Yeast Viability Research Articles

Yeast Propagation Control: Low Frequency Electrochemical Impedance Spectroscopy as an Alternative for Cell Counting Chambers in Brewery Applications

Electrochemical impedance spectroscopy is a powerful tool in life science for cell and pathogen detection, as well as for cell counting. The measurement principles and techniques using impedance spectroscopy are highly diverse. Differences can be found in used frequency range (β or α regime), analyzed quantities, like charge transfer resistance, dielectric permittivity of double layer capacitance and in off- or online usage. In recent contributions, applications of low-frequency impedance spectroscopy in the α regime were tested for determination of cell counts and metabolic burden in Escherichia coli and Saccharomyces cerevisiae. The established easy to use methods showed reasonable potential in the lab scale, especially for S. cerevisiae. However, until now, measurements for cell counts in food science are generally based on Thoma cell counting chambers. These microscopic cell counting methods decelerate an easy and quick prediction of yeast viability, as they are labor intensive and result in a time delayed response signal. In this contribution we tested our developed method using low frequency impedance spectroscopy locally at an industrial brewery propagation site and compared results to classic cell counting procedures.

Modeling the inactivation of Lactobacillus brevis DSM 6235 and retaining the viability of brewing pitching yeast submitted to acid and chlorine washing.

This study aimed to model the inactivation of Lactobacillus brevis DSM 6235 while retaining the viability of yeasts during washing brewer's yeast with phosphoric acid and chlorine dioxide. The independent variables in the acid washing were pH (1-3) and temperature (1-9°C), whereas in the washing with chlorine dioxide, concentration (10-90mg/L) and temperature (5-25°C) were assessed. The predictive models obtained for the four response variables γLA, γCl (decimal reduction of L. brevis DSM 6235), Vf/V0LA, and Vf/V0Cl (brewer's yeast viability ratio) were found to have R2 > 0.80 and values of Fcalc > Freference. Then, the models were considered predictive and statistically significant (p < 0.10). Our results indicated that phosphoric acid and chlorine dioxide washing resulted in up to 7 and 6.4 (log CFU/mL) decimal reductions of L. brevis DSM 6235, respectively. On the other hand, the viability of the brewer's yeast ranged from 22.3 to 99.4%. L. brevis DSM 6235 inactivation was significantly influenced by parameters pH(Q) and T°C(Q) when phosphoric acid was applied, and by parameters mg/L(L), mg/L(Q), T°C(Q), and mg/L × T°C when ClO2 was applied. The validation of the models resulted in bias (γLA, 0.93/Vf/V0LA, 0.99 - γCl, 1.0/Vf/V0Cl, 0.99) and accuracy values (γLA, 1.12/Vf/V0LA, 1.01 - γCl, 1.08/Vf/V0Cl, 1.03). The results of this study indicate that it might be possible to decontaminate brewer's yeast through acid and chlorine dioxide washing while keeping its viability. This procedure will result in the reduction of costs and the lower generation of brewer's waste.

Influence of Storage Temperature and Packaging on Bacteria and Yeast Viability in a Plant-Based Fermented Food.

Optimization of food storage has become a central issue for food science and biotechnology, especially in the field of functional foods. The aim of this work was to investigate the influence of different storage strategies in a fermented food product (FFP) and further determine whether the regular storage (room temperature (RT) and standard packaging (SP)) could be refined. Eight experimental conditions (four different temperatures × two packaging) were simulated and changes in FFP’s microbial ecology (total bacteria, lactic acid bacteria (LAB), and yeasts) and physicochemical characteristics (pH and moisture content (MC)) were determined following 1, 3, 6, and 12 months. All conditions tested showed a decline in microbial content due to the effect of the temperature, 37 °C being the most detrimental condition, while −20 and 4 °C seemed to be better than RT in some parameters. Vacuum packaging (VP) only had a major effect on MC and we found that VP preserved greater MC values than SP at 3, 6, and 12 months. The correlation analysis revealed that total bacteria, LAB, and yeasts were positively associated, and also both pH and MC showed a correlation. According to our results and with the purpose to maintain the load of viable microorganisms, we observed that the best storage conditions should contemplate SP and freezing or cooling temperature during a period no longer than 3 months.

