Yeast extract was produced from brewer's yeast of a beer factory by combined enzymatic treatments using endoprotease, exoprotease, 5'-phosphodiesterase, and adenosine monophosphate (AMP)-deaminase. Effects of enzyme combination, enzyme dosages and treatment sequence on the recovery of solid and protein, flavor and compositional characteristics were investigated. Exoprotease dosage strongly affected the recovery of protein and degree of hydrolysis (DH) and sensory characteristics. When the yeast cells were treated using optimal combination of endoprotease and exoprotease (0.6% Protamex and 0.6% Flavourzyme), high solid recovery (48.3-53.1%) and the best flavor profile were obtained. Among various treatment sequences using multiple enzymes, treatment with protease followed by nuclease resulted in the highest 5'-guanosine monophosphate (5'-GMP) content. The optimal concentrations of both 5'-phosphodiesterase and AMP-deaminase were found to be 0.03%. After treatments using optimal combination of enzyme, enzyme dosages and treatment sequence for four enzymes, a high solid yield of 55.1% and 5'-nucleotides content of 3.67% were obtained.

Succinic acid (SA) is one of the most important biobuilding blocks in biorefinery. Its production from fermentation of renewable biomass sources is becoming a consolidated alternative that is more sustainable and potentially more economic than the traditional petroleum-based path for SA production. Fermentative production of SA has been successfully commercialized and a large and increasing number of SA-derivatives are promoting the economic stability of this production. However, the companies producing SA from fermentation are targeting specialized markets and the production is far from large-scale bulk chemical synthesis. In order to develop optimized and economic processes, the best candidates in every step of the SA production process must be identified. In this paper, the most promising biomass sources, pretreatment methods, fermentation conditions (i.e. host microorganism, fermenter design and operative mode) and separation techniques for industrial SA production are critically reviewed. Selection of the host microorganism is a key factor for SA production. However, the availability, potential and sustainability of feedstocks, fermentation and separation process must also be carefully evaluated for a cost-effective and environmentally sustainable SA production.

Background: The fermentation of sugars to alcohols by microbial systems underpins many biofuel initiatives. Short chain alcohols, like n-butanol, isobutanol and isopropanol, offer significant advantages over ethanol in terms of fuel attributes. However, production of ethanol from resistant Saccharomyces cerevisiae strains is significantly less complicated than for these alternative alcohols. Results: In this study, we have transplanted an n-butanol synthesis pathway largely from Clostridial sp. to the genome of an S. cerevisiae strain. Production of n-butanol is only observed when additional genetic manipulations are made to restore any redox imbalance and to drive acetyl-CoA production. We have used this butanol production strain to address a key question regarding the sensitivity of cells to short chain alcohols. In the past, we have defined specific point mutations in the translation initiation factor eIF2B based upon phenotypic resistance/sensitivity to high concentrations of exogenously added n-butanol. Here, we show that even during endogenous butanol production, a butanol resistant strain generates more butanol than a butanol sensitive strain. Conclusion: These studies demonstrate that appreciable levels of n-butanol can be achieved in S. cerevisiae but that significant metabolic manipulation is required outside of the pathway converting acetyl-CoA to butanol. Furthermore, this work shows that the regulation of protein synthesis by short chain alcohols in yeast is a critical consideration if higher yields of these alcohols are to be attained.

Biomass production by baker's yeast in a fed-batch reactor depends on the metabolic regime determined by the concentration of glucose and dissolved oxygen in the reactor. Achieving high biomass concentration in turn is dependent on the dynamic interaction between the glucose and dissolved oxygen concentration. Taking this into account, we present in this paper the implementation of a decoupled input-output linearizing controller (DIOLC) for maximizing biomass in a fed-batch yeast process. The decoupling is based on the inversion of 2×2 input-output matrix resulting from global linearization. The DIOLC was implemented online using a platform created in LabVIEW employing a TCP/IP protocol via the reactor's built-in electronic system. An improvement in biomass yield by 23% was obtained compared to that using a PID controller. The results demonstrate superior capability of the DIOLC and that the cumulative effect of smoother control action contributes to biomass maximization.Copyright © 2015 Elsevier Ltd. All rights reserved.

