Computer-aided baker's yeast fermentations.

The economics of yeast production depend heavily upon the cellular yield coefficient on the carbon source and the volumetric productivity of the process. The application of an on-line computer to maximize these two terms during the fermentation requires a continuous method of measuring cell density and growth rate. Unfortunately, a direct sensor for biomass concentration suitable for use in industrial fermentations is not available. Material balancing, with the aid of on-line computer monitoring, offers an indirect method of measurement. Laboratory results from baker's yeast production in a 14-liter fermentor (with a PDP-11/10 computer for on-line analyses) show this indirect measurement technique to be a viable alternative. From the oxygen uptake and carbon dioxide production data, gas flow rate, and ammonia addition rate, the cell density during the fermentation has been estimated and found to compare well with actual fermentation data.     

Experimental simulation of oxygen profiles and their influence on baker's yeast production: I. One-fermentor system

In production-scale bioreactors microorganisms are exposed to a continually changing environment. This may cause loss of viability, reduction of the yield of biomass or desired metabolites, and an increase in the formation of by-products. In fed-batch production of baker's yeast, profiles may occur in substrate and oxygen concentrations and in pH. This article deals with the influence of a periodically changing oxygen concentration on the growth of baker's yeast in a continuous culture. Also, influences on the production of ethanol, glycerol, acetic acid, and on the composition of the cells were investigated. It was found that relatively fast fluctuations between oxygen-unlimited and oxygen-limited conditions with a frequency of 1 or 2 min had a distinct influence on the biomass and metabolite production. However, RNA, protein, and carbohydrate contents measured in cells exposed to fluctuations differed little from those in cells from an oxygen-unlimited or an oxygen-limited culture. The respiration and fermentation capacities of cells exposed to fluctuations can be larger than the capacities of cells grown under oxygen-unlimited conditions.     

Gas phase biotransformation reaction catalyzed by baker's yeast

The gas phase continuous production of acetaldehyde from ethanol and hexanol from hexanal using dried baker's yeast was studied as an alternative approach to conventional processes. The effects of water activity, activity of substrates, and amount of yeast on the performance of the continuous bioreactor were investigated. The extent of yeast hydration and ethanol activity are the most important factors affecting yeast activity and stability.     

Phosphate mediation of the aerobic fermentation in baker's yeast

Summary If the supply of phosphate were restricted in a glucose-limited chemostat culture of baker's yeast to an extent that residual phosphate in the medium could hardly be observed, the value of critical dilution rate was apparently enhanced. This observation suggests a possible mediation by phosphate between anaerobic and aerobic functions of the baker's yeast.     

Experimental and simulation studies of multivariable adaptive optimization of continuous bioreactors using bilevel forgetting factors

A multivariable on-line adaptive optimization algorithm using a bilevel forgetting factor method was developed and applied to a continuous baker's yeast culture in simulation and experimental studies to maximize the cellular productivity by manipulating the dilution rate and the temperature. The algorithm showed a good optimization speed and a good adaptability and reoptimization capability. The algorithm was able to stably maintain the process around the optimum point for an extended period of time. Two cases were investigated: an unconstrained and a constrained optimization. In the constrained optimization the ethanol concentration was used as an index for the baking quality of yeast cells. An equality constraint with a quadratic penalty was imposed on the ethanol concentration to keep its level close to a hypothetical "optimum" value. The developed algorithm was experimentally applied to a baker's yeast culture to demonstrate its validity. Only unconstrained optimization was carried out experimentally. A set of tuning parameter values was suggested after evaluating the results from several experimental runs. With those tuning parameter values the optimization took 50-90 h. At the attained steady state the dilution rate was 0.310 h(-1) the temperature 32.8 degrees C, and the cellular productivity 1.50 g/L/h.     

