Effect of 'loss of function' mutation in SER in wine yeast: fermentation outcomes in co-inoculation with non-Saccharomyces

Published: 20 April 2022In wine fermentation, improved wine complexity and sensorial properties can arise from the use of non-Saccharomyces yeast. Generally less alcohol tolerant, such strains often do not finish fermentation, therefore requiring a second inoculation with the more robust Saccharomyces cerevisiae, usually added on Day 3. This sequential approach affords non-Saccharomyces time to make an impact before being overtaken by S. cerevisiae. However, two inoculations are inconvenient; therefore the identification of a slow growing S. cerevisiae strain that can be used in a single co-inoculation with the non-Saccharomyces yeast is highly attractive. In this study we investigated the use of the naturally occurring ‘loss of function’ SER1 variant, identified in a Sake yeast, for the purposes of carrying out co-inoculated wine fermentations. The SER1-232(G > C; G78R) change was introduced into the commonly used wine strain, EC1118, via CRISPR/Cas9 editing. In a chemically defined grape juice medium, the SER1(G78R) mutant grew and fermented more slowly and increased acetic acid, succinic acid and glycerol concentrations. Simultaneous inoculation with the slower-growing mutant with a Metschnikowia pulcherrima or Lachancea thermotolerans strain in sterile Sauvignon blanc juice resulted in differences in sensorial compounds, most likely derived from the presence of non-Saccharomyces yeasts. The EC1118 SER1 (G78R) mutant completed fermentation with M. pulcherrima, MP2, and in fact improved the viability of MP2 compared to when it was used as a monoculture. The SER1 (G78R) mutant also promoted both the growth of the SO2-sensitive L. thermotolerans strain, Viniflora® Concerto™, in a juice high in SO2 and its subsequent dominance during fermentation. In co-fermentations with wild-type EC1118, the Concerto™ population was substantially reduced with no significant changes in wine properties. This research adds to our understanding of the use of a novel slow-growing S. cerevisiae yeast in wine fermentations co-inoculated with non-Saccharomyces strains.Tom A. Lang, Michelle E. Walker, Paul K. Boss and Vladimir Jirane

Genomic analysis of Kazachstania aerobia and Kazachstania servazzii reveals duplication of genes related to acetate ester production

Kazachstania aerobia and Kazachstania servazzii can affect wine aroma by increasing acetate ester concentrations, most remarkably phenylethyl acetate and isoamyl acetate. The genetic basis of this is unknown, there being little to no sequence data available on the genome architecture. We report for the first time the near-complete genome sequence of the two species using long-read (PacBio) sequencing (K. aerobia 20 contigs, one scaffold; and K. servazzii 22 contigs, one scaffold). The annotated genomes of K. aerobia (12.5Mb) and K. servazzii (12.3Mb) were compared to Saccharomyces cerevisiae genomes (laboratory strain S288C and wine strain EC1118). Whilst a comparison of the two Kazachstania spp. genomes revealed few differences between them, divergence was evident in relation to the genes involved in ester biosynthesis, for which gene duplications or absences were apparent. The annotations of these genomes are valuable resources for future research into the evolutionary biology of Kazachstania and other yeast species (comparative genomics) as well as understanding the metabolic processes associated with alcoholic fermentation and the production of secondary ‘aromatic’ metabolites (transcriptomics, proteomics and metabolomics).Mandy Man-Hsi Lin, Michelle E. Walker, Vladimir Jiranek, and Krista M. Sumb

Localized surface states in HTSC: Alternative mechanism of zero-bias conductance peaks

It is shown that the quasiparticle states localized in the vicinity of surface imperfections of atomic size can be responsible for the zero-bias tunneling conductance peaks in high-Tc superconductors. The contribution from these states can be easily separated from other mechanisms using their qualitatively different response on an external magnetic field.Comment: REVTeX, 4 pages, 2 figs; to be published in PR

Oscillations of a solid sphere falling through a wormlike micellar fluid

We present an experimental study of the motion of a solid sphere falling through a wormlike micellar fluid. While smaller or lighter spheres quickly reach a terminal velocity, larger or heavier spheres are found to oscillate in the direction of their falling motion. The onset of this instability correlates with a critical value of the velocity gradient scale $\Gamma_{c}\sim 1$ s$^{-1}$. We relate this condition to the known complex rheology of wormlike micellar fluids, and suggest that the unsteady motion of the sphere is caused by the formation and breaking of flow-induced structures.Comment: 4 pages, 4 figure

Modern yeast development: finding the balance between tradition and innovation in contemporary winemaking

MinireviewA key driver of quality in wines is the microbial population that undertakes fermentation of grape must. Winemakers can utilise both indigenous and purposefully inoculated yeasts to undertake alcoholic fermentation, imparting wines with aromas, flavours and palate structure and in many cases contributing to complexity and uniqueness. Importantly, having a toolbox of microbes helps winemakers make best use of the grapes they are presented with, and tackle fermentation difficulties with flexibility and efficiency. Each year the number of strains available commercially expands and more recently, includes strains of non-Saccharomyces, strains that have been improved using both classical and modern yeast technology and mixed cultures. Here we review what is available commercially, and what may be in the future, by exploring recent advances in fermentation relevant strain improvement technologies. We also report on the current use of microbes in the Australian wine industry, as reported by winemakers, as well as regulations around, and sentiment about the potential use of genetically modified organisms in the future.Jennifer M. Gardner, Lucien Alperstein, Michelle E.Walker, Jin Zhang, Vladimir Jirane

Surface and capillary transitions in an associating binary mixture model

We investigate the phase diagram of a two-component associating fluid mixture in the presence of selectively adsorbing substrates. The mixture is characterized by a bulk phase diagram which displays peculiar features such as closed loops of immiscibility. The presence of the substrates may interfere the physical mechanism involved in the appearance of these phase diagrams, leading to an enhanced tendency to phase separate below the lower critical solution point. Three different cases are considered: a planar solid surface in contact with a bulk fluid, while the other two represent two models of porous systems, namely a slit and an array on infinitely long parallel cylinders. We confirm that surface transitions, as well as capillary transitions for a large area/volume ratio, are stabilized in the one-phase region. Applicability of our results to experiments reported in the literature is discussed.Comment: 12 two-column pages, 12 figures, accepted for publication in Physical Review E; corrected versio

Microwave Conductivity due to Scattering from Extended Linear Defects in d-Wave Superconductors

Recent microwave conductivity measurements of detwinned, high-purity, slightly overdoped YBa$_{2}$Cu$_{3}$O$_{6.993}$ crystals reveal a linear temperature dependence and a near-Drude lineshape for temperatures between 1 and 20 K and frequencies ranging from 1 to 75 GHz. Prior theoretical work has shown that simple models of scattering by point defects (impurities) in d-wave superconductors are inconsistent with these results. It has therefore been suggested that scattering by extended defects such as twin boundary remnants, left over from the detwinning process, may also be important. We calculate the self-energy and microwave conductivity in the self-consistent Born approximation (including vertex corrections) for a d-wave superconductor in the presence of scattering from extended linear defects. We find that in the experimentally relevant limit ($\Omega, 1/\tau \ll T \ll \Delta_{0}$), the resulting microwave conductivity has a linear temperature dependence and a near-Drude frequency dependence that agrees well with experiment.Comment: 13 pages, 7 figure