Commercial Wine Alcohol Reduction Strategies

With A Focus On Evaporative Perstraction

article by rocco longo ph.d & rob gore

Introduction

Alcohol levels in wines have consistently climbed over the last few decades by up to 2% v/v, with Australian red wines being among the most alcoholic in the world (alcohol levels are typically greater than 13.5% v/v) (Alston et al., 2015). These increases are often linked to the hotter temperatures and seasonal occurrences (such as more frequent heatwaves), which can produce asynchronous flavour and sugar accumulation in grapes. As a result, winemakers must postpone harvest, with grape sugar concentrations at harvest frequently surpassing targets. In many cases, though, winemakers pick their crops later on purpose, hoping for wines with riper flavours that develop naturally with grape ripening.

Simultaneously with an increase in alcohol levels in wine, the World Health Organization (WHO) developed a global campaign in 2010 to limit the population’s alcohol consumption, prompting many nations to pass legislation such as boosting alcohol prices or requiring nutritional labelling for all beverages served (WHO, 2010). Due to these alcohol-related regulatory policies, as well as increased health consciousness among society, consumer perceptions of low-alcohol beverages may have shifted (Chrysochou, 2014). As a result, lighter wines with less than 12 percent v/v alcohol have grown in popularity (Bruwer et al., 2014), with an Australian study predicting that acceptance of wines with 3 to 8% v/v alcohol would rise from 16% to 40% if they tasted like regular alcohol wines (Bruwer et al., 2014). (Saliba et al., 2013).

The foregoing factors sparked a greater interest in solutions that could assist winemakers in better controlling the alcohol concentration of their wines (Longo et al., 2017). Many winemaking procedures have been implemented, including a modified irrigation strategy, juice water dilution, the use of low-ethanol-yielding non-Saccharomyces cerevisiae, spinning cone column (vacuum distillation), nanofiltration, and evaporative perstraction (Table 1). This article will go over some of the commercially accessible technologies for the Australian wine industry, with a particular emphasis on evaporative perstraction.

Table 1: A summary of possible ways for limiting the amount of alcohol in wine (Adapted from Longo et al., 2017)

Evaporative Perstraction

Evaporative perstraction (EP) is a relatively novel method for lowering the alcohol content of wine by up to 4-5% v/v. EP is a membrane-based process in which a feed (wine) and a stripping solution (usually water) pass through a microporous, hydrophobic hollow fibre membrane contactor on opposite sides (Figure 1). This method is also known as “isothermal membrane distillation” or “osmotic distillation.” When referring to the elimination of alcohol from wine, however, the term “evaporative perstraction” should be used (Diban et al., 2008).

The vapour pressure difference between the volatile solute (alcohol) in the feed (wine) and stripping solution (water) drives the mass transfer (Varavuth et al., 2009). The working pressure is normally kept lower than the capillary penetration pressure of liquid (wine) into the pores, so the hydrophobic membrane does not get wet. As a result, air gaps are generated in the pores to allow alcohol vapour from wine to flow through to the stripping solution (Diban et al., 2008). Because alcohol (ethyl alcohol) is one of the most volatile substances in wine, it moves through membrane pores faster than water. In addition, the solubility of aromatic molecules in wine is higher than in pure water, limiting their loss during the dealcoholisation process. Other factors may also limit the loss of aroma compounds such as the running conditions. Reducing the wine and water flow rates from 600 to 300 L/h, lowering the pH of the water stream from 7 to 3, and changing the feed/stripping volume ratio from 1.5:1 to 1:4.7 all assist offsetting the loss of volatile aroma components in wine (Diban et al., 2013).

Figure 1: Evaporative perstraction process

When the effect of EP on wine aroma was studied, a 2% v/v alcohol removal from a Merlot wine (which started out at 13.3% v/v alcohol) was accompanied by percentage losses ranging from 0.9% to 5.6% for 2-phenyl ethanol (the least volatile of the compounds studied) and from 57% to 98.l% for ethyl octanoate (the most volatile) (Diban et al., 2008). The hydrophobic property of the various compounds explains these discrepancies, with ethyl octanoate being the most hydrophobic. These chemicals moved through the pore air gaps into the stripping water with alcohol due to their attraction for the membrane contactor and their high volatility. Their adsorption onto the membrane could possibly account for a small loss of 2% to 3%. The authors stated that despite the aroma losses observed in the decreased alcohol wine, a sensory panel could not detect any difference between control and reduced alcohol wine. Lisanti et al. (2013) obtained a similar result. When tested by 12 panellists, a 2% v/v alcohol removal by EP had no significant effect on the sensory profile of an Aglianico red wine, according to the authors. When 5% v/v alcohol was removed, however, the intensity of various aroma descriptors diminished.

