Bioconversion of acid-hydrolyzed poplar hemicellulose to acetic acid byClostridium thermoaceticum

Summary Clostridium thermoaceticum was used to ferment carbohydrate released from pretreated oat splet xylan and hemicellulose isolated from hybrid poplar. Hydrolysis with dilute sulfuric acid (2.5% (v/v) for oat spelt xylan and 4.0% (v/v) for poplar hemicellulose) at 100°C for 60 min was found to release the highest concentration of fermentable substrate.C. thermoaceticum, when grown in non-pH controlled batch culture at 55°C under a headspace of 100% CO₂, typically produced 14gl^–1 acetic acid during a 48 h fermentation in medium containing 2% xylose. In fed-batch fermentations this organism was able to produce 42gl^–1 acetic acid after 116h when the concentration of xylose was maintained at approximately 2% and the pH was controlled at 7.0.     

Single-cell protein of Rhodotorula sp. Y-38 from ethanol, acetic acid and acetaldehyde

Summary Continuous cultivation of Rhodotorula sp. Y-38 was carried out on ethanol, acetic acid or acetaldehyde. At a feed concentration of 1.0 % (w/v) ethanol, the cell yield of 64 g/100 g ethanol and crude protein of 52 g/100 g biomass were obtained at D=0.5 h^-1. The respective value of the content of amino acids and nucleic acids was 42.6 and 9.4 g/100 g biomass. At 2.0 % (w/v) acetic acid, cell yield was found to be 50 g/100 g acetic acid at D=0.4 h^-1. The optimum dilution rate ranged between 0.3 and 0.4 h^-1. At 0.05 % (w/v) acetaldehyde, the maximum cell yield was obtained at D=0.14 h^-1.     

Effects of acetic acid and lactic acid on the growth of Saccharomyces cerevisiae in a minimal medium

Specific growth rates (μ) of two strains of Saccharomyces cerevisiae decreased exponentially (R ²>0.9) as the concentrations of acetic acid or lactic acid were increased in minimal media at 30°C. Moreover, the length of the lag phase of each growth curve (h) increased exponentially as increasing concentrations of acetic or lactic acid were added to the media. The minimum inhibitory concentration (MIC) of acetic acid for yeast growth was 0.6% w/v (100 mM) and that of lactic acid was 2.5% w/v (278 mM) for both strains of yeast. However, acetic acid at concentrations as low as 0.05–0.1% w/v and lactic acid at concentrations of 0.2–0.8% w/v begin to stress the yeasts as seen by reduced growth rates and decreased rates of glucose consumption and ethanol production as the concentration of acetic or lactic acid in the media was raised. In the presence of increasing acetic acid, all the glucose in the medium was eventually consumed even though the rates of consumption differed. However, this was not observed in the presence of increasing lactic acid where glucose consumption was extremely protracted even at a concentration of 0.6% w/v (66 mM). A response surface central composite design was used to evaluate the interaction between acetic and lactic acids on the specific growth rate of both yeast strains at 30C. The data were analysed using the General Linear Models (GLM) procedure. From the analysis, the interaction between acetic acid and lactic acid was statistically significant (P≤0.001), i.e., the inhibitory effect of the two acids present together in a medium is highly synergistic. Journal of Industrial Microbiology & Biotechnology (2001) 26, 171–177. Received 06 June 2000/ Accepted in revised form 21 September 2000     

Fermentative production of P(3HB-co-3HV) from propionic acid by Alcaligenes eutrophus in fed-batch culture with pH-stat continuous substrate feeding method

The feeding of propionic acid for production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] by Alcaligenes eutrophus ATCC17697 was optimized using a fed-batch culture system. The concentration of propionic acid was maintained at 3 g l^–1 as growth was inhibited by propionic acid in the broth. A pH-stat substrate feeding system was used in which propionic acid was fed automatically to maintain a pH of the culture broth at 7.0. By feeding a substrate solution containing 20% (w/v) propionic acid, 4.9% (w/v) ammonia water [at a molar ratio of carbon to nitrogen (C/N molar ratio) of 10] in cell growth phase, the concentration of propionic acid in the broth was maintained at 3 g l^–1 giving a specific growth rate of 0.4 h^–1. To promote P(3HB-co-3HV) production, two stage fed-batch culture which consisted of the stage for the cell growth and the stage for the P(3HB-co-3HV) accumulation was carried out. When the substrate solution whose C/N molar ratio was 50 was fed in P(3HB-co-3HV) accumulation phase, the cell concentration and the P(3HB-co-3HV) content in the cells reached 64 g l^–1 and 58% (w/w) in 55.5 h, respectively.     

