Ethanol production from sweet sorghum juice in batch and fed-batch fermentations by <Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

Sweet sorghum juice supplemented with 0.5% ammonium sulphate was used as a substrate for ethanol production by Saccharomyces cerevisiae TISTR 5048. In batch fermentation, kinetic parameters for ethanol production depended on initial cell and sugar concentrations. The optimum initial cell and sugar concentrations in the batch fermentation were 1 × 10⁸ cells ml⁻¹ and 24 °Bx respectively. At these conditions, ethanol concentration produced (P), yield (Y _ps) and productivity (Q _p ) were 100 g l⁻¹, 0.42 g g⁻¹ and 1.67 g l⁻¹ h⁻¹ respectively. In fed-batch fermentation, the optimum substrate feeding strategy for ethanol production at the initial sugar concentration of 24 °Bx was one-time substrate feeding, where P, Y _ps and Q _p were 120 g l⁻¹, 0.48 g g⁻¹ and 1.11 g l⁻¹ h⁻¹ respectively. These findings suggest that fed-batch fermentation improves the efficiency of ethanol production in terms of ethanol concentration and product yield.     

Influence of the ratio of the initial substrate concentration to biomass concentration on the performance of a sequencing batch reactor

Biomass behaviour and COD removal in a benchscale activated sludge reactor have been studied alternating anaerobic and aerobic conditions. Particular attention has been paid to the influence of the ratio of the initial substrate concentration (S ₀) to the initial biomass concentration (X ₀) on the reactor performance. Tests at very low ratios (S ₀/X ₀<2) demonstrate the existence of a threshold below which the reactor performance is seriously affected (S ₀/X ₀=0.5). Under conditions of total suppression of cell duplication, substrate maintenance requirements have also been calculated for the microbial consortium present in the activated sludges. The results obtained show that stressed biomass can survive conditions of substrate lack better than unstressed biomass.List of Symbols b h^–1 specific death rate - COD g/l chemical oxygen demand - DO g/l dissolved oxygen concentration - K _s g/l Monod saturation constant - MLSS g/l mixed liquor suspended solid concentration - P g/l phosphorus concentration - S g/l substrate concentration - S ₀ g/l initial substrate concentration - SS g/l suspended solid concentration - t h time - X g/l biomass concentration - X ₀ g/l initial biomass concentration - Y _SX g/g yield of growth on substrate - _max h^–1 maximum specific growth rate     

Fermentation ofd-xylose,d-glucose andl-arabinose mixture byPichia stipitis Y 7124: sugar tolerance

Summary The effect of substrate concentration (S ₀) on the fermentation parameters of a sugar mixture byPichia stipitis Y 7124 was investigated under anaerobic and microaerobic conditions. Under microaerobiosisP. stipitis maintained high ethanol yield and productivity when initial substrate concentration did not exceed 150 g/l; ethanol yield of about 0.40 g/g and volumetric productivity up to 0.39 g/l per hour were obtained. Optimal specific ethanol productivity (0.2 g/g per hour) was observed withS ₀=110 g/l. Under anaerobic conditionsP. stipitis exhibited the highest fermentative performances atS ₀=20 g/l; it produced ethanol with a yield of 0.42 g/g, with a specific rate of 1.1 g/g per day. When the initial substrate level increased, specific ethanol productivity declined gradually and ethanol yield was dependent on the degree of utilization of each sugar in the mixture.Abbreviations E _m maximum produced ethanol (g/l) - E ₀ initial ethanol (g/l) - E _v evaporated ethanol (g/l) - Q _p volumetric productivity of ethanol (g ethanol/l per hour or g/l per day) - q _p specific productivity of ethanol (g ethanol/g cells per hour) - q _pm maximum specific productivity of ethanol (g/l per hour) - S ₀ initial substrate concentration (g/l) - t _f time at which produced ethanol is maximum (h) - Y _p/s ethanol yield (g ethanol produced/g substrate utilized) - Y _x/s cell yeild (g cells produced/g substrate utilized) - Y _xo/xy xylitol yield (g xylitol produced/g xylose utilized) - probability coefficient - specific growth rate coefficient (h^-1 or d^-1)     

