Question

Raw materials, chemical reaction and process description of submerged fermentation process for citric acid

Answer 1

INTRODUCTION
Citric acid (2-hydroxy-propane-1,2,3-
tricarboxylic acid) derives its name from
the Latin word citrus , a tree whose fruit
is like the lemon. Citric acid is a
tricarboxylic acid (Figure 1 ) with a
molecular weight of 210.14 g/mol, which
contains three carboxylic functional
groups with three different values of
pK (3.1, 4.7, and 6.4). It is a primary
metabolic product formed in the
tricarboxylic acid (or Krebs) cycle and is
found in small quantities in virtually all
plants and animals, being isolated from
lemon juice in 1784.
Figure 1
Chemical structure of citric acid.
Citric acid was first commercially-
produced in England around 1826 from
imported Italian lemons (lemons contain
7–9% citric acid). Lemon juice remained
the commercial source of citric acid until
1919, when the first industrial process
using Aspergillus niger started in
Belgium. Currently, the extraction of
citric acid is limited to some small
factories in Mexico and Africa.
Citric acid was synthesized from glycerol
by Grimoux and Adams (21 ) and later
from symmetrical dicloroacetone. Other
routes have been published from various
synthetic materials since then, but
chemical methods have so far proved
uncompetitive.
Wehmer (106 ) was the first to
demonstrate that Citromyces (now
Penicillium ) accumulated citric acid in a
medium containing sugar and inorganic
salts. Since then, many organisms have
been found to accumulate citric acid: A.
niger, Aspergillus awamori, Aspergillus
nidulans , Aspergillus fonsecaeus ,
Aspergillus luchensis , Aspergillus
phoenicus , Aspergillus wentii , Aspergillus
saitoi , Aspergillus flavus, Absidia sp.,
Acremonium sp., Botrytis sp.,
Eupenicillium sp., Mucor piriformis ,
Penicillium janthinellum , Penicillium
restrictum , Talaromyces sp., Trichoderma
viride and Ustulina vulgaris (73 ).
Currie (15 ) found that some strains of A.
niger were able to grow in a medium
containing sugars and salts at an initial
pH of 2.5–3.5. Throughout their growth,
these strains excreted large amounts of
citric acid, which established the basis
for industrial production.
Besides fungi, it is known that several
yeasts produce citric acid from n-alkanes
and carbohydrates (63 ), especially
species belonging to the genera Candida ,
Hansenula, Pichia , Debaromyces, Torula,
Torulopsis , Kloekera , Saccharomyces,
Zygosaccharomyces and Yarrowia .
During the '60s and '70s oil was cheap,
and citric acid was produced industrially
from this source by Candida sp.,
including C. tropicalis , C. catenula , C.
guilliermondii and C. intermediate (73 ).
Today, this production is not economical.
As a disadvantage, the fermentation by
yeasts led to the formation of large
quantities of isocitric acid as an
unwanted byproduct, so mutant strains
with low aconitase activity should be
used.
Although many microorganisms can be
employed to produce citric acid, A. niger
is still the main industrial producer. In
fact, specific strains that are able to
overproduce citric acid in different types
of fermentation processes have been
developed. The theoretical yield is 112 g
of anhydrous citric acid per 100 g of
sucrose. However, in practice, due to
losses during trophophase, the yield of
citric acid from these strains often does
not exceed 70% of the theoretical yield
on carbon source. Despite a long and
successful history of producing citric
acid, there is not unanimous explanation
of the biochemical basis of the process.
FACTORS AFFECTING CITRIC ACID
FERMENTATION
The conditions for citric acid
fermentation were established during
the '30s and '40s when the effects of
various components of the fermentation
media were evaluated.
The accumulation of citric acid is
strongly influenced by the composition
of the medium, especially in submerged
fermentation processes. However, with
the exception of early studies by Currie
(15 ), there were no other systematic
studies on the composition of the
medium until the 40s (92 , 93 ). These
authors developed a medium that was
the basis for further research on the
production of citric acid. It was shown
that the factors mainly affecting the
citric fermentation are the type and
concentration of carbon source, nitrogen
and phosphate limitation, pH, aeration,
oligoelements concentration, and
morphology of the producing
microorganism. Certain nutrients have
to be in excess (such as sugars, protons
or oxygen), other at limiting levels (such
as nitrogen and phosphate) and others
below well-established threshold values
(such as trace metals, particularly
manganese).
Carbon source
The carbon source for citric
fermentation has been the subject of
many studies, especially regarding the
use of polysaccharides. In general, only
the sugars that are quickly assimilated by
the microorganism allow high final yield
of citric acid (62 ).. Cations usually
come from insoluble r ).