Using yeast to make crystal clear wine

Ms Thulile Ndlovu: Faculty of AgriSciences, Stellenbosch University

Why bother about wine clarity?

“White wine clarity is crucial to the winemaker, because a bottle showing haziness is likely to be rejected by the consumer, even though this unattractive haze does not affect the taste and other wine quality characteristics,” Ms Ndlovu says.

“People will in all likelihood not drink murky-looking wine, even if you tell them that it is one of the finest wines ever produced."

An easy definition of wine clarity?

  •  Wine clarity refers to the amount of suspended particles in a wine and is used as a measure of quality.
  • Wine haze or cloudiness results in great loss for the wine industry.
  • It is caused when the grape proteins in wine interact with other wine components to form unstable structures. These clump together in such large molecules that they are visible as wine haze.

Wine haze in the industry

This student in the Faculty of AgriSciences says it is standard practice in the wine industry to address wine haze by using fining agents. These are added before bottling to adsorb insoluble particles such as proteins and to stabilise compounds that may form wine haze. Among the fining agents used are bentonite, egg whites, casein derived from milk, gelatine and isinglass obtained from fish bladders. “Fining agents have their disadvantages, though, because their use is labour intensive and costly,” says Ms Ndlovu. Bentonite, a type of clay, is by far the most commonly used agent, but results in loss of wine volume and may cause a loss of flavour and aroma compounds. It is also non-recyclable and cannot be reused.

“Other products are animal-derived, and have raised health concerns since they may cause allergies,” she says. “Developing a viable alternative treatment to remove the grape proteins from wine prior to bottling would therefore be highly beneficial to the wine industry.”

Her research

According to Ms Ndlovu, there are naturally occurring components in wine that reduce haziness. These protein-sugar compounds, called mannoproteins, are found in the cell walls of yeast. They are released during yeast growth, for example during the fermentation of grapes and also on wines aged on lees. However, commercially available yeast strains tend to deliver too low levels of mannoproteins to effectively prevent haze formation.

In her research, Ms Ndlovu assessed various yeast strains, including some novel strains, for their potential to protect against haze. The research team’s work on the use of a novel yeast strain, FB20, as starter cultures for fermentation or in yeast breeding processes, is currently being patented. Meanwhile, more studies will be needed to determine the impact of this strain in large scale fermentations simulating wine industry conditions.

“We now know that some new yeast strains are much better at protecting against haze than those currently used in the industry,” she summarises her findings. “Our results show that wine haze formation not only depends on wine grape proteins, but varies according to the yeast strain used for fermenting the wines.”

A patent is already pending to protect the intellectual property generated by Ms Ndlovu's doctoral degree research on wine haziness, which she is doing under supervision of Dr Benoit Divol and Prof Florian Bauer of the Institute for Wine Biotechnology at Stellenbosch University.