The transformation of wine yeasts with tannase and laccase genes to eliminate the instability of wines caused by oxidizable polyphenols


Project Number
IWBT 01-12

Project title
The transformation of wine yeasts with tannase and laccase genes to eliminate the instability of wines caused by oxidizable polyphenols

Project leader
Van Rensburg, P

Institution
University of Stellenbosch, Faculty of AgriSciences, Institute for Wine Biotechnology

Team members
Crous, M J
Howard, R L
Myburgh, J
Abotsi, E K
Kgasi, M N S
Muvhango, N M
Gama, A D

Project description
The development of recombinant wine yeast strains that can produce tannase and laccase. The use of such wine yeast strains could help to produce wines that have better organoleptic madeirisation-resistant features, more stable colour and to eliminate the instability in white wines caused by oxidisable polyphenols.

Phenols in wine predominantly arise from the grapes and are primarily flavonoids. They are crucial components in wines because of their basic importance to the colour and flavour. Flavonoids (catechins and proanthocyanidins) are also responsible for discolouration, turbidity and flavour changes in wines. Chemical and enzymatic oxidation of phenolic compounds leads to the formation of highly reactive intermediates (quinones), which interact with amino acids to reduce sugars and alcohol and give rise to chromophores and volatile substances. This results in a loss of freshness, anywhere from yellowing to browning of the wine and turbidity. The removal of polyphenols is a common practice in winemaking processes to prevent discolouration and/or to obtain stabilisation against oxidation, especially in white and rosé wines. Wine technology tends toward the adoption of systems for extraction of the must which limit the solubilisation of oxidisable substances and toward the use of fining (gelatin and casein), adsorbants (active carbon) and polyvinylpolypyrrolidone (PVPP). The use of these additives in wine production is aimed at reducing the level of polyphenols and compounds that catalyse their transformation, particularly enzymes and heavy metals. In order to limit the amount of treatment with chemical-physical agents, it has been suggested that enzymes active against polyphenols (oxidases, hydrolases and transferases) be used in the prefermentative phase. The aim of this is to set off a process of enzymatic oxidation of polyphenols in the must under controlled conditions, so as to destabilise them and accelerate the process of polymerisation and flocculation. The possibility of eliminating or achieving reactivity loss of the polyphenols responsible for wine instability has been tested on the bench and pilot scale using tannase, phenolase and laccase. The laccase was more effective than the other enzymes tested and produces a wine with a more stable colour. The wines treated with laccase had different and better organoleptic madeirisation-resistant features. Results showed that the enzymatic treatment is highly effective, preferable, or practically identical to traditional processing, especially when using must without the addition of sulphur dioxide.

Article
Van Rensburg, P, Pretorius, I S. 2000. Enzymes in winemaking: Harnessing natural catalysts for efficient biotransformations – a review, South African Journal of Enology and Viticulture, v. 21 Special issue (p. 72-53)

Note
Published article serve as Final Report.

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