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Literature Review 15 ___________________________________________________________________________ 2 LITERATURE REVIEW Acetic acid is an important raw material in the chemical industry. It is normally produced by chemical synthesis since the traditional vinegar fermentation process suffers from low product yield and high energy cost. However, the balance of economic advantage will not perpetually favor the petrochemical industry and the prospect of exhaustion of fossil fuel will soon or later force the use of fermentation process for renewable substrates as a route to useful chemicals. So, this literature review first documents the acetic acid historical background and its production processes with a short description of renewable substrates available for a homoacetogen fermentation. The ultimate requirement of organic compounds and the actual human trend for towards environment protection have triggered a renewed interest in the study of whey (or milk) permeate valorization by fermentation. Thus, the second purpose of this chapter is to show the potential use of milk permeate as a substrate for organic acids production due to its advantageous chemical composition. Significant advances have recently been made in the development of potential use of thermophilic clostridia for organic compounds production from renewable resources like corn ...
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Literature Review 15 ___________________________________________________________________________
2 LITERATURE REVIEW
Acetic acid is an important raw material in the chemical industry. It is normally produced by chemical synthesis since the traditional vinegar fermentation process suffers from low product yield and high energy cost. However, the balance of economic advantage will not perpetually favor the petrochemical industry and the prospect of exhaustion of fossil fuel will soon or later force the use of fermentation process for renewable substrates as a route to useful chemicals. So, this literature review first documents the acetic acid historical background and its production processes with a short description of renewable substrates available for a homoacetogen fermentation. The ultimate requirement of organic compounds and the actual human trend for towards environment protection have triggered a renewed interest in the study of whey (or milk) permeate valorization by fermentation. Thus, the second purpose of this chapter is to show the potential use of milk permeate as a substrate for organic acids production due to its advantageous chemical composition. Significant advances have recently been made in the development of potential use of thermophilic clostridia for organic compounds production from renewable resources like corn starch or cellulose for industrial purposes. Since lactose appears to be a difficult substrate for fermentation compared to glucose or sucrose, the final aim of this chapter is to consider and describe the possibilities of lactose utilization for acetic acid production by clostridia to investigate some new process technologies. 2.1 Calcium-Magnesium-Acetate (CMA)
2.1.1 Acetic acid historical background Acetic acid has been produced as long as wine making has been practiced and therefore dates back to at least 10,000 BC (Nickol, 1979, Agreda and Zoeller, 1993), although it could be predated by fermented foods made from milk (Allgeier at al., 1974). It is assumed that the first vinegar, which is an aqueous solution of acetic acid, resulted from spoiled wine (Ghose and Bhadra, 1985), given that the latin wordacetum sour or sharp wine. It means initially functioned as a medicinal agent and was most likely the first known antibiotic. For most of human history, all acetic acid was derived by the same age-old process of sugar fermentation to ethyl alcohol and subsequent oxidation to acetic acid by micro-organisms to produce vinegar. This was the sole source of acetic acid. Late in the nineteenth century, this process was supplemented by the advent of wood distillation, which provided an additional source of acetic acid (Agreda and Zoeller, 1993). In 1916, the first dedicated plant for the production of acetic acid by chemical rather than biological means became commercial (LeMonnier, 1965). This method was based on acetylene-derived acetaldehyde, and it marks the advent of inexpensive, industrial-grade acetic acid and the birth of viable industry based on its use (Agreda and Zoeller, 1993). The advantages of chemical synthetic routes include high acetate concentrations (35-45% by weight), high production rates, and acetic acid generated in the free-acid form. The major disadvantages are the need for high temperatures and high pressures, strong agitation, the risk
Literature Review 16 ___________________________________________________________________________ of explosion, the high cost of catalysts, and the dependence on nonrenewable uncertain sources of raw materials (crude oil). Fermentation production routes have traditionally been aimed at the food market. Vinegar production usually requires lower capital investment, has shorter start-up times, and can generate different types and flavors of vinegar when different carbohydrate sources are used. Furthermore, the raw material (e.g., corn) is a renewable resource. The cost of acetic acid from chemical synthesis is about 50 cents/kg whereas it reaches 70 cents/kg for aerobic fermentation. Clearly the latter value must be decreased if fermentation production is to supply the demand for nonfood uses. In 1989, the worldwide production of acetic acid by the petrochemical route approximated 4 millions tonnes (Busche, 1991). The industrial importance of acetic acid can be understood from Figure 2.1. The major outlet today is for vinyl acetate, which is used for vinyl plastics, adhesives, textiles finishes, and latex paints. This market has grown rapidly during the past few years due to the demand for synthetic fibers. In 1979, calcium magnesium acetate (CMA) was identified as a non-corrosive environmental-friendly alternative to chloride salts for deicing roads (Marynowski et al., 1985). Liquid potassium acetate is being used now as a deicer for airport runways and as a heat exchange fluid; in the latter role it could serve as a partial replacement for ethylene glycol. In addition, there are reports that CMA or calcium acetate can also be used as an additive to coal-fired combustion units, for example, boilers used by electrical utilities (Levendis, 1991; Manivanan and Wise, 1991; Sharma, 1991). Here calcium acts as a grabber for sulfur in the coal, reduces sulfur dioxide emissions, and partially relieves the problem of acid-rain pollution. If these environment-related substitutions take place, the demand for acetic acid would increase tremendously.