The influence of calcium and magnesium ions on resistance to dehydration in the yeast, Saccharomyces cerevisiae, was investigated. Magnesium ion availability directly influenced yeast cells' resistance to dehydration and, when additionally supplemented with calcium ions, this provided further significant increase of yeast resistance to dehydration. Gradual rehydration of dry yeast cells in water vapour indicated that both magnesium and calcium may be important for the stabilization of yeast cell membranes. In particular, calcium ions were shown for the first time to increase the resistance of yeast cells to dehydration in stress-sensitive cultures from exponential growth phases. It is concluded that magnesium and calcium ion supplementations in nutrient media may increase the dehydration stress tolerance of S. cerevisiae cells significantly, and this finding is important for the production of active dry yeast preparations for food and fermentation industries.

Drying processes are one of the main consumers of heat energy in production. Any decreases in heat consumption during the drying process will considerably decrease production costs. This study analyzes the high consumption of heat in the drying of baker's yeast. The main task is to minimize the energy demand and lower the price of the final products with partial heat recovery. These changes will require system modifications. One of the most popular and effective methods that can be used in this case is heat process integration with Pinch Technology. In this study, a reference system was simulated with a mathematical model and analyzed for waste heat streams. This paper suggests the redesigning of a drying system for production of active dry yeast. Selected streams that satisfy conditions for heat process integration were involved in the evaluation for a better solution. Two different scenarios were proposed as possible solutions. The suggested solutions are retrofit designs of Heat Exchanger Networks. These Heat Exchanger Networks include already installed heat exchangers as well as new heat transfer units. The selection of better design was made with economic analysis of investment. The proposed scenarios of the analyzed sub-system give improvement in heat energy recovery. The best determined solution reduces the cost and thus has the highest profitability, but not the highest heat energy recovery.

In general, the genetic characteristics, the phenotype and the microbial purity of the production brewing yeast strains are among the most important factors in maintaining a consistently good quality of products. Analysis of restriction fragment length polymorphism (RFLP) patterns of 18S rRNA-coding DNA was investigated to group ale and lager strains. All production brewing yeast strains showed the same RFLP pattern as the type strain and synonym type strains of S. cerevisiae, and were quite different from the type and synonym type strains of S. pastorianus. Based on these data, all production brewing yeast strains investigated in this study appeared to belong to S. cerevisiae. Electrophoretic karyotyping and random amplified polymorphic DNA (RAPD) analysis appeared to be suitable methods for distinguishing not only the type and synonym type strain of S. cerevisiae and S. pastorianus, but also the ale and the lager strains.

Yeasts rarely encounter ideal physiological conditions during their industrial life span; therefore, their ability to adapt to changing conditions determines their usefulness and applicability. This is especially true for baking strains of Saccharomyces cerevisiae. The success of this yeast in the ancient art of bread making is based on its capacity to rapidly transform carbohydrates into CO2 rather than its unusual resistance to environmental stresses. Moreover, baker's yeast must exhibit efficient respiratory metabolism during yeast manufacturing, which determines biomass yield. However, optimal growth conditions often have negative consequences in other commercially important aspects, such as fermentative power or stress tolerance. This article reviews the genetic and physiological characteristics of baking yeast strains, emphasizing the activation of regulatory mechanisms in response to carbon source and stress signaling and their importance in defining targets for strain selection and improvement.

This study investigated the possibility of using yeast strains in fermented milks to obtain products with high Angiotensin I-converting enzyme (ACE) inhibitory activity and low bitter taste. Ninety-three yeast strains isolated from Colombian Kumis in different geographic regions were molecularly identified, and their milk fermentation performances were determined. Molecular identification evidenced that Galactomyces geotrichum, Pichia kudriavzevii, Clavispora lusitaniae and Candida tropicalis, were the dominant species. Eighteen out of 93 strains produced fermented milk with ACE-inhibitory (ACEI) activity values ranging from 8.69 to 88.19%. Digestion of fermented milk samples by pepsin and pancreatin demonstrated an increase in ACEI activity, with C. lusitaniae KL4A as the best producer of ACEI peptides. Moreover, sensory analysis of the products containing the major ACE-inhibitory activity pointed out that P. kudriavzevii KL84A and Kluyveromyces marxianus KL26A could be selected as potential adjunct starter cultures in Kumis, since they made a considerable contribution to the ACE inhibitory activity and produced fermented milk without bitter taste. In this study we observed that Colombian Kumis can be an excellent vehicle for the isolation of yeasts with a potential to enhance bioactive peptides produced during milk fermentation.Copyright © 2012 Elsevier B.V. All rights reserved.

