Biomedical Engineering Reference
In-Depth Information
Polylactic acid does not exist in nature. It is generally synthesized from lactic
acid. PLA is a synthetic straight-chain aliphatic polyester. The high molecular
weight of PLA can be obtained by polymerization of lactic acid cyclization or
direct polymerization of lactic acid. PLA has good biodegradability and good
biocompatibility and bioabsorbability; thus, it will not leave any environmental
problems when degraded. In the medical field, it has been considered to be the most
promising biodegradable polymer for making surgical suture materials, such as for
wound healing. Thus, its research and development are extremely active.
7.3.1.1
Microorganisms Producing Lactic Acid
Many microbes can produce lactic acid, and the most widely used in industry are the
strains of lactobacilli, which have a high lactic acid synthesis rate and yield. They are
not easily contaminated in the desired fermentation conditions (temperature higher
than 40
ı
C, low pH, low oxygen concentration, and high concentration of lactic
acid). The
Lactobacillus
spp. include those with obligatory homofermentation,
facultative heterofermentation, and obligatory homofermentation. The first two have
value for industrial production because these bacteria ferment on hexose, and the
main product is lactic acid. Benchmarks for breeding these two types of bacteria are
production of a stereoisomer of lactic acid; the type of carbohydrates utilized; and
complex nutrients (such as vitamins and amino acids). Fermentation by-products
and the tolerance level of lactic acid should also be considered. In short, the best
strains are able to take advantage of cheap raw materials, ferment quickly and fully,
and need fewer other nutrients. The best strains should also have such features as
production of a mass of specific three-dimensional lactic acid, small cell yield, and
almost no by-products.
7.3.1.2
Microbial Transformation of Lactic Acid
Conventional lactic acid fermentation is mainly based on starch as the raw material.
In recent years, lignocellulosic biomass has been used as raw material for lactic
fermentation, which saves food and reduces feedstock cost. So, it became the focus
of the study of lactic acid fermentation. Cellulose from straw as a raw material for
lactic acid fermentation is described in this section. Figure
7.2
shows the mechanism
of lactic acid production from cellulose materials.
Before lactic acid fermentation, cellulose raw materials should be pretreated by
physical or chemical methods, then the right amount of cellulase is added, and
hydrolyzation occurs at a certain temperature (usually 50
ı
C) for a certain time.
After that, the saccharification liquid can be obtained. Then, a certain amount (typi-
cally 10 % v/v) of bevel seed is added after activation to the saccharification liquid,
and culture occurs under the optimum growth temperature of the fermentation
bacteria.
