Industrial Microbiology


General Types Of Industrial Microbiology
Types of Fermentation Process
Food and Food Additives

Alcoholic Fermentation

Production of Vinegar
Manufacture  of  Various Chemicals
Single Cell Protein


General Types Of Industrial Processes

There are several types of industrial processes where microorga­nisms ate used to produce desirable end products which have well defined industrial uses and applications. These may be broadly classi­fied into the following groups.

1. Foods and food additives. Microorganisms themselves are cultivated on a large scale for use as food and animal feed. Yeast, fungi, and algae are produced from media containing inorganic nitrogen source and other readily available and cheap nutrients. Such food products are good sources of protein,, vitamins, and other organic nutrients. In recent years microbial processes are employed for large scale production of amino acids.

2. Alcoholic beverages. Fruit juices and extract of grains are fermented   with the   production of   beer,   wine,   and other alcoholic beverages.

3.  Manufacture of various chemicals. Microorganisms fer­ment various substances,   usually carbohydrates,   in nutrient   media. They produce a variety   of chemicals   (various alcohols, lactic acid, acetic acid, citric acid, gluconic acid, etc.)   which are being recovered, purified and sold.

4. Therapeutic compounds.  Antibiotics, vitamins, and ster­oid drugs are prominent in this category.

5. Industrial enzymes.  A number of microbial,enzymes have industrial applications, and are produced on a large scale



Types  Of Fermentation   Processes

The fermentation unit in industrial microbiology is analogous to a chemical plant in the chemical industry. A fermentation process is a biological process and, therefore, has requirements of sterility and use of cellular enzymic reactions instead of chemical reactions aided by inanimate catalysts, sometimes operating at elevated temperature and pressure. Industrial fermentation processes may be divided into two main types, with various combinations and modifications. These are batch fermentations and continuous fermentations.

Batch fermentations

A tank of fermenter is filled with the prepared mash of raw mate­rials to be fermented. The temperature and pH for microbial fermen­tation is properly adjusted, and occassionally nutritive supplements are added to the prepared mash. The mash is steam-sterilized in a pure culture process. The inoculum of a pure culture is added to the fermenter, from a separate pure culture vessel. Fermentation proceeds, and after the proper time the contents of the fermenter, are taken out for further processing. The fermenter is cleaned and the process is repeated. Thus each fermentation is a discontinuous process divided into batches.

Continuous fermentation

Growth of microorganisms during batch fermentation confirms to the characteristic growth curve, with a lag phase followed by a loga­rithmic phase. This, in turn, is terminated by progressive decrements in the rate of growth until the stationary phase is reached. This is because of limitation of one or more of the essential nutrients. In continuous fermentation, the substrate is added to the fermneter continously at a fixed rate. This maintains the organisms in the logari­thmic growth phase. The fermentation products are taken out conti­nuously. The design and arrangements for continuous fermentation are somewhat complex.

Aerobic fermentations

A number of industrial processes, although called 'fermentations',

Fig. 1. Aerobic fermenter.

are carried on by microorganisms under aerobic conditions. In older aerobic processes it was necessary to furnish a large surface area by exposing fermentation media to air. In modern fermentation processes aerobic conditions are maintained in a closed fermenter with submerged cultures. The contents of the fermenter are agitated with au impeller and aerated by forcing sterilized air (Fig 1).

Anaerobic fermentations

Basically a fermenter designed to operate under micro-aerophilic or anaerobic conditions will be the same as that designed to operate under aerobic conditions, except that arrangements for intense agitation and aeration are unnecessary. Many anaerobic fermentations do, how­ever, require mild aeration for the initial growth phase, and sufficient N agitation for mixing and maintenance of temperature.                        




Food  And   Food  Additives

Baker's yeast

The production of baker's yeast is the largest domestic use of a microorganism for food purposes. Baker's yeast is a strain of Sacchawinyces certvisiae The strain of the yeast is carefully selected for its capacity to produce abundant gas quickly, its viability during ordinary storage, and its ability to produce desirable flavour. The organisms are mixed with bread dough to bring about vigorous sugar fermentation. The carbon dioxide produced during the fermentation is responsible for leavening or rising of the dough.

