ABSTRACT
References ........................................................................................................................395
15.1 INTRODUCTION
Citric acid (2-hydroxy-1,2,3-propanetricarboxylic acid), a tricarboxylic acid, has the following structure:
1 C H2 C OOH
2 OH-– C –– C OOH
3 C H2 C OOH
It is naturally found in fruits such as lemon, orange, pineapple, plum, and pear; in the seeds of different vegetables; and in animal bone, muscle, and blood (1,2). Citric acid was first commercially obtained from lemon juice, crystallized in 1784 by Carl Wilhelm Scheele, a Swedish chemist (2,3). Citric acid produced from fruits is known as “natural citric acid,” in contrast to that produced by microorganisms (3). Although citric acid is currently being chemically synthesized, there is no chemical method that is superior to microbial fermentation. In 1893 Wehmer investigated citric acid production from sucrose with strains of Mucor and Penicillium (1). This experiment was not successful and was abandoned in 1903. Difficulties included the selection of proper microorganisms, microbial degeneration, contamination, long fermentation time, high plant construction cost, and an insufficient spread between raw material cost and the price of finished product (4). Wehmer reported that until 1910 no successful citric acid fermentation process was carried out. Until 1920 all commercially produced citric acid came from lemon or lime juice (5). In 1917 Currie described the first production of citric acid with Aspergillus niger. He reported that different strains of A. niger, grown in low pH medium supplemented with 15% (w/v) sucrose plus nutrients, converted 55% of the sugar to citric acid (4). Citric acid was isolated from the fermentation broth as insoluble calcium citrate by adding calcium hydroxide, followed by regeneration of the acid using sulfuric acid. Citric acid crystallizes into two forms: anhydrous and monohydrous. The anhydrous form is produced at a temperature exceeding 36.6°C while the monohydrate is produced at temperatures of up to 36.0°C (1,5). By 1930 a number of manufacturers used the submerged A. niger fermentation process for the production of citric acid from sucrose (5,6). In 1952, America Miles Company (Miles Laboratories, Inc., Elkhart, IN) used the submerged fermentation process in a pilot program on a plant scale to produce citric acid. Little information exists about the processes they used. From the results claimed in their patents, however, it is clear that they used pretreated molasses with a cation exchanger as a substrate deficient in phosphate, iron, and manganese
(3,4). After 1952, many countries used glucose or beet and cane molasses as substrates to produce citric acid. Among the microorganisms producing citric acid, strains of A. niger have been used, but more recently, some yeast strains (Yarrowia, Hansenula, Pichia, Torula) have also been employed. These strains, which were isolated from soil or plant material, were further improved by mutation (7). Recently, different strains of yeasts have been used instead of A. niger for the production of citric acid (8-11). Today, most citric acid used in food and other industries is produced by microbial fermentation.