Fermentation of molasses using a thermotolerant yeast,Kluyveromyces marxianus IMB3: simplex optimisation of media supplements

 The use of molasses as a substrate for ethanol production by the thermotolerant yeast Kluyveromyces marxianus var. marxianus was investigated at 45°C. A maximum ethanol concentration of 7.4% (v/v) was produced from unsupplemented molasses at a concentration of 23% (v/v). The effect on ethanol production of increasing the sucrose concentration in 23% (v/v) molasses was determined. Increased sucrose concentration had a similar detrimental effect on the final ethanol produced as the increase in molasses concentration. This indicated that the effect may be due to increased osmotic activity as opposed to other components in the molasses. The optimum concentration of the supplements nitrogen, magnesium, potassium and fatty acid for maximum ethanol production rate was determined using the Nelder and Mead (Computer J 7:308–313, 1965) simplex optimisation method. The optimum concentrations of the supplements were 0.576 g l^-1 magnesium sulphate, 0.288 g l^-1 potassium dihydrogen phosphate and 0.36% (v/v) linseed oil. Added nitrogen in the form of ammonium sulphate did not affect the ethanol production rate. Received: 29 January 1996/Received revision: 23 April 1996/Accepted: 29 April 1996     

A novel immobilised design for the production of the heterologous protein lysozyme by a genetically engineered Aspergillus niger strain

Abstract A novel immobilisation design for increasing the final concentration of the heterologous protein lysozyme by a genetically engineered fungus, Aspergillus niger B1, was developed. A central composition design was used to investigate different immobilised polymer types (alginate and pectate), polymer concentration [24% and 4% (w/v)], inoculum support ratios (1:2 and 1:4) and gel-inducing agent concentration [CaCl₂, 2% and 3.5% (w/v)]. Studies of the kinetics of production showed that optimum lysozyme productivity occurred after 10 days. Lysozyme production was significantly affected by polymer type, polymer concentration, and inoculum support ratio. Overall, immobilisation in Ca-pectate resulted in higher lysozyme production compared to that in Ca-alginate. Similar effects were observed when the polymer concentration was reduced. Regardless of polymer type and concentration, increasing the fungal inoculum level increased lysozyme production. A significantly higher lysozyme yield was achieved with Ca-pectate in comparison to Ca-alginate (approximately 20–23 mg l^–1 and 0.5–2 mg l^–1, respectively). The maximum lysozyme yield achieved was about 23 mg l^–1 by immobilisation in Ca-pectate 2% (w/v) with 33% (v/v) mycelium and 3.5% (w/v) gel-inducing agent (CaCl₂). Response surface methodology was used to investigate the effect of pH and water activity (a_w). The best medium pH was 4.5–5.0, and bead a_w for optimum lysozyme yield was 0.94, regardless of polymer type.     

Growth and Fermentation Characteristics of a Strain of the Wine Yeast Kluyveromyces thermotolerans Isolated in Greece

Summary During the single culture fermentation of grape must K. thermotolerans, strain TH941, isolated in a wine-producing region in northern Greece, reached a very high cell concentration of 8.4 log (c.f.u ml⁻¹), followed by a rapid decline of the viable cells. The yeast produced 9.6 g L-lactic acid l⁻¹ during the growth phase, 7.58% v/v of ethanol and showed a limited degradation of L-malic acid as well as a low production of volatile acidity. In the presence of 3% v/v and 6% v/v of ethanol the K. thermotolerans isolate was able to grow. At 9% v/v of ethanol it could not grow but showed no loss of viability for 10 days.     

