Let the yeast do the work for you: Less alcohol per bottle

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Let the yeast do the work for you: Less alcohol per bottle

Danie Malherbe

Danie Malherbe and Pierre van Rensburg
Institute for Wine Biotechnology, Stellenbosch University
danie@sun.ac.za
Key words: Yeast, alcohol, glucose oxidase, genetically modified organism (GMO)

"Bottling sunshine"
The winemaking process constitutes a unique ecological niche that involves the interaction of yeasts, lactic acid bacteria (LAB) and acetic acid bacteria (AAB). Saccharomyces cerevisiae has established its importance as a wine yeast and also proven itself as a reliable starter culture organism. Its primary role is to convert the grape sugar into alcohol and, secondly, its metabolic activities result in the production of higher alcohols, fatty acids and esters - important flavour and aroma compounds that are essential for consistent and predictable wine quality.
In an effort to "bottle sunshine", grape must is typically prepared from fully matured grapes. This approach not only gives the high flavour intensity required but also occasionally results in a more than adequate concentration of sugar. This high concentration of sugar, however, invariably leads to the production of wines that contain high levels of alcohol.

This has several implications: first, a high ethanol concentration can affect the sensory properties of the wine. Furthermore, the high alcohol content can mask the overall aroma and flavour of the wine. Second, health consciousness and increasingly strict road traffic laws pertaining to "drinking and driving" seem to be the main reasons for a worldwide decline in the consumption of alcohol. This has increased the demand for wines containing less alcohol, putting a great deal of pressure on wine producers, particularly those in warm climate wine-producing regions, as well as countries that export wine to Europe such as South Africa and Australia, where grape sugar levels can become high.
A demand for wines containing less alcohol
As there is a growing consumer demand worldwide for wine containing lower levels of alcohol and chemical preservatives, it is important to create a fast, reliable and inexpensive method to reduce the alcohol in wine.
Several physical processes are used for the removal or reduction of alcohol in wine and some of them are sometimes used in combination. These processes tend to involve expensive equipment and can be intensive from a processing point of view.

This is cutting edge and applaudable research but note should be taken that the commercial use of GMO products are not allowed in the South African Wine Industry.
Technical Consultant: Wynboer/Tegnies

The enzyme Glucose oxidase (GOX) has received considerable research interest regarding its potential application in the wine industry to reduce alcohol levels and as a bio-control agent. An alternative approach was introduced with the concept of treating grape must with GOX to reduce the glucose content of the must (the enzyme converts glucose to gluconic acid before the yeast cells are able to metabolise the glucose to ethanol), and therefore produce a wine with a reduced alcohol content after fermentation. This method was met with success, but is still labour intensive, as the winemaker has to add the enzyme to the must at a specific time.
Due to the demanding nature of modern winemaking practices and sophisticated wine markets, there is an ever-growing quest for specialised wine yeast strains possessing a wide range of optimised, improved or novel oenological properties. Thus the idea was born to develop a wine yeast that is able to reduce the amount of alcohol during fermentation.
Developing a reduced-alcohol wine yeast
This project forms part of important research on reduced alcohol wines within the Institute for Wine Biotechnology and is also funded by Winetech (Wine Industry Network of Expertise and Technology). During this ongoing study, we have attempted to genetically enhance a yeast strain from Saccharomyces cerevisiae to reach this goal. The possibility of controlling wine spoilage bacteria, such as lactic acid bacteria and acetic acid bacteria was also investigated.
Using molecular biology techniques, the Aspergillus niger glucose oxidase gene was successfully integrated into a Saccharomyces cerevisiae (laboratory strain) genome. Results obtained indicated the production of biologically active glucose oxidase and showed that it was secreted into the medium.
Microvinifications were performed with Chardonnay grapes inoculated with a S. cerevisiae strain (control), as well as a genetically enhanced S. cerevisiae strain. After 14 days the sugar concentrations were determined. The sugar concentration for both microvinifications was reduced from 230 g L-1 to 64 g L-1. The alcohol concentration of the genetically enhanced yeast was 10.1% (vol/vol), while the control yeast produced wine with an alcohol concentration of 11.9% (vol/vol). Thus the genetically enhanced yeast producing the glucose oxidase enzyme produced ñ1.8 % vol/vol less ethanol than the control yeast.
Stopping wine spoilage
Using plate assays, it was also noted that the genetically enhanced yeasts producing glucose oxidase seem to inhibit the growth of acetic acid bacteria (AAB) and lactic acid bacteria (LAB), which are the major bacterial spoilage organisms in wine.
Inhibition is linked to the presence of hydrogen peroxide (H2O2) in the medium; it is the final product of the glucose oxidase enzymatic reaction and is a known antimicrobial agent. The hydrogen peroxide that is produced leads to hyperbaric oxygen toxicity - a result of the peroxidation of the membrane lipid - and a strong oxidising effect on the bacterial cell, which is the cause of the destruction of basic molecular structures, such as nucleic acids and cell proteins. Varying degrees of inhibition for lactic acid bacteria (LAB) was observed. The inhibitory activity is dependent on the concentrations of the enzyme and the glucose in the medium.
Fermenting the New Millennium
In this exciting age of molecular yeast genetics and modern biotechnology, this study could pave the way for the development of wine yeast starter culture strains for the production of wine with a lower alcohol content and reduced levels of chemical preservatives, such as sulphur dioxide. The use of genetically modified organisms (GMOs) within the wine industry is a limiting factor at present and credible means must be found to effectively address the concerns of traditionalists within the wine industry and the negative overreaction by some consumer groups. There is a vast potential benefit to the wine consumer and industry alike. The first recombinant wine products therefore should unmistakably demonstrate safe products free of potentially harmful compounds, and have organoleptic, hygienic and economic advantages for both the wine producer and consumer.
Furthermore, it is very important to realise that it is very dangerous to have unrealistic expectations about rapid commercialisation and short-term benefits for recombinant DNA technology in the wine industry. The successful application, as well as the commercialisation of transgenic wine yeasts, should not affect the wine's most enchanting and fascinating aspects, namely its diversity of style, flavour and aroma.
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