In this article we will discuss about:- 1. Introduction to Citric Acid 2. Chemical Structure of Citric Acid 3. Fermentation Process 4. Uses.
Introduction to Citric Acid:
Citric acid is formed as an intermediate in the Kreb’s cycle, but it is accumulated in greater quantities in the fungus Aspergillus niger, may be due to metabolic abnormality. This aspect of the fungus is being exploited for the fermentative production of citric acid. It is also found as a natural product in many fruits specially citrus fruits. Prior to the development of fermentation technology, citric acid was extracted from the juice of these fruits. Mexico contributes 1% of world production of citric acid.
Citric acid can also be produced from glycero, but it is very expensive. Though Aspergillus wentii, A. clavatus, Penicillium divaricatum, P. citrinum, P. luteum, Mucor pyriformis, Citromyces pleffencinus, Candida guilleirmondii, Saccharomyces lipolytica, Trichoderma viride, Arthrobacter paraffinicius and Corynebacterium spp. can produce citric acid, A. niger is employed extensively.
Citric acid production by surface fermentation was started in 1923, while deep fermentation in 1930. Citric acid produced in 1929 was 5000 tons, which has increased to 4.0 lakhs tons by 1992. Sixty percent of citric acid produced is used in food and beverage industry as a flavouring agent and preservative, while 10% in pharmaceutical industry in the form of iron citrate, about 25% of citric acid is used in chemical industry.
Chemical Structure of Citric Acid:
Structurally, citric acid is a hydroxypropane-1, 2, 3 tricarboxylic acid and is shown in Fig. 4.1.
Fermentation Process of Citric Acid:
Various fermentation processes used for the manufacture of citric acid are shown in Fig. 4.2.
The ratio of sugar consumed to amount of citric acid produced is normally observed in the ratio of 1:1.
Aspergillus niger is employed in most of the processes for the following reasons:
1. Can easily be cultivated.
2. Process uniform biochemical properties.
3. Produces only of small amount of oxalic acid under controlled conditions.
4. Yield large amount of citric acid.
Surface culture process employing solid medium and submerged culture process employing stirred reactor are briefly described here:
1. Surface Culture Process:
Though it is an old process, it is still employed. It is a kind of stationary fermentation process.
This process consists of four phases:
(i) Inoculum production,
(ii) Preparation of medium,
(iii) Fermentation process and
(iv) Harvest and recovery.
(i) Inoculum Production:
Spore suspension is used as inoculum for the production of citric acid. Suitable and high yielding strain of A. niger is selected from a stock culture. The stock culture is inoculated on to the surface of a sporulating medium present in glass bottles. The bottles are incubated for 10-14 days at 25°C.
The composition of trace elements like salts of manganese, zinc or iron in sporulating medium should be suitably maintained, otherwise they will affect the yield of citric acid in the actual fermentation. Suspension of spores is obtained by suspending the grown up spores in a suitable diluents such as water containing a wetting agent, sodium lauryl sulphate. Besides the total number, the viability of spore crop is critical.
(ii) Preparation of Medium:
The medium, used in the production of citric acid, should have carbohydrate source and inorganic salts. A variety of materials can be used as carbon source. But, generally sucrose and beet molasses are used as carbon source.
Sucrose is the best source of carbon among different organic substances tested. A medium with less than 15% sucrose is reported to give high yield of citric acid. Reduced yield of citric acid is observed when a part of sucrose was substituted by fructose or glucose. Commercially, beet molasses is also extensively used as carbon source in the production of citric acid employing A.niger.
Apart from sugars, beet molasses also contains excessive amounts of inorganic salts. To eliminate these excessive inorganic salts, it is treated with ferrocyanide or ferricyanide before it is employed in the medium preparation. Alternatively, the inorganic salts are removed by passing the beet molasses through a cation exchange resin.
The elements like nitrogen, potassium, phosphorus and magnesium are also needed in the medium, apart from carbon source. They are added in the form of ammonium nitrate, potassium dihydrogen phosphate or potassium monohydrogen phosphate and magnesium sulphate into the medium in minimum quantity as given in table 4.2.
The presence of these elements at higher concentration lowers the yield of citric acid and increases the yield of oxalic acid.
pH of the medium should be adjusted to 3.4-3.5 by using hydrochloric acid. Low pH is reported to be most favourable because it facilitates less contamination, formation of more citric acid, suppression of formation of oxalic acid and easy sterilization of the medium.
Currie (1917) reported a fermentation medium table 4.2 with the following composition for the production of citric acid as the most favourable. Salts and sugars are dissolved in one liter of distilled water. The medium is to be sterilized at 55-103 to 69-103 Nm-2 steam pressure per square inch for 30 min.
(iii) Fermentation Process:
The production medium is placed in shallow pans in such a way that a thin layer of medium with a depth of 1 to 2.5 cm is formed. The spores of inoculum are added to the medium to keep them floating on its surface. This is achieved by suitable modulating devices. Incubation is done in the incubation chambers at 30-40°C. Figure 4.3 shows the layout of the typical fermentation.
The temperature is kept constant at 30°C during the fermentation. Air current ventilation is also important for gas exchange because the rate of citric acid production falls if CO2 in the atmosphere increases to 10%. Within 24 hrs after inoculation the germinating spore form a thin layer of mycelium on the surface of the nutrient solution. As a result of the uptake of ammonium ions, the pH in the culture liquid fall to 1.5 to 2.0.