Performance,kinetics, and substrate utilization in a continuous yeast fermentation with cell recycle by ultrafiltration membranes

Summary A continuous single stage yeast fermentation with cell recycle by ultrafiltration membranes was operated at various recycle ratios. Cell concentration was increased 10.6 times, and ethanol concentration and fermentor productivity both 5.3 times with 97% recycle as compared to no recycle. Both specific growth rate and specific ethanol productivity followed the exponential ethanol inhibition form (specific productivity was constant up to 37.5 g/l of ethanol before decreasing), similar to that obtained without recycle, but with greater inhibition constants most likely due to toxins retained in the system at hight recycle ratios.By analyzing steady state data, the fractions of substrate used for cell growth, ethanol formation, and what which were wasted were accounted for. Yeast metabolism varied from mostly aerobic at low recycle ratios to mostly anaerobic at high recycle ratios at a constant dissolved oxygen concentration of 0.8 mg/kg. By increasing the cell recycle ratio, wasted substrate was reduced. When applied to ethanol fermentation, the familiar terminology of substrate used for Maintenance must be used with caution: it is not the same as the wasted substrate reported here.A general method for determining the best recycle ratio is presented; a balance among fermentor productivity, specific productivity, and wasted substrate needs to be made in recycle systems to approach an optimal design.Nomenclature B Bleed flow rate, l/h - C _T Concentration of toxins, arbitrary units - D Dilution rate, h^-1 - F Filtrate or permeate flow rate, removed from system, l/h - F _o Total feed flow rate to system, l/h - K _s Monod form constant, g/l - P Product (ethanol) concentration, g/l - P _o Ethanol concentration in feed, g/l - PP} Adjusted product concentration, g/l - PD Fermentor productivity, g/l-h - R Recycle ratio, F/F _o - S Substrate concentration in fermentor, g/l - S _o Substrate concentration in feed, g/l - V Working volume of fermentor, l - V _MB Viability based on methylene blue test - X Cell concentration, g dry cell/l - X _o Cell concentration in feed, g/l - Y _ATP Cellular yield from ATP, g cells/mol ATP - Y _ATPS Yield of ATP from substrate, mole ATP/mole glucose - Y _G True growth yield or maximum yield of cells from substrate, g cell/g glucose - Y _P Maximum theoretical yield of ethanol from glucose, 0.511 g ethanol/g glucose - Y _P/S Experimental yield of product from substrate, g ethanol/g glucose - Y _x/s Experimental yield of cells from substrate, g cell/g glucose - S _NP/X Non-product associated substrate utilization, g glucose/g cell - k ₁, k₂, k₃, k₄ Constants - k ₁ ^APP , k ₂ ^APP Apparent k ₁, k₃ - k ₁ ^TRUE True k ₁ - m Maintenance coefficient, g glucose/g cell-h - m ^* Coefficient of substrate not used for growth nor for ethanol formation, g glucose/g cell-h - Specific growth rate, g cells/g cells-h, reported as h^-1 - _m Maximum specific growth rate, h^-1 - v Specific productivity, g ethanol/g cell-h, reported as h^-1 - v _m Maximum specific productivity, h^-1     

