Richard Muhlack

Degradation of white wine haze proteins by Aspergillopepsin I and II during juice flash pasteurization

Bentonite is commonly used to remove grape proteins responsible for haze formation in white wines. Proteases potentially represent an alternative to bentonite, but so far none has shown satisfactory activity under winemaking conditions. A promising candidate is AGP, a mixture of Aspergillopepsins I and II.; a food grade, well characterized and inexpensive protease, active at wine pH and at high temperatures (60-80°C). AGP was added to two clarified grape juices with and without heat treatments (75°C, 1min) prior to fermentation. AGP showed some activity at fermentation temperatures (≈20% total protein reduction compared to control wine) and excellent activity when combined with juice heating (≈90% total protein reduction). The more heat stable grape proteins, i.e. those not contributing to wine hazing, were not affected by the treatments and therefore accounted for the remaining 10% of protein still in solution after the treatments. The main physicochemical parameters and sensorial characteristics of wines produced with AGP were not different from controls.

Sustainable wineries through waste valorisation: a review of grape marc utilisation for value-added products.

Grapes are one of the most cultivated fruits worldwide, with one third of total production used in winemaking. Both red and white winemaking processes result in substantial quantities of solid organic waste, such as grape marc (pomace) and stalks, which requires suitable disposal. Grape marc accounts for approximately 10-30% of the mass of grapes crushed and contains unfermented sugars, alcohol, polyphenols, tannins, pigments, and other valuable products. Being a natural plant product rich in lignocellulosic compounds, grape marc is also a promising feedstock for renewable energy production. However, despite grape marc having such potential, advanced technologies to exploit this have not been widely adopted in wineries and allied industries. This review covers opportunities beyond traditional composting and animal feed, and examines value-added uses via the extraction of useful components from grape marc, as well as thermochemical and biological treatments for energy recovery, fuel or beverage alcohol production, and specialty novel products and applications such as biosurfactants and environmental remediation. New advances in relevant technology for each of these processes are discussed, and future directions proposed at both individual producer and regional facility scales, including advanced processing techniques for integrated ethanol production followed by bioenergy generation from the spent marc.

Optimal Conditions for Controlling Haze-Forming Wine Protein with Bentonite Treatment: Investigation of Matrix Effects and Interactions Using a Factorial Design

Protein instability in white wine can result in unsightly haze formation, and therefore, its prevention by adsorption of haze proteins onto bentonite is an important unit operation in commercial wine production. Optimisation of this process is challenging due to the performance impact of environmental factors and matrix effects which are difficult to control and study in wine systems. These issues are addressed in the present study; the effect of different factors on adsorption behaviour of a purified thaumatin-like grape protein (VVTL1) by sodium bentonite in a chemically defined model wine solution was investigated using a factorial design with surface response analysis. Bentonite adsorption of VVTL1 was well characterised by a multi-factor Langmuir adsorption model. The main effects of pH, temperature, potassium concentration as well as the pH*potassium matrix interaction all had a significant effect (p < 0.05) on the adsorption capacity, as did the aging of bentonite slurry before use. Observations support the hypothesis that VVTL1 adsorption onto sodium bentonite is affected by steric mass action and local interactions of exposed protein charge, with pH and temperature effects related to changes in protein conformation under those conditions. Variation in potassium concentration can cause similar effects and influence adsorption capacity by affecting bentonite swelling and charge potential, providing a greater surface area for adsorption. From a processing perspective, results suggest bentonite treatment efficiency will be optimised by treating wines at higher temperatures rather than during cold storage, at the lower pH and before cold (tartrate) stabilisation.

Modelling the Mass Transfer Process of Malvidin-3-Glucoside during Simulated Extraction from Fresh Grape Solids under Wine-Like Conditions

Extraction of grape components is a key consideration for red winemaking. The impact of changing process variables on mass transfer properties of anthocyanins from fresh pre-fermentative red grape solids under forced convective conditions was explored using the dominant red grape anthocyanin, malvidin-3-glucoside (M3G) as a model solute. A two level full factorial design was implemented to investigate effects of temperature, sugar and ethanol on mass transfer properties. Factor levels were chosen to simulate conditions found at various points during the maceration and fermentation steps of the red winemaking process. A rigorous mathematical model was developed and applied to experimental extraction curves, allowing the separation of mass transport properties in liquid and solid phases in a wine-like system, for the first time. In all cases, the coefficient of determination exceeded 0.92, indicating good agreement between experimental and mathematically-solved M3G concentrations. For the conditions studied, internal mass transfer was found to limit M3G extraction and changes to the liquid phase composition and temperature influence the distribution constant. Surface response models of mass transfer parameters were developed to allow future simulations of fermentation scenarios aimed at maximising the extraction potential of M3G.