Ju.Cla.S Mastermind Remove

While some aroma components will be lost throughout the process, membrane-based technologies are still the most often used commercially to extract up to 4% v/v alcohol from wine (Longo et., 2017). EP technology, such as Mastermind® Remove (MMR) from Ju.cla.s (Verona, Italy) (Figure 2), is designed to remove alcohol from 0.1 to 4% v/v via direct passage of the wine on the membrane contactor (as illustrated in Figure 1).Due to the operating ambient temperature of 15-16°C and pressure of 0.2-0.3 bar, this procedure has negligible thermal damage to wine components and low energy requirement. Furthermore, the polymer that makes up the MMR’s membrane has no electric charge and has no interaction with the colloid system. As a result, the method has no influence on the colour or structure of the wine. MMR typically runs in the 15-18°C range, with higher temperatures projected to boost operation speed. While turbidity has no effect on the alcohol removal process by MMR, it is best to work with a wine that is free of abrasive materials (e.g., PVPP, Bentonite, silica soil) and has an NTU below 1 to ensure membrane longevity.

Figure 2: Evaporative perstraction with direct passage on membrane contactor (adapted from Wollan, 2010)

Figure 3: Ju.Cla.S Mastermind Remove

Technologies That Are Commercially Available

Reverse osmosis (RO) followed by evaporative perstraction (EP) is another commercial technology for reducing the alcohol content of wine (Wollan, 2005) (Figure 3). First, the wine is separated into retentate (concentrate) and permeate (filtrate) by RO. The retentate is returned to the feed tank, while the permeate containing alcohol and volatile compounds passes through a membrane contactor (Liqui-Cel®, Celgard), with a counter-flow of strip water on the other side. Alcohol and, a small percentage of desirable aroma compounds (depending on the process extent), pass through the membrane pore air gaps and condense in the stripping solution, which is subsequently discharged (Longo et al., 2018; Saha et al., 2018). The dealcoholised permeate is chilled and mixed back into the feed.

The key difference between this 2-stage method and the direct passage on membrane contactor is the RO treatment, which can expose the wine to trans membrane pressures of up to 50-70 bar, resulting in temperature increases. Aside from the additional operational and maintenance costs associated with using two membranes in the process, separating the wine into permeate and retentate (and then blending both streams back together at the end) means wines take longer to re-equilibrate before attaining their full sensory potential.

Figure 4: Reverse Osmosis followed by evaporative perstraction process (adapted from Wollan, 2010)

The spinning cone column (SCC) (as offered by Flavourtech) is the most suitable way for eliminating more than 4-5% v/v alcohol of all commercially available technologies. SCC is made up of up to 22 inverted and pointed downwards cones on a vertical, rotating shaft (Schmidtke et al., 2012). As seen in Figure 4, there is also a fixed inverted cone between each pair of cones. A stripping vapour stream is injected from the bottom of the column during the procedure. The vapour (whose temperature is controlled by the vacuum applied in the column) passes through the cones, collecting any volatile chemicals it has picked up from the wine at the top of the column. The wine is passed through the SCC for the first time at 28°C, with the goal of recovering the volatile aroma component. The second pass takes place at 38°C and is designed to remove the alcohol. The final wine is the result of blending the recovered aroma with the dearomatized and dealcoholised base (Schmidtke et al., 2012).

Although SCC can recover up to 100% of the wine’s total aroma fraction and remove all of the alcohol (Belisario-Sánchez et al., 2012), it requires a large initial capital investment and its operating costs (energy consumption can be three times that of reverse osmosis and up to 46 times that of evaporative perstraction) mean SCC is best suited to high-volume operations (Margallo et al, 2015).

Figure 5: Mechanical layout of the spinning cone column. 1, wine in; 2, wine out; 3, vapour in; 4, vapour plus volatiles out; 5, rotating shaft; 6, stationery cones; 7, rotating cones. Courtesy Flavourtech, Lenehan Road, Griffith, Australia

Conclusions

Winemakers are responding to a growing awareness of the negative health and societal consequences of excessive alcohol consumption, as well as a growing desire in lighter wine styles, by producing lower alcohol wines. Even though several winemaking techniques have been adopted, membrane-based systems such as the Mastermind® Remove are the most commercially viable options for removing up to 4% v/v alcohol. While a small fraction of aroma components is inevitably lost throughout the process, using evaporative perstraction with a direct route on a membrane contactor is an easy-to-adopt, adaptable, and cost-effective option. However, vacuum distillation using a spinning cone column is currently the sole way to remove more than 4-5% alcohol by volume.

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This article was originally published in the August edition of Grapegrower & Winemaker Magazine.