The yeast cell wall of Saccharomyces cerevisiae is an important source of β-d-glucan, a glucose homopolymer with many functional, nutritional and human health benefits. In the present study, the yeast cell wall fractionation process involving enzymatic treatments (savinase and lipolase enzymes) affected most of the physical and functional characteristics of extracted fractions. Thus, the fractionation process showed that β-d-glucan fraction F4 had significantly higher swelling power and fat binding capacity compared to other fractions (F1, F2 and F3). It also exhibited a viscosity of 652.12mPas and a high degree of brightness of extracted β-d-glucan fraction. Moreover, the fractionation process seemed to have an effect on structural and thermal properties of extracted fractions. Overall, results showed that yeast β-d-glucan had good potential for use as a prebiotic ingredient in food, as well as medicinal and pharmaceutical products.Copyright © 2015 Elsevier Ltd. All rights reserved.

Carboxylic acids such as citric, lactic, succinic and itaconic acids are useful products and are obtained on large scale by fermentation. This review describes the options for recovering these and other fermentative carboxylic acids. After cell removal, often a primary recovery step is performed, using liquid-liquid extraction, adsorption, precipitation or conventional electrodialysis. If the carboxylate is formed rather than the carboxylic acid, the recovery process involves a step for removing the cation of the formed carboxylate. Then, bipolar electrodialysis and thermal methods for salt splitting can prevent that waste inorganic salts are co-produced. Final carboxylic acid purification requires either distillation or crystallization, usually involving evaporation of water. Process steps can often be combined synergistically. In-situ removal of carboxylic acid by extraction during fermentation is the most popular approach. Recovery of the extractant can easily lead to waste inorganic salt formation, which counteracts the advantage of the in-situ removal. For industrial production, various recovery principles and configurations are used, because the fermentation conditions and physical properties of specific carboxylic acids differ. Copyright © 2014 Elsevier Inc. All rights reserved.

Bio-based succinate is still a matter of special emphasis in biotechnology and adjacent research areas. The vast majority of natural and engineered producers are bacterial strains that accumulate succinate under anaerobic conditions. Recently, we succeeded in obtaining an aerobic yeast strain capable of producing succinic acid at low pH. Herein, we discuss some difficulties and advantages of microbial pathways producing "succinic acid" rather than "succinate." It was concluded that the peculiar properties of the constructed yeast strain could be clarified in view of a distorted energy balance. There is evidence that in an acidic environment, the majority of the cellular energy available as ATP will be spent for proton and anion efflux. The decreased ATP:ADP ratio could essentially reduce the growth rate or even completely inhibit growth. In the same way, the preference of this elaborated strain for certain carbon sources could be explained in terms of energy balance. Nevertheless, the opportunity to exclude alkali and mineral acid waste from microbial succinate production seems environmentally friendly and cost-effective.

Development of xylose-fermenting yeast strains that are tolerant to the inhibitors present in lignocellulosic hydrolysates is crucial to achieve efficient bioethanol production processes. In this study, the importance of the propagation strategy for obtaining robust cells was studied. Addition of hydrolysate during propagation of the cells adapted them to the inhibitors, resulting in more tolerant cells with shorter lag phases and higher specific growth rates in minimal medium containing acetic acid and vanillin than unadapted cells. Addition of hydrolysate during propagation also resulted in cells with better fermentation capabilities. Cells propagated without hydrolysate were unable to consume xylose in wheat straw hydrolysate fermentations, whereas 40.3% and 97.7% of the xylose was consumed when 12% and 23% (v/v) hydrolysate, respectively, was added during propagation. Quantitative polymerase chain reaction revealed changes in gene expression, depending on the concentration of hydrolysate added during propagation. This study highlights the importance of using an appropriate propagation strategy for the optimum performance of yeast in fermentation of lignocellulosic hydrolysates. © 2015 The Authors. Microbial Biotechnology published by John Wiley & Sons Ltd and Society for Applied Microbiology.