A pure culture of the selected strain of yeast is first grown in the laboratory and gradually built up to larger and larger volume by transfer from the test tube to the fermenter tank. Great care is taken to prevent contamination at any stage of development of the culture. During manufacturing, the strain is inoculated into a medium which frequently contains molasses and corn-steep liquor as sources of carbon, nitrogen, and mineral salts. The reaction of the medium is adjusted to pH 4.4 to 4.6. The inoculated medium is incubated at a temperature of 25 to 26C, and is aerated during the incubation period. Yeasts oxidize sugars under aerobic conditions with the liberation of energy. A large part of this energy is utilized for the synthesis of cell protoplasm. The yeast cells multiply rapidly and exhaust the sugar supply within 10 hours.

At the end of incubation the yeast cells   are removed   from the fermented medium by centrifugation, washed and mixed with starch or corn meal, and  then being pressed into cake form. Yeast cakes must be kept cool to preserve the cells and to prevent spoilage by other micro­organisms. They may also be dried. Dried yeast remains viable for several months. Yeasts are rich in vitamins and in most of the essential amino acids required by man and animals.

Food and fodder yeast

Yeast propagated essentially for food purposes is known as food yeast. Yeast produced chiefly to feed animals is called fodder yeast. The mass cultivation of yeast for use as food, more specifically as a food supplement, is to compensate for the dietary inadequacies of cheap food materials, especially in the regions of the world where human malnutrition is chronic. Secondly, there is great emphasis and interest on lowering the BOD of the effluents from industrial plants. Thus several processes to manufacture yeast from industrial and agricultural wastes have been undertaken. This converts waste into products of value, and at the same time prevents environmental pollution.

Foods from waste

Food and fodder yeast is also manufactured from waste mate­rials such as wood shavings and sawdust, straw, corn cobs, and, other agricultural wastes. A variety of substrates can be used. Yeasts capable of assimilating diverse sources of carbon are more accep­table than S cerevisiae. The asporogenous yeast, Candida utilis (Torulopsis utilis) is commonly used because of its marked ability to assimi­late various carbon sources, including xylose, galactose. and a wide range of organic acids. Torula are grown on sulphite liquor from pulp and paper mills. After removal of wood fibres for paper, the waste sulphite liquor contains valuable wood sugar. Yields of upto 50 percent of the total reducing sugar consumed, in terms of dry Torula, are obtainable.

Amino acid   production

In recent years there has been a rapid development of the prod­uction of particular amino acids by fermentation. Microorganisms can synthesize amino acids from inorganic nitrogen compounds. The rate and the amount of synthesis of some amino acids may exceed the cell's need for protein synthesis, whereupon the amino acids are excre­ted into the medium. Some microorganisms are capable of producing sufficient amounts of certain amino acids to justify their commercial production. The amino acids can be obtained from hydrolysing protein or from chemical synthesis, but in several instances the microbial process is more economical. Secondly, the microbiological method yields the naturally occurring L-amino acids. The demand for amino acids for use in foods, feeds, and in the pharmaceutical industries is expanding; moreover, when production costs decrease, a new usage is anticipated, as raw material for amino acid polymers.

Microbial production of amino acids shows two outstanding feat­ures which are not usually encountered in the development of other microbiological processes. One is the importance of autotrophic micro­organisms, These microorganisms were known only as useful tools in the analysis of metabolic pathways and in genetics but, today, they are proving of great value. The second feature is that a knowledge of meta­bolic control mechanisms can now be used to good purpose in indus­trial microbiological processes.

Glutamic acid

Interest in glutamic acid production was stimulated by the incre­asing demand for monosodium glutamate as a flavour-enhancing agent. One-stage fermentation used in Japan employs sweet potatoes as the chief raw material. A strain of Micrococcus, which requires 0.5 to 1.5 g liter of biotin is used. The nitrogen source should contain any one of the following substances : ammonium salts, urea, peptone, corn-steep liquor, digested soybean or fish meal. The pH is kept in the range of 7.0 to 8.5 and the temperature is maintained at 27 to 30C. Aerobic conditions are used. Yields of 21 to 30 per cent, based on glucose, have been reported.