A heuristic approach to fed-batch optimisation of streptokinase fermentation

Previous studies have shown that the rate of formation of streptokinase, a secondary metabolite, in batch fermentation is proportional to the specific growth rate of the biomass, which in turn is inhibited by its substrate and the primary product (lactic acid). These kinetics suggest the suitability of fed-batch operation to increase the yield of streptokinase. A near-optimal feed policy has been calculated by the chemotaxis algorithm, and it shows a substrate feed rate decreasing nonlinearly and vanishing after 11 hours. This is followed by batch fermentation for a further 8 hours, at the end of which 12% more streptokinase is generated than by purely batch fermentation. Further improvements in productivity are possible.List of Symbols k _dh^–1 decay constant for active cells - k _ph^–1 decay constant for streptokinase - K _Igl^–1 inhibition constant for lactic acid - K_S gl^–1 inhibition constant for substrate - M gl^–1 lactic acid concentration - P gl^–1 streptokinase concentration - Q 1h^–1 substrate feed rate - S gl^–1 substrate concentration - S _ingl^–1 inlet concentration of substrate - t h time - t _bh end-point of batch fermentation - t _fh end-point of fed-batch fermentation - V l volume of broth in fermenter - V ₀ l initial value of V (at t=0) - V _ml maximum value of V - X gl^–1 total biomass concentration - X _agl^–1 concentration of active biomass - Y _MX yield coefficient for lactic acid from biomass - Y _PX yield coefficient for streptokinase from biomass - Y _XS yield coefficient for biomass from substrate Greek Letters h^–1 specific growth rate of biomass - _mh^–1 maximum specific growth rate     

Phenol degradation by a psychrotrophic strain of Pseudomonas putida

Summary Cell growth and phenol degradation kinetics were studied at 10°C for a psychrotrophic bacterium, Pseudomonas putida Q5. The batch studies were conducted for initial phenol concentrations, S_o, ranging from 14 to 1000 mg/1. The experimental data for 14<=S_o<=200 mg/1 were fitted by non-linear regression to the integrated Haldane substrate inhibition growth rate model. The values of the kinetic parameters were found to be: _m=0.119 h^–1, K _S=5.27 mg/1 and K _I=377 mg/1. The yield factor of dry biomass from substrate consumed was Y=0.55. Compared to mesophilic pseudomonads previously studied, the psychrotrophic strain grows on and degrades phenol at rates that are ca. 65–80% lower. However, use of the psychrotrophic microorganism may still be economically advantageous for waste-water treatment processes installed in cold climatic regions, and in cases where influent waste-water temperatures exhibit seasonal variation in the range 10–30°C.Nomenclature K _S saturation constant (mg/l) - K _I substrate inhibition constant (mg/l) - specific growth rate (h^–1) - _m maximum specific growth rate without substrate inhibition (h^–1) - _max maximum achievable specific growth rate with substrate inhibition (h^–1) - S substrate (phenol) concentration (mg/l) - S_o initial substrate concentration (mg/l) - S_max substrate concentration corresponding to _max (mg/l) - t time (h) - X cell concentration, dry basis (mg DW/l) - X_f final cell concentration, dry basis (mg DW/l) - X_o initial cell concentration, dry basis (mg DW/l) - Y yield factor (mg DW cell produced/mg substrate consumed)     

Kinetics of <Emphasis Type="Italic">Bacillus thuringiensis</Emphasis> var. <Emphasis Type="Italic">israelensis</Emphasis> growth on high glucose concentrations

The kinetic and general growth features of Bacillus thuringiensis var. israelensis were evaluated. Initial glucose concentration (S ₀) in fermentation media varied from 10 to 152 g/l. The results afforded to characterize four morphologically and physiologically well-defined culture phases, independent of S ₀ values: Phase I, vegetative growth; Phase II, transition to sporulation; Phase III, sporulation; and Phase IV, spores maturation and cell lysis. Important process parameters were also determined. The maximum specific growth rates (μ _X,m) were not affected with S ₀ up to 75 g/l (1.0–1.1 per hour), but higher glucose concentrations resulted in growth inhibition by substrate, revealed by a reduction in μ _X,m values. These higher S ₀ values led to longer Phases III and IV and delayed sporulation. Similar biomass concentrations (X _m = 15.2–15.9 g/l) were achieved with S ₀ over 30.8 g/l, with increasing residual substrate, suggesting a limitation in some other nutrients and the use of glucose to form other metabolites. In this case, with S ₀ from 30.8 to 152 g/l, cell yield (Y _X/S ) decreased from 0.58 to 0.41 g/g. On the other hand, with S ₀ = 10 g/l growth was limited by substrate, and Y _X/S has shown its maximum value (0.83 g/g).     

