Industrial Biotechnology CHAPTER 8 Production of Organic Acids

Industrial Biotechnology CHAPTER 8 Production of Organic Acids and Industrial Alcohol

Production of Citric Acid

Introduction • Citric acid is a tribasic acid with the structure It crystallizes with the large rhombic crystals containing one molecule of water of crystallization, which is lost when it is heated to 130°C. At temperatures as high as 175°C it is converted to itaconic acid, aconitic acid, and other compounds.

Uses of Citric Acid Uses in the food industry Used as acidulant in the manufacture of jellies, jams, sweets, and soft drinks. It is used for artificial flavoring in various foods including soft drinks. Sodium citrate is employed in processed cheese manufacture. Uses in medicine and pharmacy Sodium citrate used in blood transfusion and bacteriology for the prevention of blood clotting. • The acid is used in efferverscent powers which depend for their efferverscence on the CO 2 produced from the reaction between citric acid and sodium bicarbonate. • Since it is almost universally present in living things, it is rapidly and completely metabolized in the human body and can therefore serve as a source of energy. • • •

Uses of Citric Acid • Uses in the cosmetic industry • It is used in astringent lotions such as aftershave lotions because of its low p. H. • Citric acid is used in hair rinses and hair and wig setting fluids. • Miscellaneous uses in industry • In neutral or low p. H conditions the acid has a strong tendency to form complexes hence it is widely used in electroplating, leather tanning, and in the removal of iron clogging the pores of the sand face in old oil wells. • Citric acid has recently formed the basis of manufacture of detergents in place of phosphates, because the presence of the latter in effluents gives rise to eutrophication.

Biochemical Basis of the Production of Citric Acid • Since it is an intermediate of Krebs cycle, so the acid can be accumulated by using one of the following methods • By mutation – giving rise to mutant organisms which may only use part of a metabolic pathway, or regulatory mutants; that is using a mutant lacking an enzyme of the cycle. • By inhibiting the free-flow of the cycle through altering the environmental conditions, e. g. temperature, p. H, medium composition (especially the elimination of ions and cofactors considered essential for particular enzymes). • The following are some of such environmental conditions which are applied to increase citric acid production: • The concentrations of iron, manganese, magnesium, zinc, and phosphate must be limited. To ensure their removal the medium is treated with ferro-cyanide or by ion exchange fresins. • These metal ions are required as prosthetic groups in the following enzymes of the TCA: Mn++ or Mg++ by oxalosuccinic decarboxylase, Fe+++ is required for succinic dehydrogenase, while phosphate is required for the conversion of GDP to GTP

Citric acid can be caused to accumulate by using a mutant lacking an enzyme of the cycle or by inhibiting the flow of the cycle

Biochemical Basis of the Production of Citric Acid • The dehydrogenases, especially isocitrate dehydrogenase, are inhibited by anaerobiosis, hence limited aeration is done on the fermentation so as to increase the yield of citric acid. • Low p. H and especially the presence of citric acid itself inhibits the TCA and hence encourages the production of more citric acid; the p. H of the fermentation must therefore be kept low throughout the fermentation by preventing the precipitation of the citric acid formed. • Many of the enzymes of the TCA can be directly inhibited by various compounds and this phenomenon is exploited to increase citric acid production. • Thus, isocitric dehydrogenase is inhibited by ferrocyanide as well as citric acid; aconitase is inhibited by fluorocitrate and succinic dehydrogenase by malonate. • These at enzyme antagonists may be added to the fermentation.

Fermentation for Citric Acid Production • For a long time the production of citric acid has been based on the use of molasses and various strains of Aspergillus niger and occasionally Asp. wenti. • Production by Penicillium is available, in practice are not used because of low productivity. • Recently yeasts, especially Candida spp. (including Candida quillermondi) have been used to produce the acid from sugar. • Japanese workers described a method to produce the acid by paraffins by bacteria and yeasts. Among the bacteria were Arthrobacter paraffineus and corynebacteria; the yeasts include Candida lipolytica and Candida oleiphila. • Fermentation with molasses and other sugar sources can be either surface or submerged. Fermentation with paraffins however is submerged. • (a) Surface fermentation: Surface fermentation using Aspergillus niger may be done on rice bran as is the case in Japan, or in liquid solution in flat aluminium or stainless steel pans. • Special strains of Asp. niger which can produce citric acid despite the high content of trace metals in rice bran are used. The citric acid is extracted from the bran by leaching and is then precipitated from the resulting solution as calcium citrate.