Two-stage fermentation, wherby a-ketoglutaric acid is produced and subsequently converted to glutamic acid, has received the most attention. A wide variety of bacteria produce a-ketoglularic acid, e.g. Pseudomonas fluorescens, Bacerium ketoglutaricum, Proteus spp. and certain coliforms and Kluyvera citrophila. Glutamic acid is for­med from a-ketoglutaric acid by transamination or reductive amination by glutamic acid dehydrogenase. Escherichia coU and Bacterium keto­glutaricum produce glutamic acid by transamination with aspartic acid or alanine as the-amino donor. Cultures belonging to the following genera produce glutamic acid by reductive amination of a-ketoglutarate: Pseudomonas, Serratia, Erwinia, Escherichia, Agrobactenum, Saccha-romyces, Aspergillm, Penicillium, and Rhizopus.

Many commercial processes have now been developed for the production of lysine, threonine, methionine, tryptophan, alanine.etc. Microorganisms possess the capacity to synthesize amounts of various metabolites far in excess of those normally made. Because of the exquisite control mechanism that function in microbial cells, overproduction of the metabolites is limited. Thus microbial accumulation of specific amino acids will depend in a large measure on a method of selecting strains which have lost feed back regulation of amino acid in question. The most significant factor in developing microbial processes for amino acids has been the use of autotrophs. The mutational and selection techniques help in obtaining the desired strain with specific mutational lesions.



Alcoholic Fermentation

Industrial alcohol

Ethyl alcohol can be produced by fermentation of any carbohy­drate containing a fermentable sugar, or a polysaccharide that can be hydrolysed to a fermentable sugar. The equation that describes the net result of alcoholic fermentation by yeast is :

C6H12 O6 2C2H5OH + 2 CO2

It indicates that a sugar is the substrate and that the process is anaerobic. Selected strains of Saccharomyces cerevisiae are commonly employed for fermentation. It is imperative that the strain must have a high tolerance for alcohol, must grow vigorously and produce a large quan­tity of alcohol.

In recent years the production of industrial alcohol by the fermentation process has declined because of the-increased cost of raw materials and the rapid developments of synthetic ethanol production. Industrial alcohol will probably continue to be obtained on a diminished scale from certain processes. For example alcohol is obtained as the end product in the processes designed to reduce biological oxygen demand (BOD) of some industrial wastes, including whey and sulphite waste of paper mills. The large amount of carbon dioxide evolved from decarboxylation of pyruvate during the fermentation period is recovered and converted to solid carbon dioxide.

Alcoholic beverages

These products of alcoholic fermentations originated in sponta­neous fermentation processes are of great antiquity. However, it is only in recent years that modern methods of industrial microbiology have been applied to their manufacture. In principle, the production of alcoholic beverages is similar to the production of industrial ethyl alcohol. These fermentation processes do not suffer competition from synthetics products. This is because the character of the beverage is depe­ndent upon interactions between a variety of biological factors that have not yet been denned in chemical or physical terms. In beverage production refinements are introduced with respect to flavour, aroma, colour, and sanitation that are not necessary in the making of industrial alcohol.

The type of beverage produced is determined by the nature of the plant material employed for fermentation. In all these processes the method of preparing the fermentation medium is a factor of prime importance.

Beer. Beef is made by the yeast fermentation of grains to ethanol and carbon dioxide. There are five major steps in the manufacture of beer or ale from grain. These are malting, mashing, 'fermenting, mat­uring, and finishing. Malting and mashing are concerned with the conversion of starch into fermentable form such as maltose or glucose. The chief raw material is malt, which is germinated barley that has been dried and ground. It contains stanch, proteins, and high concentration of amylases and proteinases. Amylases convert the starch into ferm­entable sugar. Mould amylase derived from Aspergillus oryzae is sometimes used for the same purpose. Ground malt is mashed in warm water to bring about the digestion of starch and proteins. The aqueous extract contains dextrins, maltose, and other sugars, protein breakdown products, minerals and various growth factors. This is a rich nutrient medium and is called beer wort. The beer wort is filtered and hops are added, Hops are the flowers of Humulus lupulus. They are added for flavour, colour, and for aroma and for mild antibacterial activity to prevent the growth of spoilage bacteria.

