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A Technical Guide for Wine Producers
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Fructophilic yeasts being developed by Nietvoorbij microbiologists
Small-scale wine production trials with fructophilic yeasts are currently underway in the Nietvoorbij experimental wine cellar. Preliminary results look promising.
The glucose:fructose sugar ratio in grapes usually equals one, however, during ripening the fructose component can increase faster resulting in grapes with more fructose than glucose. It is uncertain to what extent these imbalances are found in South African grapes as separate glucose and fructose analyses are not routinely done by South African wine cellars. It is also not known what other factors may play a role, for example vintage, vineyard location (terroir), scion-rootstock combinations, canopy densities, irrigation practices and fertilisation.
During fermentation the glucophilic wine yeast, S. cerevisiae, uses both glucose and fructose concurrently, but has a preference for glucose. Higher levels of fructose at the start of fermentation could therefore lead to a larger glucose:fructose imbalance, especially towards the end of fermentation. When the glucose:fructose ratio reaches 0.1 it has been shown that the fermentation slows down and finally stops. This manifests itself as a wine with a residual sugar composed mostly of fructose as has been found throughout the industry during the 2006 harvest.
It is already known that certain non-Saccharomyces yeasts are fructophilic (i.e. have a preference for fructose before glucose). Examples include Candida stellata and Zygosaccharomyces bailii and it has already been shown that inoculation with these yeasts can address the sugar imbalance during a lagging fermentation. Unfortunately, some strains also produce excessive volatile acidity which is not desirable. However, numerous other non-Saccharomyces yeast species are also found in grape must and can be investigated for their ability to utilise fructose.
The microbiology group at ARC Infruitec-Nietvoorbij is currently busy with selection and screening for non-Saccharomyces fructophilic yeasts that do not produce negative metabolites. The use of these selected strains will be used to address the fructose imbalances in grape must. In the proactive approach the selected yeast will be used at the start of fermentation where a glucose:fructose imbalance is suspected from historical data or is known from current analyses. This fructophyllic yeast will be inoculated together with the normal wine yeast to ensure that the imbalance is rectified during the initial stages of the fermentation.
The reactive approach is aimed at a situation where a lagging fermentation has already occurred and needs to be cured. The yeast for this purpose will necessarily need to be ethanol and sulphur tolerant and an adaptation protocol will be needed before it is added to the wine. It is envisioned that large-scale trials will be implemented in 2007.
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