Microorganisms in the production of biochemicals
Many products of microbial metabolism find an application in the food and other industries. These include amino acids, steroids, enzymes and antibiotics (Table 17.4). Microbial growth conditions are adjusted so that production of the metabolite in ques-tion takes place at an optimal rate. Often an unnaturally high rate of production is achieved by the use of a mutated or genetically engineered strain of microorganism, or by manipulating culture conditions to favour excess metabolite production.
The development of a microbial means of producing acetone was vital to the allied effort in the First World War. Acetone was a crucial precursor in explosives manufacture and the demands of war soon outstripped supply by traditional methods. The problem was solved when Chaim Weismann isolated a strain of Clostridium acetobutylicum that could ferment molasses to acetone and butanol (another industrially useful product). Nowadays, acetone is made more cheaply from petrochemicals.
Microbially produced amino acids are used in the food industry, in medicine and as raw materials in the chemical industry. The one produced in the greatest quantities by far is glutamic acid (in excess of half a billion tonnes per year), with most of it ending up as the flavour enhancer monosodium glutamate. The amino acids aspartic acid and phenylalanine are components of the artificial sweetener aspartame and are also synthesised on a large scale.
A number of organic acids are produced industrially by microbial means, most no-tably citric acid, which has a wide range of applications in the food and pharmaceutical industries. This is mostly produced as a secondary metabolite by the large-scale culture of the mould Aspergillus niger.
Certain microorganisms serve as a ready source of vitamins. In many cases these can be synthesised less expensively by chemical means; however, riboflavin (by the mould Ashbya gossypii) and vitamin B12 (by the bacteria Propionibacterium shermanii and Pseudomonas denitrificans) are produced by large-scale microbial fermentation. Microorganisms play a partial role in the production of ascorbic acid (vitamin C). Initially, glucose is reduced chemically to sorbitol, which is then oxidised by a strain of Acetobacter suboxydans to the hexose sorbose. Chemical modifications convert this toascorbic acid (Figure 17.4).
Enzymes of fungal and bacterial origin have been utilised for many centuries in a variety of processes. It is now possible to isolate and purify the enzymes needed for a specific process and the worldwide market is cur-rently worth around a billion pounds. The most useful industrial enzymes include proteases, amylases, lipases and pectinase (Table17.4). Some applications of enzymes are listed in Table 17.5, and two examples are briefly described below.
Syrups and modified starches are used in a wide range of foodstuffs, including soft drinks, confectionery and ice cream, as well as having a wealth of other applications. Different enzymes or combinations of enzymes are used to produce the desired consis-tencies and physical properties. High fructose corn syrup (HFCS) is a sweetener used in a multitude of food products. It is some 75 per cent sweeter than sucrose and has sev-eral other advantages. HFCS is a mixture of fructose, dextrose (a form of glucose) and disaccharides, and is produced by the action of a series of three enzymes on the starch (amylose and amylopectin) of corn (maize). Alpha amylase hydrolyses the internal α-1, 4-glycosidic bonds of starch, but is not able to degrade ends of the chain. The resulting di- and oligosaccharides are broken down to the monomer glucose by the action of glucoamylase, then finally glucose isomerase converts some of the glucose to its isomer, fructose.
Enzymes have been added to cleaning products such as washing powders, carpet shampoos and stain removers since the 1960s, and this remains one of the principal industrial applications of enzymes.
Proteases are the most widely used enzymes in this context; working in combination with a surfactant, they hydrolyse protein-based stains such as blood, sweat and various foods. Greasy and oily stains present a different challenge, made all the more difficult by the move towards lower washing temperatures. The inclusion of lipases aids the removal of stains such as butter, salad dressing and lipstick, while amylases deal with starch-based stains such as cereal or custard. The food and detergent industries between them account for around 80 per cent of all enzyme usage.
We have already seen that antibiotics are now produced on a huge scale worldwide. Figure 17.5 outlines the stages in the isolation, development and production of an antibiotic.
Isolating an antibiotic from a natural source is not all that difficult, but finding a new one that is therapeutically useful is another matter. Initially, the antimicrobial properties of a new isolate are assessed by streaking it across an agar plate, then inoculating a range
of bacteria at right angles (Figure 17.6). As the antibiotic diffuses through the agar, it will inhibit growth of any susceptible species. Isolates that still show potential are then grown up in a laboratory scale fermenter; it is essential for commercial culture that the antibiotic-producing organism can be cultured in this way.
Before committing to large-scale production, exhaustive further tests must be carried out on two fronts: to ascertain the potency of the preparation and the breadth of its antimicrobial spectrum, and to determine its therapeutic index by carrying out toxicity testing on animals. The final stages of development involve pilot-scale production, followed by clinical trials on human volunteers.
When an antibiotic or any other fermentation product finally goes into production, it is cultured in huge stirred fermenters or bioreactors, which may be as large as 200 000 litres. A typical stirred fermenter has impellers for mixing the culture, an air line for aeration and microprocessor-controlled probes for the continuous monitoring and regulation of temperature, pH and oxygen content (Figure 17.7). Cultures with a high protein content may also have an antifoaming agent added. The process of scale-up is a complex operation, and not simply a matter of growing the microorganism in question in ever-larger vessels. Factors such as temperature, pH, aeration, must all be considered at the level of the individual cell if scale-up is to be successful. Fermenters are usually made from stainless steel, which can withstand heat sterilisation; the economic consequences of microbial contamination when working on such a large scale can be immense.
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