Literature Review 17 ___________________________________________________________________________
Vinyl Acetate
Textile Finishes Vinyl Plastics Latex Paints Adhesives
ACETIC ACID
Chloroacetic Acetic Acetate Acid Anhydride Esters
Pharmaceuticals Plasticizers Textiles Vinegar
Cellulose Solvents Acetate
Molding Compounds Transparent Sheets Textile Fibers Photo Film Lacquers
Figure 2.1. Uses of acetic acid.
Acetate Salts
Heat Transfer Liquids Meat Preservative Neutralyser Fungicide De-icers
Literature Review 18 ___________________________________________________________________________
2.1.2 Acetic acid production processes Acetic acid as an industrial chemical is presently produced from fossil fuels and chemicals by three processes : acetaldehyde oxidation, hydrocarbon oxidation, and methanol carbonylation. The shortage of crude oil caused by economical or political circumstances has triggered the search for substitutes for natural gases. Acetic acid produced by the fermentation of renewable resources is considered as appropriate gasoline substitutes. It can be produced by three biological routes, but at present time, neither process competes with the production costs of the synthetic process (see Table 2.1 for the summary of the published results) : Two-step aerobic mesophilic process from glucose.Food-grade acetic acid is produced by the two-step vinegar process (Allgeier et al., 1974; Ebner and Follman, 1983; Crueger and Crueger, 1990; Shreve and Brink, 1977). The first step is the production of ethanol from a carbohydrate source such as glucose. This is carried out at 30°C using the yeast Saccharomyces cerevisiae : C6H12O6 ®2CO2 +2CH3CH2OH. The second step is the oxidation of ethanol to acetic acid. Although a variety of bacteria can produce acetic acid, only members ofobetteacr cAare used commercially, typically the aerobic bacteriumAcetobacter acetiat 27-37°C. This fermentation is an incomplete oxidation because the reducing equivalents generated are transferred to oxygen and not to carbon dioxide : 2CH3CH2OH+O2 ®2CH3COOH+2H2O. The overall theoretical yield is 0.67 g acetic acid per gram glucose. At the more realistic yield of 76% (of 0.67, i.e., 0.51 g per gram glucose), this process requires 2.0 kg of sugar or 0.9 kg of ethyl alcohol per kg of acetic acid produced (Busche et al., 1982). Complete aeration and strict control of the oxygen concentration during fermentation are important to maximize yields and keep the bacteria viable (Muraoka et al., 1980; Osuga et al., 1984). Submerged fermentation has almost completely replaced surface fermentation methods. The draw-and-fill mode of operation can produce acetic acid at concentrations up to 10% wt/wt in continuous culture at pH 4.5 in about 35 hours (Crueger and Crueger, 1990; Ebner and Follman, 1983; Nickol, 1979). However, the productivity is low at high acetic acid concentration and the culture is extremely sensitive to oxygen depletion (Ghose and Badhra, 1985). One-step anaerobic thermophilic process from glucose.In the 1980s, another process for production of acetic acid emerged based on anaerobic fermentation usingMoorella.These organisms, in contrast to the vinegar process, can convert glucose, xylose, and some other hexoses and pentoses almost quantitatively to acetate according to the following reaction : C6H12O6 ®3CH3COOH. This equation is described on Figure 2.2. Glucose is first oxidized to pyruvate via glycolyse (box A). Pyruvate is subsequently decarboxylated and oxidized to acetyl-CoA, which is then subject to phosphorylation and conversion to acetate via acetate-kinase (box B). Processes conceptualized in boxes A and B collectively yield 2 moles of acetate per mole of glucose. CO2 is subsequently used as the terminal sink for the eight reducing equivalents
