scientists. Even in the short term, bioplastics will have increasingly diverse highvalue applications. There is plenty of written material on bioplastics, including text
books. Therefore, here, the discussion is confined to the materials that are potentially
produced by alkaliphilic microbes. The term “bioplastic” is used here in the narrow
meaning for biodegradable bio-based plastics.
Even at the time when the general problems of plastics were not comprehended,
many proposals for bioplastics were presented. Polylactic acid is one of the most
intensively studied materials obtained from the precursor lactic acid by fermentation.
Several chemical ester polymerization routes are known. L(+)-Lactic acid can be
produced by alkaliphiles maintained at pH 8 during the sugar-lactate conversion with
excellent yield and optical purity. The high optical purity is advantageous for
properties of polylactic acid used for bioplastics [15]. Lactic acid fermentation is
profoundly studied in neutral and acidic media in industrial level for the use of lactic
acid in food industry. In food and feed industry, there are strict limitations on
microbial species that may be employed. However, for bioplastics production, the
microbial selection can be wider, and also nutrients of less value can be used.
Commercial bioplastics based on lactic acid are available for various purposes.
Their properties vary depending on the optical purity of the precursor and polymerization methods. Examples of alkaliphilic producers of optically pure L and
D-lactates were described in the first section above describing the sugar metabolism.
As stated, alkaliphilic microbes can produce high-quality lactates with very high
yields from sugar. Strain improvement and genetic engineering may be still
exploited with process optimization with cheap substrates to get lactic acid. Keeping
lactic acid as the only precursor of polymers may also be a too trivial approach.
There are other biological compounds which can be produced industrially (or be
side-products) and polymerized with chemical bonds which enzymes can break.
Polyhydroxyalkanoates (PHAs) are accumulated intracellularly as energy storage
compounds up to 10–75% of cell weight, especially under unbalanced nutritional
conditions [17, 18]. The polymers are formed from hydroxy acid monomers
(hydroxy butyrate, -valerate, etc.) by biological esterification. Due to the bifunctional structure of the monohydroxy acids, linear polymers are formed. The amount
of complex hydroxy acids (more hydroxy groups per molecule) in the structure
defines the grafting degree and increases the spectrum of properties of the PHAs.
PHAs, as a group, have a great variety of characteristics, and due to their sustainability, biodegradability, and biocompatibility, many industries are interested in
them, for example:
• Single use packaging for foods, beverages, and consumer products
• Medical applications like sutures, bone marrow scaffolds, and bone plates
• Agricultural foils and films
Some of PHAs exhibit features that are similar to those of the conventional
synthetic plastics but with a significantly lower environmental impact on the production process. However, many challenges have restrained PHAs’ applications,
including complicated genetic engineering processes for producing new PHAs and
inadequate mechanical strength. The high production cost and poor processability of
166
E. Khalikova et al.
books. Therefore, here, the discussion is confined to the materials that are potentially
produced by alkaliphilic microbes. The term “bioplastic” is used here in the narrow
meaning for biodegradable bio-based plastics.
Even at the time when the general problems of plastics were not comprehended,
many proposals for bioplastics were presented. Polylactic acid is one of the most
intensively studied materials obtained from the precursor lactic acid by fermentation.
Several chemical ester polymerization routes are known. L(+)-Lactic acid can be
produced by alkaliphiles maintained at pH 8 during the sugar-lactate conversion with
excellent yield and optical purity. The high optical purity is advantageous for
properties of polylactic acid used for bioplastics [15]. Lactic acid fermentation is
profoundly studied in neutral and acidic media in industrial level for the use of lactic
acid in food industry. In food and feed industry, there are strict limitations on
microbial species that may be employed. However, for bioplastics production, the
microbial selection can be wider, and also nutrients of less value can be used.
Commercial bioplastics based on lactic acid are available for various purposes.
Their properties vary depending on the optical purity of the precursor and polymerization methods. Examples of alkaliphilic producers of optically pure L and
D-lactates were described in the first section above describing the sugar metabolism.
As stated, alkaliphilic microbes can produce high-quality lactates with very high
yields from sugar. Strain improvement and genetic engineering may be still
exploited with process optimization with cheap substrates to get lactic acid. Keeping
lactic acid as the only precursor of polymers may also be a too trivial approach.
There are other biological compounds which can be produced industrially (or be
side-products) and polymerized with chemical bonds which enzymes can break.
Polyhydroxyalkanoates (PHAs) are accumulated intracellularly as energy storage
compounds up to 10–75% of cell weight, especially under unbalanced nutritional
conditions [17, 18]. The polymers are formed from hydroxy acid monomers
(hydroxy butyrate, -valerate, etc.) by biological esterification. Due to the bifunctional structure of the monohydroxy acids, linear polymers are formed. The amount
of complex hydroxy acids (more hydroxy groups per molecule) in the structure
defines the grafting degree and increases the spectrum of properties of the PHAs.
PHAs, as a group, have a great variety of characteristics, and due to their sustainability, biodegradability, and biocompatibility, many industries are interested in
them, for example:
• Single use packaging for foods, beverages, and consumer products
• Medical applications like sutures, bone marrow scaffolds, and bone plates
• Agricultural foils and films
Some of PHAs exhibit features that are similar to those of the conventional
synthetic plastics but with a significantly lower environmental impact on the production process. However, many challenges have restrained PHAs’ applications,
including complicated genetic engineering processes for producing new PHAs and
inadequate mechanical strength. The high production cost and poor processability of
166
E. Khalikova et al.