A robust flow cytometry-based biomass monitoring tool enables rapid at-line characterization of S. cerevisiae physiology during continuous bioprocessing of spent sulfite liquor

Assessment of viable biomass is challenging in bioprocesses involving complex media with distinct biomass and media particle populations. Biomass monitoring in these circumstances usually requires elaborate offline methods or sophisticated inline sensors. Reliable monitoring tools in an at-line capacity represent a promising alternative but are still scarce to date. In this study, a flow cytometry-based method for biomass monitoring in spent sulfite liquor medium as feedstock for second generation bioethanol production with yeast was developed. The method is capable of (i) yeast cell quantification against medium background, (ii) determination of yeast viability, and (iii) assessment of yeast physiology though morphological analysis of the budding division process. Thus, enhanced insight into physiology and morphology is provided which is not accessible through common online and offline biomass monitoring methods. To demonstrate the capabilities of this method, firstly, a continuous ethanol fermentation process of Saccharomyces cerevisiae with filtered and unfiltered spent sulfite liquor media was analyzed. Subsequently, at-line process monitoring of viability in a retentostat cultivation was conducted. The obtained information was used for a simple control based on addition of essential nutrients in relation to viability. Thereby, inter-dependencies between nutrient supply, physiology, and specific ethanol productivity that are essential for process design could be illuminated.Graphical abstract

Sodium alginate coatings added with Meyerozyma caribbica: Postharvest biocontrol of Colletotrichum gloeosporioides in avocado (Persea americana Mill. cv. Hass)

Abstract Edible coatings have been used as a medium for the incorporation of functional compounds and biocontrol agents to postharvest diseases control on fruits and to maintain their quality parameters. However, there are few reports about the use of biocontrol agents entrapped in polymeric matrices; to our knowledge, there is no report of their application to anthracnose control in avocado fruit. Hence the aims of this study were to investigate the production of volatile organic compounds (VOCs) by Meyerozyma caribbica and the ability of the yeast entrapped in sodium alginate (SA) coatings to control anthracnose caused by Colletotrichum gloeosporioides Pa14 in avocado fruit. The yeast viability, biocontrol activity, effect on weight loss as well as the efficacy of the bioactive coatings to preserve postharvest quality by prevention or cure of C. gloeosporioides infection were assessed. The main VOCs identified were alcohols (1-Butanol, 3-methyl- and phenethyl alcohol) and esters (ethyl acetate). Results revealed SA as a matrix able to maintain the yeast viability during the storage on coated avocado fruit, with minimal reductions between 0.39 and 1 Log10 CFU mL−1, depending on storage conditions. Moreover, it was demonstrated the ability of the yeast to increase its population during the ripening of avocado fruit previously-stored at 6 °C. Films with M. caribbica were capable to inhibit C. gloeosporioides growth in vitro. Meanwhile, in vivo, the preventive treatments were more effective than curative treatments in anthracnose controlling; at 25 °C the severity was halved, while at 6 °C the reduction reached 100 %. Additionally, in avocado fruit with the bioactive coatings, the weight loss was reduced by 2–3.7 % respect to the control. In conclusion, this study demonstrates the ability of M. caribbica to produce VOCs as a mechanism of action against C. gloeosporioides; additionally, SA coatings with M. caribbica were effective to reduce the weight loss and its potential as an alternative to control of postharvest diseases in avocado fruit.