Literature Review 19 ___________________________________________________________________________ generated during glycolysis and the oxidation of pyruvate (box C). This last process, that of box C, is the acetyl-CoA pathway and is responsible for the formation of the third molecule of acetate. Typical acidogenic bacteria areClostridium aceticum(Braun et al., 1981),Moorella thermoacetica (Fontaine et al., 1942; Andreesen et al., 1973),Clostridium formicoaceticum (Andreesen et al., 1970) andAcetobacterium woodii(Balch et al., 1977). Many can also reduce carbon dioxide and the one-carbon compounds to acetate (Ljungdahl, 1983). This fermentation route has several advantages. It is anaerobic and thus should have lower fermentation cost. The theoretical yields are higher than the aerobic fermentation : 3 moles of acetic acid are produced per mole of glucose consumed, that is, 1 g acetic acid/g glucose (Brownell and Nakas, 1991; Brumm, 1988; Parekh and Cheryan, 1990a, b, 1991; Schwartz and Keller, 1982b; Wise et al., 1991). Actual yields withM. thermoacetica range from 0.85 (Fontaine et al., 1942; Ljungdahl et al., 1986; Wang et al., 1978) to 0.90 g acetic acid per gram glucose and greater (Parekh and Cheryan, 1990a, 1994; Shah and Cheryan, 1995). Until 1967,M. thermoaceticathe only acetogen easily available for study was (Ljungdahl, 1986). Consequently, the most detailed studies on acetate biosynthesis have been performed with this organism. However, a basic problem with the acetogenic process is that the acetogens evaluated are inhibited by high concentrations of acetate and low pH (Wang and Wang, 1984). Many studies have addressed this problem with batch, reactor, and immobilized cell systems (Wang and Wang, 1983; Sugaya et al., 1986; Ljungdahl et al., 1985, 1989), acid tolerant mutants of M. thermoacetica have been obtained and characterized (Schwartz and Keller, 1982b; Brumm, 1988; Parekh and Cheryan, 1991). Additional and similar work has been conducted withAcetogenium kivui(Klemps et al., 1987; Eysmondt et al., 1990; Ibba and Fynn, 1991). In this regard, it is postulated that production costs of the acetogenic process could outcompete the synthetic process should an acid tolerant, high acetic acid-producing acetogen (or mutant) be obtained (Bushe, 1991). The possible use of calcium-magnesium-acetate as an environmentally safe road deicer and in the potential control of sulfur emissions from the combustion of high-sulfur coal continues to focus attention on future applications of acetogenesis for the commercial production of acetic acid.
Literature Review 20 ___________________________________________________________________________
A
B
glucose glycolysis and pyruvate ferredoxin oxidoreductase 2 ATP (SLP)
C 2 acetyl-CoA 2 CO2 + 8 [H] phosphotrans-acetylaseacetyl-CoA 2 acetylphosphate pathway 2 ATP acetate (SLP)kinaseacetate 2 acetate
Figure 2.2. Overview of the three main processes of acetogens that collectively yield three molecules of acetate per molecule of glucose. SLP, substrate-level phosphorylation (from Drake, 1993).
Table 2.1. Approaches for acetate production with anaerobes. Substrate Strain Fermentation Acetic acid results Reference GlucoseM. thermoacetica g/(L.h) and 7.1 g/L 14.3 and Boogdan, 1985 Reed systemand Cell-recycling M. thermoautotrophica GlucoseM. thermoaceticamutant g/L at pH 4Free cells in continuous et al., 1987 Reed5.3 and 2.5 g/L at pH 4.7 culture GlucoseM. thermoaceticaImmobilized cells on 6.9 g/h at a dilution rate of 0.37 h-1, 19 g/L, 60 g Wang et Wang, 1983 carrageenan gel cells/L gel GlucoseM. thermoacetica and Wang, 1984and Fed-batch 0.8 fermentation Wang g/(L.h) and 56 g/L at pH 6.9 after 125 h M. thermoautotrophica GlucoseM. thermoacetica Wiegel et al., 1991 at pH 4.8, 3.9 g/(L.h), 8.1 g/L - cells to disksand Attached M. thermoautotrophica at pH 5 and T a rotary fermentor - inrof 1.3 h, 5 g/(L.h), 100 mM - at pH 6.6 and Trof 30 min, 11 g/(L.h), 50 mM Hydrolyzed Acetogenium kivui g/(L.h) and 9 g/L Wiegel et al., 1991Continuous cultivation 4.8 corn starch GlucoseM. thermoacetica cellsmutant Free et al., 1985 Keller pH 4.5, 4.5 g/L At MethanolM. thermoautotrophica g/L SavageFree cells 1.8 and Drake, 1986 GlucoseM. thermoacetica g/L and 4 g/(L.h) at pH 5.6-7.2 18 et al., 1986 Ljungdahl cyc- ellu Dionti M. thermoautotrophica 3.8fermentation mode g/(L.h) Acetogenium kivui4 g/(L.h) GlucoseM. thermoacetica Parekh and Cheryan, 1990a, 1990bFed-batch