Enhancing freeze-thaw tolerance in baker's yeast: strategies and perspectives

Frozen dough technology is important in modern bakery operations, facilitating the transportation of dough at low temperatures to downstream sales points. However, the freeze-thaw process imposes significant stress on baker's yeast, resulting in diminished viability and fermentation capacity. Understanding the mechanisms underlying freeze-thaw stress is essential for mitigating its adverse effects on yeast performance. This review delves into the intricate mechanisms underlying freeze-thaw stress, focusing specifically on Saccharomyces cerevisiae, the primary yeast used in baking, and presents a wide range of biotechnological approaches to enhance freeze-thaw resistance in S. cerevisiae. Strategies include manipulating intracellular metabolites, altering membrane composition, managing antioxidant defenses, mediating aquaporin expression, and employing adaptive evolutionary and breeding techniques. Addressing challenges and strategies associated with freeze-thaw stress, this review provides valuable insights for future research endeavors, aiming to enhance the freeze-thaw tolerance of baker's yeast and contribute to the advancement of bakery science.

Enhancing cell cryopreservation with acidic polyamino acids integrated liquid marbles

Cryopreservation is highly desired for long-term maintenance of the viability of living biosamples, while effective cell cryopreservation still relies heavily on the addition of dimethyl sulfoxide (DMSO) and fetal bovine serum (FBS). However, the intrinsic toxicity of DMSO is still a bottleneck, which could not only cause the clinical side effect but also induce cell genetic variants. In the meantime, the addition of FBS may bring potentially the risk of pathogenic microorganism contamination. The liquid marbles (LMs), a novel biotechnology tool for cell cryopreservation, which not only have a small volume system that facilitated recovery, but the hydrophobic shell also resisted the harm to cells caused by adverse environments. Previous LM-based cell cryopreservation relied heavily on the addition of FBS. In this work, we introduced acidic polyaspartic acid and polyglutamic acid as cryoprotectants to construct LM systems. LMs could burst in an instant to facilitate and achieve ultrarapid recovery process, and the hydrophilic carboxyl groups of the cryoprotectants could form hydrogen bonds with water molecules and further inhibit ice growth/formation to protect cells from cryoinjuries. The L929 cells could be well cryopreserved by acidic polyamino acid-based LMs. This new biotechnology platform is expected to be widely used for cell cryopreservation, which has the potential to propel LMs for the preservation of various functional cells in the future.

Rheology, physicochemical properties, and microstructure of fish gelatin emulsion gel modified by γ-polyglutamic acid

In this work, the effect of the addition of γ-polyglutamic acid (γ-PGA) on the rheology, physicochemical properties, and microstructure of fish gelatin (FG) emulsion gel was investigated. Samples of the emulsion gel were evaluated for rheological behavior and stability prior to gelation. The mechanical properties and water-holding capacity (WHC) of the emulsion were determined after gelation. The microstructure of the emulsion gel was further examined using confocal laser scanning microscopy (CLSM). The results indicated a gradual increase in the apparent viscosity and gelation temperature of the emulsion at a higher concentration of γ-PGA. Additionally, frequency scan results revealed that on the addition of γ-PGA, FG emulsion exhibited a stronger structure. The emulsion containing 0.1% γ-PGA exhibited higher stability than that of the control samples. The WHC and gel strength of the emulsion gel increased on increasing the γ-PGA concentration. CLSM images showed that the addition of γ-PGA modified the structure of the emulsion gel, and the droplets containing 0.1% γ-PGA were evenly distributed. Moreover, γ-PGA could regulate the droplet size of the FG emulsion and its size distribution. These findings suggest that the viscoelasticity and structure of FG emulsion gels could be regulated by adjusting the γ-PGA concentration. The γ-PGA-modified FG emulsion gel also exhibited improved rheology and physicochemical properties. The results showed that γ-PGA-modified FG emulsion gel may find potential applications in food, medicine, cosmetics, and other industries.

Recent development in evaluation methods, influencing factors and control measures for freeze denaturation of food protein

Frozen storage is most widely adopted preservation method to maintain food freshness and nutritional attributes. However, at low temperature, food is prone to chemical changes such as protein denaturation and lipid oxidation. In this review, we discussed the reasons and influencing factors that cause protein denaturation during freezing, such as freezing rate, freezing temperature, freezing method, etc. From the previous literatures, it was found that frozen storage is commonly used to prevent freeze induced protein denaturation by adding cryoprotectants to food. Some widely used cryoprotectants (for example, sucrose and sorbitol) have been reported with higher sweetness and weaker cryoprotective abilities. Therefore, this article comprehensively discusses the new cryopreservation methods and providing comparative study to the conventional frozen storage. Meanwhile, this article sheds light on the freeze induced alterations, such as change in functional and gelling properties. In addition, this article could be helpful for the prolonged frozen storage of food with minimum quality related changes. Meanwhile, it could also improve the commercial values and consumer satisfaction of frozen food as well.