Effects of culture conditions on the co-fermentation of a glucose and xylose mixture to ethanol by a mutant of Saccharomyces diastaticus associated with Pichia stipitis

Substrates that contain hexose as well as pentose sugars can form an interesting substrate for the production of ethanol. Pichia stipitis and a respiratory-deficient mutant of Saccharomyces diastaticus were used to convert such a substrate into ethanol under continuous culture conditions. With a sugar mixture (glucose 70%/xylose 30%) at 50 g/l, the xylose was entirely consumed when the dilution rate (D) did not exceed 0.006 h^–1 whereas the glucose was entirely consumed whatever the D. The study of influence of initial substrate concentration (S₀) was performed at D = 0.015 h^–1. Under these conditions the substrate was entirely consumed when its initial concentration did not exceed 20 g/l. With S₀ = 80 g/l the residual xylose concentration reached 20.5 g/l. At low D or at low S₀, P. stipitis was the dominant species in the fermentor. Increasing the D or S₀ resulted in the wash-out of P. stipitis mainly because of its low ethanol tolerance. Correspondence to: J. P. Delgenes     

Ethanol fermentation by flocculating yeast: Performance and stability dependence on a critical fermentation rate

Summary A system coupling fermentor and decantor permitted strong accumulation of yeast flocs that were homogeneously suspended in the reactional volume. At 100–190 g/l glucose feed practically total substrate conversion was attained. At 130 g/l glucose feed the highest productivity (18.4 g.l.h) and the highest ethanol yield (90.6%) were reached with biomass levels of 80–90 g/l. We observed that the stability of this system is limited when a critical fermentation rate (D.S_o) close to 39–40 g/l.h (with corresponding ethanol productivities of 19–20 g/l.h) is reached. Higher fermentation rates provoked de-flocculation and lost of biomass.Symbols D dilution rate (h^–1) - E ethanol (g/l) - S_r residual substrate (g/l) - S_o substrate in the feed (g/l) - X biomass (g/l) - ethanol yield (%) - DS_o fermentation rate (g/l.h) (for S_r0) - P_E ethanol productivity (g/l.h)     

Optimization of process variables for minimization of byproduct formation during fermentation of blackstrap molasses to ethanol at industrial scale

Aims: To investigate the effect of molasses concentration, initial pH of molasses medium, and inoculum’s size to maximize ethanol and minimize methanol, fusel alcohols, acetic acid and aldehydes in the fermentation mash in industrial fermentors. Methods and Results: Initial studies to optimize temperature, nitrogen source, phosphorous source, sulfur supplement and minerals were performed. The essential nutrients were urea (2 kg in 60 m³), 0·5 l each of commercial phosphoric acid and sulfuric acid (for pH control) added at the inoculum preparation stage only. Yields of ethanol, methanol, fusel alcohols, total acids and aldehydes per 100‐l fermentation broth were monitored. Molasses at 29°Brix (degree of dissolved sugars in water), initial pH 4·5, inoculum size 30% (v/v) and anaerobic fermentation supported maximum ethanol (7·8%) with Y_P/S = 238 l ethanol per tonne molasses (96·5% yield) (8·2% increase in yield), and had significantly lower values of byproducts than those in control experiments. Conclusions: Optimization of process variables resulted in higher ethanol yield (8·2%) and reduced yield of methanol, fusel alcohols, acids and aldehydes. Significance and Impact of the Study: More than 5% substrate is converted into byproducts. Eliminating or reducing their formation can increase ethanol yield by Saccharomyces cerevisiae, decrease the overall cost of fermentation process and improve the quality of ethanol.     

