Bentonites, interaction with wine proteins and fermentation aromas

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Despite its technological importance, much is still to be clarifi ed about the constitution, the behaviour and the evolution of colloidal substances in wine products. In fact, the subject is very complicated and it is one of the knottiest questions to be solved in oenology. As well known, the composition of the grapes include some colloidal substances, generally with complex structure, whose molecular weights range in prevalence between 10,000 and 20,00 Dalton, with peaks that can reach the million. The nitrogenous colloids with average of high molecular weight and glucidic colloids, or “protectors” are essential for the structure and the colloidal stability of the wines. First of all, it is important to highlight that the former ones, i.e. proteins, are subject to easy coagulation and fl occulate and, therefore, can give way to turbidity and sedimentations in the wine, while the latter ones, i.e. glucidic colloids, like the relevant hydrolysis products, rather seldom insolubilise. Therefore, the danger of alterations of the limpidity derives almost exclusively from proteinic colloids (1). Besides concentration, the instability of the wines depends also on the nature of proteins. Most authors agree thinking that the proteins with lower molecular mass (between 15,000 and 35.000 Dalton) and lower isoelectric point are those most responsible for the formation of particulate (2; 3). Only a part of the proteinic fraction seems to the involved in colloidal instability, a phenomena including also a series of non-proteinic compounds, such as polysaccharides and polyphenols, that can modify the efficiency of the stabilization treatments. Since the pH of the wine is lower than most isoelectric points of the proteins, they have positive charges and can interact with various agents with opposite charge to bind with each other and then precipitate. In most cases, the stabilization of rosé or white wines as to the proteinic class, is obtained through bentonite, whose mechanism of action is mainly based on the principles of above-described electrostatic interaction. Bentonite is a natural clay-based mineral belonging to the group of the montmorillonites (aluminium hydrate silicates) swelling in water, getting more or less large, and jellifying. From the morphological point of view, it has a lamellar structures alternated with exchangeable cations and hydration water. The nature of ions, prevalently calcium and sodium, strongly infl uences some properties of clay, like, for instance, available surface and exchange capacity (1). Being electronegative, the lamellas of bentonite can interact with the proteinic molecules with positive charge to the pH of the wine; as a consequence of that, the proteins combined are eliminated from the liquid through precipitation with the particles of bentonite (4). As cation exchanger, bentonite is not specifi c for the proteins only, yet it removes also other charged species or aggregates.

Therefore, high additions of them can induce a reduction in the organoleptic properties of the wines debasing the confi guration of their aromatic compounds (5). Studies demonstrated that some macromolecules of the wine play an important role in the maintenance of the aromatic balance of the product; however, said macromolecules can be removed in more or less important quantity during the stabilization treatment. The removal can be ascribed to both a direct interaction between bentonite and the aromatic compounds and an indirect removal linked to the elimination of proteins (6). So, it is interesting to understand more details of the interaction of bentonites with proteins having different molecular weight that are naturally contained in white wines and evaluating the possible impact on the aromatic compounds originated from the fermentation.

Research

The experimental tests are part of a wide research project developed in cooperation between the Institute of Oenology and Agrofood Engineering of the Università Cattolica del Sacro Cuore di Piacenza and Azienda Dal Cin Gildo S.p.A. di Sesto San Giovanni (MI).

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