Improvement of Multiple-Stress Tolerance and Ethanol Production in Yeast during Very-High-Gravity Fermentation by Supplementation of Wheat-Gluten Hydrolysates and Their Ultrafiltration Fractions

Transcriptome profiling unravels improved ethanol production and acetic acid tolerance in yeast by preculture of wheat gluten hydrolysates

The effects of wheat gluten hydrolysates (WGH) preculture on yeast acetic acid tolerance and fermentation performances were investigated. Results showed that WGH preculture significantly increased yeast growth and viability under acetic acid stress. Particularly, the WGH fraction precipitated with 90 % (v/v) gradient ethanol (WGH-C) preculture significantly improved yeast cell membrane integrity and H+-ATPase activity, thereby decreasing the intracellular accumulation of ROS and acetic acid. Meanwhile, WGH-C preculture promoted the ethanol production efficiency, shortening the fermentation lag time by 12 h and increasing the ethanol yield by 37.46 %. These improvements were attributed to that WGH-C preculture regulated intracellular amino acid composition and transport protein related gene expression of yeast. Transcriptome profiling demonstrated that the cell wall and plasma membrane structures were remodeled, reducing the oxidative stress induced by acetic acid. Furthermore, regulation of energy metabolism and transporter activity are prime mechanisms in improving acetic acid tolerance and fermentation efficiency of yeast.

Naringenin and caffeic acid increase ethanol production in yeast cells by reducing very high gravity fermentation-related oxidative stress

Very high gravity (VHG) fermentation is an industrial-scale process utilizing a sugar concentration above 250 g/L to attain a significant ethanol concentration, with the advantages of decreased labor, production costs, water usage, bacterial contamination, and energy consumption. Saccharomyces cerevisiae is one of the most extensively employed organisms in ethanol fermentation through VHG technology. Conversely, high glucose exposure leads to numerous stress factors that negatively impact the ethanol production efficiency of this organism. Here, the impact of various phytochemicals added to the VHG medium on viability, glucose consumption, ethanol production efficiency, total antioxidant-oxidant status (TAS and TOS), and the response of the enzymatic antioxidant system of yeast were investigated. 2.0 mM naringenin and caffeic acid increased ethanol production by 2.453 ± 0.198 and 1.261 ± 0.138-fold, respectively. The glucose consumption rate exhibited a direct relationship with ethanol production in the naringenin-supplemented group. The highest TAS was determined as 0.734 ± 0.044 mmol Trolox Eq./L in the same group. Furthermore, both phytochemical compounds exhibited robust positive correlations with TAS (rnaringenin = 0.9986; rcaffeic acid = 0.9553) and TOS levels (rnaringenin = -0.9824; rcaffeic acid = -0.9791). While naringenin caused statistically significant increases in glutathione reductase (GR) and thioredoxin reductase (TrxR) activities, caffeic acid significantly increased TrxR and superoxide dismutase (SOD). Both phytochemicals seem to impact the ethanol production ability by regulating the redox status of the cells. We believe that the incorporation of particularly cost-effective antioxidants into the fermentation medium may serve as an alternative way to enhance the efficiency of bioethanol production using VHG technology.

Soy protein hydrolysates induce menaquinone-7 biosynthesis by enhancing the biofilm formation of Bacillus subtilis natto

Menaquinone-7 (MK-7) is a form of vitamin K2 with health-beneficial effects. A novel fermentation strategy based on combining soy protein hydrolysates (SPHs) with biofilm-based fermentation was investigated to enhance menaquinone-7 (MK-7) biosynthesis by Bacillus subtilis natto. Results showed the SPHs increased MK-7 yield by 199.4% in two-stage aeration fermentation as compared to the SP-based medium in submerged fermentation, which was related to the formation of robust biofilm with wrinkles and the enhancement of cell viability. Moreover, there was a significant correlation between key genes related to MK-7 and biofilm synthesis, and the quorum sensing (QS) related genes, Spo0A and SinR, were downregulated by 0.64-fold and 0.39-fold respectively, which promoted biofilm matrix synthesis. Meanwhile, SPHs also enhanced the MK-7 precursor, isoprene side chain, supply, and MK-7 assembly efficiency. Improved fermentation performances of bacterial cells during fermentation were attributed to abundant oligopeptides (Mw < 1 kDa) and moderate amino acids, particularly Arg, Asp, and Phe in SPHs. All these results revealed that SPHs were a potential and superior nitrogen source for MK-7 production by Bacillus subtilis natto.