Fermentation for Citric Acid Production • (b) Submerged fermentation: As in all other processes where citric acid is made the fermentation the fermentor is made of acid-resistant materials such as stainless steel. • The carbohydrate sources are molasses decationized by ion exchange, sucrose or glucose. Mg. SO 4, 7 H 2 O and KH 2 PO 4 at about 1% and 0. 05 -2% respectively are added (in submerged fermentation phosphate restriction is not necessary). • The p. H is never allowed higher than 3. 5. • Copper is used at up to 500 ppm as an antagonist of the enzyme aconitase which requires iron. • 1 -5% of methanol, isopranol or ethanol when added to fermentations containing unpurified materials increase the yield; the yields are reduced in media with purified materials. • As high aeration is deleterious to citric acid production, mechanical agitation is not necessary and air may be bubbled through. Anti-form is added. • The fungus occurs as a uniform dispersal of pellets in the medium. • The fermentation lasts for five to fourteen days.

Extraction • The broth is filtered until clear. • Calcium citrate is precipitated by the addition of magnesiumfree Ca(OH)2. • Since magnesium is more soluble than calcium, some acid may be lost in the solution as magnesium citrate if magnesium is added. • Calcium citrate is filtered and the filter cake is treated with sulfuric acid to precipitate the calcium. • The dilute solution containing citric acid is purified by treatment with activated carbon and passing through iron exchange beds. • The purified dilute acid is evaporated to yield crystals of citric acid. • Further purification may be required to meet pharmaceutical stipulations.

Production of Lactic Acid

Properties and chemical reactions of lactic acid • (i) Lactic acid is a three carbon organic acid: one terminal carbon atom is part of an acid or carboxyl group; the other terminal carbon atom is part of a methyl or hydrocarbon group; and a central carbon atom having an alcohol carbon group. Lactic acid exists in two optically active isomeric forms. • (ii) Lactic acid is soluble in water and water miscible organic solvents but insoluble in other organic solvents. • (iii) It exhibits low volatility. • (iv) The various reactions characteristic of an alcohol which lactic acid (or it esters or amides) may undergo are xanthation with carbon bisulphide, esterification with organic acids and dehdrogenation or oxygenation to form pyruvic acid or its derivatives. • (v) The acid reactions of lactic acid are those that form salts and undergo esterification with various alcohols. • (v) Liquid chromatography and its various techniques can be used for quantitative analysis and separation of its optical isomers

Properties and chemical reactions of lactic acid • Technical grade lactic acid is used as an acidulant in vegetable and leather tanning industries. • Various textile finishing operations and acid dyeing of food require low cost technical grade lactic acid to compete with cheaper inorganic acid. • Lactic acid is being used in many small scale applications like p. H adjustment, hardening baths for cellophanes used in food packaging, terminating agent for phenol formaldehyde resins, alkyl resin modifier, solder flux, lithographic and textile printing developers, adhesive formulations, electroplating and electropolishing baths, detergent builders. • Lactic acid has many pharmaceutical and cosmetic applications and formulations in topical ointments, lotions, anti acne solutions, humectants, parenteral solutions and dialysis applications, and anti carries agents. • Calcium lactate can be used for calcium deficiency therapy, and as an anti caries agent. • Its biodegradable polymer has medical applications as sutures, orthopedic implants, controlled drug release, etc.

Properties and chemical reactions of lactic acid • Polymers of lactic acids are biodegradable thermoplastics. These polymers are transparent and their degradation can be controlled by adjusting the composition, and the molecular weight. • Their properties approach those of petroleum derived plastics. Lactic acid esters like ethyl/butyl lactate can be used as green solvents. • They are high boiling, non-toxic and degradable components. Poly L-lactic acid with low degree of polymerization can help in controlled release or degradable mulch films for large-scale agricultural applications. • Lactic acid was among the earliest materials to be produced commercially by fermentation and the first organic acid to be produced by fermentation. • Chemical processing has offered and continues to offer stiff competition to fermentation lactic acid. • Very few firms around the world produce it fermentatively, but this could change when the hydrocarbon-based raw material, lactonitrile, used in the chemical preparation becomes too expensive because of the increase in petroleum prices.

