5.1.4.1 Nanofiltration
Nanofiltration as a post-fermentation technique works on a similar principle as for pre-fermentation technique (see 5.1.2.3), whereas by wine processing two fractions are obtained after separation, the high- ethanol fraction (permeate) and low-ethanol fraction (retentate). Similarly to all membrane separation process that have goal to reduce alcohol level in wines, the efficiency of nanofiltration depends on mixture of factors such are ethanol rejection coefficient, other wine compounds rejection coefficient (e.g. aroma compounds, acids, phenolic compounds) permeate flux, operating conditions (e.g. temperature, pressure, time) and membrane characteristics (e.g. material, pore size). In that regard, Catarino and Mendes (2011) conducted a study aiming to evaluate the efficiency of several nanofiltration membranes by regulating several of mentioned factors. Authors concluded that certain membranes may be used for the production of low-alcohol wines, especially if nanofiltration is combined with pervaporation (Catarino and Mendes, 2011). However, this additional equipment (e.g. pervaporation) may increase initial investment which is a down side of this combined approach. Other study reported that utilization of nanofiltration as a single technique may decrease ethanol content until 8% v/v which is followed by less than 15% w/v aroma compounds content decrease (Labanda et al., 2009).
5.1.4.2 Reverse osmosis
Reverse osmosis is a similar technique to nanofiltration and requires utilization of semi-permeable membranes with smaller pores size (0.1–1nm) when compared to nanofiltration. Thus reverse osmosis requires higher operating pressure and higher energy consumption when compared to nanofiltration which is one of the down sides of this technique (Gonçalves et al., 2013). Other down sides of reverse osmosis may be related lower permeate flux when compared to nanofiltation (Catarino and Mendes, 2011). However, several studies reported that utilization of reverse osmosis may be used for partial dealcoholization of wines (~2% v/v reduction) with hardly detectable sensorial differences (Gil et al., 2013) and with lack of differences in phenolic compounds content (Bogianchini et al., 2011) when compared to original wines.
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5.1.4.3 Pervaporation
Pervaporation is another membrane technique, however comparing nanofiltration and reverse osmosis membrane, pervaporation needs utilization of hydrophobic membranes which are not allowing liquid (e.g. wine) passage through membrane pores. Instead, ethanol and other wine volatile compounds (e.g. aroma compounds) are partially evaporating on relatively low temperatures (~40°C) and migrating through the membrane as a vapor due to differences in a partial pressure created by the vacuum on the other side of membrane. Vapor rich in ethanol and with a certain amount of aromatic compounds is afterwards condensed (Takács et al., 2007). Takács et al. (2007) evaluated possibilities to apply pervaporation as a technique to remove ethanol content from Tokaji Hárslevelű wines and concluded that working temperature plays a key role on the process efficiency, whereas 40°C was optimal to produce almost free alcohol product that matches organoleptic characteristics of a wines. Authors are also pointing the down side of this technique which is related to high initial economic investments (315k€)(Takács et al., 2007). Other study, investigated the possibility to use pervaporation in combination with nanofiltation to remove excessive ethanol from a red wine, whereas high-quality low-alcohol wines were produced (Catarino and Mendes, 2011). However, initial economic investments are most likely even higher when compared to single pervaporation technique.
5.1.4.4 Evaporative perstraction
Evaporative perstraction or also called osmotic distillation is a technique like pervaporation that use hydrophobic membranes, whereas separation of volatile compounds (e.g. ethanol, aromatic compounds) from the liquid (e.g. wine) is achieved by vapor pressure gradient between two sides of the membrane. Differences between two techniques are utilization of water that flows as stripping fluid in contra current on membrane side opposite to wine, and absorbs volatile permeate compounds. The up side of this technique is the fact that solubility of aroma compounds is higher in wine (feed fluid) when compared to pure water (stripping fluid), so the transfer of aromatic compounds in the water phase is limited (Diban et al., 2008). Thus, evaporative perstraction may be used for production partially dealcoholized wines (2% v/v removal) with good sensory characteristics. In fact, Diban et al. (2008) reported that despite certain aroma compounds losses in Merlot wines during the partial alcohol removal (2% v/v), there was a lack of differences in wine sensory characteristics. Other studies also reported lack of difference in wine sensory characteristics as well (Liguori et al., 2013; Lisanti et al., 2013), but also in volatile acidity, organic acids concentration, total phenolic content and color (Liguori et al., 2013a) in Aglianico wines once ethanol content was removed up to 2% v/v. However, Lisanti et al. (2013) also reported that differences in wine sensory characteristics were noticeable once ethanol content was reduced by 5% v/v, indicating that this technique might be suitable only for ‘mild’ ethanol removal from wines (up to 2% v/v). In fact, total dealcoholization (0.2% v/v remaining ethanol content) of Aglianico wines by evaporative perstraction caused reduction of aroma compounds by 98% (Liguori et al., 2013b). Another study also reported a significant aroma compounds losses (44–70%) in red wine once ethanol content was reduced up to 38% (Varavuth et al., 2009).
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5.1.4.5 Spinning cone column
Spinning cone column is based on the production of low-alcohol wines in two steps. The first step presents dearomatization of wine in spinning cone column under vacuum and low temperatures (26°C). The products of the first step are the gas fraction (stripping agent and volatile compounds) and liquid fraction (dearomatized wine). The second step presents ethanol removal from the dearomatized wine in spinning cone column at equal pressure and slightly higher temperature (~30°C). Dealcoholized and dearomatized wine is afterwards mixed with aromatic fraction to obtain lower-alcohol level wines (Belisario-Sánchez et al., 2012, 2009). Lower alcohol level wines produced by a spinning cone column may have acceptable antioxidative ability, phenolic compound content (Belisario-Sánchez et al., 2009), and aromatic compounds content when compared to raw wines (Belisario-Sánchez et al., 2012). However, spinning cone column has a high demand of energy when compared to other techniques related to physical removal of ethanol (e.g. evaporative perstraction) which is down side of this technique (Diban et al., 2013).
5.1.4.6 Vacuum-distillation and supercritical CO2 extraction
The combination of vacuum-distillation and supercritical extraction with CO2 may also serve as technique to remove excessive alcohol from wine. The working principle is based on two-step processing. The first step presents vacuum distillation at a certain temperature range (24–28°C) and high vacuum (35–50mbar) that separates wine on a low-volatile fraction (wine base) and high-volatile fraction (alcohol and volatile aromas) due to differences in boiling temperatures. The second step presents supercritical CO2 extraction at high pressure (80–100bar) and certain temperature range (25–35°C) that separate high-volatile fraction on liquid ethanol-water mixture and gas mixture (CO2 and aromas) due to differences in extraction features. The gas mixture is afterwards adequately separated and aromas added into wine (Seidlitz et al., 1992). As for the majority of post-fermentation techniques down sides are certain sensorial differences that may occur due to partial removal of aromas and aimed ethanol removal (Medina and Martinez, 1997) and a high capital cost of the process (e.g. high-vacuum distillation) (Schmidtke et al., 2012).