fermentation 46 g/L at pH 6.6 after 192 h GlucoseAcetogenium kivuiContinuous culture 34.5 g/L with a 78.7% yield, 3.55 g/(L.h) with a Eysmondt et al., 1990 dilution rate of 0.42 h-1 Oat speltM. thermoacetica Brownell and Nakas, 1991Free cells g/L in 72h 12 xylan HemicelluloseM. thermoacetica 14.4 g/L in 72hFree cells and Nakas, 1991 Brownell XyloseM. thermoacetica 42Fed-batch mode and Nakas, 1991 Brownell g/L in 116h at pH 7.0 Corn starchC. thermosaacharolyricumFree cells Yield of 2.73 acetate/mol glucose, 300 mM Wiegel et al., 1984 andM. thermoautotrophica GlucoseM. thermoacetica Parekh g/L, 80% yield, 0.7 g/(L.h) and Cheryan, 1994 83-100Fed batch fermentation Fed-batch with CSTR 1.12 g/(L.h) and 38 g/L Shah and Cheryan, 1995 WheyLactobacillus lactis et al., 1988 Tang g/L in 80 h at pH 7.6, 95% yield 30 cellsand Free permeateC. formicoaceticum Immobilized cells 1.23 g/(L.h) and 75 g/L, 95% yield Huang and Yang, 1998
Literature Review 22 ___________________________________________________________________________ Two-step anaerobic mesophilic process from lactose (of whey permeate).A novel approach for acetic acid production has been reported by Tang et al., 1988 : they have used a mixed culture ofLactobacillus lactis andClostridium formicoaceticum the conversion of sweet for whey permeate to acetic acid under mesophilic condition (37°C). The former is a homolactic bacterium that can convert 90% of the lactose present in whey into lactate. The latter cannot ferment lactose, but can produce acetate from lactate. When the mixed fermentation was controlled at pH 7.6, an acetic acid concentration of 20 g/L was produced within 20h. Production then slowed due to product inhibition, but after 80h, acetate was present at 30 g/L, plus lactate at 20 g/L. Recently, this co-culture fermentation process has been used in a fibrous-bed bioreactor (Huang and Yang, 1998): the final acetate concentration obtained in the fed-batch fermentation was 75 g/L, with an overall acetic acid yield from lactose and a productivity of 0.9 g/g and 1.23 g/(L.h), respectively. Hence, this novel approach to acetic acid production from whey seems to be promising. 2.1.3 Renewable substrates for CMA production Organic residues. present, no commercial sources of CMA exist, because it is notAt available in adequate quantities or at low enough cost to be considered as a potential raw material for highway deicing use. Consequently, inexpensive sources of acetic acid using fermentation routes of biomass were searched for. Many organic residues are readily available as cheap sources for the fermentation process wherein the main product is acetic acid. They may be categorized by the amounts available, the ease with which they can be converted, and the extent of pretreatment required before fermentation. These residues are (DeSouza and Wise, 1991) : ·: large amounts available which need little or no pretreatmentSewage sludge ·as far as supply is concerned, but requires pretreatmentCorn : an excellent source ·Wood residue : an almost unlimited source which exceeds the demand but requires pretreatment prior to fermentation ·Municipal solid waste : consists of nearly 40-80 % of cellulosic matter which does not need as severe a pretreatment as corn or wood chips · :Cheese whey or milk permeate an easy-to-convert dairy by-product which essentially requires no pretreatment Milk permeate, a potential substrate for biotechnology.Although in comparaison with other carbon substrates (like glucose in corn steep liquor), milk lactose can be assimilated only by specific micro-organisms (Moulin and Galzy, 1981), permeate, due to its animal origin, contains all the minerals, trace elements and water-soluble vitamins required for the growth of micro-organisms and thus constitutes an excellent basal medium for any type of culture required by the fermentation industry, whereas it appears difficult to produce a culture medium with wide applications from starchy materials without a preliminary treatment. In addition, sterilization of the medium is not necessary since the ultrafiltration process eliminates bacteria.