Solution properties and structure of brain proteolipids

Bovine white matter proteolipids have been studied by several physical methods and have been found to exist as micelles in 2 : 1 (v/v) chloroform–methanol solution. The data would indicate the existence of a critical micelle concentration at 0.017–0.022 g/100 ml. The curve appears linear in the range 0.017–0.2 g/100 ml, but from the data at higher concentrations it would appear that a change in slope is occurring in the region 0.2–0.3 g/100 ml. Light-scattering measurements on 2 : 1 (v/v) chloroform–methanol solutions containing more than 0.2 g/100 ml of proteolipid yielded a weight-average aggregate weight of 2.9 × 10⁶and a radius of gyration of 64.5 Å. The intrinsic viscosity of the solutions was 0.32 dl/g and the Huggins constant was 1.085. Light-scattering measurements in 88.5% formic acid–0.5M sodium formate yielded a weight-average aggregate weight of 7.1 × 10⁶ and a radius of gyration of 241 Å. The intrinsic viscosity observed for this solvent system is 0.14 dl/g and the Huggins constant is 1.005. Osmotic pressure measurements in 2 : 1 (v/v) chloroform–methanol containing less than 0.2 g/ 100 ml of proteolipid yielded a number-average aggregate weight of 7.2 × 10⁴ Ultracentrifugal analysis in 1.5:1 (v/v) methylene chloride–methanol showed two broad peaks with, s values of s_1.5% = 20.05 S, s_2% = 19.79 S for the minor peak and s_1.5%,2% = 1.86 S for the major peak. Optical rotatory dispersion studies revealed large changes in b₀ with change in solvent and proteolipid concentration. The present data suggest that the mode of attachment of protein to lipid is primarily of a noncovalent type. The results of this investigation also suggest that the proteolipid micelle above 0.2 g/100 ml is cylindrical (prolate ellipsoid) in 2:1 (v/v) chloroform-methanol and approaches a more spherical shape in 88.5% formic acid. A structure for the proteolipid micellar complex above concentrations of 0.2 g/100 ml is proposed.     

Optimization of furfural production from hemicellulose extracted from delignified palm pressed fiber using a two-stage process

This study aims to optimize the conditions for furfural production from hemicellulose extracted from delignified palm pressed fiber (dPPF) via two-stage process: acid hydrolysis followed by dehydration, using response surface methodology (RSM). The extracted hemicellulose contained 80.8% xylose. In order to convert hemicellulose to xylose in the acid hydrolysis step, there were four important parameters consisting of reaction temperature (100–150 °C), sulfuric acid concentration (1–10% v/v), ratio of sulfuric acid to hemicellulose (L/S ratio) (10, 9, and 8 v/w), and reaction time (30–120 min). The maximum xylose production (12.58 g/L) was achieved at 125 °C, 5.5% sulfuric acid, L/S ratio of 9 mL/g for 30 min with the determination coefficient (R²) value of 0.90. For the dehydration process, two parameters; reaction temperature (120–160 °C) and reaction time (30–150 min), were optimized. The maximum furfural production (8.67 g/L) was achieved at a reaction temperature of 140 °C for 90 min with the determination coefficient (R²) value of 0.93.     

Growth and butyric acid production by Clostridium populeti

The growth of Clostridium populeti in 2% (w/v) glucose medium containing 0.2% (w/v) yeast extract was optimal with 10 mM NH₄Cl as the nitrogen source. Although the maximum specific growth rate (=0.32 h^-1) with 5 mM NH₄Cl was similar, the biomass yield was about 30% lower than that at the optimum. Either sodium sulphide or cysteine-HCl at an optimum concentration of 0.33 mM and 5.0 mM respectively, could serve as the sole sulphur source for growth. The growth rate was unaffected by initial glucose concentrations of up to 10% (w/v), but in the presence of 15% glucose it declined by about 35%. The molar yield of butyric acid (mol/mol glucose) declined from 0.70 in 1% (w/v) initial glucose medium to 0.39 in 10% glucose medium. In 5.7% initial glucose medium, butyric acid levels of 6.3 g/l were obtained (0.56 mol butyrate/mol glucose) after 72 h of incubation in 2.5 l batch cultures. A decrease of about 50% in the maximum specific growth rate of C. populeti was observed in the presence of an initial concentration of either 1.2 g/l of butyric acid or 18.9 g/l of acetic acid.This paper is issued as NRCC No. 29032     

Sucrose-induced osmotic stress affects biomass,metabolite, and antioxidant levels in root suspension cultures of <Emphasis Type="Italic">Hypericum perforatum</Emphasis> L.

