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1
Department of Food Engineering, Faculty of Engineering and Architecture, Abant Izzet Baysal University, Golkoy
14280, Bolu, Turkey.
2
Department of Chemical Engineering, Faculty of Engineering, Hacettepe University, Beytepe 06532, Ankara, Turkey.
3
Department of Food Engineering, Faculty of Engineering, Hacettepe University, Beytepe 06532, Ankara, Turkey.
*Corresponding author: e/mail: yesim@hacettepe.edu.tr, Phone: +90 312 297 71 12, Fax: +90 312 299 21 23
Citric acid is the most important organic acid produced by fermentation, widely used in food, pharmaceutical and chemical
industries. Although is the traditional producer of citric acid, during the last 30 years the interest of
researchers has been attracted by the use of yeasts for citric acid fermentation processes. Among the yeast species,
is known as a potential producer of citric acid. Environmental factors that have been shown to exert an
effect on citric acid production are the type and concentration of carbon source of the fermentation medium, nitrogen and
phosphate limitations, aeration, trace elements, initial pH and temperature. Besides the regulation of product formation by
environmental conditions, strain selection and improvement has become the important factor. The improvement of citric
acid producing yeast strains has been carried out by mutagenesis and selection. Because of annual growths in demand of
citric acid, using alternative processes and strains for its production are in progress.
& citric acid; yeasts; ; fermentation parameters
Citric acid is a commercially valuable organic acid, widely used in food, pharmaceutical and beverage industries [1]. It
is the main additive used in the food industry. Citric acid is widely used to impart a pleasant, tart flavour to foods and
beverages. It also contributes to the formulation of many foods as an acidulant, antioxidant, emulsifier or preservative
[2, 3]. Among the uses of citric acid, about 70% is used in the food industry, and 10% in the cosmetics and
pharmaceuticals [4]. There is a great worldwide demand for citric acid consumption due to its low toxicity when
compared with other acidulants [3]. It is reported that the supply of natural citric acid is very limited and the demand
can only be satisfied by biotechnological fermentation processes. Citric acid is known as the most important organic
acid produced in tonnage by fermentation and is the most exploited biochemical product [3, 5]. The annual production
of citric acid was reported as 700 thousand tons in 1993 [6], 1.4 million tons in 2004 [3], and 1.6 million tons in 2008
[7, 8] .
A large number of microorganisms have been employed for citric acid production, but a few of them can produce
citric acid in industrial scale [3]. It is reported that is almost exclusively used for industrial scale
production of citric acid [5], but during the last 30 years the interest of researchers has been attracted by the use of
yeasts as citric acid producers [9]. A number of different strains, mostly belonging to the () genus
have been used for citric acid production, mainly in conventional batch processes, but also in continuous culture, and
with immobilized cells [10]. The yeast species which were reported to produce citric acid are; ()
, ,
, , , , and some species [6, 11].
Among the yeast species, is known as a potential producer of citric acid [1, 12] and has been
developed as a microbial cell factory for citric acid production in recent years [7]. The main advantages of using yeasts
are mentioned as follows: Yeasts are characterized by greater resistance to high substrate concentrations than fungi,
with comparable conversion rates and have greater tolerance to metal ions that allows the use of less refined substrates.
Using yeasts also gives a better process control due to their unicellular nature [13, 14]. It was reported that citric acid
production by yeast could be in the future an alternative to one, especially if the yeast biomass became an
additive to animal food and not a by/product [15]. However, the major disadvantage of using yeasts is the simultaneous
production of citric and isocitric acids [13, 16]. It is reported that the ratio of citric:isocitric acid can vary between 1:1 to
20:1 according to the yeast strain, carbon source and micronutrient concentration [13]. Selection of a yeast strain with
high citric acid production and giving high citric acid:isocitric acid ratios has been reported as the principal step of a
citric acid production process.
It is reported that more than 90% of the citric acid produced in the world is obtained by fermentation. The industrial
citric acid production can be carried out in three different ways: by submerged fermentation, surface fermentation and
solid/state fermentation or “Koji” process [3]. It is estimated that about 80% of the world citric acid production is
obtained by submerged fermentation in stirred tanks of 40/200 m
3
or larger airlift fermentors of 200/900 m
3
capacity
[17]. Submerged fermentation can be carried out in batch, fed batch or continuous systems, although the batch mode is
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