Laboratory culture of fairy shirmps using baker's yeast as basic food in a flow-through system

We designed and standardized a culture method for freshwater anostracans using diets free of live algae.Thamnocephalus platyurus andBranchinecta lindahli were used as test organisms.We used baker's yeast as basic food and added inert particles (clay or amorphic silicium dioxide) to improve the digestion of the yeast. A flow-through culture system was used, according to a fixed feeding schedule, to supply separately, culture medium (tap water), food, and inert particle suspensions. Three variants with baker's yeast as basic, food were compared for survival, growth, and reproduction. A diet of solely baker's yeast (diet 1) or baker's yeast supplemented with vegetal oil containing ß-carotene (diet 2) was unsuitable for reproduction ofT. platyurus. Cyst production was only achieved when diet 2 was supplemented with fish oil andSpirulina powder (diet 3). This suggests that not only a digestibility problem, but also nutritional deficiencies are present in baker's yeast.     

Profile control of the specific growth rate in fed-batch experiments

Summary The aim of this paper is to apply a computer control scheme to a laboratory scale fermentor so that the specific growth rate in a baker's yeast fed-batch culture, which cannot be measured directly, will follow as accurately as possible the desired profile specified in advance. Using an extended Kalman filter and programmed controller/feedback compensator (PF) system proposed previously, profile control of the specific growth rate () was achieved experimentally in a baker's yeast fed-batch culture. Also, bang-bang type profile control of minimized the proportion of budding cells, which have a strong correlation with the fermentative activity in bread-making.     

Adaptive optimization of a continuous culture with the use of a bilevel forgetting factor

An adaptive optimization algorithm using a dynamic identification scheme with a bilevel forgetting factor (BFF) has been developed. The simulation results show superiority of this method to other methods when applied to maximize the cellular productivity of a continuous culture of baker's yeast, Saccharomyces cerievisiae. Within the limited ranges of tuning parameters tested the BFF algorithm is found to be superior in terms of initial optimization speed and accuracy and reoptimization speed and accuracy when there is an external change and long term stability (removal of "blowing up" phenomena). Algorithms tested include those based on a constant forgetting factor, an adaptive variable forgetting factor (VFF) and moving window (MW) identification.     

Experience in using an ethanol sensor to control molasses feed-rates in baker's yeast production

An ethanol sensor has been tested for feed-rate control of baker's yeast prouction. The yeast was grown on molasses in an 8 dm³ fed-batch reactor up to a cell concentration of 60–70 kg/m³. Studies were made on three levels: reliability of the sensor system, characterisation of the control problem, and evaluation of ethanol-controlled cultivations in terms of yield and production rate. Arguments are given for the conceptual advantages of ethanol control compared to other methods of substrate control. It is also shown that ethanol control allows for a simple regulator structure. In fact, a PID regulator, with constant parameters, was used around an exponential dosage scheme. Tuning of the regulator parameters was performed by using simulation on a simplified model of the process. Several cultivations have been carried out. Results from four comparable cultivations are given in detail, and the experience from many others is summarized.     

A real-time feed rate control system for pilot plant baker's yeast production

Summary An on-line feed rate control system for baker's yeast production using the molasses uptake rate as a feeding index was developed. The optimal feed rate was obtained by maximising the feeding index. The experiments were performed to test this control system in fermenter of 30 m³ total capacity. In baker's yeast process 2760 kg M₅₀ was consumed and 2852 kg compressed yeast (D_c) was produced. Cell yield, final molasses dilution and final yeast concentration were 1.0 kg D_c/kg M₅₀, 1:6.5 and 52 g D₁₀₀/l, respectively. These results found that the developed feed rate control system is to be successful.     