In this study, we investigated the influence of initial sucrose concentration on the accumulation of biomass, phenols, flavonoids, chlorogenic acid, and hypericin in adventitious root cultures of Hypericum perforatum L. Cultures were initiated in shake flasks by using half-strength Murashige and Skoog (MS) medium, 1.0 mg l⁻¹ indolebutyric acid (IBA), 0.1 m g l⁻¹ kinetin, and different concentrations 0, 1, 3, 5, 7, or 9% in w/v) of sucrose and were maintained in darkness. The medium supplemented with 3% (w/v) sucrose resulted in the optimum biomass accumulation, but higher sucrose concentrations (5, 7, and 9%) inhibited biomass accumulation due to the relatively higher osmotic pressure. However, the amount of total phenols, flavonoids, chlorogenic acid, and total hypericin was increased with the roots grown in the medium supplemented with 5, 7, and 9% (w/v) sucrose. The antioxidant potential of methanolic extract [1,1-diphenyl-2-picrylhydrazyl (DPPH) and 2,2-azino-bis-(3-ethylbenzothiazoline-6-sulfonic acid; ABTS) radical scavenging activities] of H. perforatum adventitious roots was also assessed and correlated with the metabolite accumulation. Cultures maintained with higher initial sucrose concentration (5, 7, and 9% w/v) showed increased accumulation of phenols, flavonoids, chlorogenic acid, and total hypericin, and this might be due to the osmotic stress at elevated sucrose concentrations. To verify the effect of osmotic stress on lipid peroxidation, the levels of hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and proline were determined in the adventitious roots and the results revealed a marked increase in the concentrations of these compounds. These results suggest that optimal adventitious root biomass could be achieved in the MS medium with 3% (w/v) sucrose and increased sucrose concentration resulted in osmotic stress and, in turn, induces the accumulation of secondary metabolites.     

MODELING AND OPTIMIZATION OF GLUTAMIC ACID PRODUCTION USING MIXED CULTURE OF Corynebacterium glutamicum NCIM2168 AND Pseudomonas reptilivora NCIM2598

In this study, a hybrid system of response surface methodology followed by genetic algorithm has been adopted to optimize the production medium for L-glutamic acid fermentation with mixed cultures of Corynebacterium glutamicum and Pseudomonas reptilovora. The optimal combination of media components for maximal production of L-glutamic acid was found to be 49.99 g L^?1 of glucose, 10 g L^?1 of urea, 18.06% (v/v) of salt solution, and 4.99% (v/v) of inoculum size. The experimental glutamic acid yield at optimum condition was 19.69 g L^?1, which coincided well to the value predicted by the model (19.61 g L^?1). Using this methodology, a nonlinear regression model was developed for the glutamic acid production. The model was validated statistically and the determination coefficient (R ²) was found to be 0.99.     

Isolation of (2S,4R)-2-amino-4-methyl-hex-5-enoic acid,a nonprotein amino acid,as an allelochemical from the fruiting bodies of Boletus fraternus Peck.

The mushroom Boletus fraternus Peck. shows allelopathy and suppresses the growth of broad leaf plants in nature. According to a bioassay-guided fractionation of the fruiting body of the fungus, a rare nonprotein amino acid was isolated as a major allelochemical. The chemical structure of the compound was determined to be (2S,4R)-2-amino-4-methyl-hex-5-enoic acid (5-dehydrohomoleucine) by analysis of ¹H- and ¹³C-nuclear magnetic resonance spectra and comparison with data from the literature. The allelochemical caused 50% inhibition of lettuce seedling radicle growth at a concentration of 34 ppm (w/v). Further, since radicle growth was directed away from the filter paper to prevent contact with the allelochemical at concentrations higher than 300 ppm (w/v), the fungus may use the allelochemical to protect its immediate environment from contamination by other plants.     