Degradation of 4-chlorophenol by enriched mixed cultures utilizing phenol and glucose as added growth substrate

Two mixed cultures, phenol-oxidizing (PO) and glucose-oxidizing (GO), were cultivated in two parallel chemostat reactors. The PO culture was enriched on phenol, and the GO culture was enriched on glucose. Batch biodegradation experiments were conducted to examine the degradation of 4-chlorophenol (4-CP) under various substrate conditions. The results indicate that in the absence of added growth substrate, 4-CP transformation by PO culture was complete at S _c ^o /X _o (initial 4-CP concentration/initial biomass concentration) 0.27 and that by GO culture was complete at S _c ^o /X _o = 0.09. In the presence of 5–500 mg phenol/l, the phenol dosage required to achieve the complete transformation of 4-CP was 60 mg/l at S _c ^o /X _o = 1, increasing to 120 mg/l at S _c ^o /X _o = 2, and to 180 mg/l at S _c ^o /X _o = 5. As glucose was added to the GO culture at a concentration of over 5–500 mg chemical oxygen demand (COD)/l, 4-CP was not completely transformed at S _c ^o /X _o = 5 [S _c ^o = 50 mg/l, X _o = 10 mg/l volatile suspended solids (VSS)]. These two cultures in utilizing added growth substrate were easily switched between glucose and phenol. Overall, the capacity of PO culture to degrade 4-CP, expressed as T _c (4-CP mass consumed /biomass inactivated, having unit of mg 4-CP/mg VSS), was 0.15–0.80, which compares with T _c values of 0.05–0.26 for GO culture. This work shows that adding phenol as a growth substrate is preferable over adding glucose, as it enhances 4-CP transformation, but a final choice should take into account both degradation efficiency and the risk of phenol toxicity.     

A kinetic model and simulation of starch saccharification and simultaneous ethanol fermentation by amyloglucosidase and Zymomonas mobilis

A mathematical model is described for the simultaneous saccharification and ethanol fermentation (SSF) of sago starch using amyloglucosidase (AMG) and Zymomonas mobilis. By introducing the degree of polymerization (DP) of oligosaccharides produced from sago starch treated with -amylase, a series of Michaelis-Menten equations were obtained. After determining kinetic parameters from the results of simple experiments carried out at various substrate and enzyme concentrations and from the subsite mapping theory, this model was adapted to simulate the SSF process. The results of simulation for SSF are in good agreement with experimental results.List of Symbols g/g rate coefficient of production - _max 1/h maximum specific growth rate - E %, v/w AMG concentration - G ₁ mmol/l glucose concentration - G _c mmol/l glucose concentration consumed - G _f mmol/l glucose concentration formed - G _n mmol/l n-mer maltooligosaccharide concentration - K _i g/l ethanol inhibition constant for ethanol production - K _g mmol/l glucose inhibition constant for glucose production - K _p mmol/l glucose limitation constant for ethanol production - K _x mmol/l glucose limitation constant for cell growth - K _m,n mmol/l Michaelis-Menten constant for n-mer oligosaccharide - k _e %, v/w enzyme limitation constant - k _es proportional constant - k _{max, n} 1/s maximal velocity for n-mer digestion - k _s g/l substrate limitation constant - m _s g/g maintenance energy - MW _n g/mol molecular weight of n-mer oligosaccharide - P g/l ethanol concentration - P ₀ g/l initial ethanol concentration - P _m g/l maximal ethanol concentration - Q _pm g/(g · h) maximum specific ethanol production rate - S _n mmol/h branched n-mer oligosaccharide concentration - S ₀ g/l initial starch concentration - S _sta g/l starch concentration - S _tot g/l total sugar concentration - V _{max, n} 1/h maximum digestion rate of n-mer oligosaccharide - V ₀ g/(l · h) initial glucose formation rate - X g/l cell mass - X ₀ g/l initial cell mass - Y _p/s g/g ethanol yield - Y _x/s g/g cell mass yield     

