Measurement of Wine Proteins by Size Exclusion HPLC
Haze in white wine caused by denatured wine proteins is a major problem for the wine industry. The majority of the wine proteins responsible for haze are grape pathogenesis related proteins, thaumatin-like proteins and chitinases. The concentration of protein in untreated white wines differs by variety and ranges up to 300 mg/L. The current method of haze prevention is to add bentonite, a montmorillonite clay, to remove the proteins before bottling. This batch process can result in voluminous lees of 5 to 10%. Even if the occluded wine is separated from the bentonite lees by rotary drum vacuum filtration, it is considered reduced in quality and thus value. There is therefore considerable interest in optimising the use of bentonite through different contacting and separation techniques, and in finding alternatives to bentonite treatment to prevent wine protein haze formation. As part of our work on improved process technologies for wineries, we developed a new time-efficient size exclusion HPLC method to quantify wine proteins. A reversed phase HPLC method was used for comparison. The size exclusion method separated grape invertase from the heat-unstable thaumatin-like and chitinase proteins but the reversed phase method achieved superior separation of the similarly sized thaumatin-like and chitinase proteins. In a bentonite fining trial, the thaumatin-like and chitinase proteins were removed with similar doses of bentonite while grape invertase required higher doses for removal. In a heating trial, the chitinase proteins were the most heat labile followed by the thaumatin-like proteins. The grape invertase was relatively thermostable.