Sourdough derived strains of Saccharomyces cerevisiae and their potential for farmhouse ale brewing

The Finnish farmhouse ale sahti is unique in that it is fermented with baking, rather than brewing strains of Saccharomyces cerevisae. The custom of maintaining farmhouse yeast cultures is however no longer practiced in Finland, and much yeast derived diversity in sahti beers has presumably been lost as a consequence. Here, the brewing potential of a number of sourdough derived strains was tested with respect to a number of different fermentation traits. Seven strains originally isolated from Finnish or Italian sourdough cultures were used to ferment high gravity sahti wort (20°P), and fermentation performance together with production of volatile compounds were assessed and compared with a reference baking yeast. Strains differed in terms of fermentation rate, yield, yeast viability and beer flavour profile. All were maltotriose positive, but utilisation varied so that alcohol yield could be greater or lower than that of the reference strain, with values ranging from 6.6 to 7.9% (v/v). Production of aroma compounds was also variable so that it was possible to identify strains producing high levels of esters and those with lower production, which could be used to emphasise flavours originating from raw materials. All strains generated 4-vinyl guaiacol and so would be suitable for other beers where this is a part of the normal flavour profile. Results suggest that sourdough isolates of S. cerevisiae are suitable for sahti production, but could also be applied to other beer styles as a way to differentiate products. © 2020 The Authors. Journal of the Institute of Brewing published byJohn Wiley & Sons Ltd on behalf of The Institute of Brewing & Distilling

Temperature profiled simultaneous saccharification and co-fermentation of corn stover increases ethanol production at high solid loading

Trade-off temperature used in traditional SScF process always resulted in inhibition on enzymatic hydrolysis (low temperature inhibition, LTI). Thus, effect of temperature and solid loading on LTI was investigated in this study. Results showed that LTI was more severe at lower temperature and higher solid loading. Accordingly, a novel temperature profiled SScF process was designed. Two specific yeast strains, high temperature resistant Saccharomyces cerevisiae and xylose utilizing S. cerevisiae were co-cultured in the novel SScF process. Through eliminating sugars accumulation and alleviating ethanol repression by process optimization, 59.8 g/L ethanol was achieved in temperature profiled SScF at 12% glucan loading. Cell viability of yeast in co-culture SScF was ten times, sometimes even thousands of times, of that in mono-culture SScF during the late phase of the fermentation (48–108 h). It suggested that high temperature resistant strain helped the xylose utilizing strain maintain cell viability in SScF at high temperature (42 ℃). Based on the results, the designed novel temperature profiled SScF process showed appealing potential for bioethanol production at high solid loading and high temperature.

Effect of pH Condition on Natural Indigo (Indigofera tinctoria) Reduction by Yeast (Saccharomyces cerevisiae)

Ultimate goal of this research is to develop an efficient microbial system for indigo reduction by using yeast (Saccharomyces cerevisiae). For this purpose, we investigated the effect of pH condition on natural indigo (Indigofera tinctoria) reduction by yeast. We also checked the viability of yeast at high pH condition by observing yeast cell in the indigo reduction bath with an optical microscope. The pH of indigo reduction bath was initially set from 10.2 to 12.0 with the buffer solution. The pH value dropped drastically up to 5.5–6.5 at the initial stage of reduction. After adjusting pH once within 1–2 days, we measured some parameters related to reducing power such as the color yield (K/S value) of ramie fabric dyed with the supernatant of bath, the oxidation/reduction potential (ORP), and the pH values changed. When the initial pH was set at lower than 11 (10.2, 10.7), the baths adjusted to pH 11 showed better reducing power than the baths of pH other than 11. And also, when the initial pH was set higher than 11 (11.3 and 12.0), the baths adjusted to pH 11 showed better reducing power than the baths of pH other than 11. It took 5–11 days to reach the maximum dye uptake after the initiation of reduction, and, during the corresponding time, the ORP was from -563 to -599 mV. All the initial and the adjusted pH values were not maintained but decreased continuously, and the maximum dye uptake reached at pH 9.17-9.43. The initial and the adjusted pH greatly affected the indigo reducing power, and the highest color yield was obtained with indigo reduction bath of initial pH 10.7. When the pH of the bath was adjusted to pH 11.0, the maximum reducing power was obtained most efficiently. Also, for the pH adjustment, use of sodium hydroxide showed higher reducing power than use of calcium hydroxide.