Properties and chemical reactions of lactic acid • Lactic acid exists in two forms, the D-form and the L-form. When the symbols (+) or (-) are used, they refer to the optical rotation of the acid in a refractometer. • However optical rotation in lactic acid is difficult to determine because the pure acid has low optical properties. • The acid also spontaneously polymerizes in aqueous solutions; furthermore, salts, esters, and polymers have rotational properties opposite to that of the pure acid from which they are derived. All this makes it difficult to use optical rotation for characterizing lactic acid. • Many organisms produce either the D-or the L-form of the acid. However, a few organisms such as Lactobacillus plantarum produce both. When both the D - and L- form of lactic acid are mixed it is a racemic mixture. • The DL form which is optically inactive is the form in which lactic acid is commercially marketed.

Uses of lactic acid • (i) It is used in the baking industry. Originally fermentation lactic acid was produced to replace tartarates in baking powder with calcium lactate. Later it was used to produce calcium stearyl 2 - lactylate, a bread additive. • (ii) In medicine it is sometimes used to introduce calcium in to the body in the form of calcium lactate, in diseases of calcium deficiency. • (iii) Esters of lactic acid are also used in the food industry as emulsifiers. • (iv) Lactic acid is used in the manufacture of rye bread. • (v) It is used in the manufacture of plastics. • (vi) Lactic acid is used as acidulant/ flavoring/ p. H buffering agent or inhibitor of bacterial spoilage in a wide variety of processed foods. It has the advantage, in contrast to other food acids in having a mild acidic taste. • (vii) It is non-volatile odorless and is classified as GRAS (generally regarded as safe) by the FDA. • (viii) It is a very good preservative and pickling agent. Addition of lactic acid aqueous solution to the packaging of poultry and fish increases their shelf life.

Uses of lactic acid • (ix) The esters of lactic acid are used as emulsifying agents in baking foods (stearoyl -2 -lactylate, glyceryl lactostearate, glyceryl lactopalmitate). The manufacture of these emulsifiers requires heat stable lactic acid, hence only the synthetic or the heat stable fermentation grades can be used for this application. • (x) Lactic acid has many pharmaceutical and cosmetic applications and formulations in topical ointments, lotions, anti acne solutions, humectants, parenteral solutions and dialysis applications, for anti carries agent. • (xi) Calcium lactate can be used for calcium deficiency therapy and as anti caries agent. • (xii) Its biodegradable polymer has medical applications as sutures, orthopaedic implants, controlled drug release, etc. • (xiii) Polymers of lactic acids are biodegradable thermoplastics. These polymers are transparent and their degradation can be controlled by adjusting the composition, and the molecular weight. Their properties approach those of petroleum derived plastics. • (xiv) Lactic acid esters like ethyl/butyl lactate can be used as environment-friendly solvents. They are high boiling, non-toxic and degradable components. • (xv) Poly L-lactic acid with low degree of polymerization can help in controlled release or degradable mulch films for large-scale agricultural applications.

Fermentation for lactic acid • The organisms which produce adequate amounts and are therefore used in industry are the homofermentative lactic acid bacteria, Lactobacillus spp. , especially L. delbruckii. • In recent times Rhizopus oryzae has been used. Both organisms produce the L- form of the acid, but Rhizopus fermentation has the advantage of being much shorter in duration; further, the isolation of the acid is much easier when the fungus is used. • Lactic acid is very corrosive and the fermentor, which is usually between 25, 000 and 110, 000 liters in capacity is made of wood. Alternatively special stainless steel (type 316) may be used. • They are sterilized by steaming before the introduction of the broth as contamination with thermophilic clostridia yielding butanol and butyric acid is common. Such contamination drastically reduces the value of the product.

