Protein haze formation in white wines is a common concern in wineries. The problem is merely a visual defect, which does not affect taste and aroma traits, nor does it represent a health risk; however, consumers find this defect unacceptable. Understanding the main components, phenomena, and mechanisms involved in protein haze formation is essential to analyse and develop new protein stabilization technologies.
Which proteins become unstable?
The haze formation is mainly attributed to the slow denaturation of unstable proteins, which can occur during the storage or transport of white wines. The predominant haze-causing proteins are thaumatin-like proteins (TLPs) and chitinases (CHIs), which are grapes’ derived pathogenesis-related (PR) proteins. Other PR-proteins, such as β-glucanase, grape ripening-related proteins (GRIPs) like GRIP22 and GRIP32, invertase, and lipid transfer proteins (LTPs), have been identified as minor contributors to haze formation. Haze formation is mainly associated with CHIs and TLPs in the range of 15–30 kDa, where the latter is less susceptible to heat-induced haze than CHIs, even when the wine is subjected to temperatures of 30 °C for 22 h. These proteins have different unfolding properties; some TLPs have reversible behaviour, that is, they can be refolded after heating, whereas CHIs are characterized by irreversible behaviour, that is, they, as class IV chitinase, remain unfolded after heating. This characteristic could explain why CHIs are more susceptible to haze formation.
What are the factors affecting haze formation?
Non-protein related factors that affect haze formation include polysaccharides, polyphenols, sulphates, pH changes, and factors that increase the concentration of proteins in grapes, such as machine harvesting, long-distance transport, grape infections, and climate change. Phenolic compounds and sulphate promote the growth of protein aggregates and increase haze, while polysaccharides can interfere with the aggregation process but not prevent it. Additionally, a slight change in pH could affect proteins’ solubility due to their interaction with other macromolecules.
During ripening, grapes are prone to fungal infections, injury, or stress, leading to high concentrations of PR-proteins. For instance, the powdery mildew grapes infection—caused by Uncinula necator fungal pathogen—triggers an increase in PR-proteins. Sulphates also influence haze formation, which, in the presence of CHIs, produces a slight haze increase in white wines.
What is the current treatment?
Batch addition of sodium bentonite is the most used method to treat white wine instability; however, it is associated with negative environmental impacts, wine losses, and quality degradation. Additionally, bentonite addition is a relatively expensive method; besides bentonite, other relevant costs are intensive labour, operating cost, and waste management. Consequently, new methods are required to reduce the cost and environmental impact of the current protein stabilisation technology. Various methods have been researched over the past few years but none of them have been able to replace bentonite usage. They are:
- Improving traditional stabilisation (bentonite)
- High power ultrasound treatments
- Heat plus enzyme treatments
- Immobilized enzyme treatments supported on chitosan
- Magnetic nanoparticles coated with acrylic acid by plasma polymerisation
- Zeolites
- Grape seeds powder
- Zirconium oxide
- Mannoproteins
- Carrageenan and pectin
- Chitin and chitosan
- Yeast strains with high chitin levels
Reference:
Silva-Barbieri D, Salazar FN, López F, Brossard N, Escalona N, Pérez-Correa JR. Advances in White Wine Protein Stabilization Technologies. Molecules. 2022; 27(4):1251. https://doi.org/10.3390/molecules27041251
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