Showing papers by "Isak S. Pretorius published in 2014"

Not your ordinary yeast: non-Saccharomyces yeasts in wine production uncovered

Abstract: Saccharomyces cerevisiae and grape juice are ‘natural companions’ and make a happy wine marriage. However, this relationship can be enriched by allowing ‘wild’ non- Saccharomyces yeast to participate in a sequential manner in the early phases of grape must fermentation. However, such a triangular relationship is complex and can only be taken to ‘the next level’ if there are no spoilage yeast present and if the ‘wine yeast’ – S. cerevisiae – is able to exert its dominance in time to successfully complete the alcoholic fermentation. Winemakers apply various ‘matchmaking’ strategies (e.g. cellar hygiene, pH, SO2, temperature and nutrient management) to keep ‘spoilers’ (e.g. Dekkera bruxellensis ) at bay, and allow ‘compatible’ wild yeast (e.g. Torulaspora delbrueckii, Pichia kluyveri, Lachancea thermotolerans and Candida/Metschnikowia pulcherrima ) to harmonize with potent S. cerevisiae wine yeast and bring the best out in wine. Mismatching can lead to a ‘two is company, three is a crowd’ scenario. More than 40 of the 1500 known yeast species have been isolated from grape must. In this article, we review the specific flavour-active characteristics of those non- Saccharomyces species that might play a positive role in both spontaneous and inoculated wine ferments. We seek to present ‘single-species’ and ‘multi-species’ ferments in a new light and a new context, and we raise important questions about the direction of mixed-fermentation research to address market trends regarding so-called ‘natural’ wines. This review also highlights that, despite the fact that most frontier research and technological developments are often focussed primarily on S. cerevisiae , non- Saccharomyces research can benefit from the techniques and knowledge developed by research on the former.

Genomic insights into the evolution of industrial yeast species Brettanomyces bruxellensis

Abstract: Brettanomyces bruxellensis, like its wine yeast counterpart Saccharomyces cerevisiae, is intrinsically linked with industrial fermentations. In wine, B. bruxellensis is generally considered to contribute negative influences on wine quality, whereas for some styles of beer, it is an essential contributor. More recently, it has shown some potential for bioethanol production. Our relatively poor understanding of B. bruxellensis biology, at least when compared with S. cerevisiae, is partly due to a lack of laboratory tools. As it is a nonmodel organism, efforts to develop methods for sporulation and transformation have been sporadic and largely unsuccessful. Recent genome sequencing efforts are now providing B. bruxellensis researchers unprecedented access to gene catalogues, the possibility of performing transcriptomic studies and new insights into evolutionary drivers. This review summarises these findings, emphasises the rich data sets already available yet largely unexplored and looks over the horizon at what might be learnt soon through comprehensive population genomics of B. bruxellensis and related species.

Wine, Beer and Cider: Unravelling the Aroma Profile

Abstract: The aroma profile of alcoholic beverages is a major factor that distinguishes one product from another, and it is a key attribute that drives consumer preference at points of sale. A longstanding objective has, therefore, been to identify those aromatic compounds that are important to particular olfactory attributes of different styles of wine, beer and cider—whether perceived ortho- or retro-nasally—and to modulate them according to consumer preferences. That this has been achieved only to a relatively small extent to date is partly a reflection on the complexity of the perception of aroma mixtures and also the presence of very low concentrations of potent aroma compounds in these products. It is known, although perhaps not appreciated as widely as it should be, that aroma compounds will interact with each other, with masking or suppressing effects being probably universal for compounds at supra-threshold concentrations, together with additive interactions for compounds at sub-threshold concentrations. Thus it is likely that volatile compounds with marginal aroma impact when isolated, can together provide an influence on aroma. Some of these aroma-active compounds are produced during fermentation. Different yeasts produce differing ranges of aroma-active substances, which may greatly affect the complex flavour of a fermented product such as wine, beer and cider. While these secondary metabolites are often formed only in trace amounts, their concentrations may well determine the distinct aroma of these beverages. This chapter reviews the production of the most important aroma-active compounds produced by yeast at molecular level and seeks to understand how they might be perceived by consumers.