Project number
P04000019
Project leader
Dr Francois van Jaarsveld
Team member
Anton Nel
EXECUTIVE SUMMARY
Objectives and Rationale
The aim was to investigate winemaking processes that affect the pinking susceptibility of white wine, and to provide practical guidelines for the prevention of pinking to enable cellars to establish pinking risk practices. Sensory evaluations aimed at confirming the notion that pinking does not affect wine flavour, to confirm the optimal absorbance for visually pinked wines, and if tasters can differentiate between pinking and other wine faults. In addition, the identification of the compounds responsible for pinking was attempted.
Methods
Winemaking processes applied throughout the course of this project to investigate their impact on pinking include harvesting at different stages of grape maturity, applying different temperature treatments, subjecting grapes/pomace to different crushing and pressing methods and skin contact times, reductive and oxidative winemaking, and fermentation using different yeast strains. Fining agents and antioxidants as curative agents for pinking were investigated. Samples were subjected to different chemical analyses during the course of the project, including standard wine parameters (WineScan), volatile (GC) and less volatile (HPLC) compounds, with further characterisation of compounds using LC-MS and NMR. Three sensory evaluation experiments were conducted to confirm the notion that pinking does not affect wine aroma and taste, to confirm the optimal absorbance for visual pinking assays, and test whether or not tasters can differentiate between pinking and other wine faults. The impact of Botrytis cinerea infection on pinking potential of wines was determined in fermentation trials using must from infected and non-infected B. cinearae grapes, inoculated with five yeast strains.
Key Results
Treatments, i.e. grape maturity, temperature and time of skin contact, and their interactions, affected oenological data and colour parameters in both juice and wine, with groupings observed on PCA. Guidelines provided in this study gives some insight into the best fining agents and stages of application thereof to use for the prevention of pinking. Practical guidelines for the prevention of pinking to enable cellars to establish pinking risk practices have already been communicated in popular style in Wineland. Pinking influenced the aroma of the wine, but not the taste. Most tasters visually identified pinking at an AU of 0.03 (67%). Panellists also attributed other wine “faults” to pinked wines. Yeast strains in winemaking can contribute significantly to the protection of wine from pinking in both non-infected and infected Botrytis cinerea samples. Sunlight (UV) does decrease the pink colour considerably as determined spectrophotometrically, but not visually, questioning the effectiveness as a post-pinking treatment method. Naturally pinked wines are chemically distinct from control or non-pinked wines as shown by WineScan and LC-MS analysis and clear grouping on PCA and DA. Although chemical differences are noted, the compounds causing pinking is still evasive, but further investigated.
Key Conclusion of Discussion
Winemaking processes do affect the pinking susceptibility of white wine. Natural pinking under winemaking conditions influence the wine sensorially.
Recommendation to Industry / Key take-home message
Several winemaking processes affect pinking and this information needs to be communicated to industry/winemakers to enable them to take necessary precautions of which some will be a more economically viable option than the use of fining agents/antioxidants.