Plant-derived antioxidant dipeptides provide lager yeast with osmotic stress tolerance for very high gravity fermentation

Osmotic stress in the yeast limits productivity in industrial beer production under very high gravity brewing. This study focused on assessing the protective impacts of eleven plant-derived antioxidant dipeptides (PADs) on the osmotic stress tolerance of lager yeast. The results showed that PADs provided yeast with stress tolerance under osmotic stress. PADs supplementation enhanced cell membrane integrity and reduced oxidative damage. PADs upregulated the expression of SOD2, PEX11 and CTT1 genes under osmotic stress. Moreover, the volatile compounds contents and antioxidant activities of beers were improved by PADs, suggesting favorable quality characteristics. Especially, Phe-Cys and Leu-His could increase the DPPH radical scavenging activity of beer by 41.92% and 18.78% respectively, compared with control. Therefore, PADs are industrially scalable enhancers to improve the ability of yeast to resist osmotic stress and beer quality during very high gravity brewing.

Bioactive dipeptides enhance the tolerance of lager yeast to ethanol-oxidation cross-stress by regulating the multilevel defense system

Although high gravity brewing technology has been widely used for beer industries due to its economic benefits, yeast cells are subjected to multiple environmental stresses throughout the fermentation process. Eleven bioactive dipeptides (LH, HH, AY, LY, IY, AH, PW, TY, HL, VY, FC) were selected to evaluate their effects on cell proliferation, cell membrane defense system, antioxidant defense system and intracellular protective agents of lager yeast against ethanol-oxidation cross-stress. Results showed that the multiple stresses tolerance and fermentation performance of lager yeast were enhanced by bioactive dipeptides. Cell membrane integrity was improved by bioactive dipeptides through altering the structure of macromolecular compounds of the cell membrane. Intracellular reactive oxygen species (ROS) accumulation was significantly decreased by bioactive dipeptides, especially for FC, decreasing by 33.1%, compared with the control. The decrease of ROS was closely related to the increase of mitochondrial membrane potential, intracellular antioxidant enzyme activities including superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD), and glycerol level. In addition, bioactive dipeptides could regulate the expression of key genes (GPD1, OLE1, SOD2, PEX11, CTT1, HSP12) to enhance the multilevel defense systems under ethanol-oxidation cross-stress. Therefore, bioactive dipeptides should be potentially efficient and feasible bioactive ingredients to improve the multiple stresses tolerance of lager yeast during high gravity fermentation.

1,4-α-Glucosidase from Fusarium solani for Controllable Biosynthesis of Silver Nanoparticles and Their Multifunctional Applications

In the study, monodispersed silver nanoparticles (AgNPs) with an average diameter of 9.57 nm were efficiently and controllably biosynthesized by a reductase from Fusarium solani DO7 only in the presence of β-NADPH and polyvinyl pyrrolidone (PVP). The reductase responsible for AgNP formation in F. solani DO7 was further confirmed as 1,4-α-glucosidase. Meanwhile, based on the debate on the antibacterial mechanism of AgNPs, this study elucidated in further depth that antibacterial action of AgNPs was achieved by absorbing to the cell membrane and destabilizing the membrane, leading to cell death. Moreover, AgNPs could accelerate the catalytic reaction of 4-nitroaniline, and 86.9% of 4-nitroaniline was converted to p-phenylene diamine in only 20 min by AgNPs of controllable size and morphology. Our study highlights a simple, green, and cost-effective process for biosynthesizing AgNPs with uniform sizes and excellent antibacterial activity and catalytic reduction of 4-nitroaniline.

Protective effects of peptides on the cell wall structure of yeast under osmotic stress

Three peptides (LL, LML, and LLL) were used to examine their influences on the osmotic stress tolerance and cell wall properties of brewer's yeast. Results suggested that peptide supplementation improved the osmotic stress tolerance of yeast through enhancing the integrity and stability of the cell wall. Transmission electron micrographs showed that the thickness of yeast cell wall was increased by peptide addition under osmotic stress. Additionally, quantitative analysis of cell wall polysaccharide components in the LL and LLL groups revealed that they had 27.34% and 24.41% higher chitin levels, 25.73% and 22.59% higher mannan levels, and 17.86% and 21.35% higher β-1,3-glucan levels, respectively, than the control. Furthermore, peptide supplementation could positively modulate the cell wall integrity pathway and up-regulate the expressions of cell wall remodeling-related genes, including FKS1, FKS2, KRE6, MNN9, and CRH1. Thus, these results demonstrated that peptides improved the osmotic stress tolerance of yeast via remodeling the yeast cell wall and reinforcing the structure of the cell wall. KEY POINTS: • Peptide supplementation improved yeast osmotic stress tolerance via cell wall remodeling. • Peptide supplementation enhanced cell wall thickness and stability under osmotic stress. • Peptide supplementation positively modulated the CWI pathway under osmotic stress.

