Issue of unstable proteins
◦ No hazard to health of consumers
BUT ◦ Potential to form haze, which is
considered as wine fault
SO ◦ Have to achieve protein stability
positive charge; when pH condition above its isoelectric point, the protein will carry a net negative charge. ◦ Most of proteins in wine have their pI above the wine pH, thus they are positive charged
◦ Protein stabilization: process used in white wine production to remove the excess proteins to avoid any potential protein haze formation after bottling
◦ The pI of a protein is the pH at which the net charge of a protein is zero. ◦ For a protein, when pH condition below its isoelectric point, the protein will carry net
◦ Cold stabilization: process which involves chilling below 0°C to promote formation and precipitation of salt crystals (tartrates)ห้องสมุดไป่ตู้and then racking/filtering to remove them
• alpha helix or beta sheet • Hydrogen bonds
◦ Tertiary structure
• Weak interactions: Hydrogen bonds and Van der Waals interactions
• Ionic bonds: + and – charged amino acid side-chain groups • Disulphide bonds
insoluble ones precipitate in lees ◦ Little proteins are from yeast autolysis
How much of proteins in white juice and wine?
In the range of 30-700 mg/L in juice and 10-250 mg/L in wine, which could be influenced by:
Protein stability
White wine only issue!
What about red wine?
◦ Phenolic compounds extracted from skin could interact with proteins, and lead to precipitation
◦ Quaternary structure
• two or more polypeptide chains to form a complex
Isoelectric point (pI)
◦ The ionisable side-chain groups. A protein with more + than – ionised side-chain groups would have a net + charge, and vice versa. If there is no net charge the protein is neutral and becomes insoluble.
before bottling
Unstable proteins in wine
◦ Pathogenesis-related (PR) proteins: main soluble proteins in wine
◦ The season ◦ Variety of grape ◦ Grape growing region ◦ Ripeness ◦ Crop yield
Protein structure
◦ Primary structure
• sequence of amino acids
◦ Secondary structure
STABILIZATION
Bin Tian, PhD in Wine Science WINE202, 2015
Stabilization
The process of treating a wine to make it stable and clear for years after bottling, including heat (protein) stability and cold stability.
◦ Protein stability is not an issue in red wine
Where are proteins from?
◦ Major of proteins are from grape berries ◦ During crushing: soluble proteins dissolve in juice and
If the pI of the protein below If the pI of the protein above wine pH, it is - charged wine pH, it is + charged
Wine pH
+ + + + + + ++++ - - - - - - - - - -