Yeast Stress Response and Fermentation Efficiency: How to Survive the Making of Wine - A Review

Fermentation predictability and wine quality are directly dependent on wine yeast attributes that assist in the rapid establishment of numerical dominance in the early phase of wine fermentation, and that determine the ability to conduct an even and efficient fermentation to obtain a desirable alcohol degree. It is therefore not surprising that the primary selection criteria applied to most wine yeast strain development programmes relate to the overallobjective of achieving an efficient conversion of grape sugar to alcohol and carbon dioxide, at a controlled rate and without the development of off-flavours. Numerous factors influence the fermentation performance of wine yeast.  Following a successful inoculation of grape must with an appropriate starter culture strain, the ability of a wine yeast to adapt to and cope with the hostile environment and stress conditions prevailing in grape juice fermentation are of vital importance to fermentation performance. There is a direct correlation between fermentation efficiency and stress resistance, which refers to the ability of a yeast strain to adapt efficiently to a changing environment and unfavourable growth conditions. Successful yeast cellular adaptation to changes in extracellular parameters during wine fermentation requires the timely perception (sensing) of chemical or physical environmental parameters, followed by accurate transmission of the information to the relevant compartments of the cell.  Chemical parameters perceived during wine fermentation include the availability/concentration of certain nutrients (e.g., fermentable sugars, assimilable nitrogen, oxygen, vitamins, minerals, ergosterol and unsaturated fatty acids) and the presence of inhibitory substances (e.g., ethanol, acetic acid, fatty acids, sulfite, phenolic phytoalexins, mycotoxins, bacterial toxins and agrochemical residues). Signals of a physical nature include temperature, pH, agitation and osmotic pressure. The sensing of these environmental signals is carried out by specific receptor proteins, most of them situated on the cellular surface. Once perceived, the information is transmitted by a network of dedicated, interconnected signal transduction pathways to the relevant cellular compartments which implement theadaptive response, a process referred to as "stress response". Intensive research has focused on elucidating the molecular mechanisms involved in stress responses, which are evolutionarily well conserved. Besides furthering our understanding of the fundamental strategies for adaptation to hostile, industrial environments, and the biological resilience of Saccharomyces cerevisiae, the data are of key importance to the future improvement of wine yeast strains. This review describes the different types of stress experienced by wine yeast cells during their life cycles, summarises our current knowledge of some of the most important molecular processes required for the survival of the yeast cell, and highlights the potential benefits for future yeast strain development which can be derived from this research

Microbial Spoilage and Preservation of Wine: Using Weapons from Nature's Own Arsenal -A Review

The winemaking process includes multiple stages at which microbial spoilage can occur, altering the quality and hygienic status of the wine and rendering it unacceptable. The major spoilage organisms include species and strains of the yeast genera Brettanomyces, Candida, Hanseniaspora, Pichia, Zygosaccharomyces etc., the lactic acid bacterial genera Lactobacillus, Leuconostoc, Pediococcus, etc. and the acetic acid bacterial genera Acetobacter and Gluconobacter. The faults caused include bitterness and off.flavours (mousiness, ester taint, phenolic, vinegary, buttery, geranium tone), and cosmetic problems such as turbidity, viscosity, sediment and film formation. These spoilage organisms can also affect the wholesomeness of wine by producing biogenic amines and precursors of ethyl carbamate. The judicious use of chemical preservatives such as sulphur dioxide (S02) during the winemaking process decreases the risk of microbial spoilage, but strains vary considerably in their S02 sensitivity. There is,moreover, mounting consumer bias against chemical preservatives, and this review focuses on the possible use of biopreservatives in complying with the consumers' demand for "clean and green" products

The Occurrence of Non-Saccharomyces cerevisiae Yeast Species Over Three Vintages in Four Vineyards and Grape Musts From Four Production Regions of the Western Cape, South Africa

The role of non-Saccharomyces yeasts in wine production has been extensively debated and there is growing evidence that non-Saccharomyces yeasts play an important role in wine quality. It has been suggested that metabolites formed by some non-Saccharomyces species may contribute to wine quality. Recently a comprehensive, longterm research programme was launched by role players in the South African wine industry, whose aims include the isolation, characterisation and preservation of the natural yeast biodiversity of the Western Cape. As part of the programme, this paper investigates the presence of non-Saccharomyces yeast species over three vintages in four vineyards and musts in four distinct areas of the Western Cape. Samples were taken and the non-Saccharomyces yeast isolates were characterised by biochemical profiling and pulse field gel electrophoresis. In total 720 yeasts representing 24 species were isolated. Predominant species found in the must samples, i.e. Candida stellata, Kloeckera apiculata, Candida pulcherrima and Candida colliculosa, should have the most impact on subsequent fermentation

The Effect of Non-Saccharomyces Yeasts on Fermentation and Wine Quality

Research has shown that non-Saccharomyces yeast strains can be detected throughout wine fermentation.  Non-Saccharomyces yeasts can therefore influence the course of fermentation and also the character of the resultant wine. Previously it was shown that four non-Saccharomyces species, i.e. Kloeckera apiculata, Candida stellata, Candida pulcherrima and Candida colliculosa, predominated in grape must at the start of fermentation. In this study these four yeasts were used singularly and in combination with an industrial wine yeast (Saccharomyces cerevisiae strain VIN 13) to ferment must on a laboratory scale. The resultant wine was analysed for ethanol, volatile acidity, total S02 and glycerol. Results show that, in comparison with the industrial wine yeast, the non-Saccharomyces yeast strains could not ferment all the sugar. Furthermore, while the individualnon-Saccharomyces-fermented wines had different chemical analyses, the wines fermented by the combinations were similar to the wine produced by the industrial yeast only. In subsequent, small-scale winemaking trials some of the wines produced by combined fermentations were judged to be of better quality than those produced by the S. cerevisiae only. However, this quality increase could not be linked to increased ester levels

Oxygen in Must and Wine: A review

Oxygen can play an important role during the winemaking process. It can influence the composition and quality of the must and wine. Phenolic compounds are the main substrates for oxidation in must and wine. Oxygen addition leads to colour changes and the polymerisation of phenolic molecules in wine. Oxygen can, however, also influence the flavour and microbial composition of wine drastically, with certain off-flavours being formed and spoilage micro-organisms able to grow at too high oxygen additions to wine. A state-of-the-art, up-to-date review on the effects of oxygen in must and wine has, however, not been published recently. This review focuses on the effects of oxygen in must, during alcoholic fermentation, extended lees contact and during ageing of white and red wines. The effects it has on acetic acid bacteria and Brettanomyces are also discussed, as well as micro-oxygenation, a relative new technique used in wine production

Localization of yeast glucoamylase genes by PFGE and OFAGE

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