Fermentation for lactic acid • During the step-wise preparation of the inoculum, which forms about 5% of the total beer, calcium carbonate is added to the medium to maintain the p. H at around 5. 5 -6. 5. • The carbon source used in the broth has varied widely and have included whey, sugars in potato and corn hydrolysates, sulfite liquour, and molasses. • However, because of the problems of recovery for high quality lactic acid, purified sugar and a minimum of other nutrients are used. • Lactobacillus requires the addition of vitamins and growth factors for growth. • These requirements along with that of nitrogen are often met with ground vegetable materials such as ground malt sprouts or malt rootlets. • To aid recovery the initial sugar content of the broth is not more than 12% to enable its exhaustion at the end of 72 hours. • Fermentation with Lactobacillus delbruckii is usually for 5 to 10 days whereas with Rhizopus oryzae, it is about two days.

Fermentation for lactic acid • Although lactic fermentation is anaerobic, the organisms involved are facultative and while air is excluded as much as possible, complete anaerobiosis is not necessary. • The temperature of the fermentation is high in comparison with other fermentation, and is around 45°C. • Contamination is therefore not a problem, except by thermophilic clostridia.

Extraction • Recovery is the main problem in fermentative lactic acid production. • Lactic acid is crystallized with great difficulty and in low yield. The purest forms are usually colorless syrups which readily absorb water. • At the end of the fermentation when the sugar content is about 0. 1%, the spent medium is pumped into settling tanks. • Calcium hydroxide at p. H 10 is mixed in and the mixture is allowed to settle. The clear calcium lactate is decanted off and combined with the filtrate from the slurry. • It is then treated with sodium sulfide, decolorized by adsorption with activated charcoal, acidified to p. H 6. 2 with lactic acid and filtered. • The calcium lactate liquor may then be spray-dried

Extraction • For technical grade lactic acid the calcium is precipitated as Ca. SO 4. 2 H 2 O which is filtered off. • It is 44 -45% total acidity. Food grade acid has a total acidity of about 50%. • It is made from the fermentation of higher grade sugar and bleached with activated carbon. • Metals especially iron and copper are removed by treatment with ferrocyanide. • It is then filtered. • Plastic grade is obtained by esterification with methanol after concentration. • High-grade lactic acid is made by various methods: steam distillation under high vacuum, solvent extraction etc.

INDUSTRIAL ALCOHOL PRODUCTION

Introduction • Ethyl alcohol, CH 3 CH 2 OH (synonyms: ethanol, methyl carbinol, grain alcohol, molasses alcohol, grain neutral spirits, cologne spirit, wine spirit), is a colorless, neutral, mobile flammable liquid with a molecular weight of 46. 47, a boiling point of 78. 3 and a sharp burning taste. • Although known from antiquity as the intoxicating component of alcoholic beverages, its formula was worked out in 1808. • It is rarely found in nature, being found only in the unripe seeds of Heracleum giganteun and H. spondylium.

Properties of Ethanol • Ethyl alcohol undergoes a wide range of reactions, which makes it useful as a raw material in the chemical industry. • Some of the reactions are as follow: • (i) Oxidation: Ethanol may be oxidized to acetaldehyde by oxidation with copper or silver as a catalyst: • (ii) Halogenation: Halides of hydrogen, phosphorous and other compounds react with ethanol to replace the – OH group with a halogen:

Properties of Ethanol • (iv) Haloform Reaction: Hypohalides will react with ethanol to yield first acetaldehyde and finally the haloform reaction: • (v) Esters: Ethanol reacts with organic and inorganic acids to give esters: • (vi) Ethers: Ethanol may be dehydrated to give ethers

Properties of Ethanol • (vii) Alkylation: Ethanol alkylates (adds alkyl-group to) a large number of compounds:

Uses of Ethanol • (i) Use as a chemical feed stock: In the chemical industry, ethanol is an intermediate in many chemical processes because of its great reactivity as shown above. It is thus a very important chemical feed stock. • (ii) Solvent use: Ethanol is widely used in industry as a solvent for dyes, oils, waxes, explosives, cosmetics etc. • (iii) General utility: Alcohol is used as a disinfectant in hospitals, for cleaning and lighting in the home, and in the laboratory second only to water as a solvent. • (iv) Fuel: Ethanol is mixed with petrol or gasoline up to 10% and known as gasohol and used in automobiles.

