Transcript Industrial Biotechnology CHAPTER 8 Production of Organic Acids and Industrial Alcohol Production of Citric Acid.

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 CO2 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
pH.
• Citric acid is used in hair rinses and hair and wig setting fluids.
• Miscellaneous uses in industry
• In neutral or low pH 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, pH, 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 pH and especially the presence of citric acid itself inhibits the
TCA and hence encourages the production of more citric acid; the pH
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. MgSO4, 7H2O and KH2PO4 at about 1% and 0.05-2% respectively are
added (in submerged fermentation phosphate restriction is not necessary).
• The pH 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 pH 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 Dand 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/ pH 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
pH 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 pH 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 pH 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
CaSO4.2H2O 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, CH3 CH2 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 the 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 106
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 pH is usually in the range 4.5-4.7, when the buffering capacity of the medium
is high.
• Higher pH values tend to lead to higher glycerol formation.
• When the buffering capacity is lower, the initial pH 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 acceptance.
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