Karvakrol İlavesinin Zygosaccharomyces bailii ile Kontamine Edilmiş Elma Suyunda Bazı Fizikokimyasal ve Biyoaktif Özelliklerin Değişimi Üzerindeki Etkisinin İncelenmesi

Bu çalışmada yüksek şeker konsantrasyonuna, düşük aside, etanole ve pastörizasyon uygulamasına karşı direnç gösterebilen Z. bailii inhibisyonunda kullanılan karvakrolün, elma suyunun depolama süresi boyunca bazı fizikokimyasal ve biyoaktif özellikleri ile maya sayılarındaki değişimine olan etkisi incelenmiştir. Bu amaçla tepki yüzey yöntemi uygulanmış ve proses değişkeni olarak depolama süresi (1-41 gün), depolama sıcaklığı (4-20°C), pozitif kontrol olarak sodyum benzoat miktarı (%0-0,1) ve karvakrol miktarı (0-750 ppm) seçilmiştir. Elma suyu örneklerinde depolama boyunca maya canlılığına bağlı olarak fizikokimyasal özelliklerinde önemli değişiklikler meydana gelmiştir. Karvakrol miktarının artışı maya sayısında yaklaşık 7 log düzeyinde bir azalma meydana getirmiş ve meyve sularının bozulmalarını önlemiş, ancak karvakrol içermeyen örneklerde briks değeri maya faaliyeti sonucu yaklaşık %50 düzeyinde azalma göstermiş ve meyve suyu tüketilemeyecek duruma gelmiştir. Toplam fenolik madde değerleri 136,7-645,7 mg GAE/L aralığında tespit edilmiş olup en düşük toplam fenolik madde miktarı karvakrol içermeyen 11 no‘lu örnekte tespit edilmişken en yüksek değer ise en yüksek seviyede karvakrol ihtiva eden elma suyu örneğinde belirlenmiştir. Yapılan optimizasyon işlemi sonucunda örneklerin maksimum düzeyde karvakrol ilavesi ile Z. bailii‘nin bozucu faaliyetlerinin büyük ölçüde engellenebileceği görülmüştür.

Brazilian “Flex Mills”: Ethanol from Sugarcane Molasses and Corn Mash

This manuscript shows an ethanol production from two simultaneously processed raw materials: corn grains and molasses residue from sugarcane production. This process is new in countries such as Brazil, which has industrialized these raw materials in the same plant (Flex Mills), and the effects promoted on ethanol production (fermentation process) have not been studied. This paper shows that the uses of yeast CAT-1 (isolated for ethanol production using sugarcane) are not adequate to fermentation of broth obtained from 100% corn. However, the mixture of 50% corn broth and 50% molasses broth results in increase of cell viability, budding and bud viability of yeast, and the ethanol yield process. Another point to highlight is the high level of phenolic compounds showed by molasses, which could decrease cell viability and yeast budding rate during the recycling of this microorganism. The mixture with corn broth standardizes the phenolic compounds in the molasses broth.

Stress-induced upregulation of the ubiquitin-relative Hub1 modulates pre-mRNA splicing and facilitates cadmium tolerance in Saccharomyces cerevisiae

Splicing is a fundamental RNA-processing step for eukaryotic gene expression involved in the removal of intronic sequences of pre-mRNA. As the process is utilized for quantitative and qualitative regulation of gene expression, uncontrolled splicing can result in potential cellular dysfunctions. Accumulating evidence suggests that fidelity of splicing is regulated by a family of DEAD/DExH-box RNA helicases. Recently, we have shown that the evolutionarily-conserved ubiquitin-relative Hub1 binds directly to the DEAD-box RNA helicase Prp5, a key regulator of splicing fidelity, and stimulates its ATPase activity. When overexpressed, Hub1 enhances splicing efficiency and relaxes the constraints on splice-site and branch-site usages; yet physiological relevance of cellular Hub1 overexpression remains unknown. Here we show that Hub1 is upregulated at the transcriptional level via the yeast-specific AP1 regulon upon oxidative and heavy metal stresses, and promotes efficient splicing of introns with non-canonical splice-sites. While nonessential for yeast viability, Hub1 becomes important for cadmium tolerance when metallothionein-mediated defense system is impaired. Moreover, mutant variants of other splicing factors also showed a similar cadmium sensitivity, suggesting the role of splicing in facilitating tolerance of heavy metal stress. Taken together, we propose that cells adjust gene expression landscape required for heavy metal detoxification by promoting intron-specific splicing through the stress-induced overexpression of Hub1.