Denatured Alcohol • All over the world and even in ancient times, governments have derived revenue from potable alcohol. For this reason when alcohol is used in large quantities it is denatured or rendered unpleasant to drink. • The base of denatured alcohol is usually 95% alcohol with 5% water; for domestic burning or hospital use denatured alcohol is dispended as methylated spirit, which contains a 10% solution of methanol, pyridine and coloring material. • For industrial purpose methanol is used as the denaturant. • In the United States alcohol may be completely denatured (C. D. A. – completely denatured alcohol) when it cannot be used orally because of a foul taste or four smelling additives. • It may be specially denatured (S. D. A. – specially denatured alcohol) when it can still be used for special purposes such as vinegar manufacture without being suitable for consumption.

Manufacture of Ethanol • Ethanol may be produced by either synthetic chemical method or by fermentation. • Fermentation was until about 1930 the main means of alcohol production. • In 1939, for example 75% of the ethanol produced in the US was by fermentation, in 1968 over 90% was made by synthesis from ethylene. • Due to the increase in price of crude petroleum, the source of ethylene used for alcohol production, attention has turned worldwide to the production of alcohol by fermentation. • Fermentation alcohol has the potential to replace two important needs currently satisfied by petroleum, namely the provision of fuel and that of feedstock in the chemical industry. • The production of gasohol (gasoline – alcohol blend) appears to have received more attention than alcohol use as a feed stock. • Nevertheless, the latter will also surely assume more importance if petroleum price continues to ride.

Manufacture of Ethanol • Governments the world over have set up programs designed to conserve petroleum and to seek other energy sources. • One of the most widely publicized programs designed to utilize a new source of energy is the Brazilian National Ethanol Program. Set-up in 1975, the first phase of this program aims at extending gasoline by blending it with ethanol to the extent of 20% by volume. • The United States government also introduced the gasoline programme based on corn fermentation in 1980 following the embargo on grain sales to then Soviet Union.

Substrates • The substrate used will vary among countries. • In Brazil sugar cane, already widely grown in the country, is the major source of fermentation alcohol, while it is planned to use cassava and sweet sorghum. • In the United States enormous quantities of corn and other cereals are grown and these are the obvious substrates. • Cassava grows in many tropical countries and since it is high yielding it is an important source in tropical countries where sugar cane is not grown. • It is recognized that two important conditions must be met before fermentation alcohol can play a major role in the economy either as gasohol or as a chemical feedstock. • First, the production of the crop to be used must be available to produce the crop without extensive and excessive deforestation. • Secondly, the substrate should not compete with human food.

Fermentation • The sterilized fermentable sugars are pumped or allowed to flow by gravity into fermentation tanks and yeast is inoculated or ‘pitched in’ at a rate of 7 -15 x 10 6 yeast cells/ml, usually collected from a previous process. • These broths are inoculated with up to 5% (v/v) of thick yeast broth. • Although yeast is re-used there is still a need for regular inocula. • In general the inocula are made of selected alcohol-tolerant yeast strains usually Sacch. cerevisiae grown aerobically with agitation and in a molasses base. • Progressively larger volumes of culture may be developed before the desired volume is attained. • When the nitrogen content of the medium is insufficient nitrogen is added usually in the form of an ammonium salt. • As in all alcohol fermentations the heat released must be reduced by cooling and temperatures are generally not permitted to exceed 35 -37°C. • The p. H is usually in the range 4. 5 -4. 7, when the buffering capacity of the medium is high. • Higher p. H values tend to lead to higher glycerol formation. • When the buffering capacity is lower, the initial p. H is 5. 5 but this usually falls to about 3. 5. During the fermentation contaminations can have

Distillation • After fermentation the fermented liquor or ‘beer’ contains alcohol as well as low boiling point volatile compounds such as acetaldeydes, esters and the higher boiling, fusel oils. • The alcohol is obtained by several operations. • First, steam is passed through the beer which is said to be steamstripped. • The result is a dilute alcohol solution which still contains part of the undesirable volatile compounds. • Secondly, the dilute alcohol solution is passed into the center of a multi-plate aldehyde column in which the following fractions are separated: esters and aldehydes, fusel oil, water, and an ethanol solution containing about 25% ethanol. • Thirdly, the dilute alcohol solution is passed into a rectifying column where a constant boiling mixture, an azeotrope, distils off at 95. 6% alcohol concentration.