A large inoculum of selected strain of Saccharomyces cerevisiae is added to the wort to bring about a vigorous fermentation. Yeasts are classified as 'top yeasts' or 'bottom yeast*.Top yeasts float on the surface of a fermenting mixture and are employed in making ale. Bottom yeasts settle in the fermentation tank and are used in making beer. Beer fermentation takes place at 6 to 12C., whereas ale fermentation is complete in five to seven days at 14 to 23C. The alcoholic content of beer is between 3 to 6 percent, that of ale is somewhat higher.

The fermented wort is refrigerated at 0C for two weeks to several months to remove the harsh flavour and other undesirable characteristics. Some of the harshness attributed to higher alcohols disappears as they are oxidized or esterified during aging. Finishing process consists of carbonation, cooling, filtering and dispensing into barrels, bottles, and cans. Bottled or canned beer is usually pasteuri­zed at 60C for 20 minutes to kill yeasts and other microorganisms. As an alternative, the beer may be passed through a filter to remove microorganisms, and then aseptically dispensed into sterile cans. The composition of American lager beer is as follows :

Alcohol       3.8 percent          

Dextrins       4:3 percent

Proteins       0.3 percent          

Ash              0.3 percent

C02             0.4 percent

It also contains appreciable amounts of vitamins, particularly riboflavin. In addition, there area number of minor constituents, some of which are important for flavour and aroma.

Wines. Wine is the product made by the normal alcoholic fermentation of the juice of sound, ripe grapes and the usual cellar treatment. Beverages produced by the alcoholic fermentation of other fruits and certain vegetable products-are also called wines for example, peach wine, orange wine, cherry wine. Wine making is a much simpler process. It can be made by a direct fermentation of sugars, i.e. glucose and fructose, instead of starch which requires hydrolysis to yield sugars. Many fruits have the wine yeast Soccharomyces cerevisiae var. ellipsoideus on them. All that is necessary is to crush the fruits. An alcoholic fermentation starts spontaneously. The characteristic qualities of famous wines are attributed in part to strains of yeast found in certain localities. However, undesirable moulds, wild yeasts, and bacteria are also likely to be present and the fermentation may not give a predictably good product. Many wine makers now destroy natural yeasts by adding sulphur dioxide to the raw juice.

The grapes are crushed carefully and the juice is collected. To the raw juice or must; sulphur dioxide is added as sodium metabisulphite. The must is then inoculated with a starter culture-of a selected strain of S. cerevisiae var. elliposideus. At the start the must is aerated slightly to promote vigorous yeast growth. Once the ferment­ation sets in, the rapid production of carbon dioxide maintains anaerobic condition. The temperature of fermentation is usually 25 to 30C and the process may extend from few days to 2 weeks. The yield of ethanol varies from 7 to 15 percent (by volume). The wine is placed in large casks to settle, clarify and age for two to five years to develop a good flavour and aroma.

Wines are endless in their varieties and differ in so many attributes that it is difficult to classify them. According to colour, the two most basic types are red and while wine. In making red wines the grapes are crushed and stemmed but the skins and seeds are left in the must. While wines are made from white grapes or from the juice of grapes from which the skins have been removed. Dry wines are those which contains too little sugar to be detected by taste. In sweet wines the sugar content is high enough to be detected by taste. Sparkling wines contain carbon dioxide. They are made effervescent by secondary fermentation in closed containers, generally in the bottle itself. Still wines are those which do not contain carbon dioxide. Fortified wines contain added alcohol in the form of brandy.