Xylitol production from D-xylose byCandida guillermondii: Fermentation behaviour

Summary The ability ofCandida guillermondii to produce xylitol from xylose and to ferment individual non xylose hemicellulosic derived sugars was investigated in microaerobic conditions. Xylose was converted into xylitol with a yield of 0,63 g/g and ethanol was produced in negligible amounts. The strain did not convert glucose, mannose and galactose into their corresponding polyols but only into ethanol and cell mass. By contrast, fermentation of arabinose lead to the formation of arabitol. On D-xylose medium,Candida guillermondii exhibited high yield and rate of xylitol production when the initial sugar concentration exceeded 110 g/l. A final xylitol concentration of 221 g/l was obtained from 300 g/l D-xylose with a yield of 82,6% of theoretical and an average specific rate of 0,19 g/g.h.Nomenclature Q_p average volumetric productivity of xylitol (g xylitol/l per hour) - q_p average specific productivity of xylitol (g xylitol/g of cells per hour) - So initial xylose concentration (g/l) - tf incubation time (hours) - Y_P/S xylitol yield (g of xylitol produced/g of xylose utilized) - Y_E/S ethanol yield (g of ethanol produced/g of substrate utilized) - Y_X/S cells yield (g of cells/g of substrate utilized) - specific growth rate coefficient (h^–1) - _max maximum specific growth rate coefficient (h^–1)     

High concentration cultivation of Bifidobacterium longum in fermenter with cross-flow filtration

Summary High concentration cultivation of Bifidobacterium longum in a fermenter with cross-flow filtration using a ceramic filter is described. Continuous cross-flow filtration allowed complete recycling of the cells to the fermenter and also continuous separation of inhibitory metabolites. The final cell concentration attained in the cultivation was 54.4 g dry wt./l; this was seven times as high as that without cross-flow filtration. The time course of the cultivation with cross-flow filtration was predicted, based on the assumption that the specific growth rate can be expressed only as a function of concentrations of metabolites (acetate and lactate) in a culture broth.Nomenclature D dilution rate (h^-1) - m maintenance coefficient (h^-1) - OD ₅₇₀ optimal density at 570 nm - P _A acetate concentration (g/l) - P _A0 initial acetate concentration (g/l) - P _L lactate concentration (g/l) - P _L0 initial lactate concentration (g/l) - S lactose (substrate) concentration (g/l) - S ₀ initial lactose (substrate) concentration (g/l) - t cultivation time (h) - Y _x/s growth yield (g/g) - X dry cell concentration (g/l) - X ₀ initial dry cell concentration (g/l) - constant - constant     

A pulsing device for packed-bed bioreactors: II. Application to alcoholic fermentation

When the immobilized cells are employed in packed-bed bioreactors several problems appear. To overcome these drawbacks, a new bioreactor based on the use of pulsed systems was developed [1]. In this work, we study the glucose fermentation by immobilized Saccharomyces cerevisiae in a packed-bed bioreactor. A comparative study was then carried out for continuous fermentation in two packed-bed bioreactors, one of them with pulsed flow. The determination of the axial dispersion coefficients indicates that by introducing the pulsation, the hydraulic behaviour is closer to the plug flow model. In both cases, the residence time tested varied from 0.8 to 2.6 h. A higher ethanol concentration and productivity (increases up to 16%) were achieved with the pulsated reactors. The volumes occupied by the CO₂ were 5.22% and 9.45% for fermentation with/without pulsation respectively. An activity test of the particles from the different sections revealed that the concentration and viability of bioparticles from the two bioreactors are similar. From the results we conclude that the improvements of the process are attributable to a mechanical effect rather than to physiological changes of microorganisms.List of Symbols D m²/s dispersion coefficient - K _is l/g inhibition substrate constant - K _ip l/g inhibition ethanol constant - K _s g/l Apparent affinity constant - P g/l ethanol concentration - q _p g/(gh) specific ethanol productivity - Q _p g/(lh) overall ethanol productivity - q _s g/(gh) specific glucose consumption rate - Q _s g/(lh) glucose consumption rate - S g/l residual glucose concentration - S(in0) g/l initial glucose concentration - V _max g/(lh) maximum rate - Y _p/s g/g yield in product     