Investigation of biomass concentration,lipid production,and cellulose content in Chlorella vulgaris cultures using response surface methodology

The microalgae Chlorella vulgaris produce lipids that after extraction from cells can be converted into biodiesel. However, these lipids cannot be efficiently extracted from cells due to the presence of the microalgae cell wall, which acts as a barrier for lipid removal when traditional extraction methods are employed. Therefore, a microalgae system with high lipid productivity and thinner cell walls could be more suitable for lipid production from microalgae. This study addresses the effect of culture conditions, specifically carbon dioxide and sodium nitrate concentrations, on biomass concentration and the ratio of lipid productivity/cellulose content. Optimization of culture conditions was done by response surface methodology. The empirical model for biomass concentration (R² = 96.0%) led to a predicted maximum of 1123.2 mg dw L^?1 when carbon dioxide and sodium nitrate concentrations were 2.33% (v/v) and 5.77 mM, respectively. For lipid productivity/cellulose content ratio (R² = 95.2%) the maximum predicted value was 0.46 (mg lipid L^?1 day^?1)(mg cellulose mg biomass^?1)^?1 when carbon dioxide concentration was 4.02% (v/v) and sodium nitrate concentration was 3.21 mM. A common optimum point for both variables (biomass concentration and lipid productivity/cellulose content ratio) was also found, predicting a biomass concentration of 1119.7 mg dw L^?1 and lipid productivity/cellulose content ratio of 0.44 (mg lipid L^?1 day^?1)(mg cellulose mg biomass^?1)^?1 for culture conditions of 3.77% (v/v) carbon dioxide and 4.01 mM sodium nitrate. The models were experimentally validated and results supported their accuracy. This study shows that it is possible to improve lipid productivity/cellulose content by manipulation of culture conditions, which may be applicable to any scale of bioreactors. Biotechnol. Bioeng. 2013; 110: 2114–2122. © 2013 Wiley Periodicals, Inc.     

Enhanced solid‐state citric acid bio‐production using apple pomace waste through surface response methodology

Aims: To evaluate the potential of apple pomace (AP) supplemented with rice husk for hyper citric acid production through solid‐state fermentation by Aspergillus niger NRRL‐567. Optimization of two key parameters, such as moisture content and inducer (ethanol and methanol) concentration was carried out by response surface methodology. Methods and Results: In this study, the effect of two crucial process parameters for solid‐state citric acid fermentation by A. niger using AP waste supplemented with rice husk were thoroughly investigated in Erlenmeyer flasks through response surface methodology. Moisture and methanol had significant positive effect on citric acid production by A. niger grown on AP (P < 0·05). Higher values of citric acid on AP by A. niger (342·41 g kg^?1 and 248·42 g kg^?1 dry substrate) were obtained with 75% (v/w) moisture along with two inducers [3% (v/w) methanol and 3% (v/w) ethanol] with fermentation efficiency of 93·90% and 66·42%, respectively depending upon the total carbon utilized after 144 h of incubation period. With the same optimized parameters, conventional tray fermentation was conducted. The citric acid concentration of 187·96 g kg^?1 dry substrate with 3% (v/w) ethanol and 303·34 g kg^?1 dry substrate with 3% (v/w) methanol were achieved representing fermentation efficiency of 50·80% and 82·89% in tray fermentation depending upon carbon utilization after 120 h of incubation period. Conclusions: Apple pomace proved to be the promising substrate for the hyper production of citric acid through solid‐state tray fermentation, which is an economical technique and does not require any sophisticated instrumentation. Significance and Impact of the Study: The study established that the utilization of agro‐industrial wastes have positive repercussions on the economy and will help to meet the increasing demands of citric acid and moreover will help to alleviate the environmental problems resulting from the disposal of agro‐industrial wastes.     

Inhibition of yeast by lactic acid bacteria in continuous culture: nutrient depletion and/or acid toxicity?