After 30 hrs of fermentation, if the iron concentration is more oxalic acid and yellowish pigment is formed which in turn hinders the recovery process. The fully developed mycelium floats as thick convoluted white layer on the liquid medium. The fermentation is stopped after 8-14 days.
The rate of bioconversion of sugar to citric acid depends on the ratio of surface area to the volume of the medium. There will be higher yield of citric acid if the ratio is lower. In this shallow pan method, the ratio of surface area to the volume of the medium is lower due to which large surface area of the mycelial mat is exposed to shallow layer of the medium.
Under these conditions more and more of sugar is converted into citric acid. That is why this process is considered to be superior to the submerged culture process. Yield per hour from this process amounts to 1.2 – 1.5 kg citric acid monohydrate per square metre of fermentation surface.
(iv) Harvest and Recovery:
The mycelium is separated from the fermentation broth. Any intracellular citric acid present in the mycelium is obtained by pressing the mycelium. The filtered broth is treated with calcium hydroxide. It is filtered and washed. It is then treated with equal volume of sulphuric acid to liberate citric acid. Calcium sulphate is formed as a precipitate in this process.
The precipitate is separated by filtration. An impure solution of citric acid is obtained which is decolorized by treating with activated carbon and also demineralized. Finally pure citric acid crystals are produced by evaporation. It is also recovered alternatively by counter current extraction method.
2. Submerged Culture Process:
In this method A. niger is made to grow uniformly dispersed throughout the liquid production medium. Fermentation is generally carried out in large fermenters having 4klt a capacity of thousands of gallons and are provided with mechanical agitator and sparges. Eighty percent of the world’s supply of citric acid is produced by this process. Cost of production decreases by 25% by this method. It involves low labour cost, longer incubation period, more energy consumption and sophisticated techniques.
Three factors are important for production in submerged culture process. They are quality of the metal used for the construction of fermenter, mycelium structure and oxygen supply. Candida lipolytica, an alkane utilizing fungus can also be employed in citric acid production under continuous fermentation. It yields, 45% higher than normal citric acid production.
(i) Inoculum Production:
Mycelial mats called pellets are used as inoculum for fermentation in this process. Suitable and high yielding strains of A. niger are selected from a stock culture. The spores are induced to germinate in a seed fermenter. A nutrient solution containing 15% sugar from molasses is used in this seed fermenter. To induce the formation of mycelial pellets, cyanide ions are added to the medium.
Pellet formation largely depends upon the concentration of cyanide ions in the medium. Lower yield of citric acid occurs if the cyanide ions are in less concentration. This is because lower concentration of cyanide ions induce formation of normal mycelium instead of pellets. The spores germinate at 32°C and form pellets of 0.2 — 0.5 mm diameter within 24 hrs. During this period the pH falls to 4.3. These pellets are then used as inoculum for production fermenters.
(ii) Preparation of Medium:
The medium, employed for surface culture process is also employed in this process.
(iii) Fermentation Process:
Mostly fermenters used for citric acid production are constructed in the range of 10 — 220 klt. They must be made of stainless steel to prevent leaching of heavy metals. Normal steel, if it is used in the construction of fermenters, at low pH level of 1-2 may inhibit the formation of citric acid. Small fermenters with a capacity upto 1000 lt should have plastic lining even though they are made of stainless steel because of large surface/volume ratio. However, such plastic lining is not necessary for large stainless steel fermenters.
The mycelial pellets developed in the seed tank are transferred aseptically to the fermenters and incubated at a constant temperature, 30°C. The structure of the mycelium that forms in the fermenter is vital to a successful production process. Little citric acid is produced if the mycelium is loose and filamentous with limited branches and no chlamydospores.
Optimal citric acid is formed if the mycelium is in the form of pellets. The ratio of iron to copper in the medium determines the nature of mycelium. In some cases, production fermenters are inoculated directly with spores.
Although A. niger requires relatively little oxygen, it is sensitive to oxygen deficiency. There must be minimum oxygen concentration of 20 to 25% of the saturation value throughout the fermentation process. Short interruptions in the oxygen supply ceases the production irreversibly. The aeration rate should be 0.2-1.0 volume per min. during the acid production phase. Due to low viscosity, stirring is not necessary. Thus, although some plants use stirred fermenters, airlift reactors can also be used.
Foaming is a problem in the submerged culture process. However, it can be controlled by adding antifoam agents such as lard oil at frequent intervals. A foam chamber, 1/3rd the size of the fermenter volume, is needed in both airlift and stirred bioreacters. Mechanical antifoam devices can also be used. Progress of the fermentation process is monitored regularly by calculating the content of sugar and citric acid in the fermentation.
Uses of Citric Acid in Industries:
Industrially, citric acid is used in the following ways:
1. It is used in the production of carbonated beverages.
2. As a chelating and sequestering agent in the tanning and textile industry.
3. Citrate esters are used as plasticizer.
4. It is abundantly used in food industry as an acidulent in the preparation of food items like jams, preserved fruits and fruit juices etc.
5. It is used in frozen foods to prevent its change in colour and flavour.
6. Metal painting industry
7. In pharmaceutical industry
8. In the manufacture of astringent, hair rinsers and hair setting fluids.
9. In beverage industry as a preservative to prevent oxidation of alcohol, emulsifier of dairy products like cheese and ice creams.
10. It is used as preservative and to prevent change in colour, flavour and in the oxidation of alcohol.