Distilled liquors. Yeast action is limited by the amount of alcohol present, and at about the level of 18 percent by volume its action ceases. To produce the so called hard liquor, for higher levels of alcohol, distillation is required. Distilled alcoholic beverages may be divided into three major classes depending on the nature of the solution distilled :

1.    The products starting from a starchy substance  and needing enzymes.

2.    The products starting directly from a sugar substrate.3.    The type of liquor produced  by adding flavour substances to quite pure ethanol,   which  has been   obtained by distillation and rectification.

Malt whisky is prepared by fermentation and subsequent distillation of malted barley. Grain whisky is prepared in a similar manner from a mixture of malted and unmalted barley with unmalted maize. Malt and grain whisky are matured and finally blended to form Scotch whisky. Bourbon is whisky prepared from a mash in which maize is the predominant grain. Irish Whisky is manufactured from a mash in which rye grain predominates. Arrak (Far East) and sake (Japan) are fermented beverages piepared from rice. Rice starch is hydrolysed by amylases derived from moulds, principally Aspergills oryzae.

Brandy is obtained from distillation of fermented fruit juice, that is wine. Rum is produced by distillation of fermented molasses or other sugarcane byproducts. Gin is prepared by extracting juniper berries with alcohol and distillation of alcohol. Cordials and liqueurs are sweetened alcoholic distillates from fruits flowers, leaves, etc



Production Of Vinegar

Vinegar may be defined as the condiment made from sugary or starchy material by alcoholic and subsequent acetic acid fermenta­tions. The word vinegar is derived from French term vinaigre, meaning, 'sour wine' (vin=wine, aigre=sour). Vinegar is the product resulting from the conversion of ethyl alcohol to acetic acid by a group of widely distributed bacteria of the genus Acetobacter. Thus it can be produced from any alcoholic material, ranging from alcohol-water mixtures to various fruit wines. The composition of a vinegar will depend somewhat upon on she nature of the raw material that has undergone alcoholic and acetous fermentations. Vinegar is a solution containing at least 4 percent acetic acid and small amounts of alcohol glycerol, esters, reducing sugars, pentosans, salt, and other substances. Depending on the raw materials, vinegars are differentiated as wine vinegar, apple cider vinegar, malt vinegar, and others.

The microorganisms that produce acetic acid from ethyl alcohol are species of Acctobac.ct,A. orkannt, A. orleansis, A.schutzenbachi, A.Aceti and others. The biochemical reaction by which they form acetic acid from ethanol is as follows :-:-

2CH3 CH2OH + 02 2CH3CHO + 2H2O


Some of the Acelobacter species do not stop With acid production but continue the oxidation to carbon dioxide.

CH3COOH + O2 2CO2 + 2H2O

Thus selection of proper organisms is important for vinegar fermentation. The organisms should carry the reaction as near to completion without destroying acetic acid by oxidation. In addition they must be tolerant to ethanol.

The basic methods of vinegar production are known as the slow process: or Orleans method, the rapid generator process and submerged fermentation in an acetatof. Vinegar is commonly made at home from cider, grape juice, etc. in a barrel. A yeast fermentation is used for the production of alcohol. The alcoholic solution is transferred to a vinegar barrel and alcohol concentration is adjusted between 10 to 13 per cent. The alcoholic solution is inoculated and acidified by adding 10 to 25 per cent of pure vinegar. When vinegar fermentation is complete the vinegar is bottled and stoppered tightly. This is to prevent further oxidation of acetic acid by Acetobacter when the alcohol concentration drops to 1 to 2 per cent. During acetic fermentation the bacteria develop as a gelatinous pellicle on the surface of the liquid. The organisms thus have access to both ethyl alcohol and oxygen If the pellicle is disturbed and sinks to the bottom ('Mother of Vinegar), acetification stops, until another pellicle forms.

The Orleans or the French process employs wooden vats or asks of about 200 litres capacity. These are filled one-third with a good grade of vinegar. This constitutes the starter or the culture. At weekly intervals, 10 to 15 liters of wine are added. After five weeks 10 to 15 liters of vinegar are drawn off each week and the same amount of wine is added. Air is admitted to the barrels through holes above the level of the vinegar medium. This is a slow continuous process and requires constant attention and maintenance. However it produces a high quality of vinegar.

