The effect of must sterol content on alcoholic fermentation



By Karien O'Kennedy
The aim of the study was to investigate the phytosterol content of grape solids that form part of grape must turbidity and how it affects yeast cell viability and fermentation rate.

Project layout:
  • Solid particles from seven different musts were obtained: two red, two rosé and three white musts.
  • Fermentations were conducted in synthetic grape must with 250 g/L sugar (equal concentrations of glucose and fructose) and a YAN of 425 mg/L (72% amino acids and 28% ammoniacal nitrogen).
  • A preliminary experiment was set up with a negative control (synthetic must with no sterol or solid particle addition), as well as synthetic must with 1.5, 3.0, and 6.0 mg/L synthetic β-sitosterol respectively.
  • The first set of experiments was set up with a negative control (synthetic must with no sterol or solid particle addition), a positive control (synthetic must with 1mg/L synthetic β-sitosterol), and synthetic must with solid particles from the different real grape musts with a final phytosterol content of 1 mg/L.
  • The second set of experiments comprised a negative control and synthetic musts containing the different real grape must solid particles with turbidity levels of 30 NTU.
  • Fermentations were conducted in 1.1 L cylindrical glass bottles with Lalvin EC 1118 at 24°C.


Main findings:
  • The phytosterol content of the different grape solids (sludges) varied considerably.
  • Β-sitosterol is the main sterol in grape must.
  • In the preliminary experiment it was found that increasing synthetic phytosterol concentration increased the maximum yeast population.
  • In the first set of experiments must turbidities varied from 14 to 38 NTU after the addition of the different grape must solids to obtain 1 mg/L phytosterols. This finding indicates that no simple relationship between sterol content and must turbidity exists.
  • Grape solid addition increased nitrogen assimilation as well as maximum yeast population.
  • None of the fermentations went to dryness indicating that the amount of phytosterols (1 mg/L), regardless of origin, were not enough to allow completion of fermentation. Yeast normally needs approximately 5 mg/L phytosterols to complete fermentation if must nitrogen content is 300 mg/L. When the nitrogen content of musts is higher, larger concentrations of grape solids and lipids are needed to complete fermentation.
  • All fermentations containing phytosterols, synthetic or natural, fermented faster than the negative control with no sterols added.
  • In the second set of experiments (all at 30 NTU) the fermentation kinetics varied depending on the origin of the grape solids, with the fastest fermentations being the synthetic must containing the highest sterol content.

Significance of the study
  • The study indicates that the sterol content of grape musts play a very important role in yeast viability and fermentation speed.
  • Excessive clarification without the addition of exogenous sterols (e.g. from yeast) can lead to sluggish or stuck fermentations as a result of low phytosterol content.
  • There must be a balance between sterol and nitrogen content. Higher YAN’s require higher sterol content.
Reference:


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