The inability of fermenting microorganisms to use mixed carbon components derived from lignocellulosic biomass is a major technical barrier that hinders the development of economically viable cellulosic biofuel production. In this study, we integrated the fermentation pathways of both hexose and pentose sugars and an acetic acid reduction pathway into one Saccharomyces cerevisiae strain for the first time using synthetic biology and metabolic engineering approaches. The engineered strain coutilized cellobiose, xylose, and acetic acid to produce ethanol with a substantially higher yield and productivity than the control strains, and the results showed the unique synergistic effects of pathway coexpression. The mixed substrate coutilization strategy is important for making complete and efficient use of cellulosic carbon and will contribute to the development of consolidated bioprocessing for cellulosic biofuel. The study also presents an innovative metabolic engineering approach whereby multiple substrate consumption pathways can be integrated in a synergistic way for enhanced bioconversion.

Terpenes are natural products with a remarkable diversity in their chemical structures and they hold a significant market share commercially owing to their distinct applications. These potential molecules are usually derived from terrestrial plants, marine and microbial sources. In vitro production of terpenes using plant tissue culture and plant metabolic engineering, although receiving some success, the complexity in downstream processing because of the interference of phenolics and product commercialization due to regulations that are significant concerns. Industrial workhorses' viz., Escherichia coli and Saccharomyces cerevisiae have become microorganisms to produce non-native terpenes in order to address critical issues such as demand-supply imbalance, sustainability and commercial viability. S. cerevisiae enjoys several advantages for synthesizing non-native terpenes with the most significant being the compatibility for expressing cytochrome P450 enzymes from plant origin. Moreover, achievement of high titers such as 40 g/l of amorphadiene, a sesquiterpene, boosts commercial interest and encourages the researchers to envisage both molecular and process strategies for developing yeast cell factories to produce these compounds. This review contains a brief consideration of existing strategies to engineer S. cerevisiae toward the synthesis of terpene molecules. Some of the common targets for synthesis of terpenes in S. cerevisiae are as follows: overexpression of tHMG1, ERG20, upc2-1 in case of all classes of terpenes; repression of ERG9 by replacement of the native promoter with a repressive methionine promoter in case of mono-, di- and sesquiterpenes; overexpression of BTS1 in case of di- and tetraterpenes. Site-directed mutagenesis such as Upc2p (G888A) in case of all classes of terpenes, ERG20p (K197G) in case of monoterpenes, HMG2p (K6R) in case of mono-, di- and sesquiterpenes could be some generic targets. Efforts are made to consolidate various studies (including patents) on this subject to understand the similarities, to identify novel strategies and to contemplate potential possibilities to build a robust yeast cell factory for terpene or terpenoid production. Emphasis is not restricted to metabolic engineering strategies pertaining to sterol and mevalonate pathway, but also other holistic approaches for elsewhere exploitation in the S. cerevisiae genome are discussed. This review also focuses on process considerations and challenges during the mass production of these potential compounds from the engineered strain for commercial exploitation.

Microbial fermentation of renewable feedstocks into plastic monomers can decrease our fossil dependence and reduce global CO2 emissions. 3-Hydroxypropionic acid (3HP) is a potential chemical building block for sustainable production of superabsorbent polymers and acrylic plastics. With the objective of developing Saccharomyces cerevisiae as an efficient cell factory for high-level production of 3HP, we identified the β-alanine biosynthetic route as the most economically attractive according to the metabolic modeling. We engineered and optimized a synthetic pathway for de novo biosynthesis of β-alanine and its subsequent conversion into 3HP using a novel β-alanine-pyruvate aminotransferase discovered in Bacillus cereus. The final strain produced 3HP at a titer of 13.7±0.3gL(-1) with a 0.14±0.0C-molC-mol(-1) yield on glucose in 80h in controlled fed-batch fermentation in mineral medium at pH 5, and this work therefore lays the basis for developing a process for biological 3HP production. Copyright © 2014 The Authors. Published by Elsevier Inc. All rights reserved.