Vinegar is manufactured by more rapid methods, using the generator (German process). Generators are of various sizes and shapes. They may be as large as 15 feet in diameter and 20 feet high. The generator is equipped with a false perforated bottom, through which air enters and supports beechwood shavings. Near the top of the generator, there is a false top or perforated plate over which is arranged a rotating sprinkler, or sparger This produces a uniform distribution of vinegar stock (vinegar plus alcohol containing substrate) over the shavings (Fig ).

Fig. The   generator

The generator  is loosely  filled with wooden shavings,  through which the medium is circulated.   The air is blown through, counter current to the liquid flow. The acetic acid bacteria grow as a thin film over the wooden shavings. A large area of cells is thus simultaneously exposed to the medium and to air. The bacterial oxidation of alcohol to acetic acid evolves heat. Cooling coils are necessary to keep the temperature within the favourable range of 25 to 30 C. Several passages through a generator are required to produce vinegar of desired strength. This is accomplished by recirculation by the use of several generators in series.

In recent years vinegar is produced in acetalors by submerged fermentation. This allows a much stricter control of aeration and temperature. It is reported that the same quantity of alcoholic subs­trate may be fermented 30 times faster by this process.

The vinegar is used in pickling, in preserving meats and vege­tables, and in the manufacture of salad dressing and catsup.

Wine vinegar, is the aristocrat of vinegars and commands a premium price. Commercial development of such products as orange, grape, fruit, tangerine, and orange peel vinegar may take place in future. Properly processed, these vinegar will compete with wine vinegars.

Vinegar fermentation, however, is not a competitive process for the production of acetic acid. Synthesis from ethylene or oxidation of synthetic ethanol is more economical



Manufacture Of Various Chemicals

Lactic acid production

The use of lactic acid fermentation as a good preservation method is another ancient art of unknown origin. Lactic acid fermen­tation was investigated by Pasteur as one of his first microbiological problems. Lactic acid is commonly produced from the usual cheap sources of fermentable carbohydrates such as acid or enzyme hydrolysed corn and potato starches, molasses, and whey. Whey, the watery part of milk separated from curd during cheese making, is widely used in the manufacture of lactic acid. Whey represents a satisfactory medium for the growth of certain bacteria. It contains a relati­vely large amount of lactose and proteiuaceous substances, minerals, and some essential vitamins. The homofermeutative lactobaeilli such as Lactobacillus bufgariau, L. delbrueckii, etc., grow raipdly and convert the lactose to the single end product, lactic acid.

The typical fermentation process involved in making commercial calcium lactate and the principal grades of lactic acid is described in brief. Pasteurized whey is inoculated with a starter containing L. bulgaricus. To prepare a sufficient amount of inoculum the culture is built up by successive transfers in sterile skim milk, pasteurized skim milk, and finally, when fermentation is carried out at a temperature of 430C to discourage the growth of undesirable organisms. Fermenters and accessory equipment are fabricated with type 316 stainless steel to resist the corrosiveness of lactic acid. Combinations of glass and fluorocarbon resins (teflon) are also employed in the design of piping system, valve, filters, etc. During the fermentation, lime (Ca(OH)2) is added intermittently to neutralize the acid and to promote a good yield of calcium lactate, At the end of fermentation, the lactalbumin is coagulated by heat, When lactalbumin settles, the solution of calcium lactate is decanted off and filtered. It is then treated with decolourizing carbon and filter aids, filtered, evaporated, and crystallized. The crystals are further purified and sold as calcium lactate or converted to lactic acid. Various procedures are followed in producing the different grades of lactates and lactic acid.

Lactic acid has many uses.   It is used as ail acidulant in confect­ionery, fruit juices, and essences. It may be used in the curing of meat and  in  canned vegetable   and fish products.   Lactic acid is used in various chemical industries.    The lactates also  have  important uses. Calcium  lactate is used in  baking powders  and bread,   and in the treatment of calcium deficiency. Iron lactate is used in the treatment of anemia. Sodium lactate is used to help in the retention of moisture by such products as tobacco and as a plasticizer. Lactic acid fermentation from whey also helps in the removal of pollution of our environment. Untreated whey disposed off in our waterways would cause dangerous consequences because whey is very high in BOD.                       




