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Talking Biotech
Breakthroughs in grapevine improvement
The Institute for Wine Biotechnology (IWBT) at Stellenbosch University was founded in 1995 through a joint venture between the Wine Industry, US and the Department of Trade and Industry. It was headed by Prof IS Pretorius, who became the IWBT's founding director. In 1997 the IWBT moved into a newly refurbished laboratory in the historic JH Neethling building and consisted of 7 staff members - two full-time academics, two post-doctoral fellows and 22 postgraduate students. Since those early days, the Institute has gone from strength to strength. Today the IWBT
In today's instalment of the Biotech Brief, the spotlight will be placed on three interesting applied yeast projects that hold great promise for the South African wine industry. The author would like to thank Danie Malherbe, Campbell Louw and John Becker for respectively contributing the following sections.
Sober Saccharomyces
The cultivation of wine grape cultivars in the climate of Southern Africa (i.e. relatively warm summers and moderate, wet winters) normally results in grapes with concentrated flavour compounds and high sugar concentrations. Typically these grapes yield high quality full-bodied wines, but with relatively high alcohol levels. Although high alcohol levels is not a serious problem for the traditional wine connoisseur (or the average Stellenbosch student), the demand for quality, low alcohol wines is increasing among modern day wine consumers. Alcohol can be physically reduced in high alcohol wines, but these practises are costly and may result in quality loss. As a possible alternative to post-fermentation alcohol reduction processes, the IWBT undertook a project to create a yeast strain capable of producing reduced (low) alcohol wines. To accomplish this, the glucose oxidase gene (GOX) from another fungus, Aspergillus niger (Saccharomyces is also considered a fungus!), was cloned and integrated into the genome of a laboratory S. cerevisiae strain. GOX converts glucose to gluconic acid, preventing the conversion of glucose to ethanol. Encouraging results from micro-vinification experiments using the transformed yeast were obtained. In fact, wines made using the genetically modified yeast contained on average 1.8 % (vol/vol) less alcohol than the control fermentation. Following the success of the genetically modified laboratory strain, work has already commenced to create a similar wine yeast strain that will allow for the routine production of high quality low alcohol wines.
Polysaccharide reduction during fermentation
To compensate for S. cerevisiae's lacklustre degradation of glucan and xylan (polysaccharide compounds that commonly cause filter blockage during wine filtration), commercial preparations of xylanases and glucanases are sometimes applied during winemaking. This typically improves the clarification and processing of wine, helps to release varietal aromas from precursor compounds, and increases colour intensity. Previous studies showed that heterologous genes coding for these enzymes can be successfully expressed in laboratory strains of S. cerevisiae. In this study, separate combinations of the Trichoderma reesei xylanase gene (xyn2) and the Butyrivibrio fibrisolvens glucanase gene (end1) under control of different promoters were integrated into the genome of S. cerevisiae VIN13.
Wine made with the recombinant strains using Pinot Noir, Ruby Cabernet and Muscat d' Alexandria grapes showed promising improvements in both wine processing and colour extraction. Significant alterations in the higher alcohol, acid and ester profiles of these wines were, however, also obtained.
Resveratrol for all
The beneficial action of moderate wine consumption is a well-debated and equally well researched subject. Among the numerous compounds suspected of being involved, resveratrol has enjoyed considerable attention. Numerous publications point to its efficacy as a cancer chemo-preventative agent. It has the ability to inhibit the growth of cancerous cell lines, or cause apoptosis in these lines. Recently, resveratrol was shown to increase DNA stability and significantly increase yeast lifespan. The compound mimics the effect of calorie restriction, previously shown to extend the lifespan of many organisms, including mammals. Any effort, therefore, with the aim of increasing the levels of this compound in wine could be considered beneficial to human health. Genes necessary for the production of the compound were introduced into laboratory yeast strains. These strains were grown in synthetic media and tested for the production of the polyphenolic compound resveratrol. Mass spectrometric analyses revealed the presence of the b-glucoside of resveratrol, termed piceid. These glucose moieties are typically bound to resveratrol in plants. Enzymes with the capability of liberating these glucose molecules from resveratrol are present in grape must and wine, but could also be added to yeast strains containing proteins for resveratrol production. Based on the proof of concept obtained, this work is currently protected by a provisional patent. Ongoing work is focusing on introducing these genes into industrial wine yeast strains and testing the wines for increased levels of resveratrol.
Albert Joubert IWBT, (021) 808-2188, e-mail albert@sun.ac.za
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