The affinity of white wine proteins for bentonite is dependent on wine composition and is directly related to their thermal stability / sensitivity

Abstract

Bentonite fining is commonly used in oenology to remove all or parts of white wine proteins, which are known to be involved in haze formation. This fining is effective, but has disadvantages: it is not selective, thus molecules responsible for aroma are also removed, it causes substantial volume losses, and finally it generates wastes. Over the last decades, the knowledge of wine proteins has increased: they have been identified, their structures are known, some of them have been crystallized. 

However, haze formation is not only a question of protein composition and concentration. It depends on many other factors, such as pH, wine composition (polyphenols, polysaccharides,…). Heat or chemical tests used to adjust the bentonite dose often leads to an overestimation, because they aim at removing all the proteins, even the ones that are stable in the range 60-80 °C and are not involved in spontaneous haze. 

In this study, we analyzed and quantified the proteins in 7 white wines (3 varieties, 4 areas), treated with four bentonite doses ranging from 5 to 80 g/hL. In parallel, samples of wines were heated during 30 minutes at 40, 60 and 80 °C and the residual proteins analyzed. 

The wines differed in their protein composition. In each wine, when they were present, the proteins were adsorbed on bentonite in this order: chitinase and β-glucanase, Lipid Transfer Protein (LTP), Thaumatin Like (TL) 22 kDa, TL 19 kDa and Invertase. 

The adsorption of a given protein was wine dependent. This could be due to wine pH and ionic strength (different in the studied wines), which changes electrostatic interactions that drive the protein adsorption onto bentonite, but also to other differences in composition (ethanol, polysaccharides, polyphenols, metals…). Experiments performed at pH 2.5 indicated that pH is not the only cause of such different adsorption behaviours: indeed adsorption isotherms were different. 

Protein adsorption on bentonite was compared to their thermal sensitivity. It was ranked as previously: β-glucanase ~ Chitinase > TL22 > TL19 ~ Invertase > LTP. It is worth noting that the most thermostable proteins are the ones which need the highest doses of bentonite on a wide panel of wines. These stable proteins do not need to be removed and thus bentonite doses could be reduced. More specific tests, which would take into account only the most sensitive proteins need to be developed.