Construction of a Stable alpha-Galactosidase-Producing Baker's Yeast Strain

Molasses is widely used as a substrate for commercial yeast production. The complete hydrolysis of raffinose, which is present in beet molasses, by Saccharomyces strains requires the secretion of alpha-galactosidase, in addition to the secretion of invertase. Raffinose is not completely utilized by commercially available yeast strains used for baking, which are Mel. In this study we integrated the yeast MEL1 gene, which codes for alpha-galactosidase, into a commercial mel baker's yeast strain. The Mel phenotype of the new strain was stable. The MEL1 gene was expressed when the new Mel baker's yeast was grown in molasses medium under conditions similar to those used for baker's yeast production at commercial factories. The alpha-galactosidase produced by this novel baker's yeast strain hydrolyzed all the melibiose that normally accumulates in the growth medium. As a consequence, additional carbohydrate was available to the yeasts for growth. The new strain also produced considerably more alpha-galactosidase than did a wild-type Mel strain and may prove useful for commercial production of alpha-galactosidase.     

Candida krusei in Baker's yeast production

Summary Candida krusei is a harmful contaminant in baker's yeast manufacture, because it grows much faster than Saccharomyces cerevisiae under production conditions. This investigation showed that C. krusei utilizes the ethanol produced by baker's yeast as sole carbon source when molasses is used as a substrate. When the alcohol concentration in the effluent air is used as a parameter for controlling the aeration of the culture, conditions become favourable for the dominance of wild yeast because some of the ethanol produced by the baker's yeast is consumed immediately by C. krusei and aeration is then automatically reduced, leading to increased growth of the wild yeast.     

Computer control for continuous cultivation ofSaccharomyces cerevisiae

Summary An on-line respiratory quotient control system has been developed for the continuous cultivation of baker's yeast. This system is based on moving identification of the microbial dynamics. The optimal dilution rate that was selected as the control variable was determined by minimizing a performance index. Without resorting to complicated microbial analysis, a simple and practical moving model is obtained by continually updating the input and output data. The experimental results indicate the satisfactory controllability of the present system and the possible extention of the proposed method to other bioprocesses.     

Development of macrocapsules containing bioflavors generated in situ by immobilized cells

An alternative approach to microbial production of bioflavors, eliminating the need for lengthy product purification, is presented. It is based on co-immobilization of precursors for bioflavor generation by microbial cells, traditionally employed for food and beverage processing, within beads made of food-grade gel matrix. Following incubation under controlled conditions the bioflavor — or bioflavor mixture — is generated and accumulated within the beads. The flavor-retaining bead may then be employed as a food additive. A feasibility study demonstrated this approach with ethanol production by baker's yeast co-immobilized with glucose medium. Means and conditions for bead preparation and control of ethanol levels and production rate are presented. Complex bioflavor generation was also demonstrated by baker's yeast co-immobilized with apple juice, generating cider flavors. Beads providing beer taste were also readily made via co-immobilization of commercial brewing yeast with malt. Furthermore, the potential inherent in bioflavor generation by co-immobilization of filamentous fungi with an emulsion of oily precursor was demonstrated by γ-decalactone production from castor oil.     

Preparation of an organic solvent-tolerant strain from baker's yeast

By using immobilized baker's yeast repeatedly in isooctane with occasional reactivation by cultivation, we succeeded in the preparation of an organic solvent-tolerant strain, named KK21, which could grow in the presence of isooctane. This is the first report on an organic solvent-tolerant strain from baker's yeast. Strain KK21 showed high tolerance to organic solvents and maintained a high and stable activity on continuous reduction of n-butyl 3-oxobutanoate in an isooctane-medium two-phase system. Although the morphology of strain KK21 was the same as that of baker's yeast, the saturated fatty acid occupancy (SFA occupancy), which is defined as the percentage of saturated fatty acids in the total fatty acids of phospholipids, of strain KK21 was significantly higher than that of parental baker's yeast when strain KK21 was grown in the presence of isooctane, suggesting that a decrease in fluidity of the cell membrane might play an important role in the tolerance to organic solvents.     