Matrices (re)loaded: Durability, viability, and fermentative capacity of yeast encapsulated in beads of different composition during long-term fed-batch culture.

Encapsulated microbes have been used for decades to produce commodities ranging from methyl ketone to beer. Encapsulated cells undergo limited replication, which enables them to more efficiently convert substrate to product than planktonic cells and which contributes to their stress resistance. To determine how encapsulated yeast supports long‐term, repeated fed‐batch ethanologenic fermentation, and whether different matrices influence that process, fermentation and indicators of matrix durability and cell viability were monitored in high‐dextrose, fed‐batch culture over 7 weeks. At most timepoints, ethanol yield (g/g) in encapsulated cultures exceeded that in planktonic cultures. And frequently, ethanol yield differed among the four matrices tested: sodium alginate crosslinked with Ca2+ and chitosan, sodium alginate crosslinked with Ca2+, Protanal alginate crosslinked with Ca2+ and chitosan, Protanal alginate crosslinked with Ca2+, with the last of these consistently demonstrating the highest values. Young's modulus and viscosity were higher for matrices crosslinked with chitosan over the first week; thereafter values for both parameters declined and were indistinguishable among treatments. Encapsulated cells exhibited greater heat shock tolerance at 50°C than planktonic cells in either stationary or exponential phase, with similar thermotolerance observed across all four matrix types. Altogether, these data demonstrate the feasibility of re‐using encapsulated yeast to convert dextrose to ethanol over at least 7 weeks.

Optimization of temperature and drying time of indigenous cocktail yeast mold culture using response surface methodology (RSM)

The study was conducted to obtain an optimal combination of time and temperature of the drying process of indigenous cocktail yeast mold culture using RSM. The cocktail yeast mold culture was dried using an oven. The cocktail cultures contain Penicillium citrinum, Aspergillus niger, Acremonium strictum, and Candida famata, namely AC (Amylolytic Culture). The Response Surface Methods (RSM) with Design-Expert® 7.00 software, namely Mixture design with D-optimal was performed. The drying time was between 24- 48 hrs and the drying temperature was between 40-50oC. The total of 16 formulas of the combination of drying time and temperature for processing the dried cultures were produced by RSM. The response chosen was total viability of mold and yeast, water content, water activity, and pH. The result of optimization and verification was obtained by the model: pH (AC) = -0.058A - 1.56 x 10-003B + 7.13, where A = drying temperature ( oC), B = drying time (hr). The AC optimization was achieved at a combination of drying temperatures and time of 50oC for 48 hrs. Desirability values were 0.729. The optimum formula for AC has viability of total yeast mold of 7.39 x 106 CFU/g, moisture content of 5.62%, aw 0.303, and pH 4.18.

Rapid Beer Fermentation: The Effect of Vacuum Pressure on a Pilot Scale Lager Fermentation

Changes to industrial fermentation processes are rare, largely due to the impact on viability for Saccharomyces spp., related to environmental stresses during the process such as: osmotic pressure, ethanol concentration, dissolved CO2. This research studied yeast behavior during a High Gravity (HG) fermentation under partial vacuum pressure during standard brewing operations. This was accomplished by controlling the temperature and pressure of 30 L pilot fermentations. An experimental lager wort recipe with “Diamond” lager yeast (Saccharomyces pastorianus) was used in this study. The modified process conducted at 24.1 kPa of pressure in the headspace was compared to a control process at standard atmospheric pressure (101.3 kPa). This research showed that at 15 °C, lager fermentations (14–15°P) fermented up to 30% faster under vacuum, with an increase in the number of yeast cells in suspension of up to 100%. The rate of sugar consumption under vacuum was greater compared to the control, thus increasing the rate of ethanol production. The removal of volatile CO2 is one hypothesized mechanism for these results. The total ethanol production, pH and yeast viability did not differ significantly between the control and modified process. The application of vacuum during brewing resulted in an increased fermentation rate and number of yeast cells in suspension during fermentation.