Distillation • To obtain 200° proof alcohol, such as is used in gasohol blending, the 96. 58% alcohol is obtained by azeotropic distillation. • The principle of this method is to add an organic solvent which will form a ternary (three-membered) azeotrope with most of the water, but with only a small proportion of the alcohol. • Benzene, carbon tetrachloride, chloroform, and cyclohezane may be used, but in practice, benzene is used. • Azeotropes usually have lower boiling point than their individual components and that of benzene-ethanol-water is 64. 6°C. • On condensation, it separates into two layers. • The upper layer, which has about 84% of the condensate, has the following percentage composition: benzene 85%, ethanol 18%, water 1%. • The heavier, lower portion, constituting 16% of the condensate, has the following composition: benzene 11%, ethanol 53%, and water 36%.

Distillation • In practice, the condensate is not allowed to separate out, but the arrangement of plates within the columns enable separation of the alcohol. Four columns are usually used. • The first and second columns remove aldehydes and fusel oils, respectively, while the last two towers are for the concentration of the alcohol. • A flow diagram of conventional absolute alcohol production from molasses is given in Fig. 20. 4

Distillation

Some Developments in Alcohol Production • Due to the current interest in the potential of ethanol as a fuel and a chemical feedstock, research aimed at improving the conventional method of production has been undertaken, and more will, most certainly, be undertaken. Some of the techniques aimed at improving productivity are the following: • (i) Developments of new strains of yeast of Saccharomyces uvarum able to ferment sugar rapidly, to tolerate high alcohol concentrations, flocculate rapidly, and whose regulatory system permits it to produce alcohol during growth. • (ii) The use of continuous fermentation with recycle using the rapidly flocculating yeasts. • (iii) Continuous vacuum fermentation in which alcohol is continuously evaporated under low pressure from the fermentation broth. • (iv) The use of immobilized Saccharomyces cerevisiae in a packed column, instead of in a conventional stirred tank fermentor. Higher productivity consequent on a higher cell concentration was said to be the advantage.

Some Developments in Alcohol Production • (v) In the ‘Ex-ferm’ process sugar cane chips are fermented directly with a yeast without first expressing the cane juice. • The chips may be dried and used in the offseason period of cane production. • It is claimed that there is no need to add nutrients as would be the case with molasses, since these are derived from the cane itself. • A more complete extraction of the sugar, resulting in a 10% increase in alcohol yield, is also claimed. • (vi) The use of Zymomonas mobilis, a Gram-negative bacterium which is found in some tropical alcoholic beverages, rather than yeast is advocated. • The advantages claimed for the use of Zymomonas are the following: • (a) Higher specific rates of glucose uptake and ethanol production than reported for yeasts. • Up to 300% more ethanol is claimed for Zymomonas than for yeasts in continuous fermentation with all recycle.

Some Developments in Alcohol Production • (b) Higher ethanol yields and lower biomass than with yeasts. • This deduction is based on Fig. 20. 5 where, although the same quantity of alcohol is produced by the two organisms in 30 -40 hours, the biomass of Zymomonas required for this level of production is much less than with yeast. • The lower biomass appears to be due to the lower energy available for growth. • Zymomonas utilized glucose by the Enthner-Duodoroff pathway (Fig. 5. 4) which yields one mole of ATP/mole glucose, whereas yeasts utilize glucose anaerobically via the glycolytic pathway (Fig. 5. 1) to give two ATP/mole glucose. Its use does not appear to have gained general

Some Developments in Alcohol Production • (c) Ethanol tolerance is at least as high or even higher [up to 16% (v/v)] in some strains of the bacterium than with yeast. • (d) Zymomonas also tolerates high glocuse concentration and many cultures grow in sugar solutions of up to 40% (w/v) glucose which should lead to high ethanol production. • (e) Zymomonas grows anaerobically and, unlike yeasts, does not require the controlled addition of oxygen for viability at the high cell concentrations used in cell recycle. • (f) The many techniques for genetic engineering already worked out in bacteria can be easily applied to Zymomonas for greater productivity.

Some Developments in Alcohol Production