Effect of free and immobilised stem bromelain on protein haze in white wine

Background and Aims: Several studies have investigated the effectiveness of several proteolytic enzymes in free form as a treatment alternative to bentonite fining for wine stabilisation. This work tested the feasibility of stem bromelain, free or immobilised on a chitosan support, to reduce white wine protein haze potential. Methods and Results: The kinetic behaviour of stem bromelain, in free form and covalently immobilised on chitosan beads, was studied in 12 unfined white table wines fortified with a tripeptide chromogenic substrate. The effectiveness of the protease on wine hazing potential was determined by treating the wines in a laboratory-scale stirred reactor, and the heat test was applied to evaluate the decrease in nephelometric turbidity units. Immobilised protease was active towards the synthetic substrate and in reducing hazing potential of the wine. The effect of wine components on bromelain proteolytic behaviour, as investigated by principal component analysis, was much more noticeable for the free enzyme (in decreasing order of importance: pH, total phenolic substances, free and total sulfur dioxide) than for the immobilised form. Conclusions: An amount of 10 g/L biocatalyst (bromelain immobilised on chitosan beads), after a 24-h treatment in a laboratory-scale stirred reactor, revealed a high capacity to reduce wine haze potential (approximately 70%), which was unaffected by wine composition. Significance of the Study: Immobilised bromelain appears a suitable treatment alternative to bentonite fining for white wine haze stabilisation

Preventing protein haze in bottled white wine

Slow denaturation of wine proteins is thought to lead to protein aggregation, flocculation into a hazy suspension and formation of precipitates. The majority of wine proteins responsible for haze are grapederived, have low isoelectric points and molecular weight. They are grape pathogenesis-related (PR) proteins that are expressed throughout the ripening period post véraison, and are highly resistant to low pH and enzymatic or non-enzymatic proteolysis. Protein levels in un-fined white wine differ by variety and range up to 300 mg/L. Infection with some common grapevine pathogens or skin contact, such as occurs during transport of mechanically harvested fruit, results in enhanced concentrations of some PR proteins in juice and wine. Oenological control of protein instability is achieved through adsorption of wine proteins onto bentonite. The adsorption of proteins onto bentonite occurs within several minutes, suggesting that a continuous contacting process could be developed. The addition of proteolytic enzyme during short term heat exposure, to induce PR protein denaturation, showed promise as an alternative to bentonite fining. The addition of haze-protective factors, yeast mannoproteins, to wines results in decreased particle size of haze, probably by competition with wine proteins for other non-proteinaceous wine components required for the formation of large insoluble aggregations of protein. Other wine components likely to influence haze formation are ethanol concentration, pH, metal ions and phenolic compounds.E.J. Waters, G. Alexander, R. Muhlack, K.F. Pocock, C. Colby, B.K. O'Neill, P.B. Høp and P. Jone