Citric acid used in the food industry, especially in processing, is generally dissolved beforehand. For such applications the use of the expensive
crystalline citric acid can be avoided. Recently, 50% solution of citric acid, both technical grade as well as for food processing, has been introduced in the international market.
Citric acid is used in food processing, pharmaceuticals and industrial usage like plasticizers in plastics industry, detergents, metal cleaning, textile
printing and dyeing, photography, sulfurdioxide absorption and others
Usage of Citric Acid in various Sectors Globally:
Food, beverages and confectionery 70%
Industrial applications 18%
Citric acid can be manufactured by three different kind of processes:
(a) Extraction from citrus fruits
In case of extraction, yields are low and costs of production high. It is uneconomical and rarely used. At present about 7% of world production is
by extraction. Technically, it is quite feasible to produce citric acid by purely chemical reactions with no elements of byconversion.- However, the present level of production costs practically rules out this method of producing citric acid. All the synthetic methods proposed have eventually turned out to be expensive, hazardous and low yielding.
Fermentation continues to be the predominant way of producing citric acid and accounts for over 90% of the world production. Fermentation is obviously a complex process because several reactions are sandwitched into one single bio-reaction, for which operating conditions have to be tailored precisely and controlled severely. Variations in results often occur even under apparently similar operating conditions. All plants prefer to rely upon their own experience rather than the socalled 'proven' facts.
Presently, there are three different types of fermentation processes in use. These are:
(a) Surface Fermentation
(b) Submerged Fermentation
(c) Solid state Fermentation.
Surface and submerged fermentation processes are used quite extensively, while solid state process is still being developed, particularly in Japan.
Despite fermentation being a relatively old technology, interestingly, advances in Science and Technology have kept it in the forefront - the modern
molecular biology, or the biotechnology as it is known, has facilitated precise failuring of microbes for specific bioconversion tasks.
The surface and submerged processes continue to co-exist though the submerged process has steadily increased its share. The surface process still
accounts for a substantial part of fermentation capacity worldwide. From yield (obtainable) point of view, surface and submerged processes are rated
equally. The yield depends upon several factors, such as the origin of the molasses, the chemical treatment, the height of the substrate in the trays,
activity of the micro-organism and the conditioning of the air.
From a practical point of view, pure sugars such as glucose and sucrose are generally too expensive to use industrially. The raw materials can be
classified into two groups:
(i) Raw materials with low ash content like corn sugar, cane or beet sugar, and refined starch hydrolysates yielding very pure dextrose qualities.
(ii) Raw materials with high ash content and high amounts of other non-sugar substances, such as cane and beet molasses, high test molasses etc.
Sugarcane juice is in fact, a better raw material for citric acid production by fermentation, compared to beet or cane molasses, particularly for large plants
with capacities of over 10,000 tonnes of citric acid per annum. Molasses is one of the most widely used sources of sugar, for production of
citric acid. Being a by-product of sugar refining, it is relatively cheap. There are several kinds of molasses obtained as by-products of different types of
sugar production, such as beet, cane etc. They vary widely in quality and composition, depending upon the stage of sugar production at which molas-
ses have been taken out. Composition also varies from season to season and factory to factory. In Europe and North America, beet molasses is used
widely for citric acid production.
Eversince Curie's discovery of A. Niger in the second decade of the current century, A. Niger has been used most extensively for manufacturing citric
acid. However, it is now recognized that many species of yeasts also accumulate citric acid in their growth, albeit alongwith relatively large
amounts of iso-citric acid. Fermentation by A. Niger under surface or submerged conditions continues to be the predominant process even today.
The fermented liquid contains, besides citric acid, several unwanted materi- als, including oxalic acid, residual sugars and mycelium extraction from the
fermented liquid is, therefore, a complex process, carried out in several. stages.
There are three different processes that can be followed for recovering citric acid from the fermented liquor. These are:
(i) precipitation (classical process)
(ii) ion exchange
(iii) solvent extraction.
Precipitation is the most commonly used method. As it is more economical to remove calcium citrate by lime precipitation than it is by ion-exchange
treatment, the ion exchange method may be used for treating lime juice rather than filtered acid juice.
Solvent extraction is a possible alternative to the classical method but because the available solvents tend to extract some of the impurities too, it is easier to apply to products from glucose or alkane based substrates. The advantage of this process is that it avoids the use of lime and sulfuric acid and the concomitant problem of gypsum disposal.
Present world capacity is about 4,00,000 tonnes out of which USA has 1,70,000 tonnes and West Europe 1,00,000 tonnes. Some 30 countries produce citric acid.
Pfizer and Miles Laboratories are the majors, together accounting for 40% of world capacity. Both have plants in several countries - directly or through
subsidiaries. Japan has no large production. Small quantities are being produced in Japan by Solid State Fermentation. East Europe's production is
small. USSR produces about 20,000 to 25,000 tonnes yearly. Other sugarcane producing countries are Brazil, Cuba, Pakistan, China, Mexico etc., India being the largest producer.
Internationally, the process control systems are well advanced and auto- mated. Computerised control systems are very common. But there is no basic
change since the fifties in the process technology; raw material, micro- organism remain unaltered. Process cycle time has also not reduced. While
West Germany and East European countries use surface fermentation process, elsewhere submerged process is employed. Present research efforts are directed at finding out new micro-organisms, a continuous process (against the present batch process) and search for cost reduction.