Wine haze related to proteins

Nitrogen compounds in wine include proteins. Proteins contribute significantly to the concentration of nitrogen not digestible by yeasts in must and wine. They are the most common cause of clouding in white wines.

Protein blurring is a problem of serious economic importance for wine production as clarity is an important quality characteristic of wine. The identity, origin and factors influencing the formation of proteins associated with wine turbidities were examined by Hoj et al. (2000).

Most of the proteins synthesized in the rail are the same ones identified as being responsible for turbidity formation in white wines (Waters et al. 1996, 1998). These proteins are stable in the pH of the wine and resist proteolysis, and therefore do not deteriorate during winemaking processes, thus being added to the nitrogen not digestible by the yeasts in the wine.

Their agglomeration during storage in the bottle can lead to the formation of turbidity.

The synthesis of proteins in the rail begins during veraison and their quantity continues to increase as maturation progresses (Tattersall et al. 1997, Hoj et al. 2000, Pocock et al. 2000). Thus, the addition of nitrogen to the vine is expected to magnify the levels of accumulated protein as the rail matures, thereby increasing the unwanted nitrogen content in nitrogen not digestible by the yeasts and possibly the risk of turbidity.

Only one study studied the effect of adding nitrogen to the vineyard on soluble must proteins (Spayd et al. 1994). The concentration of soluble proteins in wine from nitrogen-treated plants was 4.8 times higher than in wines from plants not to which nitrogen was not added. As a result, the requirement for bentonite increased by 6 times.

The assessment of the amount of nitrogen to be added to the vineyard should take into account the specific characteristics of the variety. For example, Sauvignon blanc berries accumulate about twice as much protein as Sultana rails  when both grow in the same conditions (Pocock et al. 2000).

In addition, 45-54% of the total protein content associated with turbidities was found in the cortices (Pocock et al. 1998). Thus, the addition of nitrogen to any white wine grape variety characterized by the accumulation of high concentrations of protein, in which pre-fermentation extraction is applied and/or transported over long distances without special care, can increase the likelihood of clouding in the bottle, compared to a variety that accumulates lower protein concentrations.

In conclusion, when nitrogen is added to the vineyard, the viticulturist should take into account the specific characteristics of each variety regarding protein accumulation and apply strategies that minimize contact with the bark under inappropriate transport conditions (Sally-Jean Bell and Paul A. Henscke, 2005).

Source: Bell S-J and P.A. Henschke , 2005. Implications of nitrogen nutrition for grapes , fermentation and wine. Australian Journal of Grape and Wine Research 11 , 242-295.