Lactic acid was added to batch very high gravity (VHG) fermentations and to continuous VHG fermentations equilibrated to steady state with Saccharomyces cerevisiae. A 53% reduction in colony-forming units (CFU) ml^–1 of S. cerevisiae was observed in continuous fermentation at an undissociated lactic acid concentration of 3.44% w/v; and greater than 99.9% reduction was evident at 5.35% w/v lactic acid. The differences in yeast cell number in these fermentations were not due to pH, since batch fermentations over a pH range of 2.5–5.0 did not lead to changes in growth rate. Similar fermentations performed in batch showed that growth inhibition with added lactic acid was nearly identical. This indicates that the apparent high resistance of S. cerevisiae to lactic acid in continuous VHG fermentations is not a function of culture mode. Although the total amount of ethanol decreased from 48.7 g l^–1 to 14.5 g l^–1 when 4.74% w/v undissociated lactic acid was added, the specific ethanol productivity increased ca. 3.2-fold (from 7.42×10^–7 g to 24.0×10^–7 g ethanol CFU^–1 h^–1), which indicated that lactic acid stress improved the ethanol production of each surviving cell. In multistage continuous fermentations, lactic acid was not responsible for the 83% (CFU ml^–1) reduction in viable S. cerevisiae yeasts when Lactobacillus paracasei was introduced to the system at a controlled pH of 6.0. The competition for trace nutrients in those fermentations and not lactic acid produced by L. paracasei likely caused the yeast inhibition.     

Lignin Deposition and Effect of Postharvest Treatment on Lignification of Green Asparagus (Asparagus officinalis L.)

To understand how lignin synthesis is regulated after harvest, detached green asparagus stalks (Asparagus officinalis L.) were treated with 1 μl l⁻¹ of 1-methylcyclopropene (1-MCP), 50 μg l⁻¹ gibberellic acid (GA₃), 2% (v:v) ethanol or 1 μl l⁻¹ ethylene. The results showed that lignin concentration in asparagus stalks stored at room temperature rapidly increased. Three conventional precursors of lignin, 4-hydroxycinnamic acid (coumaric acid), 3,4-dihydroxycinnamic acid (caffeic acid) and 4-hydroxy-3-mythoxycinnamic acid (ferulic acid), were found to be the major phenolics in the asparagus stalks. Furthermore, the concentrations of O₂⁻ in asparagus stalks steadily increased during the storage. Deposition of lignin in harvested asparagus was significantly reduced by treating the stalks with GA₃, 1-MCP or ethanol. The concentration of lignin in stalks treated with GA₃, 1-MCP or ethanol was 32, 20 or 27% lower, respectively, than in controls 3 days after treatment. Treating stalks with ethylene enhanced lignin synthesis (p<0.05). The concentration of total phenol in stalks was also significantly reduced by GA₃, 1-MCP and ethanol, but was enhanced by ethylene treatment. However, the concentration of active oxygen (O₂^−⋅) in stalks was significantly reduced by treatment with GA₃, 1-MCP and ethanol, but was enhanced by treatment with ethylene. Our study show that postharvest treatment with 1-MCP, GA₃ or ethanol may be applied to improve the quality of green asparagus.     

Inhibition of Pigment Biosynthesis in Cucumber Cotyledons by Isoxaflutole

Isoxaflutole [5-cyclopropyl-4-(2-methylsulphonyl-4-trifluromethylbenzoyl)isoxazole] is a new preemergence herbicide for broad-spectrum weed control in maize. The effect of isoxaflutole on chlorophyll (Chl) and carotenoid (Car) biosynthesis was investigated in cucumber (Cucumis sativus L.) cotyledons. Etiolated tissue was incubated with 5 mM isoxaflutole for 24 h and irradiated (60 mol m^-2 s^-1). The irradiation for 3 h did not reduce Chl a, Chl b, and Car contents, but after a 28-h irradiation the contents of Chl a and Car decreased by 35 and 15 %, respectively, and the content of Chl b increased by 24 %. Increasing the concentration of isoxaflutole beyond 5 mM resulted in reduction of Chl a (71 %), Chl b (20 %), and Car (31 %) contents. Similarly, increase in irradiance from 60 to 180 mol m^-2 s^-1 resulted in larger reduction of Chl and Car contents. Exogenously supplied 5-aminolevulinic acid did not reverse the isoxaflutole-inhibited Chl synthesis, whereas an exogenously supplied homogentisic acid lactone reversed the inhibition of pigment synthesis due to isoxaflutole.     

