substrates can bring environmental and economical benefits in the near future (Hu et al. 2019;
Egermeier et al. 2017; Khanna et al. 2012;
Almeida et al. 2012).
13 Yeast Cell Factories
323