The aim of this study was to investigate the possibility that chitin contained in wine yeast cell walls can reduce protein haze formation by binding to the main cause of protein instability, i.e. chitinases in grape juice. Chitinases, in addition to thaumatin-like proteins and beta-glucanases that originate from grape juice, have been documented to be the main culprits in wine haze formation during prolonged storage, especially at elevated temperatures. It was also previously demonstrated that the addition of 1 g/l chitin can reduce wine haze with almost 50%. Chitin addition to grape juice or wine is however not permitted. Chitosan, a derivative of chitin, is permitted for use in winemaking, but only if it is of fungal origin. It has been demonstrated recently that 1g/l chitosan can render a white wine protein stable. Compared to bentonite addition of the same dosage, chitosan addition is not economically viable.
It has been observed that wine aged on yeast lees is more protein stable than the same wine not kept on the lees. It has been the belief that this is the result of intracellular yeast proteases released during yeast autolysis. The current study demonstrated the effect of yeast cell wall chitin in the observed increased stability.
- Chardonnay juice was fermented to dryness with various yeast strains comprising commercial Saccharomyces cerevisiae strains, Saccharomyces paradoxus strains, as well as a cerevisiae / paradoxus interspecies hybrid. Significant differences were observed in protein haze formed in the different wines.
- Similar protein haze formation was observed when the experiment was repeated in Sauvignon blanc confirming the strain effect.
- In general the paradoxus strains and hybrid produced more stable wines than the cerevisiae strains.
- The higher the chitin content in the yeast strain cell walls, the more stable the wines after heat treatment.
- The same yeast strains also removed commercial chitinase added to a model wine solution in a similar fashion with the paradoxus strains being the most effective.
- In order to confirm that it is indeed the cell wall chitin removing the chitinases and not an enzyme inhibitor secreted by the yeast cells, chitinases were added after removal of yeast cells. No significant differences could be observed in chitinase levels for the different strains, indicating that it is not an enzyme inhibitor secreted into the wine causing the chitinase removal.
- It was found that inactivated versions of the live strains also removed chitinases, but to a lesser extent than the live cells.
SIGNIFICANCE OF THE STUDY:
This study has shown that the use of wine yeast strains with high chitin content in their cell walls can reduce white wine protein haze potential and thus bentonite usage. The possibility also exists to produce yeast hulls from such strains to be used as a fining agent to reduce wine haze potential. The process has been patented by Stellenbosch University.
Ndlovu T, Divol B, Bauer FF. 2018. Yeast cell wall chitin reduces wine haze formation. Appl Environ Microbiol 84:e00668-18. https://doi.org/10.1128/AEM.00668-18.