Utilization of agricultural residues for poly(3-hydroxybutyrate) production by Halomonas boliviensis LC1

Aims: Utilization of cheap and readily available agricultural residues as cheap carbon sources for poly(3‐hydroxybutyrate) (PHB) production by Halomonas boliviensis. Methods and Results: Wheat bran was hydrolysed by a crude enzyme preparation from Aspergillus oryzae NM1 to provide a mixture of reducing sugars composed mainly of glucose, mannose, xylose and arabinose. Growth of H. boliviensis using a mixture of glucose (0·75% w/v) and xylose (0·25% w/v) in the medium led to a PHB content and concentration of 45 wt% and 1 g l^?1, respectively, after 30 h. A similar PHB concentration was attained when H. boliviensis was grown on wheat bran hydrolysate but with a lower PHB content, 34 wt%. In a batch cultivation mode in a fermentor, using 1·8% (w/v) reducing sugars, the maximum PHB accumulation by H. boliviensis was attained in 20 h, but was reduced to about 30 wt%. By adding butyric acid (0·8% v/v), sodium acetate (0·8% w/v) and decreasing the reducing sugars concentration to 1·0% w/v in the medium, PHB accumulation and concentration were increased to 50 wt% and 4 g l^?1, respectively, after 20 h. Butyric acid and sodium acetate for PHB production could also be provided by anaerobic digestion of solid potato waste. Conclusions: Cheap and readily available agricultural residues can be used as substrates to produce PHB. The production of PHB by H. boliviensis using wheat bran hydrolysate as source of carbon is expected to reduce the production cost and motivates further studies. Significance and Impact of the Study: Large‐scale commercial utilization of PHB is mainly hampered by its high production cost. Carbon source for PHB production accounts up to 50% of the total production costs. Thus, the use of waste agricultural residues can substantially reduce the substrate cost (and in turn even provide value to the waste), and can downsize the production costs. This improves the market competitiveness. Studies on PHB production by moderate halophiles were recently initiated with H. boliviensis and findings show that it has potential for commercial exploitation. PHB production by H. boliviensis using wheat bran and potato waste is hence interesting.     

Effects of ethanol,octanoic and decanoic acids on fermentation and the passive influx of protons through the plasma membrane of Saccharomyces cerevisiae

Ethanol, octanoic and decanoic acids are known toxic products of alcoholic fermentation and inhibit yeast functions such as growth and fermentation. pH-stat measurements showed that, in a concentration range up to 20 mg/l, octanoic and decanoic acids increase the rate of passive H⁺ influx across the plasma membrane of Saccharomyces cerevisiae IGC 3507. Decanoic acid was more active than octanoic acid, which agrees with its higher liposolubility. The fatty acids probably act as H⁺ carriers, since the magnitude of the effect depended on pH and correlated with the concentration of protonated fatty acids. Esterification of the fatty acids partially abolished the enhancing effect on passive H⁺ influx. Passive H⁺ influx showed saturation kinetics with half-maximal activity at 6.6 M H⁺ (pH 5.2). Contrary to previous findings, ethanol inhibited H⁺ influx exponentially up to a concentration of 8% (v/v). At higher concentrations, ethanol reactivated H⁺ influx; the original rate of H⁺ uptake was reached at 14% (v/v) ethanol. In the same concentration ranges that affected passive H⁺ influx, ethanol, octanoic and decanoic acids inhibited the fermentation rate. This inhibitory effect of the fatty acids on fermentation rate depended on liposolubility, pH, and esterification in the same way as that found for their effect on passive H⁺ influx. Inhibition of fermentation by octanoic and decanoic acids could therefore result from their effect on the rate of passive H⁺ influx. Correspondence to: S. Stevens