Improved osmotic stress tolerance in brewer's yeast induced by wheat gluten peptides

The influences of three wheat gluten peptides (WGP-LL, WGP-LML, and WGP-LLL) on the osmotic stress tolerance and membrane lipid component in brewer's yeast were investigated. The results demonstrated that the growth and survival of yeast under osmotic stress were enhanced by WGP supplementation. The addition of WGP upregulated the expressions of OLE1 (encoded the delta-9 fatty acid desaturase) and ERG1 (encoded squalene epoxidase) genes under osmotic stress. At the same time, WGP addition enhanced palmitoleic acid (C16:1) content, unsaturated fatty acids/saturated fatty acids ratio, and the amount of ergosterol in yeast cells under osmotic stress. Furthermore, yeast cells in WGP-LL and WGP-LLL groups were more resistant to osmotic stress. WGP-LL and WGP-LLL addition caused 25.08% and 27.02% increase in membrane fluidity, 22.36% and 29.54% reduction in membrane permeability, 18.38% and 14.26% rise in membrane integrity in yeast cells, respectively. In addition, scanning electron microscopy analysis revealed that the addition of WGP was capable of maintaining yeast cell morphology and reducing cell membrane damage under osmotic stress. Thus, alteration of membrane lipid component by WGP was an effective approach for increasing the growth and survival of yeast cells under osmotic stress. KEY POINTS: •WGP addition enhanced cell growth and survival of yeast under osmotic stress. •WGP addition increased unsaturated fatty acids and ergosterol contents in yeast. •WGP supplementation improved membrane homeostasis in yeast at osmotic stress.

Wheat Gluten Peptides Enhance Ethanol Stress Tolerance by Regulating the Membrane Lipid Composition in Yeast

Wheat gluten peptides (WGPs), identified as Leu-Leu (LL), Leu-Leu-Leu (LLL), and Leu-Met-Leu (LML), were tested for their impacts on cell growth, membrane lipid composition, and membrane homeostasis of yeast under ethanol stress. The results showed that WGP supplementation could strengthen cell growth and viability and enhance the ethanol stress tolerance of yeast. WGP supplementation increased the expressions of OLE1 and ERG1 and enhanced the levels of oleic acid (C18:1) and ergosterol in yeast cell membranes. Moreover, LLL and LML exhibited a better protective effect for yeast under ethanol stress compared to LL. LLL and LML supplementation led to 20.3 ± 1.5% and 18.9 ± 1.7% enhancement in cell membrane fluidity, 21.8 ± 1.6% and 30.5 ± 1.1% increase in membrane integrity, and 26.3 ± 4.8% and 27.6 ± 4.6% decrease in membrane permeability in yeast under ethanol stress, respectively. The results from scanning electron microscopy (SEM) elucidated that WGP supplementation is favorable for the maintenance of yeast cell morphology under ethanol stress. All of these results revealed that WGP is an efficient enhancer for improving the ethanol stress tolerance of yeast by regulating the membrane lipid composition.

Protein kinases Elm1 and Sak1 of Saccharomyces cerevisiae exerted different functions under high-glucose and heat shock stresses

Phosphorylation catalyzed by protein kinases is the most common and important regulatory pathway in the adaptive physiological responses to the changes in nutrition and environment of yeast. This study focused on the functions of Elm1, Sak1, and Tos3, which are three upstream protein kinases of Snf1 in Saccharomyces cerevisiae, in response to high-glucose and heat shock stresses. Results suggested that changing the gene dosage of ELM1/SAK1/TOS3 had different effects under high-glucose and heat shock stresses. ELM1 and SAK1 overexpressions could enhance the tolerance of S. cerevisiae to high-glucose and heat shock stresses, respectively. Nevertheless, the overexpression of TOS3 decreased the tolerance to high-glucose stress, and a native level of Tos3 was important for the normal adaptation to heat shock condition. The overexpression of ELM1 increased the accumulation of trehalose and ergosterol and altered the composition of fatty acids with altered gene expressions involved in the metabolism of three metabolites. Enhanced resistance to heat shock stress in SAK1 overexpression might be related to the enhanced accumulation of trehalose and ergosterol and upregulated transcription of genes related to the metabolism of trehalose and ergosterol. Furthermore, Elm1 might regulate the metabolism of trehalose, ergosterol, and fatty acids in a Snf1-independent form under high-glucose stress. A Snf1-independent pathway might be involved in the regulation of trehalose metabolism by Sak1 under heat shock condition. However, Sak1 and Snf1 may have an indirect relationship in the regulation of ergosterol synthesis. KEY POINTS: • Altering the gene dosage of ELM1/SAK1/TOS3 had different effects on stress responses • Elm1 regulated high-glucose response in a Snf1-independent manner • Sak1 and Snf1 had an indirect relationship in the regulation of heat shock response.

