Trends in Biotechnology
ReviewMeeting the consumer challenge through genetically customized wine-yeast strains
Section snippets
The genetic improvement of wine-yeast strains
Yeasts are predominant during wine fermentation. In spontaneous fermentations, there is a progressive growth pattern of indigenous yeasts originating from the surfaces of grape berries and the winery equipment 2., 3.. Yeasts of the genera Kloeckera, Hanseniaspora and Candida predominate in the early stages, followed by several species of Metschnikowia and Pichia in the middle stages, when the ethanol rises to 3–4% 2., 3., 4.. The latter stages of spontaneous wine fermentations are invariably
The genetic features and techniques for the analysis and development of starter yeast strains
The majority of laboratory-bred strains of S. cerevisiae are either haploid or diploid, whereas industrial wine-yeast strains are predominantly diploid or aneuploid, and occasionally polyploid 11., 12.. The nucleotide sequence of the entire genome of S. cerevisiae is known. It has a relatively small, compact genome (∼13 000 kb), a large number of chromosomes (16 linear chromosomes, varying in length from 200 to 2200 kb), a small number of genes (∼6000 protein-encoding genes), little repetitive
Targets for the genetic improvement of wine yeasts
Generally, the targets of strain development all relate to the improved economics of production and wine quality. Table 2 highlights some of the improvements that can be achieved using genetically engineered wine yeasts. These targets include increasing the efficiency of the fermentation process, the processing of wine and control of microbial spoilage, in addition to enhancement of wholesomeness and sensory quality 2., 6., 7., 8., 9., 10., 11., 12., 14., 15..
Improvement of fermentation performance
Wine fermentations generally proceed at a rate greater than desired and are usually controlled by lowering the fermentation temperature. ‘Runaway’ fermentations have a commercial implication because fermentor space is reduced owing to foaming and volatile aroma compounds being lost by entrainment with the evolving carbon dioxide [2]. However, wine fermentation sometimes ceases prematurely or proceeds too slowly. The financial losses caused by ‘stuck’, ‘sluggish’ or incomplete wine fermentations
Improvement of processing efficiency
The main objectives of fining (addition of adsorptive compounds, followed by settling or precipitation) and clarification (sedimentation, racking, centrifugation and filtration) during wine processing include the removal of excess amounts of certain components and microbial cells to achieve clarity and to ensure the physicochemical stability of the end product. The fining and clarification of wine often include expensive and laborious practices that generate large volumes of lees for disposal,
Improving biological control of wine-spoilage microorganisms
Uncontrolled microbial growth before, during or after wine fermentation can alter the chemical composition of the end product, detracting from its sensory properties of appearance, aroma and flavour. Healthy grapes, cellar hygiene and sound oenological practices are the cornerstones of the winemaker's strategy against the uncontrolled proliferation of spoilage microbes. Added safety is provided by the addition of chemical preservatives, such as sulfur dioxide, dimethyl dicarbonate, benzoic
Improvement of wholesomeness
It is generally accepted that moderate wine drinking is socially beneficial and can be effective in the management of stress and the reduction of coronary heart disease. The principal protective compounds found in wine include the phenolic compounds, resveratrol, salicylic acid and alcohol. However, some unwanted compounds, including suspected carcinogens (e.g. ethyl carbamate), neurotoxins (e.g. biogenic amines) and asthmatic chemical preservatives (e.g. sulfites) can sometimes also be found
Improvement of sensory quality
The single most important factor in winemaking is the organoleptic quality (appearance, aroma and flavour) of the final product. The endless variety of flavours stems from a complex, completely non-linear system of interactions among many hundreds of compounds. The bouquet of a wine is determined by the presence of a well-balanced ratio of flavour compounds and metabolites [30]. With the exception of terpenes in the aromatic grape varieties and alkoxypyrazines in the herbaceous cultivars,
Conclusion
The tailoring of wine yeast strains will undoubtedly help the wine industry meet the technical and consumer challenges of the 21st century. Over the past few years, considerable progress has been made in improving wine yeasts. However, owing to a multitude of complex scientific, technical, economic, marketing, safety, legal and ethical issues, no recombinant wine yeast has been used on a commercial scale to date. Given the current deeply rooted concerns of consumers and traditionalists, the
Acknowledgements
Marisa Honey for critical reading and editing of this article. The research conducted in the Institute for Wine Biotechnology is supported by grants from the National Research Foundation (NRF) and the South African Wine Industry (Winetech).
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