Unstable proteins in wine and their removal by bentonite and gallotannins

Abstract

A range of commercially available 'new generation' gallotannins were trialled as a bentonite substitute for protein stabilisation of white wines. Gallotannins can precipitate proteins from model protein solutions and unfined wines. Unlike the model protein solution complete stabilisation of wine using solely gallotannins was not achieved and heat induced haze increased. A haze formed immediately gallotannins came into contact with proteins and a precipitate developed at low temperatures and/or high gallotannin-protein concentrations. The increase in heat stable haze was attributable to some proteins failing to have available gallotannin binding sites until they unfolded at high temperatures where upon unbound gallotannins rapidly attached to precipitate the proteins. SDS-PAGE demonstrated that gallotannins are most effective at removing low molecular weight (<20kDa) wine protein fractions. Of the eight gallotannins tested Brewtan and Brewtan C removed the greatest amount of proteins from wine. Tanal W2, Tanal W2S and Tanal W4 showed the least potential for protein binding.

Various methods of removing unprecipitated gallotannins from wine were trailed including polyvinyl polypyrrolidone (PVPP) and casein precipitation. However, these methods were unsuccessful. Immobilisation of gallotannin onto glass and cellulose was attempted but the presence of ethanol removed large quantities of gallotannin from the matrices.

Reductions of approximately 50% in the bentonite required for heat stabilisation of wine were achieved by co-fining with gallotannins. Centrifugation is necessary for clarification when gallotannin is used in conjunction with bentonite. Even without gallotannin treatment centrifugation of bentonite in comparison with decanting, could reduce requirements by approximately 25%.