Recycling and Conversion of Yeasts into Organic Nitrogen Sources for Wine Fermentation: Effects on Molecular and Sensory Attributes

Organic nitrogen plays a significant role in the fermentation performance and production of esters and higher alcohols. This study assessed the use of yeast protein hydrolysate (YPH) as a nitrogen source for grape must fermentation. In this study, we prepared an enzymatic protein hydrolysate using yeasts recovered from a previous fermentation of wine. Three treatments were performed. DAP supplementation was used as a control, while two YPH treatments were used. Low (LDH) and high degrees of hydrolysis (HDH), 3.5% and 10%, respectively, were chosen. Gas chromatography and principal component analysis indicated a significant positive influence of YPH-supplementations on the production of esters and higher alcohols. Significantly high concentrations of 3-methyl-1-penthanol, isoamyl alcohol, isobutanol, and 2-phenylethanol were observed. Significant odorant activity was obtained for 3-methyl-1-pentanol and ethyl-2-hexenoate. The use of YPH as nitrogen supplementation is justified as a recycling yeasts technique by the increase in volatile compounds.

Wheat gluten hydrolysates promotes fermentation performance of brewer's yeast in very high gravity worts

The effects of wheat gluten hydrolysates (WGH) and their ethanol elution fractions obtained on XAD-16 resin on physiological activity and fermentation performance of brewer's yeast during very-high-gravity (VHG) worts fermentation were investigated. The results showed that the addition of WGH and their elution fractions in VHG worts significantly enhanced yeast biomass and viability, and further increased the fermentability, ethanol yield and productivity of yeast. Supplementation with 40% ethanol fraction exhibited the highest biomass (6.9 g/L dry cell), cell viability, fermentability (82.05%), ethanol titer (12.19%, v/v) and ethanol productivity during VHG worts fermentation. In addition, 40% ethanol fraction supplementation also caused the most consumption of amino acid and the highest accumulation of intracellular glycerol and trehalose, 15.39% of increase in cell-membrane integrity, 39.61% of enhancement in mitochondrial membrane potential (MMP), and 18.94% of reduction in intracellular reactive oxygen species (ROS) level in yeast under VHG conditions. Therefore, WGH supplementation was an efficient method to improve fermentation performance of brewer's yeast during VHG worts.

Tartary buckwheat protein hydrolysates enhance the salt tolerance of the soy sauce fermentation yeast Zygosaccharomyces rouxii

Supplementation of protein hydrolysate is an important strategy to improve the salt tolerance of soy sauce aroma-producing yeast. In the present study, Tartary buckwheat protein hydrolysates (BPHs) were prepared and separated by ultrafiltration into LM-1 (<1 kDa) and HM-2 (1-300 kDa) fractions. The supplementation of HM-2 fraction could significantly improve cell growth and fermentation of soy sauce aroma-producing yeast Zygosaccharomyces rouxii As2.180 under high salt (12%, w/w) conditions. However, the LM-1 fraction inhibited strain growth and fermentation. The addition of HM-2 promoted yeast cell accumulation of K⁺, removal of cytosolic Na⁺ and accumulation of glycerol. Furthermore, the HM-2 fraction improved the cell membrane integrity and mitochondrial membrane and decreased intracellular ROS accumulation of the strain. The above results indicated that the supplementation of BPHs with a molecular weight of 1-300 kDa is a potentially effective and feasible strategy for improving the salt tolerance of soy sauce aroma-producing yeast Z. rouxii.

Enhanced multi-stress tolerance and glucose utilization of Saccharomyces cerevisiae by overexpression of the SNF1 gene and varied beta isoform of Snf1 dominates in stresses

Background

The Saccharomyces cerevisiae Snf1 complex is a member of the AMP-activated protein kinase family and plays an important role in response to environmental stress. The α catalytic subunit Snf1 regulates the activity of the protein kinase, while the β regulatory subunits Sip1/Sip2/Gal83 specify substrate preferences and stress response capacities of Snf1. In this study, we aim to investigate the effects of SNF1 overexpression on the cell tolerance and glucose consumption of S. cerevisiae in high glucose, ethanol, and heat stresses and to explore the valid Snf1 form in the light of β subunits in these stresses.