Recognition and selective extraction of poly-γ-glutamic acid based on molecular imprinting technology

Poly-γ-glutamic acid (γ-PGA) is one of the few bacterial polymers in nature with high added value of biodegradability. Especially, the traditional method of extracting γ-PGA is organic solvent extraction, etc., which has the disadvantages of low extraction rate and serious environmental pollution. With the expansion of γ-PGA industrial fermentation, an efficient and environmentally friendly method is required to be adopted. In this contribution, we report a novel method of separation of γ-PGA from fermentation broth based on molecular imprinting technology. The molecular imprinted polymer (MIP) was synthesized from chitosan (CS) and glutaraldehyde in the presence of γ-PGA. A nonimprinted polymer (NIP) was also synthesized by the same procedure in the absence of γ-PGA. The chemical structures and morphological structures of both MIP and NIP were examined by FTIR spectroscopy and scanning electron microscopy. The adsorption isotherms showed that the maximum adsorption capacity of MIP was 137.85 mg/g. The maximum adsorption capacity in the adsorption of NIP was 68.92 mg/g, which indicates that MIP shows specific selectivity for γ-PGA. A high saturated absorption capacity (Qmax=140.90 mg/g) was calculated from Freundlich isotherm equation. The imprinting factor of MIP was 4.76, indicating that MIP possess good recognition ability and selectivity for γ-PGA. The adsorption capacity decreased slightly (17.0%), which suggests the satisfactory reusability of γ-PGA after 5 cycles of reuse. Our study indicates that molecularly imprinted polymers present development prospects in the effective and selective separation of γ-PGA from fermentation broth compared with organic solvent precipitation.

MAL62 overexpression enhances uridine diphosphoglucose-dependent trehalose synthesis and glycerol metabolism for cryoprotection of baker's yeast in lean dough

BACKGROUND

In Saccharomyces cerevisiae, alpha-glucosidase (maltase) is a key enzyme in maltose metabolism. In addition, the overexpression of the alpha-glucosidase-encoding gene MAL62 has been shown to increase the freezing tolerance of yeast in lean dough. However, its cryoprotection mechanism is still not clear.

RESULTS

RNA sequencing (RNA-seq) revealed that MAL62 overexpression increased uridine diphosphoglucose (UDPG)-dependent trehalose synthesis. The changes in transcript abundance were confirmed by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and enzyme activity assays. When the UDPG-dependent trehalose synthase activity was abolished, MAL62 overexpression failed to promote the synthesis of intracellular trehalose. Moreover, in strains lacking trehalose synthesis, the cell viability in the late phase of prefermentation freezing coupled with MAL62 overexpression was slightly reduced, which can be explained by the increase in the intracellular glycerol concentration. This result was consistent with the elevated transcription of glycerol synthesis pathway members.

CONCLUSIONS

The increased freezing tolerance by MAL62 overexpression is mainly achieved by the increased trehalose content via the UDPG-dependent pathway, and glycerol also plays an important role. These findings shed new light on the mechanism of yeast response to freezing in lean bread dough and can help to improve industrial yeast strains.

The effect of γ-[Glu](1≤n≤5)-Gln on the physicochemical characteristics of frozen dough and the quality of baked bread

This study was the first to examine the effects of γ-[Glu]_(1≤n≤5)-Gln (GGP, a taste enhancer; added at 0.5% or 5.0%) on the breadmaking using frozen dough. γ-[Glu]_(1≤n≤5)-Gln was produced using the method established in our research center. The addition of GGP at either level increased yeast viability, freezable water content and storage and loss moduli, decreased the free sulfhydryl content of dough during the frozen storage and freeze-thaw cycles, and improved the microstructure of frozen dough and texture of the baked bread. The addition of GGP at 0.5% led to a dough having the highest extensibility, and most complete and uniform starch-gluten network, and a baked bread crumb with the lowest hardness, best texture, and most uniform organization. These results indicated that GGP has great potential as a food-derived cryoprotectant/antifreeze agent for the baking industry.

MAL62 Overexpression Enhances Freezing Tolerance of Baker's Yeast in Lean Dough by Enhancing Tps1 Activity and Maltose Metabolism

Trehalose plays a crucial role in response to freezing stress in baker's yeast. MAL62, a gene involved in the adenosine diphosphoglucose-dependent trehalose synthesis pathway, can increase trehalose content. However, the difference between MAL62-related trehalose synthesis and traditional uridine diphosphoglucose-dependent trehalose synthesis is not well-understood. MAL62 overexpression showed less effect in enhancing intracellular trehalose compared to TPS1 overexpression. However, MAL62 overexpression elicited trehalose synthesis before fermentation with enhanced maltose metabolism and had a similar effect on cell viability after freezing. Furthermore, MAL62 and TPS1 overexpression in the NTH1 deletion background further strengthened freezing tolerance and improved leavening ability. Our results suggest that the enhancement in freezing tolerance by MAL62 overexpression may involve multiple pathways rather than simply enhancing trehalose synthesis. The results reveal valuable insights into the relationship between maltose metabolism and freezing tolerance and may help to develop better yeast strains for enhancing fermentation characteristics of frozen dough.

Characterization of a Regulator pgsR on Endogenous Plasmid p2Sip and Its Complementation for Poly(γ-glutamic acid) Accumulation in Bacillus amyloliquefaciens

Bacillus amyloliquefaciens NX-2S154 is a promising poly(γ-glutamic acid) (γ-PGA) producing strain discovered in previous studies. However, the wild-type strain contains an unknown endogenous plasmid, p2Sip, which causes low transformation efficiency and instability of exogenous plasmids. In our study, p2Sip is 5622 bp with 41% G+C content and contains four putative open reading frames (ORFs), including genes repB, hsp, and mobB and γ-PGA-synthesis regulator, pgsR. Elimination of p2Sip from strain NX-2S154 delayed γ-PGA secretion and decreased production of γ-PGA by 18.1%. Integration of a pgsR expression element into the genomic BamHI locus using marker-free manipulation based on pheS* increased the γ-PGA titer by 8%. pgsR overexpression upregulated the expression of γ-PGA synthase pgsB, regulator degQ, and glutamic acid synthase gltA, thus increasing the γ-PGA production in B. amyloliquefaciens NB. Our results indicated that pgsR from p2Sip plays an important regulatory role in γ-PGA synthesis in B. amyloliquefaciens.