Combination of natural antimicrobials for contamination control in ethanol production.

Presence of bacterial contaminants at levels > 107 colony forming units per milliliter (CFU/mL) during ethanol production processes reduces the alcoholic fermentation yield by 30%. Antibiotics are currently used to control contamination, but their residues may be detected in yeast extract, restricting this by-product trade to several countries. Thus, the objective of this study was to assess antimicrobial activity of the natural compounds hops extract, 4-hydroxybenzoic acid, nisin Z, and lysozyme against Lactobacillus fermentum, Leuconostoc mesenteroides, and Saccharomyces cerevisiae, aiming development of a formula. Minimum Inhibitory Concentration of each antimicrobial was determined for bacteria and subsequently, nisin (30mg/L) and hops extract (5mg/L) were tested together, showing inhibitory effects combining doses of each antimicrobial that were equivalent to an eightfold reduction of their original Minimum Inhibitory Concentrations (3.75 and 0.625mg/L, respectively), resulting in a FICIndex of 0.25. Thereon, a formula containing both compounds was developed and tested in fermentation assays, promoting reductions on bacterial population and no severe interferences in yeast viability or population even at extreme doses. Therefore, these compounds have great potential to successfully substitute conventional antibiotics in the ethanol industry.

Trehalose Protects the Probiotic Yeast Saccharomyces boulardii against Oxidative Stress-Induced Cell Death.

Saccharomyces boulardii is the only probiotic yeast with US Food and Drug Administration approval. It is routinely used to prevent or treat acute diarrhea and other gastrointestinal disorders, including the antibiotic-associated diarrhea caused by Clostridium difficile infections. The formation of reactive oxygen species (ROS), specifically H2O2 during normal aerobic metabolism, contributes to programmed cell death and represents a risk to the viability of the probiotic microbe. Moreover, a loss of viability reduces the efficacy of the probiotic treatment. Therefore, inhibiting the accumulation of ROS in the oxidant environment could improve the viability of the probiotic yeast and lead to more efficacious treatment. Here, we provide evidence that supplementation with a non-reducing disaccharide, namely trehalose, enhanced the viability of S. boulardii exposed to an oxidative environment by preventing metacaspase YCA1-mediated programmed cell death through inhibition of intracellular ROS production. Our results suggest that supplementation with S. boulardii together with trehalose could increase the viability of the organism, and thus improve its effectiveness as a probiotic and as a treatment for acute diarrhea and other gastrointestinal disorders.

Improvement of Bioethanol Production from Sweet Sorghum Juice under Very High Gravity Fermentation: Effect of Nitrogen, Osmoprotectant, and Aeration

To improve ethanol production fermentation efficiency from sweet sorghum juice under a very high gravity (VHG, 280 g/L of total sugar) condition by Saccharomyces cerevisiae NP01, dried spent yeast (DSY), yeast extract, and glycine concentrations were optimized using an L9 (34) orthogonal array design. The results showed that the order of influence on the ethanol concentration (PE) was yeast extract &gt; glycine &gt; DSY. The optimal nutrient concentrations for ethanol production were determined as follows: yeast extract, 3; DSY, 4; and glycine, 5 g/L. When a verification experiment under the projected optimal conditions was done, the P, ethanol yield (Yp/s), and ethanol productivity (Qp) values were 120.1 g/L, 0.47, and 2.50 g/L·h, respectively. These values were similar to those of the positive control experiment with yeast extract supplementation at 9 g/L. The yeast viability under the optimal condition was higher than that of the control experiment. To improve sugar utilization and ethanol production, aeration at 2.5 vvm for 4 h was applied under the optimal nutrient supplementation. The P, Yp/s, and Qp values were significantly increased to 134.3 g/L, 0.50, and 2.80 g/L·h, respectively.