Results

The results suggest that overexpression of SNF1 is effective to improve cell resistance and glucose consumption of S. cerevisiae in high glucose, ethanol, and heat stresses, which might be related to the changed accumulation of fatty acids and amino acids and altered expression levels of genes involved in glucose transport and glycolysis. However, different form of β regulatory subunits dominated in stresses with regard to cell tolerance and glucose utilization. The Sip1 isoform was more necessary to the growth and glucose consumption in ethanol stress. The glucose uptake largely depended on the Sip2 isoform in high sugar and ethanol stresses. The Gal83 isoform only contributed inferior effect on the growth in ethanol stress. Therefore, redundancy and synergistic effect of β subunits might occur in high glucose, ethanol, and heat stresses, but each subunit showed specificity under various stresses.

Conclusions

This study enriches the understanding of the function of Snf1 protein kinase and provides an insight to breed multi-stress tolerant yeast strains.

Enhancing Activities of Salt-Tolerant Proteases Secreted by Aspergillus oryzae Using Atmospheric and Room-Temperature Plasma Mutagenesis

Aspergillus oryzae 3.042 was mutagenized using atmospheric and room-temperature plasma (ARTP) technology to enhance its salt-tolerant proteases activity. Compared to the starting strain, mutant H8 subjected to 180 s of ARTP treatment exhibited excellent genetic stability (15 generations), growth rate, and significantly increased activities of neutral proteases, alkaline proteases, and aspartyl aminopeptidase during fermentation. Mutant H8 significantly enhanced the contents of 1-5 kDa peptides, aspartic acid, serine, threonine, and cysteine in soy sauce by 16.61, 7.69, 17.30, 8.61, and 45.00%, respectively, but it had no effects on the contents of the other 14 free amino acids (FAAs) due to its slightly enhanced acidic proteases activity. Analyses of transcriptional expressions of salt-tolerant alkaline protease gene (AP, gi: 217809) and aspartyl aminopeptidase gene (AAP, gi: 6165646) indicated that their expression levels were increased by approximately 30 and 27%, respectively. But no mutation was found in the sequences of AP and AAP expression cassettes, suggesting that the increased activities of proteases in mutant H8 should be partially attributed to the increased expression of proteases. ARTP technology showed great potential in enhancing the activities of salt-tolerant proteases from A. oryzae.

Improvement of Ethanol Tolerance by Inactive Protoplast Fusion in Saccharomyces cerevisiae

Saccharomyces cerevisiae is a typical fermentation yeast in beer production. Improving ethanol tolerance of S. cerevisiae will increase fermentation efficiency, thereby reducing capital costs. Here, we found that S. cerevisiae strain L exhibited a higher ethanol tolerance (14%, v/v) than the fermentative strain Q (10%, v/v). In order to enhance the strain Q ethanol tolerance but preserve its fermentation property, protoplast fusion was performed with haploids from strain Q and L. The fusant Q/L-f2 with 14% ethanol tolerance was obtained. Meanwhile, the fermentation properties (flocculability, SO2 production, α-N assimilation rate, GSH production, etc.) of Q/L-f2 were similar to those of strain Q. Therefore, our works established a series of high ethanol-tolerant strains in beer production. Moreover, this demonstration of inactivated protoplast fusion in industrial S. cerevisiae strain opens many doors for yeast-based biotechnological applications.

Evolution of oxidative and structural characteristics of proteins, especially lipid transfer protein 1 (LTP1) in beer during forced‐ageing

The evolution of oxidative and structural characteristics of proteins, especially lipid transfer protein 1 (LTP1), in beer during forced‐ageing was examined. The oxidative characteristics of beer and proteins were evaluated by DPPH radical scavenging activity and ABTS radical cation scavenging activity. Results showed that the levels of proteins, thiols, LTP1 and antioxidant activity decreased gradually. This was accompanied by the degradation of macromolecular proteins in beer during forced‐ageing. Results from circular dichroism (CD), Fourier‐transform infrared spectroscopy (FTIR), surface hydrophobicity (S0) and ζ‐potential further indicated that the secondary and tertiary structure of LTP1 changed drastically during forced‐ageing, with the reduction of the S0, α‐helix and β‐sheet contents and the increase in negative ζ‐potential and random coil. Thus, the proteins, especially LTP1, might play important roles in maintaining oxidative stability of beer.