Advanced control of glutathione fermentation process

A study was performed to understand the fermentation process for production of glutathione fermentation (GSH) with an improved strain of baker's yeast. Simultaneous utilization of sugar and ethanol has been found to be a key factor in the industrial process to produce GSH using Saccharomyces cerevisiae KY6186. Based on this observation, the optimal sugar feed profile for the fed-batch operation has been determined. A feedforward/feedback control system was developed to regulate the sugar feed rate so as to maximize GSH production yields. Using the feedforward/feedback control system and the on-line data of oxygen and ethanol concentration in exhaust gas, the successful scaleup to the production level was accomplished. An average of 40% improvement of glutathione production compared to a conventionally programmed control of exponential fed-batch operation was found in the new process. (c) 1992 John Wiley & Sons, Inc.     

Immobilization of living whole cells in an epoxy matrix

Summary An improved method for the mild immobilization of living microbial cells in an amine-cured epoxy resin is described. To demonstrate the viability of cells, immobilized baker's yeast was used for the production of ethanol. A repeated and continuous reincubation of the biocatalyst was possible.     

A model for continuous fermentations with amylolytic yeasts

A two-stage, associative fermentation process is more effective for continuous yeast biomass production from starch than a single-stage mixed culture fermentation process. By operating two stages, competition for the same growth limiting substrate is reduced leading to efficient starch utilization. In this article, a mathematical model has been proposed for continuous, two-stage fermentation with a pure culture, amylolytic yeast in the first stage and a mixed culture second stage with a faster growing, nonamylolytic yeast. The model parameters were determined for Saccharomycopsis fibuligera and Candida utilis in continuous, single-stage, pure cultures. In the two-stage model, the effects of changes in dilution rate on biomass, amylase, reducing sugar, and starch concentration, and ratio of stage volumes on microbial composition are discussed and compared with experimental data.     

Alpha-glucosidase inhibitory activity of Syzygium cumini (Linn.) Skeels seed kernel in vitro and in Goto-Kakizaki (GK) rats

Syzygium cumini seed kernel extracts were evaluated for the inhibition of alpha-glucosidase from mammalian (rat intestine), bacterial (Bacillus stearothermophilus), and yeast (Saccharomyces cerevisiae, baker's yeast). In vitro studies using the mammalian alpha-glucosidase from rat intestine showed the extracts to be more effective in inhibiting maltase when compared to the acarbose control. Since acarbose is inactive against both the bacterial and the yeast enzymes, the extracts were compared to 1-deoxynojirimycin. We found all extracts to be more potent against alpha-glucosidase derived from B. stearothermophilus than that against the enzymes from either baker's yeast or rat intestine. In an in vivo study using Goto-Kakizaki (GK) rats, the acetone extract was found to be a potent inhibitor of alpha-glucosidase hydrolysis of maltose when compared to untreated control animals. Therefore, these results point to the inhibition of alpha-glucosidase as a possible mechanism by which this herb acts as an anti-diabetic agent.     

Multiple forms of carboxypeptidase Y from Saccharomyces cerevisiae. Kinetic demonstration of effects of carbohydrate residues on the catalytic mechanism of a glycoenzyme.

In the course of our further investigation of the active site titration of carboxypeptidase Y, using 4-nitrophenyl trimethylacetate, we have found that carboxypeptidase Y can be isolated in different molecular forms. Carboxypeptidase Y obtained from Fleischmann baker's yeast has a molecular weight of 53,000, as compared to 64,000 for an enzyme species isolated from Anheuser-Busch baker's yeast. The amino acid analyses of both enzymes were essentially identical and very similar to those reported by others. However, we have found that the molecular weight difference is due to a variation in carbohydrate content as determined by gas chromatography. When carboxypeptidase Y was isolated from a single source, Anheuser-Busch baker's yeast, we observed a smaller variation in carbohydrate content. In all cases, sugar analyses revealed only mannose and N-acetylglucosamine to be present. The effect of the enzyme's carbohydrate content on the "burst kinetics" of the 4-nitrophenyl trimethylacetate reaction has been examined. In general, the Anheuser-Busch enzyme, containing more carbohydrate than the Fleischmann enzyme, reacts with a larger apparent bimolecular rate constant, kcat/Km. On the other hand, the deacylation rate constant, k3, is affected only slightly.