In this article we will discuss about the commercial production of various types of organic acid. The types of organic acid whose commercial production has been discussed are: 1. Lactic Acid 2. Gluconic Acid 3. Citric Acid.
Organic Acid: Type # 1. Lactic Acid:
Lactic acid is α-hydroxypropionic acid (CH3.CHOH.COOH) and is commonly produced from the usual cheap sources of fermentable carbohydrate, such as, corn and potato starch, molasses, and whey. Whey, the watery part of milk separated from curd during cheese manufacture, is widely used for the commercial production of lactic acid.
Whey represents a satisfactory medium for the growth of certain bacteria because it contains carbohydrate (lactose), nitrogenous substances including vitamins and salts. Lactobacillus bulgaricus, L. delbrueckii, etc. grow rapidly in whey and convert its carbohydrate (lactose) to the single end product, the lactic acid.
Reaction:
The biochemical reactions performed by the fermentative bacteria, the lactobacilli, in production of lactic acid can be represented as follows:
Method of Commercial Production (Fig. 40.3):
Pasteurized whey is inoculated with a starter culture containing lactobacilli, e.g., Lactobacillus bulgaricus and L. delbrueckii. To prepare a sufficient amount of inoculum for addition to main fermentation tank, the culture is successively transferred in sterile skim milk, pasteurized skim milk, and finally whey.
The inoculum from whey is now added to the main fermentation tank containing large amount of whey. The temperature of the main fermentation tank is maintained at 43-50°C to prevent the growth of many extraneous microorganisms.
During the fermentation, a slurry of lime [calcium hydroxide; Ca (OH)2] is added intermittantly to prevent the accumulation of acid otherwise the latter would retard fermentation.
When the fermentation is completed (in about 2-4 days), the fermented liquid is boiled at about 82°C to allow the coagulation of protein, the lactalbumin, which is then filtered and processed for use as animal-feed supplement.
The filtrate containing the calcium lactate is spray-dried after treating with sodium sulfide. To obtain lactic acid, the calcium lactate is treated with sulphuric acid and the lactic acid thus obtained is further purified.
Uses:
(i) It is used as an acidulent in fruit juices, confectionery and essences.
(ii) Derivatives of lactic acid such as calcium lactate and iron lactate are used in the treatment of calcium deficiency and anemia respectively.
(iii) Sodium lactate is used to help retention of moisture by such products as tobacco and as a platicizer.
(iv) Lactic acid production from whey also helps removal of pollution of our environment because untreated whey disposed off in our waterways would cause dangerous consequences. It is so because whey is very high in Biological Oxygen Demand (BOD).
Organic Acid: Type # 2. Gluconic Acid:
Gluconic acid is produced commercially by glucose fermentation with the help of strains of Aspergillus niger, Penicillium sp., or selected bacteria.
Method of Commercial Production:
A nutrient solution containing 25-40% glucose, corn steep liquor, a nitrogen source and salts is dispensed into either shallow pans (aerobic condition) or large tanks (submerged-culture condition) like in citric acid production. Now a selected strain of the mould is added to the pan or tank solution.
Fermentation starts and is carried out at 33-34°C. The pH is maintained at about 4.5 and is controlled by the addition of a strong sodium hydroxide solution. After the fermentation is completed, the fermented broth is centrifuged and processed to obtain gluconic acid.
Precaution:
The medium compositions and fermentation conditions determine the production of other acids, e.g., citric acid and oxalic acid. It is important, therefore, to select the proper mould-strain and fermentation conditions so that the production of unwarranted organic acid in the solution could be avoided.
Uses:
(i) It is used in pharmaceutical industries, dairy industry, photography, and leather and textile printings.
(ii) Gluconic acid is also used in the manufacture of toothpaste.
Organic Acid: Type # 3. Citric Acid:
Citric acid was originally produced from lemons by an Italian cartel, the discovery of its accumulation by Aspergillus niger (then named citromyces) in the early 1920s led to a rapid development of a fermentation process which, 15 years later, accounted for more than 95% of the world’s production of citric acid.
Today, citric acid is produced in bulk amounts with an estimated worldwide production of 400,000 tonnes per year, most of which is produced by fermentation with the fungus Aspergillus niger.
Commercial Production:
Two different types of fermentation processes are basically used for commercial production of citric acid, e.g., the surface fermentations and the submerged fermentations. In addition, some citric acid is also produced by solid state fermentations, particularly in less developed rural areas such as some East Asian countries. Citric acid production by yeast is exclusively done by submerged cultivation.
Surface fermentation (Fig. 40.4) is the older and more labour intensive version of citric acid fermentation, yet it is still in use, even by some major producers of citric acid. The main reasons for this are the lower power requirements and the higher reproducibility of the process due to its lower susceptibility to interference by trace metal ions and variations in the dissolved O2 tension.
Their fermentation is usually carried out in aluminium trays (2 m x 2.5 m), filled with nutrient medium to a depth of between 50 and 200 cm. Molasses solution containing inorganic nitrogenous salts is used as fermentation medium.
The pH is maintained at about 2. The fermentation medium is first freed of trace elements (e.g., Mn++, Zn++, Fe++, phosphates) through ion exchange resin because the trace elements exert inhibition of citric acid formation above a critical level.
Following this, the requisite concentrations of metal ions are added as salts in the medium. Spores are now distributed over the surface of the trays, and sterile air (serving both as an oxygen supply as well as a cooling aid) is passed over them. The mycelium develops as a coherent felt, becoming progressively more convoluted. The fermentation is completed within a period of 7 to 12 days at 30°C.
The culture fluid is taken out from the bottom of the trays, leaving the mycelial mat of A. niger more or less undisturbed. Citric acid is recovered from the fluid by precipitation. Fresh medium is added into the trays for the next batch of fermentation.
The submerged fermentation process (Fig. 40.5) is desirable because of its higher efficacy due to higher susceptibility to automatisation. Yet the severe influence of trace metal ions and other impurities present in the carbohydrate raw materials and its disturbance by variations in O2 supply make it more difficult to manage particularly because the quality of the carbohydrate source is variable.
There are two types of fermentors in use:
i. Stirred tanks and
ii. Aerated tower fermentors.
Both types are constructed of high-grade stainless steel and contain facilities for cooling. Sparging with O2 occurs from the base.
One of the most prominent features of submerged fermentation is the mycelial development which shows a characteristic pattern: the germinating spores form stubby, forked and bulbous hyphae, which aggregate to small (0.2-0.5 mm) pellets having a firm, smooth surface, and sediment quickly when harvested.
This striking morphology has been shown to be critical for attaining high yields by submerged fermentation and is dependent on an appropriate nutrient composition. It is therefore a convenient indicator for the progress of fermentation, e.g., by procedures involving microscopy. A final yield of 0.8-0.9 kg kg-1 is obtained after 7 to 10 days.
Uses:
Due to its pleasant taste, low toxicity and excellent palatability, citric acid is widely used in industry for the preparation of food and sugar confectionery (21% of total production) and beverages (45%). Other major applications are in the pharmaceutical and detergent/cleaning industry (8 and 19%, respectively).
It is also able to complex heavy metal ions, such as iron and copper, and therefore is applied in the stabilisation of oils and fats or ascorbic acid against metal ion-catalysed oxidation. In addition, citric acid esters of a wide range of alcohols are known and can be employed as non-toxic plasticisers.
Finally, some of its salts have commercial importance, e.g., trisodium citrate as a blood preservative which prevents blood clotting by complexing calcium, or as a stabiliser of emulsions in the manufacture of cheese.