Modelling of cyclic fed-batch plus batch polygalacturonase production by Aureobasidium pullulans on raw orange peel

Summary Cyclic fed-batch plus batch polygalacturonase production by Aureobasidium pullulans in slurry fermentation systems using raw orange peel as substrate was studied in a 3-dm³ stirred fermentor by setting the main operating variables (T=297°K; pH₀=3.2; OP₀=3% w/v; n=700 rpm) to optimal values determined previously. In this way, it was possible to stabilize enzyme excretion at 130–140 VU cm^–3. The time course of this fermentation process in terms of cell growth, substrate consumption and enzyme synthesis was reconstructed with a mean standard error less than 10%, by applying an unstructured model set up in a batch run and further refined in a series of cyclic fed-batch plus batch operations. In particular, the enzyme formation rate was related to the effect of reducing sugars as inhibitors at higher concentrations and as activators at lower levels by using an exponential equation. Moreover, the consumption rate of reducing sugars was found to be linearly related to the cell growth rate, its specific date being of pseudo-first order with respect to the reducing sugar concentration.Offprint requests to: M. Moresi     

Updated model of the batch acetone-butanol fermentation

Summary The recent models of the Acetone-Butanol fermentation did not adequately describe the culture inhibition by the accumulating metabolites and were unable to simulate the acidogenic culture dynamics at elevated pH levels. The present updated modification of the model features a generalised inhibition term and a pH dependent terms for intracellular conversion of undissociated acids into solvent products. The culture dynamics predictions by the developed model compared well with experimental results from an unconventional acidogenic fermentation ofC. acetobutylicum.Nomenclature A acetone concentration in the fermentation broth, [g/L] - AA total concentration of dissociated and undissociated acetic acid, [g/L] - AA _undiss concentration of undissociated acetic acid, [g/L] - APS Absolute Parameter Sensitivity - AT acetoin concentration in the fermentation broth, [g/L] - B butanol concentration in the fermentation broth, [g/L] - BA total concentration of dissociated and undissociated butyric acid, [g/L] - BA _undiss concentration of undissociated butyric acid, [g/L] - E ethanol concentration in the fermentation broth, [g/L] - f(T) inhibition function as defined in Equation (2) - k ₁ constant in Equation (4), [g substrate/g biomass] - k ₂ constant in Equation (4), [g substrate/(g biomass.h)] - k ₁ constant in Equation (5), [g substrate/(g biomass] - k ₂ constant in Equation (5), [g substrate/(g biomass.h)] - k ₃ constant in Equation (6), [g butyric acid/g substrate] - k ₄ constant in Equation (6), [g butyric acid/(g biomass.h)] - k ₅ constant in Equation (7), [g butanol/g substrate] - k ₆ constant in Equation (8), [g acetic acid/g substrate] - k ₇ constant in Equation (8), [g acetic acid/(g biomass.h)] - k ₈ constant in Equation (9), [g acetone/g substrate] - k ₉ constant in Equation (10), [g ethanol/g substrate] - k ₁₀ constant in Equation (11), [g acetoin/g substrate] - k ₁₁ constant in Equation (12), [g lactic acid/g substrate] - K _I Inhibition constant, [g inhibitory products/L] - ke maintenance energy requirement for the cell, [g substrate/(g biomass.h)] - K _AA acetic acid saturation constant, [g acetic acid/L] - K _BA butyric acid saturation constant, [g butyric acid/L] - K _S Monod's saturation constant, [g substrate/L] - LA lactic acid concentration in the fermentation broth, [g/L] - m _i ,n _i constants in Equation (14) - n empirical constant, dependent on degree of inhibition. - P concentration of inhibitory products (B+BA+AA), [g/L] - P _max maximum value of product concentration to inhibit the fermentation, [g/L] - pK_a equilibrium constant - r _A rate of acetone production, [g acetone/L.h] - r _AA rate of acetic acid production, [g acetic acid/L.h] - r _AT rate of acetoin production, [g acetoin/L.h] - r _B rate of butanol production, [g butanol/L.h] - r _BA rate of butyric acid production, [g butyric acid/L.h] - r _E rate of ethanol production, [g ethanol/L.h] - RPS Relative Parameter Sensitivity - r _LA rate of lactic acid production, [g lactic acid/L.h] - r _S dS/dt=total substrate consumption rate, [g substrate/L.h] - r _S substrate utilization rate, [g substrate/L.h] - S substrate concentration in the fermentation broth, [g substrate/L] - S ₀ initial substrate concentration, [substrate/L] - t time, [h] - X biomass concentration, [g/L] - Y _X yield of biomass with respect to substrate, [g biomass/g substrate] - Y _{P i} yield of metabolic product with respect to substrate, [g product/g substrate] Derivatives dX/dt rate of biomass production, [g biomass/L.h] - dP _i /dt rate of product formation, [g product/L.h] Greek letters specific growth rate of the culture, [h^–1] - I specific growth rate of the culture in the presence of the inhibitory products, [h^–1] - µ_max maximum specific growth rate of the culture, [h^–1]     