Single Cell Protein

SCP are the dried cells of selected micro-organisms such as algae, yeast, bacteria molds, and higher fungi, that can be used as rich protein sources to humans and animals.

                        There are various angles at which the sources of the SCP can be looked at. Some use simpler methods such as suggesting whether the source is a bacteria or fungi or yeast etc. While others based it on the photosynthetic ability of producing organisms.

                        Photosynthetic microorganisms which could be exploited commercially as sources as SCP include algae such as Scenedesmus, Spirulina, Chlorella etc, and bacteria such as those belonging to the such as Rhodopseudomonas, Bacillus and Azatobacteria. Non-photosynthetic microorganisms useful for SCP production include bacteria, such as Methylococcus and Axinetobacteria species etc. Yeasts belonging to the genera Saccharomyces, Candida and Kluyveromyces.

                          However, it is important to note that these organisms must not be pathogenic to plants, animals and humans besides not containing any toxins.

Production:- It is significant enough to consider the production of SCP from various microbial sources independently owing to the variation in the process used.

                        Algae grown photosynthetically in a medium containing organic compounds and inorganic nutrients such as Iron, Magnesium, Sodium, Potassium etc, sources of Nitrogen such as ammonia and nitrates and sources of Phosphorous and Sulfur. A pH value of about 11 is considered optimum for Spirulina while a value of 8 is considered best for Scenedesmus.

                        Artificial illumination for a light source has not been cost-effective and hence natural light is preferred. Culture system could be batch, pond or semi-continuous types, all types using an agitator to ensure uniform supply of nutrients.

                        Mixed cultures and contaminated by bacteria are looked upon as major problems in the production.

                        Bacteria are also grown photosynthetically as pure cultures with an organic carbon source or in mixed cultures with other bacteria. The most suitable growth parameters are temperature range of 25 to 300C and a pH range of 6 to 8.5.

                        Non-photosynthetic bacteria are cultured aerobically using many designs of bioreactors and fermentors. The cultures are operated as batch, continuous or fed-batch modes.

            Yields from non-photosynthetic bacteria such as Methylococcus are in the orders of one gram per gram of substrate used (menthol). However, bacterial SCP is extremely difficult to recover.

            Yeasts are cultivated using variety of carbon sources such as xylose, glucose, sucrose and lactose. Important factors for selection of a yeast strain (some as those for any other source of SCP) are its growth rate, yield, pH and temperature tolerance, aeration requirements, genetic stability and non-toxigenicity. Typical yeast cultures yield 10-15g (dry weight) per liter of medium used.

            Some higher fungi such as Coprinus have also been propagated as a source of SCP. The optimal parameters include a temperature of 25 to 300C and a pH of 3 to 7 units.


1)      SCP forms one of the richest sources of proteins or amino acids. They had been used as a part of feeds to live stocks, chickens, eggs-laying hens and pigs. The SCP provide the essential amino acids at various concentrations depending on the source, which is expressed as Biological value (BV) and Protein Efficiency Ratio (PER).

2)       Human feeding studies have been conducted with algae, yeasts and molds. However, the palatability has remained a question. Also digestability and amino acids content have remained points of controversy. Since SCPs are significant sources of proteins for man, it is advised to avoid an intake exceeding 2g per day, which would otherwise lead to stone formation in kidneys and gut.

Modifications of SCPs such as succinylation (addition of succinic acid) and Phosphorylation are being attempted to improve functional properties. However, licensed approval is awaited.





1.      Ananth.n, A text book of microbiology-Vol IV

2.      JaiPrakash nath Introdutory microbiology.

3.      Larry Mc kane and Judy kandel Microbiology Essentials and applications

4.      Pelczar  Microbiology

5.      Prescott Harley klein  Microbiology

6.      Power and Danginawala General micrbiology

7.      Sundara Rajan. S College Microbiology

8.      Sharma P.D. Microbiology

9.   Tortora .Funke.Case Microbiology an introduction