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 grape‐derived, 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.

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Background and Aims: Precipitation of unstable proteins present in white wine after bottling can cause cloudiness, which is generally considered commercially unacceptable. Winemakers therefore use bentonite to remove protein prior to bottling, however, this can result in losses of wine volume and/or sensory quality. As such, there is interest in alternate strategies for protein stabilisation. This study evaluated the potential for ultrafiltration (UF), in combination with heat and proteolytic enzymes, to remove haze-forming proteins and stabilise white wine. Methods and Results: Heat-unstable white wines were fractionated using UF membranes with nominal 10 or 20 kDa molecular mass cutoff specifications, to first assess protein removal. Fractionation of wines with the 10 kDa membrane generated heat-stable permeate and protein-rich retentate. Pilot and semi-commercial scale trials were therefore conducted with 10 kDa molecular mass cutoff membranes and conditions optimised for heat and enzyme treatment of retentate. Heating retentate at 62 C for 10 min (with or without enzyme addition) achieved significant protein removal (30-96%, depending on the protein concentration of wine). Recombination of treated retentate with permeate gave wine that was almost heatstable, such that a substantially reduced amount of bentonite (~50-60%) was required to achieve full heat stabilisation. Conclusions: Ultrafiltration can fractionate white wine, enabling targeted heat and enzyme treatment of a protein-rich fraction of wine, thereby mitigating the impacts of traditional stabilisation treatments on wine aroma and flavour. Significance of the Study: This is the first study to evaluate the combined use of UF, heat and proteolytic enzyme treatments as an alternative approach to protein stabilisation of white wine.

Grapevine cane and stalks were considered for pyrolysis at 400 to 700 °C to produce biochar for increasing the water holding capacity of vineyard soil. Feedstocks were pyrolysed using a continuous feed reactor and the resulting biochars characterized in terms of physico-chemical properties, including water retention performance. Hydrophobicity was found in biochar from both feedstocks pyrolysed at 400 °C, but not at higher temperatures. At low soil matric potential, the pyrolysis temperature was the defining variable in determining water retention whereas at higher pressures, the feedstock was the more important variable. Available water content (AWC) of biochar increased with increasing pyrolysis temperatures, with optimal results obtained from grapevine cane at a pyrolysis temperature of 700 °C, which had an AWC 23% higher than a typical clay type soil. Principal component analysis showed variability in water retention of these biochars to be closely associated with the zeta potential, as well as the carbon and ionic content, suggesting that surface charge and hydrophobicity are key properties determining water holding capacity. Pure biochars were superior in water retention performance to typical sandy soils, and so biochar amendment of these soil types may improve water holding (particularly at field capacity). Further study with pot or field trials is recommended to confirm water retention behaviour and assess the feasibility of application under different viticultural scenarios.

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