Cashew apple bagasse as a source of sugars for ethanol production by Kluyveromyces marxianus CE025

The potential of cashew apple bagasse as a source of sugars for ethanol production by Kluyveromyces marxianus CE025 was evaluated in this work. This strain was preliminarily cultivated in a synthetic medium containing glucose and xylose and was able to produce ethanol and xylitol at pH 4.5. Next, cashew apple bagasse hydrolysate (CABH) was prepared by a diluted sulfuric acid pretreatment and used as fermentation media. This hydrolysate is rich in glucose, xylose, and arabinose and contains traces of formic acid and acetic acid. In batch fermentations of CABH at pH 4.5, the strain produced only ethanol. The effects of temperature on the kinetic parameters of ethanol fermentation by K. marxianus CE025 using CABH were also evaluated. Maximum specific growth rate (μ_max), overall yields of ethanol based on glucose consumption Y_{P \mathord}

Rapid ethanol fermentation of cellulose hydrolysate. II. Product and substrate inhibition and optimization of fermentor design

High concentrations of both ethanol and sugar in the fermentation broth inhibit the growth of yeast cells and the rate of product formation. Inhibitory effects of ethanol on the yeast strain Saccharomyces cerevisiae NRRL-Y-132 were studied in batch and continuous chemostat cultures. Growth was limited by either glucose or ethanol. Feed medium was supplemented with different ethanol concentrations. Ethanol was found to inhibit growth and the activity of yeast to produce ethanol in a noncompetitive manner. A linear kinetic pattern for growth and product formation was observed according to μ = μ_m (1 – P/P_m) and v = v_m (1 – P/P_m′), where μ_m is the maximum specific growth rate at P = 0 (hr^?1); P_m is the maximum specific product formation rate at P = 0 (hr^?1); P_m is the maximum ethanol concentration above which cells do not grow (g/liter); P_m′ is the maximum ethanol concentration above which cells do not produce ethanol (g/liter). Substrate inhibition studies were carried out using short-time experimental techniques under aerobic and anaerobic condition. The degree of substrate inhibition was found to be higher than that has been reported for ethanol fermentation of pure sugar. The kinetic relationships thus obtained were used to compute growth, substrate utilization, and alcohol production patterns and have been discussed with reference to batch and continuous fermentation of enzymatically produced bagasse hydrolysate.