Microbial Lactic Acid Production from Renewable Resources - Sustainable Biotechnology: Sources of Renewable Energy - page 212

applications in food, chemical, textile, and pharmaceutical industries [1]. Recently,
there is an increasing demand for lactic acid for the manufacture of biodegradable
polymer, polylactic acid (PLA). The current worldwide demand for lactic acid is
estimated roughly to be 130,000-150,000 tons per year [2, 3].
Lactic acid can be produced by chemical synthesis or microbial fermentation
(Fig. 1). For the chemical synthesis, acetaldehyde and hydrogen cyanide are reacted
in the presence of a base under high pressure to produce lactonitrile. Distillation
is used to purify this crude lactonitrile. The purified lactonitrile is then hydrolyzed
with sulfuric acid to produce lactic acid. A byproduct of ammonium salt is also
produced [3, 4].
Feedstocks such as cheese whey and starchy and lignocellulosic biomass have
been studied for microbial lactic acid production in the past decades. When cheese
whey or simple sugars are used, lactose, glucose, or sucrose can be directly fer-
mented to lactic acid (Fig. 1b, route 3). The starchy biomass can be hydrolyzed to
glucose with enzymatic liquefaction and saccharification and then fermented to pro-
duce lactic acid (Fig. 1b, route 2). Starchy biomass can also be directly fermented to
lactic acid by amylolytic lactic acid bacteria. When lignocellulosic biomass is used,
a pretreatment process is required to break up the linkage among cellulose, hemi-
cellulose, and lignin. The cellulose and hemicellulose are hydrolyzed by cellulase
and hemicellulase to fermentable sugars. The obtained sugars can then be fermented
with the appropriate microorganism to produce optically pure L (+) or D (-) lactic
acid (Fig. 1b, route 1). Simultaneous saccharification and fermentation (SSF) can
also be applied to allow the enzymatic hydrolysis and fermentation processes to be
conducted in the same reactor. Separation and purification of lactic acid is required
for microbial fermentation process.
Fig. 1 Lactic acid production processes