Silver Nanoparticles Addition in Poly(Methyl Methacrylate) Dental Matrix: Topographic and Antimycotic Studies.

The widespread use of nanoparticles (NPs) in medical devices has opened a new scenario in the treatment and prevention of many diseases and infections owing to unique physico-chemical properties of NPs. In this way, silver nanoparticles (AgNPs) are known to have a strong antimicrobial activity, even at low concentrations, due to their ability to selectively destroy cellular membranes. In particular, in the field of dental medicine, the use of AgNPs in different kinds of dental prosthesis matrixes could be a fundamental tool in immunodepressed patients that suffer of different oral infections. Candida albicans (C. albicans), an opportunistic pathogenic yeast with high colonization ability, is one of the causative agents of oral cavity infection. In our work, we added monodispersed citrate-capping AgNPs with a size of 20 nm at two concentrations (3 wt% and 3.5 wt%) in poly(methyl methacrylate) (PMMA), the common resin used to develop dental prostheses. After AgNPs characterization, we evaluated the topographical modification of PMMA and PMMA with the addition of AgNPs by means of atomic force microscopy (AFM), showing the reduction of surface roughness. The C. albicans colonization on PMMA surfaces was assessed by the Miles and Misra technique as well as by scanning electron microscopy (SEM) at 24 h and 48 h with encouraging results on the reduction of yeast viability after AgNPs exposure.

Enhancing bioethanol productivity by a yeast-immobilized catalytically active membrane in a fermentation-pervaporation coupling process

Bioethanol is a promising substitute for fossil fuels and is mainly produced through yeast fermentation. However, in conventional batch fermentation process extracellular ethanol accumulation restricts yeast excretion of intracellular ethanol, and results in suppressed yeast viability and ethanol productivity. The present study reports a yeast-immobilized catalytically active membrane for efficient extracellular ethanol removal towards enhanced bioethanol productivity in pervaporation membrane reactor. The membrane was fabricated by immersion phase inversion through coating a porous yeast/polyethersulfone catalytic layer on a polydimethylsiloxane (PDMS) pervaporation membrane. Benefited from the high porosity of biocatalytic layer, the immobilized yeast exhibited superior fermentation capacity in high substrate concentration when compared with free yeasts. Most importantly, by comparing the fermentation performances in catalytically active membrane reactor (CAMR), inert membrane reactor and batch reactor, CAMR exhibited the highest ethanol productivity (3.05 g L−1 h−1) and yeast mass concentration (26.41 g L−1), indicating a superior yeast growth rate and viability in CAMR. We attribute this to a “genuine” in situ removal ability of the bio-catalytically active membrane: the extracellular ethanol was promptly removed from the vicinity of the yeast, promoting a more rapid excretion of the inhibitor (intracellular ethanol) and thus maximally eliminated undesirable inhibition of the intracellular ethanol.

Effect of Thermosonication and Physicochemical Properties of Wine on Culturability, Viability, and Metabolic Activity of Brettanomyces bruxellensis Yeast in Red Wines

The aim of this research was to investigate the short- and long-term effects of thermosonication and different physicochemical properties of wine on culturability, viability, and metabolic activity of Brettanomyces bruxellensis yeast. Thermosonication was conducted at 43 °C during 1, 2, and 3 min, while wine variations included several pH, alcohol, and sugar levels. Cell culturability and viability were determined immediately after treatment and during 90 days of storage, while metabolic activity was determined after 90 days of storage. Results showed that, although culturability was not confirmed in dry wines immediately after 3 min of treatment, thermosonication did not result in complete inactivation of the B. bruxellensis population. Herein, the first evidence of a viable but not culturable (VBNC) state of B. bruxellensis after thermosonication exposure was observed. Moreover, thermosonication reduced the production of volatile phenols. Obtained results suggest application of thermosonication for reduction of the B. bruxellensis population only in early stages of wine contamination.