To ferment hot, or not...

By Karien O'Kennedy
The main aim of this study was to determine if it is indeed necessary to ferment at low temperatures to achieve the optimal balance between fruity and green varietal characters in S. blanc and whether chemical and sensory data correlate for the attributes high fruitiness / low greenness.

Project layout:
  • Two different S. blanc juices from the 2008 vintage (results only published in 2017) were fermented with four different yeast strains: EC1118, Zymaflore X5, Enoferm M2 and L-1528.
  • Fermentations were conducted at 12.5°C and 25°C in 750 ml bottles.
  • Basic chemical analysis was conducted using a Foss WineScan spectrometer.
  • Volatile compounds were analysed using a GC-MS.
  • A trained sensory panel consisting of 11 students were used for sensory analysis.

Results:
  • Only EC1118 yeast fermented to dryness. This is in accordance with the manufacturer’s temperature range recommendations (10 – 30°C).
  • All three the other yeasts got stuck at 12.5°C as the minimum temperature recommendations of the manufacturers of M2 (15°C min.) and X5 (13°C min.) were not followed. L-1528 is not a commercial yeast.
  • With the exception of EC1118, known for its robustness, all three the other yeasts also got stuck at the high (but very favourable for S. cerevisiae) fermentation temperature of 25°C, despite the very low initial sugar concentrations of 19 and 22°Brix for the two musts used. This finding indicates a possible flaw in the experimental design since both M2 and X5 regularly ferment such sugars to dryness in practice. The experimental layout does not indicate the clarity of juice after cold settling (NTU), nor were any yeast nutrients added to fermentation. Both factors could have influenced fermentation security.
  • Basic wine chemistry results therefore differed between the different treatments due to the stuck fermentations, especially in terms of residual sugar and alcohol levels.
  • The 12.5°C wines had lower 3MH levels than the 25°C wines, but had higher levels of fruity acetate esters.
  • In the sensory analysis (only sniffing, not tasting) the tasters could not distinguish between the lower and the higher fermentation temperatures for the same wine in 75% of the wines analysed despite differences in chemical analyses.
  • In 25% of the wines where the panel did pick up a difference they described the 25°C as fruitier, mostly like as a result of higher 3MH levels. 
Conclusion
The researchers concluded that cold fermentation does not necessarily enhance the aroma profile of S. blanc. Cold fermentation however, may promote an optimal balance between tropical and green notes. The wines were not evaluated in terms of balance and preference and the researchers are also mindful that the differences in sugar levels between the wines could have affected aroma volatility. They do see the possibility that cellars can reduce their energy consumption by not cooling fermentations down so extensively and free up more tank space by fermenting warmer.

Significance of the study
The study is interesting but leaves the reader with a few questions:
  • Why were the recommendations from yeast suppliers not followed in terms of minimum temperatures for fermentation?
  • Why did the 25°C fermentations get stuck?
  • Why were the triplicate wines pooled and not individually analysed and standard deviations indicated on average values?
  • If a different panel was used, would they also not have picked up a difference between the 12.5°C and 25°C wines?
  • If the trial was done on semi-commercial scale, would the results be similar?
  • Why was the study only published in 2017 when the grapes were harvested in 2008?
Unfortunately at this stage the study leaves too many questions and uncertainties to recommend to cellars to shift from their tried and trusted cool fermentations (15 – 16°C) to above 20°C for S. blanc.

Reference
Rebecca C. Deed, Bruno Fedrizzi, and Richard C. Gardner (2017). Influence of Fermentation Temperature, Yeast Strain, and Grape Juice on the Aroma Chemistry and Sensory Profile of Sauvignon Blanc Wines. Journal of Agricultural and Food Chemistry 65 (40), 8902-8912.
DOI: 10.1021/acs.jafc.7b03229

Image: Shutterstock



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