Literature Review 23 ___________________________________________________________________________ 2.2 Dairy products from milk 2.2.1 Milk permeate composition Milk permeate is a by-product of the cheese-making industry following the separation of protein from milk by ultrafiltration. Its composition varies according to its origin (ewe, goat or cow) and to the cheese-making technique employed. The average composition (see Table 2.2) shows the importance of two factors: the high content of lactose and a lack of proteins removed during the ultrafiltration process. UF permeate contains vitamins (Table 2.3) and trace elements (Table 2.4). It is therefore extremely valuable nutritionally, and thus it can be used for the industrial production of many micro-organisms capable of using lactose or its hydrolysis products. Table 2.2. Average composition of UF permeate (from Coton, 1980). Component Percentage Lactose 4.9 Protein 0.03 Non protein N 0.1 Ash 0.5 Fat 0.01 < Lactic acid 0.15 Total solids 5.7 Table 2.3. Average vitamin content of UF permeate (from Moulin and Galzy, 1984). Vitamin mg/100 g of dry matter Vitamin A 80 Thiamin 5-6 Pyridoxin 5-10 Riboflavin 15-20 Calcium pantothenate 50-60 Biotin 0.1-0.3 Cobalamin 0.02-0.05 Vitamin C 20-40 Table 2.4. Average content of the main trace elements in UF permeate (from Moulin and Galzy, 1984). Element mg/100 g of dry matter Iron 3-11 Copper 1-3 Zinc 30-33 Manganese 0.5-0.8
Literature Review 24 ___________________________________________________________________________
2.2.2 Milk permeate utilizations Ultrafiltration is being used on an increasing scale to process milk for the production of special cheese. For good economics, each UF plant must have an associated process for utilization of permeate, and the expansion in the use of UF has been hindered by the difficulty of using permeate profitably. Figure 2.3 shows the principal possible uses for permeate . PERMEATES
Pig feed Concentration
Fermentation
Lactose extraction Hydrolysis
Drying Crystallisation to cattle lick Reaction with urea
Reaction to give ammonium lactate Biomass Anaerobic - Methane for fuel Other organic chemicals
ANIMAL USE
INDUSTRIAL
DIRECT HUMAN USE
Figure 2.3. Principal possible uses for milk permeate (from Coton, 1980).
Literature Review 25 ___________________________________________________________________________ Pig feed :Milk permeate has been shown to be suitable for pig feeding, though obviously additional protein must be provided to replace that extracted by ultrafiltration. After allowing for transport of the permeates, profit is very small and with the increase in size of cheese creameries, it becomes impossible to rely on pig feeding as an outlet for permeates. Drying :and costly, on its own it is thereforeThe drying of permeate is difficult commercially unattractive. Drying can be made easier if other material (soya protein) is added. In this case, the process could be profitable, the economics of these possibilities vary with individuals circumstances. Crystallization to cattle lick :The permeate could be utilized by concentration to 65-70% after which it was to be poured into moulds, and allowed to set by crystallization to give blocks containing a residual 23-30% water. These blocks can be used as cattle lick. This is a very simple method of permeate utilization but does not appear to have been taken up commercially. Reaction with urea :Urea is widely used as a source of non-protein nitrogen in the feeding of ruminants but its use suffers from the signifiant disadvantage that ammonia is produced in the rumen by microbial breakdown more rapidly than it can be utilized by the rumen micro-organisms and may attain levels which are toxic to the animal. Reaction to give ammonium lactate : similar approach, which concerns more with A whey than permeate utilization is to produce a fermented ammoniates condensed whey by the controlled anaerobic fermentation of lactose byLactobacillus bulgaricus, and the contiuous neutralization with ammonia of the lactic acid formed by the fermentation. The fermentation liquor is then evaporated to a total solids level of about 60 %, and this, it is claimed, can be incorporated in animals feeds at up 25 %, and fed so as to provide up to 8 % of dietary, crude protein in dairy cattle, without the toxic effects often associated with a non-protein nitrogen source. It is possible to produce this type of material from permeate although the ratio of protein N to NPN will be changed. Biomass production : By fermenting the lactose in permeate under strongly aerobic conditions with an appropriate yeast strain, a biomass is formed which can be separated and dried, or the total fermentation liquor can be evaporated and dried. This biomass has a crude protein content of about 45 % and can be used satisfactorily in animal feed in place of soya meal. After hydrolysis the biomass can be used in certain specialist human foods or for food flavouring, although demand in human foods is quite small. The technical problem in producing biomass is to provide sufficient oxygen in solution to convert substrate efficiently to biomass. Provision of this oxygen requires a high energy input, and the fermentation plant also requires considerable capital. Methane production :of increasing energy costs, methods for the these days In production of energy from renewable resources have a particular appeal. Production of methane by anaerobic fermentation of permeate is one possibility. Hydrolysis : Lactose permeate can be hydrolyzed in glucose and galactose by using an enzymatic process to produce syrup or other products. But none of these products was quite successful commercially.
